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Arjo-Dedessa Irrigation Project Feasibility Study Report
VOLUME - III
Water Resources
VOLUME - III (b)
Annexure - 9 Dam and Appurtenant Works Part - I Report
4
May. 2007
Addis Ababa
WATER WORKS DESIGN & SUPERVISION ENTERPRISE
IN ASSOCIATION WITH
INTERCONTINENTAL CONSULT ANTS ANO TECHNOCRATS INDIA PVT ITO
P O Box 2561 Addis Ababa Ethiopia
Tc) (251)1 614501/631890 Fax (1251)1 6l537IE-mail u u d sct/tclccom net cl1
ARJO-DEDESSA IRRIGATION PROJECT FEASIBILITY STUDY REPORT
CONTENTS OF THE REPORT
SERIAL NO.
VOLUME NO.
PARTICULARS
1
2
3
EXECUTIVE SUMMARY
MAIN REPORT
Part I - Report
Part II - Maps & Drawings
ANNEXURES
VOLUME -1
SURVEY AND INVETIGATION (ANNEXURES - 1 to 3)
4
Volume -1 (a)
Topograpnic Survey Geomorphological Studies &
Geological & Geotechnical Investigation
Part I - Report
5
Volume -1 (b;
Pan II - Appendices
6
VOLUME -II
NATURAL RESOURCES (ANNEXURES - 4 to 6)
Forestry Energy & Catchment Development Plan
VOLUME - III               WATER RESOURCES (ANNEXURES - 7 to 11)
7 Volume - III (aj Meteorological & Hydrological & Hydrogeological Studies 
Dam & Appurtenant Works
Volume - III (b)
Part I - Report
9
Part II - Drawings
Irrigation & Drainage
10
11
Volume-III (c)
Part I - Report
Part II - Drawings
Hydraulic Structures
12
13
Volume - III (d)
Part I - Report
Part II - Drawings
VOLUME IV
AGRICUTLRUE (ANNEXURES - 12 to 18)
14
15
16
Volume - IV (a)
Soil Survey & Land Evaluation
Volume - IV (b)
Agricultural Planning
Volume - IV (c)
Livestock, Fisheries Agricultural Mechanization &
Agncultural Marketing
VOLUME - V
ENVIRONMENTAL AND SOCIO - ECONOMIC ASPECTS
(ANNEXURES-19 to 25)
17
18
Volume -  v (a)
Environment, Health & Socio-economic Aspects
Volume-v (b)
Organization & Management, Physical Infrastructure
Resettlement, Financial & Economic AnalysisArJo Dedessa Irrigation Project
Dam Appurtenant Works
May 2007
TABLE OF CONTENTS
TABLE OF CONTENTS............................................................................................................................................................ ...... I
LIST OF TABLE............................................................................................................................................................................. iv
LIST OF FIGURE............................................................................................................................................................................ IV
1  INTRODUCTION......................................................................................................................................................................... I
1.1 General............................................................................................................................................ -................................... I
1.2   Background & Review of Previous Studies..............................................................................................................I
12.1 Type of Dam................................................................... -............................................................................................  5
/ 2.2 Dam Section..................................................................................................................................................................  5
1.2.3          Dam Foundation Treatment................................................................................................................................. 5
1.3 The Present Dam Site........................................................................................................................................................ 5
2   GEOLOGY......................................................................................... ..................................................................... .................. IO
2 I General............................................................................................................................................................................... IO
2.2 Dam site.............................................................................................................................................................................. 10
2.2.1 Left Abutment.............................................................................................................................................................. 10
2.2.2 Valley Floor and River bed.......................................................................................................................................  12
2.2.3 The Right abutment....................................................................................................................................................  J3
2.3 Saddle Dam Sites............................................................................................................................................................. 13
2.4 Geological Structures................................................................................................................................................. 14
3.          DAM FOUNDATION........................................................................................................................................................ 15
3.1 General...............................................................................................................................................................................15
3.2 Stripping For Dam Foundation..................................................................................................................................... 15
3.2. 1 Excavation Below Core Seating.............................................................................................................. ................... 15
3.2.2 Excavation below Shell Zone........................................ .................................................................... „....................... !6
3.2.3 Foundation Treatment.................................................................................................................................................!7
3.3 Grouting Requirements..................................................................................................................................................18
3.3   4 Curtain Grouting........................................................................................................................................................18
4    CONSTRUCTION MATERIALS....................................................................................................................... _.................... 20
4.1    Borrow areas for clay............................................................................................................................................... 20
4.1             1 ATE area........................................................................................................................................................   22
4.1.2          Chewo village...................................................................................................................................................... 23
4.               /. 3 Dendessa Area...............................................................................................................................................24
4 1.4 Maribo village............................................................................................................................................................... 25
4.1.5 Obose Area.................................................................................................................................................................... 26
4.2    Borrow Areas for Rock Fill Shell Material................. .................................................. ........ .......................... 26
4.21 AD SH Q area...............................................................................
4.2.2          Guche Area.........................................................................
...28
.28
4.3 Quarry Sites for Rock Rip Rap......................................... ....
4.4 Quarry Site for Coarse aggregate......................................
4.5 Sand...........................................................................................
4.6 Analysis Test Results of Materials for Construction
...28 
...29 
...29 
...30
4.6. i Gradation of Clay Material.........................................................
4 6.2 Plasticity Index.............................................................................
4 6.3 Permeability.................................................................................
4.6.4 Dispersivity..................................................................................
4.6.5 Classification of clay mineralogy...............................................
4.6.6 Swelling Property........................................................................
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30
.30
. 31 31
.31
4.7 Shell Material..........................................................................
...3?
4.8 Sand...........................................................................................
...37
4.8.1          Gradation curve of sand material......................................
37
4.9 Design of Filter........................................................................
...37
4.9.1 General........................................................................................................................................................................
 37
4.9.2 Sand Filter Design......................................................................................................................................................
 38
4.9.3 Design of Transition Filler ................................................................................................................. .. 
40
5   SEISMICITY............................................................................................................................................................................
 43
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5.1 General............................................................................................................................................................................. 43
5.2 Seismicity of the Region............................................................................................................................................... 43
5.3 Design seismic Coefficients......................................................................................................................................... 45
5.4 Seismic Hazard Map of Ethiopia.................................................................................................................................. 45
5.5 Horizontal seismic coefficient.................................................................................................................................... 45
6    DAM HEIGHT.............................................................................................................................. .. .......................................... 47
6.1 General.............................................................................................................................................................................. 47
6.2 A review of the previous studies made....................................................................................................................... 47
6.3 The Present Dam height................................... «............................................................................................................ 47
6.4 Free Board......................................................................................................................................................................... 47
6 4 I Methodfor Free Board Computation...........................................  48
6 4.2 Computation for Normal Free Board at FRL............................................................................................  48
6.4.3          Compulation of Minimum free Board at MWL.................................................................................................. 5!
6 4 4 Minimum Free Board at MWL...............................................................................................................................  54
6 4 5 Settlement Due to Earthquake.................................................................................................................................. 54
6.4             6 Top Bank Level.................................................................................................................................................. 54
6.4             7 Full Reservoir Level ......................................................................................................................................... 55
6.4.8 Minimum Draw Down Level (MDDL)......................................................................................................................... 55
6.4             9 Control Level.................................................................................................. -................................................. 56
7  DESIGN ASPECTS......................................................................................................................................................................58
7.1 General........................................................ ................................................................................................................. 58
711 Embankment Features................................. ................................................................................................................. 58
7 111 Crest Width
..............................................................................................................58
7.1 1.2 Berms
..................................
7.1 1 3 CamDer
.................................
........................................................................................................................... 60
7 1.14 Parapet Wall........................... ..............
7.1 1.5 Guard Stones............ ......... ................................................~........... .............. .................... ...................................60
7.2 Design Parameter................................................................-........................................................................................... 60
7.3 Basic Design requirement....................................................................................... .......................................................t>o
7.3.1 Safety against overtopping  .....................................................................................................~.................................. 61
7.3.1. I Free Board................................. ................................................................................................................................61
7.3.2 Embankment Section............................................................................................................... .....................................61
7.3.3 Zoning of Dam Section.............................................. ........................................................................................ ........  61
7.3.4 impervious Clay Core................................................................................................................................................... 63
7.3.5   Shell Zones................................................................................................................................................................. 64
7 4 Foundation Treatment................................................................................................................................................     64
7.4.1 Core Seating Treatment......................................................................................... .......................................................64
7.4.2 Shell Foundation Treatment........................................................................................................................................ 65
7.4.3 Cutoff Trench............................................................................................................................................................... 65
7.4.4     Location of Cut off Trench................................................................ ......................................................................66
7.4.5 Curtain grouting........................................................................................................................................................... 67
7.5 Stability of the earth and rock fill Dam Section....................................................................................................67
7.6 Safety Against internal Erosion...............................................................................................................................  70
7.6.1          Seepage Control Measures and Drainage Arrangement....................................................................................70
7 6.2 Inclined Filter............................................................................................................................................................... 71
7 6.2.1 Compaction of Filter..................................... „............................. ....... ................................................ -...... ......... 72
7.6.3 Horizontal Filter........................................................................................................................................................... 73
7.7           Safety of slopes
7.7.1 Upstream Slope Protection........................................................................
7.7.2 The rock for rip rap...................................................................................
7.7.3 The Dumped rock rip rap thickness..........................................................
7.7.4 Placing of Rock Rip rap.............................................................................
7 7.5 Upstream slope of cofferdam.....................................................................
7.8      Stability Analysis................................................ ~..... .....................
7.8.1          Factor of safety method........................................................................................................................................
7 8.2 Steady State Condition..................................................................................................................................................
7.8.3 Construction condition.................................................................................................................................................
7.8.4 Sudden Draw Down Condition.....................................................................................................................................
7.9      Properties of Materials.................................................................................................................... ...........
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iiArjo Dedessa Irrigation Project
Dam Appurtenant Works
May 2007
8  RIVER DIVERSION ...............................................................................................................................................................    84
8.1             River Diversion Arrangements......................................................................................................................... 85
9  INSTRUMENTATION............................................................................................................................................................... 90
9 1 General
90
9.2 Purpose of Instrumentation.............................................................................................................................. 90
9.3 Types of Measurement
9l
9.3.1 Pore water pressure ...................................................................................................................................................... 91
9 3.2 Movements....................................................................................................................................................................  91
9.3.3 Seepage and leakage..................................................................................................................................................... 91
9 3 4 Strains and stresses....................................................................................................................................................... 92
9 3.5 Dynamic load (Seismic).........................................................................................................................................  92
9.3.6 Other measurements .....................................................................................................................................................92
9.4   Planning of instrumentations for the Dam..........
92
9 4 1 Pore water pressure....................................................................................................................................................... 92
9 4 2 Seepage.......................................................................................................................................................................... 93
9 4 3 Surface movement measurement.................................................................................................................................. 93
9 4 4 Parapet Settlement Point..................................................................................................................................-........... 93
9 4 5 Horizontal and Vertical Movement Measurements..................................................................................................... 94
9 4 6 Earthquake Measurement ..........................................................................................................................................  94
9 4 7 Other Measurement...................................................................................................................................................... 94
10 SPILLWAYS FACILITIES
95
10.1 General............................................................................................................................................................................. 95
10.2   Location....................................................-.....................................................................................................................95
10.3 Spillway layout................................................................._........................................................................................... 95
10.4 Design inflow Flood Discharge.................................................................................................................................96
10.5 Gated v/s Ungated Spillway.......................................... .............. :......................... .......................................97
10.6 Energy Dissipation Arrangement...........................................................................................................................  98
10.7 Approach Channel........................................................................................................................................... .-........... 98
10.8 Spillway Structure...................................................................................................................................................   99
10.9   Ogee Wejr Profile..................................................................................................................................................... 99
10.10  Spillway Chute........................................................................................................................................................  10I
10.11 Structural Requirements................................... .......... ....... ........................................................ . ........................ 101
10.12  Floor lining .................................................................................................................................. ~............ 101
10.13 Water stops and joints.................................................................................................................... ........... -.......... 102
1014 Drainage System..................... ................................................................................................... . ............................... 102
10.15  Hydraulic Model Studies of spillway structure.......................................................................... ..................103
10.16  Zoning of Material for the spillway facilities.................................................................................................104
11 HYDRAULIC DESIGN....................... .................-................................................... . ..................................... ....................... 106
11.1 hydraulic Design of Spillway Faciuties......... ............................................................................................106
11.2   Irrigation Outlets...................................... ........................................................ . ................................ .....................114
11.3 irrigation Outlet for Right Bank Primary Canal....................................................................»......................... 114
11.4 irrigation Outlet for left Bank Primary Canal.................................................. _..............................................1l5
11.5 Hydraulic Design of Irrigation Outlet.................. ...... ........................................................................................ 115
11.6 hydraulic Design of irrigation Outlet for............................................... ........................................................... 118
12     HYDRO POWER GENERATION................................................................................................................ ..................... 122
12.1 Planning & Design for irrigation....................................... . ................................... .............................................. 122
12.2 Incidental Power Generation........................................................................................................... -.................... 123
12.3 Generation of Optimum hydro-power.......................................................................................... ......................... 123
12.4 Need for Pre-Feasibility study..........................................................................................................................—123
REFERENCES........................................................................................................................................... . .....................-........... 126
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LIST OF TABLE
Table 4.1 location of Borrow Areas For CONSTRUCTION MATERIALS....................................................................... 2i
Table 4.2       Selected Design Sand Filter Band................................................................................................................. 40
Table 6 1       Effective Fetch Length (fe)Computation for normal Free Board........................................................ 49
Table 6.2       Effective Fetch Length (f/) Computation for minimum Freeboard........................................................ 52
Table 6.3       Free Board Computations................................................................................................................................ 57
Table 7.1      Zoning of Dam Section - Material Types.......................................................................................................62
Table 7.2 Minimum Desired Values of Factors of Safety and Type of Shear Strength for Various
loading Conditions.............................................................................................................................................. 69
Table 7.3 Properties of the design parameter for the clay core..................................................................................78
Table 7.4       Rock Design Parameter.................................................................................................................................... 78
Table 7.5       Stability Analysis - upstream Slope..............................................................................................................82
Table 7.6       Stability Analysis - Downstream Slope....................................................................................................... 82
Table 8 1      Routed Design Flood for River Diversion.................................................................................................. 85
Table io. 1 Routed Design flood Based on LLG Distribution Peak out flow 1950 m3/sec........................................ 96
Table 111 Arjo Dedessa irrigation Project.................................................................... ............................................ 121
LIST OF FIGURE
FIGURE 2.1 Map showing Drill holes location along Dam axis..................................................................................... 11
FIGURE 4.1 MAP SHOWING BORROW AREAS..................................................................................................................... 27
FIGURE 4.2 PLASTICITY CHART................. _......................................................................................................................... 32
FIGURE 4.3 CLAY SAMPLE FROM CHEWO BORROW......................................................................................................... 33
FIGURE 4 4 ACTIVITY CHART....................................................................................... ............................................................34
FIGURE 4.5 CLASSIFICATION CHART FOR SWELLING POTENTIAL..............................................................................35
FIGURE 4.6 SAND FROM ALL SOURCES...............................................................................................................................36
FIGURE 4.7 DESIGN OF GRADATION OF SAND FILTER FOR CLAY................................................................................ 41
FIGURE 5.1 SEISMICITY OF ETHIOPIA................................................................................................................................... 44
FIGURE 5.2 HAZARD MAP OF ETHIOPIA....................... ............................................................................................. .......... 46
FIGURE 7.1 DAM SECTION STABILITY UPSTREAM SLOPE.............................................................................................. 79
FIGURE 7.2 DAM SECTION STABILITY ANALYSIS OF DAM SLOPE...............................................................................80
FIGURE 7.3 DAM SECTION STABILITY ANALYSIS - DOWN STREAM SLOPE.............................................................. 81
FIGURE 12.1 Elevation-Area-Capacity Curves of Arjo Dedessa Reservoir........................................................... 125
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LIST OF DRAWINGS
S. No
Drawing Details
Drawing
No
1
Location Map of Project
AD-F-Dam /
01
2
Layout Plan of Dam & Appurtenant Structure
ii
02
3
Dam Layout Plan
it
03
4
Dam cross section at Deepest river bed
••
04
5
Dam Cross Section A A to D D
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05
6
Geological Section along axis of dam showing lugeon values
H
06
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Geological section along axis and AD-2
••
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8
Geological section along axis of dam & AD-7
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Geological section along Saddle Area
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Geological section along Spillway
ii
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11
Dam - Longitudinal section & curtain grouting
I*
11
12
Cut off trench - grouting pattern
ii
12
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Dam Crest & Toe details
ii
13
14
Surface drainage Plan on D/s slope of Dam
ii
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Surface drainage Drain Section
H
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Plan showing Surface ana parapet settlement points
ii
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Section A-A
H
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Section B - B
H
18
19
Section C - C
H
19
20
Spillway Layout plan
•I
20
21
Spillway Elevation and section
ii
21
22
Spillway Longitudinal section and Plan
ii
22
23
Spillway - D/s channel sections (1/2)
ii
23
24
Spillway - D/s channel section (2/2) layout plan
••
24
25
Right Bank irrigation outlet
ii
25
26
Rignt Bank imgation outlet Longitudinal section
•I
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Left Bank irrigation outlet Layout plan
ii
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Left Bank irrigation outlet Longitudinal section
ii
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Construction Joint Details
H
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River Diversion during construction - Layout Plan
ti
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River Diversion during construction- Longitudinal section
•i
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River Diversion Duct Details
II
32
PHOTOGRAPHS
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PHOTO I              DAM SITE AND ABUTMENT LANDFORM
PHOTO 2 ROCK OUTCROP AT RIVER BED UPSTREAM OF DAM SITE PHOTO3 ROCK OUTCROP AT RIVERBED UPSTREAM OF DAM SITE PHOTO4 RESERVOIR AREA UPSTREAM OF DAM SITE
PHOTO 5              Ridge of Right abutment
May 2007
APPENDIX
APPENDIX -a             a Laboratory Tests Results of Borrow area materials
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ACRONYMS
May 2007
Xh
oc v
horizontal Seismic Coefficient Vertical Seismic Coefficient
BCEOM                      French Engineering Consultant in association with ISL and BRGM
BIS C'
Bureau of Indian Standard Effective Cohesion
CDSCO                      Construction Design in Share Company CFRD                         Concrete faced Rock fill dam
CUSEC                      Cubic Meter per Second
Cu
Coefficient of Uniformity
CV                              Coefficient of Curvature
Dm D/S EL EQ F, Fm FOS F Fe FRL GPS GLE Hs Ho ha
Average Depth of Water Down Stream
Elevation
Earth Quake
Horizontal force equilibrium Moment Equilibnum Factor of Safety
Fetch Length in Km Effective fetch length
Full Reservoir Level Global Positioning System General Limit Equilibrium Significant Wave height design Wave height Hectare
H                                Height of Dam
Hd
IS
Ls
LL
Head over Crest Indian Standard Wave Length Length of Basin Liquid Limit
M                                Million
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MOWR Ministry of Water Resources
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MWL
Maximum Water Level
MDD                          Maximum Dry Density MDDL                        Minimum Draw Down Level
NMC
Natural Moisture Content
OMC                         Optimum Moisture Content
PI PL PMF
4>
Plasticity Index
Plastic Limit
Probable Maximum Flood
Angle of Internal Friction
Q
Discharge
R
Wave Run up
RCC
Reinforced Cement Concrete
RD
Relative Density
Ru
S
Pore Water Pressure
Slope
TBL
Top of Bund Level
TEL
Total Energy Line
U/S
Upstream
USBR
United States Bureau of Reclamation
UTM
Universal Transfer Mercator
V
Velocity of Flow
VES
Vertical Electrical Sounding
W
Top Width of Crest of Dam
WAPCOS
Water and Power Consultancy Services (India) Limited.
WWDSE
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1 INTRODUCTION
1.1 General
Arjo-Dedessa Irrigation Project comprises of 40 0 m high earth and rock fill dam on river
Dedessa for impounding inflows of the river during the monsoon period The stored water will
be diverted into canal system on both the right and left bank for providing wat^r through a
network of canal system for irrigation of a command of about 13,655 hectares of irrigable land
The right bank of the dam is located in TVama Maribo Kebele of Limu Seka Woreda unaer Jima Zone The left bank is located is Chitu Bosona kebele of Guchi Woreda of lllubor zone of
Oromiya Regional state of Ethiopia. The project is located about 360 kms from Addis Ababa
and is accessible via Bedele which is about 40 kms from the project site. Location Map of the Project Area is available at Fig. AD-F-DAM/01
The aam comprises of
i)          Chute Spillway on the right flank located in the natural saddle No ; for passing floods  waters  that  may  impinge  the  reservoir  at  full  reservoir  level  to  avoid overtopping of the aam.
ii)         An irrigation outlet located in the saddle No. II in close proximity to saddle No I for  drawing  water  from  the  reservoir  for  the  right  bank  primary  canal  and  the distribution system of canal network
iii)        A second irrigation outlet taking off at the left abutment of the dam for drawing water from the reservoir for the left bank primary canal and distribution system of canal network.
iv)        The  general  layout  plan  of  the  dam  and  appurtenant  structure  are  shown  in drawing No. AD-F-DAM/02
1.2 Background & Review of Previous Studies
Various  studies  have been made since 1964 by  different organizations  for development of water resources in the country United States Bureau of Reclamation (USBR) made the first study titled 'Land and water resources of the Blue Nile Basin" in 1964 Dunng their study of Blue Nile Basin, number of potential sites  have been identified for development of irrigation ana hydroelectric  power generation in Dedessa valley  Amongst the numerous  sites  identified USBR has proposed dam site DD 11 on river Dedessa for development of irrigation and power
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They  had proposed a site DD-13 as  an alternative to DD-11 and called it Duia dam site Another site mentioned in their report is located about 1.5 km on river Dedessa upstream of the confluence of Deaessa river and Wama river
The second study  “Preliminary  Water Resources Development Master Plan for Ethiopia" was made by Water ana Power Consultancy service (India) Limited (WAPCOS) in the year 1990 They proposed a site for location of the dam on river Dedessa about 50 kms upstream of the dam site proposed by USBR The main reason for snifting the dam site is statea to be due to geological considerations. Due to the upstream location of the dam there has been an increase in the irrigation command area by covering areas at higher levels under irngation The project is planned exclusively for providing irrigation without any generation of hydroelectric power
The third and the last stuay was made by BCEOM in the year 1999 In their study report titled “Abbay River Basin investigation Development Master Plan" the proposed dam site is at the same location as has been suggested by USBR. In their review report an observation has been made on the fracturations wnicn is highly developed in the Basalt rock and will require large and intensive grouting work. Faults discovered in the left and nght bank abutments will result is possible leakage points, In addition, the faulting system detected on both the banks and highly fracturing pattern already  mentioned by  USBR, will increase the risk  of leakage Due to the extensive drilling investigations  program required to stuay  the geology  ana possible leakage problems the site has not been considered.
The dam site DD-11 on river Dedessa identified initially  by  United States  Bureau of Reclamation and later reviewed by  BCEOM were at reconnaissance level and no field investigations  have been done. Due to a major fault passing through the right and left abutments  and highly  developed fracturation pattern in Basalt rock  requiring extensive and intensive grouting work, the site is considered not favourable.
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1.2.1 Type of Dam
May 2007
The dam site is  located in a narrow gorge formed by  the erosion of river flow and the topography  suggests  adopting a concrete dam section However, the foundation rock  in general at the dam site location as  revealed during foundation investigation is  found to be fractured and vesicular Both the abutment along the dam axis are covered by Trachyte and
Tracny  - Basalt and the pan which are characterized by  Trachy-Basalt and vesicular is  not conducive to construction of a concrete dam. The valley  floor is  covered with soil cover of varying thickness. Thus the choice of dam type gets limited to either an earthfill dam or an earth and rock fill dam with a central core of impervious earth matenal and is governed pnmarily by the relative economy that can be effected by making best use of the available materials in the vicinity of the dam site
The materials for construction of an earth ano rockfill dam are available in adequate quantities Impervious clay with the required engineering properties is available in sufficient quantity in the adjoining borrow areas in as well outside the reservoir area The rockfill material for the shell
zone and rock riprap for upstream slope protection of the dam are also available in the near by quarry area in adequate quantities meeting the requirements.
In the previous studies made, a flexible dam structure has been proposed An earth and rockfill cam with top width of 10.0m and central rolled earthfill section protected by with rockfill material was proposed by USBR. A dam with central rolled earthfill section with rockfill protection was suggested by  WAPCOS. BCEOM had proposed a compacted rockfill dam with upstream concrete facing (CFRD) and a filter has  been provided all along the upstream facing underneath the concrete facing. The rockfill dam is founded on firm foundation with layers of filter material provided above the firm foundation.
Therefore considering the vanous aspects, a central rolled earth fill section of impervious clay core flanked by rockfill as shell material with filter on both upstream and downstream face of central clay core has been adopted for the Arjo - Dedessa Dam
1.2.2 Dam Section
In the previous studies earned out an earth and rock fill dam with central core of rolled earth fill had been suggested by USBR. The central core is provided with side slopes on both faces of
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l/2H:1V. The upstream face of the dam section is  provided with 2 3H :1V slope, while downstream face of dam section slope is  2H:1V upto certain height and there after slope of 2.5H:1V upto ground level
The central rolled earth fill core is covered with shell material consisting of rock fill material Doth on the upstream and downstream of rolled earth fill core. A clay blanket has been provided in the river bed and at abutments  on the upstream. The dam has  been provided with cut off trench upto firm foundation with side slopes in the cut off trench of 1|H.1V and grout curtain with grout cap has been provided.
WAPCOS had suggested an earth and rock fill dam with crest of 10 0 m width and the central impervious  clay  core is  provided with side slopes  on both faces  of 1H.1V The upstream surface slope has been provided slope of 3.0 H:1V and downstream face slope of 2H:1V upto certain height and thereafter 3.0 H:1V upto ground level. A horizontal drainage filter ending into the toe drain has  been provided. The impervious  clay  core is  flanked by  rock  fill as  shell material. The cut off trench has been taken to firm rock foundation.
BCEOM (1999) had suggested a compacted rock  fill dam section with upstream concrete facing (CFRD) of 3.0 m thickness and crest width of 8.0 m. A filter has been provided below the upstream concrete facing all along the upstream face and for a short length on the downstream face at the downstream toe of dam. Upstream face of the dam has been provided a slope of 1.5 H:1V and downstream face slope of 1.3 H:1V. The upstream surface of the dam is protected by concrete facing all along the slope. The rock fill dam has been founded on firm rock foundation and provided with three rows of curtain grouting at the toe of the dam upstream of the concrete facing with a deep central grout curtain and the other two adjoining grout curtains to check the seepage at the foundation level.
The availability and suitability of impervious clay material for the core and rock fill for the shell of the dam has  been established within reasonable distance Hence a dam with a central impervious clay core with rock fill forming the shell zone has been adopted A filter is provided on both upstream and downstream faces of the central clay core. The dam crest of 10.0 m width has been proposed. The parameters of the earth and rock fill dam and other appurtenant works has been described in subsequent chapters of the report. Upstream surface slope of the dam has been proposed to have a slope of 2.0 H:1V upto EL 1340.6 and thereafter 2.5H.1V upto ground level The downstream face slope of 1.5 H: 1V has been adopted after carrying
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out the stability analysis of the maximum dam section using SLOPE/W and SEEP/W software developed by Geo-slope international (Canada)
1.2.3 Dam Foundation Treatment
USBR (1964) had proposed an earth and rock fill dam with a positive cut off by taking the cut off trench to firm rock  foundation for site DD-11 The cut off trench has  been proposed covering the full width of the central core of rolled earth fill.
WAPCOS had suggested an rolled earth fill dam section with positive cutoff by taking the cutoff trench covering the full width of rolled earth fill section up to rock foundation BECOM in their study  had proposed the entire rock  fill dam section founded on firm rock  foundation and provided with three rows of curtain grouting in a trench width of 10.0 m on the upstream toe of the dam in continuity with the upstream concrete facing The central grout curtain is taken 60 0 m deep with the adjoining grout curtain on the upstream and downstream to 30.0 m depth to check the seepage flow in the dam foundation.
The overburden of loose material in the river bed varying from 5.0 m to 10 0 m depth at the dam site location is encountered. The central impervious clay core of the dam founded on firm rock foundation with curtain grouting tn two rows proposed to be provided central reach of the valley and grout cap provided to check dam foundation seepage The grout curtain has been provided up to depth of H and the two adjoining grout curtain rows to be taken to H/2 depth (H being the hydraulic  height of the dam) in the valley  terrace and river bed reach. Blanket grouting covering the full length of cut off trench along the full length of axis of dam has been provided.
1.3 The Present Dam Site
Subsequently during the year 2005, a team of experts from MOWR and Water Works Design and Supervision Enterprise (WWDSE) during their visit to the project area identified a dam site which is located 1.5 km upstream from the confluence of Dedessa river and Wama river near Arjo in Jima zone This site for location of dam and appurtenant works was subsequently visited by a team experts from ICT (India) & after examining various aspects the site was considered suitable and recommended for development. The dam axis has been aligned considering the topography of the ridge and both nght and left abutments The orientation and axis of the dam is defined by point DA-1 on the left abutment and DA-2 on the right abutment. The latitude ano
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1
1
1
1
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longitude and the coordinates of the dam axis in dangers and in UTM geographic coordinate system are given below
Left Abutment Point DA-1    Latitude         36° - 39’- 57.31" E
Longitude
8° -31'- 04.24" N
Easting
243048 m
Northing
942237 m
Right Abutment Point DA-2 Latitude         36° - 40’ - 06-38" E
Longitude
80- 31’-17.48" N
Easting
243328 m
Northing
942653 m
The layout plan of the dam and its orientation is snown in drawing No. AD-F-DAM/03 The field investigations of the dam site revealed that materials of construction for an earth and rock fill dam with central impervious clay core are available in sufficient quantity and is not a constraint. The stability analysis of the dam section, slope protection works and seepage control measures beiow foundation are described in subsequent chapters of this report.
Arjo-Dedessa dam site is located in the western plateau of Ethiopia and is far away from the Afar and main Ethiopian rift. The dam lies in an area of medium to low seismic activity A horizontal seismic coefficient of 0.05 g nas been adopted and vertical seismic coefficient taken as half of horizontal seismic coefficient of 0.025 g is considered. These seismic coefficients values have been considered as per the seismic hazard map of Ethiopia and dam section analysis has been carried out with these values
The topographical surveys, geological and geo-tecnmcal investigations  and other related studies have been carried out for the earth and rock fill dam at this location for the preparation of the feasibility study report
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Photographs - Ridge of Right AbutmentArjo Dedessa Irrigation Project
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2 GEOLOGY
2.1 General
May 2007
igneous rocks, different volcanic flows and oasic intrusions that come through the regional fault lines and localized fissures typify the bedrock geology of the project area. The rest of the area it is covered by alluvial, residual and colluvial soils The volcanic rocks are exposea in creeks, abutments, river bed and hills of the project area Further down stream of the dam axis, arouna mid way of command area, following the river Dedessa and on the right side metamorphic rock is also out cropping
Geological mapping was  done on dam axis, saddle dam, ana reservoir area and on construction material sites. This geological mapping targeted on identifying different rock and soil units, measuring fractures and joints on the out crops and verifying regional and local geological structures  in the project area More ever exploratory  boreholes  were drilled on different part of the project area
2.2 Dam site
Geomorphologically the aam site is characterized by two relatively steep hills, which are known as the abutments, and relatively flat nver channel in between these abutments. Alkaline vertical dikes cover the slope and top of the abutments, which gave steep landform. Five exploratory boreholes were drilled along the dam axis in order to verify the type and quality of the rocks, permeability of the ground and other geotechnical parameters and are indicated in Figure 2.1
2.2.1 Left Abutment
Three exploratory boreholes were drilled on the left abutment in order to investigate the ground conditions. In addition to drilling the boreholes, vertical electncal soundings (VES) survey was conducted in between the boreholes. The result of investigations shows that alkaline volcanic rocks, which are composed of alkaline basalt, trachyte basalt with vesicular nature and aphanatic basalt, cover the abutment. The alkaline volcanic rocks, which are exposed at the dam axis occur as a dike, and it align in NE direction. On the exposures, the alkaline volcanic rock is highly to completely weathered, the fresh part is pinkish in color, composed of alkali
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244150
248000
Figure 2.1 Borehole Location MapArjo Dedessa Irrigation Project
Dam Appurtenant Works
May 2007
K-feldspars  and affected by  closely  spaced fractures  and joints  No fresn rock  had been encountered, even by drilling up to the depth 40 m in bore holes AD-2 and AD-1 except fresh basaltic /doleritic/ intrusions in the borehole AD-7 Just 70m downstream from the borenole AD-2 the depth in which this doleretic rock is encountered is 9m It is not observed in the other boreholes along the dam axis This basic rock from the borenole AD-7 is very strong, fresh to slightly weathered and occurs as a dike
The amount of leakage in the boreholes AD-1 and AD-2 especially from the depth 6 to 13 m is unacceptable it is in the range of 38 and 26 Lugeon Further deep the test result shows tnat the Lugeon value is getting smaller and finally it is 0 lugeon, wmch implies that the formation is becoming impermeable It can be said that this abutment is weathered to different weatnering profile, and the resulted clay and silt fill the openings
2.2.2 Valley Floor and River bed
Upper part of the Left and right valley floor is covered by alluvial deposit and it’s thickness ranges from three to five meters These alluvial deposits are composed of silt and clay which were transported by  the nver Dedessa. Due to its  origin and means  of deposition, the formation is very loose Basaltic trachyte occurs under the alluvial deposit and the river bed Up to the depth of 20 meter, this formation is fractured, slightly to moderately weathered and strong Two strike slip fault/lineaments  are observed at the foot of both abutments. Due to these geological structures the cores recovered from the borenoles drilled on the valley floor are sneared and filled with white precipitates The rock formation below the basaltic trachyte is trachyte, and it is solid, moderately strong to strong, fresh to slightly weathered with radiai porphyritic  texture. The fractures  are mostly  healed, inter connected and filled with white materials
Water pressure tests were conducted in both boreholes (AD-3 and AD-4) at every 5 meters The test results generally show that the intake up to the depth 50 m is moderate to high, i.e , in the range of 3 to 10 Lugeon in AD-3 and 6 to 34 Lugeon in that of the borehole AD-4 Lugeon value and tested elevation interval for each test is presented on the Drawing AD-F-DAM/06
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2.2.3 The Right Abutment
May 2007
One exploratory borehol was drilled on the right abutment. Lithologically the formation on the
right abutment is uniform throughout the hole and it can be grouped as pinxish to paie grey 
alkali volcanic rock. The rock as it observed from the exposure and cores are strong
moderately weathered and fractured.
Colluvial deposits  cover upstream and downstream flanks  of the abutment. These colluvial deposits are decomposed into clay sand and gravel. The clay materials fill the openings in fractures, joints, and discontinuities  on noth faces  of the abutment. Based on water intake amount from the packer test in the borehole AD-5 the pans from the surface to the elevation 1335 m and below the elevation 1325 m are characterized by high Lugeon value, i.e 12 to 27 Lugeon. Accordingly the amount of leakage is not acceptable and needs remedy like grouting along the line of stripping
2.3 Saddle Dam Sites
Two Saddles are identified on the right side of the river Dedessa. Both Saddles are separated by  small hill with the maximum elevation of 1380m amsl. Geological and Geotechnical investigation, which comprise geological mapping, exploratory  boreholes  drilling with insitu testing and test pit excavation was  done along the saddles  and the hill. Based on the investigation the following three geological units are identified and discussed below as follows
Soil cover
The upper top part, with thickness varying from 1 to 4m is covered by black silty clay. This soil
is resulted from the weathering of alkali basalt. It is very soft, expansive and needs to be
removed for the construction of the structures
Trachyte I Rhyolite
This rock is exposed on the hill in between the saddles and along the saddle areas under the soil cover. Its thickness  ranges  from 7m in the hole ADS-1 to 60 m at the hill. The rocks exposed at the hill vary from trachyte to rhyolite with in a few distances and fractured. The rock cores  recovered from both drill holes  are weak  and decomposed to silty  sandy  gravel The permeability tests /falling head tests/ result shows that the permeability rate vary from 3.85 E- 05 to 1.07 E-04 cm/sec.
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Basalt
May 2007
The last formation, which was encountered by drilling and mapped in the lower elevation, at the bed of Wama River, is basalt. This rock was found at an approximate elevation 1338 m amsl in the borehole ADS-1 and ADS-2. It is  fractured in the upper pan with some vertical joints, strong to moderately weak, moderately to slightly weathered witn some vesicular zone, which are coated by  reddish to yellowish and some time whitish materials. However oeiow the elevation 1333 m it was found blocky, very strong with calcite infillings at discontinuities ana vesicular zones  Concerning the water intake, the upper part is  permeable with the Lugeon vaiue ranging from 8 to 12.
2.4 Geological Structures
Faults, fractures, lineaments, bedding planes are geological structures that affect the stability of dam and other infrastructure. Consequently regional faults, local lineaments, and fractures are identified by desk study using Areal photos and satellite imagery and this had been confirmed on the field through measuring joints, fractures and faults and in exploratory boreholes drilled.
Based on the investigations, two joint set directions (NE-SW major and E-W minor fracture directions) were recorded at the dam site According to the measurements and analysis the average dip amount of the entire joint plane is  greater or equal to 70°. Analysis  ana stereographic projection is presented on main geological report. Two E-W Lineaments cross the dam axis at the foot of both abutments. Consequently the water intake in the boreholes close to the Geological structures is high to very high. More ever, one lineament with E-W direction is also verified at the middle of right abutment beyond the end point of dam axis. Borehole AD-6 drilled at this lineament, and the falling head test result ranges from 1.58E-04 to 6.48E-05 cm/sec. From the observation of the core of the hole AD-6, and the result of falling head water intake, the lineament is filled by clay and sandy materials, and hence no serious leakage problem.
Other sets of geological structure in the area are faults, which are aligned in parallel direction and located upstream and downstream of the dam axis. These faults flow major fault direction However, since the dip amount is vertical to closely vertical, it is not expected to cause major problem on main dam except for the saddle dams. As the result, remedy is recommended along the saddle.
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3. DAM FOUNDATION
3.1 General
Ma J 2007
The Arjo-Dedessa dam comprises of right abutment, right valley floor river Ded, left valley floor and the left abutment. All these components have different characteristics and require varying treatment in regara to stripping and protection measures  against seepage below tne oam foundation The ground characteristics of the various sections of the dam foundation and the geological set up have been described in the earlier chapter-2 on 'Geology'
3.2 Stripping For Dam Foundation
The different sections of the dam foundation calls for stripping of ground surface accordingly to the rock grade aiong the dam axis. The excavation required to reach an acceptable foundation level is different for the dam seating below impervious clay core and vary for the dam seating in rock fill shell. In the river part it is considered on geological assessment of the rock grade for the cut off trench.
3.2.1 Excavation Below Core Seating
The stripping in the core trench part would be considered on geological set up and depends upon the ground conditions
•    The stripping for foundation for the core trench is to be carried out to remove talus, loose matenal and weathered rock upto a level of fresh rock, which can take up the grouting treatment.  The  depth  of  excavation  required  to  obtain  suitable  core  foundation  is  to considered on the basis of assessment of the geotechnical Engineer/Geologist
•    The core and filter foundation surface is to be thoroughly cleaned using high pressure air water  jets  and  mechanized  equipment.  Open  joints  and  minor  shear  zones  are  to  be excavated and cleaned to a minimum depth of three times the opening width and sealed by filling with slush grout back fill concrete. Shaping of abutments to provide gradual changes in slopes is important where materials with large difference in deformation modulus ana compressibility are being used such as rock fill and a sloping clay core with high moisture content. Substantial shaping with concrete may be needed depending upon the contours.
The abutments of Arjo-Dedessa Dam are not strong as revealed by the drill holes investigations and therefore would require special measures in detailing the adjoining embankment zones to be assessed jointly by the Engineer-In-Charge along with the geologist. Particularly in case of left abutment as the rock of the left abutment is weaker as compared to the rocks in the right
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abutment. Stability of the dam section has been enhanced by providing widening the fill at ends. This may be considered more economical than removing weak foundation material
The abutment and valley slopes for excavation below core seat are trimmed or presplit to 1 1 or preferably flatter Changes in valley slopes at any point is to be not more than the 20° degrees, cavities  and depressions  are to be filled with dental concrete Over hangs  and outcrops  of distressed and fractured rocks on hill slopes along the axis of the dam need to be removed
In the event of getting irregular rock surface in certain reaches then 0.5 m concrete thick over the foundation surface supporting the core and filter on both upstream and downstream of core has been provided.
The arrangement of excavation below the core seating of the dam in the cut off trencn based on the aDove consideration and the geotechnical assessment has  been indicated in the Drawing No. AD-F-Dam/ The arrangement provides  for removal of all loose material and weathered rock on the right and left abutment upto groutable rock grade. The depth of the weathered rock is varying in thickness from 4 0 m to 6 0 m on the right bank. The depth of the weathered and loose rock on the left abutment is varying in thickness from 15.0m to 40.0 m. The left bank  is  found to have more depth of weathered rock  as  compared to the right abutment. The exact required depth of excavation in the cut off trench along the dam axis requires to be assessed by the Engineer in charge and the geologist at the time of actual construction
On the right and left abutment, the excavation of the loose rock in the cut off trench is be carried out by manual wedging or by controlled blasting to avoid any shattering and damage to loosely jointed rock.
3.2.2 Excavation below Shell Zone
Foundation excavation under the shell zone will be required to reach foundation materials having shear strength and compressibility at least comparable to those of the compacted rock fill. Talus deposit and soils like weathered rock shall be removed and founded on moderately weathered rock along the abutment side slopes.
In the river channel, all alluvium deposits is to excavated and the shell zone to be founded on competent, moderately  weathered rock. Where shear zones  filled with pipable materials underlie the downstream shell, it should be excavated and a filter placed over the area to
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prevent piping Before laying the first layer of shell matenal in the river valley, the foundation material is to be thoroughly compacted by giving appropriate number of passes of a vibratory roller after applying moisture to the required extent.
In the valley area there is an overburden of riverine alluvium deposit of about 3.0 m to 5.0 m Complete removal of this alluvium deposit may not be required below the seat of rock fill shell zone. Only that much depth of alluvium where relative density (RD) of the insitu material is found to be less than 60% neea to be removed in order to safe guard against liquefaction of this soil in the event of an occurrence of earthquake of high intensity. The field relative density value by standard penetration test may not give correct values in arid zones. In that event the relative density may be determined by the relationship given under -
Dd = dma*
r
r.-r,
d a min
r .          “ r •
L d max a min J
Where, Dd = relative density
r = dry unit weight in densest state
d max
d        = dry unit weight in place
r,
. = dry unit weight in loosest state
r,
a min
3.2.3 Foundation Treatment
The foundation is to be treated for providing minimum settlement to the earth & rock fill dam
The cracks, faults or deep pits need to be removed and backfilled with concrete. All joints and cracks beneath the core and filters are to be cleaned and filled with concrete The foundation treatment must be sufficient to satisfy the following criteria.
i)  Minimum leakage
ii)  Prevention of piping
iii) Limited settlement
iv) Sufficient friction development between abutments ano
foundation to insure sliding stability
The foundation has been treated for preventing under seepage by cement grouting beneath the cutoff. Besides  pervious  zones  upstream from the impervious  membrane can be blanketed with impervious material. The foundation surface in the river valley zone will be moistened to the required extent before starting the operation of compacting by heavy vibratory rollers The ____________________________________________________________________________ 17
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clay  material in layers  of 60 cm depth is  placed at optimum moisture content (OMC) in the impervious  core. The shell material layer is  placed in the shell zone in 1.0 m lifts  for compaction by requisite number of passes by heavy vibratory rollers to be determined by trial testing.
3.3 Grouting Requirements
Grouting is carried out to achieve the following conditions for facilitating the safe design of the dam
i)       To reduce the erosion potential of seepage
ii)       To reduce the leakage through the dam foundation.
iii)        To reduce the settlement and strengthen the aam foundation.
The geological assessment of both left and right abutment of Arjo-Dedessa dam based on geotechnical investigations  reveal the fact that the rock  on both abutments  is  highly  jointed, scoriaceous, fractured and vesicular Trachy Basalt. The rock on the left aDutment is weak in companson to the rock on the right abutment of the dam. There are alternate layers of Trachy Basalt (boulder) and Trachy  Basalt as  revealed in the borehole loggings  The permeability tests  conducted on the drill holes  exhibit varying degrees  of permeability  values  at different depths in the same bore holes indicating there by that the rock is highly jointed and fissured. Three bore holes have been drilled on left bank. The permeability values exhibit high values varying from 54 Lugeon at a depth of 6.0 m to 40 Lugeons at 12.0m, depth in bore hole ADT at GPS elevation of 1358.0m. on left abutment.
From the borehole logging and the permeability values it is evident that both the nght and left abutments  and in the river bed zone there is  need for extensive grouting. Consolidation grouting to minimize the risk of excessive seepage from the water face of impervious core with the foundation is  required between the abutments  along the dam axis. The arrangement for consolidation grouting and the curtain grouting proposed for the dam is indicated in the drawing No AD-F-DAM/1.
3.3.4  Curtain Grouting
The grouting shall be earned out through the grout holes  drilled from the cut off trench consisting of two rows of grout curtain to depth upto 'H' in the main river reach or as per the Engineer-In-Charge. The number of rows of curtain grouting and the depth upto which grouting would be done shall depend upon the assessment of the foundation condition, permeability of ground strata and the geological set up forming the foundation of the dam at the time of
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construction. The pattern of grout holes on abutment and in the river valley is shown in the drawing Depending upon the width of the cut off trench, grouting pattern nas been adopted
For the Arjo-Dedessa dam foundation, one row of curtain grouting holes in the abutment reach where the cut off trench width is less than 9.0 m is proposed. The grout holes shali be drilled at 3 0 m center to center spacing and grouted These holes are primary holes. After tne grout is set the percolation test would be carried out in check holes. If the strata shows after grouting the permeability of more than 3 Lugeons and the quantity of grout absorption of tnese primary holes shows significant increase. Then secondary holes shall be drilled in the middle of the two rows of the grout holes at the intersection of diagonals of the square and grouting shall be continued in these secondary holes until the designed degree of impermeablization is acnievea For grouting with cement 38 mm diameter holes are used. The advantage gained by drilling large noles does not often justify the increase in drilling costs. In long holes the diameter at the top of the holes may have to be larger than the final diameter at the bottom of the hole to facilitate telescoping or to allow for wear of the bit.
In the central portion of the valley including the river where the core trencn gets wider it is proposed to adopt two rows of curtain grouting in order to seal larger number of joints. The depth upto which curtain grouting in done would depend upon the geotechnical condition of the foundation rock. The additional rows  of grout holes  on the upstream side and on the downstream of two lines will also be drilled. The grout holes in these additional added rows is utilized for consolidation grouting The number of additional rows  of grout holes  for consolidation grouting is taken upto 10.0 m depth vertically Thus two pattern of grout holes have been adopted for Arjo-Dedessa dam. The spacing of grout holes would be changed depending upon the results of the percolation tests and grout absorption rate regularly
The grouting should start with a low initial pressure of say 0.1 to 0.25 kg/cm2/m of burden and build up the pressure gradually. Initially the rate of intake may be 20 //min to 30 Z/min In order to avoid the premature build of high pressure a general guideline should be followed that the pressure should be raised only when the intake rate falls below 5//min. In the case of surface leak developing the pressure should immediately be reduced
The limiting value of pressure for each zone and depth may be decided initially on the basis of categorization of rock and initial values to be confirmed by trial and observation. The choice of pressure may also be established by results of trial grouting along with observation of upheaval by  uplift gauge. Limiting pressures  may  be decided by  continuous  review of the trends  of pressure and rate of intake during grouting.
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4 CONSTRUCTION MATERIALS
May 2007
The assessment of availability of construction material at proximal distance to the dam site was one of the major tasks  in the surface geological mapping, geological and geo-technical investigations  The major construction material such as  clay, shell material, rock  for coarse aggregate rip rap and sand were identified in the upstream and downstream areas  from the dam axis. Extensive field traversing has  been undertaken to locate the sources  for different types  of construction materials  required for the dam and appurtenant works  These locations for the borrow areas  for construction materials  for dam are shown on the map available at Figure 4.1.
The clay  matenals  are residual soils  which are insitu formed soils  found in small hills  where new settler settles on most of them The selected sites  for clay  material are in the upstream area from the dam axis  on both left and right of sides  of riverbanks. The shell material is fractured and weathered alkaline basalt downstream of the dam axis. The rock  for coarse aggregate is aphanatic basalt located close to ATE village. The rock for rip rap proposed is the remnant trachytic hill in the command area and the sand material is in the downstream area of Dedessa valley  The details  of each construction material are described in the following sections
4.1   Borrow areas for clay
Five  areas  have  been  identified  as  potential  source  for  obtaining  the  clay  core  matenal  for construction of the dam. The areas identified and their locations are given in Table 4 1.
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Table 4.1: Location of Borrow Areas For CONSTRUCTION MATERIALS
May 2007
Item
no
Location
Site name
Estimated
3
Volume (m )
Distance
from dam
site (km)
Easting
Northing
Type/name  of  the material
Clay for core
1
Ate
31,541.58
8
244000
935329
Silty clay
2
Chewe
345.327.50
5
243550
938640
Silty clay
3         Maribo
502.603.13
10
250000
937910
Silty clay
4
Dhendessa
1 062,455 00
6
249725
940140       Silty clay
5
Obose
140 567 58
4
244805
946300
Silty ciay
Sum
2,082,494.79
Aggregate
1
ADCAQ
>1,000,000
9
243582
934698
Basalt
2
Maribo
>1,000 000
2
247500
942350
Basalt
>2.000.000
Shell material
1
ADQ1
>6624000
1
242190
943100
Trachy basalt
2
Guche
>3 000,000
12
234489
943400
Basalt
>9.642 000
Riprap
1
Buketa
180 000
6
249320
941079
Phonolite/Trachyte
2
Colisiri
120 000
26
224180
946815
Phonolite/Trachyte
300.000
Sand
1
Borche
•
60
214802
959157
-
2
Loko
sefera
*
60
211860
960600
-
3
Loko
bridge
•
60
215289
961337
-
4
Tullu
senbete
•
65
$
-
-
* Not estimated
$ Not accessible in wet seasons
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Observation of borrows areas during the field visit to the project area showed that clay material can be borrowed from the upstream areas  from the dam site The residual soils  at gently sloping small hills on both sides of the river is available at many places. Location of the borrow sites is shown in Figure 4.1. Accordingly, the following sites were identified for clay borrow material investigation
■   ATE village area
(AtB)
■   Chewo village area
(ChB)
• Dhendhensa village
(DhB)
■   Wama-Maribo school area
(MB)
■   Obose village area
(ATB Obose)
The sites have been marked on the map and have been designated as DhB MB, ChB, AtB and (AtB obose) The proposed borrow area sites  AtB and ChB are located in the left side of Dedessa reservoir area ana other two sites namely, DhB and MB are locatea on the right side of Dedessa reservoir area. For the investigation of the quality and volume of the clay materials about 46 test pits  are dug at the 5 sites. Test pit digging, logging and bulk  sampling are conducted. Different types of samples were collected from most of the test pits and from each layer in the pits. The samples were analyzed at CDSCo s laboratory The summary of the laboratory test results of construction materials is shown in Annexure - A.
4.1.1  ATE area
The area is located about 8 km upstream of and on the left side of the dam axis adjoins to Chewo village and is  1.0 km upstream of dry  stream Adosa' The river Siden is  on the southern side of the village flowing at a distance of about 3.0 km. Red silty clay with depths varying has been located in 16 Nos representative test pits dug indicate depth of the silty clay varying in these pits. Samples have been collected and these on testing in the laboratory have indicated the following average properties.
1.          Grain size distribution
Sand
5%
Finer
95%
Gravel
0.0 %
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2         Atterberg's Limits
i. Liquid Limit (LL)
ii. Plastic Limits (PL)
iii. Plasticity Index (PI)
May 2007
63.0 %
38 0 %
25.0 %
3    Free Swell
46%
4    Linear shrinkage
46.9 %
5.   Specific gravity
2.63
6      Permeability
2.85x1 O'6 cm/sec
7.     Standard proctor test
i. Natural moisture content
ii. OMC moisture content
iii. Maximum Dry Density (MDD) 8   i. Cu
ii.     u
9.  i. Cu
ii.
10. Coefficient of consolidation 11.   Dispersivity
15.0 %
38.0 %
1346 kg/m3
38.8 KN/m2 26.6°
26.5 KN/m2 30°.0
0.234
Non despersive
4.1.2  Chewo village
The area is located about 5.0 km up stream of the axis of the dam and is on the left bank area. It is close to Ate village and is on the down stream of dry river stream Adosa which is very close by The borrow area lies between Ate village and Chewe village Test pits have been dug in the area and silty clay in these test pits in depth varying has been established. The laboratory test conducted on these samples have indicated the following average values
1 Grain size distribution Sand Fines Gravel
2. Atterbergs Limits
iv Liquid Limits (LL) v. Plastic Limits (PL) vi. Plasticity Index (PI)
3. Free Swell
4. Linear shrinkage
5.7% 94.25% 0.0%
56.0 % 36.0 % 20.0 %
- 47 86 %
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5. Specific gravity
6    Permeability
7    Standard Proctors test
i.      Natural moisture content
ii.     OMC moisture content
iii.      MDD
8. i. Cu
ii. p u
9 i. Cu'
ii. <pu'
10. Coefficient of consolidation 11.  Dispersivity
4.1.3 Dendessa Area
May 2007
2 76
1.59x1 O'6 cm/sec
8.0 %
34 48 %
1429 kg/m3
35.5 KN/m2 26° 0
37.0 KN/m2 30°. 0
0.248
Non despersive
The area is situated about 6.0 km of dam axis and is on the right bank area of Didessa River The dry  river stream Chora is  on the left side of the area The area lies  close to Amo and Maribo villages  12 Tests  pits  have been dug in the area and these indicate presence of brownish red-to-red silty clay material varying in aepths over the area.
Laboratory tests conducted on samples collected from these area have indicated the following properties
1. Grain size distribution Gravel Sand Fines
2. Atterberg Limits (%)
i.      Liquid Limits (LL)
ii.      Plastic Limits (PL)
iii.      Plasticity Index(PL)
3 Free Swell
4.    Linear shrinkage 5.  Specific gravity 6.  Permeability
0.0%
3.5%
96.5%
64.0 %
41.0%
23.0 %
47.0%
52.13%
2.70 %
3 14x1 O’8 cm/sec
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7 Standard Proctor's test
i. Natural moisture
ii. OMC
iii. MDD
i. Cu
ii. <f> u
i. Cu'
ii. fu'
Coefficient of consolidation Dispersivity
4.1.4   Maribo village
May 2007
16 0%
38.0 %
1378 kg/m3 * 5
38.5 KN/m2 27°.O
30 0 KN/m2
:       31°.0
0.231
Non despersive
The area is located about 7 0 km down stream of the axis of the dam and lies near the village Dale and Amo village on the right bank area of the river. There is a dry river stream 'Chora' near the area and river Deaessa is flowing about 2.0 km from the borrow area Representative test pits in the area has been dug and these indicate the presence of brownish red and rea silty  clay  with the depth of silty  clay  material varying over the area has  been established Laboratory tests samples from these pits collected from the area have indicated the following properties.
1. Grain size distribution Gravei Sand Fines
2. Atterberg's Limits
vii.     Liquid Limits (LL)
viii.     Plastic Limits (PL)
ix.   Plasticity Index(PI)
3 Free Swell
4. Linear shrinkage
5 Specific gravity
6.  Permeability
7.   Standard Proctor's test
i.           Natural moisture
12.0%
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ii.     OMC
May 2007
36.0%
iii.      MDD
1435 kg/m3
8.
i. Cu
46 0 KN/m2
ii 0 u
28.0°
9.
i. Cu'
27.5 KN/m2
ii <p u'
28.5°
10 D
ispersivity
Non dispersive
4.1.5   Obose Area
The area is located about 4 km from the dam axis and lies on the right Dank area of the river The area lies between Sertumi and Ketketo and is 2 kms from Ketketo on Warna Adhera high way going to Ketketo. It is near Ketketo springs, 3 test pits have Deen dug in the area which indicate. Laboratory tests conducted on samples collected from the Obose area have indicated the following properties.
1. Grain size distribution Sand
Not done
Fine
Not done
Gravel
Not done
2. Atterberg
Limits (%)
iv.
Liquid Limits (LL)
62.9 %
V.
Plastic Limits (PL)
:
41.58%
vi.
Plasticity Index (PI)
:
21.32%
4.2   Borrow Areas for Rock Fill Shell Material
Shell material
The remnant hills in the reservoir area and the remnant ridges downstream but close to the left abutment can be assessed for rock fill material. The size of the ridge is quite large and there will not be quantity  problem. One exploratory  borehole labeled as  ADQ1 was  drilled to investigate the quality and quantity of the material. The total volume of the material is depicted in Table 4.2. The site has large matenal reserve designated as ADSHQ as shown on the map at Fig. 4.1
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LEGEND
• Borehole location
A
QU  greo«te Quarry area Shell materiel Quarry Clay care borrow area Riprap Quarry site Main River
Scon clary River /v* Dam axis
SCALE
1 55000
700 0 too raoo
Figure 4 i Bonow areas location mapArjo-Dedessa Irrigation Project
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May 2007
Two areas have been identified as potential source for obtaining rock material for the shell zone of the dam
The areas identified are;
Sr
No
Village
Name
Easting
Northing
Elevation
GPS       From dam Axis
Distance
i.
ADSHQ
242190
943100
1354
3  km  aown  stream  of  dam axis on the left bank
ii.
Guche
234489
943400
12      km      down      stream distance of dam Axis
4.2.1 AD SH Q area
AD SH Q area is located about 3.0 km on the downstream axis of dam and is on the left bank of the river It is  one km downstream of confluence of Mama river and Dedessa River and upstream of stream 'Seyo which is a dry river stream Laboratory tests from this borrow areas have not been earned out but the characteristics of the rock and its properties are reported to be similar to the rock at drill holes ADt to ADS and the rock found in the drill holes ADS1 and ADS2 at Saddle No-1 and Saddle No-2 respectively The average laboratory test results of drill
holes AD1 to AD5 and ADS1 & ADS2 are given below:
1.  Sound ness (%)
4.04
2.  Point Load (Dry)
830 KN/m2
3.  SSD
780 KN/m2
4    Unconfined Compression test
30375 KN/m2
4.2.2 Guche Area
Guche site has also been identified as a reserve site for shell material in case of requirement of additional quantity. The site is located at the coordinates above mentioned and at a distance of 12.0 km for the dam axis on the left side on the downstream
4.3    Quarry Sites for Rock Rip Rap
The remnant tracnytic plug at kole are of the command area can be exploited for construction np rap material. The quality for material is investigated at CDSCo's laboratory. Two sites have
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been identified for quarry area for rock to be used for rip-rap. These sites have been known by the name Buketo and Colisir Buketo is located about 6 km upstream of the dam axis on the right side where as Colisir is 26km far down stream from the dam axis on the left side The absolute location of this site of colisir is at UTM 224180 m E, 946815m N and location of Buketo is 249320 m E and 941079 m N and elevation at the foothill 1352.0 m The laboratory tests conducted on samples have indicated the following average values. Location of borehole CRQ 1/5 & BRQ 1/2.
Porosity (%)
3.75
Unit weight (kg/m2)             2761
Water absorption (%)
1.5
Specific gravity
2.74
Abrasion (%)
31.5
Sound ness (%)
2.59
4.4 Quarry Site for Coarse Aggregate
Rock for coarse aggregate is identified at uphill area ATE village on the way to Yanfa. The rock is jointed but only slightly weathered, fine grained maffic basalt. The center of this rock out crop is at UTM 243582 m E, 934698 m N and elevation 1473m. It is workaDle for opening the quarry and suitable for coarse aggregates The site for coarse aggregate for concrete work identified is designated as ADCAQ1 and Maribo and depicted on map at Fig 4.1
4.5 Sand
The Dedessa river valley is the potential source for sand. Sand deposit is observed close to the Dedessa bndge on Bedele- Arjo road and upstream (About 6.0 km from the branching off from Bedele- Arjo Road) through the right probable main canal line. Borche, Loko Sefera, Loko Bndge and Tullu Senbete has been identified as quany sites for sand. Tullu Senbete is located in the bed of the river Dedessa, while the other two are located in the bed of tributanes Borche and Loko. The approximate distance of all the four sites from the dam axis is about 60 km. Sand samples have been collected from the three sites Borche, Lokosefera and Loko Bridge.
The laboratory test conducted on samples collected from these areas indicated the following values:-
1. Grain size distribution:
Gravel Sand
4.1% :          87.5%
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Specific Gravity Organic Content Unit weight Relative Density
May 2007
Fine
Loose Field Dense
Effective shear stress Cu'
Effective angle of internal friction
8.3%
2.55
1.8% 1261 1475 1485
0.67 kg/m 3.5 KN/m
60.33°
4.6  Analysis Test Results of Materials for Construction
4.6.1 Gradation of Clay Material
The samples obtained from potential sources of clay material for construction of the dam have been tested in the laboratory and have indicated properties, which show large percentage of finer particles. The gradation curve of clay materials for all borrow areas are shown in Figure
4.2 the soil samples report have indicated hign values of liquid limit, which show that the clay soils are highly compressible The values of properties of materials contained in the final report of Construction Design Share Company indicate very high value of shnnkage. The average shrinkage value of 52.64 % is considered to be very high as compared to the recommended value of 8-10%. The average value of free swell is about 49.9 % for the all borrow sites tested, which is also high. Nearly all of them plot below A-line signifying property of high plasticity silt.
4.6.2 Plasticity Index
The average LL values are in general greater than 50% signifying high plasticity properties The PI values of clay from chewo area are lower than from other sources The Plasticity chart for clay samples from all borrow areas are shown in Figure 4.3.
4.6.3 Permeability
The coefficient of permeability of day samples from borrow areas is quite low tan average of 10.0"8 cm/sec) making it quite useable and suitable as core matenal to minimize seepage. Due to this property of material, workability problem in general could be expected. This however can be overcome by making a test embankment prior to the construction of the actual dam and assessing the best way to conduct the compaction work.
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4.6.4 Dispersivity
May 2007
The gradation curve of the clay materials indicate the materials to be very fine For the very fine clays materials from the borrow areas, the dispersion tests were arranged to be conducted on the WWDSE soil testing laboratory by pin hole test, and double hydro meter tests as per the ASTM standard. The tests results revealed that the fine clays are not dispersive in nature.
4.6.5 Classification of clay mineralogy
For the classification of clay mineralogy for different borrow areas, the plot of Plasticity Index v/s clay sized particles (<0 002 mm) was made which indicted that the clay classification is Kaolinite in borrow areas of Ate, Chewo where as it is Kaolinite and Illite in character in case of Dhendessa ana Maribo borrow areas These are indicated in Figure 4.4
4.6.6  Swelling Property
For assessing the swelling property  of ciay  materials  for different borrow areas  the plot of activity  ana the percent of clay  sizes  from different borrow areas  indicate medium to high percentage (1.5 to 5) of swelling potential. These are indicated in Figure 4.5.
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Fig.4 3CLAY SAMPLE FROM CHEWO BORROWArjo Dedessa Irrigation Project
May 2007
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Figure 4.4 ACTIVITY CHART
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TEST CODUCTED ON CLAY MATERIAL FROM ALL BORROWS
CLASSIFICATION CHART FOR SWELLING POTENTIAL
CLAY ACTIVITY Versus PERCENT CLAY SIZES (FINER THAN 0 002mm)
F-3 MS■■«rrr prrrpFF■RRwww■
M M M M * * ___^
—♦—  LOO  BRIDGE  SAND  NO  2 —*- LOKO SEFERA SAND NO 1
—•—  LOO  BRIDGE  SAND  NO  1 —•- BORCHA SAND NO 2
LOO SEFERA SAND NO 2 —L— BORCHA SAND NO 1
PARTICLE SIZE (mm)
SAND GRADATION
ARJO DEDESSA IRRIGATION PROJFCT
Fig.4 &SAND FROM ALL SOURCESArjo Dedessa Irrigation Project
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4.7 Shell Material
May 2007
From the test results of the shell material it is observed that they are generally weak in strength. The point load tests conducted also indicate (830 Kpa) the rock to be of medium strengtn class Since it is in abundant supply its use has been made in the shell zone of the dam by accepting lower strength of the rock material, and the dam section has Deen designed accordingly
4.8 Sand
4.8.1 Gradation curve of sand material
The gradation curve plotting for sand samples from different borrow areas have been shown in figure 6 6 The plotting indicate that contents  of fines  material in the sand samples  collected from vanous identified borrow areas indicate very high percentage of fine material in the sand sample and this sano material do not meet the filter criteria for its use for filter design. As such coarser sano material is to identified for use along with this or else crushed material from the quarry may be used as admixture with the fine sandy material sucn that the admixture meets the filter design criteria for using it for filtering
4.9  Design of Filter
4.9.1 General
The prospecting for suitable sand sources  has  been done during the geo-technical investigations in the project area. The prospecting has concentrated on stream deposited sand locations  of Borche, Loko Safre Loko Bridge and Tullu Senbete areas. The tests  on sand material from these sources  have been conducted at the CDSCO laboratory  in Addis  Ababa. The full laboratory test results are contained in the laboratory reports
The Arjo-Dedessa earth and rock fill dam has been provided with a central clay core matenal with upstream and downstream filter and transitions. The transition material of intermediate gradation need to be provided between sand filter and the rockfill shell material both on the upstream and downstream. One transition between the sand filter and the rock  fill shell has been proposed at this  stage The sand filter is  proposed to act as  drain and the 2.5 meter thickness of sand filter will also function as crack filler. The rock fill shell material and transition matenals  are to be obtained by  blasting of the available volcanic  rocks  since these are only suitable matenal available locally in the vicinity of the project area.
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The thickness of upstream and downstream filters has Deen proposed to be 2.50 m considering easy of construction and meeting the drainage capacity Further refinement of filter thickness may De considered at the detailed design stage
The gradation of the naturally available sand material for use in filter is also considered not suitable as it contains large percentage of fine materials and it does not fit the designed filter bana, making it not suitable for the purpose. Further investigation of suitable sand material for use as filter need to be carried out during the detailed design stage If it proves difficult to obtain suitable sand material for use in filter then crushing of available rock  may  be considered
4.9.2  Sand Filter Design
The filter to be used in earth and rock fill dam should meet the two basic function of retention and permeability
In order to retain the fine clay material in the central core of the dam, the design of sand filter band has been adopted taking into consideration the filtenng, segregation, permeability ana stability requirement.
The gradation curves of clay materials from the different identified borrow areas exhibit clayey materials to De very fine For very fine clay Dase soils the filter design criteria used here is based on laboratory study carried out by Dunningan, Talbot and sherard ana:
Z).,.
•    For Filtering, Maximum —9 but not less than 0.20 mm From the filter provided
Dn5(f) = 0 15 mm Des(b) = 0.02 mm
. g|S(/)_ 0 15 ' D (b) 0.02
ti
Df
•    For Permeability, Minimum —> 4 but not less than 0 10 mm D f = 0.15 mm
is
Dis b = 0.003
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.^/   A1L 5O>5
l5
=
=
D b 0.003
The filter provided meets the permeability criteria
•    The allowable filter design Dand must be Kept relatively narrow to prevent use of gap- graded filter The designed filter bano range of particle size has been kept narrow
•     The  designed  filter  band  must  not  have  extremely  broad  range  of  particle  sizes  to prevent use of possibly gap graded filter. This aspect is to be taken care of during construction stage
•    Segregation of filter dunng construction shall be minimized. For segregation of filter dunng construction to be minimized
D10f=0.07
Then maximum D  (f) should be 20 mm.
M
In this case
D9o(f) is less than 20 mm and is in order
•     Coefficient of Uniformity (cu) =         > 4 Ao
Dw(f)
=0.3 mm
Dio(f)
= 0.07 mm
0-3
Z), (/)
o
0.07
= 4.3 >4
This meets the enteria that coefficient of uniformity for filter material should be more than 4 (7>3O( n
Coefficient of curvature (Cv) =--------- —>1
X
^«O(/) Ao /)
(
DM(f) = 0.2 mm
DM(f) = 0.30 mm
Dio(f) = 0.07 mm
(0.2)’ Coefficient curvature CV = ——-—
0.3x0.07 39
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0.04
" 0.021
=2>1
and satisfies the criteria’s that coefficient of curvature should be more than 1
May 2007
The selected filter band for clay materials proposed for use as a core is providea at Tabie 4 2. The filter band proposed is made wider to accommodate wide range of filter material and shown in figure 4.7
Table 4.2: Selected Design Sand Filter Band
Sieve No.
Sieve opening (mm)
percentage Passing
1"
25
100
3/8"
9.5
90-100
No 4
4.75
85-100
No 10
2
75-100
No 16
1.18
70-100
No 20
0.84
65-100
No 30
0.6
50-100
No 40
0.42
40-100
No 50
0.3
30-80
No 60
0.25
25-70
No 100
0.15
5-35
No 200
0.075
up to 5
4.9.3  Design of Transition Filter
Transition filter has been provided between the sand filter and the rock fill shell material both on the upstream and downstream of sand filter. The transition filter of 2.50 m thickness has been provided from consideration of construction convenience so that at least 2.0 m thickness of transition filter is available considering the intermixing during construction of this filter material with the adjoining materials dunng construction and compaction.
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CH8 TP9
F-3 4 T
PERCENTAGE FINER (%)Arjo Dedessa Irrigation Project
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May 2007
The transition filter material has been designed to meet the requirement of filtering and permeability the same way as has been oone for design of sand filter material to meets the requirements. Besides the filter material satisfy the coefficient of uniformity and the coefficient of curvature requirement considering these aspects the selected design filter band for the transition filter material provided is given below in Table 4.3
Table 4.3 Selected Design Transition Filter bank
Sieve No
Sieve opening (mm)
% age passing
If
1
25
100
NO. 4
4.75
90-100
I No 10
2
75-100
No. 16
1.18
65-100
No. 20
084
60-100
No. 40
0.42
50-100
No. 50
030
45-100
No. 100
0.15
20-80
No. 200
0.075
0-20
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5 SEISMICITY
5.1 General
May 2007
The project lies in a zone of medium to low seismicity. The dam nas Deen designed to De safe under anticipated seismic loading condition Keeping in view the seismicity, the design features like provision of extra free Doard, wide and properly  designed filters, flaring of core at the abutment contact area etc. nave been provided for.
5.2 Seismicity of the Region
The Arjo-Dedessa dam site is located in the western plateau of Ethiopia, outside and far away from tne Afar and main Ethiopian rift, whicn constitutes  the northern part of the African Rift valley However the chance cannot be ruled out of this dam site area being hit by a damaging earthquake, wmch may rupture in tne rift valley
Generally Afar and the main Ethiopian rift are seismically active but incomparable with what is along the Pacific  Rim. Earthquakes  that have occurred in the region are reported to cause relatively less Foss of life and damages than equivalent size shocks in other earthquake prone area This is not Decause they were not destructive but is due to the then sparse population distribution and little development of infrastructure As  the pace of urbanization and infrastructure development picks  up in the country  it is  prudent to consider the possible earthquake threats  in the sub region. The Arjo-Deoessa dam site is  about 340 km away  far from the 5.6 magnitude earthquake of 1987 in south-western Ethiopia. The other 6.4 magnitude earthquake of 1961 in Kara kore is  also very  far away  Figure 5.1 snows  the earthquake location in black  dots  along Afar and the main Ethiopian rift while the block  star represent location of Dedessa dam site (Seismicity  of Ethiopia from Ayde 2005, Earthquake catalogue plotted on 90 metre resolution Digital Elevation Model DEM) data.
The  two  earthquake  locations  are  hign  seismic  potential  areas  for  the  dam  sites  Afar  is generally seismically active and a damaging earthquake can occurs any time.
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FIGURE 5.1 SEISMICITY OF ETHIOPIA
Ayele, 200S earthquake catalogue (see annex) plotted on the 90 meters resolution DEM (Digital Elevation Model) data. Black dots are earthquake location aiong  Afar and the main Ethiopian rift while the black stars represent locations for Dedessa  &. Gumera dam sites
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5.3 Design seismic Coefficients
May 2007
The earthquake force experienced by a structure depends on its own dynamic characteristics in addition to those of ground motion. Response spectrum method takes  into account these characteristics and is recommended for use in case where it is desired to take such effects into account. For design of other structures, an equivalent static  approach employing use of seismic coefficient is adopted
5.4     Seismic Hazard Map of Ethiopia
A  reference  was  made  to  Geophysical  observatory,  Addis  Ababa  University,  Addis  Ababa Ethiopia for seismic coefficient to be adopted for the design of the Arjo-Dedessa Dam located in Jima Area In their report dated June 29, 2006 they have enclosed ’’Seismic Hazard Map of Ethiopia and its Northern and Eastern Neighboring Countries". The hazard is for a probability of occurrence of 0.0033 (Return period of 300 years). The contours in the seismic hazard map indicate pear horizontal acceleration as a fraction of g. In Figure 5.2 for the Arjo-Dedessa Dam site the nearest contour of 0.05 of peak honzontal acceleration on the hazard map has been shown which is considered for the stability analysis of the dam section. The site is in an area seismically at iess dangerous location of the country but the possibility of damaging earthquake from the adjacent rift cannot be ruled out.
Several attempts have been made to produce seismic hazard map of Ethiopia. The results can be influenced by the limitation in data both in quality of instrumentation and time span covered. The seismic hazard map of Ethiopia from all available seismotectonic data is shown in the above figure.
5.5   Horizontal seismic coefficient
In the light of above the horizontal seismic coefficient ’ocA’ of 0.05 g and vertical seismic coefficient « h = 1/2 of a v i.e. = 0.025 g has been considered in the stability analysis of dam section for Arjo-Dedessa Dam Project. Dynamic Analysis of the aam section is not considered as the project area is in a zone of medium to low intensity.
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FIGURE 5.2 HAZARD MAP OF ETHIOPIA
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6 DAM HEIGHT
6.1 General
May 2007
3
The main parameters that nave been considered for deciding the height of the dam include, topography of the area, the geological and geotechnical set up of the foundation rock, the river inflows, irrigation and other demands of water siltation rate in the reservoir during the active life of the reservoir and the evaporation losses in the reservoir. The height of the Arjo-Dedessa dam has been considered after carrying out reservoir operation studies considering the various inputs.
6.2 A review of the previous studies made
The studies carried out in the past reveal that USBR had considered a height of 79.0m for Arjo-Deaessa Dam site DD-11 providing storage of 1859.0 Mm3 for meeting irrigation requirement of 16850 na and for generation of electricity. WAPCOS proposed a dam height of 88.0m at a site 50 km upstream of site proposed by USBR providing a storage of 705.11 Mm3 exclusively for irrigation command of 139800 ha. BCEOM suggested 85.0 m high dam with a storage of 2190 Mm for irrigation a command of 14280 ha at a location same as that proposed by USBR
6.3 The Present Dam Height
The present dam height considered now is of 40.6 m height above the river bed level at a location on river Dedessa 1.5 km upstream of confluence of River Dedessa and Wama River providing a storage of 1341.16 Mm at full reservoir level of 1356 0 m for an irrigation command of about 14,700 ha. and for generation of hydro electricity.
The level of river bed at the dam site is at EL 1320.0 the dam height from the riverbed is 40.6 m. However the dam height from the foundation would be about 43.6 m considering the depth of weathered rock in the river bed.
6.4 Free Board
The free board provided above Full Reservoir Level (FRL) and Maximum Water Level (MWL) should be adequate enough to safeguard against overtopping of the dam under wave conditions. The normal free board is computed at FRl and minimum free board above MWL. The free board which has given the highest requirement of TBL (Top of Bund level) is finally adopted
3
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The factor that are considered for the estimation of the free board are.- a)  Wave cnaracteristics, wave height and wave length
May 2007
b)  Height of wind set up above still water level adopted as free board reference
elevation and
c)  Slope of the dam and roughness of the pitching on the slope.
6.4.1 Method for Free Board Computation
Out of the available methods for free board computation assistance has been derived from T Saville s method which is widely used for free board calculations of embankment dams Free board computations have Deen maae as per the Indian standard IS: 10635 “Guide lines for Free Boara requirement in Embankment Dams" This requires that Normal Free Board at FRL shall be calculated by adopting design wave height (Ho) as 1.67 times the significant wave height (Hs) and Normal Free Board should not be taken less  than 2.0m. For calculating minimum freeboard at MWL half to two third wind velocity is to be adopted. As the region may experience higher wind velocities  curing the period water level in the reservoir is  at Full Reservoir Level (FRL), half velocity has been considered for computing free board at MWL This free board has been subject to a minimum of 1.5 m. The design wave height (Ho) is to be taken as 1.27 times the significant wave height (Hs).
6.4.2Computation for Normal Free Board at FRL
The fetch length is computed as the straight-line distance along the wind direction over open water on which wind blows, which is marked to cover the maximum reservoir water, spread area within 45° on either side of the fetch. There after the effective fetch (f.) is computed by the following
Cos a Cos a
Cos a
Where Xi denotes the length any radial, which is at an angle from the central radial. The values of Xi are read from the reservoir spread of the Arjo- Dedessa dam for different angles of the radials f. is the effective fetch length. These are tabulated in Table 6.1.
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Table 6:1 Effective Fetch Length (^Computation for Normal Free Board
May 2007
■p—
Xi Cos a Cos a
Cos a                   Xi
42
0743                     0.6               033
36
0 809                    1.55
1.01
30
0 866                    545
409
24
0.914                    5.6
468
18
0.951                    8.1
7.33
12
0978                     10.25
9.08
6
0 995                    10.55
1044
0
1 00                      1377
13 77
6
0.995                    995
9.85
12
0.978                    6.95
6.65
18
0.951                    3.70
3 35
24
0.914                    3.35
2.80
30
0.866                    2.7
2.02
36
0.809
2.1
1.37
42
0.743                    1.5
0.83
^Cosa =13.512                       £ Xi CosciCosk = 78.32
78.32
13.512
= 5.796 km Say 5.8 kms
The data on wind speed has  been obtained from National Meteorological Service Agency Addis Ababa, Ethiopia and has been used for calculating of the wave height. From the data the maximum wind speed in the project area observed is 6.0 km/hour This for a 50 year return period has been computed to 30.0 km /hour and this wind velocity has been considered on the land surface in the design computation.
Wind velocity  on water surface (V) is  computed by  multiplying it with wind coefficient Q' corresponding to effective fetch to the wind velocity on land (U). The wind coefficient 'Q‘ for an effective fetch in km (f.) of 5.8 km is given as 1.27 in the IS: 10635 code. Therefore design wind velocity on land surface works to 1.27x30 = 38.1 km/hr or 10.58 m/sec.
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Significant wave (H ) is computed by the relation as below
May 2007
s
g Hs/v2 = 0.0026 <g 4 )047
V
For v = 10.58 m/sec.
f e = 5800 m
Hs = 0.555 m
Wave period (Ts) is computed by the following relationship
2 020
g Ts/v = 0 45 (g. Uv )
for v = 10.58 m/sec and f. = 5800 m
Ts = 2.78 seconds
Wave length (Ls) is computed by the relationship:
Ls = 1.56 Ts2
For Ts = 2.78
Ls = 12.04 m
Design Wave height (Ho) is computed by the relationship
Ho = 1 67 Hs
For Hs = 0.555
Ho = 1.67 x 0.555
= 0.927 m
Wave steepness ratio Ho/Ls = °'927
12.04
= 0.077
Relative run up R/Ho for embankment stope of 3H:1V and Ho/Ls = 0.077 is read on the graph of wave run up ratio versus wave steepness and embankment Slope and is read as
R/Ho = 1.40
Where R is the run up Therefore R = 1.40 x 0.927
=1.298
Designed  ’Ra’  considering  dumped  stone  riprap  for  up  stream  protection  is  computed  by multiplying 'Ra‘ with the surface roughness coefficient.
Surface run off co-efficient for dumped rip rap is = 0.50
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Therefore, Designed Ra = 0.5 x 1.298 = 0 649
May 2007
2
Since the wave sum on rough surface (Rn) works out to be less than the designed wave height (Ho) Therefore Rd is kept = Ho i.e 0 927m
The average depth of the reservoir 'D’ in m has been worked out as 25.23m Therefore wino set up 'S' in m is computed by the relationship
Wind set up S = V  F/62000. D m
Where V = wind velocity in km/hour = 30.0
F = Maximum Fetch in km = 13.77 km
D = Average Depth of Reservoir in m = 25.23 m
Wind set up S =        38.1 x 13.77
62000 x 25.23
= 0.012 m
Normal Free-board required = Ra + S
=        0.927 + 0.012 m
=       0.939 m
This is less than 2.0 m
Hence Minimum Normal Free board of 2.0 m at FRL shall be provided.
Normal Free board provided = 2.0 m
6.4.3  Computation of Minimum free Board at MWL
The fetch length is computed as the straight line distance along the wind direction over open water on which wind blows which is marked to cover the maximum reservoir water spread area within 450 on either side of the fetch. There after the effective fetch (f.) is computed by the following.
V XiCosa Cosa
f,=
Where Xi denotes the length any radial, which is at an angle from the central radial. The values of Xi are read from the reservoir spread of the Arjo-Dedessa dam for different angles of the radials for MWL at EL: 1358.6 m as tabulated below:
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Table 6.2: Effective Fetch Length (f ) Computation for minimum Freeboard
e
Xi Cos a Cos a
Cos a
Xi
42
0743
1.4
0.771
36
0.809
3.4
2.225
30
0.866
56
4.199
24
0.914
78
6.516
18
0.951
8.1
7.326
12
0.978
10.2
9.756
6
0.995
11.75
11.632
0
1.00
13.95
13.950
6
0.995
8.65
8.564
12
0.978
6.95
6.648
18
0.951
3.70
3.346
24
0.914
3.40
2.840
30
0.866
2.70
2.025
36
0.809
1.95
1.276
42
0.743
1.45
0.798
Y,Cosa = 13.512                          £ Xi Cosa Cos = 18.872
Fetch Length: 13.95 km
Effective Fetch
81.872
13.512
= 6.059fon Say 6.06 km
For MWL condition the wind speed on land is considered half of at FRL condition as per the IS code and this has been considered in the design computation.
Wind velocity on water surface (v) is computed by multiplying it with wind coefficient Q‘ corresponding to effective fetch (f.) to the wind velocity on land (U). The wind coefficient Q' for an effective fetch in km of 6.06 km is given as 1.27 as per the IS: 10635 code Therefore, design wind velocity on water surface works to - x 30.0 x 1.27 = 19.05 km/hr or 5.29 m/sec 
2
Significant wave (H ) is computed as below.
s
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2
g Hs/V  = 0.0026 (g.^-) v
v = 5.29 m/sec fe = 6060m
Hs = 0.272m
May 2007
047
Wave period (Ts) is computed by the following relationship
2 028
g. Ts/v = 0.45 (g.fe/v )
for v = 5.29 m/sec ana fe = 6060 m
Ts = 2 07 seconds
Wave length (Ls) is computed by the relation ship:
Ls = 1.56 Ts2
For Ts = 2.07
Ls = 6.69 m
Design Wave height (Ho) is computed by the relationship Ho = 1.67 Hs
For Hs = 0.272 m
Ho = 1.67 x 0.272 = 0.454 m
Wave steepness ratio Ho/Ls is computed Which is = 0.068
0.454
6.69
Relative run up R/Ho for embankment slope of 3H:1 and Ho/Ls = 0.068 read on the graph of wave run up ratio versus wave steepness and embankment slope and is read as R/Ho = 1 43 where R is the run up.
Therefore R* = 1.43x0.454 m
= 0.649 m
As this is more than Ho, therefore value of R.= 0.649 as computed has been considered The average depth 'Dm' of reservoir in m has been worked out as 25.0m. Therefore wind set up 'S’
in m is computed by the relationship wind set up S =      v2F
xD.
62000
where v=wind velocity on water surface 15x1.27 = 19 05 km/hr
F= Maximum fetch in km = 13.95
Dm = Average depth of reservoir in m = 25.0m
Wind set up
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_ 19.05x13.95
62000 x 25
= 0 003 m
Free Board Required = Ra + S
= 0.649 + 0.003
= 0.652m
This free board computed is less than the minimum free board at MWL and shall De more than or equal to 1.5m. Therefore the Minimum Free Board shall be 1.5m. Considering settlement
due to earthquake of 0.5m the free board provided above for MWL = 1.5 + 0 5m
= 2.00m
The free board computation for Normal Free Board is given is Table 6.3
6.4.4   Minimum Free Board at MWL
Minimum free board at MWL has Deen computed in the same way as for Free Board for normal water level The fetch length (F) and effective fetch (f,) is  calculated at MWL. The wind velocity on land in this case has been considered half of the considered for Normal Free Board computation. The computation for minimum freeboard are presented in Table 6.3. In case the minimum freeboard computed is less than 1.5m then at least 1.5 m freeboard is provided
6.4.5 Settlement Due to Earthquake
Since Free board computed above does not account for effect of earthquake induced waves (seiches) settlement of dam and dam foundation Hence allowance nas  been provided for these aspects in fixing the free board. For an earth and rock fill dam settlement of 0.5m is considered in computing the freeboard for settlement due to earthquake-induced waves
Hence Freeboard provided above
i)   Full Reservoir Level (FRL)= 2.0 + 0.5 = 2.50 m
ii)   Maximum Water Level (MWL) 1.5 + 05 = 2.00 m
6.4.6  Top Bank Level
Considering the normal free board the top of dam works to EL is 1356 + 2.5 = 1358.5 and on considering the minimum free board above MWL the top of dam works to EL 1358 60m ♦ 2 00 = 1360.6m. It is seen from above that the criteria of Minimum free board above MWL becomes the criteria for deciding the top of dam elevation, which come to 1360.6m Therefore Dam crest has been kept at EL 1360.6m.
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Reservoir Operation
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The following control levels are required for reservoir operation for fixing the maximum live storage of reservoir aunng the useful life of the dam and required freeboard
(i)     Full Reservoir Level (FRL)
(ii)    Minimum Draw Down Level (MDDL)
(iii)   Maximum Water Level (MWL)
6.4.7 Full Reservoir Level
The main objective is to achieve maximum live storage from the available annual river flows in a monsoon period after meeting the irngation demand in the command. The water over and above the irrigation demand is stored in the reservoir for meeting the requirements during a dry/lean year and for generation of nydroelectric  power. However the site topography, geological consideration and safety aspects are the limiting factors for the height of the dam. Hence, considering these aspects the top of the dam has been fixed at El. 1360.60 m
6.4.8 Minimum Draw Down Level (MDDL)
The  following  consideration  have  been  kept  in  view  for  fixing  minimum  draw  down  level (MDDL)
(i)       The function of irngation outlet is not affected due to the sedimentation of the useful life of the reservoir
(ii)      Adequate depth of water above the irrigation outlet is provided to prevent any vortex formation when drawing water from the irrigation outlet.
Keeping the above aspects the minimum draw down level (MDDL) in the reservoir has been fixed at EL 1350.0 m. The storage in the reservoir at FRL = 1341.16 Mm . The active storage in the reservoir between FRL at 1356.0m and MDDL at 1350.0 m is 466.49 Mm3 at the start of operation. The dead storage has been fixed at EL 1350.0m and the storage at Dead storage lend is 874.67 Mm3. With the active life of the dam considered for 100 year the sedimentation in the reservoir would attain a level of EL 1330.67 m and the active storage in the reservoir would be adequate enough In meeting the irrigation demands. In addition to meeting the irrigation demand for water the surplus water in the reservoir could be utilized for generation of power as a secondary source for generation of revenue These aspects may be examined in greater details at the time of preparation of detailed project report.
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6.4.9  Control Level
The various control levels of Arjo-Dedessa Dam and reservoir are given as under (i) Arjo-Deaessa Dam
May 2007
Length of Dam
Riverbed level:
Top of Dam:
Height of Dam above rivers bea
Height of Dam above deepest foundation
(iii)    Reservoir
Maximum water level (MWL) m Full reservoir ievel (FRL Minimum Draw Down Level (MDDL) Dead storage Mm3
Live storage Mm3
: 512 0 m
EL 1320 00 m
EL. 1360.60 m
40.6 m
43.6 m
EL: 1358.60 m
EL 1356.0 m
EL: 1350.0 m 874.7 Mm3 466 49 Mm3
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Table 6: 3 Free Board Computations Freeboard Computation (according to IS 10635:1993)
Normal Free Board (at FRL) shall be > 2m
Minimum Free Board (at MWL) shall be > 1.5m
Full Reservoir Level (FRL) 1356.00 m
U/s Slope of Dam: 2.5H:1V
May 2007
s. Step-wise computation
No
Normal
Free board
Minimum Free Remarks
Board at MWL
1        Fetch length (F) in Km
2        Effective Fetch (fe) in km
3        Wind Velocity Over land (U) in km/h
4        Wind Coefficient (Q)
5        Wind velocity over water surface (v) in km/h
6        Wind velocity over water surface (v) in m/s
7        Significant wave height (Hs) in m
8        Wave period (Ts) in seconds
9        Wave length (Ls) in m
10      Design Wave heignt (Ho) in m
11      Wave steepness Ho/Ls
12      Relative Run-Up R/Ho from diagram
13      Run-up on smooth surface R in m
14      Designed  Ra  considering  dumped  pitching  for U/S slope protection (R x 0.50)
Since Ra works out to be iess than designed
wave height (Ho), Ra is kept = Ho = 0.927m
15      Average Reservoir Depth (D) in m
16      Wind set-up in m
17      Free board Required in m
18      Permissible Free board
19     Top of dam (as calculated)
20      Top of dam required, considering settlement
21      Top of dam adopted, m
13.77                  13 95
580                   6.06
30.00                15.00
1.266                1.27
38.10                19.05
10.58                5.29
0.555               0.272
2.78                  2.07
12.04               669
0.927               0454
0 077               0.068
1.40                  1.43
1.298               0.649                     U/s slope = 2.5H:1V
Since    ha
is        less 0.649               0.325                    than Ho, 25.23               25.00                     ..value of 0.008               0 003                    Ho=0.927 0.935               0.652                    has been 2.00                  1.50                      adopted 1358.00           1360.1
1358.50           1360.6
EL 1360.6
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7 DESIGN ASPECTS
7.1 General
May 2007
The  design  of  earth  and  rock  fill  embankment  oam  would  be  governec  by  the  following considerations
•     The dam matenals should have sufficient shear strength so that the requirement of stability under various static condition and earthquake condition to which the dam and its foundation is subjected, should be met.
•     The  design  flexibility  should  be  sufficient  to  allow  the  fullest  possible  utilization  of  all required excavation and readily available borrow matenals
•     The core material in to be sufficiently impermeable to resist seepage through tne dam ano there shall be a general increase in permeaDility from the core toward the exterior slopes of the dam
•      Filter  requirements  between  adjacent  zones  and  between  the  foundation  fill  materials should be satisfied in order to avoid migration of fine materials into coarser ones.
•      The  transition   zones   should  be   thick   enough   to  accommodate  all   conceivable  fault movements
The earth and rock fill dam section adopted is a conventional section consisting of impervious day  core material ano protected by  thick  filter transitions  on the upstream, downstream ano shell of rock  fill material. This  is  considered to be the appropriate dam type for the Arjo- Dedessa Dam Project due to suitable topography and availability of construction materials for the dam in the vicinity of the area The upstream cofferdam is proposed to be incorporated into the main dam section.
7.1.1       Embankment Features
7.1.1.1   Crest Width
The general consideration in adopting crest width depends upon the working space required at
top of dam and also on the functional purpose the dam serves like the public high way and requirement of future needs.
a)       The width of dam at crest as per IS 8826 - 1978 "Guide lines for design of large earth and rock fill dams" should be fixed according to the working space required at top and the crest width should not be less than 6.0 m.
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b)       According to USBR 'Design of small oams the crest width to be adopted in case of small earth dams is given by
^=y+lCinft
where,
V\Z = Width of crest in ft
H = Height of dam in feet above nver bed, this in case of Arjo-Dedessa Dam = 40.0 m
40x3.28
W =------------4-10 ft
5
= 36.0 ft
or « 10 98 m
Say 11.0 m
c)        US army manual "Earth Embankment EM 1110-2300 31st July 1994, provides that top width of an earth and rock fill dam, depending upon the height of dam, the minimum top width should be between 25 ft (7 62 m) and 40 ft (12.195 m).
Keeping In view the above considerations, the top width of crest for Arjo-Dedessa Dam has been adopted as 10.0 m. The dam crest details are shown in drawing No. AD-F-DAM/13.
7.1.1.2 Berms
Berms shall be provided for serving the following purpose:
•    For providing level surface for construction and maintenance of the dam section
•    For reducing the surface erosion in case of downstream slope and breaking the continuity of the slope
•    To protect the lower edge of the rip rap ana for preventing it from undermining in case of the upstream slope
Berms of width 5.0 m have been provided at every vertical elevation of 10 0 m A berm 6.0 m wide at top elevation of rock toe has been provided.
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7.1.1.3  Camber
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Major portion of earth and rock fill dam consolidation takes place during the construction period Therefore camber shall be proviaea along the crest of an earth and rock fill dam to ensure that the free board provided is not reduced due to foundation settlement or consolidation of an earth ano rock fill dam Therefore camber shall be provided along the axis of the dam from zero height at the abutments to maximum at the central section of the earth and rock fill dam. Observation on existing rock fill dam indicate that settlements ranging from 0.2 % to 1.0 % of the dam heignt have been experienced For Arjo-Dedessa Dam a camber of 0.5 m has been provided at the maximum section for settlement purpose
7.1.1.4 Parapet Wall
A parapet wall of 1.5 m height over and above the free board requirements has been provided as an additional safety. The reservoir face of the parapet wall has been provided with a parabolic curvature so that the splashes resulting from reservoir waves if any are returned back to the reservoir
7.1.1.5  Guard Stones
Guard stones of size 300mm x 300 x 750mm has been provided in cement concrete blocks
1:3:6 at spacing of 3.0 m centre to centre on the downstream edge of top width of dam.
7.2  Design Parameter
i)    Free board
ii)   Properties of Materials
iii)  Seismic Coefficient
The above design parameter has been described at paras 4.4 5.12 and 11.5 respectively
7.3  Basic Design requirement
The basic requirements in the design of Arjo-Dedessa Dam is to achieve safety with economy under various conditions of operations of the reservoir. The criteria adopted for the design of and earth and rock fill dam is given as under:*
a)  Safety against overtopping of dam dunng inflow design flood
b)  Stability of slopes
c)   Safety of dam against internal erosion
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7.3.1 Safety against overtopping
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To ensure safety against overtopping adequate spillway capacity nas been provided to prevent overtopping of the earth and rock  fill dam during ana after construction The free board provided is sufficient to prevent overtopping by waves and shall also take into consideration the settlement of the dam ana foundation.
7.3.1.1 Free Board
The free board has been calculated by 'T. Savelie s" method, which is widely used for free board computation of earth and rock fill dams. The details of procedure and computations have been carried out according to Indian standard IS 10635 1993 "Guide lines for Free board requirement in embankment dam " and the computation for free board for FRL and MWL are contained in Chapter - 4 of this report
7.3.2 Embankment Section
The dam section has been mainly divided into zones namely the central impervious core ana the graded shell matenai of rock fill/gravelly temace matenai. Besides, the coarse and fine filter have been proposed at both upstream and downstream face of the impervious core Rip rap has been proposed on upstream siope face of the dam above the shell material with a filter underneath the rock rip rap. Hand placed stones have been proposed on the downstream face of the dam to provide protection against erosion resulting from rainfall or wind.
7.3.3 Zoning of Dam Section
A summary of the required matenai types in different zones of the dam section is given below in Table 7 1
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Table 7.1: Zoning of Dam Section - Material Types
May 2007
Zone No
Deception
Material type
1
Impervious core
Clayey material
1A
Coffer cam
Clay, soil as for zone 1
2
Fine filter u/s & d/s face of core
Sand,  filter   5   mm   maximum   size  and  silt  content (< 0.075 mm) less than 5%
3A
Transition zone
Small rock fill 75 mm maximum size
3B
Transition zone
Same as at 3A
4
Coarse filter
coarse sands gravel upto 75 mm size and silt 15%
4A
Upstream shell
Rock  fill  400  mm  maximum  size  50%  maximum  finer than 25 mm
4B
Downstream shell
Rock  fill  500  mm  maximum  size  50%  maximum  finer than 25 mm
5
Dumped Rock rip rap
Maximum size 1000mm 40 to. 50% > 800mm 50 to, 60% > 300-800mm, 0-10%, > 300 mm sand, rock dust <
5%
6
Blanket below u/s rip rap
Crushed rock or natural gravel maximum size 80 mm
7
Hand placed rock rip rap
Rock fragments
To prevent piping of the fine fraction of the core, a transition zone is proposed. A fine filter consisting of sand (zone 2) is provided downstream of the core. A transition filter (zone 3B) is proposed downstream of fine filter to collect the seepage through the core and convey it to the base of the downstream shell of the dam. A transition zone consisting of small rock fill may also be required to be provide size compatibility between the transition filter and the large rock fill sizes of the shell at the detailed design stage
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Filter layer (zone 2A and 2B) are provided to separate core from the upstream shell and downstream The fine filter (zone 2A) adjacent to the core on the u/s side will act as a crack filler for any cracks, wnich might form in the core resulting from differential settlements.
The proposed zoning system is aimed at maximizing the use of material obtained from the required excavation. The weathered rock obtained from excavation would be placed adjacent to the inner part of the downstream shell (zone 4A) while the sound unweathered rock would be placed adjacent to the coarse filter (zone 3A, 3B) and toward the outer part of the shell.
A separate rock fill zone (zone 4) is proposed at the base of downstream shell to provide adequate drainage capacity This rock fill zone is constructed using the more resistant basaltic rock.
The upstream face of the dam has been zoned (zone 5) to protect it from the erosion effect of the wave action, which will aiso impart drain ability in the event of instantaneous build up of pore pressure under earth quake condition. Rip rap material will be obtained as over size from excavation of basalt rock. The zoning of upstream coffer dam will be similar to main cam. The earth and rockfill dam section nas been shown in Drawing No AD-F-DAM/04
7.3.4 Impervious Clay Core
The compacted impervious clay core provides an impermeable bamer within the body of the dam. Impervious clays available locally are very suitable for the core. However, clays having high compressibility and liquid limit tend to swell and are prone to hydraulic fracturing of clay core. Clay soils having organic content are aiso not to be used as core material.
A central core is preferred over the inclined core as it has the advantage of providing higher pressures at the contact between the core and foundation and reduce the possibility of leakage and piping. Since the availability of impervious clay core matenai locally is not a constraint at site, a thicker core of compacted clay with upstream and downstream slopes of 0.5V: 1V has been provided for resisting the piping action. The top elevation of the clay core has been kept 1.0 metre above the maximum water level to prevent seepage due to capillary syphoning. The minimum top width of core of 5.0 m has been provided as per Indian standards.
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7.3.5 Shell Zones
May 2007
The function of shell is  to impart stability  and protect the impervious  core The rock  fill materials, which are not subjected to cracking on direct exposure to atmospnere, are suitable for use as snell material. In Arjo-Dedessa Dam locally available rock material is made use of in the shell zones both in upstream and downstream of impervious clay core The rock material shall be hard, sound and durable so as to resist excessive break down during the handling and placing operations  Dust and smaller fragments  of rock  shall not be more than 10%. The angular bulky  rocks  are preferred as  against flat, elongated rocks  or rounded boulders  If rounded cobbles or boulders are used, they should be scattered throughout the rock fill and not concentrated in pockets.
7.4    Foundation Treatment
7.4.1 Core Seating Treatment
The stripping below core seating will be done by  removing the alluvium talus  and loose deposits. The foundation for the impervious core, upstream filter and downstream filter will be excavated through the upper weathered rock  to fresh rock, which can take up the grout Suitable core foundation shall be based on sub-surface geo-technical data and shall be obtained from the geo-technical investigation team. The depth of excavation required to obtain a suitable core foundation shall De estimated based on the assessment of the geo-technical engineer / geologist and the longitudinal profile along the central line of dam axis indicate the depth up to which the core trench excavation snail be done.
The core and filter zone foundation surface should be thoroughly cleaned using mechanized equipment and high-pressure air water jets. Open joints  and minor shear zones  will be excavated and cleaned to a minimum depth of three times the opening width and should be sealed by filling with slush grout back fill concrete In case some major discontinuities/shear zones are encountered then these would require special treatment. All depressions in the area of core foundation and filter foundation will be filled with dental concrete treatment to provide an even and unerodable surface and core material shall be placed ano compacted.
In the event of getting megular rock surface, 50 mm - 75 mm of shotcrete thickness over the foundation surface supporting the core and on both upstream and downstream filter has been
provided. This mat of shot crete will serve as a grout cap for blanket grouting operations Blanket grouting has been provided to a depth of 10.0 m under the core and filter zones and
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shall be based on the sub surface geo-technical information and sampling obtained from the field investigation conducted by  WWDSE and tests  conducted on samples  by  the sub contractor M/S Construction Design Share Company  (CDSCO) Addis  Ababa, Ethiopia. Consolidation grouting has been provided to reduce the potential flow through open joints in the vicinity  of core and filter and thus  minimize the potential for erosion of core matenai at its contact with the foundation. Upstream and downstream filter zones enveloping the core nave been provided to serve additional security against core erosion The grout curtain comprising of two rows of grout holes is considered appropriate on the basis of the availaole data in the valley area and single grout curtain in the abutment area The spacing of the grout hole has been provided at 3.0 m centre to centre. Final spacing will depend on the grout intake and test holes and will be determined during actual grouting operations.
7.4.2  Shell Foundation Treatment
Foundation excavation under the shell zone will oe required to reach foundation materials having shear strength and compressibility at least comparable to those of the compacted rock fill. Talus deposit and soils like weathered rock shall De removed, and founded on moderately weathered rock along the abutment side slopes
In the river channel area, all alluvium deposits will be excavated and the shell zone will be founded on competent, moderately  weathered rock. Where shear zones  filled with pipable materials underlie the downstream shell it shall be excavated and a filter will be placed over these zones to prevent piping. Before laying the first layer of shell matenai in the nver valley the foundation material will be thoroughly compacted by giving appropriate number of passes of a vibratory roller after applying moisture to the required extent
7.4.3  Cutoff Trench
Cut off trench shall be provided at the foundation of the impervious clay core and serve the purpose given as under
•     To  reduce  the  seepage  of  water  resulting  from  impoundment  of  water  in  the  reservoir through foundation and abutments water faces with impervious core and
•    To prevent the sub surface erosion by piping
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A positive cut off shall be provided along the axis of the dam for the full length of the dam taking into consideration the geological set up of dam foundation and abutment as  revealed by  the bore hole logs drilled on the left and right, valley bank and the river bed
The valley at the dam site has an overburden of 3 0 m to 5 0 m depth consisting of alluvial deposits comprising of silty  clay  intermixed with gravel materiai. The positive cut off shall be provided up to groutable rock to prevent any seepage from the reservoir at the water face with impervious core.
Based on the geotechnical investigations  of the dam foundation and abutment area and the permeability  values, consolidation grouting shall be provided in the cut off trench area The pattern of grouting will depend upon tne permeability  of the foundation strata. The grout curtains  shall be provided to depths  depending upon the geological set up of the foundation rock and its permeability The grout noies shall be covered with a concrete cap.
The bottom width of the cutoff trench nas been provided for full width of the impervious core as, the availability of core matenai is not a constraint. The side slope of the cutoff trench has been kept as 1H: 1V.
After excavation of the cut off trench the same shall be filled back with impervious clay core material and shall be compacted to optimum dry density The compaction shall be done with moisture content of 2% plus or minus to provide an impervious stratum in the foundation.
7.4.4  Location of Cut off Trench
a)        The location of cutoff trench plays a significant role in reducing seepage pressures ana for  the  structural  stability  of  the  clay  core.  The  cut  off  trench  can  be  placed  on  the upstream  face  just  after  the  upstream  toe  of  impervious  core.  This  minimizes  the seepage pressures but ft is more liable to rupture if there are small movements or partial failure   of   upstream   face   of   the   embankment.   It   also   reduces   the   resistance   of foundation.  In  general  the  impervious  soils  have  low  shear  strength  (frictional)  and therefore its location in this zone is not considered favourable.
b)        The cut off trench coinciding with the centre line of embankment, it will have less effect in  reducing  seepage  pressure  through  the  embankment  but  ft  will  be  less  liable  to rupture and less likely to weaken the foundation.
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c)        A cut off trench placed on downstream side just before aownstream toe of core, has no advantage and is dangerous because it allows the transmission of seepage pressure in larger part of the foundation and reduces tne stability of the downstream half of the dam
In view of above merits and demerits of above locations it is proposed to locate the centre line of the dam coincide with the centre line of the cut off trench. This location of the cut of trench provides satisfactory performance as per the expenence and has been adopted
7.4.5  Curtain grouting
Pressure grouting of rock foundation is normally carried out to fill discontinuities, cavities or voids  in rock  mass  by  suitable material. The grouting aims  at satisfying the design requirements economically and in conformity with the rest of construction schedule.
Curtain grouting is earned out mainly to acnieve conditions for safe design of the dam.
(a) To reduce the seepage through the dam foundation
(b) To reduce the erosion potential of seepage and safe guard the foundation against erodibility hazard.
(c) To strengthen the dam foundation and reduce settlement in the foundation
On both the abutment and along the axis of the dam as revealed by the geological setup, the rocks appear highly fractured and jointed, sconacious and of vesicular basalt type with poor core recovery  It is  therefore considered necessary  that in order to minimize the risk  of seepage across the dam axis, abutment and foundation, consolidation and curtain grouting shall be required to be carried out. below core and filter seating and the abutment. This shall be carried out through the grout holes drilled from the cut off trench. Curtain grouting has been provided in the valley reach in the cut off trench consisting of two rows of grout curtain and single row of grout curtain in the abutment depending upon foundation conditions  and permeability  of the strata and geo-technical set up forming the foundation of the dam. Consolidation grouting shall be provided in the cut off trench along the axis of dam by drilling grout holes up to 10.0 m depth.
7.5    Stability of the earth and rock fill Dam Section
An earth and rock fill dam shall be safe and stable during all phases of construction and operation of the reservoir. Hence the analysis has been done for the most critical combination
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of  external  forces,  which  are  likely  to  occur  in  practice.  The  following  conditions  are  usually critical for the stability of an earth ana rock fill aam
a)       Case I - Construction condition with or without partial pool (for upstream ana aownstream slopes).
b)
c)
d)
e)
Case // - Reservoir partial pool (for upstream slope), Case III - Sudden drawdown (for upstream slope), Case IV - Steady seepage (for aownstream siope), Case V - Steady seepage with sustained rainfall (for
aownstream slope; (where annual rainfall is 200 cm or more), and
f) Case VI - Earthquake condition (for upstream and
downstream slopes;
The various design conditions of analysis for upstream and aownstream slope along with the minimum values of factors of safety to be aimed at and use of type of shear strength for each condition of analysis  has  been provided This  is  given in Appendix-A of the IS code No 7894 - 1975 which is reproduced in Table 7.2.
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Table 7.2: Minimum Desired Values of Factors of Safety and Type of Shear Strength
for Various Loading Conditions
Case
No.
Loading Condition of Dam
Slope Most
Likely to be
Critical
Type of Shear
Strength Test to
be Adopted
Minimum
Desired
Factor of
Safety
I
Construction    condition    with    or without partial poor
Upstream and downstream upstream
PT?;
1.0
II
Reservoir partial pool
Upstream
rs;
13
III
Sudden draw aown
a)      Maximum   heaa   water   to minimum
with tail water at maximum
b) Maximum tail water to minimum with reservoir full
Upstream Downstream
rs;
rs:
1.3
1.3
IV
Steady seepage with reservoir full
Downstream
rs;
1.5
V
Steady seepage with sustained
rainfall
Downstream
rs;
1.3
VI
Earth Quake condition a) Steady seepage
b) Reservoir full
Downstream Upstream
rs;
rs:
69
•Where  the  reservoir  is  likely  to  be  filled  immediately  after  completion  of  the  dam, construction  pore  pressure  would  not  have  dissipated  ana  these  should  be  taken  into consideration.
t This is to be adopted for failure plane passing through
impervious foundation layer
* S test may be adopted only in cases where the material is
cohesion less and free draining
§ Values are according to IS: 1893-1975 MCriteria for earthquake resistant design of structures (third revision)”.
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The various types of shear tests performed under different conditions of loading and drainage are designated by letters Q, R and S as follows
Q test unconsolidated undrained test,
R test: Consolidatea undrained test, and
S test Consolidated drained test
The dam is located in an area of medium to low seismicity and therefore psuedo static analysis nas  been carried out with earthquake condition IS 1893 - 1984 out lines  the procedure for evaluating the seismic coefficient and depend upon the shear wave velocity the height of the dam and tne lowest point of the aam foundation through which the failure circle is supposed to pass
Arjo-Dedessa dam site is located in western plateau of Ethiopia and is far away from the Afar and the main Ethiopian rift. The hazard map of Ethiopia and its  northern and eastern neighboring countries for a probability of exceedence of 0.0033 indicate a horizontal seismic coefficient ah of about 0.05 g (As  per the report of Geophysical observatory  Addis  Ababa University Ethiopia) The vertical seismic Coefficient is generally taken as half of a h i.e. a v = 0 025 g and the same has been considered in stability analysis of dam.
7.6     Safety Against Internal Erosion
7.6.1 Seepage Control Measures and Drainage Arrangement
Seepage control measures has been provided to protect the dam from any undesirable effect of seepage occurring through the dam body or through the foundation and abutments. Drainage system has  been so devised as  to tackle the problem of seepage and or migration of fine particles through the body of the dam or foundation. The design is governed by the type of base material, height of water in the reservoir and topographical features  of dam site. The following arrangement has been provided.
-    Impervious clay core
-    Inclined filter upstream of clay core
-    Transition filter downstream of clay core
-    Horizontal filter
-    Toe Drain
Impervious clay core: This has been described at: 7.3.4
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7.6.2 Inclined Filter
May 2007
Inclined filter is provided on downstream face of impervious clay core to collect water coming out of the core resulting from seepage and thereby keeping the downstream shell relatively dry The filter on the upstream face of impervious core acts as crack filler The filter should be cleaned and well graded. The minimum thickness of filter is determined from considerations of
(i)  The thickness of filter should have allowance for intermixing with the adjoining zones The allowance shall be varying depending on compaction equipment used for the purpose.
(ii) Minimum width required for compaction depending upon the equipment deployed
(iii) Earth quake effects
Considering the aoove aspects a minimum of 2.50 m thickness of each layer of filter has been provided from working point of view The top elevation of the filter nas been kept at the same level at that of the impervious core elevation at EL 1358 5 The filter material provided is free draining and well graded as far as possible and it shall not generally have size greater than 75 mm so as to minimize segregation during placement. The filter material provided has to satisfy the following requirements.
a)  It should be more previous than the protected base material to act as an effective drain.
b)  The gradation should be such that the particles of base material do not migrate through or dog the filter voids.
c)  It should be of thickness sufficient to provide good distribution
Filter material should satisfy the standard criteria with the base material as laid down in Indian standard fine base soils.
Two basic functions of filters and drains in earth and rock fill dams are
i) Retention function
ii) Alteration function
The    classic    Terzagi    criteria    ^—<4    addresses    ^^>4    addresses    the    permeability 
D
»
b
D„b
requirement
The filtering, segregation, permeability stability requirements have been considered in the design of sand filter band to retain the fine clay material in the core. The filter design criteria
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adopted   here   is   based   on   an   extensive   laboratory   study   carried   out   by   JL   sherard,   LP Dunnigain and JR Talbot.
As  seen  from  the  gradation  curve  Fig  No.  4.2  the  clayey  material  identified  is  very  fine  The base clay core material consists of very fine silts and clays and are more than 85% finer
For  very  five  clay  base  material  the  following  criteria  have  been  used  in  the  design  of  sand filter
•    For filtenng Maximum - < 9 but not less than 0.2 mm
D6
l5
•    For Permeability Minimum — < 4 but not less than 0.1 mm
D (*)
ls
•    The allowable filter design band must be Kept relatively narrow to prevent use of gap graded
filters
•    The designed filter band must not nave an extremely broad range of particles sizes to
prevent use of gap graded filters
•    Segregation of filter dunng confliction shall be minimum
The grain size curve of the filter provided is nearly parallel to that of the base material
Following criteria should be satisfied for well graded sand being used in the filter
(i) Co-efficient of Uniformity (Cu) =                > 4
(ii) Co-efficient of Curvature (Cv)    = >1
It  should  not  contain  any  organic  material  and  the  particles  below  0.075mm  more  than  5%  by weight.
7.6.2.1 Compaction of Filter
The compaction of sand in the filter should be done by plate vibratory rollers till specific density 
is achieved. The relative density (RD) of sand after compaction should equal to or more than
70%.
in order to obtain the desired density  with minimum compacting efforts, sand may  be placed in dry  or in saturated condition. In the case of inclined sand filters  the sand should not be allowed to flow out of specified width on either sides during compaction It should be confined
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in between two sheets  or plates  installed on either sides  of the filter, in order to ensure the specified width of the filter and to avoid contamination of filter due to mixing with adjoining materiai.
In order to maintain the prescribed slope or inclination of the filter during the construction, the impervious  core hearting zone must always  be at higher level than the level of the filter and the down stream casing zone must be kept at a level which is a bit lower than the level of the filter This  process  should be continued in the same fashion till the end of season at the end of which, the filter should be kept at least 15 cm above the adjoining zones  to avoid contamination of soil with the filter during rainy season.
It must also be ensured with utmost care that the material near the junction of filter and the adjoining zones  on either sides  of it, is  compacted to the stipulated densities  of the respective zones.
7.6.3 Horizontal Filter
The horizontal filter has  been provided on stripped ground level downstream of the impervious core/transition zone and has  been connected with inclined filter downstream of impervious  clay core. A minimum slope of 1 in 100 has  been provided towards  the toe drain for quick  disposal of seepage water. The filter material shall satisfy  the filter criteria. Though the thickness  of horizontal filter required is  small but for practical consideration a thickness  of 1,5m has  been provided. The horizontal filter has  been provided to collect seepage from the foundation and the filter and minimizes possibility of piping along the dam seat.
7.7     Safety of slopes
7.7.1 Upstream Slope Protection
Dumped rock  riprap by  far is  the most preferable and adopted type of protection for the upstream slope. It has the advantage that the energy dissipation of the wave is achieved as the wave rides  and hence wave run up on the dumped pitching is  much less  as  compared to wave run up on smooth hand placed pitching or concrete asphalt pitching. The requirement of hand placed rock  rip rap for the same wave height is  about 1.2 to 1.5 times  more than that of dumped rock  np rap due to smoother surface. However placement of rock  rip rap would require consideration of economic aspects.
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For the Arjo Dedessa Dam, dumped rock rip rap of 1000mm thickness has Deen proviaeo over a filter thickness layer of 500 mm. Thickness of rock np rap nave been computed taking in to account (i) Wave height (ii) EmDankment slope (iii) Weight of average size of rock
(iv) Rock specific gravity
A well-graded rock riprap will exert less pressure and stress on the filter
7.7.2 The rock for rip rap
The rock  np rap shall be hard, dense and durable and snail be resistant to weathering and wave pounding It should not crumble on long exposure to water, frost and air. It consists  of Doulders  of blasted rock  fragments  Filter below rock  rip rap have been provided to prevent waves action from eroding of the underlying embankment material by the suction effect.
7.7.3 The Dumped rock rip rap thickness
The rock size generally provided is given Delow in tabular form
The  dumped  rock  rip  rap  should  have  the  following  characteristics  of  dumped  stone  or  rock fragments.
i)  Quality of rock
ii) Weight or size of individual pieces
iii) Thickness of rock rip rap
iv) Shape of rock fragments or stones
v)  Effective nes ano stability of filter on which np rip in placed
Se no.
Maximum Wave Height in (m)
Minimum        Average rock size D 50 in mm
Minimum     rock     rip rap thickness (mm)
1
0-1.5
300
600
2
1.5-3.0
400
750
3
3.0 and above
700
1000
For Arjo-Dedessa Dam the minimum thickness of dumped rock rip rap provided is of 1000 mm size considering the quality of rock rip rap available in the area.
7.7.4 Placing of Rock Rip rap
The rock rip rap need not be compacted but shall be placed to grade in a manner to ensure that the larger rock fragments are uniformly distributed and the smaller rock fragments serve to fill
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the spaces  between the larger rock  fragments  in such a manner as  will result in well keyed, densely  placed, uniform layer of rip rap of specified thickness. Hand placing will be required only to the extent necessary to secure tne results above
In the placement of rock  rip rap care is  taken to prevent segregation which could result in erosion of areas  wnere small stones  are concentrated or in washing of beading material through pockets, of large stones. It is  also necessary  to provide a blanket of graded gravel underneatn the rip rap to guard against the danger of finer particles  being sucked out of voids The rip rap nas  Deen extended 1.5 m below MDDL. The rip rap accordingly  for upstream slope has been provided from crest elevation of 1360.6m to 1348 5m
7.7.5 Upstream slope of cofferdam
The upstream slope of the cofferdam is  also exposed to the wave action during the river diversion and has  been protected by  providing 1.5m thick  layer of waste material from rock quarry
The criteria of minimum rock  size implies  that the rip rap should be composed of rock, half of which size should be iarger than the recommended D50 size for a given height. The rock should be well graded from a maximum size of above 15 times  the average size varying down to 2.5 cm spells  to fill the voids  between rock  and to provide a reasonable degree of protection to the underlying filter layer. The normal size of rock  ‘D’ is  determined assuming the rock fragments to have a volume between that of sphere and a cube or
Where, r = unit weight of stones in kg/m3
and W = weight of stone in kg considering r = 2100 kg/m3
D= 0.08 w1/3
On  the  above  basis  the  thickness  and  gradation  of  dumped  rock  rip  rap  for  Arjo-Dedessa  dam proposed to be adopted are as below
I          Thickness of dumped rock rip rap =                 1000 mm
II          Maximum size of stones 40% to 50% greater than 50% to 60% for
up to 10%       less than
=         2000 kg or 1000 mm
1000 kg or 800 mm
=          45 -100 kg or 300 mm 45 kg or 300 mm.
Sand and rock dust to be less than 5% by weight of the total rock rip rap material.
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7.8 Stability Analysis
7.8.1 Factor of safety method
May 2007
Different methods have been developed for computing factor of safety SLOPE/W and SEEP/W software developed by GEOSLOPE international (Canada) for analyzing factor of safety uses different methods  and all these methods  are based on Limit Equilibrium Formulation except finite element method
The General Limit Equilibrium GLE formulation is based on two factors of safety equations and allows for a range of interslices snear-Normai force assumptions One equation gives factor of safety with respect to moment equilibrium (Fm), while the other equation gives the factor of safety  with respect to horizontal force equilibrium (Ff). The General Limit Equilibrium (GLE) method satisfies both moment ano force equilibrium by finding the cross over points of the (Fm)
and F  curves. The GLE method encompasses all other methods regardless of
f
slip circle shape.
GLE method in SLOPE/W can accommodate a wide range of different interslice forces functions & All the methods are characterized more by the equations of static satisfied and the manner is which the interslices forces handled than by the shape of the slip surface
The software analyses  large number of slip circles  based on the methodology  developed by Fellenius, Bishop, JanDu, Spenser GLE, Morganstem - Price, Corps  of engineers, lower Karafiath, Sarma and gives  the minimum factor of safety  for the critical failure surface. The method developed by  Morganstem - Price considers  both for the horizontal force equilibnum (F ) and moment equilibnum (Fm) for giving the minimum factory  of safety  His  method is 
r
considered  preferable  as  the  moment  equilibrium  on  individual  slice  is  used  to  calculate interslice shear force.
Stability analysis has been carried using SLOPE/W ana SEEP/W software As regards stability of slopes  of the dam body  although earthquake loading is  cyclic  ioading and has  dynamic loading effect, for the feasibility study report of Arjo-Dedessa Dam, only pseudo static analysis has  been carried out by  taking the horizontal seismic  coefficient value a /» = 0.05g Vertical coefficient a v is taken as half of the value of horizontal seismic coefficient, which works to be 0.025g These values of seismic coefficient have been adopted from the "Seismic Hazard Map of Ethiopia and its Northern and Eastern neighboring countries".
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7.8.2 Steady State Condition
Downstream Slope
May 2007
Stability analysis for the downstream slope of Arjo-Dedessa Dam under steady state condition has been done by considering full reservoir level (FRL) and no water level on the downstream side of the dam both for with and without earth quake loading condition.
Upstream slope
The  upstream  slope  stability  under  steady  state  condition  has  been  checked  for  both  under Normal condition and with earthquake loading condition.
7.8.3 Construction condition
On completion of construction of embankment dam the pore pressure is  partially  dissipated The residual pore pressure depends  upon the compaction methods  ano the moisture content during construction period and depends upon the rate of raising of the dam The residual pore water pressure is based on the pore water pressure ratio 'Ru'. Which is given by
U = Ru. y t. Hs
Were y t = total unit weight
Hs = the height of the soil column
The values of 'Ru* vanes for 0.1 to 0.6. In the present case tne value of 'Ru considered is 0.4 for analysis.
7.8.4 Sudden Draw Down Condition
For the stability analysis of upstream slope of the Arjo - Dedessa dam the critical condition is sudden draw down condition and the same has been considered. The factor of safety depends upon the rate of dissipation of pore water pressure and the rate at which the draw down takes place from the full reservoir level condition to minimum draw down condition by the discharging capacity  taking place over the spillway  and of the irrigation outlet. Since the imgation out let discharge is  limited, the draw down taking place below the full retention level of dam of 1356.0m to MDDL level of 1350.0 will be slow with the discharging capacity  of both the
3
irrigation outlet, being about 17.0 m /sec.
7.9   Properties of Materials
The following properties  of the clay  materials  have been adopted as  design parameter based on the test results  of clay  materials  from different borrow areas  conducted in the CDSCO laboratory. These values have been considered for stability analysis of the dam section.
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Table 7.3: Properties of the design parameter for the clay core
No
Properties
Unit
Core Material
1
V sat (Moist Unit Weight)
KN/m3
18.6
2
T sub (Submerged unit Weight)
KN/mf
8.8
3
T moist (Moist Unit Weight)
KN/m3
18.4
4
Total cohesion C
KN/m3
38
5
Total angle of internal friction
Degree
26
6
Effective cohesion C'
KN/m3
0
7
Effective Angie of friction <P’
Degree
30
Table 7.4: Rock Design Parameter
S.No
Property
Unit
Value
1
Shell Material
Kn/m3
21
2
Angle of Internal friction
Degree
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FIGURE 7.1 DAM SECTION STABILITY UPSTREAM SLOPE
May 2007
8lop«-8OO Condition W/O EQ, Elv 1320m
Arjo DtdoiM Dam
Stability Analysis
Upstioam Slope DrawOown State w/o EQ
Material# 1
Description Water
Model NoStrength
W1 9 BOZ
Piezomelnc I ine 1
Material 0 2
Description Clay Core Submerged
Model MohrCoulomb
Wt 18 4
Cohesion 5
Phi 30
Piezometric I me 1
1 462
Material • 3
Description Clay Core Moist
Model MohrCoulomb
Wt 18 4
Cohesion 5
Phi 30
Piezo met nc I Ine 1
Material# 4
Description U/S & D/S Rockfill
Model MohrCoulomb
W1 21
Coliesion 0
Phi 38
Piezomelnc Line 1
Maienal 0 5
Description Cofferdam Rockfill Model MohrCoulomb
Wt 21
Cohesion 0
Pin 38
Piezomelnc Line 1
Maleiial 0 B Descnphon Bed lock Model Bedrock Piezometnc l ine 1
55
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FIGURE 7.2 DAM SECTION STABILITY ANALYSIS OF DAM SLOPE
UZS Slope- SDH Condition W/O EQ, Fh 1330m
Arjo Dadassa Dam
Stability Analysis Upstream Slope- DrawDown State w/o EQ
• • e
U/8 Slope- SDD Condition W/O EQ, Elv 1330
Material N 1
Description Waler
Model NoStrength
Wl 9 807
Plezomeinc I. ine 1
Material • 2
Description Clay Core Submerged
Model MohrCoulomb
Wl 18 4
Cohesion 5
Phi 30
Piezometric Line 1
Material N 3
Description Clay Core Moist
Model MohrCoulomb
Wt 18 4
Cohesion 5
Phi 30
Piezometric Line 1
Material* 4
Description U/S & D/S Roc Mill
Model MohrCoulomb
Wt 21
Cohesion 0
Phi 38
Piezo metric Line 1
Malarial « 5
Deacnphon Cofferdam Rockfili
Model MohrCoulomb
Wl 21
Cohesion 0
Phi 38
Piezometnc Line 1
Matenal * 6
Description Bedrock
Model Bediock
Piozometnc Line 1
55
50
45
40
1.438
S-53 2m
MDDL 1350 0 m
60 m
Reservoir 1
80 m
GL 1320 0 m
I,
_LJ _L_L_L1 J_1 1 J_J_ L_L A          1__LL-Ll—LJ_LI I L I I. L L L 1_L_L
l. I L I I I I I I I I . I I I
-20 -10-5 0 5 101520253035404550556065707580859095 105 115 125 135 145 155 165 175 185 195 205 215 225 235 245 255 265 275 Dlstanoa (m)
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FIGURE 7.3 DAM SECTION STABILITY ANALYSIS - DOWN STREAM SLOPE
May 2007
i/S Slope Steady State W/O EQ
Arjo Dedessa Dam
Stability Analysis
Downstream Slope Steedy State w/o EQ
Material • 1
Description Clay Cota
Modal MohrCoulomb
Wt 18 4
Coh Ml on 5
Phi 30
Fiaiornetnc Lina 1
Material • 3
Detcnpton U/S A D/S RocktlO
Modal MohrCoulomb
Wt 21
Cohesion 0
Material • 5
Doecnpoon Bedrock
Phi 38
Modal Bedrock
Materials 2
Piazomotiic lino 1
□ascription U/S Clay Cora
Pieaomotnc l me 1
Material • 8
Modal MohrCoulonib
Material ■ 4
Descnption Water
Wt 18 4
DMcription Coffer Rockhll
Model Nodtrongtti
Cohesion S
Model MohrCoulomb
Wt 9 807
Phi 30
Wt 21
Pietometnc L mo 1
Piarometnc Lina 1
Cohesion 0
Phi 38
3
mO. 1 J . LJ-l-L-l 1 I I 1 I I L I I 1 I 1 1 I 1 1 L1-1 .I.Ll-l I 1 J ^1 I. 1 -1 I 1 1 I I I 1 I I I l I I I I I i 1 I / I
20      10-0 0 5101520253035404550556065707580859095
105 115 125 135 145 155 165 175 185 195 205 215 225 235 245 255 265 275
Distance (m)
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Table 7.5 Stability Analysis - Upstream Slope
May 2007
Condition of analysis
Circle      touching elevation
Minimum  FOS  as per IS code
Minimum   FOS   as   per stability analysis done
Upstream Slope
a) Suaden Draw Down b) Steady state
i) Without EQ
ii) With EQ
1320.0
1330 0
1340.6
1330.0
1.3
1.3
1.5
1.0
1.462
1.438
1.569
1.459
Upstream Slope Stability Analysis
Upstream Slope 2H :1V from crust up to 1340.6 m ana 2.5H:1V down words jp to EL: 1320.0m
FRL : EL 1356 00m
MDDL EL 1350.00m
Table 7.6 Stability Analysis - Downstream Slope Downstream Slope Stability Analysis Downstream 1.5H :1V
FRL . EL 1356.0 m
Condition of analysis
Circle     touching elevation
Minimum  FOS  as per IS code
Minimum  FOS  as  per stability analysis done
Downstream Slope a)  Steady state
Without EQ b) Steady state
Without EQ
1320 0
1325.0
1.5
1.0
1.543
1.517
•  FOS : Factor of Safety
•    EQ : Earthquake
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From the above Tables 7.5 & 7 6 it is seen that the factor of safety of upstream slope under Sudden Draw Down condition is very close to the minimum factor of safety as prescribed in IS code showing that the upstream slope adopted for the dam section is safe
For the downstream slope under steady  state condition the factor of safety, both for without Earthquake and with Earthquake situation, the factor of safety  is  very  close to the minimum factor of safety  as  provided in Indian standard code showing there Dy  that the downstream slopes adopted are safe.
The stability Analysis of the dam section  for various loading conditions have been done using Geo-studio 2004 soft ware and these are shown in Figures 7.1 to 7.4
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8 RIVER DIVERSION
May 2007
For protecting the work  ano during actual construction in the bed of a natural river it is obligatory  to isolate the river flows  away  from the work  place This  is  required so that the working area remains dry or semi dry The method and arrangement of diversion work depend pnmarily upon tne cross section of the valley, the type of dam, diversion discharge and the bed material in the river
The design flood for river during the construction period is influenced mostly by the risk ana damage factors  involved in the diversion arrangements. In case of a fill aam the risk  of overturning of emoankment during construction may result in serious damage to works partially completed. This consideration may not be so for a concrete dam wnere flood water may over top the partially constructed dam with little adverse effect if the appurtenant structure permits.
For Arjo-Dedessa Dam which is a large dam, the river diversion flood of 100 years return penod is  considered desirable as  per Indian standard code 14815.2000 and the same should be adopted for river diversion works  with suitable protection measures  taken at the end of construction season for top ano downstream of embankment aam to pass  surplus  flow considering the possibility of the flooa exceeding the design diversion flood.
However for Arjo-Dedessa dam 100 year return period routed flow has been by the hydrology group computed and for river diversion dunng construction. Table 10.1 Routed design flood computations  at cofferdam made for nver diversion dunng construction through a tunnel. As the tunnel arrangement for river diversion is not considered, twin barrel duct of 3.5 m x 3.5 n size with equivalent area of cross section m /sec has been provided with cofferdam top kept at EL 1340.60 m and with freeboard of 1.5 m provided.
3
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Table 8.1 Routed Design Flood for River Diversion
Return period 100 year peak discharge 871 m3/sec
D(m)
Q rrf/sec
Elevation (masl)
5.0
215
1338.89
5.5
232
1334 05
6.0
353
1331.19
6.5
277
1329.32
Length of pipe , L = 4.00 m
8.1   River Diversion Arrangements
The nver diversion arrangement during construction have been considered by  construction of a coffer dam on the upstream side of 20.0 m height which would ultimately  form an integral part of the main earth and rock  fill dam and a D/s  coffer dam 50 m away  from the D/s  of the dam and about 6 0m high to provide maxing area free fro water An RCC turn duct of size 3.5 m wide x  3.5 m high would be constructed on the right abutment. The riverbed level is  at EL 1320.0 m. Therefore in order to prevent silt entering the duct the invert level of the duct has been provided at EL, 1322.0m, i.e. 2.0 m above the riverbed Layout plan of the Rcc  Duct and x-sectional details are shown in drawing Vo. AD-F-DAM/30 & 31
During the lean period of 1ST year when the discharge in the river bed is  very  low, the excavation of the cut off trench upto design level in the left abutment reach for a length of 125.0 m (211.0 m to 336.0 m) and nght abutment for a length of 100.0 m (From 620.0 m to 720.0 m) involving « total length of 225.0 m is  proposed to be done. Consolidation grouting, providing concrete cap on the grout hole tops  and back  filling of trench is  to be taken up simultaneously with this  the excavation. Simultaneously  the construction of the RCC twin barrel duct, its construction and completion is  to be down. The site clearance for the cut off trench, shell zone and construction of the rock  fill dam portion upto EL 1340.6 is  to be achieved. Site clearance on the upstream of the dam axis  upto toe of the am, raising the main cofferdam which is ultimately  to form the integral part of the main dam upto EL 1340.6 along with upstream protection works is to be completed.
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in the 2nd year work  on the downstream for site clearance, core filters, horizontal filter rock  toe and downstream shell may  also be taken up after construction of the above, the routed flood discharge of 100 year return period which equal 215.0 m3/sec  shall oe diverted through the completed twin barrel box duct.
In the 3rd year of working season period remaining works, relating to consolidation grouting, grouting, cops, filter and shell reach are to De taken up and the normal raising of tne earth and rock fill dam, impervious core, filter upto maximum possible elevation feasible should be done
During 4th year the balance works  of curtain grouting and raising up of the dam upto design height, providing rock  rip rap on the upstream slope, construction of wave wall, D/S drainage and protection of slope are to be completed. Lower of duct gates  and plugging by  pumping concrete and grouting of the plug simultaneously  This  actually  is  to performed in the last three month of the working reason after the dam construction is completed upto crest level.
Spillway  structure and facilities  in saddle ADSI the work  on the spillway  structure, spillway facilities. Irrigation outlet and intake structure in saddle ASDl for Rignt Bank  canal and the Saddle dam are all independent and the construction of these does  not interfere with other components  of the work. These as  such shall be taken up simultaneously  from the very  start of the construction works of main dam.
The construction work  for the irrigation out let and its  intake structure for the left Bank  mam Canal likewise is  to be taken up independently  along with other works  from the start of the project.
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SALIENT FEATURES
May 2007
The  Dedessa  River,  flowing  northwest  is  an  important  tributary  of  River  Abbay  It  originates from  Mt.  venmo  and  Mt.  Wache  ranges  and  flows  in  an  easterly  direction  for  about  75  km Thereafter,  it  turns  rather  sharply  to  the  north  until  it  reaches  the  Abbay  River  The  Arjo- Dedessa  project  command  area  is  largely  located  downstream  of  this  sharp  turn.  The  project envisages the construction of an earth and rock fill dam, chute spillway facilities located in a saddle No.1 An irrigation outlet with an intake structure in a saddle No.2 on the right flank for the Right Bank Pomary Canal and an irrigation out let with an intake on the left abutment for Left Bank Primary Canal taking off from a left bank abutment of the dam. Other features of the left  abutment  outlet  works  are  an  upstream  pressure  conduit,  a  gate  chamber  and  a  RCC conduit discnarging in a basin from where the left bank pnmary canal takes off at a distance away from the downstream toe line of the earth and rock fill dam.
The project envisages irrigation in a command area of 13,665 hectares The project is planned for irrigation and generation of hydropower The salient feature of spillway  faculties  ano irrigation outlets are described as follows:
Type of Dam Location
Maximum height
Dam Crest width Length of crest Width of Dam (max.) Top Elevation of Dam Upstream Slope Downstream slopes
earth & Rockfill Dam with central impervious core.
Latitude : 8°-31'-12" N
Longitude . 36°-40'-O4" E
i) 40.6 m above river bed
ii) 43.6 m from the deepest foundation
10.0 m
512.0 m
180.0 m
EL. 1360.60 m
2H: IVfrom crest to EL 1340.6 m and 2.5H:1V
1.5H:1VUpto EL 1320.0 m
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Spillway (Ungated)
Location
May 2007
Right Bank Saddle No-I
Type of Spillway 
Ungated Ogee type
a; Design flood return penod
1 in 10,000 yrs
b) Peak inflow discharge
1965 m3/s
Spillway outflow routed discharge
1165.0 m3/s
Crest level
EL 1356.0 m
Width of spillway crest
125.0 m
Max. surcharge lift over crest
2.62 m
Reservoir
Catchment area Max. length
Surface area at FRL Surface area at MWL Max Water Level Normal Water Level
5278.0 sq. km
41.0 km
87.85 km2, at M.W.L. 96 01 Km2
EL 1358.60 m EL 1356.0 m
Minimum Draw Down Level(MDDL)                  EL 1350.0 m
River Bed Level
Reservoir Capacity
EL 1320.0 m
a.
At Max. W.L.
1580.3 Mm3
b.
At Normal W.L.
1341.20 Mm3
c.
At M.D.D.L.
874.7 Mm3
Out let Works
Left Bank Primary Canal
Location
Left Bank abutment
Discharging Capacity
9.015 m3/s
FSL of Canal
El. 1348.00 m
Size of the imgation out let
1.7m x 1.7m RCC Duct
Left Bank Command
6,215 ha
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Right Bank Primary Canal
i)  Location
ii) Discharge Capacity
iii)          Canal FSL
iv) Size of irrigation outlet Right Bank Command Area
May 2007
Right Bank Saddle No 2 7.52 m3/sec
EL 1348 0 m
1 5mx1.5mRCC Duct 7450 na
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9 INSTRUMENTATION
9.1 General
May 2007
A  aam  is  expected  to  withstand  tremendous  forces  during  its  operational  life  It  is  therefore imperative  that  adequate  means  are  available  in  the  dam  for  providing  information  on  continued assurance  of  its  safety  The  extent  of  internal  distress  cannot  be  directly  measured  in  a  dam. However    diagnostic    procedures    can    provide    help    in    identifying    the    problems.    Effective instrumentation   provided   in   the   dam   serves   this   objective   and   can   provide   meaDingful information  and  clues  to  the  various  problems  and  play  and  important  role  in  checking  the safety  of  structures.  The  condition  of  the  dam  ano  its  overall  safety  needs  to  be  checked regularly   to   demonstrate   that   the   dam   is   performing   safely   in   accordance   with   the   design assumptions
9.2  Purpose of Instrumentation
The monitoring instruments  selected for a given situation are required to function satisfactorily under very  harsh environmental condition and for long periods  of time For a good design it is desirable that instruments  detect systems  of distress  and where possible to relate those systems  to specific  problems  at the earliest possible stage Their prime function is  to reveal abnormalities, which may  have the potential to develop into serious  incidents  or failure. The instruments selected for a given situation should have the following characteristics
■   As simple in concept as is constant with their functions
■   Sufficiently accuracy and quality
■   Low maintenance requirements
■   Compatibility with construction techniques
■   Long term reliability and stability
■   Low cost and ruggedness
■   Simplicity and service requirements
■   Durability under adverse environmental and operating conditions
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9.3  Types of Measurement
The most significant parameters that are necessary in monitoring Oam behavior are,
•    Pore water pressure and uplift
•    movements i) Internal movement
ii) External movement
•    Seepage ana Leakage
•    Strains and stresses
•    Dynamic Loads (seismic forces)
9.3.1 Pore water pressure
May 2007
Pore water pressure is  the most important and usual measurement in embankment dams  its measurements  enables  know the seepage pattern set up in the embankment ana foundation after the reservoir impoundment. Valuable information about the behavior during construction and draw condition is obtained.
Periodical and timely  observations  will provide the means  of evaluating the behavior of the dam and enable take measures  on the basis  of observed data and comparison with the design. In addition various assumptions, which are commonly made in the design, need verifications
9.3.2 Movements
An accurate measurement of internal and external movements  resulting from the plastic deformations  of upstream and downstream slopes  under different conditions  of reservoir operations  may  provide indication of the likely  development of shear failure at weak  points  in the body of the embankment
9.3.3 Seepage and leakage
Measurement of seepage through the dam body, foundations  & abutments  of the dam may indicate erosion or blocking of downstream drains  and relief wells  by  increase or decrease of seepage respectively  at constant reservoir level. Seepage ana erosion may  take place along the lines  of poor compaction and through the cracks  in formation and fills. This  may  be indicated by  such measurement. The colour, turbidity  and changes  radically  in chemical content of the seeping water is  also an indication of seepage problem and provides  and provides useful information about the performance of the structure.
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9.3.4 Strains and stresses
May 2007
The accurate measurement of stress in earth fill & rock fill dams is difficult and distribution ot stress is complex. Furthermore stress measurement and slope of rupture planes is a matter ot considerable judgment in interpretation from the data for use in the design analysis of earth and rock fill dams Strains nowever may be calculated from displacement or measured directly
9.3.5 Dynamic load (Seismic)
Measurement of seismicity  is  important as  it causes  sudden dynamic  loading Hence instruments  for measurement and monitoring tne intensity  of ground motions  during the earthquake is required to be installed Arjo-Dedessa dam project area however lies an area of moderate to low seismicity
9.3.6 Other measurements
The other measurements, which would aid in interpretation of the main instrumentation data are necessary and are given beiow
Reservoir and Tail water level, Wave height recorders, rainfall and data about material properties, measurements of silt deposit at the upstream face of dam and other meteorological measurements, whicn are essential for interpretation of the instruments observation
9.4   Planning of Instrumentations for the Dam
9.4.1 Pore water pressure
The  porous  tube  piezometer  is  a  device  for  measuring  pore  water  pressure  primarily  in  a foundation. But is also used for measuring pore pressure in embankment. It is more sensitive to  foundation  pressures  or  ground  water  changes  and  can  withstand  silting  better  than  the conventional  observation  well.  Being  an  independent  installation  it  can  be  installed  at  any location even after completion of construction and its installation does not pose any restriction or hindrance in the construction of dam.
In Arjo-Dedessa dam it is proposed to install the pore pressure piezometer at different levels, i.e. 17 numbers at the foundation level, at EL1305.0 m and EL 1310.0 m and 24 numbers in the embankment section at EL1320.0 m EL1340.0 m & at EL1350.0 m at sections A.A. B.B and C.C All these are connected together through tubes and led to the terminal well located on the downstream side at EL 1340.60m from where these are operated upon.
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Installation of piezometer in the dam and foundation provide quantitative information indicating the magnitude and distribution of pore pressure and their variation with time The pattern of seepage, zones of potential piping and effectiveness of seepage control measures can also be indicated The uata/information obtained from such piezometer serve for taking remedial measures  purpose. The locations  and number of piezometer provided in the main dam at different section are indicated in drawings Nos AD-F-DAM/17-18 & 19.
9.4.2  Seepage
Measurement of seepage water at interface of dam and its  foundation will provide direct indication of the efficiency  of grout curtain and indicate about the necessary  remedial measures. The cnemical analysis  of water will provide the information of seepage of water through the foundation drainage arrangement and any foundation material being washed out. Corrective measures  like grouting etc. could be planned. The wet spots  on the downstream slope or at abutment locations would indicate seepage problem, and remedial measures could be suggested
Seepage measurements are made by providing notches and weirs at points where seepage is taking place and seepage discharge can be computed.
V-notches are installed in the surface drain at the toe of dam to measure seepage discharge at regular interval of time.
9.4.3 Surface movement measurement
The measurement of surface movement of the embankment dam shall be made by means of installing 22 number of surface settlement points on the dam slope, dam crest at 50.0 m center to center and shall be monitored by using a theodolite at regular intervals from bench mark established and readings each time taken shall be compared with the earlier reading to arrive at the settlement of the surface. The arrangement showing the provision of surface settlement movement points in indicated in drawing AD-F-DAM/16.
9.4.4 Parapet Settlement Point
10 Nos Parapet settlement point have been installed in the parapet wall @ 50.0 m c/c. The Parapet settlement point compressing of 18.0 mm diameter mild steel rod 0.34 m long welded to a steel plate of 300 x 300 x 6 mm. The top 450 mm of masonry wall in a length of 600 mm has  been  made  in  M-100  concrete.  The  settlement  points  embedded  in  concrete  with  MS
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projecting 40 0 mm above parapet wall with each rod crossed marked for measuring horizontal deflection.
9.4.5 Horizontal and Vertical Movement Measurements
The installation of instruments  for measuring movements  are provided at critical locations maximum vertical movement normally  occurs  at mid height of structure and maximum horizontal movement at middle of slopes. Therefore 5 Nos  horizontal movements  instruments are proposed at mid slope of structure that is at EL 1328.0, EL 1340.0 m in different sections and 6 number of vertical moment instrument at EL 1350.0 m and EL 1360 0 m. For recording vertical movement cross  arms  installation has  been proposed. Similarly  inclinometer instruments nave been proposed for registering horizontal movements
9.4.6 Earthquake Measurement
Arjo-Dedessa dam site is located in a seismic zone of medium to low intensity and there is no earthquake recording station in the vicinity of the project area. Records to show the earthquake events expenenced in the project area are not there:
The instruments for recording of seismic events proposed to be installed for the darn consists of one acceieograph at the base of the dam and one at the top of the dam. Strong motion accelerographs and structural response recorder are to installed at the base and at the top of dam. The location selected should be free from the background seismic noise erected due to vibrations  of the appurtenant works  The instrument located at the top would provide information about responses of structure resulting from earthquake.
9.4.7 Other Measurement
Wave Height Recorders: Wave height Recorders installed would be helpful in finding the wave height and in deciding the free board requirements on a more realistic way.
Rainfall: measurement of rainfall will be helpful for interpretation of pore measurement ana seepage development in earth dam
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10 SPILLWAYS FACILITIES
10.1 General
May 2007
Spillways are provided separately or in conjunction with dams to pass floodwater for reservoir regulation and for safety of dam structure The various types of spillways generally fall into one of the following types or are made of a combination of this
i) Overflow ogee spillway      ii) Gated type
iii) Shaft or glory hole
vi) Side cnannel spillway        iv) Syphon spillway           v) chute spillway
For Arjo-Dedessa dam which is an earth and rock fill dam the spillway width of 125.0m with flood lift of 2.62 m has been provided. The spillway has sufficient capacity to pass the design flood as that when the flood discharge impinges at the time when the reservoir is at FRL, the flood lift would not result in overtopping of the dam. Besides providing sufficient capacity the spillway is hydraulically and structurally adequate to dissipate the energy associated with the discharges .
It has been located such that spillway discharges will not erode or undermine the downstream toe of the dam The frequency of spillway use depends upon the sum of characterization of the drainage area. Spillway flows will result during floods or periods of sustained high run off when the capacities of other facilities are exceeded.
10.2 Location
The spillway has been located on the nght flank in the natural saddle No.1 upstream of Arjo- Dedessa dam axis. The location considered for spillway is such that it does not interfere with the functioning and performance of other structure of the project. The topograpny, geological set up and layout of other structures were the main consideration in deciding the location of the spillway  in the natural saddle No:1 on the right flank  of the dam. The discharge carrier is provided in the excavated rock and with a chute ending in an energy dissipation arrangement. The other alternative site for location of spillway examined was in a saddle on the left flank of the dam. This was not considered suitable as it involved large excavations due to its higher elevation. Considering the rock type and geo-technical investigation report, stilling basin type of energy dissipation arrangements have been considered.
10.3  Spillway Layout
The maximum spillway discharge has been computed by flood routing studies by the hydrology group. The spillway width and the flood lifts have been worked out for various combinations of
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discharges  taking into consideration the topography  and the foundation condition of the site which have influenced the location type ana other components of spillway The spillway width of 125.0 m width and flood lift of 2.62m for to the routed spillway outflow discharge of 1165 0 m3/sec  for a peak  inflow of 1950 m3/sec  for a return period of 1 in 10000 have Deen considered The different combination of spillway  width and flood lift considered are given in Table 8.10.
Table 10.1: Routed Design flood Based on LLG Distribution Peak out flow 1950 m /sec
3
s.
No
Spillway
width in
Routed out
flow Q out
(m]/s)
Flood
H(m)
Max surface
2
Area (Km )
Surcharge
storage
3
(Mm )
1
50
800
3.75
99.11
1693.0
2
75
951
3.21
97.49
1637.1
3
100
1069
2.87
96.45
1602.0
4
125
1165
2.62
95.71
1577.0
5
150
1246
2.42
95.12
1557 7
6
175
1314
2.27
94 65
1542.0
The saddle No-1, upstream of the dam axis on the right flank have provided the location at advantage for the spillway in meeting the requirement of being independent in the operation ana performance of other components  of the project The spillway  at this  location is  not interfering with the location and performance of imgation outlet from where right Dank primary canal is taking off in the near by saddle No-2.
The downstream discharge channel from the spillway  has  Deen aligned along the natural drainage and will outfall into river Wama. The natural drainage after a distance of about 1.0 km from the stilling basin has  been channelized. The general layout plan and section etc. of spillway is shown in Drawing No AD-F-DAM/20.21& 22.
10.4  Design inflow Flood Discharge
Spillway has been designed to have discharging capacity sufficient enough to pass the inflow floods corresponding to probable maximum flood (PMF), as from both the height and storage capacity aspects the proposed dam is classified as a large dam. Indian standards specify that if the failure of dam poses danger to human life, the spillway must have sufficient capacity to accommodate the routed flood discharge corresponding to probable maximum Flood (PMF) and if the failure of dam would result only in heavy damage to property but does not pose
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3
appreciable risk to life then the spillway may be designed for flood discharge corresponding to 1 in 10,000 year return period. The dam location is  in an area where failure may  probably remain restricted to the irrigation farms  and the dam area it self Loss  to human life would probably  be minimal. For such a situation and for the sake of economy, a routed flood discharge corresponding to 1 in 10,000 year return period or half of probable maximum flood (PMF) which ever is greater has been considered as there is not much habitation down stream of the dam site and the aam is located in a remote area. The Probable Maximum Flood (PMF) as computed and provided by the hydrology group is 3690 m /sec and the 1 in 10,000 year
3
return period flood value works out to 1950 m /s based on LLG distribution. Half PMF value works out to 1780 0 m3/sec since naif PMF is less than that, 1 in 10,000 year return penoa flood, peak inflow of 1 in 10,000 year return period has been considered Flood routing studies carried out with different combination of spillway widths, Flood lifts of 2.62 m and spillway width of 125.0 m is considered for hydraulic design of spillway structure.
10.5  Gated v/s Ungated Spillway
A gated spillway normally provides the most economical means of passing the design flood discharge whereas an ungated spillway is inherently safer and is the simplest form of spillway which automatically releases water wherever the reservoir water surface rises above the crest.
The ungated spillway  has  the advantage of having trouble free operation, as  there are no operating mechanisms  or malfunctioning of control gates. The main advantage lies  in the elimination of the requirement for constant regulation and attendance of the control device by an operator and the freedom from maintenance and repair. But on the other hand it has to be located at full reservoir level. This makes the flood lift in passing the flood discharge over the spillway crest high. The maximum water levels would generally become much higher than the maximum water level (MWL) in case of the gated spillway for the same FRL. This results in dam crest to be kept high above the full reservoir level.
For Arjo - Dedessa Dam the ungated spillway is considered suitable for the situation due to the remoteness of the structure. The design flood is not large and the flood lift is not very high.
The design and features of spillway structure have been worked out accordingly conforming to Indian standards. The spillway crest level of 1356.0 m has been considered This with
minimum free board above maximum water level of 1358.60 and normal free board required at FRL 1356.0 have been worked out. In both situations the top of dam works to be at 1360 60 m The spillway capacity for the chosen spillway width and flood lift is adequate enough and dam
is not over topped
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The features  of the spillway, chute and energy  dissipation arrangements  shall be tested on hydraulic  model studies  for their efficacy  and performance in regards  to their geometry distribution of flow over the ogee spillway, for air vents if any pressure distribution on inclinea chute and performance of chute, blocks, /dentated sills. The layout of the spillway  showing approach channel, control structure, chute and stilling basin and tne tail race channel are shown is Drawing No AD-F-DAM/21 & 22
10.6 Energy Dissipation Arrangement
The energy dissipation below hydraulic structure is very vital in the design and construction of dams, weirs and barrages. The energy dissipators are provided to reduce high velocity flow to a velocity  low enough to minimize erosion of the natural river bea. The reduction in velocity shall be accomplished by (a) Hydraulic jump type stilling basin or (b) Bucket type dissipators
These  arrangements  are  most  commonly  used  for  dissipation  of  energy  depending  upon  the rock and the geo-technical conditions of the downstream tail water
The type of energy dissipation arrangement considered based on the topographical and geo technical investigations  has  been proposed to be the stilling basin type. The designs  of the energy  dissipation arrangements  nave been carried out as  per Indian standards  code The hydraulic  design of stilling basin type of energy  dissipation arrangement has been done The length of stilling basin is computed and works out to 25.0 m. The depth of water in the stilling basin is computed and is equal to 6.0 m. The other parameters and feature like chute block, end sill and dentated sill have also been worked out and provided accordingly
10.7 Approach Channel
Approach channel is required to draw water from the reservoir for conveying it to the control structure. The layout of the approach channel is  selected in such a way  that the increasea velocity should be limited and the channel curvature and trasition are gradual so that the head loss through the channel is minimal and uniformity of flow over the crest of spillway is obtained. Head loss  in the approach channel has  an effect in reducing the spillway  discharge Furthermore high velocity  may  also necessitate lining of the channel depending upon the foundation condition. Uniform and even distribution of flow has  therefore been ensured over spillway crest. The arrangement of layout provides for guide walls kept at nght angles to the axis of the spillway structure in 15.0 m length on the upstream of the spillway axis and therefore are provided with curvature of radius 15.00 m. The total length of approach channel has been
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kept 15.00 m and a toe wall has been provided at the end of approach slab Stone protection of 10.0 m length over the whole width of the approach channel has been provided
The depth below crest of approach cnannel and approacn velocity have an important influence on the discharge over an over flow crest. A greater approach depth with the reduction in approach velocity results in large coefficient of discnarge value cd’ thus, for the given head over the crest a deeper approach will permit shorter crest length for a given discharge In the case of Arjo-Dedessa Dam spillway location is in a saddle and considering the topograpny and the natural ground level, the bed of the approach channel has been kept at EL 1349 0 m for a straight length of 15.0 m length and the crest of the spillway is kept at EL: 1356.0m
10.8 Spillway Structure
The overflow spillway structure adopted for Arjo-Dedessa Dam is a control weir which of an Ogee shap in profile It is located in saddle No:1 on the right flank of the dam ano 500.0 m upstream of dam axis  The geographic  location of the spillway  structure are defined by coordinates at its central point.
The Ogee profile to be acceptable, should provide maximum possible hydraulic efficiency, structural stability and economy and also to avoid formation of sub atmosphenc pressures at the surface.
The overflow spillway structure has been provided a length of 125.0 m with 5 bays of 25.0 m c/c each separated by 2.0 m thick pier 4 in numDer. The piers support the steei truss structure walkway of 1.50 m width. The crest level of Ogee weir is kept at EL: 1356.0 m. The foundation of the spillway structure has been kept on excavated firm rock and is founded at EL 1338.0 as revealed by drilling borehole at SP-1 location which is at the center of spillway structure.
10.9 Ogee Weir Profile
The upper curve of the ogee generally is made to conform closely to the profile of the lower nappe of the ventilated sheet falling from a sharp crested weir. Flow over the crest is made to adhere to the face of the profile by preventing access of air to the underside of the sheet For discharges at designed head, the flow glides over the crest with no interference from the boundary surface and attains near maximum discharge efficiency The profile below the upper curve of the Ogee is continued tangent along a slop to support the sheet on the face of the over flow. A reverse curve at the bottom of the slope turns the flow on to the chute, then to the stilling basin and thereafter to the spillway discharge channel.
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The ungated spillway crest is kept at FRL EL 1356.0 m. The overflow section adopted is Ogee shaped weir to be built in reinforced concrete 125 0 m wide with vertical upstream face The width of the spillway nas been arriveo at by various combination of flood lifts and width and using the basic weir equation for passing the design flood. Abutment at each end of the crest nas been taken to heignt above the water profile and providing free board The Ogee profile has been designed for upstream and downstream crest profile as per the IS 6934-1973
The upstream crest profile is kept tangent to the vertical face and should have zero slopes at the crest axis to ensure that there is no discontinuity along the surface of flow
Upstream profile
The upstream profile adopted conforms to the equation
y = 0.724(Z + 0.270Hdy “ + Q j0.43\5Hdar,\X + 0.274/7)°423
Where, X,Y= Cartesian coordinates
Hd = Design head
The values of Y for various values of X are obtained from the Tables in Fig 3 of the Indian standard code
Downstream profile
The down stream profile shall conform to the equation
I «3 
OI3
x = 2.0 Hd.Y
Where, Hd = design head
and , X & Y are coordinate
The profile has been designed for one value of the design head (Hd) only and has been be chosen to give the maximum practicable hydraulic  efficiency  in keeping with the operational requirement, stability  and economy  However, generally  the spillways  are operated under conditions other than the design head and at times sub atmospheric pressures developed lead to cavitations. The lower the design head of the crest profile the greater will be the discharge coefficient for the full range of heads. For design head exceeding by  25% no harmful cavitations is seen to develop. Design head is usually kept at 75% - 90% of maximum head Economy in the design of an Ogee crest may sometimes be affected by using a design head which is  less  than the maximum expected head. Use of smaller design head results  in
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increased discharges from the full range of heads. The increase in capacity makes it possible to achieve economy by reducing either the crest length or the maximum discharge head
10.10 Spillway Chute
The spillway  cnute of 125.0 m width to pass  the design flood discharge has  oeen provided below the spillway glacis It will envisage excavation in rock to reach the designed level The rock is reported to be available at EL 1338.0 m as revealed in the geological & geo-technical report. The original ground level at the saddle area is around 1350.0. The material consisting of overburden and fractured/weathered and loose rock is to be removed to lay the foundation for spillway studies, chute and stilling basin on the firm rock. The stilling basin foundation has been laid on firm rock  and thickness  of 1500 mm of concrete floor provided The same is extended upstream to form foundation for the spillway and chute the concrete floor is anchored to the rock foundation below by 25 mm p grout anchors at 1 5 m c/c and 3.0 m deep. Over the concrete floor cyclopean concrete with 60 % stone *40% concrete of C-30 grade is placed and well compacted conforming to the shape of spillway, 1500 mm thickness of reinforced cement concrete (M-50) grade is placed over the Ogee spillway and chute all around with 25 mm <p 1.5 m c/c and 3.0 m deep grout anchors as shown in the drawings of spillway structure. Under drainage arrangement has  been provided to prevent the foundation material from being leached into the pipe below. For reducing the uplift pressure, Grout curtain 25mm of 3.0 m depth @ 1.5 m c/c have been provided in the entire spillway length upstream of the Ogee Weir to prevent any seepage from the reservoir side taking place and to prevent building up of uplift pressure under the spillway  chute and stilling basin foundation. The general layout of the spillway facilities is shown in Drawing No. AD-F-DAM/20
10.11 Structural Requirements
The retaining walls  for sides  of entrance channel, discharge channel and stilling basin have been designed to with stand all possible combination of various loading like back fill earth, of rock pressure, water pressure, live load recharge, uplift pressure and forces due to horizontal and vertical seismic acceleration. The structural steel details have not been indicated in the drawings and is to be provided at detailed design stage.
10.12 Floor lining
During the spillway  flows  the floor lining is  subjected to hydrostatic  forces, boundary  drag forces, uplift and dynamic forces due to flow impingement. When there is no spill the lining is 
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subjected to the action of expansion and contraction due to temperature variations weathering and chemical deterioration, effects of settlement ano buckling or to uplift pressure due to under seepage or high ground water condition. It is recognized in the IS code that it is not possible to evaluate these various forces that might occur and also not to make the lining heavy, the thickness of lining is made on more or less empirical basis and reliance is made on providing under drains, anchors, cut offs etc. to stabilize lining.
The IS cooe provides thickness of lining in the approacn channel of 15 cm to 30 cm thick concrete under normal conditions. In the inclined chute the concrete lining thickness as been kept as 1.5 m. In the stilling basin the floor lining thickness is kept 1.50 m
In the inclined chute, the d/s profile of Ogee weir, very high velocities of flow take place causing abrasion due to suspended silt load. Once pitting starts, repairs  become difficult. It is proposed that well designed mix of hign performance concrete using silica fumes is used. Like wise, in stilling basin also, where hydraulic jump forms, such concrete should be used
10.13Water stops and joints
Water stops ano joints are provided as per the IS code which specifies that lining should be laid in panels so as to make the entrance cnannei lining a reasonably water tight upstream to reduce uplift pressure on the control structure see joints in the lining have been provided with water stops. The water stops are also to be provided in longitudinal joints, transverse joints and in the joints of side walls where seepage behind the walls is undesirable.
Water stops have been provided in the joints of stilling basin floor lining so as to avoid circulating flow under the lining due to differential head on the joint in the zone of hydraulic jump
The transverse joints have been provided with 13 mm off set as shown in the drawing so as to avoid high velocity flow striking against the edge of the lower floor, cause water to seepage through the joints under a very heavy pressure and design or uplift the lining. A cut off has been provided to prevent lifting of upstream edge of lower slab due to differential settlement.
10.14   Drainage System
The stability of floor lining is increased by providing under drams as these reduce the uplift pressure on lining. The drains under the lining consist of a network of pipe drains, which follow the joints in the lining. The drains provided are non-perforated cement sewer pipes laid with open joints in gravel and embedded on a mortars of concrete pad to prevent the foundation
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material from being leached into the pipe The drains under the inclined chute have their outlet open in discharge channel itself through the sidewalls The outlets are connected to the pipe drain The drains under the lining below maximum tail water level and stilling oasin have their outlets into the chute blocks of the basin. The drainage arrangement of stilling basin have been kept separated from that of the other floor lining. Pipe drains have also been provided at the toe of the heel of the sidewalls to collect seepage water from the backfill and relieve the wall from water pressure These pipes have their outlets provided in the discharge channel through sidewalls
Anchors bars have been provided under the foundation of chute spillway and stilling basin to increase the effective weight of the slab against displacement due to uplift ano other forces The grout ancnor of 25mm (p@ 1.5m pc/c (staggard) 3.0 m long The diameter of the hole into which anchor bars have been placed and groutea should not be less than 1/2 time the
maximum transverse dimension of the bar The spacing, diameter of holes depth of the anchor bars and type of anchor to be provided are to be decided by the actual pull out tests.
10.15 Hydraulic Model Studies of spillway structure
Hydraulic model studies of the spillway structure are required to be carried out to assess the performance and efficiency of the structure for various intensity of discharges in the spillway The model studies  carried out are helpful in carrying out modifications  in the layout ano geometry of the components of the structure. The performance and efficiency is to be studied in relation to:
i)   The flow pattern in the approach channel ana geometry of grid walls.
ii)  Flow distribution over the Ogee weir
iii) Distribution of pressure over inclined chute and need for aeration vents
to be provided if required
iv) Energy dissipation arrangement performance regarding hydraulic jump
formation and modification in designs if any called for
The hydraulic model studies in respect of above aspects need to be carried out at the time of carrying out detailed designs and before taking up the actual construction of the above spillway facilities.
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10.16 Zoning of Material for the spillway facilities
May 2007
The grades of concrete proposed to be used in different locations of the spillway facilities have been indicated below:
Sr
No
Location
Compressive striaght of
concrete (N/mm2) 28 day 
strength
Maximum size of
Aggregate (mm)
1
Spillway  Ogee crest spillway glacis, chute, eno sill & chute blocks extenor 2.0 m thickness
25
40
2
Stilling   basin   exterior   1.5   m thickness
60
(Using silica fumes)
20
3
Spillway  piers, RCC retaining walls  stilling basin except
1.5 m extenor thickness
20
40
4
The   foundation   for   filling   up crevices
15
20
5
Gravity   section   for   retaining walls
20
40
6
Approach channel floor
15
20
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Hydraulic Design Computation
for
Spillway facilities
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11 HYDRAULIC DESIGN
11.1 Hydraulic Design of Spillway Facilities
May 2007
3
The spillway  is  designed to pass  a discharge of 1 in 10,000 year return period or 14 PMF whicn ever is  greater The discharge of the 1 in 10,000 year return period is  computed as  1165.0 m3/sec  and gives  a higher value Therefore this  value of discharge has  been considered in the hydraulic design of spillway
The  routed  out  flow  discharge  as  computed  =  1165  (m3/sec)  For  a  spillway  widtn  of  125  0  m. and the surcharge height for this computed = 2.62 m.
Where, Q = cd L H372
L = 125.00 m
Le = effective width of spillway and is obtained by considering abutment and pier contraction effect.
Q = 1165 m3/sec.
The surcharge height for routed out flow discharge of 1165 m /sec and width of 125.0m = 2.62 m
Le = L - 2 Ha (Nkp + Ka) - width of pier = 125 - 2 x 2.62 (4 x 0.01 ♦ 0.1) - 2 x 4 = 125-5.24x0.14-8.0 m
= 116.27 m
Hence Q = cd x 116.35 x He372
He = (1165/2.2 x 116.27)273 =2.74 m Velocity = Q/A = 11651 8.74 x 116 27
= 1.146 m
Le = 125-2x2.74(4x0.01+0.1)-2x4
= 125-5.48 (0.14)-8
= 125-0.77-8
= 116.23 m
Va Revised = 1165/8.74 x 116.23 = 1.147 m/sec
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a
= 2.62 m
The  computation  for  the  Ogee  spillway  profile  for  both  upstream  and  downstream  have  been done and are contained in Table 11.1.
For  Arjo-Dedessa  Dam  spillway  the  good  rock  is  available  at  1338.0m  as  per  the  spillway  drill hole data at spillway location SP-1
Therefore floor of stilling basin is proposed to be kept at EL 1338 0 m The chute is proposed with slope of 2H:1 V .
Velocity at crest of Ogee weir
V = Q/A = 1165/116.23x2.54 = 3.95 m/sec
1.1472
2x9.81
= 0.067
Hd= He- Ha
= 2.74 - 0.067
= 2.673 m
For   design   of   Ogee   snape,   a   lower   design   head   is   used   as   it   increases   the   discharge coefficient for higher flows than the design flow
Adopting 98.0 % of head
Crest of Ogee = 1356 0 m
Depth of water over Ogee H  = 2.673 x 0.98
81 =0.795
Calculation for d2'
By using Bernoulli's theorem and neglecting all losses
TEL U/S = TEL + YY
U/S TEL = AZ + di + ha
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= (1356-1338)+ 2 54+ 0 795 = 18 + 3.335
= 21.335
D/S TEL = d  + ha
May 2007
22
2         2 2/2g
ha  = V
V = Q/A = 1165/125xd
2
2
/. V  = 9.32/d
2
2
2
/. ha = V /2g
22
21.335 = d  + 4.424/ d
2
2
21.335 d? =«// +4.42
2
or d  +21.335 d  +4.42 = 0
2
Solving the equation for d 2, d 2 = 0.46 m
v, =1165/125 x 0.46 = 20.26 m/sec
Froude No = J— 
>[gd
20.26 V9.81x0.46
20.26 " 2.124
= 9.54
2g
9.322
d 2x 2x9.81
= 4.42/d22
Substituting values in the equation above D/s TEL = d  + ha
2
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Using Horizontal type of Hydraulic jump
d 2=0.5 d,[^i + 8/2 - lj
= 0.5 x 0.46       + 8 x (9.94)2 - lj
= 0.5 x 0.46 x 26.00
= 5.98 m
Since Froude number is more than 4 5 Basin Type II shall be used From the graph between Froude's number and length of Basin for Froude number 9 54
Length of Basin =4.3x5.98 =25 7 say , 26.0 m
May 2007
Minimum  Tail  water  depth  required  from  graph  between  Froude  Number  and  Tail  water  depth (Design of small dam by USBR)
Tail waterdepth
0.46          “
Min Tail water depth = 13.4 x 0.46 To be achieved = 6.16 m
say 6.2 m
Basin Appurtenances (From the IS standard)
• Chute block, Height, Width and spacing
D, = 0.46m say, 0.50 m
Spacing from side wall
d. 0.50 2"2
= 0.25 m
= 0.25 m
Chute Blocks shall be provided at junction of horizontal floor and slope of chute
Basin Blocks shall be provided at a distance of 0.8 D2 form the junction of horizontal floor and slope of chute = 0.8 x 5.98
= 4.78 m. say, 5.0 m
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• Basin Block
Height of Basin Block shall be taken from Graph of Froude Number versus -t- IS code 4997- 1968
For Froude Number: 9.54
A. = 2.35
0,
hb = 2.35 x 0.50
= 1.17 say 1.20 max
Provide hD = 1.20 m
Top width = 0.02 hb
= 0.02 x 1.2 m
= 0.24 m
Provide 0 25 rr
Slope D/S shall be 1.1 and U/S shall be vertical Spacing and length of blocks W = 0.75 hb
= 0.75x1.20 = 0 90
Provide 1.0 m Spacing from sidewall = 0.375 hb
= 0.375x 1.20 = 0.45
Provide 0.5 m
May 2007
• End Detail Sill Height (h )= 0.2 D
2
t
= 0.2 x 5.98
= 1.196 m
Provide 1.25 m
Top width = 0.02 D2
= 0.02 x 5.98
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= 0119m
Provide 0 125 m
Spacing and length at bottom = 0 15 Dz
= 015x598 = 0 79 m
Provide 0.90 m D/S Slope 3:1, U/S Slope vertical
May 2007
End  sill  shall  start  right  from  side  wall  ana  no  gap  from  side  wall  shall  be  provided  The Appurtenances for stilling Basin II are snown in Fig. 11.1
• Tail Channel Design
It is assumed that in the tail channel the velocity shall be kept at 1.5 m/sec The width of the channel is kept the same as kept for the spillway chute
The width provided snail be 125.0 m Assume depth of water of 6 0 m in tail channel The discharge in tne tail channel is given by
Q = AxV
That  is  area  of  cross  section  of  the  tail  cnannei  required  is  776.7  sq  m  and  area  provided  in the tail channel is:
Area of 6.0 m depth of channel with side slopes 1.1
A = (b + nd) d
= (125 + 1.5x6) 6.0
= 784.0 ml sec
The area provided is more than required area
The bed Slope is calculated by using Manning s formula as given below
V=-A2/,S,/1 where
n
n = is the rugosity coefficient and taken as 0.025 for rough surface
Hl
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Wetted Penmeter is computed as R = —
>4                 784.0
P 125 + 2x1.41x6.0
784.0
” 125 + 16.92
= 5.52 m
/t 2ZJ = (5.52)1'3
= 3.123
v x n
1.5x0.025
3.123
0012
Bed slope S = 0.000144 i.e 1 in 6944 Say 1 in 7000
May 2007
At the end of 500 m length after the basin, bed drop will be
= -^- = 0.070 m
7000
Bed  Elevation  =  1338-0.07  m
= 1337.93 m
• Free Board In the Channel
Free board in the channel conducted flow at supercntical stage, surface roughness, wave action air bulking and spray are related to the velocity and energy content and is given by the empirical relation as below.
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Free Board (in feet) = 2.0 + 0 025 v \[d
= 2.0 +0.025 x 1.5 X
= 2.0 + 0025x 1.5x1 81
= 2.06 ft: or
=08m
say 1.0 m
Top or Bank elevation of channel
= Water elevation + Free Board
= 1344 0 + 1.0
= 1345.0 m
• Elevation at different places
Bed eievation of Tail water channel = 1344 00 - 5 98
= 1338 02 m say 1338.0 m
Water elevation in the channel = 1338.0 + TWL = 1338.0 + 6.0 =1344 00
Water level in Tail water cnannel at 500 m D/s
DIS =1344.0 - 0.07
= 1344 0 - 0 07
= 1443.93
May 2007
Bed level at 500 m D/S in tail water channel = 1343.93-6 0
= 1337.93 m
• Free Board in stilling Basin; (from USBR):
Free board in stilling basin is  provided so that stilling basin walls  are not overtopped by  the surges, splashes  and sprays  and wave action set up by  the turbulence of the jump The surface roughness  of flow is  related to the energy  dissipated in the jump and to the depth of flow in the basin.
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Free Board is calculated by the following empirical expression, which has performed
satisfactory
Freeboard (in ft) = 0 1 (v, + d,) (from USBR)
= 0.1 (20.26 + 6) x 3.28
= 8.6 ft
2.6 m
Top elevation of stilling basin walls
= Water elevation in basin * free board
= 1344 00* 2.60 m
= 1346 60 m
• Layout plan of spillway is snail in Fig 11.2.
11.2 Irrigation Outlets
Two Irrigation outlets, one near the left abutment for the Left Bank Primary Canal and the other
on the rignt flank saddle - 2 along with the intake structure for the Right Bank Primary canai are provided for withdrawal of water supplies from the reservoir in a regulated manner for meeting the Irrigation demand in the respective command areas. The control gate structure for each is provided on the upstream for regulating the releases as per the requirement of irrigation canal The gates are operated by a hydraulic hoist. As emergency gate upstream of the control gate has been provided for emergency operation if required
11.3 Irrigation Outlet for Right Bank Primary Canal
The irrigation outlet for the Right Bank Primary Canal has been located in the right flank saddle No-2. The onginal ground level at this location is at EL 1358.0 m. The drill hole ADS-2 has indicated the over burden of about 20.0 m at this  section. The Irrigation outlet has  been designed as  a cut and cover conduit and placed on hard excavated rock  stratum to carry discharge to meet the irrigation demands  for command of 7450 hectare as  per the data provided by the irrigation group The invert level of the outlet has been kept sucn that there is always  a driving head of 0.6 m available during the lean period to meet the maximum peak demand of irrigation. The MDDL of the reservoir is at EL 1350.0 m and accordingly the invert level of the right bank irrigation out let has been fixed at EL 1348.0 m.
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3
An RCC duct of 1 5 m x 1.5 m square in section has Deen provided for carrying the discharge value of 7.52 m /sec The friction losses in the conveyance system have Deen computed using Manning's  equation. Staunching rings  to increase the seepage path have Deen provided at each of the construction joint at 5m c/c interval in the impervious core downstream of contoi gate structure The RCC conduit section after the construction, is back filled properly to raise it to the top elevation of EL 1360 6 m. The arrangement of the irrigation out let have Deen shown in the Drawing No. AD-F-DAM/
11.4 Irrigation Outlet for Left Bank Primary Canal
The irrigation outlet for the Left Bank Primary Canal have been located near the left abutment The alignment has been fixed much that the Rcc conduct of provided for irrigation outlet is in cut and cover section and is aligned away for the downstream toe of the dam. The Rcc duct section of 1 7 m x 17m square shape has been designed to carry a discnarge of 9 015 m3/sec for meeting. The water demand for imgation of the left bank primary canal. The MDDL of the reservoir is at EL 1350.0. The inlet level of the irrigate outlet has Deen fixed such that the duct is always carrying discharge to in the peak imgation demand in the lean season. For this the inlet has been kept at a level so that there is always a driving head of 0 6 m
The layout of the imgation outlet and intake structure with a control gate is shown in the layout drawing No. ADIP/DAM/27 & 28.
11.5 Hydraulic Design of Irrigation Outlet
Right Bank Primary Canal
Location: Right Flank Saddle No: 2
Design Discharge proposed through this irrigation outlet as provided by irrigation group = 7.521 m /sec for designing the irrigation outlet and providing a minimum head of 0.6m during the leanest period and ensuring the imgation outlet discharges to meet the peak demand The velocity with the minimum driving head of 0.6 m is computed by the relation
3
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Velocity = y/2 g h
= -72x9.81x0.6
= 3 43 m/Sec
Area of x-section the of irrigation outlet required A' = —
7.521
“ 3.43
= 2.19 sq m
May 2007
v
Adopting a square duct of RCC of size 2.0 m x 1 5m for meeting the requirement of water 7.521
Area of cross section required for discharge of 7.521 m3/sec is =
x-area of x-section of the duct provided = 2.0 x 1 5 sq. m = 3.0 sq m
7 521 Velocity generated with this area =            —-—
=2.19 sq m Area of
of cross section provided Friction   slope   V=   -R1,3S1'2
n
A = 3 sq m P = 7m
=    2.507 m/sec
Rm=0.57
V = —— x (0.57) S*'2
0.016
s u! _ V 2-5
35.5     35.5
• 11
~ 35.5
q = 0.071
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S = (0.71)2
= 0.005
or slope = 1m
Length of Duct =100 + 105
= 205m
205
Head loss due to fraction = —-
200
= 1 03 m
The gate and groove losses are assumed to be negligible
May 2007
c
Entry loss =--------
i          03 y2
2g
= 0.3x2.52
= 2x9 81
= 0.10 m
Loss of head through the trash rack is assumed negligible
. Velocity head at exit = —
V2
2g
0.2x2.5072 2x9.81
= 0.06 m
Total head loss = 1 03 + 0.10 + 0 06
= 1 19m
say 1.20 m
Full supply levels will be kept below the level = 1350 -1.2 - 0.6 m
= 1348.2 m
But Actual FSL provided is 1348.0 m and is ok.
The top level of the RCC duct will be kept at EL = 1348.00 - 0.15 (considering 0.15 m depth below canal FSL for duct to run full)
= 1347.85 m
Bottom level taking into account the thickness from EL. 1347.85 i.e 1347.85 -(0.5 + 0.5 + 1.5)
= 1347.85-2.5 = 1345.35 m
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Depth of water in the canal = 1 4 m (assumed) Canal bea level = 134800- 1 4m
= 1346 40 m
May 2007
The general arrangement of layout plan and sectional elevation and other details  of the irrigation outlet for the Right Bank Primary Canal is shown in the Drawing No AD-F-DAM/25 & 26
11.6 Hydraulic Design of Irrigation Outlet for
Left Bank Primary Canal
Location: At the left Abutment near the crest of the aam
The discharge for the irrigation outlet for left Bank Pnmary Canal as has been provided by the irrigation group has been considered for the design of irrigation outlet.
Discharge = 9.02 m /sec
MDDL of reservoir EL. 1350.00 m
Minimum Driving head providea = 0.6 m
A cut and cover reinforced concrete duct is provided for irrigation out let from the reservoir Velocity V = y/lgh
= 72x9.81x0.6
= 3.43 m/sec
Area of cross section of the duct required for carrying the above discharge of 9.02 m3/sec
3
902                 2,o
=------- m/Sec
3.43
= 2.63 sq. m
Provide RCC duct of size 1 7 m x 1,7m
X-section area of the Rcc duct = 1.7 x 1.7 sq m
= 2.89 sq. m
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Actual Velocity of flow generated in the duct = —
9.02
” 2.89
= 3 12 m/sec Provide sill level of the duct at = 1350 - 0 6 - 1.7
= EL 1347.7 m Layout and alignment of the RCC Square Duct
May 2007
Since the ground level is nsing on the downstream side of the dam aoutment, it is proposed to take the imgation outlet duct in excavation in rock. The concrete duct will be in straight alignment for the initial 18.0 m lengtn from the center of control gate snaft and there after the duct will be aligned at an angle of 145° with the center line of the outlet and taken straight for 110.0m length. The duct will then open out in the canal at a ground elevation of about 1353.0 This point will be at distance of about 110 0 meter from the cam axis ano about 50 0 m away from the center line of the cam axis. A general layout plan of the imgation outlet duct is shown in the Drawing No. AD-F-DAM/27 & 28
A control gate operated by a hoist is provided at the upstream of the gate shaft. An emergency gate on the upstream of the control gate is provided for operation in an emergency situation The diameter of the control gate shaft housing for the control and emergency gate is proposed to be of 6.0m internal diameter and centre of the well shall be at a distance of 15 0 m from the u/s edge of the dam.
v = 3.12 =—J?2/1S,/2
n
Where:
n = rugosity coefficient and is taken = 0 016 P = Wetted perimeter of duct
A = Area of the duct S = Slope of the duct
p
A  1.7x1.7
“ P ” 4x1.7
= 0.425
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R™ =0.565
.•.3.12 = —!—   o.565 xS*' 0.016 x
nl/2 3.12x0.016
May 2007
2
or S '
12
=-------------------- 0.565
= 0.089
S = 0 0078
Total length of the duct =20 + 110
= 130 m
Loss of head due to friction = 0.0078 x 130
= 1.01 m
Adding 10 % for Dend and turn loss of head
Total loss « 1.10 m
.. The sill level on the downstream will be
=1347.7-1.10
= 1346.6 m
Full supply of canal assuming 1 4 m depth of water in the canal =1346.6 + 1.4
= 1348.00 m which is in order.
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J
I 1
] 1
1 1 1 1 1 1
1
1
Arjo-Dedessa Irrigation Project
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Table 11.1 Arjo Dedessa Irrigation Project
May 2007
Computation of Ogee Spillway Profile
**185 = 2 Hd*O85"Y For downstream profile
Hd
Upstrean
2.62 m
Downstream
Xi
Yi
Xi
Yi
-0707
-0 702
-0 694
-0 681
-0 668
-0.655
-0.642
-0.629
-0.603
-0.576
-0.550
-0.524
-0.498
-0.472
-0.445
-0.419
-0.367
-0.314
-0.262
-0.210
-0.157
-0.105
-0.052
0.000
•0.330
-0.305
-0.288
-0.266
-0248
-0.233
-0.219
-0.206
-0184
-0 163
-0.146
-0 129
-0.114
-0 101
-0.089
-0.078
-0.058
-0.041
-0.028
-0.018
-0.010
-0.004
-0.001
0.000
0000
0.250
0.500
0.750
1 000
1 250
1 500
1 750
2.000
2.250
2.500
2750
3.000
3.250
3734
0000
-0.017
-0 061
-0 130
-0.221
-0.333
-0 467
-0.621
-0 795
-0.988
-1.201
-1 433
-1.683
-1 952
-2.523
Tangent Point Coordinates
The slope of the downstream glacis = 0.8 H : 1V X                  3.734 m Corresponding Y =
m
-2.523
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i
i
FROUOE NUM0E R iF0
RECOMMENDED LENGTH FOR- BASIN II
j-BaS/n BlOCk
-4-375 r>b
CHUTE
/’'eV
—z-o-03^ w.o.15o,r
\          l>0-75*»b
/        » • (MS D-
j7
% * O’j 0;
• f-
S^OPE 1:1
□CMTaTCD
SlLu
Slope m
Dimension Sketch foil Basin IIArJoDedessa Irrigation Project
Dam Appurtenant Works
12 HYDRO POWER GENERATION 12.1 Planning & Design for Irrigation
May 2007
The dam and appurtenant works  have Deen planned ano designed basically  tor providing irngation in a gross  command area of about 17 825 ha located on both banks  of river Dedessa downstream of the dam as  required in the TOR of the Feasibility  Study  Accordingly  the values for design decision vanables, parameters and control levels have been finalized as follows
Top level of dam crest
FRL
MWL
MDDL
FSL of Canai on either side
Gross storage
Live Storage (between EL 1351-1356)
EL 1360 6 m
EL 1356 0 m
EL 1358.6 m
EL 1350 m
EL 1348 m
1341.6 Mm3 466 49 Mm3
It has  been observed that enough water resources  are available at the dam site. The mean
annual  flow  is  2328  Mm3    However  the  requirement  for  irrigation  in  final  pnase  is  only  195
Mm3. But the requirement of FSL in the canals  is  comparatively  high in order to provide irrigation by  gravity  flow. Accordingly  the MDDL has  been restricted to EL 1351m only  though the zero-elevation after the distribution silt in the reservoir works  out to EL 1346 m. Therefore, live storage only  between EL 1356 to 1350 is  utilized against total gross  storage of 1341.6 Mm3 With these control levels, the required success  in terms  of reliability  for irrigation project is achieved.
The  scope  of  increasing  the  utilization  by  expanding  the  command  area  has  been  examined But it has been found that there is no such scope due to topographic features.
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12.2 incidental Power Generation
May 2007
The   scope   and   possibility   of   power   generation   with   the   proposed   design   features   nave   oeen examined   Since   the   canals   are  contour   canals  with  comparatively  higher  value  for   FSL,  there is  very  little  difference  between  FRL  of  the  reservoir  and  FSL  of  the  canal  Thus  there  is  no scope   for   generation   of   hydropower   from   the   irrigation   water   being   released   through   the outlets.   However   the   scope   and   possibility   of   generation   of   hydropower   by   releasing   the balance  water  of  storage  as  well  as  water  available  through  run-of-the-river  into  the  river  and locating  the  powerhouse  near  the  river  bed  have  been  studied  For  this  the  centerline  of  the turbine  has  Deen  assumed  at  EL  1317m  against  the  river  bed  of  EL  1320  and  downstream normal  water  level  of  EL1323m.  The  preliminary  study  indicates  that  a  firm  power  of  5Mw  can oe   generated   with   85%   reliability   and   the   power   generation   will   be   7Mw   for   50%   reliability However omy 3Mw of hydropower will be generated with 95 to 97% reliability
12.3 Generation of Optimum Hydro-power
As  discussed above the utilization water for irrigation is  only195 Mm3 against the total annual availability  of 2398 Mm3 Thus  the site provides  scope of harnessing the balance available water which is  quite considerable, for generation of power generation But this  will require to increase the storage capacity  by  raising the height of the dam above what has  been planned for the requirement of irrigation only. Several trial runs  have been made for carrying out the simulations  studies  for power generation by  increasing the dam height successively  These studies  indicate that with FRL increased to EL 1376 m (against the presently  proposed FRL at EL 1356 m for meeting the requirement of irrigation only), a firm power generation of 20 MW can be achieved. Assuming a load factor of 0.6, the installed capacity  may  be 33.33 MW Thus  in order to harness  the available potential for hydro-power generation the dam height may  have to be increased by  about 20 m, which is  quite considerable height and will require several major changes  in the general layout and other design decision variables  and parameters.
12.4 Need for Pre-Feasibility study
In  order  to  facilitate  final  decision  about  generation  of  hydro-power  there  is  need  to  undertake  a pre-feasibility   study   to   establish   the   technical   feasibility   along   with   economic   viability   for harnessing the optimum power potential available at this site This study will take into ---------------------------------------------------------- -------------------------------------------------------------------------------123
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DamAppurtenantWork^^^—
considerations the location of the appurtenant works, along with increase in dam heig consequent increase in suDmergence. The increase in dam heignt will require to re
spillway facilities, relocate the outlets on both-left & right - banks. This will also require t the neight of saddle dam The need or otherwise of any other saddle dam may 
identified along with other aspect of submergence under the proposed reservoir Pre feasibi ty study will be able address all these relevant issues and will facilitate the final division abou inclusion hydro-power generation in the Project
(b.2) Power duration curve for FRL = 1356m - Option II
(b.3) Power duration curve for FRL ■ 1356m - Option III
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Bevation-Area-Capacity Curve for Arjo Odessa Reservoir
Volume (Million Cu.M)
Figure 12.1: Eievation-Area-Capacity Curves of Arjo Dedessa Reservoir
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REFERENCES
1 Land and Water Resources of Blue Nile Basin 1964 by USBR
May 2007
2          Preliminary Water Resources Development Master Plan for Ethiopia 1990 by WAPCOS
3. Abbay River Basin Investigation Development master Plan 1998, by BCEOM
4          'Design of Small dams’ by United states Department of Interior, Bureau of Reclamation
5         US Army Manual 'Earth Embankment EM 1110-2300' 31st duly 1994
6           US  Army  Manual  Engineering  and  Design  -  Hydraulic  design  of  reservoir  outlet  structure Engineering Manual EM 1110-2- 1602 1st August 1963
7          US Army Engineer Manual EM 1110-2- 1603 Hydraulic design of Spillways 31*’ March
1965
8.          US  Army  Engineer  Manual  Engineering  and  Design  Stability  of  Earth  and  Rock  fill  Dams EM 1110-2-1902 1st April 1970
9.            ICOLD-1994    Embankment    Dams    -    Granular    Filters    ana    Drains    -    Review    and Recommendation.
10        Sherard  JL,  Dunnigan  LP  ana  Talbot  JR  1984  (a)  Basic  properties  of  sand  and  granular filters, Journal of Geotechnical Engineering ASCE
11       Sherard JL, Wooa word RJ, Giezienski SF- "Earth and Rock fill dam"
12.     Thomas M LEPS, Rock fill Dam Design and Analysis
13       Report of Geophysical Observatory Addis Ababa University Ethiopia dt June 29, 2006
14          Earth   quake   History   of   Ethiopia   and   Hom   of   Africa   by   Pierre   Gcuin   Director   of Geophysical observatory University of Addis Ababa, Ethiopia (1957-78)
15.        Robin   Fell,   Patrick   Mac   Gregory   and   David   Stapledon   "Geotecnmcal   Engineering   of Embankment Dams."
16.      Indian Standard Codes
i)    IS 8826 -1978 " Guide Lines for design of large earth and rock fill dams ".
ii)      IS 10635: 1993" Guidelines-Free board requirement in embankment dams"
iii)     IS 7894 -1975
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iv) IS 1893 - 1984 Criteria for earth quake resistant design of structure (Third Edition)
v)      IS 6934- 1973
vi)     IS 155186 - 1994 "Criteria for Design of Chute spillways"
vii)    IS 14815-2000
viii)   IS 7500 - 2000 "Code of Practice for installation ano observation of cores and for measurement of Internal Vertical Moment in Earth Dams (First Revision)"
ix)    IS 9429-1999 "Code of Practice Drainage System For Earth ano Rock fill Dam- First Revision"
x)   IS 7356 (Part-I) - "Code of Practice for installation, maintenance and observation of Instruments for pore pressure measurements in Dam & Rock fill dam"
xi)  IS 12200-1987 "Code of practice for provision of water slops out transverse contraction joints in Masonry ana concrete dams"
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Project :- Arjo - Dedessa Irrigation Feasibility
study
Client :• Ministry Of Water
Resources
Location Arjo - Dedessa
Object :- Soil samples
LABORATORY TEST RESULTS OF CLAY SAMPLE
1 Soil samples
Grain size
distribution
Atterberg limit
Compaction
Shear strength
N*
Location of
Boro hole
Depth In
(m)
Specific
gravity
Gravel
%
Sand
%
Fine
%
LL
(%)
PL
(%)
PI
(%)
Free
Swell
(%)
Linear
shrinkage
(%)
Natural
OMC
%
MDD
kg/m3
Permeability
cm/sec
C
KN/m2
□
DEG.
C’
KN/m2
0' F DEG.
Remarks
1
?
ATB TP 1
0254)85
2.70
0
2
98
64 20
36 00
26 20
60
-
•
-
-
ATB TP 2
020-0 90
2.54
0
7
93
60.60
34.60
26.00
40
3
ATB TP 3
020-2 10
2.54
0
1
99
68.20
38 10
30 10
50
5.10
34 89
1340
461x10*
4
ATB TP 4
0.10-0 80
2.56
-
•
-
62 60
24 00
40
12 16
36 20
1486
2.31x10*
5
ATB TP 5
030-1 10
2.54
-
-
-
64.20
38 60
40.20
24 00
40
1620
38.12
1326
24
29
6
ATB TP 6
020-2.40
2.54
0
8
92
58 60
41.60
17 00
30
15.45
4300
1273
1 64x10*
7
ATB TP 7
0 30-1.80
2.54
0
2
98
61 80
38.44
23.36
40
1834
43.50
1214
-
35
19
8
ATB TP 8
030-0 90
254
0
2
98
64 00
38 10
25 90
50
18.34
43.50
1214
-
36
27
9
ATB TP 9
0.25-1 30
256
0
8
92
60.40
35.26
25.14
40
11.98
35.00
1360
-
44
30
10
ATB TP 10
0.25-1 20
269
60 86
38.20
22 66
40
17.58
3903
1322
-
-
11
ATB TP 11
0 20-1 50
269
64.20
3020
60
1993
38.00
1349
-
39
27
12
ATB TP 12
0.20-1.90
2 85
6080
34 00
38.00
30.80
60
16 24
4020
1421
-
40
30
13
ATB TP 16
0 10-1 30
2.85
66.40
40.20
2620
60
13 18
27.25
1515
-
-
29
31
14
ATB TP 17
010-0.90
269
6280
44 10
1870
40
2022
35 00
1338
-
-
15
CHBTP1
0.10-0.90
2.76
0
4
96
56.7
35.08
21 62
5 62
32 21
1377
-
34
27
16
CHB TP 2
015-1
2.77
0
10
90
538
33.44
20 36
-
•
-
-
17
CHBTP3
0 351 90
2 78
0
5
95
55.2
327
225
5.62
33 16
1409
-
-
18
CHB TP 4
2-3.25
2.75
0
4
96
55.2
38.89
1631
6 13
33 00
-
37
30
19
CHB TP 5
020-1 75
2 79
0
7
93
59.1
42 52
16 58
50 64
8 24
34.50
1412
3311
1 59x10®“
20
CHB TP 6
0.301.30
277
0
7
93
597
40.76
18 94
-
-
-
-
-
21
CHB TP 7
0.25 0 80
2.74
-
•
-
5500
32 94
22 06
15.24
41.00
1624
-
22
CHB TP 8
0 40 1 30
2 76
0
7
93
55 00
3294
22 06
-
-
-
-
-
23
24
CHB TP 9
0.20-1 60
2 75
0
2
98
585
31.34
27 16
45 08
8 24
30 00
1422
37
25
DHB TP1
0 10-2.10
269
0
4
96
60 80
40.58
20.22
40
12 02
37 50
1370
3 14x10®”“
25
DHB TP 2
0.20-1 50
2.68
-
-
-
63 80
43 68
20 12
-
-
-
-
26     DHB TP 3
0 25-1 75
2 69            0
1
99
71 401
48 9
22 5
50
-
-
•
-
-
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27
DHB TP 4
0 30-1 90
2.69
68 70
3953
29.17
60
20
DHB TP 5
0.20 3 00
269
0
9
91
54.90
51.09
381
40
29
DHB TP 6
0 15-1 40
2.69
63 60
45.62
17 98
30
DHB TP 7
0 30 1 90
2.68
63.50
43.71
19.79
50
11 97
36 O0
1440
37
24
31
DHB TP 8
020-1.00
268
60.7
40 23
20.47
52.13
32
DHB TP 9
0.85-1.45
269
669
18.79
48.11
33
DHB TP 10
0.20260
2.69
0
4
96
60.3
39 51
2079
13 27
41 50
1280
34
DHB TP 11
0 20-2 10
2.69
0
2
98
65.3
37.87
27.43
25.72
3881
1420
30
31
35
DHB TP 12
020-1.75
2.85
0
1
99
624
44 79
1761
40
-
40
30
36
MBTP1
020 1.40
2.70
66.80
42 89
23.91
40
14 26
41.00
1660
6 2x10’®
46
28
37
MBTP3
0.20-1 50
2.52
7260
41.22
31.38
60
38
MBTP4
0.15-1.70
2.64
67.30
43 84
23 46
50
57 94
12.02
40 12
1365
39
MBTP5
0.30 1 70
2.64
64 62
36 42
282
40
MBTP7
0.70-1.30
2.64
0
2
98
61 10
38 92
22 18
40
41
MBTP8
0.20-1.90
264
0
2
98
71.30
32 83
38 47
50
42
MBTP9
0 15-1 80
264
2
2
94
65 80
3948
2647
40
-
43
24 00
1295
43
MBTP10
1.10-2.20
2.64
0
6
94
64 00
42
26.32
70
1562
38 25
1420
44
OB TP 1
1-1.50
62.20
40.25
21 95
50
45
OB TP 2
0 90-1.10
63 50
43 24
2026
60
46
OB TP 3
0 90-1 30
-
-
-
-
63 00
4075
2225
60
-
-
-
-
Note: The necessary graphs and data sheets for Grain size distribution, Triaxial, Compaction and consolidation tests are
attached
129
Water Works Design & Supervision Enterprise
In Association with Intercontinental Consultants and Technocrats Pvt. Ltdu
Arjo  Dedessa  Irrigation  Project
Dam Appurtenant Works
LABORATORY TEST RESULTS OF SAND SAMPLE
2 Sand Samples
May 2007
Grain size
distribution
Organic Content
Unit weight ( Kg/m3 )
R. density
N°
Location of
Bore hole
Depth
in
( m)
Specific
gravity
Gravel
%
Sand
%
Fine
%
(%)
(•/.)
(•/.)
Free
Swell
(%)
Loose (%)
Field
m
(%)
Dense
cm/se
c
C
KN/
m2
□
DEG.
c
KN/
m2
0
DEG.
Rema
rks
47
Lokosefera 1
2 55
4
90
6
-
4
60
48
Lokosefera 2
2 55
0
95
5
3
62
49
Loko bridge 1
2.55
6
80
14
4
59
50
Loko bridge 2
2.55
6
94
0
-
51
Borche 1
2 55
5
88
7
52
Borche 2
-
2.55
4
78   Ll8
-
-
- L-
130
Water Works Design & Supervision Enterprise
In Association with Intercontinental Consultants and Technocrats Pvt LtdArjo Dedessa Irrigation Project
Dam Appurtenant Worts
Laboratory Test result of Rock Sample
May 2007
3. Rock Samples
N* Location of
Bora hole
Depth In
(m)
Porosity
<%)
Unit weigh ( Kg/m’)
Water
absorption
(%)
Specific
gravity
Point Load ( Khi/m )
Dry       SSD
ucs
(KN/m2)
Abrasion
(*)
Soundness
(%)
Organic
content
Unit weight ( kg/m  )
z
y
Loose
y
field
y
dense
y
R.Density
1
AD1
11.63-11.93
360
342
2
AD1
12.75-12.88
797
822
•
3
AD1
13.58 13 70
964
920
28480
4
AD1
17.16-1730
420
372
16800
3.19
5
AD2
11.80 12.00
736
846
24380
-
6
AD2
16.18-16.30
844
732
23122
•
7
AD2
23.30-23.50
634
680
26720
4 14
8
AD2
25.25-25.35
962
842
22236
-
9
AD2
36.65-36.92
840
726
31120
10
AD3
1355-1368
970
840
27542
-
11
AD3
21.8022.13
622
570
92182
12
AD3
24.30 24 90
1455
1380
36376
2.16
13
AO3
27.10 27.23
703
690
21082
-
14
AD3
30.73 31.10
923
870
27690
-
15
AD4
13.10 13.26
•
-
25264
-
16
AD4
17.15 17.35
876
724
28032
-
17
AD4
16.00 16 10
932
866
21015
18
AD4
21.3021.47
-
31436
-
19
20
AD4
27.52-27 75
•
-
25220
3 24
AD5
21.50-2163
832
880
26340
-
21
AD5
16.55-16.63
640
736
24936
-
22
KD5
34.70-34.85
740
620
22540
-
23
kD5
40.7540.90
936
843
20420
•
24 Z
<D5
41.18-41.33
713
684
29346
•
25 A
<05
39.40J9.50
618
723
28139
-
26 A
<D5
38 00 38 08
1120
932
31342
2 26
27 A
28 A
D51
16 78-16.92
-
-
24136
-
051
16.60 16.78
836
724
22340
-
29 A
051
22.70 23.43
-
•
19136
-
30 A
052
20.4020.67
•
64356
-
31 A
052
20.4020 67
1287
1124
59136
•
32 C
RQ 1/5
4.24
2780
1.76
2 76
•
•
-
32
2 78
33 B
RQ 1/2
-
3.26
2742
1.24
272
-
31
241
Tested by Date Checked by Date
Getachew and Mulu : 30/07/06
Abrahme Assefa 30/07/06
Approved by Date
131
Water Works Design & Supervision Enterprise
In Association with Intercontinental Consultants and Technocrats Pvt Ltd.

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