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MATERIALS TESTING DEPARTMENT AND
POL NDATG N IN’/ESHGATJON PEPAHYG.ME.NT
rxofect Arjo- dedfls-a Irrigation Feseafaility Study Client Ministry of waler Resource
Location: Dcdew
□ascription: Loko Bridge 1 sand
*
GRAJN SIZE DISTRIBUTION
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MATERIALS TESTING DEPARTMENT AND FOUNDATION INVESTIGATION DEPARTEMENT
project
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Client
Ministry of water Rusurc*
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FOUNDATION INVESTIGATION DEPARTEMENT
prci«fc
Client
Location:
Description:
Arjo- dedesa Irrigation Feseability Study Ministry erf water Resource
Dedosa
Borcho No 1 sand
____________________ _______ GFtAJNSUE D13THIBy-
Abraham As seta
Dm»:
12JO7.'O6W.CNProi- Oata
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* Eide-^vi^THE FEDERAL DEMOCRATIC REPUBLIC OF ETHIOPIA
MINISTRY OF WATER RESOURCES
ARJO DEDESSA IRRIGATION PROJECT
FEASIBILITY STUDY
Geological and Geotechnical Investigation Part II - Appendices
Appendix F: Geophysical Survey
M»> 2007
WATERWORKS DESIGN & SUPERVISION ENTERPRISE IMKSMCUITIQNWTTH
INTfHCOMTl MENTAL CQNSUITAMTS AND IICHHOCflftTS INDIA PVT UIJ P.O Bai\ 25$ I, Addii Ababa Ethiopia
T«H.25IH $HMItWlSWFax(L25m 613571 E-mail m. wd^dKOni nd «Abstract
A contract agreement was signed between Construction Design Share Company Geological Survey of Ethiopia io conduct geophysical investigation (refraction seismic
survey and Vertical Electrical Sounding) at Aijo-Dedesa and Camera irrigation project
for dam site investigation.
As part of the agreement Vertical Electrical Sounding was conducted at 10 points at AiJO- Dftfcss between June J 0 and June 20 2006.
This report presents the results the geophysical investigation (Vertical Electrical
Sounding) conducted in the area.
The interpreted resistivity layers are generally correlated with the neighboring borehole togs.
The thicknessof the overburden is estimated to be 12-17m at the spillway and about 3m at the dam site. At the saddle dam the overburden is assumed io he very thick (greater than 40m).
Electrical imaging is recommended tn addition co the planned retraction seismic Survey.
21Table of Contents
1, INTRODUCTION.................. -.......................... ........
Page
........ 1
I . 1 iS L - ■ j ■ l ■ ■ ■ ■ ■ ■ ■ ■
1
.
E. I -1 Location and AcMUftjiliiy.......................................
1.2 OBJECTIVES... r,
GEOLOGY a ran ri rri IB r. Ha III - FB !■ |l IB ■ BB ■'■ a r. . r r, I ■ ■ tJa ■ aaaaar - - h ■ -
J. CEOrHYSiCAL SURVEY.,.,.,..,.,............................... -....................„•
1.1 SURVEY GRID AND VOLUME OF WORK.
J
.1
raaa■ ■ ■ ■ aaH+ B
....... 3
3.2 VERTICAL ELECTRICAL SOUNDING (VES) ........a.....,,,...,,.
3.2.1 Instrumental ion and field procedure........... ......................... 3,2 .2 Dna processing md presentation...
3.23 Daia quality..,, , „..
r
iiiir1iiaiiaa
rrrrrvwaaa i a a • ■ a <
4+ DATA INTERPRETATION
Spfcll r4.rir....B.ii..B.Bt.Lj ■...._<■ Dant site, Lett bank. ,,. B., „. aj. Right bank
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SL CONCLUSIONS AND RECUM M FNl> ATJON L
■ — - - ■ I! B 'I. A J * 9 a H j j j , r
.7
■”■■■** ....*201. Introduction
1.1 General
LI.I Location and Accessibility
Ibe investigation site is located in lhe Oromia Regional state, about 540km fro
Ababa, geographically bounded by 36 38’SO E — J6 41 34 E and 3 32 34 t
2952 “N (Figure I.)
Il can be accessed after-driving about 4€0 km along Addis Ababa -Gore asphalt and all weather road. Turning to the right al 46£lkm from Addis Ababa, about 80km dry weather road leads to Lhe site of investigation.
1.2 Objectives
The main objectives of the geophysical survey arc
■ To determine the nature and depth of Lhe beat Tock
- To estimate the naLure and lhe thickness of ihc overburden - To dctcrcninc the nature of geologic structures if any
2. GEOLOGY
™ » ^-sed by ™k„fe TOls or „.Ml,
BwehoU
that. basaH and trachybasalt are 1hc dominant rock son the | c fl bunk f lhe right bank, however, trachyte is the dominant rock
moderately to highly fractured and the fractures am g ne
O[>
C ra|| ru
°
nunerab of different color,
*
L
I
I
k
tfroure 1 Location3. Geophysical survey
3.1 Survey grid and volume of work
Vertical Electrical Sounding (VES} was conducted al 10 poinis, Two VES observations at the spill way, four VES observations al the dam site (two VES observations on the left bank and two VES observation on the right bank} and four VES observations in the saddle darn area (Figure 2),
The following table shows the UTM coordinate of the VES points and boreholes with their elevaiions.
Table 1. Coordinates of ihe VES points and boreholes with their elevations, Arja-Dedesa
VES
No
1
2
3
4
5
6
7 9 10
ADI AD2 AD3 AIM
AD5
AD" ADS'.
' ADS2
EASTING
242437 242560
243084
243129
244858
244620 244352
.'41079
'243287 243264 243048 243103 243166^ 243226 243313 243602 243070 244259
244748
NORTHING elevation
■ remark
94I861 1368.9
941729 1377 6
13+2 4 1329 1357 2 1359,2
1ADSP I 242360
942397 942340 942446 942505 942738 942962 942613 942549
942315
942299 942370 9+2525
.942643 942642 9+2371; 942853 942503 941725
L33R 1324
1323.6
1349.7
1 365 8
13383 '
>347
1354 2 1377 3
3VAHA
LEGEND
J
f VES points
• *“ Brehole location! -------- Dan axis
Spillway
-- ------ Saddle dan
j Elevation contur \ River
I3.2 Vertical Electrical Sounding (VES)
32.1 InUruaifDtnllon and field procedure
sdoftn made TSQ-2 Tmumitttf. IPR I0A receiver and 750 W motor generator were
used for the surveys
The Schlumberger array was applied for the investigation,. Ln this configuration MN
(potential electrodes.) are kepi fixed and current electrodes (AB) are moved outward
symmetrically tn steps At some stage the voltage difference between MN will,, in
general, fall below the reading accuracy of the voltmeter in which case the distance
between MN is increased (Figure 3).
The maximum current electrode separation was limited to AB/2=22O-3J{>m due to topographic problems,
4—-AB2 ----------------------- ------------- AB/2
A MN
Fi&un: 3 Schlumberger aniy configuration
]
3,1.2 UaU processing and prr^nnxi^n
The observed apparciti ressjuvtty far cadi VES was nln,i« ,
p -iLcci 'agaiusE hill"
scparalion (AB’S) cm double logarithmic paper. Approximate auxiliary point inelhoJ was imiolly applied on ihc parameters (resistivity and ihiekncss).
S
V 'Jrrcnt elvc trade
Jam forThese layer parameters were used as initial model for more computer assisted iterative interpretation to obtain the final layer parameten.
Ftnally, the VES data are presented as geo-electric sections using these layer parameters (resistivity and thickness), The geoelectric section of the spillway, the dam site and the saddle dam are presented in Figure 4, Figure 5 & 6 and Figure 7 4 8, respectively.
J.2J Data quality.
The data quality of Vertical Electrical Sounding generally depends on the topography, orientation of Hie line, ground contact of electrodes and instrumental error. The ?Wjo-Dedesa dam site is generally characterized by nigged terrain and ground contact of electrodes was poor, which is assumed to affect the quality of Lhe data.
4. Data interpretation
Pie interpreted VES results are presented in (he form of sections (Figure-4 to Figure 8} and also summarized as shown in Tables (Table 2-Table 7). Lithological descriptions of the resistivity layers are taken from borehole records in lhe vicinity of the VES points Spillway
Subsurface information for the spillway area was obtained from two VES observations VES-1 and VES 2, located in the north-western and souh-easiem pan of the sp.Hway ' respecnvetyfFigure-^
VES-1 which is located m the north-western part and relatively at lower elevation display^ tour resistivity layers (see Figure 2 and appendix), The top layer, with 12 Ohm-
- .... and thickness, is assumed io represent the top thick soil horizon. The
1u
rtsssu* */ •"*
second layer having 9m thickness and 22 Ohm.m r„ls(ivi(y is ,
gravel. The third thick (50m) layer that is characterized bv ire
"•itrizcj by lhe restirivily
' tOC{,mprise
may compose highly weathered and fractured bus j .
3t
At the bottom the resistivity raises to 77 Ohm-m due (£l Jw rt. ,
.
and fraciunnif of the basalt.
6
m$W
NE
LEGEND
Figure 4 Geoelecrte section under v£S-i and VES-2, Spillway, ArJo-CedesaJ
z
LEGEND
Ell'ty fandy Clay
to PMXier*t^iy U««thvr«d t^achy buUl
hfahhr
tracJhy-
buutt bitvrCoiUatvd vttJh ftlty S«ndy Qf«v*L
hiQj^y frKtAr«d and vttHwetd trochy basal t
□
Figure 5 Geoeiccric section under VES-3 and VES-4.Right bank
VES-9 and VES-10 are locaicd in the eastern pan of the dam site between boreholes
and ADS ( Figure 2). Four resistivity layers are delineated under VES-9. These layers exhibit very high resistivity values (1460-8700 Ohm-m).
Thc borehole log ADS describes that thick trachyte body underlies the vicinity. Fractures observed on the trachyte arc filled with white minerals-
The high resistivity obtained suggests that the degree of weathering and fracturing of the
underlying trachyte is less in the vicinity or the fractures arc tilled by high resist!vily
minerals like silica, Figure-6 shows the four delineated resistivity layers and their
possible lithoLogjcal description.
VES-19 is Joculcd at relatively lower elevation towards the Dedcsa river (Figure-2), The four layers delineated under VES- ID are characterized by very low resistivity compared lo VES-9. The top 1.4m layer having a resistivity of 15 Ohm-m comprises silty clay.
The second 653 Ohm-m layer is 8 7m thick . This layer is associated with slightly weathered trachyte. The third layer which is characterized by low resistivity (5& Ohm-m) is very thick This layer is lithologically interpreted is highly weathered trachyte
The rise of the resistivity to 195 Ohm-m of the bottom layer indicate the decrease in degree of weathering and fracturing of the trachyte wnh depth
I
I
[ [ I
L
Saddle dam
The Com VES points of the saddle dam area are distributed in 3uch .. w
j Way
two VES observations (V F.S-J and VES-6> conducted along ESE-WNW ori
cusrem part of the saddle dam. The other two VES observe.™ vrLc,
urteni«d south V t^-7 jjpJ vpc Qv
follow the NE-SW onmtcd North Eastern part
f
o (hc
Five and four resistivity hyere are displayed under VES-5 imj (Figure The lop clay Layer is Im io 4.?m thick. Underlying
1,5-2.Im thick grave)
dam (fiI
l L L L I
L
Right bank
VES-9 and VES-10 are totaled ip the eastern part of lb*
Five and tour nesisLivity Ia>eTs arc diiphy«d under
(FilpWf 7} The top clay tay^r
l to J 7m thiefc
ls m
l.S-i-lni thick gravel.
kt)Low resistivity (16-17 Ohm-m) layer of 32-34m thickness is interpreted as sandy gravel/
highly wealhercd and decomposed and saturated basalt The bottom 50 to 75 Ohm-m
layer is assumed to comprise highly weathered basalt.
Four and three resistivity layers are delineated under VES-7 and VES-8 respectively. The
top 2,4 -2.7m thick layer is interpreted as gravel/highly weathered trachy basalt. At the
vicinity of VES-7 the fop 0.6m is covered by silty sandy clay.
The second layer from the bottom is 76-80m thick. It covers the northern part of the
saddle dam. The resistivity of this layer is between 36 Ohm-m and 44 Ohm-m as
observed under VES-7 and VES-8 and it is interpreted us highly weathered probably
saturated trachy basalt.
The bottom layer whose resistivity varies from 93 - 137 Ohm-m is assumed to comprise moderately to highly weathered basalt.
IIFigure 9 Geoelecrlc section under VES-8 and VES-7. saddle dam. Arjo-Dedesa
iTabled Interpreted VES Results of spillway, Arjo-Dedesa
VW-I Layer No
Resistivity
(Ohm-ml
Thickness
(mJ
Lithological description
remark
I
12
8
Silly sandy clay Hop soil)
2
22
9
Silty sandy gravel
J
38
50
Highly Weathered basalt
4
Tt
Moderately to highly weathered basull
VES-2
Layer
No
Resistivity
(Ohm-m)
Thickness
(mJ
Lithological description
remark
1
217
1.2
Crave 1/ weathered basalt
2
2JM
2,7
H 9b H
3
229
7.6
■b H
4
37
33
Highly weathered trachy
basalt
5
170
moderaiely weathered trachy
basaltTable 3. Interpreted VES Results of Dam Aiis» left bank, Arjo-Dedesa
VES-3 Layer No
Resistivity
(Ohm-mi
Thickness
(m)
Lithological description
remark
I
2487
1.3
Moderately -highly weathered traehy basalt
2
3923
I S.9-
Moderately -highly weathered trachy basal l
3
774
40
highly weathered Trachy basalt intercalated with gravel
J.
2355
—
Moderately weathered trachy basalt
1
1
VES-4
Layer
No
Resistivity
(Ohm-ml
Thickness
Im)
Litho logics! descrip tion
Fremark
1
17
1.7
Silly sandy clay
2
770
25
highly weathered Troche basalt
3
420
16Table*. Interpreted VES Results of dam ails, right bank, Arjo-Dedcsa
vta-y
Layer
No
Resistivity
lOhm-m]
Thickness (m)
Lithological description
remark
1
1957
0-8
highly weathered trachyte
2
8700
2.4
Moderately -slightly
weathered trachyte
3
1460
21
highly weathered trachyte
4
8314
--
Moderatcly-sJ i ghtl y weathered trachyWRhvolitc
VES -10
Layer
No
Resistivity
(Ohm-mi
Thickness (m)
Lithological description
remark
1
15.5
1.4
Silty clay
2
653
87
Slightly weathered trachyte
3
58
44.5
Highly wcaihcrcd water bearing (saluratcd) trachyte
4
195
■■
Moderately weathered and saturated trachyte
17Table 5, Interpreted VES Results of Saddle dam , Arjo-I>edesa
VES-5
Layer No
Resistivity
(Ohm-in)
Thickness
(m)
Lithological description
-J
remark
---- i —--- -
1
32
0.9
silty sandy clay
2
M
3.S
Silty day
L—
3
221
1.5
grave 1/wrathcred basil
4
i?
34
Sandy gravel/Highly weathered, decomposed and saturated basalt
5
55
--
Highly weathered basalt
VES-«
Layer
No
Resistivity
lOtim-mj
Thickness
. . (n*J
Lithological description
remark
1
7S
-
0.7
■Silty sandy clay
2
635
21
Gravcl/highly weathered basalt
3
nr
32
Sandy gravet/Highty weathered .decomposed and saturated basalt
4
75
Highly weathered basitltTable 6. Inlerpttted VES Mcsults of Saddle dam, Arjo-Dcd«a
VES-7
Layer
No
Ren Unity
jOtim-m;
lluctm*
■mj
Ldhotaiticil darn priori
.______ __..
reirurk
_______ —— |r------------- ---------
1 61
2 522
3 4J
04 Si-ltv undyclj i
l It < itjvc 1. •* cjittered ir.tch h.w .d! 76 l!ij.hL wc-LihcraJ rrjch hii.ulc
i------_
- -------- 4
J 117 — MitdcraEriy tn highly
ivejEkcrcd hiiMAlt
v >s s
t j-ycr
ttmliviE}
— T ’
ThiukELCXii 1 iiImk.^iL>il -les* ri|iiii)ii
remark,
No
•H
■ m)
a>7
Sill, tjiiil, rncl ■ ■!
L
HO ll.^hT, ATjlhcrcil Li i%_hv
H«kj||
M'*ktjiel .■ in higjil,
tcjlhrrcd li iih b.HMlI
*
?4. Conclusion and recommendation
Th* Vertical Electrical Sounding (VES) conducted in the are* delineated different resistivity layers corresponding io different lithological units, which arc generally in correlation with the neighboring borehole logs.
From the two VES observations conducted al the spillway the overburden thickness is estimated lobe 12m - 17m. The bedrock is assumed to comprise highly weathered and fractured trachy basalt.
The over burden st the dam site is very thin (about 2m thick). The bedrock is also marked by very high resistivity that is in thousands of Ohnvmeters indicating that the bedrock of the dam silc area is slightly weathered and fractured Of the fractures are filled with high resistivity minerals such *$ silica. The results obtained from VES-4 and VES-10 suggest lhat the resistivity of the bedrock at the dam sue decreases towards the river (in the vicinity of the flood plain). The saddle dam is generally marked by low resistivity (16-44 Ohm-m), 40-Bttm thick tomiation This formation is considered as soft formation and the relatively hard formation which assumed as a bedrock at saddle dam area seems deeper (about 40m deep at lhe south eastern part and about 80m in the north western part)
The unerpreicd VES results Jo not show the presence of structures such as faults and shear zones
Rech) mme a d ittofls
The authors recommertd Electrical imaging survey in the area in addition to foe p]an d seismic refraction survey io get reliable and detail subsurface information
204. Conclusion and recommendation
The Vertical Electrical Sounding (VES) conducted in the area delineated difT
rerislivity layers corresponding io different lithological units, which are generally
correlation with the neighboring borehole logs.
From the two VES observations conducted at the spillway the overburden thickness is estimated lobe 12m - 17m. The bedrock is assumed to comprise highly weathered and fractured trachy basalt,
The over burden at the dam site is very thin (about 2m thick). The bedrock is also marked by very high resistivity that is in thousands of OhnvmetcTS indicating that the bedrock of the dam site area is slightly weathered and fractured or the fractures arc tilled with high resistivity minerals such as silica, The results obtained from VES-4 und VES’lO suggest that the resistivity of the bedrock at the dam site decreases towards the river (in the vicinity of Ute Hood plain). The saddle dam is generally marked by low resistivity (16-44 Uhm-m), 4O-B0m thick formation This formation is considered as soft formation und the relatively hard formation which assumed as a bedrock al saddle dam area seems deeper {about JQjti deep al the soulh eastern part and about &Chn in the north western pan)
The interpreted VES results do not show the presence of structures such a5 faults arid shear zones.
Recommend alien*
The authors rreommend Electrical imaging survey in the area in drii
,.
.. _
a T;™ . L
j’jujiion to the planned
seismic refrxuon wvey lo get rchable and detail subsurface ihlbrnuhanAppendixr-r
L
RMS-error 2.5
10*4 |___ ~
VES'Z ARJO-DEDESA Sc hlum Derg er Configuration
No Res Truck Depth
o_ £
> is
U-J OJ
or
1 217.3 1.2 12
2 294 3 27 3.9
3 229.2 7.6 11.5
4 36.6 33.4 44.9
5 170.6 ■
10*3
10*2
10*1 •— w-*o
...-1■ -----------------------------------------------------------------------------1...................................... .......................
10*1
10*2
10*3
Current Electrode Distance (A0/2) |m]
Spillway; Ago-Dedesa Irrigation projectDam site (left bank), Arjo-Dedesa Irrigation projectRMS-error 1.6 T(M
VES-5.Saddle dam,Argo-Dedesa —1 5chlLimber.;er Configuratiqn
No Res Thick Oepth
31.0 0.3 0.9 13.6 3.B 4.7
221 2 1.5 B.2 17.8 34 7 409
48.0 ■ 4
•
Curren! Electrode Distance (ABT2) |m]
f / VI:S-5, Saddle dam, Arjo-Dedesa irrigation feasibility study4Appar. Resistor
VES-8. Saddle dam, Arjo-Dedsa Feasibility studyAppar Resisthrity lohnrrj
VES-9, Dam site, right bank, Arjo-Dedesam irrigation feasibility studyAppar RfiSisiMfy (ofunrl
VES-1O, Dame site, Atjo-Dedesa Irrigation feasibility %iu«■« ■ i
f ■ PTe«drrie
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