Arjo Dedessa Irrigation Project Feasibility Study
Project Overview
Client: Federal Democratic Republic of Ethiopia, Ministry of Water Resources
Consultants: Water Works Design & Supervision Enterprise in association with Intercontinental Consultants and Technocrats India Pvt. Ltd.
Report Date: May 2007
Location: Arjo-Dedessa, Ethiopia
Report Structure
| Volume | Content |
|---|---|
| Volume I | Survey and Investigation (Topographic Survey, Geomorphological Studies, Geological & Geotechnical Investigation) |
| Volume II | Natural Resources (Forestry, Energy & Catchment Development Plan) |
| Volume III | Water Resources (Meteorological & Hydrological Studies, Dam & Appurtenant Works, Irrigation & Drainage, Hydraulic Structures) |
| Volume IV | Agriculture (Soil Survey & Land Evaluation, Agricultural Planning, Livestock, Fisheries, etc.) |
| Volume V | Environmental and Socio-Economic Aspects |
Geological & Geotechnical Investigation Findings
The investigation included borehole logging, field permeability tests (Lugeon tests), and discontinuity analysis. Key findings include:
Borehole Data
| Borehole ID | Location | Depth (m) | Key Findings |
|---|---|---|---|
| AD1 | Left bank, Dam axis | 0-41 | Very dense silty sandy gravel with strong cores; core loss noted between 31.00m-34.40m |
| AD2 | Left bank, Dam axis | 0-28.85 | Moderately strong trachyte with whitish, greenish and brownish phenocrysts |
| AD3 | Left bank, Dam axis | 0-88 | Weak to moderately strong porphyritic trachyte with varying weathering |
| AD4 | Right bank, Dam axis | 0-48.65 | Pinkish grey moderately weathered vesicular trachyte |
Lugeon Test Results
Field permeability tests were conducted at various depths, showing variable permeability values:
- AD1 (21.5-26.5m): Average Lugeon value = 87 (Dilation flow type)
- AD2 (8.8-13.4m): Average Lugeon value = 26 (Turbulent flow)
- AD3 (35-40m): Average Lugeon value = 8 (Turbulent flow)
- AD4 (10-15m): Average Lugeon value = 35 (Dilation)
Discontinuity Analysis
Rock mass characterization identified various discontinuity types including:
- Incipient joints (45° angle)
- Fractured zones
- Healed joints
- Zones with white and green secondary mineral fillings
Core Box Photographs
The report includes extensive photographic documentation of core samples from all boreholes, with:
- Yellow blocks marking samples taken for laboratory testing
- Red blocks indicating core loss areas
Conclusion
The geological and geotechnical investigation provides comprehensive data on subsurface conditions for the proposed irrigation project, including rock quality, permeability characteristics, and structural features that will inform dam design and construction.