Birete Dam Geological Investigation Report Summary
Date: February 1981
Location: Birete River Basin, Ethiopia
Coordinates: Between latitudes 9°25'N and 9°29'N and longitudes 38°03'E and 38°20'E
1. Project Overview
The Birete Dam study was initiated by the Ethiopian Building Material Corporation in collaboration with the Ministry of Mines, Energy and Water Resources to store water (2000m³/day) for a cement plant and other water needs.
2. Basin Description
- Area: 38 square kilometers
- Altitude: 2450-2670 meters above sea level
- Topography: Undulating, gradually sloping to the north
- Main tributaries: Bora and Likimsa rivers
3. Geological Findings
General Geology
The area is part of the central Ethiopian plateau series of basalts with the following stratigraphic succession:
- Upper basic volcanites
- Ignimbrites
- Lower basic volcanites
Dam Site Geology
- 95% of mapped area covered by soil (weathering product of basalt)
- Residual clay thickness: 2-5 meters
- Joint directions and intensities were measured at three outcrops
- Mineralogical composition: Plagioclase (45%), pyroxene (40%), accessory orthopyroxene magnetite
4. Investigation Methods
- 34 pits dug along two proposed dam axes
- 9 boreholes drilled (total 173.40 linear meters)
- Permeability tests conducted in 4 boreholes
- Core samples analyzed
5. Construction Materials
Material | Location | Assessment |
---|---|---|
Rock | 500m SW and 700m N of dam site | Suitable for quarrying (10-12m depth) |
Clay (Core Material) | Kotcha Creek (1.5km NE) and Hud-gebeya (3km NW) | Satisfactory (Angle of shearing resistance: 15-26°) |
Transition Material | Biyo area and Mekoda church area | Weathered basalt suitable |
6. Key Findings
- Foundation assessed as competent for dam construction
- Average overburden thickness: ~2.50m to be stripped
- Two lava flows identified within drilled depth (30m)
- Shallow artesian groundwater found on western terrace
7. Recommendations
Foundation
- Single line grout curtain recommended for area between horizontal distance 240 to 310 from left abutment (depth 18m)
- Alternative: Dental treatment with clay blanket upstream
Materials
- Clay from Kotcha Creek (Area 3A) and Hud-gebeya (Area 3B) recommended for core
- Downstream quarry location (700m north) preferred over upstream to prevent reservoir leakage
- Transition material from area near church considered better
8. Appendices Included
- Bore Logs
- Field Permeability Test Results
- Borehole Samples Test Results
- Clay Test Results (Plant Site)
- Clay Test Results (Borrow Areas)
- Transition Material Analysis