Birete Dam Project - Geological Investigation Summary
Project Overview
The Birete Dam study was initiated by the Building Material Corporation in collaboration with the Ministry of Mines, Energy and Water Resources. The investigation aimed to identify suitable dam and reservoir sites to store sufficient water (2000m³/day) for a cement plant and other water needs.
Key Findings
1. Site Location and Basin Description
- Located in the Birete River basin (tributary of Sodoble River → Mugher River → Blue Nile)
- Coordinates: 9°25'-9°29'N, 33°03'-33°20'E
- Area: 33 km² with elevation 2450-2670m above sea level
- Topography: Undulating with northward slope, covered by residual clay from basalt weathering
2. Geological Conditions
- Central Ethiopian plateau basalts with stratigraphy:
- Upper basic volcanites
- Ignimbrites
- Lower basic volcanites
- Deep weathering has created 2-5m thick residual clays
- No major faults observed near dam site (one major fault downstream)
- Joint characteristics observed in outcrops:
Outcrop Strike Dip Spacing I 110-120° 40-55° NE 15-25cm II 210-220° Vertical 15-30cm III 290-310° 20° E 15-50cm
3. Drilling and Permeability Tests
- 9 boreholes drilled (total 173.40 linear meters)
- Permeability tests conducted in 4 boreholes (Nos. 3,4,7,8)
- Key permeability findings:
- Borehole No.4 at 18.50m: 6.96×10⁻⁴ cm/sec
- Borehole No.8 at 22m: 5.42×10⁻⁴ cm/sec (with artesian flow)
4. Construction Materials
| Material | Location | Suitability |
|---|---|---|
| Rock | 500m SW and 700m N of dam | Suitable (augite basalt), but upstream quarry may cause reservoir leakage |
| Clay (core material) | Kotcha Creek (3A) and Hud-gebeye (3B) | Suitable (shear angle 26-26.5°, cohesion 0.05 kg/cm²) |
| Transition Material | Biyo (7A) and Hekoda church (7B) | Suitable (weathered basalt) |
| Sandstone | Tuntu (14km south) | Suitable as filter layer |
5. Recommendations
- Foundation: Bedrock is generally competent
- Grout curtain recommended (depth 18m) between horizontal distance 240-310 from left abutment
- Alternative: Dental treatment with clay blanket upstream
- Materials:
- Use clay from Kotcha Creek (3A) and Hud-gebeye (3B) for core
- Use transition material from Hekoda church area (7B)
- Prefer downstream quarry (700m north) for rock to avoid reservoir leakage
Appendices Overview
- Borehole logs
- Field permeability test results
- Borehole sample test results
- Clay test results (plant site)
- Clay test results (borrow areas)
- Transition material analysis
Conclusion
The geological investigation confirms the Birete Dam site has generally competent foundation conditions with suitable construction materials available nearby. Special attention is needed for grouting in areas with higher permeability, and careful selection of quarry sites is recommended to prevent reservoir leakage.