Evaluation of the Stability and Suitability of Ashebeka Dam Foundation and Reservoir Area, Arsi Zone, Central Ethiopia

Abstract

In the countries like Ethiopia where the economy is led by agriculture and has a huge potential of resources for such activities, the construction of dams for irrigational purposes is very essential. The Ashebeka dam irrigation project is located in Degeluna Tijo Woreda, Arsi Zone of Oromia Regional State at about 30km south east of Asela Capital city of Arsi zone. The planned dam will have a height of 28m and a dam axis length of around 550m. However, the dam encountered engineering geological problems that affected the water tightness and stability of the dam axis and reservoir area.The main objective of this work is to investigate and characterize the engineering geological aspects of the proposed dam foundation and reservoir area based on detailed geological, geotechnical and geophysical investigations. The engineering geological investigation conducted in this site includes discontiunity survey, borehole drilling and laboratory testing. The area is characterized by the different rocks such as ignimbrite, tuff, vesicular basalt, residual and alluvial soils with different geological stuctures such as joints, fractures, and columnar joints were encountered. Geotechnically four layers were identified, these are overburden which comprises highly weathered ignimbrite and soil; moderately to slightly weathered ignimbrite, decomposed tuff and highly fractured and slightly weathered basalt rock mass. The result from this study showed that the average uniaxial compressive strength values range from 7.2 MPa for tuff to 109 MPa for ignimbrite rocks and the RQD values range from 24.5-88 and the RMR value ranges from 48 to 70. The estimated modulus of deformation and allowable bearing capacity ranges from 4.2 GPa to 40 GPa and 0.46 MPa to 27 MPa respectively. The permeability result of weak and highly fractured rock mass on the surface ranges from 2.9*10-1 - 4.6*103 cm/s and from falling head test at a shallow depth of the foundation showed pervious and from packer test result the lugeon value ranging from 0-2. Generally the permeability decreases down a depth.The soils found in the study area were dominated by clayey silt soils and they have a plasticity index ranging from 12.8% - 47%. The permeability of soils determined from falling head laboratory tests ranges from 1.12*10-6 cm/s to 7.25*10-5 cm/s. From slope stability analysis, both left and right abutment is stable, however both are unstable under fully saturated condition. Based on the findings of this study, ground improvement techniques have been suggested including consolidation grouting. For the water tightness and strength problems curtain and consolidation grouting were recommended in the proposed dam site, while bench stabilisation, changing the slope geometry, drainage holes, and shotcrete systems were recommended for slope stability issues.