Geotechnical And Engineering Geological Investigation Of Iyandafaro Dam Site Konso Southern Ethiopia

The Iyanda catchment covering an area of 658km2 is studied for the selected dam site. Landrnuse/ Land cover, soil and slope of the area were mapped, so that erodability for vegitation soilrncomplex is computed. Geology and hydrometheorology of the catchment was studied andrnengineering propeliies of the mapped rock and soil units are discussed. The geology andrnengineering geology of the catchment was mapped at the scale of 1:50000 and the engineeringrngeology of the reservoir at 1 :2000. The area is dominantly covered by volcanics(basalts andrnmetamorphic (gneiss) at relatively elevated areas and QuaternaIY deposits at flat plain areas.rnAreal mean annual rain fall was analyzed using Arithmetic mean and Thiessen weightedrnaverage and a value of 830.6mm and 830.5mm was obtained. Piche evaporimeter installed atrnKonso was used for the evaluation of the storage site(1680.3mm) using a pan coefficient ofrn0.85 due to the high evaporation rate at the site. The potential evapotranspiration calculated byrnthe Thornthwaite method is 1534.3mm, that is almost twice of precipitation and is typicalrnproperties of arid to semi arid climate. The soils of the study area never reaches to fieldrncapacity in general, due to this, no surplus is generated. The possible method of ground waterrnrecharge and slllface IUn off is rain fall above average and storm IUn off. The peak storm 11mrnoff for 100 year return period was 86mm. The selected reservoir site has storage capacity ofrnmore than 16Mm3 Water samples taken from the study area and its environs show decreasingrntendency from the Lake to the study area with respect to EC, TDS and dominating cations likernsodium.rnThe engineering geological mapping was carried out based on material strength, degree ofrnweathering and fi'acturing for rock units and gradation and Atterberg limit for soil material.rnFour geological units are mapped at the dam site: Gravel and sand at the river, clay and silt atrnthe flat plain area, well-graded 'soil at the right abutment and slightly fractured basaltic rocks atrnthe left abutment. The geology of the side channel spill way, selected at the left side is arncontinuation of the left abutment, which is stable, slightly weathered and fractured basalt. Thernmain canal lithology is fine grained clastics (clay and silt) which is impermeable lithological unit. The consolidation test of clayey silt soil analysis shows that, the soil is stiff with maximumrnseteliment 7mm for the aplied stress of3200kN/m2rnThe permeability coefficient computed fi·om the consolidation test is 0.7 x 10.7 cm/s. Both thernindex and engineering properties of the dam site shows that, the clayey silt soil and well gradedrnsoil at the right abutment are stable and watertight. Hence the foundation depth should be onrnthis layer. But at the main river and its levees, no watertight unit was mapped to 7m depth,rnthough the geophysical investigations show the bed rock symptom at 13.5m depth.rnThe study area catchment rock units can be classified in to five strength zones:rnZone 1 - Moderately fi·actured, slightly weathered to fresh basaltrnZone2 - Moderately to highly fractured, slightly to moderately weathered basaltrnZone 3 - Moderately fractured, slightly weathered to fresh gneissrnZone 4 - Moderately to highly fractured and weathered gneissrnZone 5 - Moderately to highly fractured and weathered gabbrosrnBased on strength classification zone 1 and zone 2 are very strong to extremely strong rocksrnand zone 3, 4 and 5 are strong to velY strong rocks. The approximate range of unconfinedrncompressive strength of extremely strong, very strong and strong rocks used for tllisrnclassification is: > 250, 100 - 250, and 50 - 100Mpa respectively.rnThe results of the electrical sounding survey presented in the form of geo-electric sectionrnwhich shows the distribution of layer resistivities and thickness. This is believed to be a closerrnapproximation to the actual geologic setting in the sub surface. The YES and profilingrninvestigation show that the over burden (soil) is thick at the flat plain areas and thin at thernabutments

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