Engineering Geological Evaluation of Bowa Dayole Dam Site and its Reservoir Area, North Shewa Zone, Oromia, Ethiopia

Abstract

Food security is a crucial challenge for developing countries like Ethiopia, where weather variations and droughts threaten food and water supply. The Bowa Dayole dam site is a masonry gravity dam which is mainly planned to irrigate the farmland downstream. In spite of this, the dam site is subjected to engineering geological issues due to the presence of highly fractured and weathered aphanitic basaltic rock and weak unwelded to welded tuff rock mass at the dam foundation. To address these issues, engineering geological evaluation of the dam site and its reservoir area was conducted on the rocks and soils of the site. In order to achieve these aims, the geological mapping, discontinuity surveying, in-situ strength test analysis, core drilling, in situ permeability test and sampling and laboratory analysis of the samples were carried out. From geological mapping, the main lithological unit of the study area were colluvial and alluvial soil deposits, aphanitic basaltic rock, and tuff unit with various degree of fracturing and weathering. The unconfined compressive strength of these rocks’ ranges from 0.16MPa to 146.88MPa and 22.13MPa to 194MPa as determined from the point load index test and Schmidt hammer rebound values which indicated very low to high strength. The Rock Quality Designation of subsurface indicated poor to excellent rock mass quality. The packer test results revealed that the hydraulic conductivity with representative Lu values of less than 1 indicated that excessive leakage at the dam foundation and abutments would not be expected. The soils found at the reservoir area were mostly covered by clayey silt soils with plasticity index values ranging from 13.84% to 26.1% with medium to high plasticity of soil and hydraulic conductivity values ranging from 1.055×10-3 cm/sec to 9.0343×10-6 cm/sec indicated semi permeable to impermeable of soil mass. The allowable bearing capacity of aphanitic basaltic rock at the dam abutment and dam foundation ranges from 0.1MPa to 22.19MPa and 0.54MPa to 6.77MPa respectively. The allowable bearing capacity of tuff rock at the dam foundation ranges from 0.037MPa to 0.11MPa. Due to the lower bearing capacity of tuff rock unit under the dam foundation, the total settlement along the dam foundation was expected but this issue can be compensated by the even load distribution by the aphanitic basaltic rock and proper foundation design. The slope stability analysis revealed that the slope sections were unstable during the fully saturated static and dynamic condition. This research recommends further detailed study of soils at the reservoir area, proper load distribution, and dam foundation design, additional evaluation of design and implementation of remedial measures such as rock bolts and shotcrete.