Abstract
Geological and geophysical investigations were conducted to assess the competence and structural integrity of the foundation site of the proposed Ife-dam at Kajola Village, Ile-Ife, Southwestern Nigeria. Geological investigation along the two (2) proposed dam axes revealed that the overburden material is loose to dense with angular shearing resistance (ɸ) of 27o to 41o. The soils are predominantly elastic silts; cohesive with considerable strength and stability. Geophysical investigation involving the Schlumberger Vertical Electrical Sounding delineated four (4) lithologies namely: topsoil with resistivity of 69 – 558 Ωm and thickness between 1.5 and 4.0 m; weathered sandy layer with resistivity from 123 – 586 Ωm and thickness between 6.5 and 20.4 m; partially weathered/ fractured basement with resistivity from 60 – 220 Ωm and thickness between 6.5 and 14.0 m; and the fresh basement rock with resistivity from 1337 – 10683 Ωm. There are indications of fractures at a depth of 32 m beneath Axis B extending to Axis A at a depth of 35 m. The subsurface materials are suitable to host a dam. Axis B is more appropriate for the dam axis, although the fracture zone should be factored into the design of the dam to prevent water seepage.
Highlights
Water supply to a community can either be from surface or groundwater sources or both. Surface water sources such as rivers, lakes and oceans occur on the surface of the earth
The geological mapping of the study area revealed that the dam site and its environs are underlain by amphibolite and granite gneiss (Fig. 1)
The geological investigation revealed that the study area is underlain by weakly foliated amphibolite and well foliated granite gneiss
Summary
Water supply to a community can either be from surface or groundwater sources or both. Surface water sources such as rivers, lakes and oceans occur on the surface of the earth. Surface water exploitation for community use involves the extraction of large volumes of water from a river or lake. Various types of dams, such as earth fill dam, rockfill dam and concrete gravity dam, can be constructed across a river. The rockfill dam has rock materials as the major foundation material, while the foundation of a concrete gravity dam is made of concrete material. The topsoil material may not be suitable as foundation material the topsoil has to be excavated in order to achieve an adequate foundation. The depth of excavation can be determined through boring and test pits, field testing of soil or rock and laboratory analysis of representative soil samples
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