Geotechnical Competence Assessment and Groundwater Prospect Deductions from Geoelectrical Sounding around Idanre Local Government Area of Ondo State, Southwestern Nigeria

Sixty vertical electrical soundings with maximum electrode spacing of 100 m; and seventy static water level measurements were undertaken in Idanre Local Government Area of Ondo State, Nigeria with the aim of assessing the engineering competence of the soil material to host civil engineering structures; and also establish notable point/location with groundwater prospect for borehole drilling and development. Typical sounding curves from the area include H, A, HA, KH, QH, and AA reflecting diverse lithological variations. The H, A, and HA curve types are the most preponderant constituting 30%, 15% and 25% respectively. The A (15%) and AA (7%) curve types suggest subsurface geoelectric configurations apparently favourable for foundation construction, as they showed an increase in resistivity (competence) with a corresponding increase in depth. The QH, KH, HA and H curve types constituting about 78% are favourable for groundwater accumulation and development. The topsoil is generally composed of sandy clay which is moderately competent soil material to harbour civil structures such as road, buildings. Although some degree of settlement is expected especially along traverses where the weathered layer is clayey. The study shows that 70% of the area in the upper 10 m is moderately suitable for civil engineering shallow foundation construction. The weathered layer is generally clay/sandy clay with resistivity less than 200 Ω-m, therefore can serve as earth medium for electrical material. The major aquifer units in the area are combined weathered layer and fractured basement (common in Aponmu and Jimgbe); a weathered layer (prominent in Imolumu, Ayefemi, Onipanu and Ajegunle) and fractured basement aquifer (found in Ajegunle and Odode). The maximum probable drilling depth to these aquifers varies between 15 m to 30 m. Areas with high static water level are characterized by thin overburden thickness (shallow depth to basement rocks). Also, areas with low static water level have corresponding thick overburden thickness (deep depth to basement rock/high depth of weathering). However substantial settlement may occur in areas with high static water level which could exceed the tolerable limit and threatens the integrity of structures during the raining season, coupled with the clayey nature of the weathered layer in many places.