Site Characterisation Using Electrical Resistivity Methods: A Case Study of Iba Nursery/Primary School, Ojo, Southwest, Nigeria

Two-dimensional (2D) resistivity imaging and vertical electrical sounding (VES) were integrated to map the subsurface lithology within Iba Nursery/Primary School, Ojo, Southwest, Nigeria, with a view to ascertaining the thickness and stratigraphy of the beds and their implications on engineering structures. Ten vertical electrical soundings, covering the entire area were conducted using Schlumberger configuration. Three 2D horizontal profiling (Wenner array) was used to qualitatively interpret the geoinformation of the lithological nature of each geoelectric layer within the study area. The VES data were processed and inverted using master curves and computer software called WinResist, while the 2D inversion was done using Diprowin. Four to five subsurface layers comprising of topsoil, clayey sand, sandy clay, sand and clay were delineated. Qualitative interpretation of VES data revealed five QHA, one QH, one KQH, one KHK, one KHA and one HA curves. The investigation of the study area has revealed that shallow foundation may not be feasible for a massive engineering structure because of the presence of clay materials that are close to the surface. But for small and medium engineering structures, the second layer is found competent due to the presence of sand with relative thickness and high resistivity value that vary from 1.2 m - 13.9 m and 88.5 Ωm – 399.4 Ωm respectively. However, good prospects exist for heavy engineering structures in the study area where the sand formation is relatively thick (19.8 m – 50 m) and has favourable resistivity values ranging between 466.2 Ωm and 560 Ωm.

Thus, the application of 2D resistivity imaging and VES has revealed both the lateral and vertical variations in depth to competent sand layers within the study area, hence providing a useful guide for the site engineers in designing appropriate foundation structures.