GEOELECTRICAL AND GEOTECHNICAL EVALUATION OF SUBSOIL COMPETENCE IN PARTS OF ILARAMOKIN, SOUTHWESTERN NIGERIA USING MULTIPLE MODELS

Abstract

Recent cases of incessant collapse of buildings associated with geotechnical competence in Nigeria brought about the importance of adopting scientific approach involving multicriteria decision analysis in geotechnical competence evaluation. Ilaramokin is a fast-growing town due to its proximity to The Federal University of Technology, Akure and the establishment of a private University in the town. Construction of buildings without proper understanding of the subsurface geotechnical competence is unsafe. Thirty (30) vertical electrical soundings, eighty-six (86) static water level data and a geological map were utilized in the evaluation. The VES results were presented as topsoil, weathered layer and bedrock resistivity maps and Iso-resistivity maps of 0.5, 1.0 and 2.0 m depth to the surface. Results delineated three to five geoelectrical layers from type-curves that comprised A, H, K, KH, QH and HKH. The resistivity of the topsoil, weathered layer and bedrock varied from 48 - 701, 31 - 1065, 14 - 139, 132 - 6582 Ωm respectively while their thickness varied from 0.4 - 4.1, 1.3 - 11.6 and 4.0 - 20.1 m in the three upper layers respectively. The VES results (weathered layer, bedrock and depth slice: 0.5, 1.0 and 2.0 m) were then combined with static water level and the geology using both TOPSIS and AHP models to produce geotechnical competence maps. Consistency test and grain size analysis were carried out on ten (10) collected soil samples to validate the geotechnical competence model maps produced. The validation showed that geotechnical model maps produced from AHP and TOPSIS models have 90 % and 80 % correlations respectively with consistency tests and grain size analysis results.