GEOLOGICAL AND GEOPHYSICAL INVESTIGATION OF FOUNDATION SUBSOIL COMPETENCE IN PARTS OF AKURE, SOUTH WESTERN, NIGERIA

Abstract

Multi Criteria Decision Analysis (MCDA) approach was adopted in generating a foundation bed competence map of northwestern part of Akure, Southwestern Nigeria. Two hundred and twenty four (224) Vertical Electrical Sounding (VES) were acquired using Schlumberger array technique with halfcurrent (AB/2)m spread of 1 to 150 m. Five factors influencing subsoil foundation competence were extracted from the VES, existing geological map of the study area and the static water level measurements. These are geology, water level, top soil resistivity, weathered layer resistivity and bedrock resistivity. The geologic map of the area identified two rock types namely migmatite-gneiss-quartzite complex and the charnockites. From the topsoil resistivity, weathered layer and bedrock resistivity maps it was observed that top soil resistivity range from 5 to 860 ohm-m, while the weathered layer resistivity varies from 3 - ∞ ohm-m and the bedrock resistivity values ranges from 7 - ∞ ohm-m. The static water level map indicated that the northwestern, southern, and central parts of the area are characterized by very low to low competence. These maps were synthesized using an additive model involving five parameters; Geology, water Level, topSoil resistivity, Weathered layer resistivity and Bedrock resistivity (GLSWB). The GLSWB-index was computed for all the VES locations and was subsequently contoured to produce GLSWB-model map which classified the area into three competence zones; moderate, high and very high competent. The low competent zone account for about 15 % of the area and are restricted to some parts of the northwestern, southern and southeastern parts of the study area, while the moderate competent zone covers about 60% of the area and they lies mainly within the western part of the study area. The high and very high competent zones cover about 25 % and are located essentially within the eastern and some pockets of western part of the study area. Overall it can be concluded that the area is moderately competent and suitable for erection of building and other engineering structures.