Application of Geotechnical and Geophysical Methods for Engineering Site Evaluation of Emure-Ile, Southwestern, Nigeria

Abstract

Subsoil competence evaluation was carried out in Emure Ile area of Ondo State, Southwestern Nigeria, with the aim of evaluating the competence of the subsurface geology in hosting civil engineering structures. Geotechnical investigation involved analysis of six soil samples for mechanical strength and soil classification test. The geophysical methods used were magnetic, Very Low Frequency Electromagnetic and Electrical resistivity. The geotechnical characteristics of the sampled soils showed that the subsoil is clay-sand with medium plasticity/compressibility at moderate compaction and strength characteristics. The magnetic and VLF – EM showed a correlated fairly conductive subsoil indicating a clayey material with few structural features. The VES interpretation results delineated four subsurface layers which included the topsoil, weathered Basement, partly weathered/fractured basement and the fresh basement. The layer resistivity values for the topsoil used in this study ranged from 30 – 5220 ohm-m with layer thicknesses of between 0.5 m and 6.1 m. The depth to bedrock was generally 20 m, but could be thicker at the central part of the town i.e. greater than 20 m. Corrosivity potential of the soil was generally minimal at 10 m; therefore this depth would suit the burial of metallic objects. The competency map of the area classified the area into moderate competence (40%) and low competence (20%). Most parts of the area were underlain by moderate competent subsoil (65%), while low competence (35%) was prominent around the northwestern – southwestern parts of the study area. Therefore this work has showed the importance of integrated geophysical and geotechnical techniques in engineering subsoil characterization.