Geohazard characterization of subsurface materials using integrated geophysical methods for post foundation studies: a case study

Abstract

Different geophysical approaches including seismic refraction, electromagnetic resistivity with very low frequency and magnetics prospecting method were used to gain a richer and more complete understanding of subsurface environment in delineating the origin of the cracks on the buildings of the Faculty of Social Sciences and Humanities, Federal University Oye- Ekiti. Three (3) profiles of seismic refraction, Five (5) Very Low Frequency-Electromagnetic traverses, twenty-one (21) Vertical Electrical Soundings and three (3) magnetic profiles were occupied and investigated around the distress building. Three major layers were delineated in the study region comprising the topsoil, the weathered/fractured layer and the fresh basement. The VES curve types obtained were A, H, K and Q curves. The buildings were found to be situated within the area of a fairly high concentration of fractures that can aid subsidence in the area and relatively low resistivity zones typical of incompetent clay formation. The geovelocity layers’ characteristics substantiate the electrical resistivity imaging and vertical electrical sounding results as it revealed a 3-layer model namely: topsoil and basement bedrock as the weathered layer is blinded. The topsoil has average velocity 363 m/s which is diagnostic of weak/unconsolidated materials presumably clayey materials and average thickness of 1.7 m. The sub-weathering/ is blinded while the third layer is the bedrock with average velocity of 1183 m/s and characterized with a displaced parallel time segment indicating fault within the bedrock. From the result, the building failure observe as cracks and foundation subsidence may have been caused by the foundation soils that made up of incompetent materials (clay) which could compress on imposing loads by differential settlement. It is noteworthy that geological features such as fractures/faults delineated within the bedrock is also inimical to the building foundation.