PRE- FOUNDATION STUDIES USING VERTICAL ELECTRICAL SOUNDING AND SOIL SAMPLE ANALYSIS

Abstract

Subsurface rock properties investigation to categories foundation competent layer for the proposed engineered structure capable of housing offices, lecture halls, and laboratories was carried out using geophysical (GPHY) and geotechnical (GTECH) techniques. Electrical resistivity GPHY technique using vertical electrical sounding (VES) field procedure was utilized in which nineteen (19) VES spots were occupied. Results of the VES revealed 2-D subsurface geoelectric sequences comprising of incompetent overburden layer of about 5 m thick with resistivity value between 10 and 210 ohm-m typical of clay soil material. Beneath this layer are sand formation and fresh bedrock with resistivity range of 750 to 1000 ohmm, which are good formations for foundation. Additionally, according to GPHY studies, western and northern portions of the region are marked by fractures/faults, which could cause building subsidence if foundation is stationed at these points. The results of the analyses of the soil samples taken in five different points in the study area revealed: (1) grain size distribution with 0.075 mm particle size passing of an average of 41% signifying the occurrence of clay, corroborating the presence of clay material within the weathered layer in GPHY investigations; (2) active and expansive nature of the soil with linear shrinkage greater than 8% revealing unsuitability of topsoil for foundation; (3) medium plasticity of soil indicating possible soil swelling; and (4) concomitant GTECH and GPHY results of the topsoil and weathered layer properties. The overburden layer with clayey materials must be removed and areas with geological structures must be taken into account.