ENGINEERING SITE INVESTIGATION OF SOIL COMPETENCE WITHIN MOUNTAIN TOP UNIVERSITY’S PERMANENT SITE AT MAKOGI-OBA, OGUN STATE, USING GEOPHYSICAL METHODS

Abstract

Geophysical methods have been used to investigate the competence of the near surface soil at Mountain Top University’s permanent site in Makogi-Oba, via Ibafo, Lagos-Ibadan Expressway, Ogun State, Nigeria with a view to determining its suitability for construction purposes. The methods adopted include the 2D electrical resistivity tomography, 1D Vertical Electrical Sounding, 2D seismic refraction tomography and the multichannel analysis of surface wave. The 2D ERT was first conducted and this was followed by the Vertical Electrical Sounding at some selected points on the 2D ERT profiles. This was followed by the seismic refraction tomography survey before the multichannel analysis of surface waves was finally done. The 2D electrical resistivity imaging delineated between three to four layers which were the clayey top soil, a clayey sand layer, a low resistivity clay and another clayey sand layer. Maximum depth probed was 49.7 m. The Vertical Electrical Sounding delineated four geoelectric layers which were interpreted as clayey topsoil, a clay layer, clayey sand layer and a bottom clay layer. From the vertical electrical soundings, top soil resistivity and depth ranged from 12 to 51 ohm-m and 0.4 to 0.8 m respectively, second layer resistivity and depth ranged from 3 to 7 ohm-m and 1.5 to 5.2 m respectively, third layer resistivity and depth ranged from 13 to 163 ohm-m and 1.6 and 9.1 m respectively. The fourth layer resistivity varied between 3 to 46 ohm-m. Though just relatively competent, the third layer is the most competent layer delineated and depth to this layer ranged between 2.2 and 5.6 m. On the seismic survey models, only two distinct layers were delineated with the discrimination property being the degree of consolidation of the earth materials. Seismic compressional wave velocity, shear wave velocity, bulk density, compressibility, bulk modulus, shear modulus and rock-mass quality within the first layer ranged between 511 and 1500 m/s, 175 and 185 m/s, 1.474 and 1.929 gcm-3, 0.235 x 10-6 and 3.150 x 10-6 Pa, 0.318 x 106 and 4.262 x 106 Pa, 52,349 and 55362 Pa, and 0.001026 and 0.01 respectively. Within the second layer, seismic compressional wave velocity, shear wave velocity, bulk density, compressibility, bulk modulus, shear modulus and rock-mass quality ranged between 1042 and 1750 m/s, 230 and 240 m/s, 1.761 and 2.005 gcm-3, 0.167 x 10-6 and 0.546 x 10-6 Pa, 1.832 x 106 and 5.993 x 106 Pa, 96519 and 110728 Pa, and 0.003484 and 0.01778 respectively. The study identified the clayey sand layer as the most competent lithologic layer within the near subsurface sequence and concluded that the near surface strata in their present state are not competent to bear the load of heavy engineering construction. It is recommended that a pile foundation which could be anchored in the clayey sand layer be considered in case the site is to be developed. It should be noted however that before any construction of any sort should be done at this site, the mechanism to take care of flooding that occurs when the Ogun river dam is spilled out should be put in place.