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