Mountain Top University Library

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
xiv
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.