Site investigation of soil competence by electrical resistivity and refraction seismic methods at a proposed building—a case study from Nigeria

Electrical resistivity and seismic refraction geophysical methods have been used to investigate the competence of the near-surface soil at the proposed College of Engineering, Mountain Top University’s permanent site, Makogi-Oba, Ogun State, Nigeria, with a view to determining its suitability for construction purposes. The techniques adopted were the 2D electrical resistivity tomography, 1D vertical electrical sounding, 2D seismic refraction tomography, and the multi-channel analysis of surface wave (MASW). 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 multi-channel analysis of surface waves was finally done. The 2D electrical resistivity imaging delineated between three to four geoelectric layers which were interpreted as the clayey topsoil, a clayey-sand/sand layer, a low-resistivity clay, and a bottom clayey-sand/sand layer. The maximum depth probed was 49.7 m. The vertical electrical sounding delineated four geoelectric layers which were interpreted as thin clayey topsoil, a clay layer, a clayey-sand/sand layer, and a bottom clay layer. From the vertical electrical soundings, topsoil 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, and third layer resistivity and depth ranged from 13 to 163 ohm-m and 1.6 to 9.1 m respectively. The fourth layer resistivity varied between 3 and 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 tomography 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 ms−1, 175 and 185 ms−1, 1474 and 1929 kgm−3, 0.5661 × 10−9 and 1.0254 × 10−9 Pa, 0.90146 × 109 and 1.69657 × 109 Pa, 49.95 and 55.36 MPa, 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 ms−1, 190 and 245 ms−1, 1761 and 2005 kgm−3, 0.1925 × 10−9 and 0.4438 × 10−9 Pa, 2.1096 × 109 and 5.09943 × 109 Pa, 65.16 and 107.62 MPa, and 0.003484 and 0.01778 respectively. The seismic tomography and MASW results confirmed that the top geoacoustic layer is loosely consolidated while the underlying layer is a more consolidated earth material which is predominantly clay. Based on the geoelectric and geoacoustic parameters, any competent material could not be established within the subgrade soils. The clayey-sand/sand layer delineated as the third layer on the VES is recommended for laboratory geotechnical testing to determine if it can support light to medium engineering structures.