SUBSOIL CHARACTERIZATION USING GEOELECTRICAL AND GEOTECHNICAL INVESTIGATIONS: IMPLICATIONS FOR FOUNDATION STUDIES

Electrical resistivity tomography (ERT) has been combined with geotechnical techniques such as cone penetrating tests, standard penetrating test and laboratory tests for detailed characterization of near-surface strata. This approach can be very helpful in conducting preliminary investigations towards a robust foundation design at a building construction site. Two ERT lines were conducted for 2D geoelectrical resistivity measurements using Wenner array configuration in combination with four cone penetrating data. Through the inversion of ERT data, two geoelectrical layers were interpreted to be loose silty sand and compacted clayey sand lithological units with the resistivity values ranging 50 – 280 Ωm and 10 – 74 Ωm respectively. A watersaturated portion with resistivity values ≤ 3 Ωm due to lagoon-water incursion was equally observed at the base of the second clayey sand layer in ERT line T2. The average cone penetrometer (CPT) value of about 110 kg/cm2 (11 MPa) with an average SPT ‘N' value of 25 was measured between 6.75 – 30.0 m, indicating that the geomaterials within this depths range are of good geotechnical properties. Laboratory tests conducted on the representative soil samples at 3.75 m depth gives a moisture content (MC) of 66%. This is attributed to the clay contents within the soil samples. The Liquid Limit (WL), Plastic Limit (WP) and Plasticity Index (PI) tests of the soil samples gives 84%, 30% and 54% respectively. The results of the proposed approach, encompassing both geophysical and geotechnical methods has helped to steer the choice of the foundation for the investigated building towards a pile-type foundation rather than a shallow one. The pile foundation will cause the higher loadings to transmit the loads to a stable soil layer within the subsurface.