Application of Integrated Geotechnical Investigation and 2D-Geoelectrical Tomography for Engineering Site (A Case Study of Ugbomro)

The current study was designed to find a permanent solution to engineering challenges involving building cracks and associated structural failures. An integrated 2-dimensional geo-electric tomography and geotechnical investigation were carried out in Ugbomro, Effurun, Delta state to aid in unravelling the causes of frequent building collapses in this fast-developing suburb, located in the Niger Delta region of Nigeria. Soil samples obtained for geotechnical investigation were tested and analysed to complement the resistivity survey using a combination of the Wenner and Dipole-Dipole arrays. This integrated approach resulted in more comprehensive data analysis and interpretations to strengthen contemporary knowledge in the study area. Wenner array provides good resolution of vertical and lateral changes in subsurface resistivity, supporting adequate delineation of horizontal structures such as sedimentary layers while the Dipole-Dipole array aids in mapping subtle subsurface features such as faults, joints and fractures and the associated geometry which could pose a threat to structural foundations and buildings within the study area. Geotechnical investigation involved drilling two boreholes to a depth of 15 metres each with samples taken at 0.8m vertical sample rate bringing the total samples collected to 32. For the electrical resistivity surveys, a spacing range of 5-30m was used with profile lengths of 100m in three different locations for both array types. Geotechnical investigation revealed predominant clayey sand composition from the surface to depths of 14m in BH 1 and 11m in BH 2 followed by predominant sand formations at the respective depths. Shallow foundational analysis carried out on both the chosen sites revealed allowable load-bearing capacities of 54 and 77kN/m2 for footing of sizes 0.5-2.0m and founded at depths of 0.5-2.0m. For the electrical resistivity survey, the Wenner array results ranged from 6.5-1199 Ωm, 27-1199Ωm, and 52.9-1499 Ωm at locations A, B and C respectively with an average RMS error of 0.213 while the Dipole-Dipole array results ranged from 98-7560Ωm, 82-7980Ωm and 3-8503Ωm at locations A, B and C with an average RMS error of 0.39. The profile maps and resistivity profiles show similarities in the lithological structures delineated using the borehole logs and the resistivity surveys, which revealed clayey sand and sand are predominant formations within the general location.