Integrated Geophysical and Geotechnical Site Characterisation for Civil Infrastructure Development in Sekondi-Takoradi, Ghana

Abstract

An integrated geotechnical site characterisation was conducted in Sekondi-Takoradi, Southwestern Ghana, to evaluate subsurface conditions for a major institutional infrastructure development project. The investigation focused on evaluating the engineering integrity, shear strength, and ultimate bearing capacity of the foundation soils to support heavy structural loads. Additionally, the study aimed to delineate critical subsurface anomalies, weak zones, and adverse geological structures that could compromise structural stability or threaten foundation longevity. The geophysical method deployed involved the use of electrical methods involving electrical imaging and vertical sounding approaches complemented with the evaluation of the geotechnical parameters of subgrade in the site based on the assessment of soil’s geotechnical indices and engineering qualities of soil samples from five trial pits from the study. Based on the results of resistivity imaging carried out using the Schlumberger dipole-dipole configuration along four traverses, the lithologic profiles of the subsurface geology revealed weathering prognosis depicting three lithologic layering characteristics of highly resistive and very thin topsoil (ferruginous sandstone/lateritic soil); intermediate highly resistive sandstone and underlying bedrock, while 1D geoelectric profiles also revealed highly resistive four lithologic units which conformed well with the 2D resistivity tomographic expressions of geologic sections in the area. The overburden thickness is usually less than 5 m, an indication that the subgrades are generally not up to 5 m in thickness. The 2D resistivity structures suggest that the subgrade materials are highly compacted in collaboration with the 1D resistivity profiles that are diagnostic of highly compacted and acidic sediments that usually possess extremely high resistivity values. Features within the subgrades are typified by undulating surfaces and fractured layers with local depressions at some parts of the site. Based on derived geophysical parameters, the subgrade could be rated as low to moderate integrity to support high load capacity. However, some parts are characterised by weak lateritic soils often associated with seepage paths that are recognised as near-surface fractures or joints in the areas. Such weak zones can have an impact on the bearing capacity of the subsoils; thus, proper measures must be considered before engineering works commence. The soil profiles revealed that the upper layers on the hilly parts of the study area are characterised by dry, loose, and coarse-grained sand, while the lower layer was discovered to be primarily residues from the parental bedrock made of reddish brown and loose fine-grained sandy clay, indicating deep weathering of leached bedrock. However, the upper sections of the slope area were revealed to be transported materials down the hill at the site. The subgrade materials exhibit specific gravity ranging from 2.69 to 2.70 g/cm − 3 , an indication that the soil is rich in quartz., while the ultimate bearing capacity of the subgrades is between 1552.7 kN/m 2 and 1281.7 kN/m 2 , which suggests that the subgrades can withstand allowable pressures between 480.6 kN/m 3 and 410.3 kN/m 3 without failure within the area.