Subsoil Competence Evaluation of Proposed Sites for Underground Utilities Using Non-destructive Integrated Geophysical Surveys and Geotechnical Method

One common cause of structural failure is inadequate soil analysis resulting in poor foundation. Therefore in order to prevent failure occurrence of a proposed communication mast and its accessories to be buried within the subsurface at a depth not less than 5 m at Isuada and Iyere towns of Ondo State, Nigeria integrated geophysical methods involving magnetic, very low frequency electromagnetic, and electrical resistivity methods were complemented with geotechnical laboratory tests adopting the British Standards methods, with the aim of evaluating the suitability of the subsoil within the chosen sites to harbour the structure. The % of fines (clay content) of the samples was generally greater than 20% with Low/Intermediate swelling potential. All the sampled soils exhibited low moisture content in their natural state (less than 10%). The geophysical results showed that the subsoil generally composed of sandy clay, clayey sand grading into laterite with thickness of 10 m and 5 m at locality 1 and 2 respectively. The results of the geotechnical analyses showed high shear strength, cohesion and compaction characteristics. Therefore considering the results of the investigation, the proposed structure can be constructed under VES 1 and 2 at locality 1 at a maximum depth of 10 m - 20 m, but fractured zone below VES 2 must be put into consideration during the design process of the proposed structure. At locality 2, construction of the structure under VES 4 could be possible provided the soil is stabilized with competent material to a depth of 15 m. Therefore geophysical investigations combine with the geotechnical results allow excellent geo-mechanical knowledge of the basement up to 10-20 m depth.