Application of geophysical and geotechnical investigations in engineering site evaluation

Geophysical and geotechnical studies were conducted at a proposed Switch station facility for Telecommunication at a site in the south-eastern part of Nigeria. The aim of the study is to evaluate the sub-soil conditions and electrical properties of the soil which may have effect on the proposed mast and switch facilities system. The geophysical investigation involved the Vertical Electrical Sounding (VES) technique using the Schlumberger configuration and a geotechnical investigation. A total of sixteen (16) VES stations were occupied within the study site. The geotechnical study involved Boreholes drilling as well as Cone Penetration Tests (CPT). A total of six (6) CPTs and three (3) Boreholes drilling were utilised within the study area. This was done to provide controls on the geophysical interpretation. Four subsurface layers were delineated within the study area which include: the topsoil (mixture of sand, silt and clay), coarse sand, clayey sand and sand. This correlated with the sub-soil investigation. The study area is underlain by a stratum of medium stiff to stiff lateritic clayey/silty sand to the depth of about 20 m as explored by the Borehole. The choice of foundation construction for the proposed structure must take care of the settlement characteristics of the clayey material. The subsurface layers up to a depth of 5 m is of moderate to high resistivity values (> 180 ohm-m) and it may not serve as a good electric earthing material, therefore there is a need to improve the subsurface conductivity of this layer most especially within the area where the electrode for the earthling system will be buried.