Abstract

Post foundation study for the preliminary causes of cracks on buildings at the Central Bank of Nigeria (old site) Jalingo branch, Northeast Nigeria, was carried out using a geophysical approach and groundwater level monitoring. The geophysical survey method employed was the electrical resistivity method. Vertical electrical sounding (VES) was carried out using a Schlumberger electrode array with a maximum half current electrode separation of 20m, vertical electrical sounding (VES) was carried out. The VES results revealed the presence of three (3) geoelectric subsurface layers in the area, namely clayey-sand, fractured/weathered basement, and crystalline basement. The resistivity of the clayey-sand varies from 45-210 Ωm, while that of fractured/weathered basement varies from 305 - 5858 Ωm and that of the crystalline basement varies from 1000-10075 Ω-m with corresponding thicknesses ranging from 0.5 - 2.3 m, 0.2 - 6 m, 2.6 – 4 m to an infinite depth respectively. The study area's field curves were analysed; H and Q-curve types with 35.7% and 64.3%, respectively. Iso-resistivity delineation at 0.5 m, 1.5 m and 3 m showed that the region with high resistivity corresponding to the crystalline basement is found in the northeastern part of the study area and is most suitable for the foundation. The results show that the investigated building was sited within the incompetent clayey-sand layer with resistivity generally less than 200 ῼm.

Key words: Vertical electrical sounding; Hydraulic head; Resistivity; Geoelectric section; Static water level.