Geo-electric Investigation of the Cause of Structural Failure Indices on a Set of Administrative Blocks

Abstract

Electrical resistivity imaging has been employed to investigate the causes of structural failure related cracks on a set of administrative buildings in southwestern Nigeria. The goals were to determine the distribution of subsurface geologic lithologies beneath the set of buildings and therefrom infer the relative strengths of such lithologies. Data were collected along four geo-electric traverses using the ABEM1000 Terrameter unit. The dipole-dipole array was utilized with an electrode separation of 5 m. Observed field data were processed and inverted using a 2.5D finite-element modeling inversion algorithm. Results indicate that the northeastern half of the site is underlain by unweathered basement rocks overlain by thin (generally < 4 m) soil cover, while the southwestern region is dominated at depths beyond 4 m by low-resistivity water-saturated clays and some weathering tills. The clays apparently receive abundant supply of water from a seasonal stream channel adjacent to the westerly administrative blocks. The block presenting with the most damage has been built out over the edge of the shallow basement rocks onto these less competent water-saturated clays in the southeastern region resulting in cantilever-style differential settling. A second damaged block is situated entirely on low resistivity water-saturated clays. Differential settling on this block is likely related to the difference in thicknesses of the clays between sites of the north and south facing walls of the structure. Beneath the northern wall, the clays are thinner and could be expected to compress less than at the southern wall. Though uncalibrated to core or log data, the results allowed a first insight into possible causes of structural failure on the buildings and it is hoped that palliative measures will benefit from these results.