FAULT PRESENCE UNDER A FAILING BUILDING COMPLEX MAPPED BY ELECTRICAL RESISTIVITY TOMOGRAPHY

Abstract

An electrical resistivity tomography supported with Vertical Electrical sounding (VEs) survey was carried out within the precincts of a Faculty building at Olabisi Onabanjo University Main campus, Ago Iwoye, Nigeria. The building is intensively affected by dangerous cracks that cause structural instability a few years after its construction. The survey aimed to image the shallow subsurface structures and determine the possible cause of the structural instability. 2D Resistivity imaging using Wenner array with smallest electrode spacing of 10m and VEs using schlumberger electrode configuration were carried out along four traverses extending about 130 meters at the precinct of the building complex. The acquired data were processed and interpreted spatially to expose the shallow structural setting of the site. Integrated interpretation led to the delineation of hazard zone in the area. This zone is interpreted as fault with vertical and sub-vertical linear features having moved both vertically and horizontally with associated resistivity values as low as 5Ωm. This feature is suggested as the main reason of the rock instability that resulted in potentially dangerous cracking of the buildings. Inferred lithologic units which include silty/clayey soil, sandy clay and the basement were mapped out at the site. compacted lateritic (hardpan) topsoil along traverse 4 (backside of the building), though not a rock unit, constitutes an important factor to the re-construction of geologic unit from ERT model. The possible cause of failure of the building was proposed to result from the underlying fault on which the whole weight of the building rests. It was also discovered that the pre-construction geotechnical investigations possibly failed to capture the zones because they are point investigations that lack continuous imaging of subsurface condition.