Abstract:
A geophysical investigation was carried out around the abandoned Ogbomoso North Local Government Secretariat Complex Building with a view to identifying the cause(s) of the failure of the foundation of the building.
Twenty (20) Vertical Electrical Soundings (VES) were carried out using Schlumberger electrode array. Dipole-dipole 2-D imaging was also undertaken along four traverses each 130 m long. The VES data were interpreted quantitatively using the partial curve matching method and computer assisted 1-D forward modeling. The VES interpretation results were used to generate geoelectric sections. The Dipole-Dipole data were inverted into 2-D subsurface images using the Dippro Software. The resulting 2-D resistivity structures image the subsurface sequence and the structural disposition of the basement bedrock.
The geoelectric sections reveal four subsurface layers which include the topsoil, with resistivity values that vary from 69 – 1161 ohm-m and thicknesses of between 0.4 and 1.8 m. The weathered layer is characterized by resistivity values ranging from 34 –364 ohm-m and thicknesses of between 0.7 and 10.1 m. The partly weathered/fractured basement was identified beneath VES 14 and 19 with resistivity values of 27 – 784 ohm-m and thicknesses of 2.4 – 4.2 m. The basement bedrock has resistivity values of between 724 - ∞ ohm – m with depth to the geoelectric bedrock of between 3.2 and 15 m. The 2-D images delineate three major subsurface layers – a topsoil (generally in blue colour band), the weathered layer (in green colour band) and a basement bedrock (in yellowish/redish/purple
colour band). The resistivity structures identify a major linear feature suspected to
be a fault that cut across the building site. The faulted zone is about20 – 30 m
wide.
It is concluded that the failure of the building foundation may have been
precipitated by differential settlement within the suspected faulted zone.
