Integration of geophysically derived parameters in characterization of foundation integrity zones: An AHP approach

Abstract

An integrated geophysical investigation involving Very Low Frequency Electromagnetic (VLF-EM), and electrical resistivity methods using Schlumberger Vertical Electrical Sounding (VES) techniques was conducted at Deeper Life Camp ground, Ipinsa, Akure, Southwestern Nigeria with the aim of developing a model map that will enhance the knowledge of the subsurface geology viz-a-viz foundation integrity for appropriate location of building within the study area. The effect of four factors including soil apparent resistivity at different depths, depth to bedrock, geology and fracture density on engineering foundation were considered. In order to achieve this, a total of ten traverses were established in approximately NW-SE, NE-SW, N-S and E-W directions with station interval of 5 m and inter-traverse separation of 10 m. One hundred and forty eight (148) station positions were occupied in all for the VLF-EM profiling. Also, fifty four (54) VES stations were occupied across the study area with current electrode spacing (AB/2) varying from 1 to 65 m. Hilbert transform, Amplitude analysis, Fraser technique and Q-Factor performed on the VLF-EM method assisted in the delineation of conductive zones that could be inimical to foundation integrity. The VES result delineated four major geo-electric layers within the study area which are: topsoil, weathered layer, fractured bedrock and the fresh bedrock. The thickness of the layers generally ranges from 0.5-19.6 m. Based on geological and geophysical investigations, foundation integrity map of the area was produced using the Multi-criteria Decision Analysis, approach of Analytical Hierarchy Process (AHP). The model map classified the foundation integrity of the study area into very low, low, moderate, high and very high foundation integrity zones. The competency model at a depth of 3 m is adjudged most suitable for foundation in the study area.