GEO-ELECTRIC ASSESSMENT FOR FOUNDATION STUDIES OFMULTISTOREY STRUCTURESWITHIN DEEPER LIFE CAMPGROUND, AKURE.

Abstract

An integrated geophysical investigation involving Very Low Frequency Electromagnetic (VLF-EM), and electrical resistivity methods using dipole–dipole array and Schlumberger Vertical Electrical Sounding (VES) techniques were conducted at Deeper Life Campground, Ipinsa, Akure, with the aim of developing a conceptual model that will enhance the knowledge of the subsurface geology viz-a-viz foundation integrity for building within the area. In order to achieve this a total of eleven traverses were established in approximately NW-SE, NE-SW, N-S and E-W orientations with station interval of 5m and traverse separation of 10m. One hundred and forty eight (148) station positions were occupied in all for VLF-EM profiling while two hundred and sixty seven (267) stations were occupied for dipole-dipole configuration. Also, fifty eight (58) VES stations were occupied across the study area with current electrode spacing (AB/2) varying from 1 to 64m. From the interpretation of VLF-EM anomalies using Hilbert transform, Fraser technique and Q-Factor,conductive zones were mapped while a total of 32 conductive zones were mapped using Amplitude analytical technique. However, variation occurred from technique to technique.The VES result delineated four major geo-electric layers within the study area. The top soil (resistivity varies from 26 to 300 Ωm and thickness ranges from 0.5 to 2.5 m); weathered layer (resistivity varies from 15 to 579 Ωm and thickness ranges from 1.3 to 14.8 m), fractured bedrock (resistivity varies from 585.3 to 777 Ωm and thickness ranges from 2.4 to 19.6 m) and fresh bedrock with resistivity 974 to 6477 Ωm and depth to bedrock 2.4 to 16.6 m). The dipoledipole results also showed the linear features. From the geophysical investigation, two major causes of potential failure in the area were identified, these are; failure precipitated by differential settlement and failure initiated by geologic features such as fractures and faults. Based on geological and geophysical investigations, foundation integrity map of the area was done using Spatial Analysis tool in Multi-criteria Decision Analysis, approach of Analytical Hierarchy Process (AHP). The consistency of judgments was checked by obtaining the consistency ratio which gives a value of 0.08 indicating that the judgments made were consistent and unbiased. The conceptual model classified the engineering integrity of the study area into poor, moderate, weak and good. A validation of the engineering conceptual model was done by comparing the result of the validation traverse and conceptual model rating with the result of the major traverses within the study area. The high percentage agreement of 75% obtained from the validation shows that the integrity map produced is accurate and reliable