FOUNDATION CHARACTERIZATION IN IKATE AREA SOUTHWESTERN NIGERIA USING ELECTRICAL RESISTIVITY TOMOGRAPHY AND CONE PENETRATION TEST

Abstract

The establishments of the nexus between the underlying geology and engineering properties are fundamentals to the safe, economic and serviceable designs of civil engineering structures. Cone Penetration Test (CPT) and Electrical Resistivity Tomography (ERT) are critical tools in foundation parameterization especially within Quaternary sediments serving as foundation of engineering structures. Both techniques were used to obtained point data and lateral continuity of geologic units within Ikate Area of Lagos Metropolis for foundation parameterization. Measurements which comprising 4 profiles of 2D resistivity imaging using the Wenner electrode configuration; 1 borehole and 12 cone penetration tests (CPT) were taken with Pasi Terrameter, percussion rig and the 10.0 tons penetrometer respectively. The ERT results interpreted using Dipro software were presented in Pseudo-sections while the cone penetration test results analysed with Microsoft Excel were represented as resistance curves. Interpretation from the two methods integrated with the borehole results reveals the presence clay, peat and sand of varying properties. The uppermost layer consists of medium dense sand followed by clay/peat of very low resistivity (< 3.5 ohm-meter) covering about 6. 0 – 8.0m in depth and terminating in another layer of sand (8.0 – 15.0m). The last sandy layer with cone resistance range between 30 – 120kg/cm2 is considered a suitable layer upon which deep foundation may be founded. Shallow foundation capacity estimates at depths 0.2 – 4.0m derived from direct CPT data using three different computation techniques gives value range from 1.7 – 49.4kg/cm2, 0.7 – 15.2kg/cm2 and 0.4 – 12.6kg/cm2 respectively. Deep foundation for skyscrapers within the area are advised to be founded within the competent layer encountered beyond 10.m depth.