An experiment to check the accuracy of non-destructive surface electrical resistivity technique in delineation of foundation beds within 5m from ground surface

Abstract

A shallow sub-surface electrical resistivity survey (ERS) was carried out at Shaukat Khanum Memorial Cancer Hospital and Research Centre (SKMCH&RC), Peshawar, Pakistan. The main objective of the survey was to capture a clearer picture of the near-surface geologic sequence to study geophysically the attributes (depth & thickness) indirectly of particular horizon lying within five meters beneath the earth's surface for an engineering study supported by actual values (bore hole data and measured values at field). Vertical electrical sounding (VES) data was acquired using Shlumberger configuration with maximum current electrodes separation (ABmax) of 16 feet in order to resolve clearly the target geoelectric layer in the very immediate vicinity of exposed geologic cross section of an excavation designed for construction of basement of massive civil structure (SKMCH&RC). Geophysical electrical resistivity technique mapped the particular layer's depth and associated thickness having values of 4.42m and 2.87 m respectively. VES results (depth & thickness) were validated by the actual field values obtained from measuring tape as well as borehole information of the studied site. Comparison showed that geophysical electrical survey technique imaged the target geologic layer accurately with uncertainty of 0.3 feet only. This study deduced the presence of inhomogeneous near-surface media covering the depth range (< 1 - 5m) where building foundations are usually placed. The error of 0.3 feet in estimation of both parameters guarantees the successful applicability of said geophysical tool in near-surface engineering geophysical investigations with high accuracy in non- destructive, cost effective and faster way. One of the objectives of the study was to introduce ERS technique and encourage utilization of other geophysical tools for such sort of engineering works at Pakistan to avoid any geotechnical hazard.