Electrical Resistivity Characterization of Peat and Clay Profiles at a Suburb of Ota, Southwest Nigeria

Abstract

Typical soft soils such as peat and some clay types have long been of great interest in geotechnical engineering as a result of their deficient hydraulic and mechanical qualities. These soils are prone to volumetric change and collapse especially in wet conditions and when loaded. Due to the need for expansion of modern cities, highways and roads have occasionally had to pass through locations underlain by pockets of these collapsible soils. In environments such as this, assessing and evaluating subsurface conditions before the start of engineering work becomes crucial. A geophysical survey involving 2D electrical resistivity imaging (ERI) and vertical electrical sounding (VES) techniques was conducted to map the spatial distribution of peat and clay zones at Koro Otun, in the vicinity of the Idiroko-Ota Highway. Twelve acquisition layouts consisting of 40 sounding stations and 12 resistivity imaging traverses were occupied using Schlumberger and Wenner electrode configurations respectively.  The results obtained reveal the presence of a very thin topsoil layer at a depth of less than 1 m in almost all surveyed locations. Peat soil characterized by resistivity and thickness in the ranges 8.8 – 9.7 Ωm and thickness 6.2 – 17.8 m respectively, was delineated at 6 locations (15 %) along 3 traverses at shallow depths of 7.8 – 24.7 m. Clay with resistivity ranging from 10.3 to 47.4 Ωm and thickness range of 1.9 – 34.8 m has more occurrences at 21 sounding stations (53 %) across 9 traverses at varying depths of 2.4 – 39.2 m, with 11 stations indicating the absence of both peat and clay. Less competent sandy clay lies beneath some places, while more competent sand or clayey sand lies beneath a few others. Deep-lying clay zones at depths greater than 20 m but less than 40 m were delineated at a few locations. Both peat and clay zones occurred mostly in the second and third subsurface layers, except at five sounding stations where clay occurred as the last layer. ERI spatial distribution depicts soft soil zones in the form of ridge/mound, trough/depression, horizontally stratified column and trapped bed along several traverses. ERI also reveals laterally extended but discontinuous distribution of clay and pockets of peat zones at a few identified locations. Even though the roads in the Idiroko border town and its surrounding areas are exposed to huge vehicular traffic, primarily from heavy-duty trucks, their lifespan and durability can still be increased if appropriate subsurface geophysical investigations are given proper consideration and their recommendations implemented before the building of roads, bridges, and other transportation facilities.