Mapping of Competent Soil Layers for Building Infrastructure Using 2D Electrical Resistivity Technique at Amansea, Anambra State, Nigeria

Abstract

Mapping of competent soil layers is a critical aspect of geotechnical site investigation for engineering construction, foundation investigation and design. The study employed six traverse lines of 2D electrical resistivity tomography (ERT) technique to delineate the subsurface conditions, and determine the thickness and depth to competent soil layers within the construction site at Amansea, Anambra State. The data was acquired using multi-electrode resistivity machine with Wenner array of equal electrode spacing of α = 3. The acquired apparent resistivity data were processed and inverted using RES2DINV software to generate 2D inverse resistivity sections of the studied areas. The interpreted resistivity sections revealed variations in subsurface lithology, including topsoil, sand clay and highly saturated zones and competent zones characterized by relatively high resistivity values. The top soil have low-resistivity values between 3.21 Ωm and 438 Ωm indicating weak, clayey, or highly weathered materials with depth range of 0 – 5 m. The intermediate layer varied across the site with resistivity range of ~ 247 - 2854 Ωm at 5 – 12 m depth, reflecting lateral heterogeneity in subsurface conditions. This transitional unit exhibits mixed sand and clay composition, providing moderate geotechnical competence. A laterally persistent, high-resistivity sandstone body (3580 – 109,354 Ωm) that thickens toward the central and eastern parts of the site at ~ 8 – 18 m depth. Information obtained from these electrical resistivity model correlated effectively with the information from the borehole log drilled around the study area. The geological model indicates that the near-surface clay-rich horizons possess poor geotechnical properties, whereas the underlying sandstone forms a competent bearing layer suitable for deep foundations. The study demonstrates that the 2D ERT technique is an effective, fast, and non-invasive tool for delineating competent foundation materials and provides valuable information for engineering site assessment and foundation planning.