Delineating Depth to Bedrock using Electrical Resistivity

An Implication for Foundation Design in Engineering Site Investigation

Abstract

The present study applied geophysical method to engineering site investigation with the aim to delineate depth to bedrock and determine area that can induce structural failure. Direct current electrical resistivity method using Vertical Electrical Soundings (VES) field procedure and Very Low Frequency Electromagnetic (VLF-EM) profiling techniques were adopted. Twelve (12) VLF-EM profiles and twenty-four (24) VES stations were occupied. The result of VLF/EM was unable to map genuine linear structure because significant peak positive threshold fall below 20 %. Four geoelectric units comprising of topsoil, lateritic/weathered layer, partially weathered layer and fresh bedrock with resistivity values which ranged from 88.4 to 680.3 Ωm, 45.9 to 597.3 Ωm, 27.9 to 1309.1 Ωm, and 625.5 to 16,450.6 Ωm; and layer thicknesses from 0.4 to 4.2 m, 1 to 15.1 m, 2.9 to 15.1 m were delineated respectively. The bedrock configuration presents a highly undulating structural pattern due to differential weathering, with depth to bedrock range between 5.0 and 22.0 m. The study location is underlain with competent earth materials of good bearing capacity and resistivity values generally > 100 Ωm except in the bedrock depressions in the north-east, east and southern flanks where resistivity values are < 100 Ωm. Foundation of engineering structure may be designed to avoid the high-risk zone in the bedrock depressions. Shallow foundation design is recommended for small to medium engineering structures in the study but deep foundation design is recommended in the bedrock depression filled with low resistivity earth materials.