Seismic Refraction and Electrical Resistivity Investigation of Building Failure: A Case Study

Abstract

Geophysical methods involving electrical resistivity and seismic refraction were adopted to investigate the cause(s) of failure of main library complex of the Olabisi Onabanjo University, Ago-Iwoye, South-Western Nigeria. The geoelectric section generated from geosounding data revealed four geologic layers. The topsoil has resistivity values varying from 128Ωm to 220Ωm and thickness between 0.9m and 2.4m. The second layer is composed of lateritic clay with resistivity values varying from 238Ωm to 410Ωm and thickness between 2.5m and 5m. The third layer is composed of weathered basement with resistivity values ranging from 56Ω-m to 88Ω-m with thickness between 9.1m and 13.5m. The fourth layer is fresh basement with resistivity values between 1110Ωm and 1200Ωm. The depth to the rock head is between 14.1m and 18.6m. The 2-D subsurface imaging revealed that the foundation soil is composed of clayey materials with resistivity values ranging from 4.3Ωm to 80Ωm, between 0.7m to 5m which correlate well with the geoelectric section. The geovelocity layers’ characteristics substantiate the electrical resistivity imaging and vertical electrical sounding results as it revealed a 4-layer model namely: topsoil, laterite and basement bedrock as the weathered layer is blinded. The topsoil has average velocity 486m/s which is diagnostic of weak/unconsolidated materials presumably clayey materials and average thickness of 2.2m. The sub-weathering/ laterite has average velocity of 1506m/s and average thickness of 7m. The third layer is the bedrock with average velocity of 2292m/s and characterized with a displaced parallel time segment indicating fault within the bedrock. Geotechnical analysis of the subsoil sample revealed that the soils have plasticity index and strength values that are typical of clay. From the result, the building failure observed as cracks and foundation subsidence may have been caused by incompetent foundation soils. It is noteworthy that such faulted zone delineated within the bedrock is also inimical to the building foundation.