Application of 2-D and 3-D Geo-electrical Resistivity Tomography and Geotechnical Soil Evaluation for Engineering Site Investigation: A Case Study of Okerenkoko Primary School, Warri-Southwest, Delta State, Nigeria

Abstract

In the design of building structures, joint efforts must be decided to resolve the depth of competent layers across the intended site to safeguard the durability of civil engineering structures and to avert the disastrous consequences of structural failure and collapse. In this study, an integrated methodology that employed DC resistivity tomography involving 2-D and 3-D techniques and geotechnical-soil analysis was used to evaluate subsoil conditions for engineering site investigation at Okerenkoko primary school, in the Warri-southwest area of Delta State, to adduce the phenomena responsible for the visible cracks/structural failure observed in the buildings. The results obtained brought to light the geological structure beneath the subsurface, which consists of four geoelectric layers identified as topsoil, dry/lithified upper sandy layer, wet sand (water-saturated) and peat/clay/sandy clayey soil (highly water-saturated). The deeply-seated peat/clay materials (ρ ≤ 20 Ωm) were delineated in the study area to the depths of 17.1 m and 19.8 m from 2-D and 3-D tomography respectively. 3-D images presented as horizontal depth slices revealed the dominance of very low resistivity materials i.e. peat/clay/sandy clay within the fourth, fifth and sixth layers at depths ranging from 8.68-12.5 m, 12.5- 16.9 m and 16.9-21.9 m respectively. The dominance of mechanically unstable peat/clay/sandy clay layers beneath the subsurface, which are highly mobile in response to volumetric changes, is responsible for the noticeable cracks/failure detected on structures within the study site. These observations were validated by a geotechnical test of soil samples in the study area. Atterberg’s limits of the samples revealed plasticity indices of zero. Thus, the soil samples within the depth analyzed were representatives of sandy soil that does not possess any plasticity. The methods justifiably provided relevant information on the subsurface geology beneath the study site and should be appropriated as major tools for engineering site assessment/geotechnical projects.