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 to competent layers across the intended site, and periodic subsidence monitoring and deformation assessment of all buildings, specifically high-rise buildings, should be a regular practice, to safeguard the durability of civil engineering structures, to avert the disastrous consequences of structural failure and collapse prevalent of late. It was this extremity that necessitated the adoption of an integrated methodology which employed DC resistivity tomography involving 2-D and 3-D techniques and geotechnical-soil analysis to evaluate subsoil properties for engineering site investigation at Okerenkoko primary school, in Warri-southwest area of Delta State, to adduce the phenomena responsible for the visible cracks/structural failure observed in the school buildings. Rectilinear set of 2-D resistivity data consisting of five (5) parallel and five (5) perpendicular lines were obtained in a 100 x 80 m2 rectangular grid using the Wenner array. Thirteen (13) Schlumberger soundings were also obtained on the site with half-current electrode separation of 200 m. The results brought to light the geological structure beneath the subsurface, which consists of four geoelectric layers identified as top soil, 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 depths of 17.1 m and 19.8 m from 2-D and 3-D imaging 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/subsidence detected on structures within the study site. The DC resistivity result was validated using geotechnical test of soil samples collected from boreholes covering the first 8.0 m on three of the profiles. Atterberg’s limits of the soil samples revealed plasticity indices of zero for all samples. Thus, the soil samples within the depth analyzed were representatives of sandy soil which does not possess any plasticity and their plasticity index is taken as zero. These findings apparently justify the subsoil conditions defined in the interpretation of 2-D and 3-D resistivity imaging data. 3-D images presented as horizontal depth slices revealed the dominance of very low resistivity materials i.e. peat/clay/sandy clay within the third, fourth and fifth layers at depths ranging from 5.38-8.68 m, 8.68-12.5 m and 12.5-16.9 m respectively. Hence, 3-D tomography amplified the degree of accuracy of the geoelectrical resistivity imaging. Resistivity contour maps of second, third and fourth layers for VES 1 to 13, displayed low resistivity direction predominantly towards the northeastern part of the site, and signifies that rocks within the northeastern part have low resistivity values, which connotes high porosity and establishes the groundwater flow trend in the study area. The methods employed in this study justifiably gave relevant information on the subsurface geology beneath the study site and its suitability for engineering practice. Thus, it is suggested that these methods should be appropriated as major tools for engineering site assessment projects and groundwater future studies.