Characterization of Subsurface Using Schlumbeger Electrical Resistivity Method and Dynamic Cone Penetration Tests

Abstract

Geophysical and Dynamic Cone Penetrometer Test (DCPT) were conducted at a site to characterize the subsurface as part of a near surface studies designed to determine the strength properties of the soil for a building foundation design. 1-D resistivity method involving the Schlumberger array was carried out at the proposed building site. Soil strength properties and grain size distribution were obtained by DCPT and laboratory analysis of soil samples respectively. Qualitative interpretation of the resistivity data suggested an A- type curve (where layer resistivity; ρ1<ρ2<ρ3), showing increasing resistivity with depth. The layer boundaries were not well defined due to poor resistivity contrast between the layers at depth. A 2-layer earth model is suggested with the average resistivity values ρ1 of 521.76 ohm-m and ρ2 of 819.94 ohm-m topping a bottom layer of a higher resistivity ρ3. Qualitative interpretation of the soundings estimated the first and second layer boundaries at 5 m and 10 m below ground respectively. Correlation of resistivity values with characteristic soils resistivity suggested a material composed of clayey, silt on the surface and sand with admixture of gravel and cobbles dominant at depth which accounted for the high resistivity. Conductive moist clay on top could account for the low resistivity values. DCPT results showed an increase in average bearing capacity from 300 to 500 kPa, to the depths of 1-2 m and decreasing from 250 to 160 kPa from 4-5 m, this suggested that the survey area is characterized by a relatively thin (1-2 m) competent top formation overlying non-cohesive materials. The non-cohesive material correlated with increasing resistivity values with depth from 4m below ground surface.  These methods discussed have suggested alternatives to the more expensive and time consuming procedures of soil characterization, as well as provide preliminary field data to limit the number of confirmatory drill holes in site investigations.