The Pacific Journal of Science and Technology –537– http://www.akamaiuniversity.us/PJST.htm Volume 11. Number 1. May 2010 (Spring) Micro-Resistivity Measurements, Near-Surface Sequence Delineation, and Empirical Relationships with Engineering Geotechnical Parameters. Inumidun B. Iginla, M.Sc. 1 ; Martins O. Olorunfemi, Ph.D. *1 ; Gabriel O. Bayowa, M.Sc. 2 ; and James O. Akintorinwa, Ph.D. 3 1 Obafemi Awolowo University, Ile – Ife, Nigeria. 2 Ladoke Akintola University of Technology, Ogbomoso, Nigeria. 3 Federal University of Technology, Akure, Nigeria. * E-mail: mlorunfe@yahoo.co.uk ABSTRACT This paper establishes empirical relationships between subsoil electrical resistivity and geotechnical parameters in a typical basement complex terrain of Nigeria. Two pits and two trenches were dug to a maximum depth of 3.0 m along and upstream of a proposed dam axis. Core cuttings (undisturbed soil samples) were collected at the base of the pits/trenches and the coefficients of Compaction, Compressibility, and Permeability were determined. Down-the-hole micro-resistivity measurements using Wenner and Pole-Dipole (Half-Wenner) arrays were made at 0.10 m interval from top to bottom of the pits/trenches with inter-electrode spacing of 0.10 m. On a linear-log plot, both coefficients of Consolidation and Compressibility increase with increase in subsoil resistivity values. The established empirical equations gave correlation coefficients that vary from 0.70 to 0.96. Beyond a threshold permeability value of around 4 x 10 -5 mm/s, subsoil permeability increases with decrease in soil resistivity while below the threshold value, soil permeability tend to increase with increase in resistivity. The study demonstrates that engineering geotechnical parameters can be estimated from resistivity measurements provided the relevant empirical equations had been established for the area of interest. (Keywords: micro-resistivity, geotechnical parameters, empirical equations) INTRODUCTION Civil engineering structures are founded on or within the earth. One of the priority considerations in the design of the foundation of such structures therefore, is the pre-construction investigation of the proposed site in order to ascertain the fitness of the host earth material. The pre-construction investigation may involve direct mechanical boring, pitting, and trenching for subsoil sequence delineation, groundwater table mapping, soil sampling and geotechnical laboratory analysis. It may also involve non-invasive geophysical investigation. Where site investigation involves both geotechnical and geophysical investigations, it is often to reduce cost (by reducing the number of borings) and improve on the subsoil imaging through 1 and 2-D geophysical data gathering and modeling (Olorunfemi and Mesida, 1987; Barker, 1997; and Olorunfemi, 2008). The pre- construction investigation provides information on the subsurface lithologies and their thicknesses, identifies the competent bedrock and determines depths to its upper interface, establishes through geotechnical parameters, and examines the degree of competence of the foundation bedrock (Aina et al., 1996; Adewumi and Olorunfemi, 2005; and Idornigie et al., 2006). In geotechnique, subsoil competence is evaluated through series of tests which include compaction, triaxial, and consolidation tests. In geophysical prospecting, the Compressional (P) and Shear (S) wave velocities in earth materials can be used to evaluate subsoil competence through the determination of the bulk modulus (Sjqgren et al., 1979 and Dutta, 1984). A compact subsoil is characterized by reduced porosity and moisture content with consequent increase in resistivity. It should therefore be possible to use resistivity measurements as indices of subsoil competence. A pre-construction geotechnical and geophysical investigation was carried out at a proposed dam