INTEGRATED GEOPHYSICAL INVESTIGATION OF PAVEMENT FAILURE ALONG OGBOMOSO-ILORIN DUAL CARRIAGE WAY, NIGERIA

Abstract

An integrated geophysical investigation was carried out at some portions of Ogbomoso-Ilorin dual carriage way that is located within the Basement Complex area of Southwestern Nigeria, with a view of delineating the cause(s) of road pavement failure observed along the road. Three failed portions and one control stable segment of the road pavement were studied. Very-low-frequency (VLF) electromagnetic (EM) and ground magnetic measurements were obtained at a regular station interval of 5 m along traverses established parallel to the road pavement. Vertical electrical sounding (VES) using Schlumberger array and 2-D resistivity imaging using dipole-dipole electrode array were employed for the electrical resistivity method. The results of the VLF-EM and magnetic methods identified presence of near surface linear features which are typical of fractures and fault zones within the unstable segments. The geoelectric sections generally delineated four geoelectric layers. These include the topsoil with resistivity values ranging from 73Ωm to 903Ωm and thickness values of 0.5m to 2.5m, weathered layer with resistivity values that vary from 10Ωm to 340Ωm and thickness values ranging from 1.9m to 16.6m, partly weathered/fractured basement with resistivity values that range from 78Ωm to 447Ωm and thickness values ranging from 2m to 6m and the fresh basement which characterized by resistivity values that vary from 228Ωm to 3083Ωm and infinite thickness. The near surface subsoil on which the road pavement is founded within the failed segments is predominantly characterized by low resistive materials (generally <150Ωm) typical of incompetent clay formation, series of resistivity highs and lows, undulating topography, a poor drainage system and shallowness of the saturated zone to the subsoil on which the road pavement is founded. Also, major geologic features which are typical of fractures, faults, joints and cavities/voids were identified from the 2D Dipole-dipole sections beneath road pavement within the unstable segments and classified stable segments. The control stable segment is however generally characterized by near subsoil with relatively high resistivity values (>150Ωm) and a locality that is characterized by relatively flat topography and well drainage system. Therefore, the geological factors responsible for the susceptibility of the pavement to failure are clayey subgrade soil beneath the road pavement, lateral inhomogeneity, near surface geological structures and changes in elevations due to fluctuation in the saturated zones.