Geophysical investigation of road pavement instability along part of Akure-Owo express way, Southwestern Nigeria

Integrated surface geophysical methods were used in investigating causes of pavement instability along a portion of Akure-Owo expressway, southwestern, Nigeria. The methods comprise of ground magnetic profiling, Very Low Frequency Electromagnetic (VLF-EM) profiling and geoelectric sounding. The magnetic profile shows a drop below 31500nT at distance 100m and above 32500nT at distances 140 and 310m, these drops in value could correspond to lithological contacts or bedrock depressions. The combined plots of raw real and filtered real VLF-EM data show conductive zones at distances 70m, 230240m and 350-360m, which could be indicative of fractured zones or conductive clay materials. The 2-D geoelectric section shows bedrock depressions at distance 80-160m (stable segment) and 240-360m (unstable segment), while bedrock fractures was delineated beneath VES points 1, 3, 5 and 10, corresponding to distances 0, 80m, 160m and 400m respectively. The topsoil resistivity varies from 83 to 865 ohm-m, while resistivity in the weathered layer materials ranges from 182-1139ohm-m along the stable segment and 27-262ohm-m along the unstable segment. The low resistivity values observed beneath the unstable segment are typical of expansive clay. Thus the instability of the road pavement along the studied portion of the road is probably precipitated by the presence of near surface bedrock depressions, occupied by low resistivity weathered materials, typical of expansive clay and sandy clay, adjudged unsuitable construction materials.