INTEGRATED GEOPHYSICAL AND GEOTECHNICAL INVESTIGATION OF LATERITIC SOIL AS SUBGRADE MATERIALS IN AKURE, SOUTHWESTERN NIGERIA

Ayodeji. K. Ogundana, Thmpson . H. T. Ogunribido*, Alvin. H. Ayowei

Abstract

An integrated geophysical and geotechnical investigation of the sub-soil was carried out to determine the suitability of the soil as sub-grade materials. Ten vertical electrical sounding points and two horizontal profiles of 100m each were carried out using a dipole-dipole configuration. Six soil samples were collected from three dug pits at 0.5m and 1.0m for laboratory analyses which included specific gravity, moisture content, compaction, grain size, and Atterberg limits. The result from the VES showed the sub-surface layer was made up of lateritic topsoil, clay, and sandy clay while the horizontal profile carried out also shows three continuous lithological units namely lateritic topsoil, clay, and sandy clay. The average resistivity values for the lateritic topsoil, clay, and sandy clay were 49 Ωm, 30 Ωm, and 95 Ωm respectively while the thicknesses for the three geo-electrical units are 1.5m, 10m and 9m respectively. The geotechnical results show that the MDD of the samples ranged from 1410 kg/m³to 1460 Kg/m³at 1m while the OMC ranged from 15-18% at 1m depth. The soils are silty clay with medium plasticity. Soil samples obtained here are fair to poor subgrade materials. The percentage of materials passing sieve no 200 for samples obtained in pits 1, 2, and 3 are 0%, 0%, and 63.6% respectively. The liquid limits values range from 35-41% while the plasticity indices range from 14 to 18%. In conclusion, soil samples are not good as subgrade materials, stabilization is necessary to improve their geotechnical properties.

Keywords: Subgrade, Geotechnical properties, Resistivity, Superstructure, Stabilization