VES

Abstract
The combination of geophysical and geotechnical methods in foundation investigation has shown to be invaluable in deciphering the depth to bedrock, characterizing the earth materials and extent of variation of allowable bearing pressure of foundation soils. Geophysical and geotechnical methods involving electrical resistivity and cone penetration test have been carried out to investigate the foundation conditions of a bridge site in Ajibode and the newly constructed Abadina-Ajibode Road, located in University of Ibadan, Ibadan, southwestern Nigeria.
Eleven vertical electrical sounding (VES) were carried out, seven at the investigated portion of the road and four at the bridge site. While four cone penetration tests were also carried out at the Bridge site. The vertical electrical sounding for investigated portion of the road revealed 2-4 different lithological layers. The first layers is topsoil which has resistivity ranges from 17-321Ωm with a mean of 220Ωm. The wide range in resistivity values of the topsoil can be due to different degree of compaction. The thickness of the topsoil ranges from 0.5-1.7m with a mean of 1.1m. The second layer resistivity from VES 1-VES 6 ranges from 19-46Ωm with a mean of 32Ωm. The resistivity of this layer is less than 100Ωm which is characteristics of clayey formation. The thickness of this layer ranges from 5.9-12.6m with a mean of 8.0m. For VES 7, the second layer is made up of lateritic pan with resistivity of 336Ωm and depth of 6.1m. The fractured/fresh basement layer resistivity ranges from 171-2364Ωm with a mean of 998Ωm and depth value ranges between 7.4-22.3m with mean depth of 10.9m. While for the investigated bridge site, the vertical electrical sounding revealed 2-3 geoelectric layers. The geoelectric layers include: the first layer is topsoil which has resistivity ranges from 31-320Ωm with a mean of 132Ωm. The wide range in resistivity of the topsoil can be due to different degree of compaction. The thickness of this layer ranges from 0.7m-2.5m with a mean of 1.5m. The second layer resistivity for VES 1-VES 2 ranges from 85-138Ωm with a mean of 116Ωm. The thickness of this layer ranges from 1.0-1.1m with a mean of 1.0m: this layer is characterized by weathered basement. The fresh basement layer resistivity ranges from 920-2853Ωm with a mean resistivity of 2067Ωm, is characterize with fresh bedrock. The cone penetrometer tests also revealed that the investigated bridge site has 2-3 different lithologies with cone resistance of 5-40Kg/cm2 with a mean of 18Kg/cm2 at depth range from 0.25-1.0m and at depth range of 1.25-1.7m, the penetrative resistance at CPT 1 and CPT 2 range from 25-250Kg/cm2 with a mean of 148Kg/cm2, this is a characteristics of competent materials which are weathered basement. CPT 3 and CPT 4 penetrative resistance range from 5-10Kg/cm2 with a mean of 9Kg/cm2 at depth range from 1.0-3.75m but from depth range between 4.0m and 5.0m, the penetrative resistance ranges from 200-250Kg/cm2 with a mean of 225Kg/cm2.
The results obtained from this study have emphasized the usefulness of geophysical methods in complementing geotechnical studies in variation in lithology accompanied by variation in the allowable bearing pressure of foundation soils.

Keywords: Ajibode, VES, Cone penetration test, Schlumberger configuration, Geotechnical survey, cone penetration test, pile foundation

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