INTEGRATED GEOPHYSICAL AND GEOTECHNICAL EVALUATION OF PAVEMENT SUSCEPTIBILITY ALONG IGBARA ODO-ILAWE ROAD SOUTHWESTERN NIGERIA

Abstract

An integrated geophysical and geotechnical method were adopted for the study. The geophysical investigation involved Electrical resistivity methods using Dipole-dipole array and Schlumberger vertical electrical sounding (VES) techniques. The dipole-dipole configuration uses inter station separation (a) of 5 m and an expansion factor (n) that varies from 1 to 5. The Schlumberger configuration was used for the VES with AB/2 varying from 1 to 65 on the right and left hand side of the road. Fourteen soundings were carried out on the study road. The vertical electrical sounding curves are A, H and KH. The geoelectric sections generally reveals three to four geologic sequence that comprises topsoil, weathered layer, partly weathered/fracture basement and fresh basement. On traverse A - A1, the topsoil on which the road is found composed of clay, sandy clay and clayey sand with resistivity value which ranges from 21 to 245 Ωm. The thickness varies from 0.9 to 1.6 m. Weathered layer resistivity varies from 9 to 1246 Ωm and the thickness of the layer ranges from 1.5 to 3.5 m. Fresh basement/fractured basement resistivity ranges from 297 Ωm to infinity. And on traverse B - B1, the topsoil resistivity ranges from 64 to 194 Ωm, with the thickness that ranges from 0.6 to 2.5 m. This layer is composed of clay, sandy clay and clayey sand. Weathered layer resistivity varies from 8 to 571 Ωm, with the thickness ranging from 1.5 to 8.7 m. This layer is composed of clay, sandy clay, and clayey sand. Fresh basement/fractured basement resistivity ranges from 429 Ωm to infinity. The dipole-dipole results were presented as pseudosection and some suspected fractures/faults at distance 50 to 60 m, 115 to 180 m, 295 to 350 m respectively on both side of the road. Geotechnical results also unravels the following: the natural moisture content of soils from classified stable segments ranges from 12.6% to 27.3% while that of failed segments ranges from 17.3% to 25%. The percentage of fines, Specific gravity, Atterberg limit test and CBR of the soils taken from classified stable segments are 28.9 - 39.6%, 2.60 – 2.70%, 32- 34%, non-plastic to 27%, 0 to 14%, 3 - 11%, 11- 18% respectively, while that of failed segments are 20.8 - 38.2%, 2.62 – 2.71%, 21 - 33%, nonplastic - 28% , 0 to 13%, 7%, 8 - 22% respectively. Therefore, from the results of the investigation of the 8 cause(s) of pavement failure in the area were identified as heterogeneity and the clayey nature of the topsoil/sub-grade material, lack of proper drainage for flood channeling and poor construction material.