GEOPHYSICAL AND GEOTECHNICAL INVESTIGATIONS OF A PAVEMENT FAILURE ALONG AKURE-IJARE ROAD, SOUTHWESTERN NIGERIA

Abstract

Geophysical and geotechnical investigations were carried out along two failed segments of Akure- Ijare road, named locality 1 and locality 2, with the aim of establishing the cause(s) of the incessant pavement failure along the road. The failed segments were extended at both ends into classified stable? segments. An integrated geophysical and geotechnical method were adopted for the study. The geophysical investigation involved the Very Low Frequency Electromagnetic (VLF-EM) and Electrical Resistivity Methods. The VLF-EM measurements were taken at intervals of 10 m along traverses parallel to road pavements. Two techniques were adopted for the electrical resistivity method namely the vertical electrical sounding (VES) and a combination of horizontal profiling and sounding using dipole-dipole configuration with inter stations 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 m and 1 to 40 m for locality 1 and 2 respectively. Nine (9) and twelve (12) VES were carried out at localities 1 and 2 respectively. The geotechnical investigation involved natural moisture content test, grain size analyses, specific gravity test, linear shrinkage test, Atterberg’s Limits, Compaction test and California Bearing Ratio (CBR) determination. The VLF-EM method revealed that the road is founded on a weakly conductive material devoid of major geological structure. The Vertical electrical sounding curves range from A, H to KH. The geoelectric sections generally identified three to four geologic sequences that comprise topsoil, weathered layer, partly weathered/fracture basement and fresh basement. At locality 1, the topsoil/subsoil on which the road is founded are of low resistivity generally less than 100 Ω-m composed of clayey materials, while the road pavement along locality 2 is within the resistive topsoil or directly on bedrock. The fresh bedrock along this locality is of infinite resistivity and generally shallow (< 2 m) with an uneven interface. The natural moisture content of soils from the classified stable segments is 4.0 % while that of failed segments ranges from 3.0 % to 5.0 %. The percentage of fines, specific gravity, Liquid Limits, Plastic Limits, Plasticity index, Linear shrinkage and CBR of the soils taken from classified stable segments are 46-48.3 %, 2.67-2.68, 32-33 %, 22-23 %, 10-11 %, 2-12 %, 46-70 % respectively, while that of failed segments are 27- 58%, 2.65-2.73, 22-54 %, 17-29 %, 5-25 %, 6-7 %, 26-57 % respectively. All the soil samples from both classified stable and the failed segments have high Maximum Dry Density at low Optimum Moisture Content. The significant overlap in the geotechnical properties of the soils in the classified stable and failed segments of the road suggests that the cause(s) of road pavement failure along the studied highway may be due to factors other than or complementary to geotechnical factors. Therefore, from the results of the investigation the causes of road failure in the studied roadway are heterogeneity and clayey nature of the topsoil/sub-grade material, lack of proper drainage at the road embankment and poor construction material.