GEOPHYSICAL INVESTIGATION OF THE FAILED SEGMENTS OF THE ILESA - AKURE HIGHWAY, SOUTHWESTERN NIGERIA

Abstract

Geophysical investigation complemented with geological and geotechnical investigations have been carried out along segments of the Ilesa-Akure highway located in the Precambrian Basement Complex of Southwestern Nigeria with a view to establishing the cause(s) of the road pavement failure. Four out of the ten failed segments established as at October, 2006 and two stable segments as control were selected for the study. A research approach that integrated remote sensing, geophysical and geotechnical methods was adopted. The remote sensing involved the processing and interpretation of the Landsat-7 ETM+ images covering the study area and its environ. The data were used to produce the lineament map of the area. The geophysical investigation involved the magnetic, Very Low Frequency Electromagnetic electrical resistivity methods. The magnetic and the VLF-EM measurement (VLF-EM) and were taken at intervals of 5 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 YES with AB/2 varying from 1 to 65 m. Twenty- two (22) and seventy-seven (77) YES were carried out at stable segments and failed localities respectively. The geotechnical investigation involved grain size analyses, Atterberge Limits, Compaction test and California Bearing Ratio (CBR) determination. The lineament map shows that lineament features were identified virtually across all the failed localities with predominant structural trend in NNE-SSW direction. The magnetic and VLF- EM results revealed that the stable segments are founded on a near homogeneous substratum devoid of major geological features while several magnetic and VLF-EM anomalies typical of linear features were identified within the failed localities. The A, H, KH, QH, HK, HA, HKH, and KHKH type curves were observed at both the stable and failed segments. The geoelectric sections generally identified four geologic sequences that comprises topsoil, weathered layer, partly weathered/fractured basement and fresh bedrock. The subsoils on which the road pavement is founded at stable segments have moderate to high resistivity exceeding 200 Q-m and are generally competent. Conversely, the subsoils on which the failed segments are founded are of low resistivity generally less than 200 Q-m and comprise incompetent clay/sandy clay formation. Networks of linear (geological) structures such as fractures and faults were identified by the 2-D resistivity structure across the failed segments. The ranges of natural moisture content, percentage of finer, Liquid Limits, Plastic Limits, Plasticity Index, Linear shrinkage and CBR of the soils taken from stable segments (control and classified stable)) are 4-31%, 25-83%, 24-56%,16-38%,8-28%,1-12% and 20-72% respectively, while that of failed segments are 4-88%, 31-70%, 20-66%, 10-41 %, 1-25%, 3-16% and 25-84%, respectively. - } The significant overlap in the geotechnical properties of the soils in the 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 complimentary to the geotechnical factors. From the results of the investigation the causes of highway pavements failure in the studied highway are, clayey topsoil/sub-grade soils (with characteristic low resistivity < 200 Q-m); near- surface linear (geological) features such as faults, fractures and lithologcal contacts beneath the highway pavements; poor drainage of run-off water from toes of the highway and excessive cut into the conductive, water absorbing clayey substratum.