GEOPHYSICAL AND GEOTECHNICAL SUBSOIL CHARACTERIZATION OF SOME SELECTED FAILED SECTIONS OF THE SAGAMU-BENIN HIGHWAY SOUTHWESTERN NIGERIA

Abstract

Geophysical and geotechnical studies have been conducted along selected segments of Sagamu- Benin highway situated in the Eastern section of Dahomey Sedimentary Basin of Southwestern Nigeria. This is with the aim of assessing causes of the incessant road pavement failures coupled with attempt at evolving empirical relationship between the two methods of study, for highway structural design considerations. Seventeen locations investigated, were categorized into stable and failed sections. The geophysical methods consist of multi-channel analysis of surface waves (MASW) and electrical resistivity (ERT). The MASW measurements were used to generate Shear Wave Velocity (SWV) tomography while the electrical resistivity method involves the vertical electrical sounding (VES) and the 2-D electrical imaging using Wenner configuration. Forty (40) VES were conducted along the selected regions using Schlumberger configuration. The geotechnical method comprised of Cone Penetrometer Tests (CPT), grain size analyses, consistency limits, compaction tests and determination of California Bearing Ratio (CBR). The results revealed that the stable segments are founded on a near homogenous substratum devoid of major geological features while some MASW anomalies associated with low velocity (Vs < 300 m/s) zones were found within the failed segments. The geoelectric section identified three to four layers comprising of the topsoil, sand/clayey sand, weathered rock/rock and bedrock respectively. The subsurface strata beneath the road pavement at the stable segments are characterized by moderate to high resistivity values ( 100 ohm-m) and are presumed to be generally competent; while the subsurface strata at the failed sections are characterized by low resistivity values ( 100 ohm-m) and assumed to be weaker materials. Geological structures such as fractures and faults were recognized by the VES and 2-D electrical imaging in some failed segments. The geotechnical results obtained from the CPT, grain size analyses, consistency limits, compaction tests and the CBR taken from the viii failed segments fall below Federal Ministry of Works and Housing (FMWH, 1997) specifications for roads and bridges design. However, the stable sections are within the acceptable standard with grain percentage passing of fine particles < 35%, liquid limit, plasticity index and CBR of the soil show ranges of 21 – 28%, 6 – 13%, and 10 - 15% respectively. The dynamic elastic properties of soil (rigidity modulus, bulk modulus, Poisson‘s ratio and the allowable bearing capacity (qa) were derived from the SWV. The derived CBR from SWV and the rigidity modulus (μ) were correlated with the CBR and Maximum Dry Density (MDD) as independent variables. Comparative Analysis Model (CAM) was developed to establish empirical relationship between geophysical and geotechnical methods using polynomial regression analysis, CBR was determined from allowable bearing capacity (qa). The coefficient of determination (R²=0.975) shows that the model is good fit and present the possibility of deriving CBR from SWV. The plausible sources of the incessant failure of the study highway were as a result of the low resistivity layers of < 100 Ωm, low shear wave velocity layers of < 360 m/s value within the upper stratum, geological features underlying the road pavement at shallow and deeper depths, lack of drainage, the steady increase in traffic volume per hour, ageing, inadequate construction materials, and environmental impacts. The result of this research work should serve as control to the design and rehabilitation of the investigated highway, proper drainage channel should be constructed on both sides of the road in order to avoid ingress of water into the subgrade/foundation of the road pavement to prevent incessant future failure.