Integrated Geotechnical and Mineralogical Evaluation of the Subgrade of some failed Pavements along Enugu-Onitsha Expressway Southeastern Nigeria

This research is focused on the constant pavement failure in parts of the Enugu-Onitsha expressway. The failed portions are constructed on the natural subgrade known to be the Shale of Enugu and Mamu Formations. Five (5) samples each were obtained from the Formations and were subjected to geotechnical test, mineralogical analysis, scanning electron microscopy (SEM) and, statistical analysis. The outcome of the geotechnical test revealed that the liquid limits (LL) are of high plasticity with values ranging from 57.69–62.61% and 53.57–56.24%, plasticity index (PI) values ranging from 20.32–24.37% and 13.37–15.32%, Slake durability index (SDI) values ranging from 0.55–31.8% and 63.4–71.6%, for Enugu and Mamu Shales respectively. From the Unconfined compressive strength (UCS) test, the cohesion (C) values ranged from 37.36–43.89KPa and 24.22- 27.07KPa, soaked California bearing ratio (CBR) values ranged from 1.03–1.22% and 0.90–1.60% for Enugu and Mamu Shales respectively. The test results are below the requirements laid down by the Federal Ministry of Works and Housing (FMWH) for pavement construction. X-Ray Diffraction (XRD) analysis revealed major minerals to be quartz and kaolinite. Moreover, kaolinite disperses and migrates when moist hence geotechnical failure. Images from SEM revealed the presence of micropores and fractures which can be used as a reference to the geotechnical test results. Statistical analysis of key geotechnical parameters such as SDI, PI, C, and CBR revealed a strong correlation. The evaluated results pointed out that the underlined natural subgrade is poor for pavement construction, and thus requires stabilization.