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.
The study involved an integration of field observations, geotechnical analysis, slope stability analysis, gully expansion rate analysis and determination of the soil erodibility factor (K). Fifteen representative gullies were studied extensively. The Grain size distribution analysis revealed that the soils are composed of gravel (5.77–17.67% and 7.01–13.65%), sand (79.90-91.01% and 82.47–88.67%), and fines (2.36–4.05% and 3.78–5.02%) for Nnewi and Nnobi respectively. The cohesion and internal friction angle values range from 1–5kPa and 2-5kPa and from 29–38° and 30–34° for Nnewi and Nnobi respectively, which suggests that the soils have low shear strength and are susceptible to shear failure. The plasticity index (PI) of the fines showed that they are nonplastic to low plastic soils and highly liquefiable with values ranging from 0–10% and 0–9% for Nnewi and Nnobi respectively. Slope stability analysis gave factor of safety (FoS) values in the range of 0.50–0.76 and 0.82–0.95 for saturated condition and 0.73–0.98 and 0.87–1.04 for unsaturated condition for both Nnewi and Nnobi respectively indicating that the slopes are generally unstable to critically stable. The erosion expansion rate analysis for a fifteen-year period (2005–2020) revealed an average longitudinal expansion rate of 36.05m/yr and 10.76m/yr for Nnewi and Nnobi gullies respectively. The soil erodibility factor (K) are 8.57x10− 2 and 1.62x10− 4 for Nnewi and Nnobi respectively indicating that the soils in Nnewi have higher erodibility potentials than those of Nnobi. Conclusively, both the Nnewi and Nnobi areas are highly prone to erosion.
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