Combined Effect of Rainfall and Shear Strength on the Stability of Highway Embankments Made of Yazoo Clay in Mississippi

Shallow slope failure repeatedly occurs in many highway slopes in Mississippi due to the abundance of expansive Yazoo Clay. The highly plastic clay soil undergoes repetitive wet–dry cycles, which reduces the shear strength to fully soften state. The current study focused on the progressive change in shear strength and the safety factor of slopes constructed out of Yazoo Clay. Undisturbed and remolded specimens were used to determine the peak, fully softened and residual shear strength, with effective normal stresses of 25, 50, and 100 kPa. The variations in shear strength were investigated with the 2D slope stability analysis software, which uses the Finite Element Method, Plaxis 2D. A highway slope in Jackson, MS was considered as the reference slope. Different rainfall volumes, 70.8 mm (2.78 in.) to 312.4 mm (12.29 in.), with a rainfall duration (30 min–7 days) based on 100-year return periods of Jackson, MS were utilized. Furthermore, three slope ratios 2H:1V, 3H:1V and, 4H:1V were selected for this study. The safety factor of the slope was determined based on peak shear strength soil test data. Later, the topsoil layer, which gets weathered within the active zone due to the repeated wet–dry cycle, was varied to fully softened and residual shear strengths. The slope stability analysis results showed that the safety factor reduces progressively from peak to residual shear strength. In addition, the factor of safety was critical when the soil reached its fully softened shear strength for 2H:1V and 3H:1V slopes with progressive rainfall. On the other hand, the 4H:1V slope reached failure at the residual phase with the presence of rainfall.