Expansive Yazoo clay soil is susceptible to volumetric deformation and is dominant in central Mississippi and other neighboring southern states of the United States. Recurring shrink-swell behavior causes a significant problem to infrastructures in the area. Although Yazoo clay causes a significant problem in the deep southern states, limited study has been conducted on the behavior of Yazoo clay, especially in the presence of rainfall. The objective of this current study is to investigate the coupled effect of changes in void ratio due to wet-dry cycles and rainfall on the stability of highway slopes made of Yazoo clay. The finite element method in Plaxis 2D by Bentley System (https://www.plaxis.com/) has been utilized to investigate the coupled effect of changes in mechanical properties and rainfall using flow-deformation and stability analysis. Reconstituted expansive clay soil samples were used for the laboratory experiment. The reconstituted Yazoo clay samples were subjected to 3, 5, and 7 wetting and drying cycles in an enclosed chamber for a 24-h period. The axial deformation of the samples and the change in void ratios at each number of the cycle was closely monitored. The strength change at each wet and dry cycle was also investigated and used for slope stability analysis in the presence of rainfall. The test results indicate that the void ratio increases with the increasing number of wet-dry cycles. A continuous increment in void ratios from 0.99 in an undisturbed state with no wet-dry cycle to 1.49 at the 7th wet-dry cycle, indicating a 48.9% increase, as the wetting and drying cycle increases was recorded; in turn, decreasing the cohesion of the soil by 77%. The factor of safety considering the effect of two total rainfall periods of Rv = 126.2 mm (2 h) and Rv = 271.7 mm (3 days) reduced from 1.7 to 1.2 and 1.68 to 1.02, considering the effect of the 7th wet-dry cycle at the topsoil. The changes in the void ratio due to the wetting and drying cycle of Yazoo clay soil reduces the shear strength to a fully softened condition, increasing the possibility of slope failure. This condition further worsens in the presence of a perched water condition due to the infiltration of rain water.

Yazoo clay soil is expansive in nature, and extended over central Mississippi, Alabama and Southern Louisiana. Shallow slope failures are frequent in embankments constructed on expansive Yazoo clay in Mississippi due to the rainfall volume and climatic variation. The current study investigates the effect of rainfall over the factor of safety of fill slopes constructed with Yazoo clay soil using the finite element method (FEM). The historical rainfall pattern of Mississippi obtained from the National Oceanic and Atmospheric Administration (NOAA), is evaluated at different intensities (2 to 50 mm/h), and durations of rainfall (30 min to 7 days) with the data being, based on a return periods of 50, 100, and 500 years. Three fill slopes were constructed using Yazoo clay soil, with a slope ratios of 2H: 1V, 3H: 1V, and 4H: 1V. The rainfall pattern is applied over the top boundary of the slope in Plaxis, using flow analysis. The preliminary investigation results indicated that rainfall water infiltrated the top part of the slope, near the crest, resulting in the reduction of both the matric suction and the factor of safety of the slope. Moreover, the factor of safety of the slope is directly influenced by the total rainfall volume, regardless of the intensity and duration.