Effect of Swell-shrink Characteristics on Landslides in Yazoo Clay

016436352018July 01, 2017 to June 30, 2018 Final Research ReportPDFTech ReportDTRT13-G-UTC50Slope stabilitySlope failureHighway maintenanceFinite element methodVoid ratiosMoisture contentVertisolsLandslidesClay soilsMontmorilloniteLeveesPlastic soilsRainfallHighway maintenanceHighway slopesWeather effectsYazoo clayMudflowsLandslidesDesignGeotechnologyUnited StatesArkansasMississippiMaritime Transportation Research and Education CenterKhan, SadikIvoke, JohnNobahar, MasoudMaritime Transportation Research and Education CenterUniversity of ArkansasJackson State University. Dept. of Civil & Environmental EngineeringUnited States. Dept. of TransportationUS Transportation CollectionSlope failures are frequent in highway embankments as well as in waterway infrastructures (levees) on expansive Yazoo clay in Mississippi which cause significant maintenance problems and require millions of state and federal dollars to fix it. After construction, the strength of the high plastic clay degrades with time due to the seasonal temperature and moisture variation, which is one of the significant factors of slope failure. However, no study is available on the strength reduction of Yazoo clay soil. The current study intends to investigate the repeated drop in the shear strength of the Yazoo clay soil with wet-dry cycles which cause slope failure. Representative Highly plastic Yazoo clay soil samples were collected from slope sites to investigate the soil mechanical properties. The high plastic Yazoo clay samples were tested at the laboratory to investigate the effect of wetting and drying cycles on the degradation of the shear strength. The study begins with the laboratory testing to quantify the progressive changes in the shear strength of the Yazoo clay soil. The test results also indicated that the highest shear strength ( c = 18.4 kPa and \u3c6 = 20.2 o ) was determined with the peak shear strength test, whereas the residual test generated the lowest strength ( c = 5.26 kPa and \u3c6 = 12.8 o ). Further laboratory testing was conducted to investigate the change in the void ratio of Yazoo clay soil under series of a wet and dry cycle, which eventually cause the changes in the shear strength. Reconstituted expansive clay soil samples were used for the experiment. The samples were subjected to 3, 5, and 7 number s of wetting and drying cycles in an enclosed chamber. During the drying process, the temperature ranged from 120-125 deg. F to simulate the typical high summer temperature in Mississippi. The test results indicated that the void ratio increases with the progressive number of wet and dry cycles. With the continuous increments in void ratios, the soil has more void spaces which reduce the shear strength of the Yazoo clay. Further study was conducted using 2D Finite Element Methods in Plaxis 2D to investigate the progressive changes in the factor of safety of the slope constructed over the Yazoo clay soil. A highway slope over I220 near US 80 in Jackson, MS was considered as a reference slope. It was 3H : 1V slope with 9.2 m height. The factor of safety of the slope was determined based on the existing soil test data, with a peak shear strength. Later the topsoil layer which gets weathered due to the repeated wet-dry cycle was changed to fully soften, and residual shear strength and the effect of each shear strength (peak, fully soften and residual) on slope stability was evaluated. The slope stability analysis results indicated that the slope is stable at the dry condition, even with the residual shear strength. However, considering a perched water condition at the topsoil due to the effect of rainfall, soil slope failed at fully soften shear strength. Thus, the fully softened shear strength with the perched water condition due to rainfall triggers the slope failures in Yazoo clay