MDOT State Study 286 - Performance Evaluation of Highway Slopes on Yazoo Clay [cover title]

US Transportation Collection2020PDFTech ReportKhan, Mohammad SadikAmini, FarshadNobahar, MasoudMississippi. Dept. of TransportationUnited States. Department of Transportation. Federal Highway AdministrationMississippi. Dept. of TransportationUnited StatesMississippiClay soilsDeformationEmbankmentsFinite element methodMoisture contentRainfallShear strengthSlope failureSoil suctionSwelling soilsPerformanceInstrumentationFEM AnalysisFHWA/MDOT-RD-20-286SPR-2017(028)/107691-1011000Final Report, Feb 27, 2018 to June 30, 2020Slope failures are frequent in highway embankments on expansive Yazoo clay in Mississippi due to frequent seasonal rainfall events associated with temperature and humidity variation causing expensive maintenance problems for the Mississippi Department of Transportation (MDOT). This State Study 286 was conducted to understand the performance of highway slopes containing Yazoo clay. Six repaired highway slopes were instrumented comprehensively to monitor the moisture, matric suction, and temperature variation, as well as monitoring the slope deformation using vertical inclinometers. Besides field monitoring, numerical modeling using the Finite Element Method (FEM) was conducted to evaluate the effect of rainfall frequency and duration (based on historical rainfall data of Mississippi) on the water infiltration and corresponding change in the factor of safety of the highway slope. Based on the field monitoring results, the data of volumetric moisture content and matric suction readings were observed constant throughout all six slopes. Certain peaks and drops in the moisture content were also observed during the summer months. The numerical modeling analyses indicates the rainfall during late summer to early fall is the most critical time in which a perched water condition will develop. As the wet-dry cycle soften the shear strength of high plastic clay to fully softened state the presence of perched water creates a slope condition vulnerable to a creeping slide failure. Based on this research, it is observed that slope failures of Highway embankments containing highly plastic clays such as Yazoo clay takes place due to the development of perched water conditions after five to seven years of construction. It is highly recommended to consider including a perched water zone up to the depth of the active zone along with the development of fully soften shear strength condition in the slope stability analysis of any slope containing high volume change clays, such as Yazoo clay120