Performance Evaluation of a Highway Slope on Expansive Soil in Mississippi

Abstract

Slope failure in highway embankments is common in central Mississippi due to the presence of the Yazoo Formation clays that are highly plastic in nature and experience extreme shrink–swell movement with seasonal moisture variation. The current study presents a performance evaluation of a repaired landslide along a highway exit ramp slope that contains Yazoo clay. It is a 3H:1V slope located along the exit of Sowell road from I55 south, which is around 32.18 km (20 mi) north of downtown Jackson, Mississippi. The slope was rebuilt and reinforced with H-piles and two layers of uniaxial geogrid. Automated soil moisture sensors and water potential probes were installed at the crest and middle of the slope to monitor the moisture content and matric suction variation. An automated rain gauge and vertical inclinometers were installed at the slope site to monitor the real-time rainfall and deformation of the slope. A sliding slope movement was observed near the toe of the slope in October 2018, a few months after the landslide repair of the slope was complete. Based on the instrumentation results, the slope was observed to be fully saturated. Considering both site investigation and field instrumentation results, a back analysis was conducted using the finite-element method in Plaxis 3D and the potential influencing factors and failure states were replicated. Based on the monitoring results, the topsoil of the slope is fully saturated, which is a clear indication of the formation of perched water conditions. The Plaxis 3D flow analysis results indicated that within a 4-month wet rainfall period of monitoring the slope, the progressive rainfall saturated the near-surface soil which reduced the soil’s matric suction significantly. The successive rainfall and buildup of a perched water zone in the slope eventually caused another failure of this recently repaired slope.