Progressive Development of the Perched Water Zone in Highway Slopes Made of Highly Plastic Clay

Based on National Oceanic and Atmospheric Administration data, after Hawaii and Louisiana, Mississippi is the rainiest state in the United States, having the most peak precipitation that occurs mainly in late winter. Development of perched water (DPW) has had a remarkable effect on the service life of highway slopes constructed on expansive clay. The objective of the current study is to map the DPW condition at highway slopes made of highly plastic clay (HPC) in Mississippi. Several highway slopes that are made of HPC in Jackson, MS, were instrumented using moisture sensors, water potential probes, and rain gauges. Based on the field investigations, it has been observed that a perched water condition exists in all the slopes constructed of Yazoo clay. To investigate the DPW condition and map the accumulation of the water within the slopes, a series of flow analyses have been conducted using the finite element method in Plaxis. The flow analysis is conducted considering the shrink/swell behavior of the Yazoo clay with the real-time rainfall events, as observed in the rain gauges. The numerical analysis was in good agreement with field monitoring results. Based on the analysis, it is observed that rainwater accumulated during the summer to fall season because of a high infiltration rate with the presence of desiccation cracks. On the other hand, the low permeability situation during the spring held the percolated water within the slopes. Repeated events of infiltration and water hold-up condition progressively develop the perched water zone in the slopes made of Yazoo clay.