Use of Hybrid Geosynthetics in Mitigating Rainfall-Induced Slope Instability

Abstract

The objective of the present study is to investigate the mechanism of rainfall-induced instability in steep mountainous slopes and to mitigate the same by the inclusion of hybrid geosynthetics. For this purpose, a typical hilly terrain subjected to heavy rainfall was chosen for case study, and the local geology, recorded rainfall data and shear strength parameters measured at the site were replicated numerically by incorporating unsaturated soil parameters related to volumetric water content, permeability and soil matric suction. Hydro-mechanical infiltration analysis was performed on the selected slope profile using finite-element software SEEP/W, and corresponding pore water pressure values obtained during rainfall were incorporated in limit equilibrium-based software SLOPE/W for monitoring the factor of safety with time. The results indicated that slope instability occurred due to rapid loss of matric suction from an initial value of −200 kPa at the onset of rainfall to −60 kPa, resulting in a safety factor lying below the critical limit of 1.0. A global failure surface passing through the toe was observed in this case, thereby indicating that rainfall-induced slope instability may not necessarily be attributed solely to increasing ground water levels. As a remedial measure, the inclusion of dual-function hybrid geosynthetics has been suggested, which exhibit both reinforcement and drainage characteristics. The hybrid geosynthetics are proposed to be installed from the bottom to the top, following the sequence of slope construction or during slope reprofiling as a part of road widening projects in hilly areas. Subsequent numerical analysis conducted on the reinforced slope depicted considerable retention of soil suction at the end of rainfall and an enhanced global stability value of 2.29, which may be attributed to the drainage potential of hybrid geosynthetics in dissipating the excess pore water pressure generated within the slope during rainfall, coupled with reinforcement action which prevents loss of soil shear strength and associated softening, thereby averting possible slope instability.