This paper presents a case study and numerical investigation of a failure of a geosynthetic-reinforced soil slope with marginal backfill subject to rainfall infiltration. The investigated slope is a 26 m high, 4 tier geogrid-reinforced slope constructed for stabilizing the existing highly weathered and frac-tured sandstone road embankment. Contrary to the backfill recommendations in design guidelines, locally available low plasticity silty clay (CL-ML) with over 60% of fines, was used as backfill in the reinforced zone. The GRS slope first experienced excessive deformation after seasons of typhoon and heavy rainfall from 2010-2013. The measured settlement and horizontal deflection at slope crest were 140 and 80 cm from June to December 2012. Although an immediate remedial action had been taken, the slope final col-lapse was caused by two sequential typhoon events with total accumulated rainfall of over 600 mm in August 2013. The failure mode was identified as the compound failure which had the failure surface par-tially cut through the reinforced zone at the lowest tier of the slope and partially passed along the inter-face between retained weathered layer and intact rock. Using recorded rainfall, site geology, and meas-ured soil and reinforcement parameters, a series of coupled hydro-mechanical finite element analyses based on the framework of unsaturated soil mechanics were performed to examine the failure mechanism and causes contributing to the failure. The numerical results indicate the slope failure was attributed to positive porewater pressure accumulation within reinforced zones. Lessons learned from this case history are discussed.
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