Infiltration into unsaturated reinforced slopes with nonwoven geotextile drains sandwiched in sand layers

The use of nonwoven geotextile drains in geosynthetic-reinforced soil (GRS) structures has been suggested to facilitate the dissipation of pore water pressure. It has also been recognised that the nonwoven geotextile may retard water penetration due to the capillary barrier effect under unsaturated soil conditions and can function as a drainage material only once the soil immediately above it is nearly saturated. In this study, numerical models of unsaturated slopes with nonwoven geotextile drains, subjected to rainfall infiltration were developed to investigate the unsaturated hydraulic behaviour and stability of slopes constructed with nonwoven geotextile drains in thin layers of highly permeable sand (i.e. sand cushions). The numerical models were first validated for their suitability for modelling water flow and the capillary barrier effect within unsaturated soils using the experimental results from a one-dimensional soil column infiltration test and full-scale infiltration tests. Next, a series of numerical simulations of unsaturated slopes with and without sand cushions and under different infiltration conditions were performed. The numerical results indicated that the sand cushions reduced the development of the capillary barrier effect by acting as an intermediate material between the backfill and the nonwoven geotextile, which bridged the gap between two materials with very different unsaturated hydraulic characteristics. The reduction of the development of the capillary barrier effect led to the accumulation of pore water pressure above the nonwoven geotextile being effectively dissipated downward. The sand cushions also acted as additional drain layers to facilitate the drainage of water within the slope system. Thus, the inclusion of sand cushions enhanced the local slope stability for soils above the top geotextile layer. Based on the numerical results, methods for determining the occurrence of the capillary barrier effect are identified from the literature and discussed.