Abstract:
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.
