Numerical study of infiltration into unsaturated clay slopes with nonwoven geotextile drains sandwiched in sand cushions: featuring the capillary barrier effect

Abstract

Economic and environmental constrains have required the use of marginal fills as backfill in geosynthetic-reinforced soil (GRS) retaining structures. The main challenge in the use of marginal fill as backfill material is their inability to quickly drain water, leading to building up of pore water pressures. The use of nonwoven geotextile drains within these backfills has been suggested but it is also recognized that the nonwoven geotextile may retard water due to capillary barrier effect under unsaturated soil conditions and start to act as a drainage material only once the soil immediately above it becomes nearly saturated. In this study, the numerical model is first calibrated using experimental results of onedimensional clay column underlain by nonwoven geotextile system subjected to infiltration to validate its suitability of modeling capillary barrier effect. Thereafter, numerical models of unsaturated clay slopes with nonwoven geotextile drains are developed to investigate the unsaturated hydraulic behavior of such systems and to evaluate the effect of sandwiching nonwoven geotextile drains in thin layers of sand (i.e., sand cushions) on the development of the capillary barrier. The numerical results indicate that inclusion of sand cushions reduced the development of capillary barrier by acting as a transition zone of pore pressure difference between clay and geotextile. As a result, the accumulation of pore water pressure within soils above nonwoven geotextiles can be dissipated downward effectively. In addition, the sand cushions also act as additional drain layers to facilitate the drainage of water within the slope system and subsequently enhance system stability.