Economic and environmental constrains have necessitated 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 until the soil immediately above it became nearly saturated. In this study, the numerical model was first calibrated using the results from two tests. First, the experimental results of one-dimensional clay column underlain by nonwoven geotextile system subjected to infiltration were used to validate numerical model’s suitability of modeling capillary barrier effect. Second, the experimental results of centrifuge model study on flow within low permeable reinforced slope were used to validate numerical model’s capability of modeling unsaturated seepage flow within soil slopes with nonwoven geotextile drains. Thereafter, numerical models of unsaturated poorly graded sand and clay slopes with nonwoven geotextile drains were 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 effect. The effect of infiltration rate on hydraulic behavior of such systems was also evaluated. The numerical results indicated that inclusion of sand cushions reduced the development of capillary barrier by acting as a transition zone of pore pressure difference between poorly draining backfill and geotextile; as a result, the accumulation of pore water pressure within soils above nonwoven geotextiles was dissipated downward effectively. In addition, the sand cushions also acted as additional drain layers to facilitate the drainage of water within the slope system and subsequently enhanced system stability. Numerical results also showed that increase in infiltration rate leads to increased pore water pressure during infiltration. In addition, it is critical to evaluate local slope stability at each layer of soil-geotextile interface. This study proposed a material selection procedure to select among backfill soil, sand cushion and geotextile in order to determine whether capillary barrier effect will occur.

Economic and environmental constrains have necessitated 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 until the soil immediately above it became nearly saturated. In this study, the numerical model was first calibrated using the results from two tests. First, the experimental results of one-dimensional clay column underlain by nonwoven geotextile system subjected to infiltration were used to validate numerical model’s suitability of modeling capillary barrier effect. Second, the experimental results of centrifuge model study on flow within low permeable reinforced slope were used to validate numerical model’s capability of modeling unsaturated seepage flow within soil slopes with nonwoven geotextile drains. Thereafter, numerical models of unsaturated poorly graded sand and clay slopes with nonwoven geotextile drains were 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 effect. The effect of infiltration rate on hydraulic behavior of such systems was also evaluated. The numerical results indicated that inclusion of sand cushions reduced the development of capillary barrier by acting as a transition zone of pore pressure difference between poorly draining backfill and geotextile; as a result, the accumulation of pore water pressure within soils above nonwoven geotextiles was dissipated downward effectively. In addition, the sand cushions also acted as additional drain layers to facilitate the drainage of water within the slope system and subsequently enhanced system stability. Numerical results also showed that increase in infiltration rate leads to increased pore water pressure during infiltration. In addition, it is critical to evaluate local slope stability at each layer of soil-geotextile interface. This study proposed a material selection procedure to select among backfill soil, sand cushion and geotextile in order to determine whether capillary barrier effect will occur.

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