Infiltration into unsaturated reinforced slopes with nonwoven geotextile drains sandwiched in sand layers J. N. Thuo 1 , K. H. Yang 2 and C. C. Huang 3 1 Graduate Student, Department of Civil and Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, Telephone: +886 928441052; Telefax: +886 227376606; E-mail: D10305813@mail.ntust.edu.tw 2 Associate Professor, Department of Civil and Construction Engineering, National Taiwan University of Science and Technology, Taipei, Taiwan, Telephone: +886 227301227; Telefax: +886 227376606; E-mail: khy@mail.ntust.edu.tw (Corresponding author) 3 Professor, Department of Civil Engineering, National Cheng Kung University, Tainan, Taiwan, Telephone: +886 62757575-63160; Telefax: +886 62358542; E-mail: samhcc@mail.ncku.edu.tw Received 19 November 2014, revised 11 April 2015, accepted 03 June 2015, published 16 September 2015 ABSTRACT: The use of nonwoven geotextile drains in geosynthetic-reinforced soil (GRS) structures has been suggested to facilitate the dissipation of porewater 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 byacting 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 porewater pressure above the nonwoven geotextile being effectively dissipated downward. The sand cushions also acted as additional drain layersto 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. KEYWORDS: Geosynthetics, Unsaturated flow, Capillary barrier effect, Infiltration, Nonwoven geotextile, Sand cushion REFERENCE: Thuo, J. N., Yang, K. H. and Huang, C. C. (2015). Infiltration into unsaturated reinforced slopes with nonwoven geotextile drains sandwiched in sand layers. Geosynthetics International, 22, No. 6, 457–474. [http://dx.doi.org/10.1680/jgein.15.00026] 1. INTRODUCTION The backfill material forms one of the major constituents of geosynthetic-reinforced soil walls and slopes and ac- counts for 30–40% of their cost (Christopher and Stuglis 2005; Raisinghani and Viswanadham 2011). Design guidelines (Elias et al. 2001; AASHTO 2002; Berg et al. 2009; NCMA 2010) limit the use of fine-grained soils as backfill material within the reinforced zone. Figure 1 shows the gradation limits specified in the design guide- lines. In addition to the gradation limits, the plasticity index of the backfill is also specified (PI ≤ 6 and 20 for walls and slopes, respectively). However, to reduce the construction cost of geosynthetic-reinforced soil (GRS) structures and minimise the transportation cost and en- vironmental impact associated with the disposal of the excavated soil, locally available soils with relatively low hydraulic conductivity (usually referred to as marginal fills) have beenused as alternative backfills. It has been reported that the low draining capacity of fine soils compromised the performance of reinforced soil walls upon rainfall infiltration because of the build-up of Geosynthetics International, 2015, 22, No. 6 1072-6349 © 2015 Thomas Telford Ltd 457 Downloaded by [ National Taiwan University of Science and technology (NTUST)] on [07/12/15]. Copyright © ICE Publishing, all rights reserved.