Influential factors affecting drainage design considerations for mechanical stabilised earth walls using geocomposites

Mechanically stabilised earth (MSE) walls are made of unsaturated soil that is strengthened with artificial materials. They have been widely used over the past three decades; however, occasional failures due to inadequate drainage during heavy rainfall have been encountered. Essentially, the reinforced zone of the MSE wall must be protected from water inflow for the system to work satisfactorily. Recent studies have reported on the alternative use of geocomposites to replace the conventional drainage system in MSE walls, typically comprising well-graded gravel. Geocomposites comprise a core material with a large flow channel, which is covered by a nonwoven geotextile. Although it is widely recognised that geotextiles possess a water retention characteristic (WRC), only a few studies have examined the effect of WRC on the seepage responses of MSE walls. In this research, large-scale physical model tests for seepage flow through an MSE wall were undertaken with and without an L-shaped geocomposite drain. Comparisons indicate that the MSE wall with a geocomposite is superior to that without a geocomposite installed. A series of numerical experiments was conducted with finite element software to investigate the effects of the hydrologic properties of the soil, geotextile, and geonet on the seepage responses in the MSE wall. The seepage responses, including effective saturation and phreatic surface, were found to be primarily governed by the WRC of the soil and the ratio between the saturate hydraulic conductivity of the geonet and that of the soil. The ‘capillary barrier’ phenomenon was found to play a secondary role in determining the effective saturation along the interface between the soil and the geocomposite.