Effect of Unsaturated Drainage Layer on Suction Distribution at Interface of a Two-Layered Slope

Abstract

The use of system with capillary barrier effect has been considered as promising solution to rainfall-induced slope failures in unsaturated residual soil slopes. The intense and prolonged nature of rainfall in tropical climate region renders the system with capillary barrier effect ineffective due to excessive percolation of water into the second layer at short diversion length. Therefore, this paper studied the effect of unsaturated drainage layer in the system with natural capillary barrier effect as a slope protection measure. A laboratory approach was followed, with grade VI (residual soil) and grade V (completely weathered rock) as fine-grained and coarse-grained soil layers, respectively. Similarly, gravel and sand were used as material for unsaturated drainage layer. These soils were arranged in a laboratory slope model to mimic the natural capillary barrier effect. The slope model was subjected to three (3) different rainfall conditions of 2-hour (short-time heavy rainfall condition), 24-hour (medium-time heavy rainfall condition) and 7-day (prolonged light rainfall condition). The results obtained show that the unsaturated drainage layer improved the performance of the system significantly and results in longer diversion length compared to the system without the unsaturated drainage layer, especially when subjected to the short-time and medium-time heavy rainfall conditions. Therefore, it could be concluded that unsaturated drainage layer improved the performance of system with natural capillary barrier effect significantly and eliminates its major shortcoming of early percolation in response to rainfall conditions.