Effect of fine-grained soil layer thickness on performance of modified capillary barrier system

Abstract

Slope instability due to rainfall infiltration constitutes major natural disasters in many tropical countries covered by residual soil. In these types of soils, groundwater table usually exist at considerable depth with significance thickness of unsaturated soil above water table. Negative pore water pressure or matric suction exist in the unsaturated soils and contributes additional shear strength to the soil. However, the matric suction reduces due to rainfall infiltration and invariably reduces the additional shear strength provided by the matric suction. The principle of capillary barrier is amongst the methods employed to reduce infiltration of rainfall into unsaturated soil and the infiltrating water into the system is stored in the upper (fine-grained) soil layer. Therefore, this paper highlights the effect of different thickness of fine-grained soil layer on performance of a modified capillary barrier system using numerical modelling approach. The capillary barrier was constructed from residual soils and subjected to three different rainfall intensities of 1 hour, 24 hour and 7 day. The thickness of fine-grained soil layer was varied from 0.1 m to 0.3 m while the coarse-grained soil layer and the unsaturated drainage layer were maintained as 0.3 m and 0.1 m respectively. The results shows that 0.3 m thick fine-grained soil layer is more effective in preventing breakthrough occurrence in case of capillary barrier without unsaturated drainage layer. However, when the capillary barrier was modified with gravel as unsaturated drainage layer, 0.1 m thick fine-grained soil layer was sufficient to prevent breakthrough occurrence for 1 hour and 24 hour rainfall intensities. In the case of 7 day rainfall intensity, breakthrough occurred after 4th day of rainfall infiltration in all the varied thicknesses of the fine-grained soil layer. Therefore modified capillary barrier with unsaturated drainage layer enhance the performance of capillary barrier system by transporting the infiltrating water before breakthrough occurrence. Similarly, it helps in reducing the thickness of fine-grained soil layer.