Feasibility study of a new unsaturated three-layer landfill cover system

As an improvement of the two-layer cover with capillary barrier effect (CCBE) (i.e. fine-grained soil overlying a coarse-grained soil), a new three-layer landfill cover system is proposed and investigated for humid climate. This new system is to add a fine-grained soil (i.e., clay) underneath a two-layer CCBE (i.e., a silt overlying a gravelly sand layer). The feasibility of this proposed cover system was investigated by conducting a one-dimensional water infiltration test. In addition, transient seepage simulations were carried out to back-analyse the test results and investigate the importance of hydraulic properties of the CCBE on the proposed cover. Based on the infiltration experiment and numerical back-analysis, it is found that no percolation was observed after 48 hours of ponding, which is equivalent to a rainfall return period of greater than 1000 years. However, the upper two-layer CCBE is only effective for a rainfall return period of about 35 years. This implies that the proposed bottom clay layer is needed for humid climate. Numerical parametric simulations reveal that increasing the saturated permeability of the upper fine-grained soil by two orders of magnitude (1.4x10-6 m/s to 2.1x10-4 m/s), the wetting front is still within the clay layer after 12 hours of constant water ponding (>1000 year rainfall) and no percolation occurred.