Abstract
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.
Highlights
Covers that are commonly placed over landfills have been identified as an important component in preventing rainfall infiltration to the buried solid waste after their closure [1,2]
The newly proposed landfill cover system is a three-layer cover system, which consists of a soil layer with a relatively low permeability that is added underneath a cover with capillary barrier effect (CCBE), as shown in
The upper two-layer CCBE was no longer functional after this ponding duration. This phenomenon has been demonstrated by researchers for two-layer CCBEs under one-dimensional conditions [22-24]
Summary
Covers that are commonly placed over landfills have been identified as an important component in preventing rainfall infiltration to the buried solid waste after their closure [1,2]. It is common to rely on naturally occurring low permeability materials such as clays. Landfill covers composed of naturally occurring materials are prone to desiccation induced cracking, which can compromise their integrity and performance [4,5]. More attention has been given to covers with capillary barrier effects (CCBEs) as an alternative landfill cover system because of their ease of construction, long service life and relatively low cost [67]. The principle of CCBE utilizes the capillary barrier effect between the two soil layers which consist of a layer of fine-grained soil over a layer of coarse-grained soil. The amount of water infiltrating the system can be minimized due to the coarse-grained soil being less permeable than the fine-grained soil under relatively dry conditions. More attention has been paid to CCBEs as an alternative cover system in semi-arid and arid regions, the performance of CCBE under humid climates has so far been unsatisfactory [912]
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