Investigation on the hydraulic response of a bioengineered landfill cover system subjected to extreme drying-wetting cycle

Construction wastes such as recycled concrete have been widely reused in geotechnical engineering to reduce environmental pollution and preserve natural resources. In this study, a one-dimensional (1D) soil column test was carried out to evaluate the hydrological performance of a three-layer landfill cover system using recycled concrete aggregates (RCA) under humid climatic conditions. This three-layer system consists of a layer of fine-grained and a layer of coarse-grained recycled aggregates (i.e. FRC and CRC) overlying the bottom silt refuse soil. Numerical simulations were conducted to analyse the model test and investigate the effects of the bottom layer and climate conditions on the performance of the three-layer landfill cover system. Consistent results were obtained between the measured data and numerical predictions. The matric suctions in the vegetated cover were nearly 80% higher than the bare cover after drying for 14 days from a nearly saturated condition. Even after the simulated extreme rainfall of Hong Kong with a return period of 100 years, a higher suction was also well-retained in vegetated cover. This was due to the initial high suction induced by evapotranspiration of plants, which led to the infiltration rates into grassed cover being up to 30% lower than the bare one. The results from the numerical parametric analysis suggest a bottom soil layer with an effective particle size (d ₁₀) value of 0.02 mm and 0.2 mm to be used in the three-layer landfill cover system under humid and semi-humid climates, respectively. For semi-arid or even arid regions, a two-layer cover with capillary barrier effects (CCBE) is already sufficient for preventing water percolation as the rainfall in those areas is not heavy.