Physical and numerical modelling of a vegetated three-layer landfill cover system using recycled aggregates without a geomembrane

It is clear that the efficient use of recycled construction waste materials can reduce waste generation and preserve natural resources. Although the use of recycled concrete aggregates (RCA) as a CCBE subjected to small intensity rainfall has been reported, no physical model test data and numerical simulation to investigate the use of RCA as a landfill cover subjected to high intensity of rainfall under humid climatic conditions. In this study, one-dimensional (1D) soil column and two-dimensional (2D) flume model tests simulating humid climatic conditions were carried out to evaluate the hydrological performance of a sustainable three-layer landfill cover system using recycled concrete aggregates (RCA) but without a geomembrane. This three-layer system consists of a layer of fine-grained and a layer of coarse-grained recycled aggregates (i.e., FRC and CRC, respectively) overlying the bottom fine-grained completely decomposed volcanic soil (CDV). The effects of vegetation on the three-layer cover system were also considered in the 1D column tests. In addition, numerical simulations were carried out to back analyse the model tests by using the modified Darcy–Richards equation in COMSOL Multiphysics to verify test results and to reveal the water flow mechanisms. Consistent results were obtained between the measured data and numerical predications. Even after the simulated extreme rainfall of Hong Kong with a return period more than 1,000-year, a higher matric suction was well-retained in vegetated cover system than that in bare cover due to evapotranspiration of plants. As revealed in the 2D flume model test and numerical simulation, the middle CRC layer can switch from a capillary barrier layer to a drainage layer to reduce infiltration into the bottom layer even under the heavy rainfall. Most of infiltrated rainfall water (i.e., more than 95% of total precipitation) can be diverted as surface runoff and lateral drainage in the two upper RCA layers. The rest of infiltrated water is stored in the cover system. No percolation is observed through the cover system using RCA during the flume test and predicted by numerical analysis. The physical model tests and numerical simulations consistently verified the effectiveness of the proposed sustainable three-layer landfill cover system using RCA without geomembrane under heavy rainfall in humid climatic conditions.