Physical and numerical modeling of a novel unsaturated multilayered cover system for mining closure

Hydraulic cover systems used in tailings storage facilities (TSFs) in cold region mines commonly depend on thick compacted layers or geosynthetic liners, both of which tend to lose functionality when subjected to prolonged snowmelt or extreme precipitation. This work introduces an alternative cover concept: a lightweight, fully unsaturated multilayered cover system (MLCS) composed predominantly of locally available tailings and waste rock. In this approach, tailings are engineered either through clay–biochar amendment or by inducing surface hydrophobicity, enabling them to function as key components of a capillary-controlled barrier. The MLCS incorporates a capillary break, and a hydrophobic interface specifically designed to trigger water entry pressure (WEP) dependent resistance during severe ponding events. Comprehensive laboratory tests including soil–water retention characteristics, hydraulic conductivity functions demonstrated that modified tailings mixtures exhibit enhanced capacity to delay wetting. The hydrophobic layer, characterized by a WEP of approximately 4.71 kPa, permitted the construction of an exceptionally thin 0.5-m cover while still providing breakthrough resistance. Performance was evaluated using a fully instrumented one-dimensional physical model subjected to a constant ponding head of 115 mm maintained for 61 days. No percolation was detected beneath the MLCS for 61 days, and the hydrophobic interface remained unsaturated even as the overlying layers approached saturation. Coupled finite-element simulations employing WEP-based hydraulic switching reproduced the temporal evolution of suction, moisture distribution, and percolation observed in the physical experiment. Overall, the study shows that the MLCS can function as a robust, climate-tolerant hydraulic barrier and offers a cost-effective closure solution that minimizes reliance on imported geomaterials.