Hydraulic buffer potential of tailings based geo-composites for mine closure

The high cost and limited availability of suitable soils for hydraulic barriers pose a major challenge for tailings storage facility (TSF) closure. This study evaluates the feasibility of using onsite tailings as alternative cover materials through four modification approaches (bentonite amendment, biochar addition, hydrophobic treatment and particle size fractionation). Laboratory testing assessed the hydraulic and geotechnical properties of these geo-composites. Among them, 5% bentonite amendment performed best as a barrier material, achieving a saturated hydraulic conductivity (k_sat) of 3.24 × 10⁻⁹ m/s and an air entry value (AEV) of 59 kPa. Partially hydrophobic samples significantly reduced infiltration, lowering k_sat by up to two orders of magnitude under both dry (w% = 0) and wet conditions (compacted at optimum moisture content (OMC) of 16.8%), indicating suitability for multilayered covers. Biochar amended tailings (5%) showed moderate k_sat (2.67 × 10⁻⁷ m/s) and high-water storage capacity (0.41 m³/m³), suggesting potential for surface layers supporting vegetation. Numerical modelling of an idealised cover system (2H:1 V sloped) with thin bentonite amended tailings layer beneath a biochar modified tailings layer demonstrated improved stability, maintaining a factor of safety of 2.5 under a 100-year rainfall event, while an uncovered slope fell below Finnish regulatory limits. Stability was achieved through suction retention and enhanced runoff, including for steeper slopes (1H:1 V). These findings highlight the potential of tailings-based geo-composite covers as sustainable alternatives for TSF closure design and support their consideration in future guidelines and regulations.