Geotechnical Response of Cemented Paste Backfill under the Multiphysics Coupled Conditions in Deep Mines

Abstract

Cemented paste backfill (CPB), a mixture of tailings, water, and binder, is widely used in underground mining to stabilize mine openings, improve ore recovery, and enable underground disposal of mine wastes. The geotechnical behaviour of CPB, including strength development, deformation response, stress-strain behavior, and pore water pressure evolution, is critical to the safe and economic design of backfill structures. In deep mines, however, CPB does not cure under conventional stress-free laboratory conditions. Instead, it hardens under coupled thermal-hydraulic-mechanical-chemical (THMC) conditions, including time-dependent vertical self-loading, closure-driven multiaxial confinement, elevated field temperature, drainage or no-drainage boundaries, and possible seepage/infiltration. These coupled processes strongly affect CPB hardening and can fundamentally alter the mechanisms governing its performance. The main objective of this thesis is to investigate the behaviour of CPB under deep-mine-representative curing conditions, with emphasis on the coupled effects of time-dependent multiaxial stress, hydraulic boundary evolution, and elevated field temperature. To achieve this objective, a novel multiaxial compressive stress curing and monitoring apparatus was developed to reproduce closure-related deep-mine curing conditions at laboratory scale. Using this system, a coordinated experimental programme was conducted to examine the effects of multiaxial stress path, horizontal closure magnitude/rate, directional anisotropy, confinement timing, drainage and seepage condition, elevated temperature, and integrated THMC factor combinations on CPB behaviour. The results show that CPB behaviour in deep mines is fundamentally stress-path dependent and multiphysics controlled. Strength development, deformation evolution, positive pore water pressure (PWP), and suction cannot be interpreted from stress magnitude, temperature, or binder content alone. Instead, the response depends on the coupled evolution of multiaxial stress path and stress history, thermal exposure, and hydraulic boundary conditions during curing. Time-dependent multiaxial confinement significantly modifies CPB hardening by promoting stress-assisted densification, changing pore structure evolution, and altering the hydraulic state during curing. The results demonstrate that confinement timing, closure intensity, and directional balance govern whether closure acts as a beneficial densification mechanism or shifts toward damage-dominated behaviour. Moderate and well-balanced confinement can enhance strength, whereas excessive or highly anisotropic closure can reduce densification efficiency, induce cracking, and compromise structural integrity. The hydraulic response of CPB under deep-mine curing is shown to be a critical component of both early-age safety and long-term performance. Closure-driven confinement amplifies and prolongs early-age positive PWP, increasing hydraulic demand on barricades, while controlled drainage reduces the magnitude and duration of this risk. After positive PWP dissipates, suction develops as hydration and/or drainage drive desaturation. Suction evolution is also stress-path dependent: mechanical confinement delays suction onset by maintaining high saturation, while stronger late-age confinement may induce a stress-related resaturation tendency. Elevated temperature accelerates hydration and can intensify both early-age PWP response and later-age suction development through faster water consumption. In contrast, seepage/infiltration acts as a major disturbance by suppressing or collapsing suction and by reducing retained ionic inventory through dilution and solute transport, thereby limiting later-age hydration and strength gain. The thesis further demonstrates that thermal and mechanical effects interact synergistically under deep-mine THMC curing. Elevated temperature strengthens CPB mainly through a hydration-controlled pathway, while multiaxial confinement strengthens CPB mainly through a structure-controlled pathway with stress-assisted hydration. When acting together, these effects reinforce each other, producing the highest strength development under deep-mine-representative conditions. Factor-isolation analysis shows that, within the tested ranges, thermal and mechanical effects are the dominant contributors to strength enhancement, while hydraulic conditions remain essential regulators of safety and performance efficiency. Binder content sets the achievable strength ceiling, but actual strength depends on whether the curing environment allows hydration products to form, remain, and be effectively integrated into a stable load-bearing skeleton. Overall, this thesis establishes that conventional stress-free laboratory curing does not reliably represent CPB behaviour in deep underground mines. It provides experimentally constrained evidence that CPB performance in deep mines is governed by the coupled evolution of multiaxial stress path, hydraulic boundary conditions, and thermal exposure during curing. The findings develop a mechanistic framework for understanding CPB response in closure-dominated deep-mine environments and provide a stronger basis for safer, more realistic, and more cost-effective backfill design.