Experimental Investigation of Compaction Deformation and Seepage Mechanism of Granular Waste Rock Backfill Materials for Mitigating Mining-Induced Water Hazards

The development of mining-induced fracture zones in the backfilling working face near the aquifer significantly increases the seepage risk of granular waste rock backfill materials (GWRBMs), posing threats to groundwater resources and security problems. This study experimentally investigates the coupled effects of overburden stress and particle size distribution on the non-Darcy seepage characteristics of GWRBMs. A novel stepwise seepage pressure testing apparatus was designed to simulate axial loading stress (0∼15 MPa) and seepage pressure (1∼8 MPa), enabling the simultaneous monitoring of displacement, porosity evolution, and flow dynamics. Key findings reveal the following. Particle size distribution-dependent response: Under a fixed axial loading stress, the initial porosity of GWRBMs increases with particle size. Within the same range of seepage pressure variation, smaller-particle GWRBMs exhibit higher porosity variation rates but smaller flow rate changes. For instance, under increasing seepage pressures from 1 to 8 MPa, the 0-5 mm GWRBMs demonstrated a porosity variation rate of 4.58% and a flow rate change of 5.441 L/h, while the larger-particle counterparts displayed an inverse trend. Permeability-particle size distribution correlation: The permeability coefficient exhibits an increasing trend with particle size, while the attenuation of liquid flow inertial effects results in a reduction of the non-Darcy factor β. GWRBMs demonstrate hybrid particle size advantages, particularly when fine particle content exceeds critical thresholds, and the seepage characteristics of GWRBMs with different particle sizes are more similar to those of small particle sizes of GWRBMs. The distribution of particle sizes of GWRBMs has a significant impact on their own seepage characteristics. Overall, the findings of this study are of great significance for the protection of water resources and the prevention of water-related hazards in the solid filling working face near the aquifer.