Utilisation of waste rock backfill materials in deep gobs for mitigating mining-induced water inrush risks: compression deformation and seepage characteristics

In deep backfilling coal mines, granular waste rocks face complex stress and water environments after being backfilled in the goaf. To characterize the compression deformation and seepage of granular waste rock backfill materials in the deep goaf, a fluid-structure interaction (FSI) test system for granular waste rocks was developed. Influences of overburden stress on deformation and seepage characteristics of granular waste rocks were investigated. The research shows that the high overburden stress in deep underground coal mines can continuously change the structure of granular waste rocks. In the process of increasing overburden stress from 15 to 22.5 MPa, the porosity of granular waste rocks decreases rapidly and the permeability reduces fast accordingly. After the axial pressure exceeds 22.5 MPa, the porosity and permeability of granular waste rocks both tend to be stable. In the seepage process of confined groundwater under single overburden stress, the increase in water pressure can improve the porosity and permeability of granular waste rocks. Small waste rocks in samples play the leading role, they can achieve a waterproof state under relatively low overburden stress. The backfilling method that combines large and small particles can be considered, which can achieve rapid water proofing at a low cost.