Backfill grouting for mining subsidence prevention

Backfill grouting for mining subsidence prevention

Mining subsidence has been a major hazard in most underground coal mines, particularly those designs and practices based on the wrong assumption of fixed, permanent and nondeteriorating coal pillars. Mining induced subsidence significantly affects mining costs where major surface structures and natural environment need to be protected. Remedial measures to manage damage caused by subsidence can often be very costly with potentially damaging impacts and irreversible consequences. Backfilling and injection of granular materials into the mining induced voids, separated beddings and cracks, as either diluted granular slurry or concrete paste, is widely used to control mine subsidence. Granular grouts and slurries made of mine and power plant wastes and rejects are viable environmental backfill solutions to both ground stability and mine waste management problems. Like concrete paste, the flowing slurry can be categorised as a generally nonlinear frictional viscous cohesive (Bingham Herschel-Bulkley) fluid. The general frictional viscous, cohesive, non-Newtonian fluid model has been applied to concrete flowability problems such as L-box and slump tests. While slump test is used in shallow foundations, L-box test is used in difficult deep foundations. It is designed to measure workability and flowability of tremie pipe concrete as an indirect index measure of concrete viscosity and plastic yield. Tremie pipes are used to control concrete flow rate and minimise bleeding and dilution when placed into deep submerged excavations. Mathematical and experimental models have been developed to not only solve the flow velocity along the L-box channel length as a function of time and distance, but also simulate the flow of the backfill material and demonstrate the detailed process of filling the voids to minimise any further subsidence.

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