Experimental research on the influence of different factors on the behaviour of broken coal and rock particles during compaction

Broken coal-rock masses are a major part of the goaf. The breaking of coal and rock particles in the compaction process affects various aspects including control of strata motion, prediction of surface subsidence, and backfill mining. Therefore, isotropic compaction experiments on broken coal-rock mass were carried out by utilising the loading part of the seepage test system for coal-rock mass to analyse influences of particle strength, particle size, mixed type, stress, and loading and unloading times on breakage of particles in the compaction process. The test results show that the crushing behaviour of coal and rock particles in the compaction process can be divided into three stages: the structural re-arrangement and crushing of particles under a low stress, particle breakage under moderate stress, and compression of particles under a high stress. The reduction rate of porosity decreases with the increase of particle strength and increase of loading and unloading times, increases with the increase of particle size, has nothing to do with two mix types of coal-rock mass after compaction. Under the same stress unloading conditions, the recovery amounts of particles from compression deformation were similar, which was not related to unloading times. Research results is significant to those analysing overlying strata motion, predicting surface subsidence, optimising the material ratio for backfill mining, etc.