Numerical Simulation of Broken Coal Strength Influence on Compaction Characteristics in Goaf

The caving zone in goaf can be regarded as a porous medium consisting of broken coal and a rock mass. During the compaction process of broken coal and a rock mass in a caving zone, the re-breakage of the rock and coal affects the compaction stress and pore characteristics of the caving zone. In this study, a discrete element numerical simulation of a broken coal sample based on the bonded particle model (BPM) was carried out to study the evolution characteristics of stress, strain, and breakage during its compaction. The grading equation of the minimum particle size during the simulation process is given based on a laboratory test. This equation can reduce the effect of the sub-particles, which cannot be further broken through the BPM. The stress–strain curve of the BPM during compaction can be divided into two stages using maximum vertical strain, em, and the stress calculation model of these two stages is given. In first stage, there was no effect of the particle strength on em. Here, em of the BPM was equal to the porosity of the BPM minus the initial porosity of the broken particles. The average slope of a straight-line section after em was proportional to the elastic modulus of the coal samples with a ratio of 1.0892. In addition, the particle breakage rate, Bs, and its calculation model were proposed to describe the breakage of the BPM during the compaction process. The evolution characteristics of particle breakage and its correlation with the stress, strain, and particle location are illustrated. The breakage rate of the model increased with increase in strain in an S-shape function. When strain was greater than em, the particles had difficulty breaking again, and the breakage rates of soft coal, medium coal, and hard coal were 96.05%, 87.21%, and 87.78%, respectively. Under the same stress conditions, the breakage rate of soft coal was clearly higher than that of hard coal. The breakage rate increased the fastest when stress was equal to the tensile strength of the coal sample during the compaction process of the BPM.