Numerical simulation of compaction and re-breakage characteristics of coal and rock samples in goaf

During the compaction process of broken coal and 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 and rock sample (BCRS) based on the bonded particle model is carried out to study the evolution characteristics of stress, strain and breakage during its compaction. The influence of coal-rock combination ratio and structure on the breakage and compaction characteristics of BCRS is analyzed. The stress–strain curve of the BCRS during compaction can be divided into two stages with the maximum vertical strain ε_m, and the stress models for these stages are given. When the strain exceeds ε_m, the stress increases linearly, and the slope of the straight line is proportional to the proportion of rock practices in the BCRS. But the proportion of rock practices has little effect on the ε_m. With the increase of strain, the breaking rate of BCRS increases in an S-shaped manner. When the strain is greater than ε_m, the coal and rock practices will be basically no longer broken. Under the same coal-rock ratio, the coal-rock combination structure has a great influence on the breaking rate of the BCRS. In the loading process of BCRS, the broken coal practices take precedence over the broken rock ones, and then produce stress relief and filling effect on the surrounding broken rock particles, which greatly reduces the breaking rate of rock samples. Finally, the fitting model for breaking rate-strain of composite BCRS is given, and the strain at the maximum increase speed value of the breaking rate is put forward to quantitatively analyze the influence of coal-rock ratio on the breaking rate.