Influence mechanism of particle size on the compaction and breakage characteristics of broken coal mass in goaf

Caving zone is generally composed of broken coal and rock mass in coal mine goaf.During the compaction process of a caving zone, the breakage characteristics of broken coal and rock mass directly affect the physical and mechanical properties and pore and flow characteristics of the caving zone.The yield criterion of granular materials based on curvature radius and contact stress is constructed, and then the laboratory tests and three-dimensional numerical simulation are further carried out to study the breakage characteristics and the influencing mechanism of broken coal samples with different particle sizes during its compaction.According to the particle breakage criterion, the main parameters that affect particle breakage are the contact curvature radius and material parameters of contact particles.It is concluded that the larger the size of the broken coal and rock mass is, the smaller the contact stress between the broken coal and rock mass is.The experimental results show that within the range of experimental particle size, under the same stress state, the larger the particle size, the less likely it is to be broken, and the smaller the breakage rate calculated according to the gradation number before and after loading.Furthermore, particle coordination number (particle contact number) is proposed and is the main reason that determines the impact of particle size on breakage rate.Small particles with less coordination number have larger deviatoric stress when transmitting the same stress, which leads to easier breakage compared with particles with multiple coordination numbers.The quantitative simulation results also show that when the original large particles are completely broken, the proportion of sub-particles larger than 100 in the three kinds of particles (10, 15 and 20 mm) is 6.7%, 24.6% and 31.5% respectively.With the loading of broken coal samples, the coordination number corresponding to the largest particle of coal samples with different particle sizes increases continuously, which makes the stress that the broken coal samples can bear far exceed their own strength.When the coordination number reaches a certain value, the broken coal sample is basically not broken, the coordination number of the coal sample particles remains basically unchanged.Based on the results of theoretical analysis, laboratory test and numerical simulation, it is concluded that the main mechanism of the influence of particle size on the breakage characteristics is the difference between contact stress and coordination number.In the early stage of loading, the difference of coordination number is smaller, the larger the size of the broken coal sample is, the smaller the contact stress of particles under the same loading stress state of the model is.In the later loading process, the coordination number of large particle size is larger than that of small particle size, which makes it more difficult to break. © 2020, Editorial Office of Journal of China Coal Society. All right reserved.