Effect of loading rate on characteristics of cyclic structural adjustment of sandstone granules

During underground coal seam mining, changes in the working face advancement rate can easily affect the compaction state of granules in the collapse zone. This is an important factor in the induction of gas disasters and surface subsidence in mining areas. In this work, a cyclic loading and unloading mechanical test of granules under different loading rates was carried out. The changes in mechanical parameters of the granules at various stages were investigated. It is shown that the strain of each group of specimens under cyclic loading shows an increasing trend and the final strain increases with the loading rate. The input energy of the granules increases under cyclic loading, and under a low loading rate, the compaction force needs to overcome interparticle friction to destroy the relatively stable structure, which results in a need for more energy to achieve the same level of deformation. The acoustic emission ringing counts of each group of granules specimens show an overall increasing trend, with the highest proportion of ringing counts in the first loading stage. The compaction of pores and filling of particles under cyclic loading is a "uniform compaction, stable change, slow adjustment" dynamic process. When the loading is slow, the relative positions of the granule particles in each stress gradient are more adequately adjusted. The results of this study provide important theoretical support for the scientific formulation of gas control strategies and the prevention of surface subsidence in air-mining zones under different mining speeds.