Re-Crushing Evolution Characteristics of Broken Rock Samples with Different Particle Sizes in Goaf during Compaction

The compaction and re-crushing characteristics of broken rock samples (BRS) in the goaf caving zone directly influence the seepage characteristics in the goaf, mining pressure on the longwall face, and surface subsidence. To study the compaction and re-crushing characteristics of BRS with different particle sizes, the experiment of BRS compaction considering particle size was carried out with the help of acoustic emission (AE) monitoring system. The experimental results indicate that BRS with larger particle sizes release more energy and have a higher number of re-crushing when subjected to the same loading stress. But the smaller the corresponding stress under the same strain conditions of the larger particle sizes. On the basis, a stress-strain model for BRS compaction considering particle size was developed. The cumulative count and energy of AE from BRS continue to increase during the broken stage. The larger the particle size, the more AE events occur. Re-crushing of BRS is mainly characterized by tensile failure. As stress increases, the proportion of shear failure continues to decrease. However, larger particle sizes show a higher proportion of shear failure. The b-value of AE indicates that the BRS undergoes a process of stable crack development during re-crushing. The acoustic emission localization experiment shows that the re-crushing of particles occurs as layered breakage in the vertical direction and uniform breakage in the horizontal direction. Horizontally, breakage starts in the boundary area and spreads evenly to the central area. Vertically, the upper layer breaks first, followed by the middle and lower layers. The research results have certain guiding significance for the overlying rock strata and surface movement, control of mining pressure manifestation, and safety management of goaf.