The rock mass in the caving zone of a goaf is relatively broken and considered a porous medium. Additionally, it has the characteristics of irregular size and shape and sharp edges, and it is easy to break. In the process of caving zone compaction, the shape characteristic of a broken rock mass is one of the most important factors affecting the evolution of the compaction characteristics and the water permeability of the caving zone. Through discrete element numerical simulation and theoretical research, the influence of the shape characteristic on compaction characteristics and the water permeability of a broken rock mass is analyzed. The research results are as follows: (1) The number of edges on a caved broken rock mass is negatively correlated with the strain limit of compaction, the initial void ratio and the final breaking ratio. It is positively correlated with the deformation modulus and the residual dilatancy coefficient. (2) The smaller the amount of edges on the broken rock mass, the more obviously the rotation movement occurs during compaction. (3) The smaller the number of edges on the broken rock mass, the faster the decline in the rate of the water permeability, and the lower the water permeability at the final stable stage. (4) With an increasing number of broken rock mass edges, the total strain energy and the dissipative strain energy of caved broken rock masses show a decreasing trend, while the elastic strain energy shows a growing trend.

Grouting is the most widely used technology for treating coal goafs. In this study, a numerical simulation method was used to establish a model of multi-seam goafs with different spacing conditions to investigate the subsidence reduction effects of various grouting schemes on multi-coal seam goafs. By varying the range and opportunity of grouting treatments, the effects of coal seam spacing, grouting treatment range, and grouting opportunity on subsidence reduction were analyzed. The results showed that: (1) With constant overburden (OB), the subsidence reduction ratio of the subgrade center increases linearly as the interburden (IB) decreases (1 ≤ OB/IB ≤ 2), then increases exponentially (2 < OB/IB), and eventually becomes stable. (2) When treatment is conducted based on the half-width of the subgrade, the width of the subgrade, and the range of the trapezoid, residual surface subsidence tends to adopt an inclined ‘W’-shape in open cutting. The surface residual subsidence exhibits a symmetrical ‘W’-shape when full-range grouting is adopted. (3) For a multi-coal seam goaf with longer mining stoppage time, the subsidence reduction ratio of the subgrade center is lower, and it is exponentially related to the grouting opportunity. As the grouting opportunity is extended and OB/IB decreases, the subsidence reduction ratio of the subgrade center declines exponentially.