The longwall caving zone formed during coal mining constitutes a complex porous medium characterised by strong spatial heterogeneity and pronounced temporal evolution. Understanding its compaction behaviour and permeability evolution is not only vital for controlling conventional mining hazards, such as gas migration and water inrush, but also critical for determining the feasibility of emerging applications, including underground water storage and carbon dioxide sequestration. However, existing continuum-based studies inherently struggle to capture the mechanisms driven by the discrete nature of broken rock masses. This thesis addresses this limitation by establishing a numerical modelling framework integrating the Discrete Element Method (DEM) with COMSOL Multiphysics. A fully parameterised rock geometry generation method is proposed to quantitatively characterise the relationships between rock-scale structural heterogeneity and macroscopic engineering response. Research findings demonstrate that rock block geometry exerts a fundamental control on mechanical stability. Unlike idealised spheres, angular blocks exhibit higher stress sensitivity due to stress concentrations that induce early-stage secondary rebreakage. This irregularity significantly alters the pore network topology, increasing the tortuosity of flflow paths. Simultaneously, Particle Size Distribution (PSD) is shown to be critical. Through a systematic analytical framework, this study establishes how the initial PSD range dictates the mechanical and hydraulic evolution of the longwall caving zone. Extending the analysis to the active caving zone, results indicate that while widely graded structures develop higher stiffness through enhanced interlocking, their pronounced heterogeneity induces significant non-Darcy flow behaviour and elevated inertial resistance under high-velocity airflow conditions. To bridge these rock-scale mechanisms with engineering-scale reality, a full-scale composite grading model representing the vertical stratification of a practical Australian longwall mine was developed using a “space-for-time” substitution strategy. The model successfully reconstructs the spatiotemporal evolution of permeability and reveals a distinct interlayer stiffness effect. It is found that the load transfer from the upper coarse-rock layers induces anomalously high strain in the underlying fine-fragment layers. This mechanism leads to the formation of pronounced vertical hydraulic zoning, where the basal layer evolves into a seepage barrier while the upper sections remain conductive. Overall, this thesis quantifies the governing relationships between rock morphology, grading structure, and macroscopic response. The findings provide a robust theoretical foundation for improved hazard mitigation in deep coal mining and for the sustainable, resource-oriented utilisation of abandoned underground space.