Field-scale modelling of compaction and permeability evolution in longwall caving zones

Abstract The geomechanical and hydraulic evolution of the longwall caving zone is vital for gas drainage, spontaneous combustion prevention, and abandoned mine utilisation. This study established layered composite particle size distribution (PSD) models, simulating the compaction and permeability evolution of the longwall caving zone under quasi-static compression. By mapping field-measured stress to longwall face advance distance, we quantitatively analysed the macroscopic and partitioned response of four composite PSD models. Macroscopic strain confirmed that final compressibility is negatively correlated with the proportion of coarse broken rocks. Partitioned strain analysis revealed a key effect of stiffness: the composite PSD model, possessing the stiffest upper skeleton, produced the largest strain in the basal fine-rock deposition region. Regarding permeability, the part of these models with the broadest gradation range exhibited the highest initial permeability but the lowest stress sensitivity, while the with the narrowest gradation range demonstrated the highest sensitivity. Streamline analysis confirmed that permeability is highly spatially heterogeneous, with upper regions forming primary conductive channels and the basal region constituting an effective seepage barrier. These findings provide essential understanding for predicting fluid migration within the caving zone.