Experimental study on seepage characteristics of broken coal and rock samples in caving zone under cyclic loading and unloading

When the coal-seams are multi-layered, the stress and permeability of the caving zone are disturbed again during the following coal seam mining. Repeated unloading and compaction cause caved coal and rock to break further in the caving zone, which in turn affects the migration of gas in goaf. This paper briefly summarizes prior research on caving zone internal stress, porosity, permeability and other parameters of the process of longwall face advancing. The forecast methods for calculating the stress and permeability of the caved zone during its compaction are listed. On the basis, this paper describes a laboratory experiment of the broken coal and rock in the caving zone of the gob in Huainan coal mine. The laboratory test results imply that the re-crushing, re-arrangement, and compressional deformation of particles in the loading process lead to a drastic drop in the caving zone porosity, which causes the permeability reduction. The permeability sensitivity to stress of broken rock (broken coal) samples was determined by their strength. The greater the proportion of coal samples in the combined coal and rock samples, the higher the permeability stress sensitivity and the smaller the permeability.With the increase of the cyclic times, the stress sensitivity gradually weakened. Based on laboratory experiments, the stress-permeability fitting model of the broken coal and rock in the caving zone during the group coal seam mining is proposed in this paper.