Permeability Characteristics of Broken Coal and Rock Under Cyclic Loading and Unloading

Abstract

During longwall mining, the permeability evaluation in a caving zone is one of the most challenging risks for mine operators because it controls the behavior of gas flow and it significantly influences gob gas extraction and spontaneous combustion. Based on the stress environment of repeated mining in the Huainan coal mine, this study discusses the results of a laboratory experiment on broken coal and rock in a caving zone of a gob. The stress–permeability relationship of broken coal, rock, and combination of coal and rock samples with various particle sizes under cyclic loading and unloading was experimentally studied. The laboratory test results imply that permeability stress sensitivity and permeability loss of the first loading and unloading were significantly greater than the succeeding cyclic loading and unloading. Re-crushing and re-arrangement were the main two factors that lead to a drastic drop in caving zone porosity causing permeability reduction in the first loading and unloading process. The initial permeability and initial pore compressibility increased logarithmically when particle size increased, while the changing rate of pore compressibility decreased exponentially. The stress sensitivity of the broken coal samples’ permeability increased when particle size increased. Based on the laboratory experiments, the stress–permeability fitting model considering particle size of broken coal and rock in a caving zone during repeated mining was proposed in this study.