Abstract: How to quantitatively describe and evaluate the mining-enhanced permeability for low-permeability coal seams has always been in a blind state, which results in the inadequacy of fracturing measurements and gas drainage during coal mining for local conditions. Mining-enhanced permeability can reflect the influence of mining or other measures on permeability enhancement and also quantitatively evaluate the effect of coal seam permeability enhancement, and its distribution and evolution can be used as a guide to the location of gas enriched area and the rational layout of gas drainage boreholes. The volumetric strain caused by mining is determined by simplifying the model of borehole with cracks. A single borehole algorithm is proposed for mining-enhanced permeability. A field test for detecting mining fracture network is carried out at mining face No.8212 of Datong coal mining group. According to the fracture data of each single borehole, the evolution of mining induced fracture network and pressure are studied, and the evolution of a single borehole mining-enhanced permeability is analyzed. The results show that the fracture network evolution presents a trend of growing out of nothing, growing from short to long, growing from narrow to wide and connecting continuously, respectively. The single borehole mining-enhanced permeability rises at first and then maintains steady as the mining face advances, thus providing a foundation for the design of gas drainage at the working face.

Key words: mining effect, overlying strata, fracture evolution, single borehole mining-enhanced permeability