Field investigation of fractures evolution in overlying strata caused by extraction of the jurassic and carboniferous coal seams and its application: Case study

Abstract

Abstract In multiple seam coal mining, the fractures induced by the mining of the lower coal seam may propagate to the goaf of the upper coal seam that is mined earlier. The fracture connection leads to the leakage of hazardous gas. To fully understand the gas leakage due to fracture evolution, the electromagnetic-based measurement system and the borehole televiewer were adopted in the panel No.8100 in Tongxin coal mine, Datong Coalfield, China. The coal seam thickness is 15\u202fm and it was mined by top-coal caving method. The resistivity and fracture pattern in distinct zones of rock stratum were detected, and the correlation between them was explained clearly. We found that the fracture evolves in two stages as the panel working face advances, i.e., fracture propagation and fracture re-closure. The maximum height of fracture propagation is 150\u202fm, in which the height of the caved and the fractured zones are 80\u202fm and 70\u202fm, respectively. These fractures start to be compacted and are re-closed at the distance of 80\u202fm behind the working face. The extraction of the panel No.8100 results in the formation of a gas-bearing zone (GBZ) with separations and shear fractures rather than connecting the goafs of the carboniferous coal seam (CCS) and the jurassic coal seam (JCS). The GBZ is located in a region from 80\u202fm to 150\u202fm above the goaf and 80\u202fm behind the working face. Based on the above findings, effective gas drainage approach is proposed to reduce the concentrations of the hazardous gases consisted of CH4 and CO. After gas drainage, the maximum concentrations of CH4 and CO drop to 0.5% and 16.63\u202fppm, with reductions of 53.7% and 33.9% respectively.