Intermediate coal pillar instability and permeability evolution in extremely thin protective seam by auger mining

Abstract

Protective seam mining method is an effective way to eliminate the coal and gas outburst hazard. Auger mining (AM) is adopted to protective seam thinner than 1.0 m. The key of AM is to set intermediate coal pillar (ICP) because they have two functions: on the one hand, ICPs have to support the roof temporarily to avoid coal auger crushing; on the other hand, they have to be broken subsequently to release the pressure of protected layer above. Theoretical analysis and numerical simulation are used to study the instability mechanism of the ICPs. On this basis, the effect of ICPs on the pressure relief and permeability evolution of the protected layer was analyzed. Results show that the reasonable ICP width is related to many factors, such as AM width, AM height, and ICP strength. The single ICP instability mode is compressive failure and the ICPs collapse one by one gradually instead of collapsing wholly and suddenly. The reasonable ICP width of Chiyu Coal Mine is 2.0 m with the conditions of 1.0 m AM height and 10.0 m AM width, namely when 4 AM rooms are mined by the coal auger and three ICPs are set. These ICPs collapse one by one and the sequence is No. 2, No. 1, and then No. 3. With the AM face advancing, the amount of ICPs increase and the pressure relief effect is also gradually improved. When some of ICPs collapse, the stress within the overburden above the goaf is released rapidly which significantly improves the performance of pressure relief.