Roof Movement Characteristic and Pressure Relief Range Determination during the Lower Long-Distance Protective Seam Mining

Protective layer mining is an important technology for gas control, while long distance, the insufficient pressure relief, and borehole construction are the problems. It is important to clear roof movement laws and pressure relief range of the long-distance protective layer mining. By theoretical analysis and numerical calculation, the paper analyzed stress distribution laws and calculation formula of pressure relief range in the long-distance protective seam mining, and discussed the roof movement characteristics and pressure relief range during working face advancing in 9# seam. The results showed that any stress point in the protected seam underwent five stages with working face advancing of the protective seam: original rock stress - stress increasing - stress reduction - stress recovery - stability. With working face advancing, the pressure relief range of the protected seam constantly expanded, while the seam stress level did not reduce, and the stress began to recover with caving rock compacting, the stress in the protected seam showed the process from stress decreasing during rock movement to stress recovering during the compaction, and the maximum pressure relief of the protected seam was 13.7% of the original rock stress. In the pressure relief range, with the working face advancing, the maximum expansion deformation rate decreased gradually to 3.34‰. The effective protected range of seam 9 was 10~177m after the working face advancing. It was calculated that the pressure relief angles of seam 9 were 79°and 82°respectively after strike mining of seam 15. The upper pressure relief angle was 77°and the lower one 72°after dip mining. The research results provided a basis for gas extraction in long-distance protective layer mining.