Study on the fracture characteristics of thick-hard limestone roof and its controlling technique

The Taiyuan group is one of the coal-bearing formations in Shanxi Province, in which coal resource is extremely abundant. No.15 coal seam is the main minable coal seam in Taiyuan group, and it develops with thick-hard limestone roof which is a threat to safe and efficient mining because of the roof failure. Aiming at the geological setting of No.15 coal seam and its surrounding rock, we conduct an experimental study on physical and mechanical properties of No.15 coal seam and its floor as well as roof, and we also have an underground measurement on roof fracture width and floor loose range of headgate. Based on experimental results and fracture mechanics theory, we established a thick-hard limestone roof cracking model and discussed the factors that affect roof fracture initiation and extension. The research results demonstrate that roof fracture height is affected by the horizontal stress, surrounding rock deformation, roof mechanical strength and roof span. The deformation of wall and floor will induce the roof fracture extension. The roof fracture extension is high in stress concentration area under the upper coal face residual coal pillar. The fracture height can be effectively controlled by reducing surrounding rock loose range and improving mechanical strength of walls.