Control mechanism of air leakage cracks at regenerated roof with coal spontaneous combustion

Spontaneous combustion of regenerated roof coal is one of the common types of mine fire, and the air leakage in the regenerated roof cracks is the key cause of oxidation and spontaneous combustion of coal left. At the present stage, the support mode of the regenerated roof is only from the angle of “strong wall support, maintain stable roof”,and the research on the control of air leakage in the regenerated roof cracks is not sufficient, especially the research on the evolution and expansion of air leakage cracks caused by stress concentration. In view of the return airway regenerated roof at No. 2304 working face in the Luwa coal mine of Shangdong Province, according to the theory of elastic mechanics, the critical failure criterion and the basis of the generalized Hooke's law, the relationship between stress concentration factors and the regenerated roof characteristic parameters of surrounding rock under the various measures is deduced respectively. Based on actual situation and the regenerated roof crushing experiments with fractal dimension, the stress concentration control and the crack evolution of regenerated roof under various measures are expounded.Resultsshow that the effect of elastic modulus on the rock stress concentration of regenerated roof is most significant. According to the calculated working conditions on site, using bolt support, the maximum stress concentration factor is 2.341,using slurry cementing support, the maximum stress concentration factor is 1.563,and using nano-slurry bubble material cementation support, the maximum stress concentration coefficient is 1.393. In addition, in the crushing test, it is found that the cracks mainly propagate from the center of cement material cementation body in all directions, the horizontal and longitudinal fractures will continue to increase, and gradually be connected. For the material of nano-slurry bubble, on the basis of original cracks, the crack evolution continues to be developed, when the stress exceeds 8 MPa, the cracks in nano-slurry bubble material cementation body will be no longer developed. Fractal dimension calculation results show that when the stress is less than 5 MPa, the porous structure of nano-slurry bubble material can make the fractal dimension significantly more than that of the cement material cementation body. When the strees is greater than 5 MPa, especially when it exceeds the compressive strength, the fractal dimension of nano-slurry bubble material cementation body will be significantly less than that of the slurry cementing body, so the evolution of crack extension and evolution rate is slower, which is beneficial to prevent air leakage in the cracks of regenerated roof.