Study on mechanical mechanism of roof fracture evolution in deep stope with variable face length

Irregular coal seam mining is easy to cause the problems such as roof stress concentration and abnormal ground pressure behavior. In order to explore the roof fracture law and the structural evolution characteristics of stope with variable length, the roof structure models with four types of boundary conditions were established and analyzed using the small-deflection thin plate bending theory in view of the geometric characteristics and mechanical causes of the abrupt stope with the inclined length of working face changing from small to large in various mining stages. Meanwhile, the roof fracture law and macro-mechanical response were analyzed by MATLAB and FLAC numerical simulation methods based on the zoning characteristics of roof ground pressure in stope with variable face length. Through systematic analysis and summary, the transmission and evolution model of overburden structure comprising “three stopes, three areas and three structures” in working face with variable length was constructed, and the regional fracturing effect of “two stopes and two laws” was put forward. Besides, the application was verified by the measured ground pressure of typical engineering cases. The results show that the stope with variable face length is divided into small-face stope, variable-face stope and large-face stope. Specifically, the roof of small-face stope is in a slow pressure structure, where the traditional “O-X” fracture occurs. The roof of variable-face stope is in an abrupt structure, where the cracks generated by the extended and drift “O-X” fractures have similar development characteristics to the cracks in the roof of large face stope in pressurized structure, and so they are integrated into a full-face stope. Besides, the roof of the full-face stope is fractured in “X-O” shape, with the cracks developing continuously to produce the extended fracture. Thus, the roof fracture theory of “two stopes and two laws” was formed. The research conclusion provides an important basis for exploring the essence of overburden migration in stope with variable face length and strengthening the roof prevention and control under the occurrence conditions of deep complex coal seams.