Abstract
Roof fracture is one of the main reasons for dynamic disaster in working face. During the process of long wall mining, due to the continuous fracture and collapse of roof, a stress shell composed of high stress is formed around the surrounding rock of the stope. The gravity of the overlying rock is transferred to the coal body around the stope through the stress shell. With the advancing of the working face, the stress shell has undergone the dynamic evolution process of the initial shell, extension shell, and flat shell. Based on the theory of shell and plate, the mechanical model of elliptical thin rock plate is constructed. The stress and bending moment distribution expressions of elliptic structure are obtained under two boundary conditions: the surrounding fixed-supported edges and simply supported edges. The results show that cracks begin to appear at the middle point of the semimajor axis and propagate along both ends of the semimajor axis. Then the fracture begins at the end of the semiminor axis, and the crack extends along the arc direction, forming an “X-O” type fracture. Finally, the mechanical nature of the stress shell instability is the result of the tensile fracture of the bedrock at the top of the shell.
Highlights
Roof fracture is one of the main reasons for dynamic disaster in working face
Based on the theory of shell and plate, the mechanical model of elliptical thin rock plate is constructed. e stress and bending moment distribution expressions of elliptic structure are obtained under two boundary conditions: the surrounding fixed-supported edges and supported edges. e results show that cracks begin to appear at the middle point of the semimajor axis and propagate along both ends of the semimajor axis. en the fracture begins at the end of the semiminor axis, and the crack extends along the arc direction, forming an “X-O” type fracture
Xie et al [17,18,19] found that there was a stress shell composed of high stress in the surrounding rock of the stope by using the comprehensive research methods of numerical simulation, similar simulation, and field measurement. e research shows that the stress shell is the main bearing capacity system of the overlying rock mass of the working face, which bears and transmits the stope pressure. e formation mechanism and evolution characteristics of the stress shell are related to the working face length, mining height, lithology, and other parameters, but the influence of the overlying rock fracture on the stress shell is ignored in the analysis
Summary
Based on the bending moment expression (9) and stress expression (10), the maximum moment and stress are obtained at the center of the elliptical rock stratum. E deflection equation of a semielliptical rock stratum satisfying all boundary conditions is as follows: ω2 A2ya2 + b2 − 1 ,. According to the internal force and bending moment formula of elasticity, the bending moment formula of equal thickness rock stratum is obtained as follows:. According to the stress derivation formula of elasticity, the expression of stress component of equal thickness rock stratum is obtained as follows: Ez z2ω z2ω σx μ2. According to formulas (18) and (19), the internal stress and bending moment of the elliptical rock stratum reach the maximum at point C. erefore, the crack appears first at point C. en the crack propagates along the periphery
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