1.School of Mines,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China
2.State Key Laboratory of Coal Mine Disaster Dynamics and Control,Chongqing University,Chongqing 400044,China
3.State Key Laboratory for Fine Exploration and Intelligent Development of Coal Resources,China University of Mining and Technology,Xuzhou,Jiangsu 221116,China
Thick and hard roofs are characterized by high strength and large thickness. During periodic fracture, the abrupt release of accumulated energy can trigger mine tremors and, in turn, induce rock-burst hazards. Existing studies generally idealize thick and hard roofs as rock strata with uniform thickness, which differs markedly from their actual occurrence characteristics. To address this limitation, based on Timoshenko beam with gradually varying cross-section, the exact calculation methods for the deflection, rotation, bending moment, shear force, and energy of thick and hard roofs during periodic fracture were developed in this study. Analytical relationships were further derived for the periodic weighting interval, elastic energy density, elastic energy, and the relationship between energy release and the impact load on hydraulic supports. The effects of cantilever length, overburden load, and stratal trend on fracture characteristics and dynamic hazards were systematically investigated. The results show that, in terms of fracture mechanics, the limiting periodic weighting interval follows a Lorentzian peak-function relationship with the stratal trend coefficient (k₁/k₂), with the peak occurring near k₁/k₂ = 1. The limiting periodic weighting interval increases as a power function of cantilever length and decreases exponentially with overburden load. In contrast, the peak bending moment increases sharply as a power function of cantilever length, increases linearly with overburden load, and is unaffected by stratal trend. In terms of energy accumulation, the elastic energy density and accumulated elastic energy exhibit similar evolution patterns: both increase as power functions of cantilever length and overburden load, but decrease as power functions of the stratal trend coefficient. In terms of impact hazard, the impact load on hydraulic supports also follows a Lorentzian peak-function relationship with stratal trend, increases linearly with cantilever length, and decreases as a power function of overburden load. The sensitivity of the impact load to the influencing factors ranks as follows: overburden load > stratal trend > cantilever length. Therefore, pre-fracturing measures should be designed to reduce the effective size of the thick and hard roof by modifying its along-strike geometry and shortening its cantilever length, thereby weakening mine-tremor energy accumulation and release at the source. These findings provide a theoretical basis and a practical control pathway for the precise prevention and control of rock-burst hazards in working faces with thick and hard roofs.
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