Abstract
Because the first-weighting of a main roof with a large mining height has obvious sudden characteristics and is more severe, which causes large-scale support crushing and has a great impact on the ecological environment of the mining area, it is necessary to conduct an in-depth analysis. This paper studies the mechanical mechanism and asymmetric fracture conditions of a main roof with a large mining height, with the first-weighting occurring in a shallow coal seam. In combination with an asymmetric three-hinged arch structural model, the main roof was regarded as a finite plate model with a crack, and a fracture-mechanics model was established. The conditions and main controlling factors of main roof fracture asymmetry were analyzed, and the determination methods of the first-weighting interval and support resistance were further analyzed. The results show that the stress concentration and the stress-intensity factor increase at the crack tip with the advancement of the face; when the stress-intensity factors increase beyond the critical value, the crack expands until the first-weighting. The sufficient condition for modeling the instability was the length s of the branch crack reaching the protection thickness H of the main roof, and the necessary condition was the activation of the crack. The calculation equations of the first-weighting interval and the support resistance were obtained. The influence weights of each parameter on the support resistance are ordered as follows: overburden load q > rock fracture toughness KC > crack length a > main roof thickness h > weighting interval l. Finally, the theoretical analysis results were verified by an in situ monitoring case of the no. 33,206 working face in the Bulianta coal mine, China. On this basis, a reasonable value of the support resistance is further calculated. The results mentioned above can provide a new method for researching the first-weighting of the main roof and can improve the accuracy of the roof control analysis. The research on the mechanisms of first-weighting and the support resistance can effectively promote the safety production of mine, which is in line with the concept of green and sustainable development of the mine.
Highlights
Shallow coal seams with burial depths of less than 150 m are widely distributed in Gansu, Ningxia and Inner Mongolia in Northwest China and are characterized by shallow burial depths, thin bedrock, and overlying thick and loose sand layers [1]
The cut down of the entire thickness of the overlying strata along the working face during the first-weighting of the main roof was a dynamic evolutionary process [33]
The asymmetric fracture mechanical mechanism of the first-weighting of the main roof was analyzed using the fracture mechanics method
Summary
Shallow coal seams with burial depths of less than 150 m are widely distributed in Gansu, Ningxia and Inner Mongolia in Northwest China and are characterized by shallow burial depths, thin bedrock, and overlying thick and loose sand layers [1]. Wang et al [14] proposed a dynamic-load method for calculating the support resistance in combination with the instability characteristics of the roof of a large mining height working face in a shallow seam. Huang et al [25,26] pointed out that an asymmetric three-hinged arch structure was formed when the main roof fractured first during shallow coal seam mining and provided a method for calculating the support resistance. From the perspective of fracture mechanics, combined with the research conclusion of the asymmetric three-hinged arch structural model, this paper studies the mechanical mechanism of formation and expansion of the damage zone in the main roof, predicts the fracturing of the main roof, and analyzes the determination method of the firstweighting interval and support resistance. Fracture Characteristics of a Main Roof with a Large Mining Height during First-Weighting
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