Analysis of the Initial Fracture Characteristics for Main Roof Overlying Fully Mechanized Face with Large Mining Height

The fully mechanized mining technique with large mining heights has become the most widely adopted mining method for thick coal seams in Shendong Coalfield of China. Increases in the mining height result in rib spalling, support instability, and other disastrous accidents, in addition to abnormal roof weighting. Knowledge of the initial fracture characteristics of the main roof is critical in the control of ground pressure. Based on an analysis of the impacts of controlled caving on fractures in the main roof, a mechanical model of rock beams that considers the supporting effects of hydraulic support and caved gangue is proposed. The initial fracture characteristics of the main roof are analyzed. The results indicate that the initial step of fracturing of the main roof is dependent on the physical and mechanical properties of the roof, the external load, and the support provided by the hydraulic support and caved gangue; the initial step increases with increases in the supporting effect and the roof-control distance of the hydraulic support and caved gangue. In combination with in situ monitoring data, the characteristics of the main roof are verified. The support provided by the caved gangue has different effects on different positions of the main roof, and this effect is smaller for higher positions.