Abstract:
Fully-mechanized caving mining with super-large mining height is an effective method for safe and efficient mining of thick and extra-thick coal seams. Aiming at the problem of poor top coal caving ability at the end of fully-mechanized caving mining in hard coal seams, this paper takes the 7.2 m fully-mechanized caving working face with super-large mining height in Shenshupan Coal Mine of Yushen Mining Area as the research background. Combined with the geological occurrence conditions of hard roof-thick hard top coal, based on the characteristics of mine pressure in 3115 fully-mechanized caving working face with super-large mining height, the deformation characteristics and fracture law of the end area of thick and hard roof are studied by medium-thick plate theory. The evolution mechanism of stress field-displacement field-crack field in top coal under special roof structure at the end is revealed by 3DEC numerical simulation. The influence of mechanical mining height and large setting force hydraulic support on the crushing effect of top coal at the end was studied. The results show that: ① The mining of 7.2 m super large space leads to the severe fracture of the overlying hard rock stratum, and the dynamic load effect of the initial pressure of the working face is significant. The periodic pressure of the working face has the characteristics of “large and small periodic pressure, large periodic continuous pressure ”. The time sequence of pressure is characterized by alternating transmission from the middle of the working face to both ends, and the spatial difference of pressure is significant, mainly concentrated in the range of No.20—No.90, and there is no pressure in the head and tail area of nearly 40 m. ② The fracture mode of hard roof is tensile-shear mixed fracture. The larger the thickness of the roof is, the more significant the spatial stiffness effect of the end is. The end area will continue to form a large-scale stable arc triangle hanging plate structure with the advance of the working face. The instability and fracture lag of the structure is significant, which demonstrates the safety and reliability of coal caving mining in the whole working face with super large mining height. The arc triangle hanging plate structure also shields the stress transfer of roof fracture instability, which leads to the nonlinear evolution of top coal in the underlying working face area, such as large caving block, lagging caving and even hanging roof. ③ By increasing the mining height of the working face and applying the hydraulic support with large initial support force, the structure and caving characteristics of the top coal body in the caving area can be actively controlled. When the mining height of the working face is increased to 7.2 m, the unloading damage of the coal body at the end of the advanced area is aggravated, and the top coal structure at the end is evolved from “ fissure beam zone ” to “block zone ” or “block zone-bulk zone ”. In the support-surrounding rock structure system of the end area, the support directly dominates the evolution path of the stress state of the top coal body and drives the deflection of its stress principal axis. The top coal in the roof control area experienced two loading-unloading, with two deflections in the direction of the maximum principal stress, and the tensile-shear mixed failure of the middle coal body occurred. The increase of the initial support force of the support effectively drives the middle and upper coal body to deconstruct and fracture. ④ The complete set of equipment for top-coal caving mining in the whole working face with super-large mining height has greatly increased the recovery rate of hard coal seam to 91.21%, with an average increase of 10.71%. At the end of single-sided mining, more than
600000 tons of coal is expected to be recovered. The new mode of fully mechanized top coal caving mining in the whole working face is realized, and the problem of difficult caving of hard top coal under the arc triangle hanging plate is solved, which plays an important role in promoting the development of extra-thick hard coal mining technology.