Dynamic collapse mechanism and prevention of shallow-buried pillar group underlying working seam floor in mined-out area

The safety problems of the shallow-buried overlying and underlying mined-out areas with pillars in the Yuanbaowan Coal Mine are investigated using some theoretical analysis, laboratory experiments, physical simulation, numerical simulation and field measurement methods. In particular, the vertical load and internal stress distribution patterns of coal pillars underlying the working seam floor in the mined-out area are explored,and the potential damage form of the coal pillars in the mined-out area is discussed based on the Mises equivalent stress. The failure mode of key pillars and the movement characteristics of fractured flakes in the mined-out areas are revealed, and the failure sequence of underlying coal pillar groups and dynamic instability evolution are clarified.Therefore, a method of strengthening the bearing capacity of the underlying coal pillars through grouting in the minedout area is proposed to recover the coal resources in the middle coal seam between the mined-out seams, and the critical filling height of the mined-out area to ensure a safe mining of No.6107 working face is given. The results show that if the mined-out area of No. 9 coal seam is not filled, the uneven subsidence of the roof strata will lead to the eccentric compression of the coal pillars, the key pillar and its fractured bodies rotate and subside eventually. Once the key pillar is damaged, the overburden load presents the characteristics of two-way transmission. A two-way domino chain-type collapse centered on the key pillar is shown in the entire coal pillar group, while the outward radiating wave-like instability expansion characteristic with the key pillar as the center is presented in the three-dimensional space. The failure and instability of the key pillars will induce the overall instability of the underlying coal pillar group, which may lead to induce such dynamic disasters as a large-scale collapse of the working face and its advanced coal rock mass. The critical filling height for the safe mining of the working face No.6107 at the mined-out area is 6 m, which is also confirmed by the actual measurement results. On this basis, for the regional area where the filling effect is poor and the filling height is less than 6 m, the secondary filling and re-inforcement treatment method is proposed to ensure the safe mining of remaining coal resources in the middle seam above the mined-out area.