## 4 Discussion  
### Destruction Mechanism of Hard-Roof Roadway Surrounding Rock in Extra-Thick Coal Seams  
The zonal fracture characteristics of roadway surrounding rock significantly affect the stability of roadway support structure [35]. The thickness of extra-thick coal seams above the roof is typically more than 4 m, which makes the roof support structure mostly arranged within the coal seam. Coal has a weak shear strength and primary fractures are often developed. Top coal affected by mining often undergoes expansion and dislocation deformation. A large number of joints and cracks develop on the microscopic level and failure and extensional deformation of the inner separation layer occur on the macro level [36]. The size of the plastic failure circle of roadway roof surrounding rock is typically larger than that of conventional roadways. The anchoring depth is therefore greater for the bearing structure of a shallow foundation supported by bolts. In this case, the supporting structure of the original scheme is limited by the rigid material and limited bolt length (2.5 m). The supporting range of the bolt remains in the fracture zone of the surrounding rock, as shown in Figure 11. However, the lack of bolt preloading force worsens the result. This causes a large number of stratum separation cracks to form within and outside of the bolt anchoring area, seriously damages the integrity of the roof rock mass, and strongly reduces the strength and stability of the shallow bearing structure constructed by the bolts. In this case, even the preload provided by the cable cannot enhance the strength of the shallow bearing structure and integrity of the rock mass. The anchor cable plays a more important role in suspending shallow fractured rock mass, but active reinforcement is not achieved. The roadway surrounding rock is thus affected by mining of the adjacent working face and large deformation and instability occur. Figure 10: Field construction and tunnel maintenance effects: (**a**) torque amplifier preloading flexible bolt; (**b**) hydraulic pit drill construction blasting drilling; (**c**) maintenance effect of sides; and (**d**) maintenance effect of roof. The long distance of suspended roofs is another important cause of the severe roadway deformation and failure. According to the voussoir beam theory proposed by [PERSON] [37], a voussoir beam structure will form on both sides of the stope after coal mining, as shown in Figure 12. In this case, the key strata of the roof are hard fine sandstone with high strength and strong self-stabilizing ability and the rock structure does not easily cave. This extends the cantilever length of the key block B that bears the high rock pressure above it. Caving rock blocks cannot effectively fill the goat owing to the large movement space of the overburden rock in the goat and small thickness of the direct roof in the fully mechanized caving face. The key block B of the voussoir beam structure cannot be supported by gangue in the goat over long time periods. The key block B will continue to rotate and sink until its weight is balanced by transferring the pressure from the overburden to the coal body and pillar. FLAC\({}^{\text{3D}}\) software was used to simulate the surrounding rock stress field changes of AR 4106 under the influence of the mining of WF 4105. The model with dimensions is 268 m \(\times\) 50 m \(\times\) 100 m. The Mohr-Coulomb model is used to simulate the rock strata and coal seam. The horizontal and bottom sides are roller constrained. The vertical load is applied to the top boundary of the model which equals to the self-weight of the overlying strata. The simulation results are shown in Figure 13. As shown in Figure 13,b, high stress concentrations formed on the side of the protective coal pillar near the roadway after mining WF 4105 and the stress environment of the surrounding rock changed. The maximum stress was 28.3 MPa and the stress concentration coefficient was 1.58.