## 3 Field Tests and Results  
### Principles for a Support System  
The roof support was completed over two construction periods in the original scheme and included an average of eight bolts per row (bolt diameter = 22 mm, length = 2500 mm, preloading force = 30 kN) with seven cables per row (cable diameter = 17.8 mm, length = 8.8 m, preloading force = 60 kN) and a row spacing of 800 mm. Despite the very high roof support density, the foundation anchor structure could not restrain the initial expansion deformation of the top coal owing to the short bolt length and insufficient preloading force. This inefficiency led to notable roadway deformation and failure during tunneling, and the bearing capacity of roadway roof support structure is poor. Furthermore, the hard roof ruptured and turned downward in response to the mining disturbance of WF 4105. This seriously aggravated the stress concentration of the coal pillar and led to the formation, propagation, and convergence of a large number of cracks in the surrounding rock, thus intensifying the roadway deformation and failure. The severe roof subsidence compressed  
Figure 6: Monitoring data of roadway deformation under original support: (**a**) roadway deformation during tunneling and (**b**) deformation of roadway during mining of adjacent working face.  
the side rock mass, which aggravated the convergent deformation of the two roadway sides and further enhanced the shrinkage of the effective roadway space.  
According to the cross-border anchoring theory of thick roofs [31], with the increase of roof bolt length, the damage zone of the bolt end will move up gradually. The key difficulty of roof support in an extra-thick coal seam is that bolts should be anchored in the deep and stable rock mass. According to previous research [32; 33], the length of the bolts should be more than 3.5 m under the condition of an extra-thick coal seam. Combined with the actual conditions and the previous experience of the research group, the flexible 4-m-long bolts are determined to replace rebar bolts in the roof. At the same time, to ensure the continuous transmission of stress between anchorage structures, the length of the cable is reduced to 6.1 m.  
In the aspect of roadway stress optimization, the long-distance cantilever structure of the main roof must be destroyed. Considering the convenience of construction, deep-hole blasting technology is selected. The depth design of blasting drilling should consider the hard fine sandstone stratum of the roof. In this case, the vertical distance between the stratum and the coal seam is 28.28 m, which is the vertical length of the blasting borehole. Combined with the previous research [34] and the actual conditions, the blasting drilling angle is determined to be 40\({}^{\circ}\), and the length of drilling and blasting is 44 m.  
Based on the above analysis, a new surrounding rock control strategy considering deep- hole blasting pressure relief and thick anchoring zone were proposed. The new support system accounts for the following principles by integrating both field experience and the results of field investigations. (1) The bearing structure of the roof foundation is constructed using long flexible bolts to suppress the initial rock mass expansion and deformation. (2) Short cables are used to further strengthen the bearing structure of the roof foundation and enhance the anti-disturbance ability of the anchor structure. (3) Deep-hole blasting is adopted to pre-split the roof, which effectively reduces the coal pillar stress concentration, optimizes the roadway surrounding rock stress field, and minimizes damage to the anchor structure.