In coal mining operations, controlling the deformation of the roadway in front of the working face is a significant challenge. The excessive stress from the surrounding rock and difficulties in caving the goaf roof have hindered safe and efficient production. To address this issue, we have established a mechanical model of the stress field in the advance roadway of the working face and analyzed the stress evolution law of the surrounding rock before and after cutting the roof of the roadway.
Through theoretical calculations, numerical simulations with 3DEC software, and field testing, we have determined key parameters such as cutting depth, cutting angle, and hole spacing for a roof cutting and pressure relief technique (TCPAF) to enhance roadway safeguarding in the advance mining face. The results indicate that the stress on the low-stress side of the leading roadway remains slightly increased, while on the high-stress side, stress decreases significantly, optimizing the stress environment of the surrounding rock. This research provides a scientific basis for similar conditions of roadway deformation.
Geological Factors
The Pingdingshan Sixth Mine is located in Pingdingshan City, Henan Province, with a three-level pentane belt hill in the coal roof of pent9-10, approximately 20–24 m away from the vertical depth of pent9-10 coal (up to 30 m in some areas). The working face 22,190 has a strike length of 676 m, a width of 90.9 m, an average coal seam thickness of 2.45 m, and a dip angle of 8° to 17°. The coal seam strikes 98° to 107°, with a roof consisting of 5.4 m of sandy mudstone and 4.1 m of fine sandstone.
During the advancing of the mining face, the main roof fractures to form an “X-O” type collapse, creating a “overhanging beam” configuration with two curved triangular masses at each end and a larger cantilever mass supported by three fixed anchors and a simple support at the front end of the retreat roadway. Under the influence of this “overhanging beam”, the surrounding rock on both sides of the working face advance roadway is affected by uneven forces, with the stress on one side (right side) being excessive, thus affecting the stability of the surrounding rock in the roadway.
Cutting Depth Considerations
To analyze the deformation and failure mechanism of the roadway surrounding rock, we established a mechanical model of the stress distribution in the advance of the working face. The rock loads on the both sides of the working face advance roadway are simplified into F1 and F2, which are the surrounding rock stresses of the two sides of the working face advance roadway.
The reduction of overlying load and overhanging beam dead weight can effectively improve the loading condition of the advance roadway. Collapse gangue from the roof, serving as a support for the overhanging beam through blasting and cutting roof, can increase the supporting force by adjusting the cutting depth, thus improving the supporting force of the filling on the overhanging beam.
Roof cutting directly affects the length of the overhanging beam. The analysis of the change of the forces F1 and F2 on the two sides of the working face advance roadway reveals that when the cutting depth decreases, the force F1 on the left side of the working face advance roadway increases slightly, while the force F2 on the right side decreases significantly.
Structural Integrity Assessment
Based on the analysis, we propose a technology named advance working face roof-cutting (AWFT) to protect the roadway by reducing advance stress. The key parameters for roof cutting include cutting depth and cutting angle.
According to the rock mass dilatation theory, the cutting depth should be determined based on the thickness of the rock layer that can effectively support the overlying rock layer after the completely filled goaf is crushed. For the 22,190 working face of Pingdingshan Sixth Mine, the cutting depth is calculated to be 16.4 m, where the rock layer is 3.2 m away from being completely cut off.
The cutting angle affects the weight of the roof overhanging beam and the lateral supporting force of the collapse debris on the overhanging beam. A too small roof-cutting angle will lead to difficulties in drilling charges and sealing holes, and the collapse gangue cannot effectively support the overhanging beam laterally. Based on the bond-beam theory and the S-R stability principle of surrounding rock structure, the optimal cutting angle is determined to be 15°.
Monitoring and Mitigation
To verify the effectiveness of the TCPAF, we conducted numerical simulations using 3DEC software and field tests at the Pingdingshan Sixth Mine. The simulation results show that when the cutting depth is 16 m, the maximum stress on the right side of the roadway experiences the greatest decrease in surrounding rock stress, and the stress state of the surrounding rock on both sides of the roadway is tending towards symmetry, greatly improving the stress state of both sides and achieving optimal cutting effect.
The field monitoring data also confirms that after the roof cutting, the stress on the left side of the roadway surrounding rock increased by 23.06%, while the stress on the right side decreased by 30.42%. This indicates that the TCPAF can effectively reduce the overhanging length, accelerate roof collapse, and optimize the stress environment of the roadway.
In summary, the TCPAF method has been successfully applied in the field at Pingdingshan No. 6 Mine, effectively reducing stress on roadway surrounding rock while maintaining efficient production. This method can be tailored to meet different engineering conditions and geological environments, providing a scientific basis for similar conditions encountered in other mines.
