5.1 Analysis of vertical stress distribution law of roof during tunneling
As shown in Fig. 8, the model is sliced along the roof of the coal seam, and the vertical stress distribution at different positions before and after the head is analyzed. The colors in the figure from red to blue indicate that the vertical stress gradually increases. There is a "C"-shaped stress concentration zone at about 0-15m head-on in front of tunneling, and there is a stress superimposed zone greater than 4.0Mpa on the side of the coal pillar of the roadway during the double-lane tunneling process.
The model roadway has a total length of 180m. When driving 90m, the vertical stress analysis is carried out at 5, 15, 25, and 35m front and rear respectively. As shown in Fig. 9, the 5m front and rear of the driving is a severely disturbed area, and the stress concentration intensity is more than 5.3Mpa. ; The front 15m is the disturbance mitigation zone; the front 25m ~ 35m is in the excavation disturbance stable zone, which is not affected by mining.
The vertical stress distribution behind the front is similar to the front. Stress concentration occurs on the left and right sides of the roadway, and the influence range gradually increases as it is far from the head of the tunnel. The stress concentration in the area 15m away from the head of the tunnel gradually weakens. Within 1.5m of the shallow surface of the two sides, there is a sudden unloading area of roadway excavation stress. This phenomenon is often accompanied by the phenomenon of fragmentation. There is a stress concentration phenomenon at 8m above the top plate at the front end. As the distance from the front end increases, the concentrated stress gradually decreases.
As shown in Fig. 10, during the double-lane tunneling of the new scheme, there are "C-shaped" vertical stress distribution areas at different positions above the roof of the working face. The stress difference is 0.02Mpa at a distance of 1.5 ~ 2.5m above the roof; the distance is 2.5 ~ At 3.5m, the stress difference is 0.2Mpa; at the distance of 3.5 ~ 4.5m, the stress difference is 0.05Mpa. Therefore, it is concluded that the excavation engineering activities have caused different levels of disturbance effects on the roof, which also explains the low-density cross-boundary roof from the front. The support meets the existing roadway control requirements.
As shown in Fig. 11, with the head-on position as the origin of the coordinates, the vertical stress of the survey lines at 0m, 1m, 2m, 3m, 4m, and 5m on the roof of the roadway is monitored. It is found that the vertical stress changes significantly within the range of 20m from the front of the roadway. In the range beyond 20m, the vertical stress is relatively stable and changes slowly.
After the roadway is excavated, the vertical stress of the roof is significantly reduced due to the pressure relief effect, and its magnitude is much smaller than the original rock stress. Taking the 0m measurement line above the roof as an example, the stress suddenly increased to -4.3Mpa at 20m behind the head, and slowly increased to -4.7Mpa at the front, while the stress at 20m in front of the head gradually decreased, and the decrease was reduced from 20m. The internal stress of ~ 40m decreases slightly, approaching − 4.5Mpa, which is the original rock stress at this moment. Within the range of 40m ~ 90m, the stress gradually stabilizes to about − 4.5Mpa.
5.2 Analysis of the vertical displacement change of the roadway during the stoping period
1)Once mining
After one mining, the displacement and deformation of the roadway are shown in Fig. 12(a). The maximum deformation of the roadway roof is 0.5 mm, and the bottom deformation is 1.2 mm. Due to the influence of the goaf on the right, the left and right sides are asymmetrically deformed. The deformation of the left side is 0.25 mm, and the deformation of the right side is 0.5 mm.
Displacement and deformation analysis of the roadway roof is sliced, as shown in Fig. 12(b). The displacement and deformation of the ultra-front end of the roadway are the same as the deformation at the initial stage of driving, both within 0.5 mm, and the deformation near the front end exceeds 1 mm. With the gradual influence of one mining operation, the deformation of the roadway gradually increases. The peak value is 1.5 mm or more.
After a mining operation, the vertical stress distribution of the roadway is rough as shown in Fig. 12(c). The stress concentration on the coal pillar side reaches 4.54 MPa, which is 1.14 times the original rock stress. The stress in the goaf on the right is about 0.07 MPa. In addition, stress concentration occurred at the super front end of the working face on the right side, reaching 4.5 MPa.
2)Secondary mining
To study the impact of secondary mining on the S1231 tunneling face, the S1231 face was mined at 0.8 m each time. After 50 steps, the mining was performed again. After 89.6 m was mined, the calculation was completed to balance.
As shown in Fig. 13(a), the maximum displacement and deformation of the roof after the stoping of this working face is 10mm, the left side deforms 2.5 mm, the right side deforms 7.5mm, and the bottom plate deforms about 2.5mm.
As shown in Fig. 13(b), there will be three main stress concentration areas after the mining face, the stress disturbance area in front of the work face, the stress distribution area of the coal pillar structure, and the stress distribution area on the side of the goaf, and the stress on the side of the coal pillar. The peak concentration peak is the largest, which can reach 7.9 MPa; the peak stress peak at the super front end of the working face is second, at about 22.5 MPa; the peak stress concentration peak in the goaf area on the right is the smallest. Except for a stress concentration area in the middle of the goaf area, most of them are Original rock stress.
5.3 Discussion
During tunneling, the front and back 5m range is the stress fluctuation area, and the 5 ~ 15m is the stress adjustment area. After 15m, it gradually approaches the original rock stress area. It can be seen that the coal body has higher strength and better integrity, so the stress transfer range is smaller. From the perspective of the layout of the stress measurement line, the deep stress of the roof within 20m behind the head has a larger increase of 0 ~ 2m, and the maximum increase at 0m is as high as 2.3Mpa. The stress fluctuation range of the area above 2m is small. This phenomenon is explained from the front. The shallow area within 2m of the roof is an important area to control the integrity and stability of the roof, and the design length of the anchor rod in the new scheme just achieves the cross-border support effect, and successfully builds a thick anchor layer.
After one mining, the maximum deformation of the roof of the roadway is only 0.5 mm, the deformation of the bottom plate is 1.2 mm, the deformation of the left side is 0.25 mm, and the deformation of the right side is 0.5 mm. After the second mining, the deformation of the roadway roof reaches 10mm, the left side deforms 2.5mm, the right side deforms 7.5mm, and the bottom plate deforms about 2.5mm. Combined with the cross-border anchoring mechanism(Xie et al., 2018), the results show that after the excavation of the tunneling project, the stress in the shallow part of the surrounding rock gradually increases. After the shallow surrounding rock produces cracks, the stress gradually transfers to the deep part, causing further extensive damage. The cross-border anchoring and supporting technology increases the length of the anchor rod under the premise of high prestress, which makes the development of shallow cracks slow during tunneling, and the degree of crack development is greatly reduced. In the later stage of mining disturbance, the roof has strong bearing performance and high integrity, so it can effectively control the deformation of the roadway.
Numerical simulation results show that the low-density cross-border support technology has a positive effect on the surrounding rock control of the S1231 tunneling face of the Caragana Tower, achieving good results, and can be tested on-site.