Y-shaped fork tunnel closed steel arch support structure and its construction method

Abstract

The present invention relates to a Y-shaped fork tunnel closed steel arch support structure and its construction method. The present invention is applicable to the field of support for Y-shaped fork tunnels. The technical solution of the present invention is a Y-shaped fork tunnel closed steel arch support structure. The Y-shaped branch tunnel has a main tunnel, a fork tunnel, a first branch tunnel and a second branch tunnel connected to the fork tunnel. It is characterized by having: a closed first steel arch, a number of first steel arches are evenly installed in the main tunnel along the axial direction of the main tunnel; a closed second variable-span steel arch, a number of second variable-span steel arches are evenly installed in the fork tunnel along the axial direction of the fork tunnel; a semi-closed third steel arch, a number of third steel arches are evenly installed in the intersection area of the fork tunnel and the branch tunnel along the axial direction of the branch tunnel; a fourth steel arch, a number of fourth steel arches are evenly installed in the branch tunnel along the axial direction of the branch tunnel, and the arches are connected by longitudinal connecting steel bars.

Description

Technical Field

[0001] The present invention relates to a closed steel arch support structure for a Y-shaped bifurcation tunnel and a construction method thereof, which is applicable to the field of support for Y-shaped bifurcation tunnels. Background Art

[0002] In water conservancy and hydropower projects, the excavation shape of a Y-shaped bifurcation tunnel is complex and the excavation span is relatively large. If the bifurcation tunnel is arranged in an area with good geological surrounding rock conditions, shotcrete-bolt support and conventional construction schemes can ensure the stability of the surrounding rock. If the geological conditions of the surrounding rock section of the bifurcation tunnel are poor, strong support measures with a relatively large stiffness such as steel arches are required. Limited by the tunnel shape of the bifurcation tunnel and the construction scheme of sectional excavation at the bifurcation, the steel arches at the bifurcation crotch of the tunnel often cannot closely follow the excavation face to form a closed support structure, and the effects of large support stiffness and timely support of the steel arches cannot be exerted. This does not conform to the tunnel design and support concept of the New Austrian Tunneling Method, and there are potential hazards in the stability of the surrounding rock and construction safety at the bifurcation tunnel. Summary of the Invention

[0003] The technical problem to be solved by the present invention is: To solve the above technical problems, the present invention provides a closed steel arch support structure for a Y-shaped bifurcation tunnel and a construction method thereof.

[0004] The technical solution adopted by the present invention is: A closed steel arch support structure for a Y-shaped bifurcation tunnel, the Y-shaped branch tunnel having a main tunnel, a bifurcation tunnel, a first branch tunnel and a second branch tunnel connected to the bifurcation tunnel, characterized by comprising:

[0005] A first steel arch, the first steel arch being closed, and a plurality of first steel arches being uniformly installed in the main tunnel along the axial direction of the main tunnel;

[0006] A second variable-span steel arch, the second variable-span steel arch being closed, and a plurality of second variable-span steel arches being uniformly installed in the bifurcation tunnel along the axial direction of the bifurcation tunnel;

[0007] A third steel arch, a plurality of third steel arches being uniformly installed in the cross region of the bifurcation tunnel and the branch tunnel along the axial direction of the branch tunnel, the third steel arch being a semi-closed arch, and the third steel arch being fixedly connected to the adjacent second variable-span steel arch;

[0008] A fourth steel arch, a plurality of fourth steel arches being uniformly installed in the branch tunnel along the axial direction of the branch tunnel;

[0009] Longitudinal connecting steel bars for fixedly connecting adjacent first steel arches, adjacent second variable-span steel arches, adjacent third steel arches, adjacent fourth steel arches, adjacent first steel arches and second variable-span steel arches, adjacent second variable-span steel arches and third steel arches, and adjacent third steel arches and fourth steel arches.

[0010] Preferably, the semi - enclosed third - type steel arch frame forms a closed structure by welding the horizontal section of the third - type steel arch frame to the adjacent second - type variable - span steel arch frame.

[0011] Preferably, the first - type steel arch frame has a first side - top arch frame and a first invert arch frame. The first side - top arch frame and the first invert arch frame are made of I - beams, and the first side - top arch frame is fixedly connected to the surrounding rock through bolts.

[0012] Preferably, the second - type variable - span steel arch frame has a second side - top arch frame and a second invert arch frame. The second side - top arch frame and the second invert arch frame are made of I - beams, and the second side - top arch frame is fixedly connected to the surrounding rock through bolts.

[0013] Preferably, the third - type steel arch frame has a third side - top arch frame and a third invert arch frame. The third side - top arch frame and the third invert arch frame are made of I - beams. The third side - top arch frame is a semi - arched steel arch frame, and the third invert arch frame is a semi - invert arch frame. The third side - top arch frame is fixedly connected to the surrounding rock through bolts, and the third side - top arch frame and the third invert arch frame are respectively welded to the horizontal sections of the adjacent second side - top arch frame and second invert arch frame.

[0014] Preferably, the fourth - type steel arch frame has a fourth side - top arch frame and a fourth invert arch frame. The fourth side - top arch frame and the fourth invert arch frame are made of I - beams, and the fourth side - top arch frame is fixedly connected to the surrounding rock through bolts.

[0015] Preferably, the bottoms of the bolts fixedly connected to the first - type steel arch frame, the second - type variable - span steel arch frame, the third - type steel arch frame and the fourth - type steel arch frame are all bent, and the horizontal parts of the bent bottoms of the bolts are respectively welded to the bottom flanges of each steel arch frame.

[0016] Preferably, the first side - top arch frame and the first invert arch frame, the second side - top arch frame and the second invert arch frame, the third side - top arch frame and the third invert arch frame, and the fourth side - top arch frame and the fourth invert arch frame are all fixedly connected by connecting steel plates. Connecting steel plates are fixedly welded at the joints between the side - top arch frames and the invert arch frames respectively. The connecting steel plates between the side - top arch frames and the invert arch frames are fixedly connected by bolts and nuts.

[0017] Preferably, both ends of the longitudinal connecting steel bars are bent, and the bent parts at both ends of the longitudinal connecting steel bars are fixedly connected to the adjacent steel arch frames through angle steels.

[0018] A support method for a Y - shaped fork - hole closed steel arch frame, characterized in that

[0019] S1. Excavate the main tunnel towards the branch tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed concrete meets the requirements, install the steel mesh, several first-shaped steel arch frames, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0020] S2. Before excavating the branch tunnel, first construct radial pipe roofs for advanced support. After the grouting strength of the pipe roofs meets the requirements, start excavating the branch tunnel.

[0021] S3. After the excavation of the branch tunnel is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, second-shaped variable-span steel arch frames, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0022] S4. After the support of the branch tunnel is completed, excavate the intersection area between the branch tunnel and the first branch tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, third-shaped steel arch frames, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0023] S5. After the support at the intersection of the branch tunnel and the first branch tunnel is completed, continue to excavate along the direction of the first branch tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, fourth-shaped steel arch frames, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete.

[0024] S6. After the support of the first branch tunnel is completed, excavate the intersection area between the branch tunnel and the second branch tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the steel fiber concrete meets the requirements, install the steel mesh, third-shaped steel arch frames, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0025] S7. After the support at the intersection of the branch tunnel and the second branch tunnel is completed, continue to excavate along the direction of the second branch tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, fourth-shaped steel arch frames, anchor bolts, and longitudinal connecting steel bars until the distance from the face to the branch tunnel is not less than three times the tunnel diameter. After completion, re-spray steel fiber concrete.

[0026] The beneficial effects of the present invention are as follows: In the area with poor geological conditions of the surrounding rock tunnel section, the main tunnel, branch tunnel, and branch tunnels are excavated and supported in sequence, which can give full play to the effects of the large support stiffness and timely support of the arch frames. The third-shaped steel arch frame at the crotch of the branch tunnel is fixedly connected to the second-shaped variable-span steel arch frame at the adjacent branch tunnel, forming a closed support structure. Description of the Drawings

[0027] Figure 1 It is a schematic plan view of the steel arch frames of the Y-shaped branch tunnel.

[0028] Figure 2 It is the sectional view of the first type of steel arch frame.

[0029] Figure 3 It is the sectional view of the second type of variable-span steel arch frame.

[0030] Figure 4 It is the sectional view of the third type of steel arch frame.

[0031] Figure 5 It is the sectional view of the fourth type of steel arch frame.

[0032] Figure 6 It is the detail drawing of the joints of each steel arch frame.

[0033] Figure 7 It is the detail drawing of the connection between each steel arch frame and the anchor rod.

[0034] Figure 8 It is the detail drawing of the longitudinal connecting steel bars.

[0035] In the figure: 1. Main tunnel, 2. Branch tunnel, 3. First branch tunnel, 4. Second branch tunnel, 5. First type of steel arch frame, 6. Second type of variable-span steel arch frame, 7. Third type of steel arch frame, 8. Fourth type of steel arch frame, 9. Anchor rod, 10. Steel fiber concrete layer, 11. Steel mesh, 12. Longitudinal connecting steel bars, 13. First side-top arch frame, 14. First invert arch frame, 15. Second side-top arch frame, 16. Second invert arch frame, 17. Third side-top arch frame, 18. Third invert arch frame, 19. Fourth side-top arch frame, 20. Fourth invert arch frame, 21. Connection steel plate, 22. Angle steel. Specific implementation mode

[0036] This embodiment is a Y-shaped branch tunnel closed steel arch frame support structure. The Y-shaped branch tunnel has a main tunnel, a branch tunnel, a branch tunnel and a steel arch frame support structure installed in the main tunnel, the branch tunnel and the branch tunnel.

[0037] In this embodiment, a number of first type of steel arch frames are evenly installed along the axis direction of the main tunnel. The first type of steel arch frame is a closed steel arch frame formed by combining a first side-top arch frame and a first invert arch frame. Both the first side-top arch frame and the first invert arch frame are made of I-beams. The first side-top arch frame is fixed to the surrounding rock through anchor rods. The bottom of the anchor rod is bent, and the horizontal part of the bent bottom of the anchor rod is welded to the bottom flange of the first side-top arch frame. Steel fiber concrete is sprayed on the inner wall of the main tunnel, and a steel mesh is embedded in the steel fiber concrete layer.

[0038] Further, the first side crown arch and the first invert arch are fixedly connected by connecting steel plates. Connecting steel plates are fixedly welded at the connection points of the first side crown arch and the first invert arch respectively. The connecting steel plates between the first side crown arch and the first invert arch are fixedly connected by bolts and nuts.

[0039] Further, a number of longitudinal connecting steel bars for connecting two adjacent first profiled steel arches are installed between two adjacent first profiled steel arches. The two ends of the longitudinal connecting steel bars are bent, and the bent parts at the two ends of the longitudinal connecting steel bars are fixedly connected to the adjacent steel arches by angle steels.

[0040] In this embodiment, a number of second variable-span steel arches are evenly installed along the axial direction of the branch tunnel in the branch tunnel. The second variable-span steel arch is an enclosed steel arch composed of a second side crown arch and a second invert arch. Both the second side crown arch and the second invert arch are made of I-beams. The second side crown arch is fixed to the surrounding rock by bolts. The bottom of the bolt is bent, and the horizontal part of the bent bottom of the bolt is welded to the bottom flange of the second side crown arch. Steel fiber concrete is sprayed on the inner wall of the branch tunnel, and a steel mesh is embedded in the steel fiber concrete layer.

[0041] Further, the second side crown arch and the second invert arch are fixedly connected by connecting steel plates. Connecting steel plates are fixedly welded at the connection points of the second side crown arch and the second invert arch respectively. The connecting steel plates between the second side crown arch and the second invert arch are fixedly connected by bolts and nuts.

[0042] Further, a number of longitudinal connecting steel bars are installed between two adjacent second variable-span steel arches and between adjacent first profiled steel arches and second variable-span steel arches. The two ends of the longitudinal connecting steel bars are bent, and the bent parts at the two ends of the longitudinal connecting steel bars are fixedly connected to the adjacent steel arches by angle steels.

[0043] In this embodiment, a number of third profiled steel arches are evenly installed along the axial direction of the branch tunnel in the intersection area of the branch tunnel and the branch cavern. The third profiled steel arch is a semi-enclosed steel arch formed by a third side crown arch and a third invert arch. Both the third side crown arch and the third invert arch are made of I-beams. The third side crown arch is fixed to the surrounding rock by bolts. The bottom of the bolt is bent, and the horizontal part of the bent bottom of the bolt is welded to the bottom flange of the third side crown arch. Steel fiber concrete is sprayed on the inner wall of the intersection area of the branch tunnel and the branch cavern, and a steel mesh is embedded in the steel fiber concrete layer.

[0044] Further, the third - type steel arch frame is fixedly connected to the adjacent second - type variable - span steel arch frame, and the third side - top arch frame and the third invert arch frame are respectively welded to the horizontal sections of the adjacent second side - top arch frame and the second invert arch frame.

[0045] Further, the third side - top arch frame and the third invert arch frame are fixedly connected by connecting steel plates. Connecting steel plates are fixedly welded at the connection parts of the third side - top arch frame and the third invert arch frame respectively. The connecting steel plates between the third side - top arch frame and the third invert arch frame are fixedly connected by bolts and nuts.

[0046] Further, a number of longitudinal connecting steel bars are installed between two adjacent third - type steel arch frames and between the adjacent second - type variable - span steel arch frames and third - type steel arch frames. The two ends of the longitudinal connecting steel bars are bent, and the bent parts at the two ends of the longitudinal connecting steel bars are fixedly connected to the adjacent steel arch frames by angle steels.

[0047] In this embodiment, a number of fourth - type steel arch frames are evenly installed along the axis direction of the branch tunnel. The fourth - type steel arch frame is an enclosed steel arch frame formed by combining a fourth side - top arch frame and a fourth invert arch frame. Both the fourth side - top arch frame and the fourth invert arch frame are made of I - shaped steel. The fourth side - top arch frame is fixed to the surrounding rock by bolts. The bottom of the bolt is bent, and the horizontal part of the bent bottom of the bolt is welded to the bottom flange of the fourth side - top arch frame. Steel - fiber concrete is sprayed on the inner wall of the branch tunnel, and a steel - bar mesh is embedded in the steel - fiber concrete layer.

[0048] Further, the fourth side - top arch frame and the fourth invert arch frame are fixedly connected by connecting steel plates. Connecting steel plates are fixedly welded at the connection parts of the fourth side - top arch frame and the fourth invert arch frame respectively. The connecting steel plates between the fourth side - top arch frame and the fourth invert arch frame are fixedly connected by bolts and nuts.

[0049] Further, a number of longitudinal connecting steel bars are installed between two adjacent fourth - type steel arch frames and between the adjacent third - type variable - span steel arch frames and fourth - type steel arch frames. The two ends of the longitudinal connecting steel bars are bent, and the bent parts at the two ends of the longitudinal connecting steel bars are fixedly connected to the adjacent steel arch frames by angle steels.

[0050] The processing method of the present invention is as follows:

[0051] S1. Excavate the main tunnel from the main - tunnel direction towards the branch - tunnel direction. After the excavation is formed, initially spray steel - fiber concrete for support. After the strength of the initially sprayed concrete meets the requirements, install the steel - bar mesh, a number of first - type steel arch frames, bolts and longitudinal connecting steel bars. After completion, re - spray steel - fiber concrete for support.

[0052] S2. Before the excavation of the turnout tunnel, first, from the side top arch part of the main tunnel towards the turnout tunnel direction, construct radial pipe roofs for advanced support, including drilling, inserting rods, grouting, and waiting for strength. After the grouting strength of the pipe roof meets the requirements, start the excavation of the turnout tunnel section. The footage per cycle shall be adapted to the spacing of the arch frames, generally not exceeding 1 m.

[0053] S3. After the excavation of the turnout tunnel is formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install steel mesh, the second type variable-span steel arch frame, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0054] S4. After the support of the turnout tunnel is completed, excavate the intersection area between the turnout tunnel and the first branch tunnel. After the excavation is formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install steel mesh, the third type steel arch frame, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0055] S5. After the support of the intersection of the turnout tunnel and the first branch tunnel is completed, continue to excavate along the direction of the first branch tunnel. After the excavation is formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install steel mesh, the fourth type steel arch frame, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete. The length from the face of the first branch tunnel to the turnout block shall be not less than three times the diameter of the branch tunnel.

[0056] S6. After the support of the first branch tunnel is completed, excavate the intersection area between the turnout tunnel and the second branch tunnel. After the excavation is formed, initially spray steel fiber concrete for support. After the strength of the steel fiber concrete meets the requirements, install steel mesh, the third type steel arch frame, anchor bolts, and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support.

[0057] S7. After the support of the intersection of the turnout tunnel and the second branch tunnel is completed, continue to excavate along the direction of the second branch tunnel. After the excavation is formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install steel mesh, the fourth type steel arch frame, anchor bolts, and longitudinal connecting steel bars. The length from the face to the turnout tunnel shall be not less than three times the tunnel diameter. After completion, re-spray steel fiber concrete.

Claims

1. A closed steel arch support structure for a Y-shaped branch tunnel. The Y-shaped branch tunnel has a main tunnel, a branch tunnel, a first branch tunnel and a second branch tunnel connected to the branch tunnel. It is characterized in that comprising: a first type of steel arch frame, which is closed, and a plurality of first type of steel arch frames are evenly installed in the main tunnel along the axial direction of the main tunnel; a second type of variable-span steel arch frame, which is closed, and a plurality of second type of variable-span steel arch frames are evenly installed in the branch tunnel along the axial direction of the branch tunnel; a third type of steel arch frame, a plurality of third type of steel arch frames are evenly installed in the cross area of the branch tunnel and the branch adit along the axial direction of the branch adit, the third type of steel arch frame is a semi-closed arch frame, and the third type of steel arch frame is fixedly connected to the adjacent second type of variable-span steel arch frame; a fourth type of steel arch frame, a plurality of fourth type of steel arch frames are evenly installed in the branch adit along the axial direction of the branch adit; longitudinal connecting steel bars, which are used to fixedly connect adjacent first type of steel arch frames, adjacent second type of variable-span steel arch frames, adjacent third type of steel arch frames, adjacent fourth type of steel arch frames, adjacent first type of steel arch frames and second type of variable-span steel arch frames, adjacent second type of variable-span steel arch frames and third type of steel arch frames, and adjacent third type of steel arch frames and fourth type of steel arch frames; the semi-closed third type of steel arch frame forms a closed structure by welding the horizontal section of the third type of steel arch frame to the adjacent second type of variable-span steel arch frame; the first type of steel arch frame has a first side-top arch frame and a first invert arch frame, the first side-top arch frame and the first invert arch frame are made of I-beams, and the first side-top arch frame is fixedly connected to the surrounding rock through bolts; the second type of variable-span steel arch frame has a second side-top arch frame and a second invert arch frame, the second side-top arch frame and the second invert arch frame are made of I-beams, and the second side-top arch frame is fixedly connected to the surrounding rock through bolts; the third type of steel arch frame has a third side-top arch frame and a third invert arch frame, the third side-top arch frame and the third invert arch frame are made of I-beams, the third side-top arch frame is a semi-arch steel arch frame, the third invert arch frame is a semi-invert arch frame, the third side-top arch frame is fixedly connected to the surrounding rock through bolts, and the third side-top arch frame and the third invert arch frame are respectively welded to the horizontal sections of the adjacent second side-top arch frame and the second invert arch frame; the fourth type of steel arch frame has a fourth side-top arch frame and a fourth invert arch frame, the fourth side-top arch frame and the fourth invert arch frame are made of I-beams, and the fourth side-top arch frame is fixedly connected to the surrounding rock through bolts.

2. The closed steel arch support structure for a Y-shaped branch tunnel according to claim 1, characterized in that the bottoms of the bolts fixedly connected to the first type of steel arch frame, the second type of variable-span steel arch frame, the third type of steel arch frame and the fourth type of steel arch frame are all bent, and the horizontal parts of the bent bottoms of the bolts are respectively welded to the bottom flanges of the respective steel arch frames.

3. The closed steel arch support structure for a Y-shaped branch tunnel according to claim 1, characterized in that the first side-top arch frame and the first invert arch frame, the second side-top arch frame and the second invert arch frame, the third side-top arch frame and the third invert arch frame, and the fourth side-top arch frame and the fourth invert arch frame are all fixedly connected by connecting steel plates. Connecting steel plates are fixedly welded at the joints between the respective side-top arch frames and the respective invert arch frames. The connecting steel plates between the side-top arch frames and the invert arch frames are fixedly connected by bolts and nuts.

4. The closed steel arch support structure for a Y-shaped branch tunnel according to claim 1, characterized in that the two ends of the longitudinal connecting steel bars are bent, and the bent parts at the two ends of the longitudinal connecting steel bars are fixedly connected to the respective adjacent steel arch frames through angle steels.

5. A method for supporting a Y-shaped branch tunnel with a closed steel arch, characterized in that S1. Excavate the main tunnel towards the branch tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed concrete meets the requirements, install the steel mesh, several first-shaped steel arch frames, anchor bolts and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support. S2. Before excavating the branch tunnel, first construct radial pipe roofs for advanced support. After the grouting strength of the pipe roofs meets the requirements, start excavating the branch tunnel. S3. After the excavation of the branch tunnel is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, second-shaped variable-span steel arch frames, anchor bolts and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support. S4. After the support of the branch tunnel is completed, excavate the intersection area between the branch tunnel and the first access tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, third-shaped steel arch frames, anchor bolts and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support. S5. After the support at the intersection of the branch tunnel and the first access tunnel is completed, continue to excavate along the direction of the first access tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, fourth-shaped steel arch frames, anchor bolts and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete. S6. After the support of the first access tunnel is completed, excavate the intersection area between the branch tunnel and the second access tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the steel fiber concrete meets the requirements, install the steel mesh, third-shaped steel arch frames, anchor bolts and longitudinal connecting steel bars. After completion, re-spray steel fiber concrete for support. S7. After the support at the intersection of the branch tunnel and the second access tunnel is completed, continue to excavate along the direction of the second access tunnel. After the excavation is completed and formed, initially spray steel fiber concrete for support. After the strength of the initially sprayed steel fiber concrete meets the requirements, install the steel mesh, fourth-shaped steel arch frames, anchor bolts and longitudinal connecting steel bars until the distance from the face to the branch tunnel is not less than three times the tunnel diameter. After completion, re-spray steel fiber concrete.

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