Construction method for entering main tunnel from parallel pilot tunnel of tunnel

Abstract

The invention discloses a construction method for entering a main tunnel from a tunnel parallel pilot tunnel, which comprises the following steps: S1, when a communicating transverse tunnel is tunneled to be close to a main tunnel construction section, gradually raising the arch crown elevation of the communicating transverse tunnel; s2, after the communicating transverse tunnel continues to be tunneled to the excavation contour line of the main tunnel, reinforced supporting is conducted, then a shed frame is adopted to enter the main tunnel, and after the shed frame climbs obliquely upwards to the top of the tunnel, excavation is conducted forwards with a flat slope to the arch foot position of a step on the outer side of the main tunnel; s3, after the tunnel is tunneled to the range of the main tunnel, the arch part of the main tunnel is temporarily supported, and then the top of the main tunnel is excavated; s4, primary support is conducted on the top of the main tunnel; s5, expanding and excavating the middle-upper rock body section by section; s6, under the primary support of the upper section, excavation and support are conducted in the main attack direction through a step normal; and S7, when the tunnel face distance of the upper section reaches 30 m, bottom falling construction of the side wall of the lower half section is started. The method is high in reliability, and safe construction of entering the main tunnel from the parallel pilot tunnel can be achieved.

Description

Technical Field

[0001] This invention belongs to the field of tunnel construction technology, specifically relating to a construction method for a parallel pilot tunnel leading into the main tunnel. Background Technology

[0002] Tunnels have experienced rapid development as a crucial transportation infrastructure. During the construction and operation of tunnels, some longer highway tunnels often incorporate parallel pilot tunnels alongside the main tunnel to serve multiple purposes, including providing geological forecasting, muck removal channels, material transportation, ventilation, and maintenance. However, the short distance between the parallel pilot tunnel and the main tunnel means that their parallel construction can sometimes interfere with each other. Furthermore, the construction process for the connecting cross passage from the parallel pilot tunnel to the main tunnel is complex and its safety is difficult to guarantee. Specifically, the current elevation of the connecting cross passage is different from that of the main tunnel, making direct entry into the main tunnel impossible. Therefore, the supports installed during the entry process have an inclination angle, but the stress on these supports differs from that of normal connecting cross passages and main tunnel supports, compromising construction safety. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and provide a construction method for a parallel pilot tunnel to enter the main tunnel. By constructing a connecting cross tunnel between the parallel pilot tunnel and the main tunnel, two new working faces can be provided for the main tunnel. Moreover, the connecting cross tunnel and the parallel pilot tunnel can serve as channels for materials, personnel, ventilation, drainage, etc., of the new working faces of the main tunnel. The construction method of this invention has high reliability and can achieve safe construction of the parallel pilot tunnel into the main tunnel.

[0004] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A construction method for a tunnel parallel to a pilot tunnel leading into the main tunnel includes the following steps: S1. In the parallel pilot tunnel, gradually excavate the connecting cross tunnel towards the main tunnel. When the connecting cross tunnel is excavated to the section close to the main tunnel construction area, start to gradually raise the arch elevation of the connecting cross tunnel and extend the steel frame. S2. After the connecting cross tunnel continues to be excavated to the outline of the main tunnel, reinforced support is installed at the junction of the connecting cross tunnel and the main tunnel. Then, a scaffold is used to enter the main tunnel. After the scaffold climbs up the slope to the top of the tunnel, it is excavated forward on a flat slope to the foot of the outer step arch of the main tunnel. S3. After the tunneling reaches the main tunnel area, temporary support is provided for the arch of the main tunnel, and then the main tunnel roof is excavated. S4. After the main tunnel roof excavation is completed, initial support shall be provided for the main tunnel roof. S5. Next, the middle and upper rock mass is gradually expanded and the initial support of the right half is extended to the arching line. Each cycle advances 1.0 to 1.5 meters. S6. Under the initial support of the upper section, the step method is used to excavate and support in the main attack direction; S7. When the distance between the upper section face and the working face reaches 30m, the construction of the lower half section sidewall bottoming begins.

[0005] Preferably, in step S1, the connecting cross tunnel and the main tunnel are set orthogonally, the cross section at the junction of the connecting cross tunnel and the main tunnel is enlarged in the form of a 75° flared mouth, and the intersection of the connecting cross tunnel and the parallel guide tunnel at the small pile side is set with a rounded corner, the radius of which is 5m.

[0006] Preferably, the flared opening at the junction of the connecting cross passage and the main tunnel is reinforced with three parallel I20 I-beam gantry frames, which also serve as the support for the main tunnel steel frame.

[0007] Preferably, in step S2, the establishment of the reinforced support includes the following steps: S21. Within the area of ​​the connecting cross passage, I20 type steel frame is used to form the cross passage arch frame. The spacing between the steel frames is set to 1.0m. The steel frames are longitudinally welded together with φ22 threaded steel bars. The circumferential spacing of the connecting bars is set to 1m. φ42 locking foot anchor pipes are used for positioning. S22. Shot C25 concrete onto the arch wall of the connecting transverse opening, with a thickness of 20cm. Then, install φ25 hollow grouting anchors in the arch and φ22 mortar anchors on the side walls. The length of both the hollow grouting anchors and the mortar anchors is 3m, and the spacing is 1.0m×1.0m, arranged in a quincunx pattern. Hang φ6 steel mesh on the arch wall, with a spacing of 20cm×20cm, overlapping 1~2 meshes, and welding point by point to complete the steel frame arch support on the connecting transverse opening. S23. A longitudinal horizontal support beam is installed on the formed steel arch support along the direction of the main tunnel. The support beam is made of I20 steel and is welded from 3 I20 steel frames. The two ends of the support beam are connected to I20 steel columns to support the bottom of the main tunnel wall, thus completing the reinforced support.

[0008] Preferably, in step S2, multiple locking anchor pipes φ42 are installed at the arch foot of the outer step of the main tunnel, and the length of the locking anchor pipes is 4m.

[0009] Preferably, in step S3, the establishment of the temporary support includes the following steps: S31. Based on the stability of the surrounding rock, I20 temporary scaffolding is used to support the top. The scaffolding is spaced 0.8m apart and the scaffolding is welded together with φ22 steel bars. S32. Combined with 5cm of initial shotcrete, the temporary scaffold beams are made of 20cm shotcrete mesh with a mesh size of 15×15cm, forming a temporary support system.

[0010] Preferably, in step S4, the initial support includes the following steps: S41. An I20 steel frame is set at the cantilever of the main tunnel to form the cantilever arch frame of the main tunnel. The spacing is 1.0m. The steel frames are welded together with φ22 longitudinal connecting bars. The circumferential spacing of the tie rods is 1m. Each node of the upper and lower step arch foot of the steel frame is positioned by 4m long φ42 locking foot anchor pipes. Two locking foot anchor pipes are set at each node. S42. Based on step S41m, spray a 15cm thick layer of C25 concrete at the arch wall. Then, install φ25 hollow grouting anchors at the arch and φ22 mortar anchors at the side walls. The length of both the hollow grouting anchors and the mortar anchors is 6m, and the spacing is 1.0m×1.0m. Both the hollow grouting anchors and the mortar anchors are arranged in a quincunx pattern. Then, hang φ6 steel mesh at the arch wall with a spacing of 20cm×20cm, overlap 1-2 meshes, and weld them point by point to form the initial support.

[0011] Preferably, in step S7, the construction steps for the lower half-section sidewall bottoming are as follows: The left and right side walls are staggered by 3-5m for bottom excavation and support, with an advance of 2m per cycle, while advancing forward.

[0012] Compared with the prior art, the advantages of this invention are as follows: (1) The construction method of parallel pilot tunnel into main tunnel provided by the present invention can provide two new working faces for main tunnel by constructing a connecting cross tunnel between parallel pilot tunnel and main tunnel. The connecting cross tunnel and parallel pilot tunnel can serve as channels for materials, personnel, ventilation and drainage of new working faces of main tunnel, which improves convenience. (2) The method provided by the present invention can connect the connecting cross tunnel to the main tunnel by using pipe roof support to extend upwards, and connect the support of the connecting cross tunnel to the main tunnel, thereby further improving the reliability of the support and enabling safe construction of the parallel pilot tunnel into the main tunnel. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the planar layout of the connecting cross tunnels leading from the parallel pilot tunnel into the main tunnel in this invention. Figure 2 for Figure 1 Cross-sectional view at section 1-1; Figure 3 for Figure 1 Cross-sectional view at section 2-2; Figure 4 for Figure 3 A magnified view of part A in the middle; Figure 5 for Figure 3 Cross-sectional view at section 3-3; Figure 6 for Figure 1 Cross-sectional view at section 4-4; In the diagram: 1. Main tunnel; 101. Main tunnel face; 102. First new working face; 103. Second new working face; 2. Parallel pilot tunnel; 201. Pilot tunnel face; 3. Connecting cross tunnel; 4. Main tunnel cantilever arch; 5. Cross tunnel arch; 6. Support beam; 7. Steel column; 8. Frame; 801. Frame crossbeam; 802. Frame column. Detailed Implementation

[0014] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below with reference to specific embodiments, but the embodiments are not intended to limit the present invention.

[0015] The construction method for a parallel pilot tunnel leading into the main tunnel provided in this embodiment of the invention includes three tunnel chambers: the main tunnel 1, the parallel pilot tunnel 2, and the connecting cross tunnel 3. Further, the main tunnel 1 includes three working faces: the main tunnel face 101, the first new working face 102, and the second new working face 102. The parallel pilot tunnel 2 has one working face: the pilot tunnel face 201.

[0016] This invention addresses a foreign tunnel, which is characterized by highways being constructed within a single tunnel. This presents a problem: rescue and maintenance cannot be carried out through the tunnel alone, necessitating the construction of an auxiliary passage. To increase the working face and facilitate future material transport, rescue, and maintenance, a connecting passage is built. However, due to the different elevation, unlike normal passages (parallel and downward-oriented), the connecting passage in this invention is constructed upwards and must connect to the main tunnel. Therefore, pipe roof support is required for upward construction. This invention provides a climbing scaffold method for upward construction, which allows the connecting passage to connect with the main tunnel support, ensuring tunnel construction safety.

[0017] like Figure 1 As shown in the figure, the construction method for parallel pilot tunnel leading into the main tunnel provided in this embodiment of the invention specifically includes the following steps: S1. Within the parallel pilot tunnel 2, a connecting cross tunnel is gradually excavated towards the main tunnel 1. When the connecting cross tunnel 3 approaches the construction section of the main tunnel 1, the elevation of the arch of the connecting cross tunnel 3 is gradually raised, and the steel frame is extended. Traditionally, parallel pilot tunnels enter the main tunnel through direct excavation, eliminating the elevation difference between the pilot tunnel and the main tunnel. However, if a connecting cross tunnel is used, during excavation, the tunnel is excavated to the face of the main tunnel before excavating to both sides. Because the elevation of the connecting cross tunnel is lower than that of the main tunnel, direct excavation becomes impossible. Since the tunnel enters the main tunnel from the connecting cross passage, there will be an inclination angle during support construction. However, the support stress is not exactly the same as that of the normal connecting cross passage and main tunnel. This results in an imbalance of support stress, leading to uneven settlement of the tunnel or even collapse. Therefore, in the specific construction of this embodiment, when the connecting cross passage is excavated to a section close to the main tunnel construction area, the elevation of the connecting cross passage arch is raised and the steel frame is lengthened. This facilitates a good connection with the main tunnel, thereby connecting the support of the connecting cross passage with the support of the main tunnel and ensuring the safety of tunnel construction.

[0018] In step S1, the connecting cross tunnel 3 and the main tunnel 1 are set in an orthogonal manner. The cross section at the junction of the connecting cross tunnel 3 and the main tunnel 1 is enlarged in the form of a 75° funnel. The intersection of the connecting cross tunnel 3 and the parallel guide tunnel 2 at the small pile side is set with a rounded corner, and the radius of the rounded corner is 5m.

[0019] Specifically, the funnel-shaped opening at the junction of the connecting cross passage 3 and the main tunnel 1 is reinforced by welding three parallel I20 I-beam gantry frames, which also serve as the support for the steel frame of the main tunnel 1.

[0020] S2. After the connecting cross tunnel continues to be excavated to the outline of the main tunnel 1, reinforced support is installed at the junction of the connecting cross tunnel 3 and the main tunnel 1. Then, the scaffold 8 is used to enter the main tunnel. After the scaffold 8 climbs up the slope to the top of the tunnel, it is excavated forward at a flat slope to the arch foot of the outer step of the main tunnel 1. Four locking anchor pipes φ42 are installed at the arch foot of the outer step of the main tunnel 1. The length of the locking anchor pipes is 4m to ensure the stability of the arch. In step S2, the establishment of the reinforced support includes the following steps: S21. Within the range of the connecting transverse tunnel 3, I20 type steel frame is used to form the transverse tunnel arch frame 5. The spacing between the steel frames is set to 1.0m. The steel frames are longitudinally welded together with φ22 threaded steel bars. The circumferential spacing of the connecting bars is set to 1m. φ42 locking foot anchor pipes are used for positioning. S22. Shot C25 concrete onto the arch wall of the connecting transverse tunnel 3, with a spray thickness of 20cm. Then, install φ25 hollow grouting anchor rods in the arch and φ22 mortar anchor rods on the side walls. The length of both the hollow grouting anchor rods and the mortar anchor rods is 3m, and the spacing is 1.0m×1.0m, arranged in a quincunx pattern. Hang φ6 steel mesh on the arch wall, with a spacing of 20cm×20cm, overlapping 1~2 meshes, and welding point by point to complete the steel frame arch support on the connecting transverse tunnel 3. S23, such as Figure 2-4 As shown, a longitudinal horizontal support beam 6 is installed along the direction of the main tunnel 1 on the formed steel arch support. The support beam 6 is made of I20 steel and is welded from 3 I20 steel frames. The main tunnel 1 steel frame rests on the support beam 6 and is welded together with the support beam. The arch spacing is 1.0m (consistent with the spacing of the main tunnel 1 steel frame). A total of 11 frames are fabricated. The two ends of the support beam 6 are connected to I20 steel columns 7 for support to the bottom of the main tunnel 1 wall. C25 concrete is sprayed to cover the connecting reinforcement rings and column steel frames. The area around the support beam is not sprayed for the time being. It will be sprayed together after the main tunnel steel frame is in place. The initial support of the connecting cross passage 3 is constructed as soon as possible to complete the reinforced support.

[0021] During the specific construction process, the longitudinal tie rod of the connecting transverse tunnel 3 extends 1m into the main tunnel 1 and is firmly welded to the steel frame or longitudinal tie rod of the main tunnel 1. The advanced small guide tube of the connecting transverse tunnel 3 extending into the main tunnel 1 is also welded to the steel frame of the main tunnel 1.

[0022] S3. After the tunneling reaches the area of ​​the main tunnel 1 (within 20m of the intersection with the connecting cross tunnel 3), I20 steel frame supports are added on both sides within this area. Then, temporary support is provided for the arch of the main tunnel 1, and then the main tunnel roof is excavated. In step S3, the establishment of the temporary support includes the following steps: S31. Based on the stability of the surrounding rock, I20 temporary scaffolding 8 is used to support the top. The scaffolding 8 includes scaffolding beams 801 and scaffolding columns 802. The spacing between the scaffolding 8 is 0.8m. The scaffolding 8 are welded together as a whole using φ22 steel bars. S32. Combined with 5cm of initial shotcrete, the temporary scaffold beam 801 uses 20cm shotcrete mesh with a mesh size of 15×15cm (the temporary scaffold beam 801 will serve as permanent support), forming a temporary support system. The scaffold structure is as follows: Figure 5 As shown, Figure 5 The dimensions of H and b are determined based on the site conditions and processed on-site.

[0023] Lightweight mini excavators were used to enter the arch of the main tunnel 1 horizontally and vertically through a small pilot tunnel, and the arch top of the pilot tunnel was excavated by lifting the top surface. Manual pneumatic picks and work platforms were used for trimming.

[0024] S4. After the excavation of the main tunnel 1 cantilever is completed, initial support shall be provided for the main tunnel 1 cantilever. After entering the main tunnel 1 area, the excavation and initial support shall be larger than the corresponding design elevation of the main tunnel 1 arch to allow for the thickness of temporary support and sufficient deformation. During the excavation process, temporary scaffolding of No. I20 I-beams shall be used for timely support. In step S4, the establishment of the initial support includes the following steps: S41. An I20 steel frame is set at the cantilever of the main tunnel 1 to form the cantilever arch frame 4 of the main tunnel, with a spacing of 1.0m. The steel frames are welded together with φ22 longitudinal connecting bars. The tie rods are spaced 1m apart in the circumferential direction. Each node of the upper and lower step arch foot of the steel frame is positioned by a 4m long φ42 locking foot anchor pipe. Two locking foot anchor pipes are set at each node. S42. Based on step S41, spray a 15cm thick layer of C25 concrete at the arch wall (spray a 25cm thick layer of C25 concrete in the range of PK443+50~PK443+70). Then, install φ25 hollow grouting anchors at the arch and φ22 mortar anchors at the side walls. The length of both the hollow grouting anchors and the mortar anchors is 6m, and the spacing is 1.0m×1.0m. Both the hollow grouting anchors and the mortar anchors are arranged in a quincunx pattern. Then, hang φ6 steel mesh at the arch wall with a spacing of 20cm×20cm, overlap 1-2 meshes, and weld them point by point to form the initial support.

[0025] The initial support operation described above shall be carried out according to the following procedures: Once the temporary support for the upper pilot tunnel of the main tunnel 1 is deemed adequate, remove the vertical support on one side of the temporary scaffold 8 and construct the initial support on that side (remove the support first). The longitudinal scaffold beam 801 at the top of the arch will serve as permanent support. The same method will be used to construct the other side.

[0026] After the initial support of the upper guide tunnel of the arch has stabilized, the upper guide tunnel of the arch will be excavated in the direction of the main attack (the side of the small pile number at the entrance) using a working platform, with an advance of 1.0 to 1.5 meters. After the excavation is qualified, initial shotcreting will be carried out immediately, and the initial support will be implemented in a timely manner.

[0027] Finally, the excavation platform is turned around and the upper guide tunnel of the arch is excavated towards the secondary attack (the side with the larger exit pile number), with an advance of 1.0 to 1.5 meters. After the excavation is deemed satisfactory, initial shotcreting is immediately carried out, and initial support is promptly implemented using the same method as above.

[0028] S5. After the excavation and support of the upper pilot tunnel within 6.0m is completed, the middle and upper rock mass is then expanded section by section, and the initial support of the right half is extended to the arching line. Each cycle advances 1.0 to 1.5m. S6. Under the initial support of the upper section, the bench method is used for excavation support in the main attack direction (side of the smaller pile number). To ensure safety, the upper bench within a 40m range of this section of the main tunnel is excavated and supported in two stages until the maximum span (arch line), such as... Figure 6 As shown.

[0029] S7. When the distance between the upper section face and the working face reaches 30m, the lower half section sidewall bottoming construction begins. Specifically, the left and right sidewalls are staggered by 3-5m for bottoming excavation and support, with an advance of 2m per cycle, while advancing forward simultaneously.

[0030] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A construction method for a parallel pilot tunnel leading into the main tunnel, characterized in that, Includes the following steps: S1. In the parallel pilot tunnel (2), the connecting cross tunnel (3) is gradually excavated towards the main tunnel (1). When the connecting cross tunnel (3) is excavated to the section close to the main tunnel (1), the arch elevation of the connecting cross tunnel (3) is gradually raised and the steel frame is extended. S2. After the connecting cross tunnel (3) continues to be excavated to the outline of the main tunnel (1), reinforced support is installed at the junction of the connecting cross tunnel (3) and the main tunnel (1). Then, a scaffold is used to enter the main tunnel (1). After the scaffold climbs up the slope to the top of the tunnel, it is excavated forward on a flat slope to the foot of the outer step of the main tunnel (1). S3. After the excavation reaches the range of the main tunnel (1), temporary support is provided for the arch of the main tunnel (1), and then the top of the main tunnel (1) is excavated. S4. After the main tunnel (1) is excavated, the main tunnel (1) is initially supported. S5. Next, the middle and upper rock mass is gradually expanded and the initial support of the right half is extended to the arching line. Each cycle advances 1.0 to 1.5 meters. S6. Under the initial support of the upper section, the step method is used to excavate and support in the main attack direction; S7. When the distance between the upper section face and the working face reaches 30m, the construction of the lower half section sidewall bottoming begins.

2. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 1, characterized in that, In step S1, the connecting cross tunnel (3) and the main tunnel (1) are set in an orthogonal manner. The cross section at the junction of the connecting cross tunnel (3) and the main tunnel (1) is enlarged in the form of a 75° horn mouth. The intersection of the connecting cross tunnel (3) and the parallel guide tunnel (2) at the small pile side is set with a rounded corner. The radius of the rounded corner is 5m.

3. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 2, characterized in that, The flared opening at the junction of the connecting cross passage (3) and the main tunnel (1) is reinforced with three parallel I20 I-beam gantry frames, which also serve as the support for the steel frame of the main tunnel (1).

4. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 1, characterized in that, In step S2, the establishment of the reinforced support includes the following steps: S21. Within the range of the connecting cross tunnel (3), I20 type steel frame is used to form the cross tunnel arch frame (5). The spacing between the steel frames is set to 1.0m. The steel frames are longitudinally welded together with φ22 threaded steel bars. The circumferential spacing of the connecting bars is set to 1m. φ42 locking foot anchor pipe is used for positioning. S22. Spray C25 concrete on the arch wall of the connecting transverse tunnel (3) with a spray thickness of 20cm. Then, install φ25 hollow grouting anchor rods in the arch and φ22 mortar anchor rods on the side wall. The length of the hollow grouting anchor rods and the mortar anchor rods are both 3m and the spacing is 1.0m×1.0m. They are arranged in a quincunx pattern. Hang φ6 steel mesh on the arch wall with a spacing of 20cm×20cm. Overlap 1~2 meshes and weld them point by point to complete the steel frame arch support on the connecting transverse tunnel (3). S23. A longitudinal horizontal support beam (6) is set on the steel frame arch support along the direction of the main tunnel (1). The support beam is made of I20 steel. The support beam (6) is welded from 3 I20 steel frames. The two ends of the support beam (6) are connected to I20 steel columns (7) to support the bottom of the wall of the main tunnel (1) to complete the reinforced support.

5. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 1, characterized in that, In step S2, multiple locking anchor pipes φ42 are installed at the arch foot of the outer step of the main tunnel (1), and the length of the locking anchor pipe is 4m.

6. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 1, characterized in that, In step S3, the establishment of the temporary support includes the following steps: S31. Based on the stability of the surrounding rock, I20 temporary scaffolding (8) is used to support the top. The spacing of the scaffolding (8) is 0.8m. The scaffolding (8) is welded together with φ22 steel bars. S32. Combined with the initial sprayed concrete of 5cm, the temporary scaffold beam (801) adopts 20cm sprayed mesh with a mesh size of 15×15cm to form a temporary support system.

7. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 1, characterized in that, In step S4, the initial support includes the following steps: S41. An I20 steel frame is set at the cantilever of the main tunnel (1) to form the cantilever arch frame (4) of the main tunnel, with a spacing of 1.0m. The steel frames are welded together with φ22 longitudinal connecting bars. The tie rods are spaced 1m apart in the circumferential direction. Each node of the upper and lower step arch foot of the steel frame is positioned by φ42 locking foot anchor pipes with a length of 4m. Two locking foot anchor pipes are set at each node. S42. Based on step S41, spray a 15cm thick layer of C25 concrete at the arch wall. Then, install φ25 hollow grouting anchors at the arch and φ22 mortar anchors at the side walls. The length of both the hollow grouting anchors and the mortar anchors is 6m, and the spacing is 1.0m×1.0m. Both the hollow grouting anchors and the mortar anchors are arranged in a quincunx pattern. Then, hang φ6 steel mesh at the arch wall with a spacing of 20cm×20cm, overlap 1-2 meshes, and weld them point by point to form the initial support.

8. The construction method for parallel pilot tunnels leading into the main tunnel according to claim 1, characterized in that, In step S7, the construction steps for the lower half section sidewall bottom are as follows: The left and right side walls are staggered by 3-5m for bottom excavation and support, with an advance of 2m per cycle, while advancing forward.

Images