Variable section construction method

Abstract

The invention relates to the technical field of underground excavation subway construction, in particular to a variable-section construction method which comprises the following steps: S1, excavating a large-section cavern of a tunnel to the junction of a large section and a small section; s2, an inclined guide wall is constructed, the middle partition wall before the section is changed is connected with the middle partition wall after the section is changed, so that the large-space cavern before the section is changed is communicated with the small-space cavern after the section is changed, and forward excavation and supporting are directly continued until all design mileage construction is completed without waiting for the situation that other caverns are constructed to the junction of the large section and the small section; and the caverns which are not overlapped before and after the section change are blocked. According to the variable-section construction method, the inclined guide wall is constructed at the variable section and penetrates through the front and rear middle partition walls at the junction of the large section and the small section, so that the overall stability of the cavern can be maintained in the variable-section switching process, direct continuous excavation and supporting can be achieved without waiting for all the caverns to reach the variable-section mileage at the same time and then breaking and blocking one by one, and the variable-section construction period is greatly shortened; and the overall tunneling speed is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of underground excavation subway construction, and in particular to a variable-section construction method. Background Art

[0002] Multi-hole tunnels are generally constructed using CRD, double-sidewall pilot tunneling and other methods. To avoid the tunnel cluster effect, these methods use staggered excavation and support methods for individual chamber faces.

[0003] When encountering the junction of a large section and a small section during tunnel construction, the middle wall at the variable section is disconnected along the tunnel section direction. The current conventional variable section construction is: when the single cavern in the front is excavated and supported to the variable section mileage, the cavern heading is closed, and when all caverns have reached the variable section mileage, excavation and support are carried out in a staggered manner in sequence on the single cavern heading until all the designed mileage is completed. This conventional variable section construction method has a long construction period, and it is necessary to wait until the large section of the tunnel before the variable section is fully constructed before the blockage can be removed to construct the subsequent tunnel. Summary of the Invention

[0004] The purpose of the present invention is to overcome the shortcomings of the traditional variable section construction method that the blockage must be removed and the subsequent tunnels must be constructed only after the large section of the tunnel before the variable section is completed, which affects the construction progress, and provide a variable section construction method.

[0005] In a first aspect, the present invention provides a variable cross-section construction method, comprising the following steps: S1: Use the double-sidewall pilot method to excavate the large-section tunnel chamber to the junction of the large and small sections; S2: Construct an inclined guide wall to connect the middle partition wall in front of the variable section with the middle partition wall behind the variable section, so that the large space cavern in front of the variable section and the small space cavern behind the variable section are connected. Without waiting for the construction of the remaining caverns to the junction of the large and small sections, continue excavation and support forward until all designed mileage construction is completed; seal the caverns that do not overlap before and after the variable section.

[0006] The variable section construction method provided by the present invention constructs an inclined guide wall at the variable section and penetrates the front and rear middle partition walls at the junction of large and small sections. The inclined guide wall and the middle partition walls connected thereto form an integral support system, which can maintain the overall stability of the cavern during the variable section switching process. There is no need to wait for all caverns to reach the variable section mileage at the same time and then remove the blockages one by one. Direct continuous excavation and support can be carried out, which eliminates the "stagnation and waiting" link in the large-section and small-section conversion stage to the greatest extent, thereby greatly shortening the variable section construction cycle and improving the overall tunnel excavation speed. The traditional method requires first sealing the completed large-section cavern, and then removing the blockage after other caverns are in place to continue constructing the next section. The variable section construction method provided by the present invention can continue to operate after connecting the front and rear middle partition walls through the inclined guide wall. There is no need to unseal and re-support, which avoids repeated sealing and support construction and reduces waste of materials and human resources.

[0007] Preferably, the inclined guide wall extends from the large section to the small section, and its extension length is 6 to 8 meters.

[0008] The 6-8m-long inclined guide wall creates a gradual transition zone between the large and small sections, avoiding stress concentration caused by sudden changes in section. Through the gradual transition of the inclined guide wall, the lateral pressure of the surrounding rock and the downward pressure of the roof in the tunnel are smoothly transferred from the large space to the small space, effectively reducing the risk of sudden increases in surrounding rock displacement, crack expansion, and local collapse at the transition section. After the 6-8m long inclined guide wall is connected to the front and rear middle partition walls, a continuous steel support-concrete composite support belt can be constructed in the section transition area, thereby enhancing the surrounding rock restraint force at the sudden change point of the cave section and improving the bearing capacity of the overall support system.

[0009] Preferably, the angle between the inclined guide wall and the tunnel extension direction is 25°~37°.

[0010] The preferred angle of the inclined guide wall is 25°~37°, which can more evenly distribute the horizontal lateral pressure and vertical roof pressure of the surrounding rock in the tunnel to the inclined guide wall and the middle partition wall, reducing the risk of local instability of the surrounding rock.

[0011] Preferably, the thickness of the inclined guide wall is the same as that of the middle partition wall before the cross-section change and the middle partition wall after the cross-section change.

[0012] The uniform thickness enables the inclined guide wall and the middle partition wall to form an integral support belt with consistent cross-section. The inclined guide wall has the same thickness as the adjacent middle partition wall, and its cross-sectional moment of inertia and bending and shear resistance are equivalent to those of the middle partition walls on both sides. In the section transition area, the load can be transmitted continuously and evenly along the wall, avoiding stiffness mutations and stress concentration caused by thickness differences, thereby reducing the risk of local cracks and deformation.

[0013] On the other hand, setting the inclined guide wall to the same thickness as the middle partition wall can also simplify construction preparation, reduce the types of templates and the number of adjustments; the cutting and binding of steel bars can also be carried out according to uniform specifications, reducing processing and installation errors and improving construction efficiency.

[0014] Preferably, in S2, the non-overlapping cavities before and after the cross-section change are sealed, comprising the following steps: Install reinforced anchor pipes with a horizontal spacing of 300±30mm and a vertical spacing of 500±50mm. The reinforced anchor pipes are arranged in a staggered plum blossom pattern with a length of 2±0.3m. After the anchor pipe is reinforced, the first steel mesh is hung and concrete is sprayed for the first time to initially close the tunnel face. Then the grid steel frame is erected. After the grid steel frame is erected, the second steel mesh is hung on the inside of the grid steel frame and concrete is sprayed for the second time to close the tunnel face.

[0015] The reinforced anchor pipes are arranged in a plum blossom shape of 300±30mm×500±50mm, forming a dense and staggered anchoring grid, which firmly anchors the shotcrete to the surrounding rock and significantly enhances the bonding strength and integrity of the plug wall and the parent surrounding rock; The first shotcrete is mainly used to initially fill the tunnel face. Together with the first layer of steel mesh, it can quickly seal the tunnel opening and form a uniform primary lining. The second shotcrete is sprayed after the second layer of steel mesh is hung on the inner side of the grid steel frame. This thickens the sealing body and eliminates pores and cold joints on the initial shotcrete surface, ensuring a consistent final lining thickness. Hanging the first layer of steel mesh and spraying concrete first can quickly form a temporary closed surface with high rigidity, providing a flat support surface for the subsequent erection of the grid steel frame, while reducing the risk of severe deformation or collapse of the surrounding rock; then construct the grid and the second layer of mesh on the relatively stable inner side of the initial spraying surface, and spray concrete again in a controlled environment to improve construction safety.

[0016] Preferably, the reinforced anchor pipe is installed with a horizontal downward inclination angle of 10±2°.

[0017] The preferred angle of the reinforced anchor pipe is a downward inclination of 10±2°. After the reinforced anchor pipe is at a certain downward inclination angle, its force direction is closer to the principal stress direction of the surrounding rock, and it can be more effectively embedded in the surrounding rock structure, increasing the friction area with the rock and soil body and improving the anchoring force; at the same time, compared with horizontal or upward inclination layout, the downward inclination reinforced anchor pipe has stronger pull-out resistance when the head wall is subjected to the thrust of the surrounding rock in front, preventing the reinforced anchor pipe from being pulled out or loosened due to the force.

[0018] Preferably, the first steel mesh is a Φ6@150×150mm steel mesh, and the thickness of the first shotcrete is 40±2mm.

[0019] The preferred first reinforcement mesh is a Φ6@150×150mm mesh, ensuring a moderate reinforcement density. This creates a uniform, crack-resistant skeleton, effectively suppressing cracking in the sprayed layer, while also preventing overcrowding that could affect concrete placement and compaction. An initial sprayed layer thickness of approximately 40mm, combined with the first reinforcement mesh, quickly establishes an initial support layer with sufficient load-bearing capacity, providing a safe and reliable temporary roof for subsequent support and monitoring.

[0020] Preferably, when erecting the grid steel frame, each grid is connected by Φ22 steel bars, the spacing between the Φ22 steel bars is 1000mm, and the Φ22 steel bars are arranged in a double layer staggered manner.

[0021] The preferred large-diameter steel bars of Φ22mm have higher section inertia moment and bending and shearing capacity. Through double-layer staggered arrangement, a strong "frame-core reinforcement" composite system can be formed in both the longitudinal and transverse directions of the grid, which significantly improves the overall stiffness of the grid steel frame, thereby more effectively resisting the impact load caused by surrounding rock pressure and concrete spraying.

[0022] Preferably, the second steel mesh is a Φ6@150×150mm steel mesh and is welded to the grid steel frame.

[0023] The welded steel mesh fits the grid steel frame, and the mesh and grid holes fit tightly together. When the shotcrete hardens, it can penetrate and bite more evenly along the mesh and grid frame, which enhances the bonding strength between the concrete and the steel structure, reduces rebound and voids, and improves the density and anti-seepage performance of the dam wall.

[0024] Preferably, after the second shotcrete spraying, single-liquid cement slurry is used for anchor pipe grouting to reinforce the soil, the grouting pressure is controlled at 0.3-0.5 MPa, and the final grouting pressure is 0.5 MPa.

[0025] Single-liquid cement slurry has a low viscosity and can diffuse evenly along the pores and microcracks of the anchor pipe, tightly bonding the loose, weathered or broken surrounding rock particles to form a continuous reinforcement body; by controlling the grouting pressure at 0.3-0.5 MPa, it can ensure that the slurry fully penetrates into the target reinforcement area, while avoiding secondary fracture of the surrounding rock due to excessive pressure, thereby significantly improving the strength and stiffness of the surrounding rock; after grouting reinforcement, the surrounding rock, anchor pipe and shotcrete are jointly subjected to force, forming an integrated support structure of "anchor pipe-slurry-surrounding rock", which improves the overall stability.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention provides a variable-section construction method. By constructing an inclined guide wall at the variable section and penetrating the middle partition walls before and after the junction of the large and small sections, the inclined guide wall and the middle partition walls connected thereto form an integral support system, which can maintain the overall stability of the cavern during the variable-section switching process. There is no need to wait for all caverns to reach the variable-section mileage at the same time and then break the blockage one by one. Direct continuous excavation and support can be carried out, which eliminates the "stagnation and waiting" link in the conversion stage between large and small sections to the greatest extent, thereby greatly shortening the variable-section construction period and improving the overall tunnel excavation speed.

[0027] 2. The present invention provides a variable-section construction method, which can continue to operate after connecting the front and rear middle partition walls through inclined guide walls. There is no need to unseal and re-support, which avoids repeated sealing and support construction and reduces waste of materials and human resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a plan view of the junction of large and small sections; Figure 2 It is a schematic diagram of the cross section at the junction of large and small sections.

[0029] Markings in the figure: 1- inclined guide wall, 2- middle partition wall before the cross section change, 3- middle partition wall after the cross section change, 4- reinforced anchor pipe, 5- grid steel frame. DETAILED DESCRIPTION

[0030] The present invention will be further described in detail below with reference to specific embodiments. However, this should not be construed as limiting the scope of the present invention to the following embodiments, as all technologies implemented based on the present invention fall within the scope of the present invention.

[0031] Unless otherwise specified, in the description of the specific embodiments of the present invention, the terms indicating the orientation or positional relationship, such as "upper", "lower", "left", "right", "center", "inside", and "outside", are based on the expressions of the orientation or positional relationship shown in the accompanying drawings, or are the orientation or positional relationship in which the invented product / device / apparatus is placed when it is conventionally used. These terms of orientation or positional relationship are merely for the purpose of facilitating the description of the scheme of the present invention or simplifying the description of the specific embodiments to facilitate the rapid understanding of the scheme by technicians, and do not indicate or imply that a specific device / component / element must have a specific orientation, or be constructed and operated in a specific positional relationship, and therefore should not be understood as limiting the present invention.

[0032] In addition, if the terms "horizontal", "vertical", "overhanging", "parallel" and the like appear, it does not mean that the corresponding devices / components / elements are required to be absolutely horizontal or vertical or overhanging or parallel, but may be slightly tilted or have deviations. For example, "horizontal" only means that its direction is more horizontal relative to "vertical", and does not mean that the structure must be completely horizontal, but may be slightly tilted. Alternatively, it can be simply understood that the corresponding devices / components / elements are set in directions such as "horizontal", "vertical", "overhanging", and "parallel", and can have an error / deviation of ±10% relative to the corresponding direction setting, more preferably an error / deviation within ±8%, more preferably an error / deviation within ±6%, more preferably an error / deviation within ±5%, and more preferably an error / deviation within ±4%. As long as the corresponding device / component / element is within the error / deviation range, it can still achieve its role in the solution of the present invention.

[0033] In addition, the expressions “first”, “second”, “third”, etc. in the terms are merely used to distinguish the description of the same or similar components, and should not be understood as emphasizing or implying the relative importance of specific components.

[0034] In addition, in the description of the embodiments of the present invention, "several," "plurality," and "a number" represent at least two. It can also be any number such as two, three, four, five, six, seven, eight, nine, or even more than nine.

[0035] Furthermore, in the description of the technical solution of the present invention, unless otherwise expressly specified, defined, or limited, the terms "disposed," "installed," "connected," "connected," "provided with," "laid," and "arranged" should be understood broadly. For example, they may refer to fixed connections, detachable connections, or integral connections. They may be welded, riveted, bolted, threaded, or other commonly used connection methods in the art. Such connections may be mechanical, electrical, or communicative; they may be direct, indirect via an intermediate medium, or internally connected between two components.

[0036] Example 1 like Figure 1 (In the figure, the A hidden excavation section is the large section, and the B hidden excavation section is the small section). Figure 2 As shown, this embodiment provides a variable cross-section construction method, comprising the following steps: S1: Use the double-side wall pilot method to excavate the large-section tunnel chamber to the junction of the large and small sections. During construction, the excavation and support are carried out by staggering the single tunnel face.

[0037] It can be understood that multi-hole blind-bored tunnels can also be constructed using the CRD method.

[0038] S2: Construct the inclined guide wall 1 to connect the middle partition wall 2 in front of the variable section with the middle partition wall 3 behind the variable section, so that the large space cavern in front of the variable section and the small space cavern behind the variable section are connected. Without waiting for the construction of the remaining caverns to the junction of the large and small sections, continue to excavate and support forward until all the designed mileage construction is completed; seal the caverns that do not overlap before and after the variable section.

[0039] The variable section construction method provided in this embodiment constructs an inclined guide wall 1 at the variable section and penetrates the front and rear middle partition walls at the junction of large and small sections. The inclined guide wall 1 and the middle partition walls connected to it form an integral support system, which can maintain the overall stability of the cavern during the variable section switching process. There is no need to wait for all caverns to reach the variable section mileage at the same time and then remove the blockage one by one. Direct continuous excavation and support can be carried out, which eliminates the "stagnation and waiting" link in the large and small section conversion stage to the greatest extent, thereby greatly shortening the variable section construction period and improving the overall tunnel excavation speed; the traditional variable section method requires first sealing the completed large section cavern, and then removing the blockage after other caverns are in place to continue construction of the next section. The variable section construction method provided in this embodiment can continue operation after connecting the front and rear middle partition walls through the inclined guide wall 1, without the need to unseal and re-support, avoiding repeated sealing and support construction, and reducing waste of materials and human resources.

[0040] Further, such as Figure 1 As shown, the inclined guide wall 1 extends from the large section (A dark excavation section) to the small section (B dark excavation section), and its extension length is 6~8m.

[0041] The inclined guide wall 1, extending 6 to 8 meters, creates a gradual transition zone between the large and small cross-sections, avoiding stress concentration caused by sudden cross-section changes. This gradual transition of the inclined guide wall 1 smoothly shifts the lateral pressure of the surrounding rock and the downward pressure of the roof from the large to the small space within the tunnel, effectively reducing the risk of sudden increases in surrounding rock displacement, crack expansion, and localized collapse at the cross-section transition.

[0042] After the 6-8m long inclined guide wall 1 is connected to the front and rear middle partition walls, a continuous steel support-concrete composite support belt can be constructed in the section transition area, thereby enhancing the surrounding rock constraint force at the sudden change of the cave section and improving the bearing capacity of the overall support system.

[0043] Further, such as Figure 1 As shown, the angle ∠α between the inclined guide wall 1 and the tunnel extension direction is 25°~37°. The preferred angle of the inclined guide wall 1 is 25°~37°, which can more evenly distribute the horizontal lateral pressure and vertical roof pressure of the surrounding rock in the tunnel to the inclined guide wall 1 and the middle partition wall, reducing the risk of local instability of the surrounding rock.

[0044] Furthermore, the thickness of the inclined guide wall 1 is the same as that of the middle partition wall 2 before the cross-section change and the middle partition wall 3 after the cross-section change. The uniform thickness allows the inclined guide wall 1 and the middle partition wall to form an integral support belt with a consistent cross-section. The inclined guide wall 1 is of the same thickness as the adjacent middle partition walls, and its cross-sectional moment of inertia and bending and shearing resistance are comparable to those of the middle partition walls on both sides. In the cross-sectional transition area, the load can be continuously and evenly transmitted along the wall, avoiding sudden stiffness changes and stress concentration caused by thickness differences, thereby reducing the risk of local cracks and deformation. On the other hand, setting the inclined guide wall 1 to have the same thickness as the middle partition wall can also simplify construction preparations, reduce the types of templates and the number of adjustments; the cutting and binding of steel bars can also be carried out according to uniform specifications, reducing processing and installation errors and improving construction efficiency.

[0045] Example 2 like Figure 2 As shown, based on Example 1, this embodiment describes the steps of constructing a plugging wall to seal the cavern that does not overlap before and after the cross-section change. The variable cross-section construction method provided in this embodiment, which seals the cavern that does not overlap before and after the cross-section change, includes the following steps: Reinforcement anchor pipes 4 were installed, with horizontal spacing of 300±30mm and vertical spacing of 500±50mm. The reinforcement anchor pipes 4 were arranged in a staggered pattern in a plum blossom pattern, with a length of 2±0.3m. The reinforcement anchor pipes 4 were arranged in a plum blossom pattern of 300±30mm x 500±50mm, forming a dense and staggered anchoring grid that firmly anchored the shotcrete to the surrounding rock, significantly enhancing the bond and integrity between the headwall and the parent surrounding rock.

[0046] After the anchor pipe 4 is strengthened, the first steel mesh is hung and concrete is sprayed for the first time to initially close the tunnel face, and then the grid steel frame 5 is erected. After the grid steel frame 5 is erected, the second steel mesh is hung inside the grid steel frame 5 and concrete is sprayed for the second time to close the tunnel face.

[0047] Furthermore, the anchor pipe 4 is reinforced when being driven with a horizontal downward inclination angle of 10±2°.

[0048] The reinforced anchor pipe 4 is preferably tilted downward at a 10±2° angle. This downward angle places the reinforced anchor pipe 4 closer to the principal stress direction of the surrounding rock, allowing it to be more effectively embedded in the surrounding rock structure, increasing the frictional area with the rock and soil and improving the anchoring force. Furthermore, compared to horizontal or upward-tilted arrangements, the downward-tilted reinforced anchor pipe 4 offers greater pullout resistance when the headwall is subjected to thrust from the surrounding rock, preventing the reinforced anchor pipe 4 from being pulled out or loosened due to the force.

[0049] The first concrete spraying is mainly used for preliminary filling of the tunnel face. Together with the first layer of steel mesh, it can quickly seal the cavern opening and form a uniform primary lining. The second concrete spraying is carried out after the second layer of steel mesh is hung on the inner side of the grid steel frame 5. This can thicken the sealing body and eliminate the pores and cold joints on the initial spraying surface, ensuring a consistent thickness of the final lining.

[0050] Hanging the first layer of steel mesh and spraying concrete first can quickly form a temporary closed surface with high rigidity, providing a flat support surface for the subsequent erection of the grid steel frame 5, while reducing the risk of severe deformation or collapse of the surrounding rock; then, the grid and the second layer of mesh are constructed inside the relatively stable initial spraying surface, and the concrete is sprayed again in a controlled environment to improve construction safety.

[0051] Furthermore, in this embodiment, the first steel mesh is a Φ6@150×150mm steel mesh (a steel mesh made by weaving or welding 6mm diameter round steel bars at a 150mm vertical and horizontal spacing), and the thickness of the first shotcrete is 40±2mm.

[0052] The preferred first reinforcement mesh is a Φ6@150×150mm mesh, ensuring a moderate reinforcement density. This creates a uniform, crack-resistant skeleton, effectively suppressing cracking in the sprayed layer, while also preventing overcrowding that could affect concrete placement and compaction. An initial sprayed layer thickness of approximately 40mm, combined with the first reinforcement mesh, quickly establishes an initial support layer with sufficient load-bearing capacity, providing a safe and reliable temporary roof for subsequent support and monitoring.

[0053] Furthermore, when erecting the grid steel frame 5, each grid is connected by Φ22 steel bars, the grid spacing can be 0.5m, the Φ22 steel bar spacing is 1000mm, and the Φ22 steel bars are arranged in a double layer staggered manner.

[0054] The preferred large-diameter steel bars of Φ22mm have higher section inertia moment and bending and shearing capacity. Through double-layer staggered arrangement, a strong "frame-core bar" composite system can be formed in both the longitudinal and transverse directions of the grid, which significantly improves the overall stiffness of the grid steel frame 5, thereby more effectively resisting the impact load caused by surrounding rock pressure and concrete spraying.

[0055] Furthermore, the second steel mesh is a Φ6@150×150mm steel mesh and is welded to the grid steel frame 5. The welded steel mesh fits the grid steel frame 5, and the mesh closely matches the grid cells. During hardening, the shotcrete can more evenly penetrate and engage with the grid skeleton along the mesh, thereby enhancing the bond between the concrete and the steel structure, reducing rebound and voids, and improving the density and impermeability of the caulking wall.

[0056] Furthermore, after the second shotcrete injection, single-liquid cement slurry is used for anchor pipe grouting to reinforce the soil. The grouting pressure is controlled at 0.3-0.5 MPa, and the final grouting pressure is 0.5 MPa. Single-liquid cement slurry has a low viscosity and can evenly diffuse along the pores and microcracks of the anchor pipe, tightly bonding loose, weathered, or broken surrounding rock particles to form a continuous reinforcement body. By controlling the grouting pressure at 0.3-0.5 MPa, it can ensure that the slurry fully penetrates the target reinforcement area while avoiding secondary fractures of the surrounding rock due to excessive pressure, significantly improving the strength and stiffness of the surrounding rock. After grouting reinforcement, the surrounding rock, anchor pipe, and shotcrete are jointly subjected to force, forming an integrated "anchor pipe-slurry-surrounding rock" support structure, improving overall stability.

[0057] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A variable cross-section construction method, characterized in that: The following steps are involved: S1: Use the double-sidewall pilot method to excavate the large-section tunnel chamber to the junction of the large and small sections; S2: Construct the inclined guide wall (1) to connect the middle partition wall (2) in front of the variable section with the middle partition wall (3) behind the variable section, so that the large space cavern in front of the variable section and the small space cavern behind the variable section are connected. Without waiting for the construction of the remaining caverns to the junction of the large and small sections, continue to excavate and support forward until all the designed mileage construction is completed; seal the caverns in front of the variable section and behind the variable section that do not overlap.

2. A variable cross-section construction method according to claim 1, characterized in that: The inclined guide wall (1) extends from the large section to the small section, and its extension length is 6 to 8 meters.

3. A variable cross-section construction method according to claim 1, characterized in that: The angle between the inclined guide wall (1) and the tunnel extension direction is 25°~37°.

4. A variable cross-section construction method according to claim 1, characterized in that: The thickness of the inclined guide wall (1) is the same as that of the middle partition wall (2) before the cross section change and the middle partition wall (3) after the cross section change.

5. A variable cross-section construction method according to claim 1, characterized in that: In S2, the non-overlapping caverns before and after the cross-section change are sealed, including the following steps: The reinforced anchor pipes (4) are installed, the horizontal spacing of the reinforced anchor pipes (4) is 300±30mm, the vertical spacing of the reinforced anchor pipes (4) is 500±50mm, the reinforced anchor pipes (4) are arranged in a staggered manner in a plum blossom shape, and the length of the reinforced anchor pipes (4) is 2±0.3m; After the reinforcement anchor pipe (4) is completed, the first steel mesh is hung and the first shotcrete is sprayed to initially close the tunnel face, and then the grid steel frame (5) is erected. After the grid steel frame (5) is erected, the second steel mesh is hung on the inner side of the grid steel frame (5) and the second shotcrete is sprayed to close the tunnel face.

6. A variable cross-section construction method according to claim 5, characterized in that: The reinforced anchor pipe (4) is installed with a horizontal downward inclination angle of 10±2°.

7. A variable cross-section construction method according to claim 5, characterized in that: The first steel mesh is Φ6@150×150mm steel mesh, and the thickness of the first shotcrete is 40±2mm.

8. The variable cross-section construction method according to claim 5, characterized in that: When erecting the grid steel frame (5), each grid is connected by Φ22 steel bars, the spacing between the Φ22 steel bars is 1000mm, and the Φ22 steel bars are arranged in a double layer staggered manner.

9. A variable cross-section construction method according to claim 2, characterized in that: The second steel mesh is a Φ6@150×150mm steel mesh and is welded to the grid steel frame (5).

10. The variable cross-section construction method according to claim 5, characterized in that: After the second shotcrete injection, single-liquid cement slurry is used for anchor pipe grouting to reinforce the soil. The grouting pressure is controlled at 0.3-0.5 MPa, and the final grouting pressure is 0.5 MPa.

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