Support Structure for Crossing Karst Cave under Tunnel and Its Construction Method

Abstract

The present invention relates to a support structure for spanning a karst cave under a tunnel and a construction method thereof, belonging to the technical field of tunnel support structures. The support structure includes a pre-support structure and a later arch support structure constructed step by step. The pre-support structure is completed when a karst cave appears below the front of the tunnel during excavation to its planned route, and the later arch support structure is completed after the tunnel excavation passes through the karst cave. During construction, first, simply clean the karst cave and fill it with soil to build the pre-support structure and form the initial support of the tunnel, allowing the tunnel to pass through the karst cave first. Then, turn back to further process the karst cave to form the arch support structure, thus finally completing the permanent support structure for the karst cave spanning under the tunnel. The support structure and its construction method are applicable to the construction environment of karst cave tunnels with a small space, fully ensuring construction safety and quality, and effectively avoiding the difficulties of large precast formwork entering the site, reducing the construction difficulty and construction cost.

Description

Technical Field

[0001] The present invention relates to a support structure and a construction method thereof established in a large karst cave when tunneling in a tunnel project encounters a large karst cave below that needs to be spanned, and belongs to the technical field of tunnel support structures. Background Art

[0002] Karst caves are widespread in the regions of Yunnan, Guizhou and Sichuan in China. During the excavation construction of tunnel projects in these regions, it may encounter the situation of large-volume karst cave cavities. At present, the commonly used construction countermeasures are to use tunnel bottom backfill or pile-raft structure to fully reinforce the tunnel bottom cavity and then carry out tunnel structure construction. However, when encountering deep, large or complex geological karst caves, the scheme of first detecting and reinforcing the karst cavity and then passing through is relatively cumbersome and the construction difficulty is relatively large, with certain limitations. The existing methods for treating tunnels spanning karst caves include tunnel bottom backfill reinforcement and pile-raft structure spanning. However, the possibility of loss of tunnel bottom filling materials and the high requirements of ballastless tracks for high-speed railways on foundation settlement make these two methods have certain limitations. For karst caves with high settlement requirements and relatively deep depths, the arch bridge spanning method can be adopted. However, due to the narrow construction space in the tunnel and the difficulty in the entry of large machinery and precast components, the bridge construction efficiency in the tunnel is low, and the traditional support erection method is difficult and it is difficult to ensure construction safety.

[0003] Chinese Patent with publication number CN11851252A discloses a steel-concrete composite arch bridge for spanning a karst cave in a tunnel and a construction method thereof. This method uses a crane to assemble the arch bridge structure, and does not further process the karst cavity under the tunnel bottom. The supporting effect of only using the arch bridge is doubtful. The components of this arch bridge are numerous, the splicing is cumbersome, and the construction process is complex. In a tunnel with a relatively small construction space, there are also problems such as difficulties in the entry of splicing machinery and precast templates. Therefore, the application conditions are limited. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: when encountering a large-volume karst cave cavity during the excavation construction of a tunnel project, how to form a karst cave support structure without affecting the tunneling of the tunnel to ensure the safety of the tunnel.

[0005] The first technical solution proposed by the present invention to solve the above technical problem is: a support structure for spanning a karst cave below a tunnel, including a pre-support structure and a post-arch bridge support structure constructed step by step;

[0006] The pre - support structure is constructed when a karst cave appears below the front of the planned route of the tunnel excavation. It includes the primary support arch erected in the karst cave passing through the area below the tunnel, the temporary inverted arch lining, the anchor rods drilled into the bedrock of the karst cave wall, and the backfill soil in the karst cave in this area. The primary support arch is erected on the stepped surface of the backfill soil and sprayed with primary support concrete. Shotcrete with wire mesh is formed at the top of the karst cave in this area. Vertical pile foundations are buried along the side of the tunnel arch waist in the backfill soil, and steel pipe columns are installed on the vertical pile foundations. After hanging wire mesh and spraying concrete on the steel pipe columns, a concrete retaining wall is formed along the side of the tunnel arch waist;

[0007] The later arch bridge support structure is constructed after the tunnel excavation passes through the karst cave. It includes the foundation pit excavated at the bottom of the karst cave, the arch seat and the arch ring formwork formed on the bottom surface of the foundation pit, the superstructure of the reinforced concrete arch ring cast on the arch ring formwork, the backfill soil and slag on the arch, and the closed wall constructed in the karst cave outside the range of the concrete retaining wall.

[0008] The second technical solution proposed by the present invention to solve the above - mentioned technical problems is: a construction method for a support structure for spanning a karst cave under a tunnel, including the following steps:

[0009] 1) When a karst cave appears below the front of the planned route of the tunnel excavation, after the karst cave is explored and it is determined that the karst cave passes through the area below the planned tunnel route, first use blasting to clean up the loose slag in the karst cave and backfill the karst cave;

[0010] 2) Backfill the karst cave passing through the area below the tunnel with soil, use shotcrete with wire mesh to reinforce the top of the karst cave in this area, and drill anchor rods into the bedrock of the karst cave wall;

[0011] 3) Erected the primary support arch of the tunnel on the stepped surface of the backfill soil, and spray the primary support concrete and the temporary inverted arch lining;

[0012] 4) If there is no karst cave below the side of the tunnel arch waist, then proceed to the next step; if there is a karst cave below the side of the tunnel arch waist, then execute this step: bury vertical pile foundations along the side of the tunnel arch waist in the backfill soil, construct steel pipe columns on the vertical pile foundations, hang wire mesh and spray concrete on the steel pipe columns to form a concrete retaining wall along the side of the tunnel arch waist, and then backfill the karst cave outside the side of the tunnel arch waist with concrete and tunnel debris;

[0013] 5) Continue the tunnel construction to make the tunnel pass through the karst cave;

[0014] 6) Return to the karst cave again, first break the temporary inverted arch lining and the backfill of the inverted arch in the karst cave, and then construct advanced support obliquely downward from the arch feet of the primary support arch to the karst cave and inject cement slurry;

[0015] 7) Excavate the foundation pit in sections at the bottom of the karst cave, and carry out shotcrete with wire mesh protection for each section of the foundation pit;

[0016] 8) After the foundation pit excavation is completed, grouting reinforcement is carried out at the bottom of the foundation pit. After the reinforcement is completed, surcharge preloading is carried out on the foundation bed, and then the ground mold concrete and the concrete piers at the construction joints of the arch ring are constructed.

[0017] 9) Bind the steel bars of the arch seats and the arch ring. After arranging the cooling devices, pour the concrete of the arch seats in sequence, and then pour the concrete of the arch ring in segments to form the ground mold of the arch ring. Continue to bind the steel bars for the backfill above the arch, and backfill the concrete above the arch to form the arch bridge and the superstructure above the arch.

[0018] 10) After the construction of the arch bridge and the superstructure above the arch is completed, if there are karst caves below the side of the tunnel arch waist, backfill the karst caves outside the side of the tunnel arch waist with muck, and construct a closed wall with concrete to complete the construction of the arch bridge support structure in the karst cave.

[0019] The beneficial effects of the present invention are as follows: When the bottom in front of the tunnel needs to cross a giant karst cave, due to the adoption of step-by-step construction of the preliminary support structure and the arch bridge support structure in the karst cave, the excavation and advancement of the tunnel can be completed first by using the preliminary support structure in the karst cave covering the area under the tunnel; after the tunnel crosses the karst cave at the bottom, then go back to construct the reinforced concrete arch bridge structure in the karst cave, so as to finally complete the permanent support for the karst cave crossing under the tunnel. The preliminary support structure and its construction method ensure that the tunnel can pass through the karst cave quickly without affecting the excavation progress of the tunnel; the arch bridge support structure is to backfill and reinforce the foundation pit after the karst cave is excavated, then construct the ground mold on the foundation bed working surface, bind the arch ring steel bars attached to the ground mold and then carry out the construction of the arch seats. After the arch seats meet the strength requirements, pour the concrete of the arch ring in segments, and finally carry out the backfill above the arch to complete the arch bridge structure. This support structure and its construction method are applicable to the construction environment of karst cave tunnels with small spaces. Using the strengthened backfill soil to create a working surface and construct the ground mold fully ensures the construction safety, guarantees the construction quality, and effectively avoids the difficulty of large precast formwork entering the site, thus reducing the construction difficulty and construction cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The following further describes a support structure and its construction method for a tunnel crossing a karst cave below in conjunction with the drawings.

[0021] Figure 1 It is a schematic diagram of the structure of a tunnel crossing a karst cave with the support structure of the present invention.

[0022] Figure 2 It is a cross-sectional view after the karst cave outside the side of the tunnel arch waist is backfilled with soil.

[0023] Figure 3 It is a cross-sectional view of the arch bridge crossing structure for the construction of the karst cave.

[0024] Figure 4 It is a schematic diagram of the excavation in the karst cave foundation pit.

[0025] Figure 5 It is the sectional construction drawing of the arch ring of an arch bridge.

[0026] In the figure: 1. Initial support arch frame of the tunnel; 2. Backfill soil in the solution cavity; 3. Steel pipe columns; 4. Concrete wall; 5. Ground formwork of the arch ring; 6. Arch bridge structure; 7. Pile foundation; 8. Backfill of waste rock; 9. Shotcrete protection; 10. Shotcrete with wire mesh; 11. Pumped concrete backfill; 12. Grouting reinforcement drilling; 13. Concrete pier columns; 14. Backfill above the arch; 15. Access road; 16. Excavation of foundation pit A unit; 17. Excavation of foundation pit B unit; 18. Excavation of foundation pit C unit; 19. Construction joint of the arch ring; 20. Closing section of the arch ring. Specific implementation mode Embodiment

[0027] A support structure for spanning a karst cave under a tunnel in this embodiment is formed step by step during construction, including a pre-support structure and a later arch bridge support structure. The construction method includes the following steps:

[0028] 1) As shown in, when a karst cave appears under the front of the planned route of the tunnel during excavation, after detecting the karst cave and determining that the karst cave passes under the planned tunnel route, first use blasting to clean the loose slag in the karst cave and backfill the karst cave; specifically: Figure 1 Adopt smooth blasting to excavate the upper bench of the tunnel, and design the excavation of the lower bench of the tunnel according to Class V surrounding rock, while cleaning the loose slag in the solution cavity within the lower bench and the side of the tunnel arch waist.

[0029]

[0030] 2) Backfill the soil 2 into the karst cave passing through the area under the tunnel, use shotcrete with wire mesh 10 to reinforce the top (roof) of the karst cave in this area, and drill anchor bolts into the rock foundation of the karst cave wall.

[0031] 3) Erect the initial support arch frame 1 of the tunnel on the step surface of the backfill soil 2, and spray the initial support concrete and the temporary inverted arch lining; ensure that the initial support arch frame 1 is closed into a ring in time.

[0032] 4) If there is no karst cave under the side of the tunnel arch waist, then proceed to the next step; if there is a karst cave under the side of the tunnel arch waist, execute this step: Bury the pile foundation 7 in the backfill soil 2, and construct the steel pipe columns 3 on the pile foundation 7; After hanging the wire mesh and spraying concrete on the steel pipe columns 3, form a concrete retaining wall 4 along the side of the tunnel arch waist; As shown in. Figure 2

[0033] Then backfill the concrete and waste rock (tunnel muck) 8 into the karst cave outside the side of the tunnel arch waist, filling and advancing step by step, and strengthening the monitoring and measurement of the roof and the affected area of the solution cavity during the advancement, finally providing a safe space for supplementary exploration for the design.

[0034] ​​To be more specific, the steel pipe column 3 is in close contact with the top (roof plate) of the cave. The steel pipe column 3 of this embodiment adopts φ630 seamless steel pipe filled with concrete. The spacing between each steel pipe column 3 is 2m, and the steel pipe columns 3 are connected by φ22 longitudinal connecting steel bars.

[0035] A further option is to provide a reinforced arch protection for the primary supporting arch frame 1 of the tunnel, wherein the reinforced arch protection is supported by pre-buried corbels in the concrete retaining wall 4 to form a temporary structural force system.

[0036] After the above steps 1)-4), the preliminary support structure of the tunnel across the cave below the present embodiment is formed, including the initial support arch frame 1 and temporary invert lining of the tunnel erected in the cave, anchor rods drilled into the rock foundation in the cave wall, and backfill soil 2 passing through the cave area below the tunnel and along the side of the tunnel arch waist. The initial support arch frame 1 is erected on the step surface of the backfill soil 2 and sprayed with initial support concrete. The top plate of the cave in this area is formed with an anchor net and sprayed concrete 10. A pile foundation 7 is buried in the backfill soil 2, and a steel pipe column 3 is installed on the pile foundation 7. A concrete retaining wall 4 is formed along the side of the tunnel arch waist by hanging a net and spraying concrete on the steel pipe column 3.

[0037] 5) Continue tunnel excavation to make the tunnel pass through the cave.

[0038] 6) Go back to the cave again, first remove the temporary invert lining and invert filling in the cave; more specifically: use anchor cables to strengthen the edge of the arch frame in the cave, set temporary cross braces at intervals of 2 steel frames at the connection plate position of the initial support arch frame of the lower step of the tunnel, and after the support construction at the first step of the bottom base of the arch ring is completed, the temporary cross braces can be removed according to the on-site monitoring situation.

[0039] Then, advance support is applied at the arch foot of the primary arch frame obliquely downward toward the cave to form a grouting reinforcement borehole 12 and inject cement slurry; further specifically: the advance support uses Ф42 steel flower pipe and injects cement slurry into the steel flower pipe. The length of the steel flower pipe is 4.5m, and the longitudinal spacing between each steel flower pipe is 0.3m.

[0040] 7) Excavate the foundation pit in sections at the bottom of the cave, and carry out anchor net spraying and concrete protection on each section of the foundation pit9. Figure 3 , 4As shown in the figure, the foundation pit is divided into three units, namely Unit A 17, Unit B 18, and Unit C 19. First, excavate Unit A 16 and Unit B 17, and then excavate Unit C 18, with an expansion excavation of 64 cm and excavation to the first step. Connect to I25b steel I-beams with a longitudinal spacing of 60 cm, and drive φ76 locking foot bolts with a length of 6 m, and hang wire mesh and spray concrete for closure; during the excavation of Unit A 16 and Unit B 17 for each layer, use anchor mesh shotcrete for protection on the vertical surface near the end of Unit C 18, spray 10 cm thick C25 concrete, use φ8 steel mesh with a size of 25 cm×25 cm, and use φ25 fiber bolts with a length of 3 m for the bolts, with a spacing of 1.2 m×1.2 m; for the side wall of the access road 15, use anchor mesh shotcrete for protection, spray 10 cm thick C25 concrete, use φ8 steel mesh with a size of 25 cm×25 cm, Φ22 mortar bolts, with a spacing of 1.2 m×1.2 m and a length of 3 m for each bolt; in the range of the radial support bedrock, set φ32 mm mortar bolts, with a bolt length of 4 m and a spacing of 1.2×1.2 m; in the non-bedrock range, use 76 mm steel pipe with holes for radial grouting, with a steel pipe length of 9 m and a spacing of 1.5×1.5 m; construct the first double-spliced I45b steel waling beam and steel support, with a longitudinal spacing of 3 m for the steel support. Subsequently, excavate Units A, B, and C in sequence according to the above steps until the foundation pit excavation is completed.

[0041] 8) After the foundation pit excavation is completed, grout and reinforce the bottom of the foundation pit. After the reinforcement is completed, conduct surcharge preloading on the foundation. Specifically, the preloading time is not less than 48 h, and the preloading load is not less than 1.1 times the self-weight of the arch ring. Use 8 m long Ф110 PVC pipe with holes for grouting.

[0042] Then construct the ground mold concrete and the concrete pier at the construction joint of the arch ring.

[0043] 9) Bind the steel bars of the arch seat and the arch ring. After arranging the cooling device, pour the concrete of the arch seat in sequence, and then pour the concrete of the arch ring in sections to form the ground mold of the arch ring. Continue to bind the steel bars for the backfill above the arch, and backfill the concrete above the arch to form the arch bridge and the superstructure above the arch. Specifically, for this step:

[0044] 9.1) Before the construction of the arch ring, construct a 20 cm thick concrete ground mold on the surface 5. The main steps are as follows: use an excavator to correct the first section of the ground, after passing the formwork erection acceptance, pour the first section of 20 cm thick concrete, then remove the formwork and conduct curing and construction joint treatment, and then start to correct the second section of the road surface. Repeat the above steps in a cycle until the arch ring construction is completed; the number of construction sections of the ground mold should be consistent with the segmentation of the arch ring. After the construction is completed, sprinkle water for moisturizing curing. After the strength reaches 75% of the design strength, the steel bars of the arch ring can be bound; the 20 cm thick concrete and the strengthened foundation form the ground mold for the arch ring construction, which serves as a temporary support to provide a working surface for the arch ring construction and bear the load of the arch ring during construction. After the construction is completed, it also serves as a part of the arch-tunnel integrated structure to increase the strength of the arch bridge.

[0045] 9.2) The steel bars of the arch seat and the arch ring are tied up at one time. Subsequently, the working cooling device is arranged, and then the concrete of the arch seat is poured. After the construction of the arch seat is completed, the concrete of the arch ring is constructed by cast-in-place in sections with a ground form. A 2m closure section 20 is set at the mid-span, 6 construction joints 19 are set, and a 13m long C55 concrete pier 13 is set at the construction joints, as Figure 3 and Figure 4 shown.

[0046] 9.3) After the construction of the arch ring is completed, the backfill concrete construction of the superstructure of the arch is carried out. The volume of the backfill concrete is large and the height is high, so it cannot be constructed at one time. Therefore, the backfill is constructed in layers, and the layer height is set in combination with the layout position of the steel supports for the foundation pit excavation. There are the first steel support and the second steel support. The setting positions of the first steel support and the second steel support are the first step and the third step. Therefore, the backfill concrete is poured in three layers.

[0047] 10) After the construction of the arch bridge and the backfill concrete of the superstructure of the arch is completed, if there are karst caves below the lower part outside the side of the arch waist of the tunnel, the karst cave cavity outside the range of the tunnel concrete retaining wall is backfilled with muck until 2m below the top of the karst cave, and then the cavity is filled with pumped C20 concrete (concrete) 11 and a closed wall is constructed to complete the construction of the support structure.

[0048] After the above steps 5)-10), the arch bridge support structure spanning the karst cave below the tunnel of this embodiment is formed, including: the foundation pits 16, 17, 18 excavated at the bottom inside the karst cave, the arch seat and the ground form 5 of the arch ring formed on the bottom surface of the foundation pit, the arch bridge structure 6 of the reinforced concrete arch ring poured on the ground form 5 of the arch ring, the backfill muck 14 on the arch and the closed wall constructed for the karst cave cavity outside the range of the tunnel concrete retaining wall.

Claims

1. A construction method for a support structure under a tunnel that crosses a karst cave. Features The following steps are involved: 1) When a karst cave appears below the planned tunnel route during tunnel excavation, if it is confirmed through exploration that the karst cave passes below the planned tunnel route, the slag in the karst cave shall be cleared by blasting and the karst cave shall be backfilled; 2) Backfill soil into the cave that passes through the area below the tunnel, use anchor nets to spray concrete to reinforce the top of the cave in this area, and drill anchor rods into the rock foundation inside the cave wall; 3) Erecting the primary arch frame of the tunnel on the step surface of the backfill soil, and spraying the primary support concrete and temporary invert lining; 4) If there is no karst cave below the side of the tunnel waist, the next step is executed; if there is a karst cave below the side of the tunnel waist, this step is executed: a pile foundation is buried along the side of the tunnel waist in the backfill soil, and a steel pipe column is constructed on the pile foundation, and a mesh is hung on the steel pipe column and concrete is sprayed to form a concrete retaining wall along the side of the tunnel waist, and then concrete and ballast are backfilled into the karst cave outside the side of the tunnel waist; 5) Continue tunnel construction to make the tunnel pass through the cave; 6) Go back to the cave again, first remove the temporary invert lining and invert filling in the cave, then carry out advance support and inject cement slurry at the arch foot of the primary arch frame diagonally downward into the cave; 7) Excavate the foundation pit in sections at the bottom of the cave, and spray concrete with anchor nets for protection section by section; 8) After the excavation of the foundation pit is completed, grouting reinforcement is carried out at the bottom of the foundation pit. After the reinforcement is completed, the base load preloading is carried out, and then the concrete piers at the ground formwork and arch ring construction joints are constructed; 9) After tying the arch seat and arch ring steel bars and arranging the cooling device, pour the arch seat concrete in sequence, then pour the arch ring concrete in sections to form the arch ring ground formwork, continue to tie the arch backfill steel bars, backfill the arch concrete to form the arch bridge and arch structure; 10) After the construction of the arch bridge and the structure above the arch is completed, if there is a cave below the tunnel arch waist, fill the cave outside the tunnel arch waist with debris, and use concrete to build a closed wall to complete the construction of the arch bridge support structure in the cave.

2. The construction method of the support structure for the tunnel crossing the karst cave under claim 1, Features: In the step 4), the steel pipe columns are in close contact with the top of the cave. The steel pipe columns are made of φ630 seamless steel pipes filled with concrete. The spacing between each steel pipe column is 2m, and the steel pipe columns are connected by φ22 longitudinal connecting steel bars.

3. The construction method of the support structure for the tunnel crossing the karst cave under claim 2, Features: In the step 4), a reinforced arch is applied to the primary supporting arch of the tunnel, and the reinforced arch is supported by pre-buried corbels in the concrete retaining wall to form a temporary structural force system.

4. The construction method of the support structure for the tunnel crossing the karst cave under claim 2, Features: In the step 6), the advance support uses Ф42 steel tubes and cement slurry is injected into the steel tubes. The length of each steel tube is 4.5m, and the longitudinal spacing between each steel tube is 0.3m.

5. The construction method of the support structure for the tunnel crossing the karst cave under claim 2, Features: In step 7), the foundation pit is divided into three units, namely Unit A, Unit B and Unit C. First, Unit A and Unit B are excavated, and then Unit C is excavated. It is excavated 64 cm wider and to the first step, and then connected to I25b steel I-beams with a longitudinal spacing of 60 cm. Lock foot bolts with a length of 6 m and a diameter of φ76 are driven, and the surface is closed by shotcrete with wire mesh; during the excavation of each layer of Unit A and Unit B, the vertical surface near the end of Unit C is protected by shotcrete with wire mesh and bolts. 10 cm thick C25 concrete is sprayed, a wire mesh of φ8 with a size of 25 cm×25 cm is used, and fiber bolts with a length of 3 m and a diameter of φ25 are used as bolts, with a spacing of 1.2 m×1.2 m; the side wall of the access road is protected by shotcrete with wire mesh and bolts. 10 cm thick C25 concrete is sprayed, a wire mesh of φ8 with a size of 25 cm×25 cm is used, Φ22 mortar bolts are used, with a spacing of 1.2 m×1.2 m and each bolt is 3 m long; in the range of the radial support bedrock, mortar bolts with a diameter of 32 mm are set, the bolt length is 4 m, and the spacing is 1.2×1.2 m; in the non-bedrock range, radial grouting is carried out with steel pipe with holes of 76 mm, the length of the steel pipe with holes is 9 m, and the spacing is 1.5×1.5 m; the first double I45b steel girders and steel supports are constructed, and the longitudinal spacing of the steel supports is 3 m; the excavation of Unit A, Unit B and Unit C is carried out in turn until the excavation of the foundation pit is completed.

6. The construction method of the support structure spanning the karst cave under the tunnel according to claim 2, characterized in that: in step 8), the preloading time is not less than 48 h, the preloading load is not less than 1.1 times the self-weight of the arch ring, and grouting is carried out with 8 m long Ф110 PVC pipes with holes.

7. The construction method of the support structure spanning the karst cave under the tunnel according to claim 2, characterized in that: in step 9), 9.1) Before the construction of the arch ring, a 20 cm thick concrete ground mold surface is constructed. The main steps are to use an excavator to correct the first section of the ground, after passing the formwork erection acceptance, pour the first section of 20 cm thick concrete, then remove the formwork and carry out curing and construction joint treatment, and then start to correct the second section of the road surface, and repeat the cycle until the construction of the arch ring is completed; the number of construction sections of the ground mold should be the same as the segmentation of the arch ring. After the construction is completed, it is watered and moisturized for curing. After the strength reaches 75% of the design strength, the steel bars of the arch ring can be tied; the 20 cm thick concrete and the strengthened foundation form the construction ground mold of the arch ring, which serves as a temporary support to provide a working surface for the construction of the arch ring and bear the load of the arch ring during construction. After the construction is completed, it also serves as a part of the arch-tunnel integrated structure to increase the strength of the arch bridge; 9.2) The steel bars of the arch seat and the arch ring are tied up at one time, then the working cooling device is arranged, and then the concrete of the arch seat is poured. After the construction of the arch seat is completed, the concrete of the arch ring is cast in segments by the ground mold. A 2 m closure section is set in the middle of the span, 6 construction joints are set, and 13 m long C55 concrete piers are set at the construction joints; 9.3) After the construction of the arch ring is completed, the backfill concrete construction of the superstructure of the arch is carried out. The backfill is constructed in three layers, and the height of each layer is set in combination with the arrangement position of the steel supports for the foundation pit excavation; the steel supports are divided into the first steel support and the second steel support, and the first steel support and the second steel support are set as the first step and the third step, and the backfill concrete is poured in three layers.

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