Construction method for tunnel underneath passing through weak silt layer

Abstract

The invention discloses a construction method for a tunnel underneath passing through a weak silt layer, and belongs to the technical field of tunnel construction, and the construction method comprises the following steps: S1, setting an enclosure structure: constructing two temporary drainage ditches along two sides of a preset tunnel foundation pit, and constructing two groups of continuous vertical waterproof curtains between the two temporary drainage ditches; s2, layered excavation is carried out, wherein earth excavation is carried out to the position 0.2-0.4 m below the designed elevation of an inner support; installing a waist beam and erecting a cross arm at the designed elevation of the inner support; s3, secondary excavation and bottom plate sealing are conducted, specifically, excavation continues to be conducted to the designed bottom elevation of the foundation pit; s4, backfill soil construction is conducted, specifically, backfill soil is conducted in gaps between the two side walls and the enclosure structure; and S5, construction of an upper structure: after the inner support is removed, constructing an anti-collision wall at the top of the construction side wall, sealing and pouring the backfill soil on the two sides of the foundation pit, and finally removing the drainage ditch to restore ground traffic. The method has the effect of effectively controlling deformation and risks in the tunnel construction process according to the characteristics of the soft sludge stratum.

Description

Technical Field

[0001] This invention relates to the field of tunnel construction, and in particular to a method for constructing a tunnel under a weak silt layer. Background Technology

[0002] With the acceleration of urbanization and the continuous improvement of the three-dimensional transportation network in my country, the demand for tunnel projects such as urban subways, integrated utility tunnels, and underground roads is increasing. When constructing tunnels in coastal, riverside, and lakeside areas, the geological conditions of the construction sites are mostly deep, weak silt layers.

[0003] Soft silt layers are rich in water, have poor bearing capacity, are highly sensitive, and exhibit thixotropic and rheological properties. Deep foundation pit excavation under soft silt layers presents a common challenge in urban underground engineering construction: ensuring the safety of the project itself while minimizing disruption to surface traffic, controlling environmental disturbance, achieving rapid construction, and guaranteeing construction quality. Therefore, the industry urgently needs a specialized tunnel construction method that effectively controls deformation and risks specific to the characteristics of soft silt strata. Summary of the Invention

[0004] In order to address the characteristics of soft silt strata and effectively control deformation and risks during tunnel construction, this application provides a construction method for tunnels passing under soft silt strata.

[0005] This application provides a construction method for tunnels passing under weak silt layers, employing the following technical solution: A method for constructing a tunnel under a weak silt layer includes the following steps: S1, setting up a retaining structure: constructing two temporary drainage ditches along both sides of the pre-designed tunnel pit, and constructing two sets of continuous vertical water-stop curtains between the two temporary drainage ditches to isolate the pit from the surrounding soil and water environment; subsequently, within the excavation range inside the pit, pre-reinforcing the weak silt layer below the pit bottom; S2, layered excavation: after the pre-reinforcement in the pit reaches the required strength, the first earthwork excavation is carried out, excavating to 0.2m to 0.4m below the design elevation of the internal support; then, installing the waist beam and erecting the horizontal bracing at the design elevation of the internal support, and subsequently, within the pit... Dewatering is carried out using the wellpoint dewatering method; S3, Secondary excavation and bottom slab closure: Excavation continues to the designed bottom elevation of the foundation pit; a concrete cushion layer is then constructed at the bottom of the foundation pit, and a waterproof layer is laid on top of the concrete cushion layer, followed by the pouring of the full-width tunnel bottom slab and side walls; S4, Backfilling construction: After the bottom slab reaches a certain strength, backfilling is carried out in the gaps between the side walls and the retaining structure; after the backfill reaches the design requirements, the internal supports are removed; S5, Superstructure construction: After the internal supports are removed, a crash barrier is constructed on top of the construction side walls, and the backfill on both sides of the foundation pit is sealed and poured, finally the drainage ditch is removed, and ground traffic is restored.

[0006] By adopting the above technical solutions, the external isolation of the vertical water-stop curtain and the internal improvement of the pre-reinforcement of the bottom mixing piles are combined to enhance the stability of the tunnel foundation construction from the root. The combination of layered excavation and immediate support further effectively suppresses the deformation of the retaining structure. Finally, the backfilling construction ensures the overall structural stability of the tunnel after construction. The solution in this application is suitable for effectively controlling the deformation and risks during the tunnel construction process, especially given the characteristics of soft silt strata with low bearing capacity and high sensitivity.

[0007] Optionally, in step S1, the main method for pre-reinforcing the weak silt layer below the pit bottom is to construct cement mixing piles inside the pit. The top of the cement mixing piles is level with the design elevation of the pit bottom, and the length of the cement mixing piles is not less than 1.0m.

[0008] By adopting the above technical solution, a pre-reinforcement layer with uniform thickness and controllable strength is constructed in the foundation pit space. This pre-reinforcement layer serves as a support for the pit bottom during subsequent excavation, which helps to improve the overall rigidity and bearing capacity of the pit bottom and effectively suppresses the rebound and plastic deformation of the pit bottom caused by excavation unloading. At the same time, it facilitates the formation of a water-stopping and reinforcement system in conjunction with the surrounding vertical water-stopping curtain, fundamentally improving the excavation geological conditions in the foundation pit and creating prerequisites for subsequent dry excavation and structural construction.

[0009] Optionally, the vertical water-stop curtain includes water-stop mixing piles and water-stop steel sheet piles. The two sets of water-stop mixing piles are located on the side away from each other of the two sets of water-stop steel sheet piles. The insertion depth of the water-stop steel sheet piles into the ground is deeper than that of the water-stop mixing piles.

[0010] By adopting the above technical solution, the water-stop mixing piles and water-stop steel sheet piles form a two-layer water-stop structure, thereby effectively achieving reliable sealing of the water-stop curtain within the construction depth range. At the same time, the water-stop mixing piles effectively block the seepage of shallow water and some upper water, and also help to reinforce the shallow soil. The water-stop steel sheet piles effectively cut off the seepage path of deep confined water, while resisting the pressure on the deep soil caused by the excavation of the foundation pit.

[0011] Optionally, the water-stop mixing pile is an interlocking pile, the water-stop mixing pile includes a water-stop pile body and a limiting and blocking part fixedly installed on the pile body, the limiting and blocking part is set near the free section at the top of the water-stop pile body, and the water-stop steel sheet pile includes multiple steel plates, each of the limiting and blocking parts abutting against one side of each steel plate.

[0012] By adopting the above technical solution, during the steel sheet pile driving process, the limiting and retaining part can effectively guide and constrain the planar position and verticality of the steel sheet pile, ensuring that the steel sheet pile can be installed quickly and accurately. At the same time, it helps to ensure the stability of the position of the steel sheet pile after installation, thus jointly bearing part of the shallow soil pressure.

[0013] Optionally, two adjacent steel plates are designated as a first plate and a second plate. The first plate is bent towards the side of the second plate and provided with a first hook. The second plate is bent towards the first plate and provided with a second hook that engages with the first hook.

[0014] By adopting the above technical solution, the connection stiffness and shear resistance of two adjacent steel plates in the horizontal plane are further enhanced, effectively suppressing the relative slippage and displacement that may occur between two adjacent steel plates under lateral water and soil pressure, and further ensuring the overall stability of the water-stop steel sheet pile composed of multiple steel plates.

[0015] Optionally, the cross brace in step S2 includes a support body, a fixed joint, a movable joint, and a pressing drive component. The fixed joint is fixedly installed at one end of the support body. Two waist beams are arranged in parallel. The fixed joint abuts against one of the waist beams. The movable joint slides and engages with the end of the support body away from the fixed joint. The pressing drive component is used to drive the movable joint to move away from the support body. The movable joint abuts against the other waist beam.

[0016] By adopting the above technical solution, the movable joint is driven to slide along the support body by the pressure-driving component, enabling the support body to stably and firmly press against the two side beams under the action of the fixed joint and the movable joint, thereby improving the overall stiffness and initial stability of the support system. In addition, the pressure-driving component allows the movable joint to make adaptive micro-adjustments to the two crossbeams with different distances, making it highly adaptable.

[0017] Optionally, the movable joint has two plug-in portions, which pass through and slide into the support body. A limiting baffle is provided between the two plug-in portions. The limiting baffle has a limiting inclined surface, which abuts against the movable joint. The side of the limiting baffle away from the limiting inclined surface abuts against the support body.

[0018] By adopting the above technical solution, the setting of the limit baffle plays a role in limiting the movement of the movable joint caused by the pressure drive component to retract due to unexpected circumstances. This helps to further ensure the stability of the position of the movable joint after support, and thus helps to ensure the support effect of the cross brace.

[0019] Optionally, the support body is fixedly installed with a fixing frame, and a limiting screw is rotatably installed on the top of the limiting baffle. The limiting screw passes through in the vertical direction and is threaded into the fixing frame.

[0020] By adopting the above technical solution, applying force to rotate the limiting screw can synchronously drive the limiting baffle to move stably in the vertical direction. The operation is simple, and the self-locking effect between the limiting screw and the fixed frame helps to further ensure the stability of the position of the limiting baffle after movement.

[0021] Optionally, the support body includes a first support rod, a second support rod, and an intermediate support rod. The two ends of the intermediate support rod are respectively inserted through and rotatably slidably engaged with the first support rod and the second support rod. A bidirectional lead screw is coaxially fixedly installed on the intermediate support rod. The two ends of the bidirectional lead screw with opposite thread directions are respectively inserted through and threadedly engaged with the first support rod and the second support rod.

[0022] By adopting the above technical solution, when the intermediate support rod is driven to rotate and the bidirectional lead screw rotates synchronously, the first support rod and the second support rod move synchronously toward each other or away from each other, thereby realizing the overall lengthening or shortening of the support body. This facilitates the adjustment of a large dimension between two crossbeams with different distance dimensions, which is beneficial to further enhance applicability. Moreover, the thread self-locking effect of the bidirectional lead screw helps to fully ensure the stability after the overall length is adjusted.

[0023] Optionally, in step S4, the backfill soil includes a plain concrete layer and a medium-coarse sand layer arranged sequentially from bottom to top.

[0024] By adopting the above technical solution, the plain concrete layer can quickly form a high-strength support after backfilling, thus providing immediate and reliable lateral restraint for the bottom of the sidewall, effectively distributing lateral pressure, and reducing the possible deformation of the sidewall after the removal of the cross bracing. The upper medium-coarse sand layer is beneficial to fully utilize its advantages of high permeability, easy compaction, and good deformation coordination, draining any small amount of water that may seep in, ensuring close contact with the sidewall and enclosure structure, and evenly transferring the load.

[0025] In summary, this application includes at least one of the following beneficial technical effects: 1. The combination of external isolation of the vertical water-stop curtain and internal improvement of pre-reinforcement by mixing piles at the bottom of the pit enhances the stability of the tunnel foundation construction from the root. The combination of layered excavation and immediate support further effectively suppresses the deformation of the retaining structure. Finally, the backfilling construction ensures the overall structural stability of the tunnel after construction, which is conducive to effectively controlling the deformation and risks during the tunnel construction process, given the characteristics of low bearing capacity and high sensitivity of soft silt strata.

[0026] 2. The water-stop mixing piles and water-stop steel sheet piles form a two-layer water-stop structure, thereby effectively achieving reliable sealing of the water-stop curtain within the construction depth range.

[0027] 3. By driving the movable joint to slide along the support body through the pressure-driving component, the support body can stably achieve stable pressure on the two side waist beams under the action of the fixed joint and the movable joint, which is conducive to improving the overall stiffness and initial stability of the support system. Attached Figure Description

[0028] Figure 1 This is a construction diagram of step S1 in the embodiment of this application.

[0029] Figure 2 This is a top view of the vertical water-stop curtain and horizontal bracing in the embodiments of this application.

[0030] Figure 3 yes Figure 2 A magnified view of part A in the diagram.

[0031] Figure 4 This is a construction diagram of step S2 in the embodiment of this application.

[0032] Figure 5 This is a three-dimensional schematic diagram of the vertical water-stop curtain and horizontal bracing in the embodiments of this application.

[0033] Figure 6 yes Figure 5 A magnified view of part B in the diagram.

[0034] Figure 7 This is a perspective view of the cross brace in an embodiment of this application.

[0035] Figure 8 This is a construction diagram of the internal support before its removal in step S4 of this application.

[0036] Figure 9 This is a schematic diagram of the structure in step S5 of the embodiment of this application.

[0037] Explanation of reference numerals in the attached figures: 1. Temporary drainage ditch; 2. Vertical water-stop curtain; 21. Water-stop mixing pile; 211. First pile body; 212. Second pile body; 213. Limiting and retaining part; 22. Water-stop steel sheet pile; 221. First sheet body; 222. Second sheet body; 223. First hook; 224. Second hook; 3. Cement mixing pile; 4. Waist beam; 5. Horizontal brace; 51. Support body; 511. First support rod; 512. Second support rod; 513. Intermediate support rod; 514. First adjustment 515. Second adjustment groove; 516. Support rod adjustment part; 52. Fixed joint; 53. Movable joint; 531. Insertion part; 532. Pressing part; 54. Pressing drive part; 6. Fixed frame; 7. Limiting screw rod; 8. Limiting baffle; 81. Limiting inclined surface; 9. Rotating handle; 10. Two-way screw rod; 11. Concrete pad layer; 12. Waterproof layer; 13. Base plate; 14. Side wall; 15. Plain concrete layer; 16. Medium-coarse sand layer; 17. Anti-collision wall. Detailed Implementation

[0038] The following is in conjunction with the appendix Figure 1-9 This application will be described in further detail.

[0039] This application discloses a method for constructing a tunnel under a weak silt layer. (Refer to...) Figure 1 The construction method for tunnels passing under weak silt layers includes the following steps: Reference Figure 1 S1, Retaining structure setup: Along both sides of the pre-set tunnel foundation pit, construct two temporary drainage ditches 1, and construct two sets of continuous vertical water-stop curtains 2 between the two temporary drainage ditches 1 to isolate the foundation pit from the surrounding water and soil environment; subsequently, within the excavation range inside the foundation pit, pre-reinforce the weak silt layer below the bottom of the pit.

[0040] Continue to refer to Figure 1 The main method for pre-reinforcing the weak silt layer below the pit bottom is to construct cement mixing piles 3 inside the pit. The top of the cement mixing piles 3 is flush with the design elevation of the pit bottom, and the length of the cement mixing piles 3 is not less than 1.0m, so as to ensure the strength of the pre-reinforcement structure at the bottom of the pit.

[0041] Continue to refer to Figure 1 Specifically, the vertical water-stop curtain 2 includes water-stop mixing piles 21 and water-stop sheet piles 22. The two sets of water-stop mixing piles 21 are located on the side of the two sets of water-stop sheet piles 22 that are far apart from each other. The insertion depth of the water-stop sheet piles 22 into the ground is deeper than that of the water-stop mixing piles 21. This is to reinforce the shallow soil while effectively cutting off the seepage path of deep confined water.

[0042] Reference Figure 1 and Figure 2Furthermore, in this embodiment, the water-stop mixing pile 21 is an interlocking pile. One water-stop mixing pile 21 is designated as the first pile body 211, and the two adjacent water-stop mixing piles 21 are designated as second pile bodies 212. Each first pile body 211 includes a water-stop pile body and an integrally fixed limiting and retaining part 213. The limiting and retaining part 213 has two free sections, both located near the top of the first pile body 211. The water-stop steel sheet pile 22 includes multiple steel plates, one of which is designated as the first plate body 221, and the two adjacent steel plates are designated as the second plate bodies 222. The two limiting and retaining parts 213 of each first pile body 211 abut against both sides of each first plate body 221 to provide limiting and guiding functions during the installation process, ensuring that the steel sheet pile can be installed quickly and accurately.

[0043] Reference Figure 2 and Figure 3 In addition, the first plate 221 is bent towards the side of the second plate 222 and is provided with a first hook 223. The second plate 222 is bent towards the first plate 221 and is provided with a second hook 224 that engages with the first hook 223. After the steel sheet pile is installed, the inner sides of the first hook 223 and the second hook 224 press against each other, so that the relative slippage and misalignment between the two adjacent steel plates are not easy to occur, further ensuring the overall stability of the water-stop steel sheet pile 22 composed of multiple steel plates.

[0044] Reference Figure 4 S2, Layered excavation: After the pre-reinforcement in the pit reaches the required strength, the first earthwork excavation is carried out, excavating to 0.2m to 0.4m below the design elevation of the inner support. In this embodiment, the excavation is specifically carried out to 0.3m below the design elevation. Then, the waist beam 4 is installed at the design elevation of the inner support and the cross bracing 5 is erected. Subsequently, the pit is dewatered by wellpoint dewatering.

[0045] Reference Figure 5 and Figure 6 Specifically, two waist beams 4 are provided and fixedly installed on one side of the two sets of sheet piles respectively. The cross brace 5 includes a support body 51, a fixed joint 52, a movable joint 53, and a pressing drive component 54. The fixed joint 52 is fixedly installed at one end of the support body 51 and abuts against one of the waist beams 4. The movable joint 53 is slidably fitted to the end of the support body 51 away from the fixed joint 52. In this embodiment, the pressing drive component 54 includes a pressing jack. Two pressing jacks are provided. One end of the cylinder of the two pressing jacks is connected to the movable joint 53, and the other end of the piston rod is connected to the support body 51, so that when the pressing jack drives its own piston rod to move, it drives the movable joint 53 to press against the other waist beam 4, thereby achieving stable pressing against the two waist beams 4.

[0046] Reference Figure 6 and Figure 7 To further ensure the stability of the movable joint 53, the movable joint 53 has two insertion parts 531, which are inserted and slidably fitted onto the support body 51. A fixing frame 6 is fixedly installed on the free end of the support body 51 near the movable joint 53. The fixing frame 6 is threadedly fitted with a limit rod 7 in the vertical direction. A limit baffle 8 is rotatably fitted at the bottom of the limit rod 7. A rotating handle 9 is fixedly installed on the top of the limit rod 7 to facilitate the rotation of the limit rod 7 by the construction personnel.

[0047] Reference Figure 7 The limiting baffle 8 is located between the two insertion parts 531. The limiting baffle 8 has a limiting inclined surface 81, which is located on the side of the limiting baffle 8 away from the support body 51. The side of the limiting baffle 8 away from the limiting inclined surface 81 abuts against and slides vertically against the support body 51. The movable joint 53 is integrally fixedly connected to the side facing the limiting baffle 8 with a pressing part 532. The limiting inclined surface 81 abuts against the pressing part 532, so that the limiting baffle 8 can limit the retraction of the pressing jack in case of accidental retraction, which helps to further ensure the stability of the position of the movable joint 53 after support.

[0048] Continue to refer to Figure 7 Furthermore, the support body 51 includes a first support rod 511, a second support rod 512, and an intermediate support rod 513. The intermediate support rod 513 is located between the first support rod 511 and the second support rod 512. Each of the first and second support rods 511 and 512 has a first adjustment groove 514 at one end facing the intermediate support rod 513. Both ends of the intermediate support rod 513 pass through and rotatably slide within the first adjustment grooves 514 of the first and second support rods 511 and 512, respectively. Each of the first and second support rods 511 and 512 has a support rod adjustment part 516 coaxially fixedly connected to one end facing the intermediate support rod 513. Both ends of the intermediate support rod 513 have second adjustment grooves 515, and both support rod adjustment parts 516 pass through and rotatably fit within the second adjustment grooves 515 of the intermediate support rod 513.

[0049] Continue to refer to Figure 7 A bidirectional lead screw 10 is coaxially fixedly mounted on the intermediate support rod 513. The two ends of the bidirectional lead screw 10, with opposite thread directions, are threaded and connected to the support rod adjustment parts 516 of the first support rod 511 and the second support rod 512. This allows the first support rod 511 and the second support rod 512 to move synchronously towards or away from each other when the bidirectional lead screw 10 rotates, thereby achieving the overall extension or shortening of the support body 51. This facilitates larger adjustments between two crossbeams of different distances, further enhancing its applicability. Furthermore, the design of the first adjustment groove 514 and the second adjustment groove 515 facilitates the concealment and storage of the bidirectional lead screw 10, making it less susceptible to external influences and ensuring its stability and service life after rotation.

[0050] Reference Figure 8 S3, Secondary excavation and sealing of the bottom slab 13: Continue excavation to the designed bottom elevation of the foundation pit; then construct a concrete cushion layer 11 at the bottom of the foundation pit, and lay a waterproof layer 12 on top of the concrete cushion layer 11. Subsequently, pour the full-width tunnel bottom slab 13 and side walls 14. In this embodiment, there are two side walls 14, which are integrally fixed to the two sides of the bottom slab 13 near the two vertical water-stop curtains 2.

[0051] Reference Figure 8 and Figure 9 S4, Backfilling construction: After the base slab 13 reaches a certain strength, backfill soil in the gap between the two side walls 14 and the retaining structure; after the backfill soil reaches the design requirements, remove the internal support.

[0052] Specifically, the backfill soil includes a plain concrete layer 15 and a medium-coarse sand layer 16 arranged sequentially from bottom to top. After backfilling, the plain concrete layer 15 can quickly form a high-strength support 51, thereby providing immediate and reliable lateral restraint for the bottom of the side wall 14 and reducing the possible deformation of the side wall 14 after the removal of the cross brace 5. The upper medium-coarse sand layer 16 helps to drain any small amount of water that may seep in, ensuring close contact with the side wall 14 and the retaining structure, and uniformly transferring the load.

[0053] Reference Figure 9 S5, Superstructure construction: After the internal supports are removed, a crash barrier 17 is constructed on top of the construction side wall 14, and the backfill soil on both sides of the foundation pit is sealed and poured. Finally, the temporary drainage ditch 1 is removed and ground traffic is restored.

[0054] Two crash barriers 17 are installed and located on the top of the two side walls 14 respectively, to effectively isolate the tunnel from the outside world.

[0055] The implementation principle of the tunnel construction method under a weak silt layer in this application embodiment is as follows: During the construction process, the external isolation of the vertical water-stop curtain 2 is combined with the internal improvement of the pre-reinforcement of the bottom mixing piles, which fundamentally enhances the stability of the tunnel foundation pit construction; at the same time, the combination of layered excavation and immediate support further effectively suppresses the deformation of the retaining structure; finally, the overall structural stability of the tunnel after construction is ensured by backfilling. The scheme in this application is convenient for effectively controlling the deformation and risks during the tunnel construction process, given the characteristics of low bearing capacity and high sensitivity of the weak silt layer.

[0056] The above are all preferred embodiments of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for constructing a tunnel under a weak silt layer, characterized in that: Includes the following steps: S1, retaining structure setting: along the two sides of the pre-set tunnel foundation pit, two temporary drainage ditches (1) are constructed, and two sets of continuous vertical water-stop curtains (2) are constructed between the two temporary drainage ditches (1) to isolate the foundation pit from the surrounding water and soil environment; then, within the excavation range inside the foundation pit, the soft silt layer below the bottom of the pit is pre-reinforced. S2, layered excavation: After the pre-reinforcement in the pit reaches the required strength, the first earthwork excavation is carried out, excavating to 0.2m to 0.4m below the design elevation of the inner support; then, the waist beam (4) is installed at the design elevation of the inner support and the cross bracing (5) is erected. Subsequently, the pit is dewatered by well point dewatering method. S3, Secondary excavation and bottom slab (13) closure: Continue excavation to the design bottom elevation of the foundation pit; then construct a concrete cushion layer (11) at the bottom of the foundation pit, and lay a waterproof layer (12) on top of the concrete cushion layer (11), and then pour the full tunnel bottom slab (13) and side walls (14). S4, Backfilling construction: After the bottom slab (13) reaches a certain strength, backfill soil in the gap between the two side walls (14) and the retaining structure; after the backfill soil meets the design requirements, remove the internal support; S5, Construction of the superstructure: After the internal support is removed, a crash barrier (17) is constructed on the top of the construction side wall (14), and the backfill soil on both sides of the foundation pit is sealed and poured. Finally, the drainage ditch is removed and ground traffic is restored.

2. The method for constructing a tunnel under a weak silt layer according to claim 1, characterized in that: In step S1, the main way to pre-reinforce the weak silt layer below the pit bottom is to construct cement mixing piles (3) in the foundation pit. The top of the cement mixing piles (3) is level with the design elevation of the bottom of the foundation pit, and the length of the cement mixing piles (3) is not less than 1.0m.

3. The method for constructing a tunnel under a weak silt layer according to claim 1, characterized in that: The vertical water-stop curtain (2) includes water-stop mixing piles (21) and water-stop steel sheet piles (22). The two sets of water-stop mixing piles (21) are located on the side away from the two sets of water-stop steel sheet piles (22). The water-stop steel sheet piles (22) are inserted into the ground at a greater depth than the water-stop mixing piles (21).

4. The method for constructing a tunnel under a weak silt layer according to claim 3, characterized in that: The water-stop mixing pile (21) is an interlocking pile. The water-stop mixing pile (21) includes a water-stop pile body and a limiting and blocking part (213) fixedly installed on the pile body. The limiting and blocking part (213) is set near the free section at the top of the water-stop pile body. The water-stop steel sheet pile (22) includes multiple steel plates. Each limiting and blocking part (213) abuts against one side of each steel plate.

5. The method for constructing a tunnel under a weak silt layer according to claim 3, characterized in that: The two adjacent steel plates are designated as a first plate (221) and a second plate (222). The first plate (221) is bent towards the second plate (222) and a first hook (223) is provided. The second plate (222) is bent towards the first plate (221) and a second hook (224) is provided to engage with the first hook (223).

6. The method for constructing a tunnel under a weak silt layer according to claim 1, characterized in that: The cross brace (5) in step S2 includes a support body (51), a fixed joint (52), a movable joint (53), and a pressing drive (54). The fixed joint (52) is fixedly installed at one end of the support body (51). There are two parallel waist beams (4). The fixed joint (52) abuts against one of the waist beams (4). The movable joint (53) slides and engages with the end of the support body (51) away from the fixed joint (52). The pressing drive (54) is used to drive the movable joint (53) to move away from the support body (51). The movable joint (53) abuts against the other waist beam (4).

7. A method for constructing a tunnel under a weak silt layer according to claim 6, characterized in that: The movable connector (53) has two plug-in parts (531), which pass through and slide into the support body (51). A limiting baffle (8) is provided between the two plug-in parts (531). The limiting baffle (8) has a limiting inclined surface (81), which abuts against the movable connector (53). The side of the limiting baffle (8) away from the limiting inclined surface (81) abuts against the support body (51).

8. A method for constructing a tunnel under a weak silt layer according to claim 6, characterized in that: The support body (51) is fixedly installed with a fixing frame (6), and the top of the limiting baffle (8) is rotatably installed with a limiting screw (7). The limiting screw (7) passes through the fixing frame (6) in the vertical direction and is threadedly engaged with it.

9. A method for constructing a tunnel under a weak silt layer according to claim 6, characterized in that: The support body (51) includes a first support rod (511), a second support rod (512), and an intermediate support rod (513). The two ends of the intermediate support rod (513) are respectively inserted through and rotated and slidably engaged with the first support rod (511) and the second support rod (512). A bidirectional screw rod (10) is coaxially fixedly installed on the intermediate support rod (513). The two ends of the bidirectional screw rod (10) with opposite thread directions are respectively inserted through and threadedly engaged with the first support rod (511) and the second support rod (512).

10. A method for constructing a tunnel under a weak silt layer according to claim 1, characterized in that: In step S4, the backfill soil includes a plain concrete layer (15) and a medium-coarse sand layer (16) arranged sequentially from bottom to top.

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