Exit construction method for ultra-shallow-buried large-section soft rock tunnel

Abstract

The invention discloses a construction method for going out of a hole of an ultra-shallow buried large-section soft rock tunnel, and relates to the technical field of tunnel construction.The construction method comprises the steps that a guide wall is constructed on a stratum where a hole to be formed is located, and a guide structure is buried in the guide wall; constructing a tunnel portal supporting structure on the stratum where the tunnel portal is located along the guide structure; concrete is poured into the first grouting pipes, so that a primary pipe shed supporting structure is formed; the tunnel face of the tunnel is excavated towards the hole opening to the position away from the hole opening by a first preset distance; a three-step method is adopted to excavate the tunnel to the position away from the portal by a second preset distance, and an upper step tunnel face, a middle step tunnel face and a lower step tunnel face are formed; a small pilot tunnel is excavated in the tunnel face of the upper step towards the portal until the tunnel is communicated with the outside; according to the method, the possibility of collapse of the ultra-shallow-buried large-section soft rock tunnel in the out-of-hole process can be reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel construction, and in particular to a construction method for exiting an ultra-shallow buried large-section soft rock tunnel. Background Art

[0002] Ultra-shallow, large-section soft rock tunnels are prone to collapse during construction due to the thin cover layer and poor self-stabilization of the surrounding rock. Furthermore, the larger cross-section of these tunnels means even poorer self-stabilization of the surrounding rock, leading to more significant deformation and greater potential for collapse during construction. This is particularly true during tunnel exit, when the tunnel opening is exposed to the external environment, disrupting the original geological equilibrium and redistributing stress in the surrounding rock. Furthermore, since the self-stabilization of the soil near the tunnel entrance is relatively weak, collapse is extremely likely to occur. Summary of the Invention

[0003] The main purpose of the present invention is to propose a construction method for ultra-shallow buried large-section soft rock tunnel exit, aiming to solve the technical problem that ultra-shallow buried large-section soft rock tunnel is very prone to collapse during the exit process.

[0004] To achieve the above-mentioned purpose, the present invention proposes a construction method for exiting an ultra-shallow large-section soft rock tunnel, comprising:

[0005] Construct a guide wall in the stratum where the cave entrance is to be formed, and bury a guide structure in the guide wall; construct a portal support structure in the stratum where the cave entrance is located along the guide structure;

[0006] A plurality of first grouting pipes are buried at intervals around the outer circumference of the tunnel at the tunnel face, and an anchoring skeleton structure is installed in each of the first grouting pipes;

[0007] pouring concrete into each of the first grouting pipes to form a preliminary pipe-roof support structure;

[0008] Excavating the tunnel face toward the tunnel entrance to a position at a first preset distance from the tunnel entrance;

[0009] Excavating the tunnel to a position at a second preset distance from the tunnel opening using a three-bench method, and forming an upper bench face, a middle bench face, and a lower bench face;

[0010] Excavating a small pilot tunnel on the upper step facing the tunnel entrance until the tunnel is connected to the outside world;

[0011] Expanding the small pilot tunnel toward the tunnel entrance until the area of ​​the tunnel corresponding to the upper bench face is completely connected to the outside world;

[0012] Continue excavating the middle step face and the lower step face toward the tunnel entrance until the areas corresponding to the middle step face and the lower step face are completely connected to the outside world, thereby completing the tunnel exit.

[0013] In one embodiment, the step of excavating the tunnel face toward the tunnel entrance to a position a first preset distance from the tunnel entrance includes:

[0014] excavating the tunnel face toward the tunnel entrance by a third preset distance;

[0015] Return to the step of burying a plurality of first grouting pipes at intervals around the outer circumference of the tunnel in the tunnel face, and installing an anchoring skeleton structure in each of the first grouting pipes, until the tunnel face is excavated to a position at a first preset distance from the opening.

[0016] In one embodiment, before the step of expanding the small pilot tunnel toward the tunnel entrance until the area of ​​the tunnel corresponding to the upper step working face is completely connected to the outside world, the method further includes:

[0017] Setting up temporary supports in the small pilot tunnel;

[0018] Remove the temporary support.

[0019] In one embodiment, the step of excavating the tunnel to a position at a second preset distance from the tunnel opening using a three-step method and forming an upper step face, a middle step face, and a lower step face comprises:

[0020] The tunnel is excavated alternately left and right using a three-step method to a position at a second preset distance from the tunnel opening, and an upper step face, a middle step face and a lower step face are formed.

[0021] In one embodiment, the step of constructing a guide wall in the stratum where the hole to be formed is located and burying a guide structure in the guide wall includes:

[0022] A guide wall is constructed in the stratum where the hole to be formed is located, and a plurality of guide pipes arranged around the hole are buried in the guide wall to form the guide structure;

[0023] burying a second grouting pipe along each guide pipe toward the stratum where the hole is located;

[0024] Concrete is poured into each of the second grouting pipes to form the portal support structure.

[0025] In one embodiment, the angle between any one of the first grouting pipes and the tunnel is A, and the angle between any one of the second grouting pipes and the tunnel is B; wherein A≥10°, 3°≥B≥1°.

[0026] In one embodiment, a plurality of grouting holes arranged at intervals are provided on the pipe wall of the first grouting pipe and the pipe wall of the second grouting pipe.

[0027] In one embodiment, the distance between any two adjacent second grouting pipes is 40 mm, the second grouting pipe is a steel pipe with a diameter of 108 mm, and the length of the second grouting pipe is 25 m.

[0028] In one embodiment, the distance between any two adjacent first grouting pipes is 40 mm, the first grouting pipes are steel pipes with a diameter of 76 mm, and the length of the first grouting pipes is 9 m.

[0029] In one embodiment, the anchoring skeleton structure includes a plurality of support rings and a plurality of support rods, wherein the plurality of support rings are arranged at intervals along the extension direction of the first grouting pipe, and the plurality of support rods are arranged at intervals circumferentially around the plurality of support rings, and each of the support rods extends along the extension direction of the first grouting pipe and is installed on the outer periphery of the plurality of support rings.

[0030] The technical solution of the present invention provides a stable support system for subsequent construction by constructing a guide wall and a portal support structure at the tunnel entrance to be formed, effectively preventing the collapse of the strata around the tunnel entrance and reducing the safety risks during the construction process. Then, by burying the first grouting pipe and installing the anchor skeleton structure at the tunnel face, and then pouring concrete to form a preliminary pipe-roof support structure, the surrounding rock in front of the tunnel face is pre-supported, which can enhance the self-stabilizing ability of the surrounding rock, reduce the deformation of the surrounding rock, and reduce the possibility of collapse during the tunnel exit process. In addition, the three-step method is used for tunnel excavation, and the large-section tunnel is decomposed into multiple relatively small steps for construction, which is conducive to controlling the stress release during the excavation process, making the deformation of the surrounding rock more uniform, and avoiding the stress concentration and instability of the surrounding rock caused by the one-time excavation of a large section. Finally, a small pilot tunnel is excavated at the upper step face to connect with the outside world, and then the upper step, middle step and lower step are gradually excavated to gradually expand the connection between the tunnel and the outside world. The stress of the surrounding rock around the tunnel entrance can be released through the small pilot tunnel first, avoiding the rapid change of surrounding rock stress at the moment of exiting the tunnel, ensuring the smooth progress of the tunnel exit process, further reducing the possibility of collapse, and improving safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0032] Figure 1 A schematic flow chart of an embodiment of a construction method for exiting an ultra-shallow, large-section soft rock tunnel provided by the present invention;

[0033] Figure 2 A schematic diagram of the construction of a three-step method in the construction method for exiting an ultra-shallow, large-section soft rock tunnel provided by the present invention;

[0034] Figure 3 A schematic diagram of the construction of a small pilot tunnel in the construction method for an ultra-shallow, large-section soft rock tunnel exit provided by the present invention;

[0035] Figure 4 A schematic structural diagram of the preliminary pipe-roof support structure in the construction method for the ultra-shallow buried large-section soft rock tunnel exit provided by the present invention;

[0036] Figure 5 A schematic structural diagram of a guide wall in an embodiment of a construction method for exiting an ultra-shallow, large-section soft rock tunnel provided by the present invention;

[0037] Figure 6 A schematic structural diagram of a second grouting pipe in an embodiment of a construction method for exiting an ultra-shallow, large-section soft rock tunnel provided by the present invention;

[0038] Figure 7 This is a structural schematic diagram of the first grouting pipe and anchoring structure in an embodiment of the construction method for ultra-shallow buried large-section soft rock tunnel exit provided by the present invention.

[0039] Description of Figure Numbers:

[0040] 10. Guide wall; 11. Guide structure; 111. Guide pipe; 20. First grouting pipe; 21. Grouting hole; 22. Anchoring skeleton structure; 221. Support ring; 222. Support rod; 30. Tunnel; 31. Upper step face; 311. Small guide hole; 32. Middle step face; 33. Lower step face; 40. Second grouting pipe.

[0041] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0042] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0043] It should be noted that if the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture. If the specific posture changes, the directional indications will also change accordingly.

[0044] In addition, if there are descriptions involving "first", "second", etc. in the embodiments of the present invention, the descriptions of "first", "second", etc. are only for descriptive purposes and cannot be understood as indicating or suggesting their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features limited to "first" and "second" may explicitly or implicitly include at least one of such features. In addition, if "and / or" or "and / or" appears in the full text, its meaning includes three parallel solutions. Taking "A and / or B" as an example, it includes solution A, solution B, or solutions that satisfy both A and B. In addition, the technical solutions between the various embodiments can be combined with each other, but it must be based on the ability of ordinary technicians in this field to implement. When the combination of technical solutions is mutually contradictory or cannot be implemented, it should be deemed that such a combination of technical solutions does not exist and is not within the scope of protection required by the present invention.

[0045] The present invention provides a construction method for exiting an ultra-shallow buried large-section soft rock tunnel.

[0046] See also Figures 1 to 5 In one embodiment of the present invention, the construction method for the ultra-shallow buried large-section soft rock tunnel exit includes:

[0047] Step S100: construct a guide wall in the stratum where the tunnel entrance is to be formed, and bury a guide structure in the guide wall; construct a guide wall 10 in the stratum where the tunnel entrance is to be formed, and accurately bury a guide structure 11 in the guide wall 10. The guide structure 11 can provide accurate directional guidance for the subsequent construction of the tunnel portal support structure.

[0048] In step S200, a portal support structure is constructed along the guide structure in the stratum where the tunnel entrance is located; based on the guide structure 11, a portal support structure is gradually constructed along its direction in the stratum where the tunnel entrance is located. The support structure has sufficient strength and stability, and can effectively support the stratum around the tunnel entrance in advance, so as to prevent the collapse of the soil around the tunnel entrance during the subsequent tunnel 30 exiting the tunnel.

[0049] Step S300: burying a plurality of first grouting pipes at intervals around the outer circumference of the tunnel at the tunnel face, and installing an anchoring skeleton structure in each of the first grouting pipes;

[0050] Step S400 involves pouring concrete into each of the first grouting pipes to form a preliminary pipe-roof support structure. Multiple first grouting pipes 20 are buried at regular intervals around the perimeter of tunnel 30 at the tunnel face. An anchoring framework 22 is installed within each first grouting pipe 20, and concrete is subsequently poured into the pipes, thereby forming a preliminary pipe-roof support structure. This preliminary pipe-roof support structure pre-reinforces the surrounding rock within a certain range in front of tunnel 30, enhancing its stability and effectively suppressing its deformation, thereby reducing the risk of collapse during subsequent tunnel exit construction.

[0051] Step S500, excavating the tunnel face toward the tunnel entrance to a position a first preset distance away from the tunnel entrance;

[0052] In step S600, the tunnel is excavated to a second preset distance from the tunnel entrance using a three-bench method, forming an upper step face, a middle step face, and a lower step face. Tunnel 30 is then excavated, with the face of tunnel 30 being excavated to a first preset distance from the tunnel entrance. Tunnel 30 is then further excavated to a second preset distance from the tunnel entrance using the three-bench method, forming an upper step face 31, a middle step face 32, and a lower step face 33. The three-bench method allows the steps to cooperate and support each other, which helps control surrounding rock deformation, disperse stress, reduce the area of ​​excavation, and effectively reduce the possibility of collapse.

[0053] Step S700 involves excavating a small pilot tunnel from the upper bench face toward the tunnel entrance until the tunnel is connected to the outside world. Excavating a small pilot tunnel from the upper bench face toward the tunnel entrance until the tunnel is connected to the outside world. Excavating the small pilot tunnel can preemptively release stress in the surrounding rock around the tunnel entrance, preventing collapse caused by sudden stress release during subsequent tunneling.

[0054] Step S800: expanding the small pilot tunnel toward the tunnel entrance until the area of ​​the tunnel corresponding to the upper bench face is completely connected to the outside world;

[0055] Step S900 , continue excavating the middle step face and the lower step face toward the tunnel entrance until the areas corresponding to the middle step face and the lower step face are completely connected to the outside world, thereby completing the tunnel exit.

[0056] The technical solution of the present invention provides a stable support system for subsequent construction by constructing a guide wall 10 and a portal support structure at the entrance to be formed, effectively preventing the collapse of the strata around the entrance and reducing the safety risks during the construction process. Then, by burying the first grouting pipe 20 and installing the anchor skeleton structure 22 at the tunnel 30 face, and then pouring concrete to form a preliminary pipe-roof support structure, the surrounding rock in front of the face is pre-supported, which can enhance the self-stabilizing ability of the surrounding rock, reduce the deformation of the surrounding rock, and reduce the possibility of collapse during the exit process. In addition, the three-step method is used to excavate the tunnel 30, decomposing the large-section tunnel 30 into multiple relatively small steps for construction, which is conducive to controlling the stress release during the excavation process, making the deformation of the surrounding rock more uniform, and avoiding the stress concentration and instability of the surrounding rock caused by the one-time excavation of a large section. Finally, a small pilot tunnel 311 is excavated at the upper step face 31 to connect with the outside world, and then the upper step, middle step, and lower step are gradually excavated to gradually expand the connection between the tunnel 30 and the outside world. The stress of the surrounding rock around the tunnel entrance can be released through the small pilot tunnel 311 first, avoiding the rapid change of the surrounding rock stress at the moment of exiting the tunnel, ensuring the smooth exit process of the tunnel 30, further reducing the possibility of collapse, and improving safety.

[0057] In one embodiment, step S500 includes:

[0058] Step S510, excavating the tunnel face toward the tunnel entrance by a third preset distance;

[0059] Return to step S300 and execute until the tunnel face is excavated to a position at a first preset distance from the tunnel entrance.

[0060] Specifically, after each excavation of the third preset distance, the first grouting pipe 20 is promptly buried and the anchoring skeleton structure 22 is installed, followed by pouring concrete to form a pipe-roof support, which effectively enhances the stability of the surrounding rock in front of the tunnel face. The above operation is repeated until the tunnel face of the tunnel 30 is excavated to the first preset distance from the tunnel entrance, thereby avoiding the large-scale instability and collapse of the surrounding rock that may result from a single long-distance excavation. The entire excavation process is broken down into multiple small cycles, and a preliminary pipe-roof support structure is applied in each small cycle to support the surrounding rock, thereby effectively limiting deformation of the surrounding rock, enhancing the self-stabilizing ability of the surrounding rock, and further reducing the possibility of instability and collapse.

[0061] In one embodiment, before step S800, the method further includes:

[0062] Step A100, setting up temporary supports in the small pilot tunnel;

[0063] Step A200: remove the temporary support.

[0064] Specifically, after the small pilot tunnel 311 is excavated and formed, temporary supports are set up inside the small pilot tunnel 311. The temporary supports can effectively support the surrounding rock around the small pilot tunnel 311, thereby relieving stress in the surrounding rock around the small pilot tunnel 311 and effectively preventing the small pilot tunnel 311 from collapsing due to stress release after formation. After the stress release of the small pilot tunnel 311 is completed, the temporary supports can be removed to allow for subsequent excavation of the small pilot tunnel 311.

[0065] In one embodiment, step S600 includes:

[0066] In step S610, the tunnel is excavated alternately left and right using a three-step method to a position at a second preset distance from the tunnel opening, and an upper step face, a middle step face, and a lower step face are formed.

[0067] Specifically, the upper step face 31, middle step face 32, and lower step face 33 are divided into two left and right sections. First, one section of the upper step face 31 is excavated to a third preset distance, and then the other section of the upper step face 31 is excavated to a third preset distance. The middle step face 32 and lower step face 33 are then excavated sequentially, following the same construction method as the upper step face 31. This alternating left and right excavation method allows for more uniform and gradual stress release in the surrounding rock of tunnel 30, avoiding stress concentration and instability caused by excavating a large area at once.

[0068] In one embodiment, step S100 includes:

[0069] Step S110, constructing a guide wall in the stratum where the hole to be formed is located, and burying a plurality of guide pipes arranged around the hole in the guide wall to form the guide structure;

[0070] Step S120: burying a second grouting pipe along each guide pipe toward the stratum where the hole is located;

[0071] Step S130: pouring concrete into each of the second grouting pipes to form the portal support structure.

[0072] Specifically, if Figure 5 As shown, a portal support structure is formed by burying a guide pipe 111 within the guide wall 10, burying a second grouting pipe 40 along the guide pipe 111, and pouring concrete into the grouting pipe. This portal support structure effectively enhances the stability of the ground at the portal entrance and prevents portal collapse. Furthermore, the construction of the guide wall 10 and guide pipe 111 provides accurate guidance for the burial of the second grouting pipe 40, ensuring that the position and angle of the grouting pipe meet design requirements, thereby improving the quality of the portal support structure.

[0073] In one embodiment, the angle between any first grouting pipe and the tunnel is A, and the angle between any second grouting pipe and the tunnel is B; wherein A≥10°, 3°≥B≥1°.

[0074] Specifically, if Figure 5 As shown, by burying the guide pipe 111 in the guide wall 10 and burying the second grouting pipe 40 along the guide pipe 111, specifically, if the angle A between any first grouting pipe 20 and the tunnel 30 is ≥10°, the first grouting pipe 20 can cover a larger range of surrounding rock in front of the face, effectively enhance the self-stabilizing ability of the surrounding rock, and reduce the deformation and collapse risk of the surrounding rock during excavation. If the angle between any second grouting pipe 40 and the tunnel 30 is between 3° and 1°, the second grouting pipe 40 can be tightly combined with the guide wall 10 and the stratum at the entrance to form a stable portal support structure. In addition, a smaller angle B causes less disturbance to the stratum during the grouting process, which is beneficial to protecting the stability of the surrounding rock at the entrance and can further reduce the possibility of collapse.

[0075] In one embodiment, a plurality of grouting holes arranged at intervals are provided on the pipe wall of the first grouting pipe and the pipe wall of the second grouting pipe.

[0076] Specifically, if Figure 6 and Figure 7 As shown, the grouting holes 21 allow the slurry to penetrate deeply into the stratum, effectively bonding with it. This integrates the grouting pipe with the stratum, enhancing the supporting function of the grouting pipe and improving the stability and load-bearing capacity of the tunnel 30. Furthermore, the multiple, spaced grouting holes 21 expand the diffusion range of the slurry in the stratum. This allows the slurry to more evenly fill the gaps around the grouting pipe and the stratum, forming a more complete and dense grouting body, thereby enhancing the strength and stability of the stratum.

[0077] In one embodiment, the interval between any two adjacent second grouting pipes is 40 mm, the second grouting pipes are steel pipes with a diameter of 108 mm, and the length of the second grouting pipes is 25 m.

[0078] Specifically, the second grouting pipe 40, with a diameter of 108 mm, has a large cross-sectional area and high strength, effectively withstanding the loads generated by stratum pressure and surrounding rock deformation. The 25-meter-long grouting pipe can penetrate deep into the stratum, forming a longer support range and enhancing the overall stability and load-bearing capacity of the portal support structure. The 40-mm spacing between adjacent grouting pipes allows for mutual support and connection between them, further improving the rigidity and stability of the support structure and effectively preventing collapse and deformation of the stratum at the portal.

[0079] In one embodiment, the distance between any two adjacent first grouting pipes is 40 mm, the first grouting pipes are steel pipes with a diameter of 76 mm, and the length of the first grouting pipes is 9 m.

[0080] Specifically, the first grouting pipe 20, with a diameter of 76 mm and a length of 9 m, has a moderate cross-sectional area and high strength, effectively withstanding the loads generated by stratum pressure and surrounding rock deformation. The 40 mm spacing between adjacent grouting pipes allows for mutual support and connection between them, further enhancing the rigidity and stability of the support structure and effectively preventing collapse and deformation of the surrounding rock at the tunnel face of tunnel 30.

[0081] In one embodiment, the anchoring skeleton structure includes a plurality of support rings and a plurality of support rods, the plurality of support rings are arranged at intervals along the extension direction of the first grouting pipe, the plurality of support rods are arranged at intervals circumferentially around the plurality of support rings, and each support rod extends along the extension direction of the first grouting pipe and is installed on the outer periphery of the plurality of support rings.

[0082] Specifically, if Figure 7 As shown, support rings 221 are spaced apart along the extension direction of the first grouting pipe 20, multiple support rods 222 are spaced apart circumferentially around the support ring 221, and steel bars are spaced apart along the extension direction of the first grouting pipe 20 and installed on the outer circumference of the support ring 221, forming a cage-like skeleton structure. This cage-like skeleton structure can be tightly integrated with the grouting pipe and the surrounding stratum, supporting and anchoring the grouting pipe from multiple directions, making the grouting pipe more stable in the stratum, providing good reinforcement for the stratum, and achieving better reinforcement effect, which can better reduce the possibility of instability and collapse.

[0083] The above description is merely an exemplary embodiment of the present invention and does not limit the scope of the present invention. Any equivalent structural transformation made by utilizing the contents of the present invention description and drawings under the technical concept of the present invention, or directly / indirectly applied in other related technical fields, is included in the scope of protection of the present invention.

Claims

1. A construction method for exiting an ultra-shallow large-section soft rock tunnel, characterized in that: The construction method comprises: Constructing a guide wall in the stratum where the hole to be formed is located, and burying a guide structure in the guide wall; Constructing a tunnel portal support structure along the guide structure in the stratum where the tunnel portal is located; A plurality of first grouting pipes are buried at intervals around the outer circumference of the tunnel at the tunnel face, and an anchoring skeleton structure is installed in each of the first grouting pipes; pouring concrete into each of the first grouting pipes to form a preliminary pipe-roof support structure; Excavating the tunnel face toward the tunnel entrance to a position at a first preset distance from the tunnel entrance; Excavating the tunnel to a position at a second preset distance from the tunnel opening using a three-bench method, and forming an upper bench face, a middle bench face, and a lower bench face; Excavating a small pilot tunnel on the upper step facing the tunnel entrance until the tunnel is connected to the outside world; Expanding the small pilot tunnel toward the tunnel entrance until the area of ​​the tunnel corresponding to the upper bench face is completely connected to the outside world; Continue excavating the middle step face and the lower step face toward the tunnel entrance until the areas corresponding to the middle step face and the lower step face are completely connected to the outside world, thereby completing the tunnel exit.

2. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to claim 1, characterized in that: The step of excavating the tunnel face toward the tunnel opening to a position a first preset distance away from the tunnel opening comprises: excavating the tunnel face toward the tunnel entrance by a third preset distance; Return to the step of burying a plurality of first grouting pipes at intervals around the outer circumference of the tunnel in the tunnel face, and installing an anchoring skeleton structure in each of the first grouting pipes, until the tunnel face is excavated to a position at a first preset distance from the opening.

3. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to claim 1, characterized in that: Before the step of expanding the small pilot tunnel toward the tunnel entrance until the area of ​​the tunnel corresponding to the upper step working face is completely connected to the outside world, the method further includes: Setting up temporary supports in the small pilot tunnel; Remove the temporary support.

4. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to claim 1, characterized in that: The step of excavating the tunnel to a position at a second preset distance from the tunnel opening by using the three-step method and forming an upper step face, a middle step face, and a lower step face comprises: The tunnel is excavated alternately left and right using a three-step method to a position at a second preset distance from the tunnel opening, and an upper step face, a middle step face and a lower step face are formed.

5. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to any one of claims 1 to 4, characterized in that: The steps of constructing a guide wall in the stratum where the hole to be formed is located and burying a guide structure in the guide wall include: A guide wall is constructed in the stratum where the hole to be formed is located, and a plurality of guide pipes arranged around the hole are buried in the guide wall to form the guide structure; burying a second grouting pipe along each guide pipe toward the stratum where the hole is located; Concrete is poured into each of the second grouting pipes to form the portal support structure.

6. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to claim 5, characterized in that: The angle between any one of the first grouting pipes and the tunnel is A, and the angle between any one of the second grouting pipes and the tunnel is B; wherein A≥10°, 3°≥B≥1°.

7. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to claim 6, characterized in that: A plurality of grouting holes arranged at intervals are provided on the pipe wall of the first grouting pipe and the pipe wall of the second grouting pipe.

8. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to claim 5, characterized in that: The distance between any two adjacent second grouting pipes is 40 mm, the second grouting pipe is a steel pipe with a diameter of 108 mm, and the length of the second grouting pipe is 25 m.

9. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to any one of claims 1 to 4, characterized in that: The distance between any two adjacent first grouting pipes is 40 mm, the first grouting pipe is a steel pipe with a diameter of 76 mm, and the length of the first grouting pipe is 9 m.

10. The construction method for exiting an ultra-shallow large-section soft rock tunnel according to any one of claims 1 to 4, characterized in that: The anchoring skeleton structure includes multiple support rings and multiple support rods. The multiple support rings are arranged at intervals along the extension direction of the first grouting pipe, and the multiple support rods are arranged at intervals circumferentially around the multiple support rings. Each support rod extends along the extension direction of the first grouting pipe and is installed on the outer periphery of the multiple support rings.

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