A construction method for large semi-filled karst caves in karst tunnels

Abstract

This invention discloses a construction method for a large semi-filled karst cave in a karst tunnel. The method includes the following steps: first, applying wire mesh and shotcrete to the cavity area of ​​the karst cave wall for protection; then, installing grouting pipes on the tunnel face, surrounding rock, and non-cavity areas of the karst cave wall, and injecting cement-water glass double-liquid grout into the surrounding soil for reinforcement, ensuring the stability of the karst cave during construction, avoiding the formation of a large plastic zone during tunnel excavation, and simultaneously blocking water; filling the karst cave below the tunnel with slag, preserving the original karst pipes, and installing C20 concrete backfill and C35 reinforced concrete slabs across the karst cave to prevent uneven settlement of the tunnel; draining karst water in the karst cave section in two parts, relying on the original karst pipes and the tunnel drainage system respectively, to avoid excessive load on the tunnel support system due to excessive water pressure inside the karst cave; and adding a steel-framed concrete arch on the outside of the tunnel's initial support to prevent unstable falling rocks inside the karst cave from causing impact loads on the main tunnel support structure.

Description

Technical Field

[0001] This invention belongs to the field of karst tunnel engineering construction technology, specifically relating to a construction method for large semi-filled karst caves in karst tunnels. Background Technology

[0002] Karst geology in my country is mainly concentrated in Yunnan, Guizhou, and Sichuan provinces, as well as parts of Guangdong, Guangxi, and Hubei provinces. With the rapid development of my country's economy, infrastructure construction in these areas has increased significantly, with large-scale highway and railway projects being planned and constructed. This necessitates addressing numerous engineering problems caused by karst geology, particularly karst tunnel engineering. Due to the increasing design standards for railways, limitations imposed by longitudinal slope and curve radius inevitably lead to long, deep-buried karst tunnels. Furthermore, limitations in exploration techniques and timeframes, coupled with the inherent irregularity of karst development, make it difficult to fully ascertain the specific karst morphology of the tunnel sections during the exploration and design phase. Consequently, the probability of encountering karst formations of varying shapes, sizes, and hazards during tunnel construction is extremely high. Therefore, the treatment of large karst caves beneath the tunnel floor during karst tunnel construction, while ensuring construction quality and tunnel operational safety, is of paramount importance.

[0003] For karst caves encountered during tunnel excavation, smaller caves are typically treated with grouted rubble backfilling, while larger caves can be crossed by bridges. However, when a tunnel needs to pass through a large, semi-filled karst cave with a large cavity, loose filling material, and poor stability, the limited space within the tunnel makes it difficult for large machinery to operate effectively, resulting in low bridge construction efficiency. This makes traditional construction methods difficult, unable to effectively control structural deformation during construction, and unable to solve the construction challenges of tunneling through large semi-filled karst caves. Summary of the Invention

[0004] The purpose of this invention is to provide a construction method for large semi-filled karst caves, addressing the problems of high construction difficulty and inability to effectively control structural deformation during construction in existing technologies.

[0005] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0006] A method for constructing a large semi-filled karst cave in a karst tunnel includes the following steps:

[0007] S1, Reinforcement treatment of the cavity area of ​​the cave wall;

[0008] S1.1, Remove unstable rocks. Use an excavator to gradually remove loose rocks and attached materials from the outside to the inside and from top to bottom on the surface of the cave wall cavity.

[0009] S1.2, initial spraying of C25 concrete onto the surface of the karst wall cavity;

[0010] S1.3, Drilling and installing medicated anchor bolts on the walls of the karst cave with initial shotcrete;

[0011] S1.4, lay steel mesh on the surface of the karst wall after initial shotcrete application, tie it to the anchor rods, and then weld the steel mesh into a net.

[0012] S1.5, spray C25 concrete onto the mesh surface after the treatment in step S1.4;

[0013] S2, reinforcement treatment of the working face, surrounding rock, and non-cavity areas of the cave wall;

[0014] S2.1, Fabrication of grouting conduit;

[0015] S2.2 According to the design spacing and design hole positions, boreholes shall be drilled in the non-cavity areas of the working face, surrounding rock, and cave walls. The depth and diameter of the boreholes shall be matched with the grouting pipe.

[0016] S2.3, Insert the grouting pipe into the borehole;

[0017] S2.4, a cement-water glass double-liquid grout is injected into the grouting pipe using a grouting pump. The cement-water glass double-liquid grout is sprayed into the soil around the pipe through the holes in the pipe wall to achieve soil reinforcement.

[0018] S3, base treatment and preservation of the original karst drainage channels;

[0019] S3.1, the karst caves below the tunnel are filled with cavitary material, and drainage steel pipes are installed according to the design location. The lower end of the drainage steel pipe is inserted into the cavitary material, and the upper end of the drainage steel pipe is connected to the karst cave. After the cavitary material is filled, water can still flow in the karst caves below the tunnel.

[0020] S3.2, cover the cavitary material with geotextile and fill the top of the geotextile with C20 concrete;

[0021] S3.3 After the C20 concrete has solidified, steel bars are tied and formwork is installed above the karst cave, and C35 concrete is poured to form a reinforced concrete slab as the foundation of the karst tunnel. The formwork is removed after the C35 concrete reaches more than 85% of the design strength. At the same time, the upper end of the drainage steel pipe is ensured to be more than 1m above the reinforced concrete slab.

[0022] S4, Support construction and introduction of tunnel drainage system for drainage;

[0023] S4.1, steel arch frames are added on the outside of the tunnel's initial support. The steel arch frames are located on one side of the karst cave and are arranged at intervals along the longitudinal direction of the tunnel. The arch top of the steel arch frame is anchored to the surrounding rock by locking foot anchor pipes, and the arch foot of the steel arch frame is anchored to the reinforced concrete slab by locking foot anchor pipes. The connection between the locking foot anchor pipes and the steel arch frame is fixed by welding with U-shaped steel bars.

[0024] S4.2, Install formwork on the outside of the steel arch frame to provide a pouring cavity for the subsequent C35 concrete pouring step; weld grouting pipes to the top of the steel arch frame; weld multiple grouting pipes at intervals along the length of the steel arch frame; install corrugated drainage pipes at the bottom of the tunnel, with the lower end of the corrugated drainage pipes connected to the tunnel drainage system and the upper end of the corrugated drainage pipes connected to the karst cave; and the upper end of the corrugated drainage pipes is higher than the upper end of the drainage steel pipes.

[0025] S4.3, C35 concrete is injected into the casting mold cavity through the grouting pipe, so that the outside of the steel arch frame is covered with C35 concrete to form a steel frame concrete arch; the formwork is removed after the strength of C35 concrete reaches more than 85% of the design strength.

[0026] S4.4, medium-grained sand is blown in as a buffer layer on the outside of the steel frame concrete arch via a dredging pipe.

[0027] As a preferred embodiment of the present invention, in step S1, the thickness of the initial sprayed C25 concrete is 3cm; the thickness of the subsequent sprayed C25 concrete is 7cm.

[0028] As a preferred embodiment of the present invention, in step S2, the grouting conduit is a hot-rolled seamless steel pipe with a wall thickness of 4 mm and a length of 4.5 mm. Multiple grouting conduits are arranged in a quincunx pattern on the working face, surrounding rock, and non-cavity areas of the karst cave wall. The front end of the grouting conduit is conical, and holes with a diameter of 15 mm are arranged alternately every 15 cm on the pipe wall except for a range of 500 mm from the rear end of the pipe opening.

[0029] As a preferred embodiment of the present invention, in step S2, the volume ratio of cement slurry to water glass in the cement-water glass double-liquid slurry is 1:0.1, wherein the water-cement ratio of the cement slurry is 1:1; and the concentration of the water glass is 35 Baume degrees.

[0030] As a preferred embodiment of the present invention, in step S3, the thickness of the C20 concrete is 1.5m; the thickness of the C35 concrete is 2.45m.

[0031] As a preferred embodiment of the present invention, in step S3, the drainage steel pipes are provided in multiple locations and are arranged at intervals along the longitudinal direction of the tunnel.

[0032] As a preferred embodiment of the present invention, in step S4, the anchor pipe and the U-shaped steel bar, as well as the steel arch frame and the U-shaped steel bar, are all subjected to double-sided continuous welding, with a weld length of not less than 10cm and a weld thickness of not less than 5mm.

[0033] As a preferred embodiment of the present invention, in step S4, the corrugated drainage pipes are provided in multiple portions and arranged at intervals along the longitudinal direction of the tunnel.

[0034] As a preferred embodiment of the present invention, in step S4, the corrugated drainage pipe is wrapped with geotextile and fixed with wire, and the section of the corrugated drainage pipe extending into the karst cave is buried in a concrete fixing platform, with the upper end of the corrugated drainage pipe extending from the top of the concrete fixing platform.

[0035] As a preferred embodiment of the present invention, in step S4, the thickness of the buffer layer is not less than 50cm.

[0036] The construction method for large semi-filled karst caves in karst tunnels provided by this invention has the following advantages compared with the prior art:

[0037] (1) The technical solution of the present invention first performs wire mesh and anchor spray protection treatment on the cavity area of ​​the karst cave wall, and then installs grouting pipes on the working face, surrounding rock and non-cavity area of ​​the karst cave wall, and injects cement-water glass double liquid grout into the soil around the pipes for reinforcement treatment, so as to ensure the stability of the karst cave during construction and avoid the generation of a large plastic zone during tunnel excavation; at the same time, by injecting cement-water glass double liquid grout to reinforce the surrounding rock and fill the gaps in the surrounding rock, the connection between rock layers becomes more precise, thereby improving the self-stabilizing ability of the surrounding rock and the water blocking effect;

[0038] (2) The technical solution of the present invention uses slag filling to fill the karst cave below the tunnel, which can preserve the original karst pipeline and ensure the smooth drainage of karst water; and sets C20 concrete backfill and C35 reinforced concrete slabs to cross the karst cave to avoid uneven settlement of the tunnel and ensure the safety of the main structure of the tunnel.

[0039] (3) The technical solution of the present invention discharges karst water in the karst cave section in two parts, relying on the original karst pipes and the tunnel drainage system respectively. The two drainage methods cooperate with each other. Especially when the rainfall increases sharply and the karst water cannot be discharged in time through the original karst pipes, the karst water is introduced into the tunnel drainage system (i.e. the drainage ditch in the tunnel) through the pre-buried corrugated drainage pipes and discharged, thereby effectively avoiding the excessive load on the tunnel support system caused by the excessive water pressure in the karst cave, which affects the safety of the tunnel.

[0040] (4) The technical solution of the present invention adds a steel frame concrete arch on the outside of the tunnel primary support, and fills medium-grained sand on the outside of the steel frame concrete arch as a buffer layer. Such a structure can avoid the impact load on the main support structure of the tunnel caused by unstable falling rocks in the karst cave, thereby playing the role of protecting the main structure of the tunnel. Attached Figure Description

[0041] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings of the embodiments will be briefly described below.

[0042] Figure 1 This is a construction schematic diagram of the construction method for a large semi-filled karst tunnel according to an embodiment of the present invention;

[0043] Figure 2 This is a construction diagram of the foundation treatment and drainage measures;

[0044] Figure 3 This is a schematic diagram of the grouting conduit. Detailed Implementation

[0045] Embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0046] In the description of this invention, it should be understood that the orientation descriptions, such as up, down, front, back, left, right, etc., are based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0047] In the description of this invention, "several" means one or more, "more than" means two or more, "greater than," "less than," and "exceeding" are understood to exclude the stated number, while "above," "below," and "within" are understood to include the stated number. The use of "first" and "second" in the description is merely for distinguishing technical features and should not be construed as indicating or implying relative importance, or implicitly indicating the number of indicated technical features, or implicitly indicating the order of the indicated technical features.

[0048] In the description of this invention, unless otherwise explicitly defined, terms such as "set up," "install," and "connect" should be interpreted broadly, and those skilled in the art can reasonably determine the specific meaning of the above terms in this invention in conjunction with the specific content of the technical solution.

[0049] like Figures 1 to 3As shown in the figure, the construction method for large semi-filled karst caves in karst tunnels according to an embodiment of the present invention includes the following steps:

[0050] S1, Reinforcement treatment of cavity area A in the cave wall;

[0051] S1.1, Remove unstable rocks. Use an excavator to gradually remove loose rocks and attachments from the outside to the inside and from top to bottom on the surface of the cave wall cavity.

[0052] S1.2, initial spraying of C25 concrete onto the surface of the karst wall cavity;

[0053] S1.3, Drilling and installing medicated anchor bolts on the walls of the karst cave with initial shotcrete;

[0054] S1.4, lay steel mesh on the surface of the karst wall after initial shotcrete application, tie it to the anchor rods, and then weld the steel mesh into a net.

[0055] S1.5, spray C25 concrete onto the mesh surface after the treatment in step S1.4.

[0056] It should be noted that in step 1, when spraying concrete, a wet spraying robot is used, and the operation is carried out in sections, patches, and layers sequentially. The spraying sequence should be from bottom to top, and the length of each section should not exceed 6m. During spraying, the low-lying areas should be roughly leveled first, and then sprayed layer by layer from bottom to top, repeating the process. The spraying speed should be appropriate to facilitate concrete compaction. Excessive air pressure increases the spraying speed and rebound; insufficient air pressure results in a low spraying speed and weak compaction, affecting the strength of the sprayed concrete. Therefore, after starting the machine, it is important to observe the air pressure. Operation can only begin after the initial air pressure reaches 0.5MPa, and the air pressure should be adjusted according to the nozzle output. Generally, the working air pressure is: 0.3–0.5MPa for sidewalls and 0.4–0.65MPa for arches. During spraying, maintain an appropriate distance between the nozzle and the sprayed surface, and keep the spraying angle as close to 90° as possible to achieve maximum compaction and minimum rebound. The distance between the nozzle and the surface to be sprayed should be 1.5–2.0 m. The nozzle should be moved continuously and slowly in a horizontal circular motion, overlapping half a circle with the other half. The circular motion drawn by the sprayer should be 40–60 cm horizontally and 15–20 cm high. To ensure the stability of the cavity in the cave wall, the initial sprayed C25 concrete should be 3 cm thick, and the subsequent sprayed C25 concrete should be 7 cm thick.

[0057] S2, reinforcement treatment of non-cavity areas of the working face, surrounding rock, and cave walls;

[0058] S2.1, Fabricate grouting conduit 1, as follows Figure 3 As shown;

[0059] S2.2 According to the design spacing and design hole positions, boreholes shall be drilled in the non-cavity area B of the working face, surrounding rock, and karst cave wall. The depth and diameter of the boreholes shall match the grouting guide pipe 1.

[0060] S2.3, Insert the grouting guide pipe 1 into the borehole;

[0061] S2.4, a cement-water glass double-liquid grout is injected into the grouting pipe 1 using a grouting pump. The cement-water glass double-liquid grout is sprayed into the soil around the pipe through the holes 101 on the pipe wall of the grouting pipe 1 to achieve soil reinforcement.

[0062] It should be noted that in step S2, the grouting conduit 1 is a hot-rolled seamless steel pipe with a wall thickness of 4mm and a length of 4.5m. Multiple grouting conduits 1 are arranged in a quincunx pattern on the working face, surrounding rock, and non-cavity area B of the karst cave wall. The front end of each grouting conduit 1 is conical, and except for a 500mm radius from the rear end, the pipe wall has 15mm diameter holes 101 spaced 15cm apart. The volume ratio of cement slurry to water glass in the cement-water glass dual-liquid grout is 1:0.1, wherein the water-cement ratio of the cement slurry is 1:1; and the concentration of the water glass is 35 Baume. To accelerate grouting, a grout distributor can be installed for simultaneous multi-pipe grouting.

[0063] S3, base treatment and preservation of the original karst drainage channels;

[0064] S3.1, the karst cave below the tunnel is filled with slag 2, and drainage steel pipe 5 is installed according to the design location. The lower end of the drainage steel pipe 5 is inserted into the slag 2, and the upper end of the drainage steel pipe 5 is connected to the karst cave. After the karst cave below the tunnel is filled with slag 2, water can still flow in it.

[0065] S3.2, geotextile is used to cover the top of the cavitary muck 2, and C20 concrete 3 is filled on top of the geotextile; the geotextile can prevent too much concrete from flowing into the cavitary muck 2 and affecting the drainage capacity of the original karst pipe.

[0066] S3.3 After the C20 concrete 3 has solidified, steel bars are tied and formwork is installed above the karst cave, and C35 concrete 4 is poured to form a reinforced concrete slab as the foundation of the karst tunnel. The formwork is removed after the C35 concrete 4 reaches more than 85% of the design strength. At the same time, the upper end of the drainage steel pipe 5 is ensured to be more than 1m above the reinforced concrete slab, so as to leave enough space for sediment to prevent silt from clogging the drainage steel pipe 5.

[0067] It should be noted that in step 3, the slag 2 must be composed of moderately weathered limestone boulders of high strength and appropriate size; strongly weathered limestone or muddy materials must not be used to ensure the drainage capacity of the existing karst conduits. Multiple drainage steel pipes 5 are provided and arranged at intervals along the longitudinal direction of the tunnel to increase the flow capacity between the karst caves and the existing karst conduits, achieving rapid drainage. To ensure the stability of the tunnel's main foundation, the thickness of the C20 concrete 3 is 1.5m; the thickness of the C35 concrete 4 is 2.45m.

[0068] S4, Support construction and introduction of tunnel drainage system 11 Drainage;

[0069] S4.1, a steel arch frame 8 is added on the outside of the tunnel's initial support 7. The steel arch frame 8 is located on one side of the karst cave and multiple steel arch frames are arranged at intervals along the longitudinal direction of the tunnel. The arch top of the steel arch frame 8 is anchored to the surrounding rock by locking foot anchor pipes 9, and the arch foot of the steel arch frame 8 is anchored to the reinforced concrete slab by locking foot anchor pipes. The connection between the locking foot anchor pipes 9 and the steel arch frame 8 is fixed by welding with U-shaped steel bars.

[0070] S4.2, a template is installed on the outside of the steel arch frame 8 to provide a pouring cavity for the subsequent C35 concrete pouring step; a grouting pipe is welded to the top of the steel arch frame 8; multiple filling pipes are welded at intervals along the length of the steel arch frame 8; a corrugated drainage pipe 6 is installed at the bottom of the tunnel, the lower end of the corrugated drainage pipe 6 is connected to the tunnel drainage system 11, and the upper end of the corrugated drainage pipe 6 is connected to the karst cave; and the upper end of the corrugated drainage pipe 6 is higher than the upper end of the drainage steel pipe 5, so as to realize a drainage method in which the karst cave water is mainly discharged by the original karst pipe and supplemented by the drainage by the tunnel drainage system 11.

[0071] S4.3, C35 concrete is injected into the casting mold cavity through the grouting pipe, so that the outside of the steel arch frame 8 is covered with C35 concrete to form a steel frame concrete arch; the formwork is removed after the strength of C35 concrete reaches more than 85% of the design strength.

[0072] S4.4, medium-grained sand is blown in as a buffer layer 10 on the outside of the steel frame concrete arch via a blow-fill pipe.

[0073] It should be noted that in step S4, the connection between the anchor pipe 9 and the U-shaped steel bar, as well as between the steel arch frame 8 and the U-shaped steel bar, is achieved through double-sided continuous welding. The weld length is not less than 10cm, and the weld thickness is not less than 5mm, thus ensuring the connection strength between the anchor pipe and the steel arch frame 8. Multiple corrugated drainage pipes 6 are provided and arranged at intervals along the longitudinal direction of the tunnel, increasing the flow capacity between the karst cave and the tunnel drainage system 11, and achieving rapid drainage. To better arrange the corrugated drainage pipes 6, they are wrapped with geotextile and secured with wire. The section of the corrugated drainage pipe 6 extending into the karst cave is buried in a concrete fixing platform 12, and the upper end of the corrugated drainage pipe 6 extends from the top of the concrete fixing platform. Furthermore, the thickness of the buffer layer 10 is not less than 50cm to ensure that the buffering performance of the buffer layer 10 meets the requirements of large impact loads.

[0074] In summary, the key to the construction method for large semi-filled karst caves in karst tunnels provided by the embodiments of the present invention lies in:

[0075] (1) The technical solution of the present invention first performs wire mesh and anchor spray protection treatment on the cavity area A of the karst cave wall, and then installs grouting pipe 1 on the working face, surrounding rock and non-cavity area B of the karst cave wall, and injects cement-water glass double liquid grout into the soil around the pipe for reinforcement treatment, so as to ensure the stability of the karst cave during construction and avoid the generation of a large plastic zone during tunnel excavation; at the same time, by injecting cement-water glass double liquid grout to reinforce the surrounding rock and fill the gaps in the surrounding rock, the connection between rock layers becomes more precise, thereby improving the self-stabilizing ability of the surrounding rock and the water blocking effect;

[0076] (2) The technical solution of the present invention uses slag 2 to fill the karst cave below the tunnel to preserve the original karst pipes and ensure the smooth drainage of karst water; and sets C20 concrete 3 for backfilling and C35 reinforced concrete slabs to cross the karst cave to avoid uneven settlement of the tunnel and ensure the safety of the main structure of the tunnel.

[0077] (3) The technical solution of the present invention discharges karst water in the karst cave section in two parts, relying on the original karst pipes and the tunnel drainage system 11 respectively. The two drainage methods cooperate with each other. Especially when the rainfall increases sharply and the karst water cannot be discharged in time through the original karst pipes, the karst water is introduced into the tunnel drainage system 11 (i.e. the drainage ditch in the tunnel) through the pre-buried corrugated drainage pipe 6 to discharge it, thereby effectively avoiding the excessive load on the tunnel support system caused by the excessive water pressure in the karst cave, which affects the safety of the tunnel.

[0078] (4) The technical solution of the present invention adds a steel frame concrete arch on the outside of the tunnel primary support 7, and fills medium-grained sand on the outside of the steel frame concrete arch as a buffer layer 10. Such a structure can avoid the impact load on the main support structure of the tunnel caused by unstable falling rocks in the karst cave, thereby playing the role of protecting the main structure of the tunnel.

[0079] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A construction method for large semi-filled karst cave of karst tunnel, characterized in that, The method comprises the following steps: S1, reinforcing treatment of the cavity area of the cave wall; S1.1, removing dangerous rocks, using excavators to remove loose rocks and attachments on the surface of the cave wall from outside to inside and from top to bottom step by step; S1.2, primary spraying of C25 concrete on the surface of the cave wall; S1.3, cartridge anchor drilling and installation on the cave wall with primary sprayed concrete; S1.4, laying steel mesh on the surface of the cave wall with primary sprayed concrete and fixing it on the anchor rod, and then welding each steel mesh into a net; S1.5, re-spraying C25 concrete on the net surface after the step S1.4 treatment; S2, reinforcing treatment of the non-cavity area of the tunnel face, surrounding rock and cave wall; S2.1, making a grouting pipe; S2.2, drilling according to the design interval and design hole position on the non-cavity area of the tunnel face, surrounding rock and cave wall, the depth and diameter of the drilling should match the grouting pipe; S2.3, inserting the grouting pipe into the drilling; S2.4, using a grouting pump to pressurize cement and water glass double-liquid slurry into the grouting pipe, the cement and water glass double-liquid slurry is sprayed to the soil around the pipe through the hole on the pipe wall of the grouting pipe to realize soil reinforcement; S3, base treatment and retaining original karst pipeline drainage; S3.1, filling the cave below the tunnel with hole slag, installing drainage steel pipes according to the design position, and inserting the lower end of the drainage steel pipe into the hole slag, the upper end of the drainage steel pipe is communicated with the cave; the cave below the tunnel can still maintain water flow after being filled with hole slag; S3.2, covering the hole slag with geotextile and filling C20 concrete on the geotextile; S3.3, after the C20 concrete is solidified, binding steel bars and installing forms on the C20 concrete above the cave, and pouring C35 concrete to form a reinforced concrete slab as the karst tunnel foundation; the forms are removed after the C35 concrete strength reaches more than 85% of the design strength; at the same time, the upper end of the drainage steel pipe is higher than the reinforced concrete slab by more than 1m; S4, support construction and drainage by introducing the tunnel drainage system; S4.1, adding steel arches outside the initial support of the tunnel, the steel arches are located on one side of the cave and are arranged in multiple along the longitudinal direction of the tunnel, the arch top of the steel arch is anchored to the surrounding rock by a lock foot anchor pipe, and the arch foot of the steel arch is anchored to the reinforced concrete slab by a lock foot anchor pipe; the connection between the lock foot anchor pipe and the steel arch is welded and fixed by U-shaped steel; S4.2, installing forms outside the steel arch to provide a pouring mold cavity for the subsequent C35 concrete pouring step; a grouting pipe is welded on the arch top of the steel arch; multiple blow fill pipes are welded at intervals in the length direction of the steel arch; a corrugated drainage pipe is installed at the bottom of the tunnel, the lower end of the corrugated drainage pipe is connected to the tunnel drainage system, and the upper end of the corrugated drainage pipe is communicated with the cave; and the upper end of the corrugated drainage pipe is higher than the upper end of the drainage steel pipe; S4.3, injecting C35 concrete into the pouring mold cavity through the grouting pipe to coat the outside of the steel arch with C35 concrete to form a steel frame concrete arch protection; the forms are removed after the C35 concrete strength reaches more than 85% of the design strength. S4.4, filling medium sand as a buffer layer on the outside of the steel frame concrete protective arch through a filling pipe.

2. The construction method of a large karst tunnel semi-filled cave according to claim 1, characterized in that, In step S1, the thickness of the initial spraying C25 concrete is 3 cm; and the thickness of the re-spraying C25 concrete is 7 cm.

3. The karst tunnel large semi-filled cave construction method of claim 1, wherein, In step S2, the grouting pipe is a hot-rolled seamless steel pipe with a wall thickness of 4 mm and a length of 4.5 m, and a plurality of the grouting pipes are arranged in a plum blossom shape on the working face, the peripheral rock, and the non-cavity area of the cave wall; the front end of the grouting pipe is in a sharp cone shape, and apart from the pipe wall within a range of 500 mm from the rear end pipe opening, the pipe wall is arranged with eyelets with a diameter of 15 mm at intervals of 15 cm.

4. The karst tunnel large semi-filled cave construction method of claim 1, wherein, In step S2, the volume ratio of the cement slurry to the water glass in the cement-water glass double-liquid slurry is 1:0.1, wherein the water-cement ratio of the cement slurry is 1:1; and the concentration of the water glass is 35 Baume degrees.

5. The karst tunnel large semi-filled cave construction method of claim 1, wherein, In step S3, the thickness of the C20 concrete is 1.5 m; and the thickness of the C35 concrete is 2.45 m.

6. The construction method of a large karst tunnel semi-filled cave according to claim 1, characterized in that, In step S3, the drainage steel pipe is provided in plurality and arranged along the longitudinal direction of the tunnel at intervals.

7. The karst tunnel large semi-filled cave construction method of claim 1, wherein, In step S4, double-sided continuous welding is adopted between the locking foot anchor pipe and the U-shaped steel bar and between the steel arch and the U-shaped steel bar, and the length of the welding seam is not less than 10 cm, and the thickness of the welding seam is not less than 5 mm.

8. The karst tunnel large semi-filled cave construction method of claim 1, wherein, In step S4, the corrugated drainage pipe is provided in plurality and arranged along the longitudinal direction of the tunnel at intervals.

9. The karst tunnel large semi-filled cave construction method of claim 1, wherein, In step S4, the corrugated drainage pipe is wrapped with geotextile and fixed by iron wire binding, and the pipe section of the corrugated drainage pipe extending into the cave is buried in the concrete fixing platform, and the upper end of the corrugated drainage pipe extends from the top of the concrete fixing platform.

10. The method of claim 1, wherein the method is a large-scale semi-filled karst cave construction method for a karst tunnel, characterized in that, In step S4, the thickness of the buffer layer is not less than 50 cm.

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