A method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave.

Abstract

This invention discloses a method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave. The method includes steps such as pile location measurement and setting out, drilling, drill bit removal, grouting, and tunnel floor construction. A steel mesh and steel pipe are used to support the piles, significantly improving their mechanical properties, resulting in better strength, toughness, and fracture resistance. During drilling, a steel mesh follow-up method is used to prevent soil and rock on the borehole wall from falling into the pile hole due to vibration, improving drilling efficiency. Openings are provided in the cylindrical steel mesh, allowing the drill bit to embed itself into the soil and rock on the borehole wall as the drill bit is pulled out, further improving the stability of the pile hole and the overall strength of the pile. Combined with the concrete foundation at the pile top, this forms a reinforced structure for the tunnel floor, exhibiting good bearing capacity and reinforcement effect. This invention has the advantages of being less prone to pile breakage, effectively controlling borehole collapse, and providing excellent reinforcement.

Description

Technical Field

[0001] This invention belongs to the field of tunnel construction technology, specifically relating to a method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave. Background Technology

[0002] Since the beginning of the 21st century, my country's transportation infrastructure construction has entered a new era, with the construction of high-standard railways and high-grade highways becoming increasingly common. In western and southwestern my country, due to challenging terrain and widespread karst formations, most sections of railway lines are traversed via tunnels, inevitably involving extensive crossings of karst strata. When railway and highway tunnels pass through large filled karst caves, the caves are filled with layers of sand, clay, black soil, and soft soil such as clay and gravel. These soft soil layers have insufficient bearing capacity, leading to weak tunnel foundations. Therefore, to ensure structural stability during operation and prevent excessive settlement from affecting operational safety, tunnel foundation reinforcement is generally required.

[0003] Currently, common methods for treating soft soil foundations at the bottom of tunnels include bridge crossing and root pile reinforcement. Root pile reinforcement, a method using small-diameter cast-in-place piles formed by pressure reduction, is named for its root-like shape. It causes minimal disturbance to the existing foundation and has a wide range of applications, particularly for treating various soft soil foundations. However, it also has the following problems: First, soft soil cannot provide adequate support for the cast-in-place piles, making them prone to breakage. Second, during drilling, encountering dense karst caves can easily lead to borehole collapse, repeatedly blocking the piles, making borehole cleaning difficult and delaying construction. Third, the grouting pressure is limited, only able to fill the cavities of the karst caves, and cannot guarantee sufficient bonding between the grout and the cave filling material to form a uniform solidified body, thus failing to achieve the desired treatment effect. Therefore, developing a method for reinforcing the tunnel bottom when crossing large filled karst caves, which is less prone to pile breakage, effectively controls borehole collapse, and provides good reinforcement, is objectively necessary. Summary of the Invention

[0004] In order to solve the problems existing in the background art, the purpose of this invention is to provide a method for reinforcing the tunnel bottom when tunnels pass through large filled karst caves, which is not prone to pile breakage, can effectively control the phenomenon of hole collapse, and has a good reinforcement effect.

[0005] The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in this invention, includes the following steps:

[0006] ① Measurement and layout of pile positions: The piles are evenly distributed in several rows along the length of the tunnel. The pile positions of adjacent rows of piles are staggered. The pile positions are measured and laid out on site in the tunnel to determine the pile position of each pile and the drilling point.

[0007] ② Drilling: Install the drilling machine according to the drilling points determined in step ①, and carry out drilling operations at the drilling points in batches. The drilling spacing should be staggered by 2 holes, and the pile position deviation should not exceed ±20mm. During the drilling process, continuously insert cylindrical steel mesh into the pile hole. The lower end of the steel mesh is connected to a guide cone with a smaller upper end and a larger lower end. The outer diameter of the lower end of the guide cone is the same as the nominal diameter of the drill bit. Openings are processed on the side walls of the steel mesh and the guide cone. The distance between the outer wall of the steel mesh and the hole wall is 3-5mm. During drilling, ensure that the distance between the lower end of the guide cone and the drill bit is always 2-4m.

[0008] ③ Exiting the drill bit: After drilling is completed in step ②, lift the drill bit upwards. After being guided by the guide cone, the drill bit enters the interior of the reinforcing mesh. During the upward lifting process, the drill bit applies an outward force to the guide cone and the reinforcing mesh, widening the openings on the reinforcing mesh and the guide cone, so that the reinforcing mesh and the guide cone are embedded in the soil layer of the borehole wall. After the drill bit leaves the pile hole opening, high-pressure air is used to clean the hole until no dust or gravel is blown out of the hole opening.

[0009] ④ Grouting: After drilling and clearing in step ③, a steel pipe is placed in the pile hole, with the lower end of the steel pipe inserted to the bottom of the pile hole. Several grouting holes are evenly machined on the side wall of the steel pipe. The diameter of the grouting holes is 12-18mm, and the distance between two adjacent grouting holes is 30-40mm. A grouting pipe is inserted into the steel pipe, with the lower end of the grouting pipe extending to the bottom of the steel pipe. Grouting is carried out from bottom to top at a pressure of 0.5-0.8MPa until the grout overflows from the hole. Then, the grouting pipe is pulled out.

[0010] ⑤ Tunnel Bottom Construction: Grouting operations were completed in all pile holes. After the grout solidified, the exposed steel pipes were cut off with a cutting machine, the tunnel bottom was excavated, and a concrete foundation was poured on top of the piles. This completed the construction of the tunnel bottom reinforcement structure for the tunnel passing through a large filled karst cave.

[0011] Furthermore, in step ②, the mesh size of the reinforcing mesh is 10mm×10mm, and the diameter of the reinforcing bars is 6~10mm.

[0012] Furthermore, in step ②, several openings are evenly distributed on the reinforcing mesh, and a retaining strip is installed inside the opening, with the lower end of the retaining strip hinged to the opening.

[0013] Furthermore, in step ④, a slurry outlet is machined on the pipe wall at the lower end of the steel pipe, and multiple vertical rods are installed at the lower end of the steel pipe.

[0014] Furthermore, in step ②, the steel mesh includes several longitudinal steel bars and several transverse steel bars that are orthogonally lapped, with the longitudinal steel bars located on the side facing the center line of the pile hole.

[0015] Furthermore, in step ①, the cast-in-place piles are arranged in groups of three along the length of the tunnel. In each group, the middle row of cast-in-place piles is set vertically downwards, while the two rows of cast-in-place piles on both sides are set inclined downwards in opposite directions. The angle between the inclined cast-in-place piles and the vertical line is 15° to 45°.

[0016] Furthermore, in step ④, several support rings are spaced apart along the length of the outer wall of the steel pipe, and through holes are machined on the support rings.

[0017] Furthermore, in step ④, after the grout overflows the hole and grouting is stopped, sealing plugs are installed between the grouting pipe and the steel pipe, and between the steel pipe and the pile hole. Then, before the grout initially sets, secondary grouting is performed again through the grouting pipe into the pile hole, with a grouting pressure of 2 to 5 MPa.

[0018] The beneficial effects of this invention are as follows:

[0019] I. In the process of drilling pile holes, the present invention adopts the method of following the steel mesh. The steel mesh can effectively block the soil and rocks on the pile hole wall, preventing these soil and rocks from falling into the pile hole due to loosening. This can prevent frequent collapse and hole shrinkage during the drilling process, eliminate the problem of repeated blockage of the pile hole, reduce the difficulty of subsequent hole cleaning, improve drilling efficiency, and shorten the construction period.

[0020] Second, this invention creates a structure where the grout encapsulates the reinforcing mesh and steel pipe after the cast-in-place pile is formed. The reinforcing mesh acts as the external skeleton of the concrete, while the steel pipe acts as the internal skeleton. These three elements are firmly integrated into a single structure. Both the reinforcing mesh and the steel pipe serve as supporting structures for the concrete, improving the strength and toughness of the cast-in-place pile. This significantly increases the pile's mechanical properties and fracture resistance, solving the problem of pile breakage caused by soft soil's inability to provide adequate support. Simultaneously, the steel pipe can easily extend into the bottom of the pile hole, providing a smooth passage for the subsequent grouting pipe and preventing the pipe's end from being blocked by the reinforcing mesh during installation.

[0021] Third, in this invention, when the drill bit is lifted, the diameter of the reinforcing mesh is smaller than the nominal diameter of the drill bit. After the drill bit enters the cylindrical reinforcing mesh, the opening of the reinforcing mesh will become larger, thereby increasing the diameter of the reinforcing mesh. Ultimately, the reinforcing mesh is pressed into the soil and rock inside the pile hole wall, improving the stability of the pile hole wall and preventing the problem of hole collapse. During grouting, the grout can penetrate through the mesh of the reinforcing mesh and seep into the gaps of the surrounding soil and rock, so that the grout and the filling material of the karst cave can be fully bonded and form a uniform solid body. It can also fill the cavity part of the karst cave, thereby making the cast-in-place pile integrated with the surrounding soil and rock, improving the overall strength of the cast-in-place pile. Combined with the concrete foundation set on the pile top, it improves the bearing capacity of the entire tunnel bottom structure, prevents problems such as tunnel bottom subsidence, deformation, and secondary lining cracking when the tunnel passes through a large filled karst cave, and extends the service life of the tunnel bottom.

[0022] In summary, this invention utilizes both reinforcing mesh and steel pipes to support the cast-in-place piles, significantly improving their mechanical properties, strength, toughness, and fracture resistance. This solves the problem of pile breakage caused by the inability of soft soil and other filling materials in large karst caves to provide adequate support. Secondly, during drilling, a reinforcing mesh is used to support the loose borehole wall, preventing soil and rock from falling into the hole due to vibration, thus reducing the difficulty of cleaning the borehole and improving drilling efficiency. Finally, openings are incorporated into the cylindrical reinforcing mesh, allowing the drill bit to expand the mesh diameter during drilling, embedding it into the soil and rock within the borehole wall. This further enhances the stability of the borehole and the overall strength of the cast-in-place pile. Combined with the concrete foundation at the pile top, it forms a reinforced structure for the tunnel floor, exhibiting good load-bearing capacity and reinforcement effect. This invention offers advantages such as reduced pile breakage, effective control of borehole collapse, and excellent reinforcement effect. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the overall structure of the cast-in-place pile 1 in this invention;

[0024] Figure 2 This is a top view of the arrangement of the cast-in-place pile 1 in this invention;

[0025] Figure 3 This is a schematic diagram of the pile hole 3 structure during drilling in this invention;

[0026] Figure 4 This is a schematic diagram of the pile hole 3 structure during grouting in this invention;

[0027] Figure 5 This is a top view of the steel mesh 4 in this invention;

[0028] In the diagram: 1-cast pile, 2-tunnel, 3-pile hole, 4-steel mesh, 5-guide cone, 6-drill bit, 7-opening, 8-support ring, 9-steel pipe, 10-grouting pipe, 11-concrete foundation, 12-clamping strip, 13-vertical rod. Implementation

[0029] The present invention will be further described below with reference to the embodiments and accompanying drawings, but this does not limit the present invention in any way. Any changes or substitutions made based on the teachings of the present invention are within the protection scope of the present invention. Example

[0030] The tunnel bottom reinforcement method for tunnels passing through large filled karst caves as described in Example 1 includes the following steps:

[0031] ① Pile Position Measurement and Layout of Cast-in-Place Piles 1: Several rows of cast-in-place piles 1 are evenly distributed along the length of the tunnel. The pile positions of adjacent rows of cast-in-place piles 1 are staggered. The staggered arrangement of cast-in-place piles 1 has two functions: firstly, it expands the grouting area during grouting, allowing the grout to penetrate more stratum fissures and improve reinforcement strength; secondly, it reduces disturbance between adjacent pile holes 3 during drilling. On-site measurement and layout are conducted inside tunnel 2 to determine the pile position of each cast-in-place pile 1 and the drilling point. Cast-in-place piles 1 are grouped into sets of three rows along the length of tunnel 2. In each set, the middle row of cast-in-place piles 1 is set vertically downwards, while the two rows on either side are set inclined downwards in opposite directions. The angle between the inclined cast-in-place piles 1 and the vertical line is 15°. For the overall reinforcement structure of the tunnel floor of tunnel 2, different inclination angles of the cast-in-place piles 1 can make the reinforcement structure more stable and robust, with higher bearing capacity. The specific inclination angle can be determined according to the actual soil structure.

[0032] ② Drilling: Install the drilling machine according to the drilling points determined in step ①, and carry out drilling operations at the drilling points in batches. The drilling spacing should be staggered by 2 holes, and the pile position deviation should not exceed ±20mm. During the drilling process, continuously insert cylindrical steel mesh 4 into the pile hole 3. The lower end of the steel mesh 4 is connected to a guide cone 5 with a smaller upper port and a larger lower port. The outer diameter of the lower port of the guide cone 5 is the same as the nominal diameter of the drill bit 6. Openings 7 are processed on the side walls of the steel mesh 4 and the guide cone 5. The distance between the outer wall of the steel mesh 4 and the hole wall of the pile hole 3 is 3mm. During drilling, ensure that the distance between the lower port of the guide cone 5 and the drill bit 6 is always 2m.

[0033] The reinforcing mesh 4 includes several longitudinal reinforcing bars and several transverse reinforcing bars that are orthogonally lapped together. The longitudinal reinforcing bars are located on the side facing the center line of the pile hole 3. The reinforcing mesh 4 is usually formed by welding several longitudinal reinforcing bars and several transverse reinforcing bars that are orthogonally lapped together. Considering that in this invention, during the drilling process, the drill bit 6 will be pulled out from the inside of the cylindrical reinforcing mesh 4, and the edge of the drill bit 6 will continuously rub against the reinforcing mesh 4, if the transverse reinforcing bars are facing the inside of the pile hole 3, then during the drilling process, the drill bit 6 may encounter problems such as jamming or even get stuck. In order to prevent the above problems, the longitudinal reinforcing bars are located on the side facing the inside of the pile hole 3. In this way, the problem of the transverse reinforcing bars jamming the drill bit 6 is solved during drilling. At the same time, the longitudinal reinforcing bars are arranged vertically and can also guide the drill bit 6, so that the drill bit 6 can be moved out of the pile hole 3 more smoothly after completing the drilling operation.

[0034] The reinforcing mesh 4 has several openings evenly distributed on it, and a retaining strip 12 is installed in each opening. The lower end of the retaining strip 12 is hinged to the opening, and the retaining strip 12 can rotate around the hinge. In this invention, when the drill bit 6 is lifted to exit the drill, since the nominal diameter of the drill bit 6 is larger than the diameter of the reinforcing mesh 4, it will have the effect of expanding the diameter of the reinforcing mesh 4. At the same time, due to the friction between the drill bit 6 and the reinforcing mesh 4, the drill bit 6 tends to pull the reinforcing mesh 4 upward when it is lifted, which will cause the reinforcing mesh 4 to loosen. In order to prevent this situation, a rotatable retaining strip 12 is provided. When the drill bit 6 is lifted, the retaining strip 12 is subjected to force and rotates outward. When the reinforcing mesh 4 moves upward, it gradually gets stuck into the hole wall of the pile hole 3, thereby preventing the reinforcing mesh 4 from moving upward with the drill bit 6, and playing the role of fixing the reinforcing mesh 4. Secondly, since the retaining strip 12 is stuck into the mud and sand around the pile hole 3, it can also improve the connection strength between the reinforcing mesh 4 and the surrounding mud and sand after pouring, thereby improving the strength of the entire cast-in-place pile 1.

[0035] Preferably, under normal circumstances, the mesh size of the reinforcing mesh 4 is 10mm×10mm, and the diameter of the reinforcing bars is 6-10mm. In actual construction, the actual particle size of the filling material inside the large-scale karst cave through which the pile hole 3 passes can be determined based on the geological exploration. Then, the mesh size and diameter of the reinforcing mesh 4 are determined based on the actual particle size of the filling material. On the one hand, the reinforcing mesh 4 can prevent the mud and sand around the pile hole 3 from falling into the pile hole 3, thus preventing the problems of hole burial and hole shrinkage. On the other hand, it can also serve as the skeleton of the cast-in-place pile 1, which only needs to meet the support strength requirements.

[0036] ③ Drilling: After drilling is completed in step ②, the drill bit 6 is lifted upwards. After being guided by the guide cone 5, the drill bit 6 enters the interior of the steel mesh 4. During the upward lifting process, the drill bit 6 applies an outward force to the guide cone 5 and the steel mesh 4, widening the opening 7 on the steel mesh 4 and the guide cone 5, so that the steel mesh 4 and the guide cone 5 are embedded in the soil layer of the pile hole 3 wall. After the drill bit 6 leaves the opening of the pile hole 3, high-pressure air is used to clean the hole until no dust or gravel is blown out of the hole.

[0037] ④ Grouting: After drilling and cleaning in step ③, steel pipe 9 is inserted into pile hole 3, with the lower end of steel pipe 9 inserted to the bottom of pile hole 3. Several grouting holes are evenly machined on the side wall of steel pipe 9. The diameter of the grouting hole is 12mm and the distance between two adjacent grouting holes is 30mm. A grouting pipe 10 is inserted into steel pipe 9, with the lower end of grouting pipe 10 extending to the bottom of steel pipe 9. Grouting is carried out from bottom to top at a pressure of 0.5MPa into pile hole 3 until the grout overflows from the hole. Grouting is then stopped and grouting pipe 10 is pulled out.

[0038] After the grout overflows the borehole and grouting is stopped, sealing plugs are installed between the grouting pipe 10 and the steel pipe 9, and between the steel pipe 9 and the pile hole 3. Then, before the grout initially sets, secondary grouting is performed again through the grouting pipe 10 into the pile hole 3 at a grouting pressure of 2 MPa. Secondary grouting is performed after the first grouting operation and before the grout initially sets. If the grout solidifies and shrinks or falls back, grouting is promptly replenished. The pressure of the secondary grouting is higher than that of the first grouting operation, allowing the grout to better fill the gaps in every corner of the pile hole 3, resulting in a denser and more compact cast-in-place pile 1. This effectively connects the reinforcing mesh 4, the steel pipe 9, and the grout into a unified whole, improving the strength and mechanical properties of the entire cast-in-place pile 1 and preventing pile breakage. Secondly, the higher grouting pressure also allows the grout to more easily penetrate into the surrounding soil and rock gaps, thereby generating numerous root-like structures, expanding the area of ​​grout injection into the soil and rock, and further improving the strength and firmness of the cast-in-place pile 1, increasing the bearing capacity and support capacity of the tunnel bottom structure of tunnel 2.

[0039] Several support rings 8 are spaced along the length of the outer wall of the steel pipe 9. The support rings 8 have through holes. The support rings 8 are set on the steel pipe 9 and have two functions. First, the outer surface of the support ring 8 contacts the steel mesh 4, thereby supporting the steel pipe 9. This can prevent the steel pipe 9 from tilting and position the steel pipe 9 in the middle of the pile hole 23. Second, after grouting is completed, the support rings 8 are wrapped with grout, forming a concrete skeleton and improving the strength of the cast-in-place pile 1.

[0040] A grout outlet is machined on the lower end of the steel pipe 9, and multiple vertical rods 13 are installed at the lower end of the steel pipe 9. The grout outlet allows the grout to flow more smoothly from the steel pipe 9 into the pile hole 3, while the vertical rods 13 have two functions. First, when the bottom of the pile hole 3 is hard rock, the vertical rods 13 provide support to prevent the lower end of the steel pipe 9 from hitting the bottom of the pile hole 3 and preventing the grout from flowing out from the lower end of the steel pipe 9. Second, when the bottom of the pile hole 3 is soft mud and sand, the lower end of the vertical rods 13 can be inserted into the mud and sand for a certain length, guiding the grout to diffuse into the deeper mud and sand below the pile hole 3 during grouting, thereby improving the strength of the cast-in-place pile 1.

[0041] ⑤ Tunnel 2 bottom construction: Grouting operations were completed in all pile holes 3, and after the grout solidified, the exposed steel pipes 9 were cut off with a cutting machine, the tunnel bottom was excavated, and a concrete foundation 11 was poured on top of the piles, thus completing the construction of the tunnel bottom reinforcement structure for tunnel 2 passing through a large filled karst cave.

[0042] After years of construction verification, when using the method described in this invention to reinforce the tunnel floor of tunnel 2 passing through a large filled karst cave, the use of steel mesh 4 and steel pipe 9 to jointly support the cast-in-place pile 1 significantly improves the mechanical properties of the cast-in-place pile 1, giving it better strength, toughness, and fracture resistance. This solves the problem that soft soil and other filling materials in large filled karst caves cannot provide adequate support for the cast-in-place pile 1, making it prone to breakage. Secondly, when drilling the pile hole 3, a method of following up with steel mesh 4 is adopted. After drilling, the following steel mesh 4 can promptly support the loose hole wall. To prevent soil and rocks on the borehole wall from falling into the pile hole 3 due to vibration, thus avoiding blockage or shrinkage, the invention reduces the difficulty of cleaning the hole and improves drilling efficiency. Finally, the invention provides an opening 7 on the cylindrical steel mesh 4, which allows the drill bit 6 to expand the diameter of the steel mesh 4 while drilling, enabling it to embed into the soil and rocks on the borehole wall of the pile hole 3. This further improves the stability of the pile hole 3, increases the overall strength of the cast-in-place pile 1, and, combined with the concrete foundation at the top of the pile, forms a reinforced structure for the bottom of the tunnel 2. This structure has good bearing capacity and reinforcement effect, and extends the service life of the bottom of the tunnel 2. Example

[0043] The tunnel bottom reinforcement method for tunnels passing through large filled karst caves as described in Example 2 includes the following steps:

[0044] ① Pile Position Measurement and Layout of Cast-in-Place Piles 1: Several rows of cast-in-place piles 1 are evenly distributed along the length of the tunnel. The pile positions of adjacent rows of cast-in-place piles 1 are staggered. The staggered arrangement of cast-in-place piles 1 has two functions: firstly, it expands the grouting area during grouting, allowing the grout to penetrate more stratum fissures and improve reinforcement strength; secondly, it reduces disturbance between adjacent pile holes 3 during drilling. On-site measurement and layout are conducted inside tunnel 2 to determine the pile position of each cast-in-place pile 1 and the drilling point. Cast-in-place piles 1 are grouped into sets of three rows along the length of tunnel 2. In each set, the middle row of cast-in-place piles 1 is set vertically downwards, while the two rows on either side are set inclined downwards in opposite directions. The angle between the inclined cast-in-place piles 1 and the vertical line is 15°. For the overall reinforcement structure of the tunnel floor of tunnel 2, different inclination angles of the cast-in-place piles 1 can make the reinforcement structure more stable and robust, with higher bearing capacity. The specific inclination angle can be determined according to the actual soil structure.

[0045] ② Drilling: Install the drilling machine according to the drilling points determined in step ①, and carry out drilling operations at the drilling points in batches. The drilling spacing should be staggered by 2 holes, and the pile position deviation should not exceed ±20mm. During the drilling process, continuously insert cylindrical steel mesh 4 into the pile hole 3. The lower end of the steel mesh 4 is connected to a guide cone 5 with a smaller upper port and a larger lower port. The outer diameter of the lower port of the guide cone 5 is the same as the nominal diameter of the drill bit 6. Openings 7 are processed on the side walls of the steel mesh 4 and the guide cone 5. The distance between the outer wall of the steel mesh 4 and the hole wall of the pile hole 3 is 4mm. During drilling, ensure that the distance between the lower port of the guide cone 5 and the drill bit 6 is always 3m.

[0046] The reinforcing mesh 4 includes several longitudinal reinforcing bars and several transverse reinforcing bars that are orthogonally lapped together. The longitudinal reinforcing bars are located on the side facing the center line of the pile hole 3. The reinforcing mesh 4 is usually formed by welding several longitudinal reinforcing bars and several transverse reinforcing bars that are orthogonally lapped together. Considering that in this invention, during the drilling process, the drill bit 6 will be pulled out from the inside of the cylindrical reinforcing mesh 4, and the edge of the drill bit 6 will continuously rub against the reinforcing mesh 4, if the transverse reinforcing bars are facing the inside of the pile hole 3, then during the drilling process, the drill bit 6 may encounter problems such as jamming or even get stuck. In order to prevent the above problems, the longitudinal reinforcing bars are located on the side facing the inside of the pile hole 3. In this way, the problem of the transverse reinforcing bars jamming the drill bit 6 is solved during drilling. At the same time, the longitudinal reinforcing bars are arranged vertically and can also guide the drill bit 6, so that the drill bit 6 can be moved out of the pile hole 3 more smoothly after completing the drilling operation.

[0047] The reinforcing mesh 4 has several openings evenly distributed on it, and a retaining strip 12 is installed in each opening. The lower end of the retaining strip 12 is hinged to the opening, and the retaining strip 12 can rotate around the hinge. In this invention, when the drill bit 6 is lifted to exit the drill, since the nominal diameter of the drill bit 6 is larger than the diameter of the reinforcing mesh 4, it will have the effect of expanding the diameter of the reinforcing mesh 4. At the same time, due to the friction between the drill bit 6 and the reinforcing mesh 4, the drill bit 6 tends to pull the reinforcing mesh 4 upward when it is lifted, which will cause the reinforcing mesh 4 to loosen. In order to prevent this situation, a rotatable retaining strip 12 is provided. When the drill bit 6 is lifted, the retaining strip 12 is subjected to force and rotates outward. When the reinforcing mesh 4 moves upward, it gradually gets stuck into the hole wall of the pile hole 3, thereby preventing the reinforcing mesh 4 from moving upward with the drill bit 6, and playing the role of fixing the reinforcing mesh 4. Secondly, since the retaining strip 12 is stuck into the mud and sand around the pile hole 3, it can also improve the connection strength between the reinforcing mesh 4 and the surrounding mud and sand after pouring, thereby improving the strength of the entire cast-in-place pile 1.

[0048] Preferably, under normal circumstances, the mesh size of the reinforcing mesh 4 is 10mm×10mm, and the diameter of the reinforcing bars is 6-10mm. In actual construction, the actual particle size of the filling material inside the large-scale karst cave through which the pile hole 3 passes can be determined based on the geological exploration. Then, the mesh size and diameter of the reinforcing mesh 4 are determined based on the actual particle size of the filling material. On the one hand, the reinforcing mesh 4 can prevent the mud and sand around the pile hole 3 from falling into the pile hole 3, thus preventing the problems of hole burial and hole shrinkage. On the other hand, it can also serve as the skeleton of the cast-in-place pile 1, which only needs to meet the support strength requirements.

[0049] ③ Drilling: After drilling is completed in step ②, the drill bit 6 is lifted upwards. After being guided by the guide cone 5, the drill bit 6 enters the interior of the steel mesh 4. During the upward lifting process, the drill bit 6 applies an outward force to the guide cone 5 and the steel mesh 4, widening the opening 7 on the steel mesh 4 and the guide cone 5, so that the steel mesh 4 and the guide cone 5 are embedded in the soil layer of the pile hole 3 wall. After the drill bit 6 leaves the opening of the pile hole 3, high-pressure air is used to clean the hole until no dust or gravel is blown out of the hole.

[0050] ④ Grouting: After drilling and cleaning in step ③, a steel pipe 9 is inserted into the pile hole 3, with the lower end of the steel pipe 9 inserted to the bottom of the pile hole 3. Several grouting holes are evenly machined on the side wall of the steel pipe 9. The diameter of the grouting hole is 15mm, and the distance between two adjacent grouting holes is 35mm. A grouting pipe 10 is inserted into the steel pipe 9, with the lower end of the grouting pipe 10 extending to the bottom of the steel pipe 9. Grouting is carried out from bottom to top at a pressure of 0.6MPa into the pile hole 3 until the grout overflows from the hole. Then, the grouting pipe 10 is pulled out.

[0051] After the grout overflows the borehole and grouting is stopped, sealing plugs are installed between the grouting pipe 10 and the steel pipe 9, and between the steel pipe 9 and the pile hole 3. Then, before the grout initially sets, secondary grouting is performed again through the grouting pipe 10 into the pile hole 3 at a grouting pressure of 2 MPa. Secondary grouting is performed after the first grouting operation and before the grout initially sets. If the grout solidifies and shrinks or falls back, grouting is promptly replenished. The pressure of the secondary grouting is higher than that of the first grouting operation, allowing the grout to better fill the gaps in every corner of the pile hole 3, resulting in a denser and more compact cast-in-place pile 1. This effectively connects the reinforcing mesh 4, the steel pipe 9, and the grout into a unified whole, improving the strength and mechanical properties of the entire cast-in-place pile 1 and preventing pile breakage. Secondly, the higher grouting pressure also allows the grout to more easily penetrate into the surrounding soil and rock gaps, thereby generating numerous root-like structures, expanding the area of ​​grout injection into the soil and rock, and further improving the strength and firmness of the cast-in-place pile 1, increasing the bearing capacity and support capacity of the tunnel bottom structure of tunnel 2.

[0052] Several support rings 8 are spaced along the length of the outer wall of the steel pipe 9. The support rings 8 have through holes. The support rings 8 are set on the steel pipe 9 and have two functions. First, the outer surface of the support ring 8 contacts the steel mesh 4, thereby supporting the steel pipe 9. This can prevent the steel pipe 9 from tilting and position the steel pipe 9 in the middle of the pile hole 23. Second, after grouting is completed, the support rings 8 are wrapped with grout, forming a concrete skeleton and improving the strength of the cast-in-place pile 1.

[0053] A grout outlet is machined on the lower end of the steel pipe 9, and multiple vertical rods 13 are installed at the lower end of the steel pipe 9. The grout outlet allows the grout to flow more smoothly from the steel pipe 9 into the pile hole 3, while the vertical rods 13 have two functions. First, when the bottom of the pile hole 3 is hard rock, the vertical rods 13 provide support to prevent the lower end of the steel pipe 9 from hitting the bottom of the pile hole 3 and preventing the grout from flowing out from the lower end of the steel pipe 9. Second, when the bottom of the pile hole 3 is soft mud and sand, the lower end of the vertical rods 13 can be inserted into the mud and sand for a certain length, guiding the grout to diffuse into the deeper mud and sand below the pile hole 3 during grouting, thereby improving the strength of the cast-in-place pile 1.

[0054] ⑤ Tunnel 2 bottom construction: Grouting operations were completed in all pile holes 3, and after the grout solidified, the exposed steel pipes 9 were cut off with a cutting machine, the tunnel bottom was excavated, and a concrete foundation 11 was poured on top of the piles, thus completing the construction of the tunnel bottom reinforcement structure for tunnel 2 passing through a large filled karst cave.

[0055] After years of construction verification, when using the method described in this invention to reinforce the tunnel floor of tunnel 2 passing through a large filled karst cave, the use of steel mesh 4 and steel pipe 9 to jointly support the cast-in-place pile 1 significantly improves the mechanical properties of the cast-in-place pile 1, giving it better strength, toughness, and fracture resistance. This solves the problem that soft soil and other filling materials in large filled karst caves cannot provide adequate support for the cast-in-place pile 1, making it prone to breakage. Secondly, when drilling the pile hole 3, a method of following up with steel mesh 4 is adopted. After drilling, the following steel mesh 4 can promptly support the loose hole wall. To prevent soil and rocks on the borehole wall from falling into the pile hole 3 due to vibration, thus avoiding blockage or shrinkage, the invention reduces the difficulty of cleaning the hole and improves drilling efficiency. Finally, the invention provides an opening 7 on the cylindrical steel mesh 4, which allows the drill bit 6 to expand the diameter of the steel mesh 4 while drilling, enabling it to embed into the soil and rocks on the borehole wall of the pile hole 3. This further improves the stability of the pile hole 3, increases the overall strength of the cast-in-place pile 1, and, combined with the concrete foundation at the top of the pile, forms a reinforced structure for the bottom of the tunnel 2. This structure has good bearing capacity and reinforcement effect, and extends the service life of the bottom of the tunnel 2. Example

[0056] The tunnel bottom reinforcement method for tunnels passing through large filled karst caves as described in Example 3 includes the following steps:

[0057] ① Pile Position Measurement and Layout of Cast-in-Place Piles 1: Several rows of cast-in-place piles 1 are evenly distributed along the length of the tunnel. The pile positions of adjacent rows of cast-in-place piles 1 are staggered. The staggered arrangement of cast-in-place piles 1 has two functions: firstly, it expands the grouting area during grouting, allowing the grout to penetrate more stratum fissures and improve reinforcement strength; secondly, it reduces disturbance between adjacent pile holes 3 during drilling. On-site measurement and layout are conducted inside tunnel 2 to determine the pile position of each cast-in-place pile 1 and the drilling point. Cast-in-place piles 1 are grouped into sets of three rows along the length of tunnel 2. In each set, the middle row of cast-in-place piles 1 is set vertically downwards, while the two rows on either side are set inclined downwards in opposite directions. The angle between the inclined cast-in-place piles 1 and the vertical line is 15°. For the overall reinforcement structure of the tunnel floor of tunnel 2, different inclination angles of the cast-in-place piles 1 can make the reinforcement structure more stable and robust, with higher bearing capacity. The specific inclination angle can be determined according to the actual soil structure.

[0058] ② Drilling: Install the drilling machine according to the drilling points determined in step ①, and carry out drilling operations at the drilling points in batches. The drilling spacing should be staggered by 2 holes, and the pile position deviation should not exceed ±20mm. During the drilling process, continuously insert cylindrical steel mesh 4 into the pile hole 3. The lower end of the steel mesh 4 is connected to a guide cone 5 with a smaller upper port and a larger lower port. The outer diameter of the lower port of the guide cone 5 is the same as the nominal diameter of the drill bit 6. Openings 7 are processed on the side walls of the steel mesh 4 and the guide cone 5. The distance between the outer wall of the steel mesh 4 and the hole wall of the pile hole 3 is 5mm. During drilling, ensure that the distance between the lower port of the guide cone 5 and the drill bit 6 is always 4m.

[0059] The reinforcing mesh 4 includes several longitudinal reinforcing bars and several transverse reinforcing bars that are orthogonally lapped together. The longitudinal reinforcing bars are located on the side facing the center line of the pile hole 3. The reinforcing mesh 4 is usually formed by welding several longitudinal reinforcing bars and several transverse reinforcing bars that are orthogonally lapped together. Considering that in this invention, during the drilling process, the drill bit 6 will be pulled out from the inside of the cylindrical reinforcing mesh 4, and the edge of the drill bit 6 will continuously rub against the reinforcing mesh 4, if the transverse reinforcing bars are facing the inside of the pile hole 3, then during the drilling process, the drill bit 6 may encounter problems such as jamming or even get stuck. In order to prevent the above problems, the longitudinal reinforcing bars are located on the side facing the inside of the pile hole 3. In this way, the problem of the transverse reinforcing bars jamming the drill bit 6 is solved during drilling. At the same time, the longitudinal reinforcing bars are arranged vertically and can also guide the drill bit 6, so that the drill bit 6 can be moved out of the pile hole 3 more smoothly after completing the drilling operation.

[0060] The reinforcing mesh 4 has several openings evenly distributed on it, and a retaining strip 12 is installed in each opening. The lower end of the retaining strip 12 is hinged to the opening, and the retaining strip 12 can rotate around the hinge. In this invention, when the drill bit 6 is lifted to exit the drill, since the nominal diameter of the drill bit 6 is larger than the diameter of the reinforcing mesh 4, it will have the effect of expanding the diameter of the reinforcing mesh 4. At the same time, due to the friction between the drill bit 6 and the reinforcing mesh 4, the drill bit 6 tends to pull the reinforcing mesh 4 upward when it is lifted, which will cause the reinforcing mesh 4 to loosen. In order to prevent this situation, a rotatable retaining strip 12 is provided. When the drill bit 6 is lifted, the retaining strip 12 is subjected to force and rotates outward. When the reinforcing mesh 4 moves upward, it gradually gets stuck into the hole wall of the pile hole 3, thereby preventing the reinforcing mesh 4 from moving upward with the drill bit 6, and playing the role of fixing the reinforcing mesh 4. Secondly, since the retaining strip 12 is stuck into the mud and sand around the pile hole 3, it can also improve the connection strength between the reinforcing mesh 4 and the surrounding mud and sand after pouring, thereby improving the strength of the entire cast-in-place pile 1.

[0061] Preferably, under normal circumstances, the mesh size of the reinforcing mesh 4 is 10mm×10mm, and the diameter of the reinforcing bars is 6-10mm. In actual construction, the actual particle size of the filling material inside the large-scale karst cave through which the pile hole 3 passes can be determined based on the geological exploration. Then, the mesh size and diameter of the reinforcing mesh 4 are determined based on the actual particle size of the filling material. On the one hand, the reinforcing mesh 4 can prevent the mud and sand around the pile hole 3 from falling into the pile hole 3, thus preventing the problems of hole burial and hole shrinkage. On the other hand, it can also serve as the skeleton of the cast-in-place pile 1, which only needs to meet the support strength requirements.

[0062] ③ Drilling: After drilling is completed in step ②, the drill bit 6 is lifted upwards. After being guided by the guide cone 5, the drill bit 6 enters the interior of the steel mesh 4. During the upward lifting process, the drill bit 6 applies an outward force to the guide cone 5 and the steel mesh 4, widening the opening 7 on the steel mesh 4 and the guide cone 5, so that the steel mesh 4 and the guide cone 5 are embedded in the soil layer of the pile hole 3 wall. After the drill bit 6 leaves the opening of the pile hole 3, high-pressure air is used to clean the hole until no dust or gravel is blown out of the hole.

[0063] ④ Grouting: After drilling and cleaning in step ③, steel pipe 9 is inserted into pile hole 3, with the lower end of steel pipe 9 inserted to the bottom of pile hole 3. Several grouting holes are evenly machined on the side wall of steel pipe 9. The diameter of the grouting hole is 18mm and the distance between two adjacent grouting holes is 40mm. A grouting pipe 10 is inserted into steel pipe 9, with the lower end of grouting pipe 10 extending to the bottom of steel pipe 9. Grouting is carried out from bottom to top at a pressure of 0.8MPa into pile hole 3 until the grout overflows from the hole. Grouting is then stopped and grouting pipe 10 is pulled out.

[0064] After the grout overflows the borehole and grouting is stopped, sealing plugs are installed between the grouting pipe 10 and the steel pipe 9, and between the steel pipe 9 and the pile hole 3. Then, before the grout initially sets, secondary grouting is performed again through the grouting pipe 10 into the pile hole 3 at a grouting pressure of 2 MPa. Secondary grouting is performed after the first grouting operation and before the grout initially sets. If the grout solidifies and shrinks or falls back, grouting is promptly replenished. The pressure of the secondary grouting is higher than that of the first grouting operation, allowing the grout to better fill the gaps in every corner of the pile hole 3, resulting in a denser and more compact cast-in-place pile 1. This effectively connects the reinforcing mesh 4, the steel pipe 9, and the grout into a unified whole, improving the strength and mechanical properties of the entire cast-in-place pile 1 and preventing pile breakage. Secondly, the higher grouting pressure also allows the grout to more easily penetrate into the surrounding soil and rock gaps, thereby generating numerous root-like structures, expanding the area of ​​grout injection into the soil and rock, and further improving the strength and firmness of the cast-in-place pile 1, increasing the bearing capacity and support capacity of the tunnel bottom structure of tunnel 2.

[0065] Several support rings 8 are spaced along the length of the outer wall of the steel pipe 9. The support rings 8 have through holes. The support rings 8 are set on the steel pipe 9 and have two functions. First, the outer surface of the support ring 8 contacts the steel mesh 4, thereby supporting the steel pipe 9. This can prevent the steel pipe 9 from tilting and position the steel pipe 9 in the middle of the pile hole 23. Second, after grouting is completed, the support rings 8 are wrapped with grout, forming a concrete skeleton and improving the strength of the cast-in-place pile 1.

[0066] A grout outlet is machined on the lower end of the steel pipe 9, and multiple vertical rods 13 are installed at the lower end of the steel pipe 9. The grout outlet allows the grout to flow more smoothly from the steel pipe 9 into the pile hole 3, while the vertical rods 13 have two functions. First, when the bottom of the pile hole 3 is hard rock, the vertical rods 13 provide support to prevent the lower end of the steel pipe 9 from hitting the bottom of the pile hole 3 and preventing the grout from flowing out from the lower end of the steel pipe 9. Second, when the bottom of the pile hole 3 is soft mud and sand, the lower end of the vertical rods 13 can be inserted into the mud and sand for a certain length, guiding the grout to diffuse into the deeper mud and sand below the pile hole 3 during grouting, thereby improving the strength of the cast-in-place pile 1.

[0067] ⑤ Tunnel 2 bottom construction: Grouting operations were completed in all pile holes 3, and after the grout solidified, the exposed steel pipes 9 were cut off with a cutting machine, the tunnel bottom was excavated, and a concrete foundation 11 was poured on top of the piles, thus completing the construction of the tunnel bottom reinforcement structure for tunnel 2 passing through a large filled karst cave.

[0068] After years of construction verification, when using the method described in this invention to reinforce the tunnel floor of tunnel 2 passing through a large filled karst cave, the use of steel mesh 4 and steel pipe 9 to jointly support the cast-in-place pile 1 significantly improves the mechanical properties of the cast-in-place pile 1, giving it better strength, toughness, and fracture resistance. This solves the problem that soft soil and other filling materials in large filled karst caves cannot provide adequate support for the cast-in-place pile 1, making it prone to breakage. Secondly, when drilling the pile hole 3, a method of following up with steel mesh 4 is adopted. After drilling, the following steel mesh 4 can promptly support the loose hole wall. To prevent soil and rocks on the borehole wall from falling into the pile hole 3 due to vibration, thus avoiding blockage or shrinkage, the invention reduces the difficulty of cleaning the hole and improves drilling efficiency. Finally, the invention provides an opening 7 on the cylindrical steel mesh 4, which allows the drill bit 6 to expand the diameter of the steel mesh 4 while drilling, enabling it to embed into the soil and rocks on the borehole wall of the pile hole 3. This further improves the stability of the pile hole 3, increases the overall strength of the cast-in-place pile 1, and, combined with the concrete foundation at the top of the pile, forms a reinforced structure for the bottom of the tunnel 2. This structure has good bearing capacity and reinforcement effect, and extends the service life of the bottom of the tunnel 2.

[0069] The methods described in Examples 1-3 above were used to reinforce the tunnel floor of Tunnel 2, which traverses a large filled karst cave. A method of following the steel mesh 4 during drilling was employed to prevent frequent collapses and hole shrinkage during the drilling process, eliminating the problem of repeated blockage of the pile hole 3, reducing the difficulty of subsequent hole cleaning, improving drilling efficiency, and shortening the construction period. The cast-in-place pile 1 forms a structure where the grout encapsulates the steel mesh 4 and steel pipe 9, which improves the strength and toughness of the cast-in-place pile 1, significantly increasing its mechanical properties and fracture resistance, and solving the problem of pile breakage due to the inability of soft soil to provide adequate support for the cast-in-place pile 1. The drill bit... When the drill is pulled up, the opening 7 of the steel mesh 4 will become larger, eventually pressing the steel mesh 4 into the soil and rock inside the pile hole 3, improving the stability of the pile hole 3 wall and preventing the problem of hole collapse. The grout and the karst filling material are fully bonded and form a uniform solid body, which can also fill the cavity part of the karst, thereby making the cast-in-place pile 1 integrated with the surrounding soil and rock, improving the overall strength of the cast-in-place pile 1. Combined with the concrete foundation 11 set on the pile top, the bearing capacity of the entire tunnel bottom structure is improved, preventing problems such as tunnel bottom subsidence, deformation, and secondary lining cracking when the tunnel 2 passes through a large filled karst. This extends the service life of the tunnel bottom.

Claims

1. A method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, characterized in that: Includes the following steps: ① Measurement and layout of pile positions of cast-in-place piles (1): The cast-in-place piles (1) are evenly distributed in several rows along the length of the tunnel. The pile positions of two adjacent rows of cast-in-place piles (1) are staggered. The pile positions of each cast-in-place pile (1) are determined on-site in the tunnel (2) to determine the drilling point. ② Drilling: Install the drilling machine according to the drilling points determined in step ①, and carry out drilling operations on the drilling points in batches. The drilling spacing is staggered by 2 holes, and the pile position deviation shall not exceed ±20mm. During the drilling process, continuously insert cylindrical steel mesh (4) into the pile hole (3). The lower end of the steel mesh (4) is connected to a guide cone (5) with a smaller upper port and a larger lower port. The outer diameter of the lower port of the guide cone (5) is the same as the nominal diameter of the drill bit (6). Openings (7) are processed on the side walls of the steel mesh (4) and the guide cone (5). The distance between the outer wall of the steel mesh (4) and the hole wall of the pile hole (3) is 3-5mm. During drilling, ensure that the distance between the lower port of the guide cone (5) and the drill bit (6) is always 2-4m. ③ Drilling: After drilling is completed in step ②, the drill bit (6) is lifted upwards. After being guided by the guide cone (5), the drill bit (6) enters the interior of the steel mesh (4). During the upward lifting process, the drill bit (6) applies an outward force to the guide cone (5) and the steel mesh (4), expanding the opening (7) on the steel mesh (4) and the guide cone (5), so that the steel mesh (4) and the guide cone (5) are embedded in the soil layer of the pile hole (3). After the drill bit (6) leaves the hole (3), high-pressure air is used to clean the hole until no dust or gravel is blown out of the hole. ④ Grouting: After drilling and cleaning in step ③, a steel pipe (9) is placed in the pile hole (3) so that the lower end of the steel pipe (9) is inserted into the bottom of the pile hole (3). Several grouting holes are uniformly processed on the side wall of the steel pipe (9). The diameter of the grouting hole is 12-18mm and the distance between two adjacent grouting holes is 30-40mm. A grouting pipe (10) is inserted into the steel pipe (9) with the lower end of the grouting pipe (10) extending into the bottom of the steel pipe (9). Grouting is carried out from bottom to top with a pressure of 0.5-0.8MPa into the pile hole (3) until the grout overflows from the hole and the grouting is stopped. The grouting pipe (10) is then pulled out. ⑤ Tunnel (2) Tunnel bottom construction: All pile holes (3) have completed grouting operations, and after the grout has solidified, the exposed steel pipes (9) are cut off with a cutting machine, the tunnel bottom is excavated, and a concrete foundation (11) is poured on the pile top, thus completing the construction of the tunnel bottom reinforcement structure of the tunnel (2) passing through the large filling karst cave.

2. The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, is characterized in that: In step ②, the mesh size of the steel mesh (4) is 10mm×10mm, and the diameter of the steel bars is 6~10mm.

3. The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, is characterized in that: In step ②, the steel mesh (4) has several openings evenly distributed on it, and a retaining strip (12) is provided in the opening. The lower end of the retaining strip (12) is hinged to the opening.

4. The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, is characterized in that: In step ④, a slurry outlet is machined on the pipe wall at the lower end of the steel pipe (9), and multiple vertical rods (13) are provided at the lower end of the steel pipe (9).

5. The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, is characterized in that: In step ②, the steel mesh (4) includes several longitudinal steel bars and several transverse steel bars that are orthogonally lapped, and the longitudinal steel bars are located on one side facing the center line of the pile hole (3).

6. The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, is characterized in that: In step ①, the cast-in-place piles (1) are arranged in groups of three rows along the length of the tunnel (2). In each group, the middle row of cast-in-place piles (1) is set vertically downwards, while the two rows of cast-in-place piles (1) on both sides are set inclined downwards in opposite directions. The angle between the inclined cast-in-place piles (1) and the vertical line is 15° to 45°.

7. The method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, is characterized in that: In step ④, a number of support rings (8) are provided at intervals along the length direction on the outer wall of the steel pipe (9), and through holes are machined on the support rings (8).

8. A method for reinforcing the tunnel floor when a tunnel passes through a large filled karst cave, as described in claim 1, characterized in that: In step ④, after the grout overflows the hole and grouting is stopped, sealing plugs are installed between the grouting pipe (10) and the steel pipe (9), and between the steel pipe (9) and the pile hole (3). Then, before the grout sets, secondary grouting is carried out again through the grouting pipe (10) into the pile hole (3), with a grouting pressure of 2 to 5 MPa.

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