Method for Retreating Intermittent Controlled Pre-grouting Construction of Pile Foundations for Ultra-Large and Ultra-Deep Karst Caves in Karst Areas

Abstract

This application relates to the technical field of karst cave reinforcement, and proposes a method for back-retreating intermittent controlled pre-grouting construction of ultra-large and ultra-deep karst cave pile foundations, including the following steps: S1: Measurement and layout of the center of the bored pile hole; S2: Using a geological drilling rig, drilling several circles of probing holes with the center of the bored pile hole as the center for sampling and probing; when the karst cave boundary is still not determined after the probing diameter changes to a certain distance, the probing is stopped; S3: Drilling vent holes connected to the karst cave next to the probing holes using a drilling rig; S4: Installation of grouting pipes; S5: Grouting pump injecting mortar into the karst cave through the grouting pipes; grouting is paused after a certain period of time, the grouting pipe is lifted upwards, and grouting is resumed; S6: Repeating steps S4-S5: until the back-retreating intermittent grouting construction through all probing holes is completed. This application has the effect of reducing the cost of karst cave reinforcement while meeting the conditions for bored pile drilling.

Description

Technical Field

[0001] This application relates to the field of karst cave reinforcement treatment, and in particular to a method for back-retreating intermittent controlled pre-grouting construction of pile foundations for ultra-large and ultra-deep karst caves in karst areas. Background Technology

[0002] Cast-in-place piles are pile foundations formed by drilling holes in the soil on-site, placing a steel cage inside the holes, and pouring concrete. They are a common treatment method for bridge foundations in soft soil strata.

[0003] In related technologies, when constructing bored piles in karst areas with numerous underground caves (mostly semi-filled or unfilled), direct construction of the bored piles carries significant risks. This is because the combined effects of the caves and the thick sand layers above them make it easy for the drilling rig to leak its drilling mud when drilling into the cave, potentially leading to cave collapse. Therefore, before drilling the bored pile, it is usually necessary to fill and reinforce the cave with grout to reduce the risk of cave collapse during subsequent pile foundation drilling.

[0004] Regarding the aforementioned technologies, the above treatment methods require injecting a large volume of mortar into the cave to fill and reinforce it when dealing with extremely large and deep karst caves, resulting in high construction costs. Summary of the Invention

[0005] To reduce the construction cost of karst cave reinforcement while meeting the requirements for borehole pile construction within karst caves, this application provides a method for retreating intermittent controlled pre-grouting construction of pile foundations in ultra-large and ultra-deep karst caves in karst areas.

[0006] The method for retreating intermittent controlled pre-grouting construction of pile foundations for ultra-large and ultra-deep karst caves in karst areas provided in this application adopts the following technical solution:

[0007] The method for back-retreating intermittent controlled pre-grouting construction of pile foundations for ultra-large and ultra-deep karst caves in karst areas includes the following steps:

[0008] S1: Measurement and layout of the center of the bored pile hole;

[0009] S2: Sampling along the edge of the karst cave: Using a geological drilling rig, several circles of exploration holes are drilled with the center of the bored pile hole as the center for sampling and exploration. When the boundary of the karst cave is still not determined after the exploration diameter has changed to a certain distance, the exploration is stopped.

[0010] S3: A venting hole connected to the karst cave is constructed next to the exploration hole using a drilling rig;

[0011] S4: Grouting pipe installation: The grouting pipe is inserted into the karst cave through the probe hole and the top of the grouting pipe is connected to the grouting pump; the bottom of the grouting pipe has several grout outlets on its outer periphery.

[0012] S5: Retreating intermittent grouting construction: The grouting pump injects mortar into the karst cave through the grouting pipe; after each injection for a certain period of time, the grouting is paused, the grouting pipe is raised and grouting is resumed; the above steps are repeated until the grouting pipe is raised to the top of the karst cave and no more mortar can be injected;

[0013] S6: Repeat steps S4-S5 until the intermittent grouting construction through all the probing holes into the karst cave is completed, so as to form a solidified body at the hole of the cast-in-place pile in the karst cave.

[0014] By adopting the above technical solution, the solidified body is used to support and reinforce the hole of the cast-in-place pile in the karst cave. During the subsequent construction of the cast-in-place pile, after the drilling rig enters the karst cave, it can continue to drill on the solidified body. On the one hand, the presence of the solidified body can limit the leakage of the drilling mud during the subsequent drilling, so as to facilitate the construction of the cast-in-place pile. On the other hand, the solidified body can provide local support for the karst cave, making it less likely for the karst cave to collapse. Compared with the traditional construction method, there is no need to perform large-scale grouting in the karst cave, which reduces the amount of grouting required and reduces the cost waste of large-scale grouting in the karst cave.

[0015] Preferably, the following steps are also included:

[0016] S7: Sampling verification of the center of the bored pile hole: Sampling is carried out by drilling a sampling hole at the center of the bored pile hole using a geological drilling rig.

[0017] S8: When the sampling results of step S7 reveal that there are still local karst caves in the solidified body of the karst cave, perform secondary grouting on the sampling hole at the center of the bored pile hole until the local karst caves in the solidified body are filled and reinforced.

[0018] By adopting the above technical solution, the forming quality of the solidified body in the karst cave can be judged by the sampling results. When the sampling results reveal that there are still local karst caves in the solidified body, secondary grouting can be used to further fill and reinforce the solidified body to ensure the overall strength of the solidified body. This will facilitate the drilling of the subsequent cast-in-place piles, and the drilling rig can then carry out further drilling construction in the solidified body.

[0019] Preferably, in step S4, before installing the grouting pipe, a steel sleeve is first inserted into the probe hole, and several clamping rods are welded to the outer periphery of the steel sleeve extending out of the probe hole to restrict the probe hole from sliding downward; then the grouting pipe is inserted into the steel sleeve and placed into the karst cave.

[0020] By adopting the above technical solution, steel casing is used to support the inner wall of the probe hole, thereby reducing the possibility of the probe hole collapsing due to construction disturbance during subsequent construction, which could lead to the grouting pipe being buried.

[0021] Preferably, in step S4, after the steel sleeve is placed into the probe hole, mortar is injected into the gap between the steel sleeve and the probe hole to seal the gap between the steel sleeve and the probe hole.

[0022] After the grouting pipe is inserted into the steel casing, the gap between the grouting pipe and the steel casing is sealed by a sealing element.

[0023] The sealing element includes an annular airbag coaxially disposed on the inner circumference of the steel sleeve; the annular airbag is connected to a venting pipe; the end of the venting pipe away from the annular airbag passes through to the outside of the steel sleeve and extends to the top of the steel sleeve.

[0024] By adopting the above technical solution, the gaps between the steel casing and the probe hole, as well as the gaps between the steel casing and the grouting pipe, are sealed. This reduces the backflow of mortar from the gaps between the steel casing and the probe hole, and between the steel casing and the grouting pipe, to the ground when the grouting pump injects grout into the karst cave through the grouting pipe. This facilitates better solidification of the mortar injected into the karst cave. Furthermore, by setting up an annular airbag, gas is injected into the annular airbag through an air pipe. The expanding annular airbag seals the gaps between the steel casing and the grouting pipe. When the grouting pipe is subsequently lifted, the gas inside the annular airbag can be discharged, allowing for smoother lifting of the grouting pipe.

[0025] Preferably, in step S4, before injecting mortar into the karst cave, high-pressure clean water is first pumped into the grouting pipe using a grouting pump.

[0026] By adopting the above technical solution, the grouting pipe is connected to the grout outlet, which facilitates the grout injected into the grouting pipe by the subsequent grouting pump to flow into the karst cave through the grout outlet.

[0027] Preferably, an annular limiting plate is coaxially connected to the outer periphery of the bottom end of the steel sleeve.

[0028] By adopting the above technical solution and setting the annular limiting plate, on the one hand, when the steel sleeve is lowered into the probe hole, it is convenient to push the outer periphery of the steel sleeve away from the probe hole wall, reducing the possibility of air pipe damage caused by friction between the exposed outer periphery of the steel sleeve and the probe hole wall. On the other hand, when mortar is subsequently injected into the gap between the steel sleeve and the probe hole, the annular limiting plate can restrict the mortar from flowing into the karst cave, making it easier for the injected mortar to better seal the gap between the steel sleeve and the probe hole.

[0029] Preferably, in step S3, after creating the vent hole, an vent pipe is inserted into the vent hole and extended to the top of the cave.

[0030] By adopting the above technical solution, the inner wall of the exhaust hole can be supported by the exhaust pipe, reducing the possibility of the exhaust hole failing due to collapse during subsequent construction.

[0031] Preferably, in step S3, the vent hole is drilled to the bottom of the cave using a drilling rig; before inserting the vent pipe into the vent hole, a support rod is inserted into the cave through the vent hole, and both ends of the support rod are inserted into the vent holes at the top and bottom of the cave, respectively.

[0032] By adopting the above technical solution, all subsequent support rods can be fixed in the solidified body, and the support rods in the solidified body can form a reinforcing skeleton, which is beneficial to improving the overall strength of the solidified body.

[0033] In summary, this application includes at least one of the following beneficial technical effects:

[0034] 1. By performing intermittent grouting into the karst cave through step-by-step probing holes, a solidified body supporting the karst cave is formed at the drilling site of the cast-in-place piles. After the drilling rig enters the karst cave, it can continue to drill holes on the solidified body. On the one hand, the solidified body can limit the leakage of drilling mud from the drilling rig; on the other hand, it can provide support for the position of the cast-in-place piles in the karst cave through the solidified body; and on the other hand, it can reduce the possibility of hole collapse in the karst cave during the drilling of the cast-in-place piles. Moreover, this application does not require complete filling of the karst cave during grouting, which helps to reduce the grouting cost of the karst cave.

[0035] 2. By setting up an annular airbag on the inner circumference of the steel casing; before injecting mortar into the karst cave through the grouting pipe, air is inflated into the annular airbag through the air pipe. The expanded annular airbag seals the gap between the steel casing and the grouting pipe, preventing the mortar injected into the karst cave from escaping to the ground through the gap between the steel casing and the grouting pipe. Before lifting the grouting pipe upwards, the gas in the annular airbag is released through the air pipe to reduce the friction between the annular airbag and the grouting pipe, thus facilitating a smoother upward lifting of the grouting pipe.

[0036] 3. Insert support rods into the cave through the vent holes, and insert the top and bottom of the support rods into the vent holes at the top and bottom of the cave, respectively. After the solidified body is formed, several support rods can be fixed in the solidified body to form a reinforcing skeleton and improve the overall strength of the solidified body. Attached Figure Description

[0037] Figure 1 This is a schematic diagram illustrating the drilling positions of the probe hole and the vent hole in an embodiment of this application.

[0038] Figure 2 This is a schematic diagram illustrating the installation relationship of the exhaust port, support rod, and exhaust pipe in an embodiment of this application.

[0039] Figure 3 This is a schematic diagram illustrating the installation relationship between the probe hole, the steel sleeve, and the grouting hole in an embodiment of this application.

[0040] Figure 4 yes Figure 3 Enlarged schematic diagram of part A in the middle.

[0041] Figure 5 This is a schematic diagram illustrating the process of intermittent grouting construction in a karst cave, as described in the embodiments of this application.

[0042] Explanation of reference numerals in the attached figures:

[0043] 1. Drilling hole location for cast-in-place pile; 10. Karst cave; 101. Consolidated body; 2. Excavation hole; 20. Grouting pipe; 201. Pipe section; 202. Grout outlet; 21. Steel sleeve; 211. Annular limiting groove; 212. Annular limiting plate; 22. Connecting rod; 23. Annular airbag; 24. Vent pipe; 3. Exhaust hole; 31. Exhaust pipe; 4. Support rod; 41. Limiting rod. Detailed Implementation

[0044] The following is in conjunction with the appendix Figure 1-5 This application will be described in further detail.

[0045] This application discloses a method for retreating intermittent controlled pre-grouting construction of pile foundations for ultra-large and ultra-deep karst caves in karst areas, including the following steps:

[0046] S1: Reference Figure 1 and Figure 2 The center of the bored pile hole 1 is measured and laid out. Before measuring and laying out the bored pile hole 1, the approximate location of the karst cave 10 should be determined by physical detection instruments. At the same time, physical detection instruments should be used to survey whether there are underground pipelines in the construction area. If there are interfering pipelines, they should be relocated and protected in a timely manner.

[0047] S2: Sampling and probing of karst cave 10: Using a geological drilling rig, drill several concentric circles of probing holes 2, centered on the center of the bored pile borehole 1, for sampling and probing. The first circle of probing holes 2 is arranged on a circle with a radius of 2.5m centered on the bored pile borehole 1. The radius of each subsequent circle increases by 2.5m, with a spacing of approximately 3m between each probing hole 2 within each circle. The depth of the probing holes 2 must not be less than the depth of the pile foundation. Probing should be stopped when the boundary of karst cave 10 is still not determined after the probing diameter has changed to within 10m of the center of the bored pile borehole 1. After sampling is completed, record the depth of karst cave 10 based on the sampling results.

[0048] S3: Drill a vent hole 3 next to the exploration hole 2, connecting it to the karst cave 10; the specific construction steps are as follows:

[0049] S3.1: Drill an exhaust hole 3 next to the exploration hole 2 using a geological drilling rig. The exhaust hole 3 is drilled to a depth of 0.3m from the bottom of the karst cave 10. Both the exhaust hole 3 and the corresponding exploration hole 2 are equidistant from the center of the bored pile hole 1.

[0050] S3.2: A support rod 4 is inserted into the karst cave 10 through the vent hole 3, and both ends of the support rod 4 are respectively inserted into the vent holes 3 at the top and bottom of the karst cave 10; several limiting rods 41 are vertically welded to the top section of the support rod 4, and the limiting rods 41 are evenly distributed around the axis of the support rod 4; when the support rod 4 is inserted into the vent hole 3, the end of the limiting rod 41 away from the support rod 4 abuts against the wall of the vent hole 3; in this embodiment, both the support rod 4 and the limiting rod 41 are steel bars;

[0051] S3.3: Insert the exhaust pipe 31 into the exhaust pipe 31 and move the exhaust pipe 31 downward until the bottom end face of the exhaust pipe 31 abuts against several limiting rods 41;

[0052] S3.4: Connect the exhaust pipe 31 to the high-pressure water pump, and pump high-pressure water into the drain pipe to achieve the conduction of the exhaust pipe 31, thereby reducing the possibility of sand, gravel, mud, etc. clogging the exhaust pipe 31 when it is lowered.

[0053] S3.5: Disconnect the exhaust pipe 31 from the high-pressure water pump.

[0054] S4: Reference Figure 3 and Figure 4 Installation of grouting pipe 20: The grouting pipe 20 is inserted into the karst cave 10 through the probe hole 2 and connected to the grouting pump; in this embodiment, the grouting pump is specifically model "Jiale HBT60.13-90S". Several grout outlets 202 are opened on the outer periphery of the bottom end of the grouting pipe 20; the several grout outlets 202 are distributed in a quincunx pattern.

[0055] The specific steps for S4 are as follows:

[0056] S4.1: Installation of steel sleeve 21: Insert steel sleeve 21 into the probe hole 2 and move steel sleeve 21 downward until the bottom end of steel sleeve 21 moves to the top of the cave 10; weld several clamping rods 22 vertically to the outer periphery of the end of steel sleeve 21 that extends out of the top of the probe hole 2 and make the clamping rods 22 abut against the ground to restrict the downward movement of steel sleeve 21;

[0057] S4.2: Inject mortar or clay into the gap between the steel sleeve 21 and the probe hole 2 to seal the gap between the steel sleeve 21 and the probe hole 2;

[0058] S4.3: Installation of grouting pipe 20; Insert the grouting pipe 20 into the steel sleeve 21 and lower the grouting pipe 20 until the bottom of the grouting pipe 20 is 0.5m away from the bottom of the karst cave 10;

[0059] S4.4: Seal the gap between the grouting pipe 20 and the steel sleeve 21 using a sealing element;

[0060] S4.5: Connect the top end of the grouting pipe 20 to the grouting pump via an adapter.

[0061] S5: Retreating Intermittent Grouting Construction: The grouting pump injects mortar into the karst cave 10 through the grouting pipe 20; after each injection for a certain period of time, the grouting is paused, the grouting pipe 20 is raised upwards, and grouting is resumed; the above steps are repeated until the grouting pipe 20 is raised to the top of the karst cave 10 and no more mortar can be injected. In this embodiment, the mortar is M10 cement mortar, with a specific mix ratio of cement:sand:admixture:water:water-reducing agent = 1:6.14:0.39:1:0.02, a mortar density of 1850kg / m3, a consistency of 90±20mm, and a 28-day compressive strength of 12.4MPa.

[0062] Reference Figure 1 and Figure 5 The specific steps for S5 are as follows:

[0063] S5.1: The grouting pump injects high-pressure clean water into the grouting pipe 20 to connect the bottom end of the grouting pipe 20 and the grout outlet 202;

[0064] S5.2: The grouting pump injects mortar into the karst cave 10 through the grouting pipe 20; the grouting speed is controlled at 400-600 liters / min; the grouting time is 90-120 minutes for each section; followed by a 20-minute interval.

[0065] S5.3: After lifting the grouting pipe 20 upwards by using a crane and a multi-functional drilling rig by 2-4m, repeat step S5.2 until the grouting pipe 20 is lifted to the top of the karst cave 10 and mortar can no longer be injected.

[0066] With the above settings, by raising the grouting pipe 20 after an interval of 20 seconds to perform grouting, the mortar injected first is initially cemented. After multiple cycles, a solidified body 101 supporting the karst cave 10 can be formed inside the cave. Compared with the traditional method of directly injecting large amounts of grout into the karst cave 10, this method can limit the waste caused by mortar flowing too far.

[0067] When performing intermittent grouting on the probe holes 2 of different rings, the principle of "outer first, then inner" is adopted. That is, the outermost probe holes 2 are constructed first, and then the inner probe holes 2 are constructed. This is so that the mortar injected into the karst cave 10 can be better formed when performing intermittent grouting on the probe holes 2 of the inner ring later.

[0068] When performing intermittent grouting on the same exploration hole 2, grouting is carried out in a skip-interval manner.

[0069] For multi-layered karst cave 10, the retreating intermittent grouting construction of the lower layer of karst cave 10 should be completed first before the retreating intermittent grouting construction of the upper layer of karst cave 10 is carried out.

[0070] The sealing element includes an annular airbag 23 coaxially disposed on the inner circumference of the bottom end of the steel sleeve 21; an annular limiting groove 211 is provided on the outer circumference of the bottom end of the steel sleeve 21 corresponding to the annular airbag 23; the outer circumference of the annular airbag 23 is fixedly connected to the annular limiting groove 211; a venting pipe 24 is also connected to the outer circumference of the annular airbag 23, the venting pipe 24 passes through the outer circumference of the steel sleeve 21 and extends to the top end of the steel sleeve 21; a section of the venting pipe 24 located on the outer circumference of the steel sleeve 21 is fixed to the outer circumference of the steel sleeve 21 by several clamps. With the above arrangement, gas is injected into the annular airbag 23 through the venting pipe 24, and the expanded annular airbag 23 seals the gap between the steel sleeve 21 and the grouting pipe 20, so as to limit the mortar from escaping to the ground from the gap between the steel sleeve 21 and the grouting pipe 20 during subsequent grouting, so that the mortar injected into the karst cave 10 can better form within the karst cave 10. When the grouting pipe 20 moves upward, the gas inside the annular airbag 23 is released through the vent pipe 24, so that the annular airbag 23 contracts, thereby reducing the friction between the annular airbag 23 and the grouting pipe 20, making it easier for the grouting pipe 20 to move upward more smoothly.

[0071] In step S4.1, before inserting the steel sleeve 21 into the probe hole 2, the annular airbag 23 is first folded and stored in the annular limiting groove 211, and several strips of adhesive tape are glued to the opening of the annular limiting groove 211 to prevent the annular airbag 23 from popping out of the annular storage groove. With the above settings, when the grouting pipe 20 is inserted into the steel sleeve 21 later, the grouting pipe 20 is less likely to rub against the annular airbag 23, and the grouting pipe 20 can be inserted into the steel sleeve 21 more smoothly. When the grouting pipe 20 is lowered into place, when air is inflated into the annular airbag 23 through the air pipe, the annular airbag 23 can automatically push out the adhesive tape.

[0072] An annular limiting plate 212 is coaxially connected to the outer periphery of the bottom end of the steel sleeve 21. With the above settings, on the one hand, when the steel sleeve 21 is placed into the probe hole 2, the annular limiting plate 212 can be used to push the vent pipe 24 away from the hole wall of the probe hole 2 to limit the friction between the vent pipe 24 and the probe hole 2. At the same time, it can reduce the amount of sealing mortar injected between the steel sleeve 21 and the probe hole 2 falling into the karst cave 10, so that the sealing mortar can better seal the gap between the steel sleeve 21 and the probe hole 2.

[0073] The grouting pipe 20 is composed of several pipe sections 201 spliced ​​together; adjacent pipe sections 201 are connected to each other at their closest ends by a threaded structure. This configuration allows for the addition or reduction of the number of pipe sections 201 as needed for construction. After the subsequent construction is completed, the grouting pipe 20 can be disassembled into several pipe sections 201 to facilitate the cleaning of residual mortar inside the pipe sections 201.

[0074] S6: Repeat steps S4-S5 until the back-reverse intermittent grouting construction is completed through all the probing holes 2 into the karst cave 10; so as to form a solidified body 101 that supports and reinforces the karst cave 10 at the hole of the cast-in-place pile in the karst cave 10.

[0075] In step S3, support rods 4 are inserted into the karst cave 10 through the vent holes 3, and the two ends of the support rods 4 are respectively inserted into the vent holes 3 at the top and bottom of the karst cave 10. After the solidified body 101 is formed, several support rods 4 are fixed inside the solidified body 101. Several support rods 4 form a reinforcing skeleton to improve the overall strength of the solidified body 101. This makes it less likely for the solidified body 101 to collapse and be damaged due to the drilling of the drill rod during the subsequent grouting pile driving process. This allows the solidified body 101 to more stably support and reinforce the karst cave 10.

[0076] S7: Sampling verification at the center of borehole 1 of cast-in-place pile: A sampling hole is drilled at the center of borehole 1 of cast-in-place pile using a geological drilling rig for sampling.

[0077] S8: When the sampling results of step S7 reveal that there is still a local karst cave 10 at the solidified body 101 inside the karst cave 10, secondary grouting is performed on the sampling hole at the center of the hole position 1 of the cast-in-place pile until the local karst cave 10 at the solidified body 101 is filled and reinforced.

[0078] This application involves sequentially injecting grout into the karst cave 10 through the exploration holes 2, thereby forming a solidified body 101 supporting the karst cave 10 at the grouting hole location. Subsequently, after the drilling rig enters the karst cave 10, it can form a hole on the solidified body 101, reducing the loss of wall-protecting mud during drilling. At the same time, the solidified body 101 supports the karst cave 10, making it less prone to hole collapse when the karst cave 10 is drilled for cast-in-place piles. Moreover, compared with traditional construction methods, the construction method of this application does not require large-volume grouting into the karst cave 10, greatly reducing the amount of grouting required and lowering construction costs.

[0079] The above are all preferred embodiments of this application, and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape and principle of this application should be covered within the scope of protection of this application.

Claims

1. A method for constructing a retreating intermittent controlled pre-grouting method for pile foundations in karst areas with ultra-large and ultra-deep karst caves, characterized in that: Includes the following steps: S1: Center measurement and layout of the borehole location for cast-in-place piles (1); S2: Sampling of the edge of the karst cave (10): Using a geological drilling rig, drill several circles of edge exploration holes (2) with the center of the hole position (1) of the cast-in-place pile as the center for sampling and edge exploration; when the edge exploration diameter reaches a certain distance and the boundary of the karst cave (10) is still not determined, stop the edge exploration; S3: A vent (3) is constructed next to the exploration hole (2) and connected to the karst cave (10) by a drilling rig; S4: Grouting pipe (20) installation: The grouting pipe (20) is inserted into the karst cave (10) through the probe hole (2) and the top of the grouting pipe (20) is connected to the grouting pump; the bottom of the grouting pipe (20) is provided with several grout outlets (202); S5: Retreating intermittent grouting construction: The grouting pump injects mortar into the karst cave (10) through the grouting pipe (20); after each injection for a certain period of time, the grouting is paused, the grouting pipe (20) is raised upwards and grouting is carried out again; the above steps are repeated until the grouting pipe (20) is raised to the top of the karst cave (10) and mortar can no longer be injected; S6: Repeat steps S4-S5 until the back-reverse intermittent grouting construction is completed through all the probing holes (2) into the karst cave (10) to form a solid body (101) at the hole of the cast-in-place pile in the karst cave (10); In step S3, after making the vent hole (3), insert the vent pipe (31) into the vent hole (3) and make the vent pipe (31) extend to the top of the cave (10); In step S3, the vent hole (3) is drilled to the bottom of the cave (10) using a drilling rig; before inserting the vent pipe (31) into the vent hole (3), the support rod (4) is inserted into the cave (10) through the vent hole (3), and the two ends of the support rod (4) are inserted into the vent holes (3) at the top and bottom of the cave (10) respectively; The top section of the support rod (4) is vertically welded with several limiting rods (41), which are evenly distributed around the axis of the support rod (4). When the support rod (4) is placed into the exhaust hole (3), the end of the limiting rod (41) away from the support rod (4) abuts against the wall of the exhaust hole (3). The exhaust pipe (31) is placed into the exhaust hole (3) and moved downward until the bottom end face of the exhaust pipe (31) abuts against the several limiting rods (41).

2. The method for constructing a retreating intermittent controlled pre-grouting method for pile foundations in karst areas with ultra-large and ultra-deep karst caves according to claim 1, characterized in that: It also includes the following steps: S7: Sampling verification of the center of the hole position (1) of the cast-in-place pile: Sampling is carried out by drilling a sampling hole at the center of the hole position (1) of the cast-in-place pile using a geological drilling rig; S8: When the sampling results of step S7 reveal that there is still a local karst cave (10) in the solidified body (101) inside the karst cave (10), perform secondary grouting on the sampling hole at the center of the hole position (1) of the cast-in-place pile until the local karst cave (10) in the solidified body (101) is filled and reinforced.

3. A method for constructing a retreating intermittent controlled pre-grouting method for pile foundations in karst areas with ultra-large and ultra-deep karst caves, as described in any one of claims 1-2, characterized in that: In step S4, before installing the grouting pipe (20), a steel sleeve (21) is first inserted into the probe hole (2), and several clamping rods (22) are welded to the outer periphery of the steel sleeve (21) extending out of the top of the probe hole (2) to restrict the probe hole (2) from sliding downward; then the grouting pipe (20) is inserted into the steel sleeve (21) and placed into the karst cave (10).

4. The method for constructing a retreating intermittent controlled pre-grouting method for pile foundations in karst areas with ultra-large and ultra-deep karst caves according to claim 3, characterized in that: In step S4, after the steel sleeve (21) is placed into the probe hole (2), mortar is injected into the gap between the steel sleeve (21) and the probe hole (2) to seal the gap between the steel sleeve (21) and the probe hole (2). After the grouting pipe (20) is inserted into the steel sleeve (21), the gap between the grouting pipe (20) and the steel sleeve (21) is sealed by the sealing element; The sealing element includes an annular airbag (23) coaxially disposed on the inner circumference of the steel sleeve (21); the annular airbag (23) is connected to a vent pipe (24); one end of the vent pipe (24) away from the annular airbag (23) passes through to the outside of the steel sleeve (21) and extends to the top of the steel sleeve (21).

5. The method for constructing a retreating intermittent controlled pre-grouting method for pile foundations in karst areas with ultra-large and ultra-deep karst caves according to claim 1, characterized in that: In step S4, before injecting mortar into the karst cave (10), high-pressure clean water is pumped into the grouting pipe (20) by a grouting pump.

6. The method for constructing a retreating intermittent controlled pre-grouting method for pile foundations in karst areas with ultra-large and ultra-deep karst caves according to claim 4, characterized in that: The bottom outer periphery of the steel sleeve (21) is also coaxially connected to an annular limiting plate (212).

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