A casing device for bored pile construction in a karst cave and a bored pile construction method

Abstract

The present invention relates to the field of foundation and basic engineering technology, and specifically discloses a casing device and a method for bored pile construction in a karst cave. The casing device includes a straight upper casing and a lower casing, and a sealing assembly arranged around the outer periphery of the lower casing, wherein: the lower casing includes an inner lining skeleton, a steel mesh layer, and a waterproof cloth layer arranged in sequence from the inside to the outside; an open pile shoe with a diameter matching the inner lining skeleton is fixed to the bottom of the inner lining skeleton; the waterproof cloth layer includes several pieces of waterproof cloth overlapped and covered on the steel mesh layer in sequence, and the waterproof cloth is tied and fixed to the inner lining skeleton; a positioning hole matching the top of the inner lining skeleton is opened at the bottom of the upper casing, and the inner lining skeleton is inserted into the positioning hole. In the present invention, the inner lining skeleton of the lower casing and the steel cage form a double-layer longitudinal force-bearing structure, and the steel mesh layer and the waterproof cloth layer provide circumferential constraints on the concrete protective layer of the bored pile, forming a bored pile with high strength, anti-buckling, and good horizontal stability.

Description

Technical Field

[0001] The present invention relates to the technical field of foundation and basic engineering, and in particular to a casing device used for bored pile construction in a karst cave and a bored pile construction method. Background Art

[0002] Karst caves are natural holes formed by soluble limestone under the influence of karst. They have peculiar shapes and sizes, and their development follows no regular pattern. In some areas, they develop intensely and multi-layered, with many caves becoming large cavities. Under the erosion of groundwater, caves are left unfilled or partially filled. Sites with intense karst development are unsuitable for engineering construction, but it is often difficult to avoid them. Treatment is necessary to ensure the stability of the building foundation site, and sufficient bearing capacity is required.

[0003] Currently, the common treatment method for large caves is to drill through the cave with a small or large bore drill and then fill the cave with sand, clay, or pour concrete and cement slurry. Most of the existing patented technologies are based on pre-cavity filling or grouting. Because the drilling rig uses a mud wall protection process, if the cave is not filled or grouted in advance, the drilling rig will inevitably leak slurry when drilling, and even cause drill drop or collapse accidents. There are also patents that use casing technology, which also adopts the method of first drilling and then inserting casing to form piles. For example, the invention patent (publication number CN109629563 A) discloses a pile foundation enclosure construction method, which uses hoisting equipment to lower a retracted variable cross-section casing into the pile hole in the cave area, inflate an air bag pre-placed in the variable cross-section casing, and use the air bag to expand the variable cross-section casing to the same size as the pile hole. However, since the airbag is invisible when inflated in the underground pile hole, it is easy to be punctured when it encounters gravel and has poor reliability; in unfilled tall caves, the variable-section casing is easy to open, and in semi-filled or mostly filled tall caves, the filling soil is unlikely to be completely squeezed open, and the variable-section casing is in an incompletely opened state, so the pile diameter becomes smaller and cannot meet the requirements of the designed pile diameter; in this scheme, the concrete of the bored pile is poured three times: lower, middle and upper, which is prone to construction quality problems such as broken piles. Before lowering the steel cage, the pile hole is cleaned through a catheter, which is unlikely to clean the sediment at the bottom of the pile, resulting in many problems. For example, an invention patent (publication number CN 112523205 A) discloses a pile foundation construction method for open cave geology with thick overburden that is prone to collapse. This patent adopts a double casing structure of bored pile casing and protective steel casing. First, the bored pile casing is constructed through the bored pile casing to a first preset position below the bedrock surface. Then, the cave is reinforced and backfilled with grouting. After the cave is filled, the normal cast-in-place pile process is used. The protective steel casing is installed along the entire length, which greatly increases the process, materials, and construction costs of the cave grouting treatment. Therefore, existing solutions generally have problems such as huge filling work volume, high material consumption, uncontrollable costs, long construction period, and poor filling quality.

[0004] Therefore, people in this field are in urgent need of finding a new technical solution to solve the above problems. Summary of the Invention

[0005] In view of the technical problems in the prior art, the present invention provides a casing device and a bored pile construction method for bored piles in a karst cave.

[0006] The present invention includes a casing device for use in cast-in-place pile construction in a karst cave, the casing device comprising a straight upper casing and a lower casing, and a plugging assembly arranged around the outer periphery of the lower casing, wherein:

[0007] The lower casing includes an inner lining frame, a steel mesh layer, and a waterproof cloth layer, which are arranged in sequence from the inside out. The inner lining frame and the steel mesh layer are arranged coaxially. An open pile shoe with a matching diameter is fixed to the bottom of the inner lining frame. The waterproof cloth layer includes several pieces of waterproof cloth that are overlapped and covered on the steel mesh layer in sequence, and the waterproof cloth is tied and fixed to the inner lining frame.

[0008] A positioning hole matching the top of the liner frame is opened at the bottom of the upper casing, and the liner frame is inserted into the positioning hole;

[0009] The plugging assembly is arranged near the connection between the upper casing and the liner frame.

[0010] Furthermore, the plugging assembly includes a first annular steel bar, a second annular steel bar, an expansion water stop ring and a binding belt, wherein:

[0011] The first annular steel bar and the second annular steel bar are tied parallel to the outside of the lower casing;

[0012] The expansion water stop ring is fixed between the first annular steel bar and the second annular steel bar by a binding belt.

[0013] Furthermore, the inner lining skeleton is a grid structure constructed by a plurality of circular steel bars arranged transversely and a plurality of long steel bars arranged longitudinally.

[0014] Furthermore, the number and position of the positioning holes match the long steel bars in the lining frame, and the aperture of the positioning holes is 1mm to 2mm larger than the diameter of the long steel bars. When the positioning holes are plugged into the long steel bars, the center line of each positioning hole coincides with the center line of the corresponding long steel bar.

[0015] Furthermore, in the waterproof cloth layer, the joints of two adjacent waterproof cloths are double-layered, and the inner waterproof cloth is provided with nail holes, and is fixed to the inner lining frame by iron wire through the nail holes. The outer waterproof cloth covers the nail holes of the inner waterproof cloth and is fixed by an outer iron wire hoop.

[0016] Furthermore, the expansion water stop ring includes a water stop strip and an elastic rubber membrane wrapped around the water stop strip, and the elastic rubber membrane is provided with a plurality of water permeable holes that are evenly arranged.

[0017] Furthermore, the inner diameters of the upper casing, the lower casing and the open pile shoe are the same.

[0018] The present invention also includes a method for constructing a bored pile in a cave. The method is implemented based on the above-mentioned casing device and includes the following steps:

[0019] S1: Pile hole protection construction: The casing device is arranged according to the positional relationship between the upper casing in the shallow soil layer and the lower casing in the karst cave. After applying butter in the positioning hole, it is connected to the liner skeleton, and the connection position is 1.0m to 2.0m below the soil-rock interface. The drilling rig includes a reaming drill bit and a drill rod with spiral blades. The drilling rig is installed and moved to the designed pile position. The open pile shoe is welded to the bottom of the liner skeleton, and the open pile shoe is located at the front end of the reaming drill bit.

[0020] S2: Pile hole drilling: Align the cone tip of the reaming drill bit with the center of the pile and drill the hole in a forward direction, allowing the lower and upper casings to sink synchronously. At the same time, clean the debris brought out of the hole by the spiral blades. When the drill rod, lower casing or upper casing sinks to a height of 0.8m to 1.2m from the ground, extend the drill rod, lower casing and upper casing accordingly until the reaming drill bit passes through the cave and enters the intact rock formation.

[0021] S3: Concrete pouring: clean the sediment at the bottom of the pile, lower the steel cage, insert the concrete pouring steel pipe, and pour the concrete using underwater pouring under the protection of the upper and lower casings;

[0022] S4: Cast-in-place pile formation: When the concrete pouring height reaches 1.0m to 1.2m or more at the joint of the upper casing and the lower casing, pour concrete while pulling up the upper casing until it is completely pulled out of the ground to form a cast-in-place pile in the cave.

[0023] Furthermore, the diameter of the reaming drill bit is 10 mm to 15 mm larger than the outer diameters of the open pile shoe, the upper casing and the lower casing, thereby forming a soil-cutting cavity.

[0024] Furthermore, the drill pipe, lower casing and upper casing are extended accordingly, including connecting the upper casing by means of threads or bolts, connecting the lining frame in the lower casing by means of welding, and overlapping the wire mesh layer in the lower casing by means of wire tying.

[0025] The invention provides a casing device and a bored pile construction method for bored piles in caves, wherein the casing device follows up synchronously with the drilling of the hole by the reaming drill bit, and there is no need for mud wall protection to form the hole, thereby achieving the purpose of wall protection in the whole process. The upper casing in the soil layer can be recycled, and the lower casing retained in the cave and the steel cage are cast into piles at one time. Even if there is no filling soil in the tall cave, there is no need to use sand, gravel, concrete materials for filling or grouting, which completely saves the material cost, machinery cost and labor cost of cave treatment. The formed bored pile can fully meet the construction requirements of steel casing of the same size, and the lining skeleton of the lower casing and the steel cage form a double-layer longitudinal force-bearing structure, and the wire mesh layer and the waterproof cloth layer provide circumferential constraint on the concrete protective layer of the bored pile, thereby ensuring the vertical compressive bearing capacity of the bored pile, and forming a bored pile with high strength, anti-buckling and good horizontal stability. In addition, compared with the existing construction scheme, the invention reduces the construction process, has a high degree of mechanization of construction equipment, and greatly shortens the construction period compared with the traditional cave treatment method. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 A schematic cross-sectional view of a casing device for use in cast-in-place pile construction in a karst cave according to an embodiment of the present invention;

[0028] Figure 2 A bottom view of the upper casing of the casing device used for the construction of cast-in-place piles in a karst cave according to an embodiment of the present invention;

[0029] Figure 3 A schematic cross-sectional view of a lower casing of a casing device used for cast-in-place pile construction in a karst cave according to an embodiment of the present invention;

[0030] Figure 4 Schematic diagram of the partial structure of the lower casing of the casing device used for the construction of cast-in-place piles in a cave according to an embodiment of the present invention (I);

[0031] Figure 5 A schematic diagram of a portion of the structure of a casing device used in the construction of cast-in-place piles in a cave according to an embodiment of the present invention;

[0032] Figure 6 for Figure 5 Schematic diagram of the middle AA section;

[0033] Figure 7 for Figure 5 Schematic diagram of the middle BB section;

[0034] Figure 8 Schematic diagram of the partial structure of the lower casing of the casing device used for the construction of cast-in-place piles in a cave according to an embodiment of the present invention (II);

[0035] Figure 9 A schematic cross-sectional view of a plugging assembly of a casing device for use in cast-in-place pile construction in a karst cave according to an embodiment of the present invention;

[0036] Figure 10 A schematic structural diagram of an elastic rubber membrane in a plugging assembly of a casing device used in cast-in-place pile construction in a karst cave according to an embodiment of the present invention;

[0037] Figure 11 Flow chart of the steps of the construction method of cast-in-place piles in a cave according to an embodiment of the present invention;

[0038] Figure 12 Schematic diagram of the construction method of the cast-in-place pile in the cave according to the embodiment of the present invention (1);

[0039] Figure 13 Schematic diagram of the construction method of the cast-in-place pile in the cave according to the embodiment of the present invention (II);

[0040] Figure 14 Schematic diagram of the construction method of the cast-in-place pile in the cave according to the embodiment of the present invention (3);

[0041] Among them: 10-upper casing, 101-positioning hole, 20-lower casing, 201-inner lining skeleton, 2011-circular steel bar, 2012-long steel bar, 202-wire mesh layer, 203-waterproof cloth layer, 2031-inner waterproof cloth, 2032-outer waterproof cloth, 2033-nail hole, 204-iron wire, 30-sealing component, 301-first ring steel bar, 302 -Second ring reinforcement, 303-Expansion water stop ring, 3031-Water stop strip, 3032-Elastic rubber membrane, 3033-Water permeable hole, 304-Binding belt, 40-Open pile shoe, 501-Bottom surface of the pedestal, 502-Soil layer, 503-Limestone, 504-Concrete, 505-Cave, 601-Expansion drill bit, 602-Drill rod, 603-Concrete-cast steel pipe, 604-Rebar cage. DETAILED DESCRIPTION

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

[0043] A casing device for construction of cast-in-place piles in a karst cave according to an embodiment of the present invention is as follows: Figures 1 to 14 As shown, the casing device includes a straight cylindrical upper casing 10 and a lower casing 20, and a sealing assembly 30 arranged around the outer periphery of the lower casing, wherein: the lower casing 20 includes an inner lining frame 201, a wire mesh layer 202 and a waterproof cloth layer 203 arranged in sequence from the inside to the outside; the inner lining frame 201 and the wire mesh layer 202 are arranged coaxially; an open pile shoe 40 with a matching diameter is fixed to the bottom of the inner lining frame 201; the waterproof cloth layer 203 includes several pieces of waterproof cloth overlapped and covered on the wire mesh layer 202 in sequence, and the waterproof cloth is tied and fixed to the inner lining frame 201; a positioning hole 101 matching the top of the inner lining frame 201 is opened at the bottom of the upper casing 10, and the inner lining frame 201 is inserted into the positioning hole 101; the sealing assembly 30 is arranged near the connection between the upper casing 10 and the inner lining frame 201.

[0044] In this embodiment of the present invention, the upper casing 10, with its positioning holes 101, serves as a temporary retaining wall within the soil layer 502, while the lower casing 20 serves as a permanent retaining wall within the cave 505, blocking water flow and silt. The open pile shoe 40 is welded to the liner frame 201, providing guidance and sealing during the lowering of the casing assembly. The liner frame 201 is inserted into the matching positioning holes 101 at its top, preventing the lower casing 20 from rotating with the drill pipe during drilling by the reamer drill bit 601.

[0045] Specifically, such as Figure 8 As shown, the lining skeleton 201 is a grid structure constructed of a plurality of circular reinforcement bars 2011 arranged horizontally and a plurality of long reinforcement bars 2012 arranged vertically. To ensure more reliable installation of the lower casing 20 and the upper casing 10, the number and position of the positioning holes 101 in this embodiment match the long reinforcement bars 2012 in the lining skeleton 201, and the aperture of the positioning holes 101 is 1mm to 2mm larger than the diameter of the long reinforcement bars 2012, so as to facilitate the insertion or removal of the lining skeleton 201 from the positioning holes 101. More preferably, when the positioning holes 101 are plugged into the long reinforcement bars 2012, the center line of each positioning hole 101 coincides with the center line of the corresponding long reinforcement bar 2012, thereby securing the upper casing 10 on the bottom surface 501 of the support.

[0046] The embodiment of the present invention does not limit the diameter and height of the upper casing 10 and the lower casing 20, and those skilled in the art can choose them at their own discretion. Preferably, in this embodiment, the inner diameters of the upper casing 10, the lower casing 20 and the open pile shoe 40 are the same to ensure that the thickness of the cast-in-place pile concrete protective layer is uniform and meets the design requirements.

[0047] Specifically, such as Figure 1As shown, in an embodiment of the present invention, the sealing assembly 30 includes a first annular steel bar 301, a second annular steel bar 302, an expansion water stop ring 303 and a binding belt 304, wherein: the first annular steel bar 301 and the second annular steel bar 302 are tied in parallel to the outside of the lower casing 20; the expansion water stop ring 303 is fixed between the first annular steel bar 301 and the second annular steel bar 302 by the binding belt 304.

[0048] The sealing assembly 30 on the outer wall of the lower casing 20 is installed at a position close to the movable connection between the upper casing 10 and the lower casing 20. The first annular steel bar 301 and the second annular steel bar 302 are tied to the outside of the waterproof cloth layer 203. The specific sizes of the first annular steel bar 301 and the second annular steel bar 302 are not limited here. In order to meet the construction strength requirements, it is preferred to use steel bars with a diameter of 8mm to 12mm, and the distance between the two steel bars is 20mm to 50mm. An expansion water stop ring 303 with a thickness of 10mm to 15mm is placed between the first annular steel bar 301 and the second annular steel bar 302. The binding belt 304 can be made of iron wire, and the expansion water stop ring 303 is tied and fixed by the iron wire.

[0049] After the plugging assembly 30 is installed, under the action of groundwater, the expansion water stop ring 303 absorbs water and expands, sealing the gap between the borehole wall and the lower casing 20, preventing cement slurry from leaking into the cave 505 from the gap when pouring concrete. Figure 9 and Figure 10 As shown, the expansion waterstop ring 303 of this embodiment of the present invention includes a waterstop strip 3031 and an elastic rubber membrane 3032 covering the waterstop strip 3031. The elastic rubber membrane 3032 is provided with a plurality of evenly spaced water permeable holes 3033. Groundwater enters the waterstop strip 3031 through the water permeable holes 3033, causing the waterstop strip 3031 to absorb water and expand. The elastic rubber membrane 3032 is elastic, so it does not rupture during the expansion of the waterstop strip 3031.

[0050] Specifically, in the waterproof cloth layer 203 of the embodiment of the present invention, the joints of two adjacent waterproof cloths are double-layered, and the inner waterproof cloth 2031 is provided with nail holes 2033, and is fastened to the inner lining frame 201 through the nail holes 2033 using iron wire. The outer waterproof cloth 2032 covers the nail holes 2033 of the inner waterproof cloth 2031 and is fixed by an outer iron wire hoop. Figure 3 As shown, the outer part of the waterproof cloth layer 203 is fixed with an iron wire 204. Figure 8As shown, the upper and lower inner layers of waterproof cloth 2031 are fixed to the inner lining frame 201 by wire through nail holes 2033. Since the nail holes 2033 are provided on the upper and lower edges of the inner layers of waterproof cloth 2031, the nail holes 2033 are covered by the outer layer of waterproof cloth 2032. Preferably, the waterproof cloth joints are double-layered within a width range of 300mm to 500mm. After the outer layer of waterproof cloth 2032 covers the nail holes 2033, it is then fixed with wire 204.

[0051] The present invention also includes an embodiment of a method for constructing a bored pile in a cave, the construction method is implemented based on the casing device of the above embodiment, such as Figures 1 to 14 As shown, the steps include:

[0052] Step S1: Pile hole protection construction: Arrange the casing device according to the positional relationship that the upper casing 10 is located in the shallow soil layer 502 and the lower casing 20 is located in the cave 505. After applying butter in the positioning hole 101, it is plugged into the lining skeleton 201, and the plugging position is located 1.0m to 2.0m below the soil-rock interface; the drilling rig includes a reaming drill bit 601 and a drill rod 602 with spiral blades. The drilling rig is installed and moved to the designed pile position, and the open pile shoe 40 is welded to the bottom of the lining skeleton 201 as a whole, and the open pile shoe 40 is located at the front end of the reaming drill bit 601.

[0053] The structures of the upper casing 10 and the lower casing 20 in this embodiment can refer to the aforementioned embodiments of the casing device, and will not be described in detail here.

[0054] Step S2: Pile hole drilling: Align the cone tip of the reaming drill bit 601 with the center of the pile and drill the hole in a forward direction, so that the lower casing 20 and the upper casing 10 sink synchronously, and at the same time clean the debris brought out of the hole by the spiral blades. When the drill rod 602, the lower casing 20 or the upper casing 10 sinks to a height of 0.8m to 1.2m from the ground, the drill rod 602, the lower casing 20 and the upper casing 10 are correspondingly extended until the reaming drill bit 601 passes through the cave 505 and enters the intact rock formation. Figure 12 As shown, when the reaming drill bit 601 reaches the complete rock formation, the positions of the upper casing 10 and the lower casing 20 are distributed as shown in the figure.

[0055] In this embodiment, the diameter of the reaming drill bit 601 is set to be 10mm to 15mm larger than the outer diameter of the open pile shoe 40, the upper casing 10 and the lower casing 20, thereby forming a soil cutting cavity and ensuring that the upper casing 10 and the lower casing 20 sink synchronously with the reaming drill bit 601.

[0056] When cleaning the debris brought out of the hole by the spiral blade, it is preferred to use a special drill bucket with a double-expanded wing scraper for cleaning.

[0057] Step S3: pouring concrete: clean the pile bottom sediment, lower the steel cage 604, insert the pouring concrete steel pipe 603, and pour concrete by underwater pouring under the protection of the upper casing 10 and the lower casing 20 connected in an upper and lower manner.

[0058] Before pouring concrete, Figure 5 As shown, the positional relationship between the steel cage 604, the lower casing 20 and the open pile shoe 40 after being lowered, Figure 6 and Figure 7 They are schematic diagrams of corresponding cross sections, and the steel cage 604 and the lower casing 20 are arranged coaxially. Figure 13 The diagram shown is a schematic diagram after pouring concrete.

[0059] Step S4: Cast-in-place pile formation: When the concrete pouring height reaches 1.0m to 1.2m above the joint of the upper casing and the lower casing, pour concrete 504 while pulling up the upper casing 10 until the upper casing 10 is completely pulled out of the ground to form a cast-in-place pile in the cave. Figure 14 After the cast-in-place pile is completed, the upper casing 10 is removed, while the lower casing 20 and the steel cage 604 remain in place, forming a cast-in-place pile with high strength, anti-buckling and good horizontal stability.

[0060] Specifically, when executing step S2, the drill rod 602, the lower casing 20 and the upper casing 10 are correspondingly extended, including connecting the upper casing 10 by means of threads or bolts when extending, connecting the lining frame 201 in the lower casing 20 by welding when extending, and overlapping the wire mesh layer 202 in the lower casing 20 by tying with wires.

[0061] Compressive tests conducted on cast-in-place piles obtained using a 2,000-ton indoor press revealed that cast-in-place piles with waterproof fabric layers exhibited a 20% to 50% increase in vertical compressive bearing capacity compared to cast-in-place piles without waterproof fabric layers. Even if large caves are not filled with soil, there is no need to use sand, gravel, or concrete for filling or grouting, significantly saving on material, machinery, and labor costs for cave treatment, reducing cave treatment costs by approximately 600 to 1,200 yuan per cubic meter. 3 The construction method of the present invention reduces the number of construction steps and utilizes a high degree of mechanization in the construction equipment. Compared to traditional cave treatment methods, the construction period can be shortened by more than 20%. Furthermore, for cast-in-place piles exceeding several tens of meters in depth, the casing device of the present invention can reduce the weight by at least 40% to 60% compared to steel casings.

[0062] The casing device and bored pile construction method used in the construction of bored piles in caves of the embodiment of the present invention are characterized by the casing device being synchronously followed by the drilling of the reaming drill bit, and no mud wall protection is required for hole formation, thereby achieving the purpose of wall protection throughout the entire process. The upper casing in the soil layer can be recycled, and the lower casing and the steel cage retained in the cave are cast into piles at one time. Even if there is no filling soil in a tall cave, there is no need to use sand, gravel, concrete materials for filling or grouting, which completely saves the material cost, machinery cost and labor cost of cave treatment. The formed bored pile can fully meet the construction requirements of steel casings of the same size, and the lining skeleton of the lower casing and the steel cage form a double-layer longitudinal load-bearing structure, and the wire mesh layer and the waterproof cloth layer provide circumferential constraints on the concrete protective layer of the bored pile, thereby ensuring the vertical compressive bearing capacity of the bored pile, and forming a bored pile with high strength, anti-buckling and good horizontal stability. In addition, compared with the existing construction scheme, this embodiment reduces the construction process, the construction equipment has a high degree of mechanization, and the construction period is greatly shortened compared with the traditional cave treatment method.

[0063] The present invention is further described above with the aid of specific embodiments. However, it should be understood that the specific description herein should not be construed as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in this field after reading this specification are all within the scope of protection of the present invention.

Claims

1. A casing device for bored pile construction in a cave, characterized in that: The casing device includes a straight upper casing and a lower casing, and a plugging assembly arranged around the outer periphery of the lower casing, wherein: The lower casing comprises an inner lining frame, a steel mesh layer and a waterproof cloth layer, which are arranged in sequence from the inside out; the inner lining frame and the steel mesh layer are arranged coaxially; an open pile shoe with a matching diameter is fixed to the bottom of the inner lining frame; the waterproof cloth layer comprises a plurality of pieces of waterproof cloth which are overlapped and covered on the steel mesh layer in sequence, and the waterproof cloth is tied and fixed to the inner lining frame; A positioning hole matching the top of the lining frame is provided at the bottom of the upper casing, and the lining frame is inserted into the positioning hole; The plugging assembly is arranged near the connection between the upper casing and the liner frame; The plugging assembly includes a first annular steel bar, a second annular steel bar, an expansion water stop ring and a binding belt, wherein: The first annular steel bar and the second annular steel bar are tied parallel to the outside of the lower casing; The expansion water stop ring is fixed between the first annular steel bar and the second annular steel bar by the binding belt; The expansion water stop ring includes a water stop strip and an elastic rubber membrane wrapped around the water stop strip, and the elastic rubber membrane is provided with a plurality of water permeable holes that are evenly arranged.

2. A casing device for bored pile construction in a cave as claimed in claim 1, characterized in that: The inner lining skeleton is a grid structure constructed by a plurality of circular steel bars arranged transversely and a plurality of long steel bars arranged longitudinally.

3. A casing device for bored pile construction in a cave as claimed in claim 2, characterized in that: The number and position of the positioning holes match the long steel bars in the lining frame, and the aperture of the positioning holes is 1 mm to 2 mm larger than the diameter of the long steel bars. When the positioning holes are plugged into the long steel bars, the center line of each positioning hole coincides with the center line of the corresponding long steel bar.

4. A casing device for construction of bored piles in a cave as claimed in claim 1, characterized in that: In the waterproof cloth layer, the joints of two adjacent waterproof cloths are double-layered, and the inner layer of the waterproof cloth is provided with nail holes, and is fixed to the inner lining frame by iron wire through the nail holes. The outer layer of the waterproof cloth covers the nail holes of the inner layer of the waterproof cloth and is fixed by an outer iron wire hoop.

5. A casing device for bored pile construction in a cave according to any one of claims 1 to 4, characterized in that: The inner diameters of the upper casing, the lower casing and the open pile shoe are the same.

6. A method for constructing cast-in-place piles in a cave, characterized in that: The construction method is implemented based on the casing device according to any one of claims 1 to 5, comprising the steps of: S1: Pile hole protection construction: The casing device is arranged according to the positional relationship that the upper casing is located in the shallow soil layer and the lower casing is located in the karst cave. After applying butter in the positioning hole, it is plugged into the liner frame, and the plugging position is located 1.0m to 2.0m below the soil-rock interface. The drilling rig includes a reaming drill bit and a drill rod with spiral blades. The drilling rig is installed and moved to the designed pile position. The open pile shoe is welded to the bottom of the liner frame as a whole, and the open pile shoe is located at the front end of the reaming drill bit. S2: Pile hole drilling: Align the cone tip of the reaming drill bit with the center of the pile, and drill the hole in a forward direction, so that the lower casing and the upper casing sink synchronously, and at the same time clean the debris brought out of the hole by the spiral blades. When the drill rod, the lower casing or the upper casing sinks to a height of 0.8m to 1.2m from the ground, the drill rod, the lower casing and the upper casing are correspondingly extended until the reaming drill bit passes through the cave and enters the intact rock formation; S3: Concrete pouring: clean the sediment at the bottom of the pile, lower the steel cage, insert the concrete pouring steel pipe, and pour the concrete using underwater pouring under the protection of the upper and lower casings; S4: Cast-in-place pile formation: When the concrete pouring height reaches 1.0m~1.2m or more at the joint of the upper casing and the lower casing, the upper casing is pulled up while pouring concrete until it is completely pulled out of the ground to form a cast-in-place pile in the cave.

7. A method for constructing a cast-in-place pile in a cave as claimed in claim 6, characterized in that: The diameter of the reaming drill bit is 10mm to 15mm larger than the outer diameters of the opening pile shoe, the upper casing and the lower casing, forming a soil cutting cavity.

8. The method for constructing a cast-in-place pile in a cave according to claim 6, wherein: The drill pipe, lower casing and upper casing are extended accordingly, including connecting the upper casing by means of threads or bolts when extending the upper casing, connecting the lining skeleton in the lower casing by means of welding when extending the inner casing, and overlapping the wire mesh layer in the lower casing by means of wire tying.

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