Karst area pile foundation construction method and operation platform

Abstract

The invention discloses a karst area pile foundation construction method and an operation platform, and relates to the field of pile foundation construction.The karst area pile foundation construction method comprises the steps that a steel sleeve is drilled into bed rock through a full-rotation casing drilling machine; a steel casing is prefabricated on the ground, a first water-resisting layer and a second water-resisting layer are laid on the inner wall and the outer wall of the steel casing respectively, and a steel sling is connected to the lower portion of the steel casing; hoisting the composite pile casing into the steel sleeve, and filling broken stones between the steel pile casing and the second water-resisting layer to form a broken stone layer; after the steel sleeve is pulled out, a reinforcement cage is lowered, and pile body concrete is poured; and after initial setting of the concrete, the steel casing is pulled out through cooperation of the steel sling and lifting equipment, and finally grouting is conducted between the two water-resisting layers. The operating platform comprises a bottom frame, an upper frame, a hydraulic lifting mechanism, a slewing mechanism and a prestress support, and the top of the steel casing is pulled out. Recycling of the steel casing is achieved, the construction cost is reduced, meanwhile, a multiple permanent protection system is formed on the periphery of the pile body, and the construction quality and durability of the pile foundation in the karst area are improved.

Description

Technical Field

[0001] This invention relates to the field of pile foundation engineering construction. More specifically, this invention relates to a pile foundation construction method and operating platform in karst areas. Background Technology

[0002] Karst areas are characterized by a wide variety of geological features, including caves, fissures, and underground rivers, resulting in uneven geological structures and poor stability. When drilling and grouting piles in such areas, the drilling mud used in conventional drilling processes can instantly leak away along fissures, causing the borehole wall to lose support and increasing the risk of borehole collapse and drill bit burial. Furthermore, the presence of cavities in the caves makes it extremely easy for over-pouring and leakage of concrete during the pouring process, potentially leading to quality defects such as mud inclusions in the pile body, pile breakage, and hollowing at the pile bottom, severely impacting the bearing capacity and durability of the pile foundation.

[0003] To ensure pile quality, the project generally adopts the full casing follow-up process, which uses a full-rotation casing drilling rig to drive the steel casing to rotate and drill, allowing the steel casing to penetrate the karst cave development area and embed itself into the underlying stable bedrock. The rigid protective wall of the steel casing isolates unfavorable strata, providing a safe working space for subsequent construction. In the full casing follow-up process, in addition to the conventional single casing process, there is also a construction method using double casing (double casing). Compared to the single casing process, the double casing method has certain advantages when dealing with fracture clusters, fluid-like filling materials, or interconnected bead-like karst cave clusters: the outer steel casing undertakes the drilling and rigid wall protection functions and is then retrieved, while the inner steel casing provides a closed forming space for concrete pouring, preventing fluid-like filling materials from flowing into the hole and preventing excessive concrete loss. For example, Chinese patent CN114673188A discloses a method for constructing pile foundations in karst areas, which involves drilling with an outer casing, installing an inner casing inside the outer casing, pouring concrete, and then pulling out the outer casing after the concrete has set, while the inner casing remains permanently inside the pile hole. Similarly, Chinese patent CN113445498A discloses a construction process for bored pile foundations in complex karst geological conditions, whose double-steel casing full-casing follow-up method also employs the process of drilling with an outer steel casing, installing an inner steel casing, pouring concrete, and then pulling out the outer steel casing.

[0004] In practice, other similar double-casing (double-casing) construction techniques are also used. The specific construction process is as follows: First, a full-rotation casing drilling rig is used to rotate and press the outer steel casing into the designed depth, while simultaneously removing the soil inside the casing. Then, a permanent steel casing with an inner diameter matching the designed pile diameter is placed inside the outer steel casing, and crushed stone is filled into the circumferential gap between the permanent steel casing and the outer steel casing. After filling, the outer steel casing is pulled out, leaving the crushed stone layer surrounding the permanent steel casing. Finally, a reinforcing cage is placed inside the permanent steel casing, and the pile body concrete is poured. This method, through the combined action of the permanent steel casing and the crushed stone layer, effectively isolates unfavorable strata such as karst caves, prevents concrete loss, and ensures the quality of pile formation.

[0005] However, all the aforementioned existing technical solutions share a common problem: whether it's the inner steel casing in the double-casing method or the permanent steel casing in the "steel casing + permanent steel casing" method, the inner steel casing that is in direct contact with the concrete is not removed after the concrete is poured; it is simply buried underground as a permanent structure. This is because pile foundations in karst areas need to penetrate karst caves and embed themselves into bedrock, and the piles are generally deep and long. The contact area between the steel casing and the concrete is larger than that of cast-in-place piles using conventional steel casings, and an adhesive bond has already formed between the two after the concrete has initially set. If only a top-mounted extraction device is used after the concrete has set, not only will the adhesive resistance easily lead to extraction failure, but the extraction force will also be transmitted to the pile body through the adhesive interface, causing irreversible damage to the already formed concrete. This is the direct reason why CN114673188A, CN113445498A, and the "steel casing + permanent steel casing" method all permanently leave the inner steel casing and do not remove it. The material cost of a single pile's steel casing can be as high as 30,000 to 80,000 yuan, which can significantly increase the engineering cost for large bridges or building complexes.

[0006] Therefore, how to ensure the quality of pile foundation construction and effectively cope with adverse geological conditions such as karst caves in karst areas, while also realizing the recycling of steel casings and reducing engineering costs, has become a pressing technical problem in this field. This invention addresses these problems by proposing a pile foundation construction method and operating platform for karst areas. Summary of the Invention

[0007] One objective of this invention is to provide a method and operating platform for pile foundation construction in karst areas. The aim is to achieve the recycling of steel casings while ensuring the integrity of the pile body through a composite structure design of recyclable steel casings and permanent waterproof layers, combined with a coordinated extraction process of steel slings and lifting equipment. At the same time, it forms a multi-layered permanent protection system around the pile body, achieving a balance of reliable quality, controllable cost, and high construction efficiency.

[0008] To achieve these and other advantages according to the invention, according to one aspect of the invention, the invention provides a method for pile foundation construction in karst areas, comprising the following steps: S1: Drive the full-rotation casing drilling rig to rotate the steel casing to cut the karst strata and drill downwards until the bedrock. During the drilling process, the soil and broken rock inside the steel casing are removed simultaneously. S2. A steel casing is prefabricated on the ground. A first waterproof layer is laid on the inner wall of the steel casing, and a second waterproof layer is laid on the outside of the steel casing. The bottom of the first and second waterproof layers are connected. The steel casing, the first waterproof layer, and the second waterproof layer form a composite casing structure. One end of a steel sling is connected to the outer wall at the lower part of the steel casing, and the other end of the steel sling extends from between the steel casing and the second waterproof layer. S3. The composite casing structure is hoisted as a whole and placed centrally inside the steel sleeve; S4. Fill the circumferential gap between the steel casing and the second waterproof layer with graded crushed stone to form a crushed stone layer; S5. After the crushed stone layer is filled and compacted, start the full-rotation casing drilling rig to rotate and pull out the steel sleeve; S6. Place the pile reinforcement cage in the space enclosed by the first waterproof layer and pour pile body concrete into the space. S7. When the pile concrete reaches the initial setting state and has not yet undergone volume expansion due to hydration heat, the steel casing is pulled out of the hole by the steel sling and the lifting equipment working together. The first waterproof layer, the second waterproof layer and the crushed stone layer between them are all left in the hole. S8. Cement grout is injected into the space between the first waterproof layer and the second waterproof layer, and the cement grout is solidified with the crushed stone layer to form a crushed stone concrete composite layer.

[0009] Preferably, both the first waterproof layer and the second waterproof layer are made of PVC polymer waterproof membrane. The PVC base layer of the first waterproof layer contacts the inner wall of the steel casing, and the anti-seepage crystalline layer of the first waterproof layer faces the inner side of the pile hole for anchoring with the pile concrete. The PVC base layer of the second waterproof layer faces the inner wall of the steel casing, and the anti-seepage crystalline layer of the second waterproof layer faces inward for anchoring with the crushed stone concrete composite layer.

[0010] Preferably, the bottom of the steel casing is provided with a base, the base including an inner cylinder, a bottom plate and several insert plates; the outer diameter of the inner cylinder is smaller than the inner diameter of the steel casing, the bottom of the steel casing is provided with several vertical insertion holes, the insert plates are horizontally protruding from the outer wall of the inner cylinder, and the insert plates are inserted into the insertion holes one by one; the bottom plate is annular, it is horizontally protruding from the bottom outer edge of the inner cylinder and extends to the outside of the steel casing, several positioning posts are vertically provided on the bottom plate, and all positioning posts are spaced apart on the outside of the steel casing; a protective sleeve is fitted around the positioning post, the bottom end of the protective sleeve is connected to the bottom plate, and the length of the protective sleeve is equal to the length of the second waterproof layer, the second waterproof layer is wrapped around the outside of all the protective sleeves.

[0011] Preferably, the sheath and the second waterproof layer are fixedly connected by a plurality of hot melt gaskets, one side of the hot melt gasket is fixedly connected to the inner wall of the second waterproof layer, and the other side of the hot melt gasket is fixedly connected to the outer wall of the sheath.

[0012] Preferably, the lower part of the steel casing is provided with a plurality of sling supports, the sling supports are adapted to the inner wall of the casing and can slide vertically within the casing, the casing has a through groove along its length, the through groove penetrates the side wall of the casing; the insert plate is disposed between the inner cylinder and the positioning post and passes through the through groove, when the casing is fitted onto the positioning post, the opening of the through groove contacts the outer wall of the steel casing; the connection part of the sling support and the steel casing passes through the through groove, the sling support is connected to the bottom end of the steel sling, and the cable of the steel sling is threaded inside the casing.

[0013] Preferably, the bottom of the first waterproof layer is disposed within the annular gap between the steel casing and the inner cylinder; the bottom of the second waterproof layer bypasses the lower edge of the bottom plate and folds upward to form an overlapping seal with the first waterproof layer.

[0014] Preferably, the crushed stone layer is filled with pebbles, which are continuously graded with a particle size of 5mm to 25mm. Among them, the mass percentage of particles with a particle size of 5mm to 10mm is 25% to 35%, the mass percentage of particles with a particle size of 10mm to 20mm is 40% to 55%, and the mass percentage of particles with a particle size of 20mm to 25mm is 10% to 20%. The pebble particles are round or sub-round, with a needle-like or flaky particle content of no more than 5%, a mud content of no more than 1%, and a mud lump content of no more than 0.5%.

[0015] Preferably, the steel slings are configured to meet the following requirements: in, n For the number of steel slings, The allowable tensile force of a single steel cable G For the self-weight of the steel casing, For the upward pull-out force of the full-rotation casing drilling rig on the steel casing, The total friction force inside the steel casing, The total friction force on the outside of the steel casing, The coefficient of friction between the steel casing and the first waterproof layer. The coefficient of friction between the steel casing and the gravel layer, The effective lateral pressure of concrete at initial setting, D For the diameter of the steel casing, h For pile length, a The pressure transmission reduction factor for the gravel layer is 0.2 to 0.5. The static lateral pressure coefficient of the gravel layer, It is a heavily gravelly layer.

[0016] Secondly, the present invention provides an operating platform for the above-mentioned pile foundation construction method in karst areas, comprising: The base frame is placed on the ground; The upper frame is positioned above the lower frame; A hydraulic lifting mechanism is connected between the bottom frame and the upper frame, and is used to drive the upper frame to rise and fall relative to the bottom frame; A rotary mechanism, mounted on the upper frame, is used to clamp the outer wall of the steel sleeve or steel protective sleeve and can drive the clamped steel sleeve or steel protective sleeve to rotate. A prestressed support is installed on the upper frame to fix the upper end of the steel cable and tighten the steel cable.

[0017] Preferably, it also includes a temporary fixing device, which is set on the bottom frame or the ground, for temporarily fixing the steel casing when the slewing mechanism releases the steel casing and the prestressed support releases the steel cable.

[0018] The present invention has at least the following beneficial effects: First, this invention provides a method for pile foundation construction in karst areas that enables the recycling of steel casings. By connecting steel slings to the lower outer wall of the steel casing, and then using the steel slings and lifting equipment in coordination after the initial setting of the pile concrete and before its secondary expansion, the steel casing can be completely recycled and reused. Compared with the existing practice of burying the steel casing underground as a permanent structure, this invention effectively avoids the waste of steel casing material for each pile foundation. Especially for large bridges or building complexes with a large number of pile foundations, its cost-saving effect is significant.

[0019] Secondly, this invention forms a crushed stone concrete composite protective layer on the periphery of the pile body, which is difficult to achieve using conventional construction methods. By filling the space between the steel casing and the second waterproof layer with graded pebbles to form a crushed stone layer, and after the steel casing is pulled out, cement grout is injected into the space between the first waterproof layer and the second waterproof layer, so that the cement grout and the crushed stone layer are solidified into one, thereby forming a dense crushed stone concrete composite layer between the second waterproof layer and the first waterproof layer. The pebbles and cement grout used are inexpensive and have good economic benefits.

[0020] Third, this invention constructs a multi-layered protection system around the pile body, which improves the durability and safety of pile foundations in karst areas. The first waterproof layer provides the first waterproof barrier for the pile body; the second waterproof layer is built around the crushed stone layer to form the second waterproof barrier, which to a certain extent isolates the erosion of the pile body by groundwater and karst caves in karst areas, and at the same time avoids the loss of grout during the concrete pouring process, thus solving the quality risks of pile foundation construction in karst areas to a certain extent.

[0021] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0022] Figure 1 This is a cross-sectional schematic diagram of one technical solution of the present invention; Figure 2 This is a cross-sectional schematic diagram of one technical solution of the present invention; Figure 3 This is a schematic diagram of the composite casing structure in one technical solution of the present invention; Figure 4 This is a schematic diagram of the installation of the base and the protective sleeve in one technical solution of the present invention; Figure 5 This is a schematic diagram of the base in one technical solution of the present invention; Figure 6 This is a schematic diagram of the steel sling and sheath in one technical solution of the present invention; Figure 7 This is a schematic diagram of the overlapping of the waterproof layer in one technical solution of the present invention; Figure 8 This is a schematic diagram of the construction platform before the steel casing is pulled out in one technical solution of the present invention; Figure 9 This is a schematic diagram of the construction platform during the steel casing extraction process in one of the technical solutions of the present invention.

[0023] Attached reference numerals: 1-karst stratum, 10-bedrock, 2-steel sleeve, 3-steel casing, 30-insertion, 31-pile reinforcement cage, 310-limiting component, 4-first waterproof layer, 41-second waterproof layer, 5-gravel layer, 6-sheath, 61-through groove, 62-hot melt gasket, 7-base, 71-inner cylinder, 72-bottom plate, 73-insertion plate, 74-positioning column, 75-lifting lug, 8-steel sling, 81-sling support, 9-construction platform, 91-bottom frame, 92-upper frame, 93-hydraulic lifting mechanism, 94-slewing mechanism, 95-prestressed support, 96-guide wheel structure. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can implement it based on the description.

[0025] It should be understood that terms such as “having,” “comprising,” and “including” as used herein do not exclude the presence or addition of one or more other elements or combinations thereof.

[0026] It should be noted that, unless otherwise specified, the experimental methods described in the following embodiments are conventional methods, and the structures and components described are commercially available. In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "setting" should be interpreted broadly. For example, they can refer to fixed connection or setting, detachable connection or setting, or integral connection or setting. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances. The terms "lateral," "longitudinal," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention and simplifying the description. They do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention.

[0027] like Figures 1-9 As shown, the present invention provides a method for pile foundation construction in karst areas, comprising the following steps: S1: Drive the full-rotation casing drilling rig to rotate the steel sleeve 2 to cut the karst strata 1 and drill downwards until the bedrock 10. During the drilling process, the soil and broken rock inside the steel sleeve 2 are removed simultaneously.

[0028] Specifically, the full-rotation casing drilling rig is a specialized pile driving machine that integrates rotation and pressure extraction. Its rotation mechanism drives the steel casing 2 to rotate 360° reciprocatingly, while the pressure extraction cylinder applies downward pressure, causing the steel casing 2 to drill downwards in a rotary cutting manner. The steel casing 2 is made of high-strength steel plate rolled and welded, with a diameter larger than the designed pile diameter, typically 10-20 cm larger, and a wall thickness of 16-25 mm selected according to geological conditions and drilling depth. During drilling, a grab bucket or rotary drilling rig is used to continuously remove soil from inside the steel casing 2, keeping the soil surface inside the steel casing 2 always a certain distance below the bottom opening of the casing to reduce drilling resistance. After the steel casing 2 passes through the karst cave development area of ​​the karst stratum 1, drilling continues until it is embedded into the lower stable bedrock 10 to a certain depth, ensuring that the bottom opening of the casing enters the intact bedrock, forming a stable rigid retaining wall.

[0029] S2. A steel casing 3 is prefabricated on the ground. A first waterproof layer 4 is laid on the inner wall of the steel casing 3, and a second waterproof layer 41 is laid on the outside of the steel casing 3. The bottom of the first waterproof layer 4 and the second waterproof layer 41 are connected. The steel casing 3, the first waterproof layer 4, and the second waterproof layer 41 form a composite casing structure. One end of a steel sling 8 is connected to the outer wall at the lower part of the steel casing 3, and the other end of the steel sling 8 extends from between the steel casing 3 and the second waterproof layer 41.

[0030] Specifically, both the first waterproof layer 4 and the second waterproof layer 41 can be made of polymer waterproof membrane. This type of waterproof membrane has the characteristics of good flexibility, high tensile strength, and corrosion resistance. One side of the polymer waterproof membrane is the base layer, and the other side is the anti-seepage crystalline layer. When laying the first waterproof layer 4, several temporary fixing points are set on the inner wall of the steel casing 3. For example, the base layer of the first waterproof layer 4 is partially fixed to the inner wall of the steel casing 3 by spot hot-melt welding or applying a peelable adhesive. The number and distribution of fixing points only need to ensure that the first waterproof layer 4 does not slip or wrinkle significantly under its own weight and during subsequent hoisting and filling operations, and full adhesion is not required. The purpose of this is to maintain a weak connection between the first waterproof layer 4 and the inner wall of the steel casing 3. When the steel casing 3 is pulled out later, the first waterproof layer 4 can be easily separated from the steel casing 3, avoiding the obstruction of extraction or damage to the newly set pile foundation concrete due to large-area adhesion. The second waterproof layer 41 is located outside the steel casing 3. The connection between the second waterproof layer 41 and the steel casing 3 can be maintained by a simple support. The diameter of the cylinder formed by the second waterproof layer 41 is compatible with the inner diameter of the steel sleeve 2. The steel sling 8 uses high-strength, low-relaxation steel strand, and its specifications and number are determined based on the pile diameter, pile length, and extraction resistance. Sling supports are pre-welded or bolted to the lower outer wall of the steel casing 3. One end of the steel sling 8 is firmly fixed to the support, and the other end of the steel sling 8 extends upwards along the gap between the outer wall of the steel casing 3 and the second waterproof layer 41, extending from the top of the steel casing 3 to provide a traction point for subsequent coordinated extraction.

[0031] S3. The composite casing structure is hoisted as a whole and placed centrally within the steel sleeve 2. Specifically, a special lifting device is set up on the top of the steel casing 3 before hoisting to ensure that the composite casing structure does not deform or get damaged during the hoisting process. After the composite casing structure is vertically hoisted, it is slowly lowered into the already drilled steel sleeve 2. During the lowering process, the position of the composite casing structure is controlled by measuring or guiding devices to keep it centered within the steel sleeve 2, that is, the axis of the steel casing 3 is basically coincident with the axis of the steel sleeve 2, ensuring that the circumferential gap of the subsequent crushed stone filling is uniform. When the composite casing structure is lowered to the bottom of the hole, its bottom contacts the bedrock 10, completing the central placement.

[0032] S4. Graded crushed stone is filled into the circumferential gap between the steel casing 3 and the second waterproof layer 41 to form a crushed stone layer 5. Specifically, crushed stone is filled into the circumferential gap between the steel casing 3 and the second waterproof layer 41 using a crushed stone filling device. The filling material is pebbles, preferably round or sub-round in shape, with strict control over the mud content and the content of needle-like and flaky particles. Layered filling and compaction are used during filling. A vibrator or small vibrator can be used to compact the crushed stone, ensuring that the crushed stone layer 5 is filled evenly and the density meets the requirements. The thickness of the crushed stone layer 5 is the width of the circumferential gap between the steel casing 3 and the second waterproof layer 41, typically 10–20 cm. During the filling process, care should be taken to protect the second waterproof layer 41 from being scratched by sharp crushed stone, while ensuring that the exit channel of the steel sling 8 remains unobstructed. The crushed stone layer 5 plays multiple roles during construction: on the one hand, the crushed stone layer 5 fills the space between the steel casing 3 and the second waterproof layer 41, using the granular characteristics of the stone particles to form a radial constraint on the steel casing 3, preventing the steel casing 3 from shifting during subsequent operations and playing a positioning role; on the other hand, when the outer steel sleeve 2 is pulled out, the crushed stone layer 5 adheres tightly to the inner side of the second waterproof layer 41, and with its own weight and the interlocking effect between the particles, it can effectively prevent the local collapse of the karst strata of the borehole wall that may occur. Furthermore, when encountering cavities or fissures, because the second waterproof layer 41 completely encloses the crushed stone layer 5, the crushed stone layer 5 will not collapse and flow into the cavity area, thereby maintaining the stability of the outer structure of the pile hole.

[0033] S5. After the gravel layer 5 is filled and compacted, start the full-rotation casing drilling rig to rotate and pull out the steel sleeve 2. During operation, the rig's rotation mechanism can be selected to statically pull or to rotate back and forth with a small amplitude. At the same time, the pressure-pulling cylinder is activated to apply upward pulling force. To reduce disturbance to the surrounding strata and the already filled gravel layer, the pulling process is carried out slowly and uniformly, controlling the pulling speed at 0.3 to 0.5 m / min, so that the steel sleeve 2 rises smoothly. The steel sleeve 2 can rise smoothly under continuous pulling force until it is completely pulled out of the hole. After the steel sleeve 2 is pulled out, the gravel layer 5 remains in place under its own weight and the constraint of the surrounding strata, forming a gravel protective wall wrapped inside the second water-proof layer 41, while the composite casing structure remains intact inside the hole.

[0034] S6. Lower the pile reinforcement cage 31 into the space enclosed by the first waterproof layer 4, and pour pile body concrete into the space. The pile reinforcement cage 31 is fabricated on the ground according to the design drawings, and its outer diameter is smaller than the inner diameter of the first waterproof layer 4. To ensure a uniform protective layer thickness between the reinforcement cage and the first waterproof layer 4 during the lowering process, and to avoid the reinforcement cage directly scraping or damaging the first waterproof layer 4, wheel-type limiting members 310 are installed circumferentially on the main bars or stirrups of the pile reinforcement cage 31. The wheel-type limiting members 310 are made of nylon or rubber, and their outer edges protrude from the surface of the reinforcement cage and form rolling contact with the inner wall of the first waterproof layer 4, thereby ensuring centered positioning and reducing frictional resistance when the reinforcement cage is lowered. A crane is used to lift the pile reinforcement cage 31 into the space enclosed by the first waterproof layer 4 in sections, and adjacent sections are connected by straight threaded sleeves or welding. The concrete slump is controlled between 180 and 220 mm. During the pouring process, the bottom of the guide pipe is always buried at a certain depth below the concrete surface (usually 2 to 6 m) to ensure that the concrete continuously and densely fills the space enclosed by the first waterproof layer 4.

[0035] S7. When the concrete of the pile body reaches the initial setting state and before volume expansion due to heat of hydration occurs, the steel casing 3 is pulled out of the hole through the coordinated action of the steel sling 8 and the lifting equipment. The first water-proof layer 4, the second water-proof layer 41, and the crushed stone layer 5 located between them are all left in the hole. The initial setting time is determined according to the concrete mix ratio and the ambient temperature, usually 5 to 8 hours after pouring. When the concrete reaches the initial setting state (which can be measured by a penetration resistance meter, or judged by the fact that pressing the surface with a finger leaves a slight fingerprint but no grout sticks), and before significant secondary expansion due to heat of hydration occurs, the pulling operation is started. The lifting equipment can be the pressure pulling system of a full-rotation casing drilling rig, hydraulic jacks, or a large crane, in coordination with the steel sling 8. During operation, firstly, the lifting equipment applies an upward first pulling force to the top of the steel casing 3, and simultaneously the steel sling 8 applies an upward second pulling force to the bottom of the steel casing 3, forming a coordinated pulling from top to bottom. Since the first waterproof layer 4 is only connected to the inner wall of the steel casing 3 through temporary fixing points and no large-area bonding has been formed, these fixing points are destroyed in the early stage of pipe pulling, and the first waterproof layer 4 is completely separated from the steel casing 3. After the steel casing 3 is completely pulled out, the first waterproof layer 4 is tightly attached to the surface of the pile body, while the second waterproof layer 41 is kept in place by the support of the outer gravel layer 5, and the gravel layer 5 sandwiched between the two is also completely left in the hole.

[0036] S8. Cement grout is injected into the space between the first waterproof layer 4 and the second waterproof layer 41, and the cement grout solidifies with the crushed stone layer 5 to form a crushed stone concrete composite layer. Specifically, grouting is carried out immediately after the steel casing 3 is pulled out. The grouting equipment uses a high-pressure grouting pump to inject cement grout into the space between the first waterproof layer 4 and the second waterproof layer 41 through the grouting channel pre-set in the composite casing structure. The cement grout can be prepared using P.O42.5 ordinary Portland cement, with a water-cement ratio controlled at 0.5 to 0.6. Appropriate amounts of water-reducing agent and expanding agent can be added to improve fluidity and filling effect. The grouting pressure is controlled at 0.5 to 1.0 MPa, so that the grout can penetrate evenly into the pores of the crushed stone layer 5. During the grouting process, the grouting pressure and grout absorption are continuously observed. When the grouting pressure rises steadily and the grout absorption decreases significantly, it indicates that the pores of the crushed stone layer 5 have been fully filled with grout, and grouting can be stopped. After the cement grout has fully set, it forms a dense crushed stone concrete composite layer together with the crushed stone layer 5. This crushed stone concrete composite layer forms a firm anchor with the impermeable crystalline layer of the second waterproof layer 41, while providing reliable lateral support for the first waterproof layer 4, and together with the first waterproof layer 4 and the second waterproof layer 41, it constitutes a multi-layered permanent protection system around the pile body.

[0037] In another technical solution, both the first waterproof layer 4 and the second waterproof layer 41 are made of PVC polymer waterproof membrane. The PVC base layer of the first waterproof layer 4 is attached to the inner wall of the steel casing 3, and the anti-seepage crystalline layer of the first waterproof layer 4 faces the inside of the pile hole to form an anchoring bond with the pile concrete. The PVC base layer of the second waterproof layer 41 faces the inner wall of the steel sleeve 2, and the anti-seepage crystalline layer of the second waterproof layer 41 faces inward to form an anchoring bond with the crushed stone concrete composite layer.

[0038] The PVC polymer waterproof membrane consists of a PVC base layer and an anti-seepage crystalline layer laminated to the surface of the base layer. The PVC base layer provides the membrane with its main strength and flexibility, while the anti-seepage crystalline layer contains active chemical substances that react with cement hydration products in humid environments to form insoluble crystals, thus achieving chemical anchoring with the concrete. The PVC base layer of the first waterproof layer 4 is adhered to the inner wall of the steel casing 3. The adhesion method can be point-like or strip-like hot-melt welding, that is, using a hot air gun to locally heat and melt the PVC base layer and the inner wall of the steel casing 3 and press them together to form several discrete fixing points. The number and distribution of the fixing points only need to ensure that the first waterproof layer 4 can overcome its own weight and not slip during static placement, subsequent hoisting, and filling operations, and it is not required to be fully adhered to the inner wall of the steel casing 3. The purpose of this setting is that when the steel casing 3 is subsequently pulled out, the first waterproof layer 4 can be easily separated from the inner wall of the steel casing 3, avoiding additional pulling resistance caused by large-area adhesion. The first waterproof layer 4 has its anti-seepage crystalline layer facing the inside of the pile hole, that is, towards the subsequently poured pile body concrete. After the pile body concrete is poured, the moisture and cement hydration products in the concrete penetrate to the surface of the anti-seepage crystalline layer, activating the active chemical substances therein, causing them to penetrate into the concrete and generate insoluble crystals, firmly anchoring the first waterproof layer 4 to the pile body concrete, thus forming the first permanent waterproof barrier of the pile body.

[0039] The impermeable crystalline layer of the second waterproof layer 41 faces inward, towards the subsequently formed crushed stone layer 5 and the injected cement grout. When the steel casing 3 is pulled out, and cement grout is injected into the space between the first waterproof layer 4 and the second waterproof layer 41, the cement grout and the crushed stone layer 5 work together. The water and hydration products in the cement grout also activate the impermeable crystalline layer of the second waterproof layer 41, allowing the active substances to penetrate into the crushed stone concrete composite layer and generate crystals. This firmly anchors the second waterproof layer 41 and the crushed stone concrete composite layer together, forming the second permanent waterproof barrier of the pile body. Through the above arrangement, the first waterproof layer 4 and the second waterproof layer 41 form a chemical anchorage with the pile body concrete and the crushed stone concrete composite layer from the inner and outer sides, respectively. This ensures stable adhesion during the service life of the pile foundation and also achieves a separable connection with the recyclable steel casing 3, providing a structural basis for the smooth extraction of the steel casing 3.

[0040] In another technical solution, the bottom of the steel casing 3 is provided with a base 7, the base 7 including an inner cylinder 71, a bottom plate 72 and a plurality of insert plates 73; the outer diameter of the inner cylinder 71 is smaller than the inner diameter of the steel casing 3, the bottom of the steel casing 3 is provided with a plurality of vertical insertion ports 30, the insert plates 73 are horizontally protruding from the outer wall of the inner cylinder 71, and the insert plates 73 and the insertion ports 30 are inserted and fitted one-to-one; the bottom plate 72 is annular, it is horizontally protruding from the bottom outer edge of the inner cylinder 71 and extends to the outside of the steel casing 3, a plurality of positioning posts 74 are vertically provided on the bottom plate 72, and all positioning posts 74 are spaced apart on the outside of the steel casing 3; a protective sleeve 6 is fitted around the positioning posts 74, the bottom end of the protective sleeve 6 is connected to the bottom plate 72, and the length of the protective sleeve 6 is equal to the length of the second waterproof layer 41, the second waterproof layer 41 is wrapped around the outside of all the protective sleeves 6.

[0041] The outer diameter of the base plate 72 matches the inner diameter of the steel sleeve 2, meaning there is only a small gap between the outer edge of the base plate 72 and the inner wall of the steel sleeve 2. This ensures a guiding fit between the base plate 72 and the inner wall of the steel sleeve 2 when the composite casing structure is hoisted into the steel sleeve 2, helping to center the composite casing structure within the steel sleeve 2 and ensuring uniform circumferential spacing of the subsequent crushed stone layer 5. Several lifting lugs 75 are also provided on the upper surface of the base plate 72, evenly distributed around its circumference. During hoisting of the composite casing structure, the lifting hook of the hoisting equipment is connected to the lifting lugs 75 on the base plate 72 via a wire rope, lifting the base 7 together with the protective sleeve 6 fitted onto it. At this time, the steel casing 3 is supported on the base 7 through the insertion fit between the insert plate 73 and the insertion port 30; that is, the weight of the steel casing 3 is transferred to the base plate 72 via the insert plate 73 and borne by the lifting lugs 75. After the composite casing structure is lowered to the bottom of the hole, the base plate 72 contacts the bedrock 10, the base 7 sits on the bottom of the hole, and the steel casing 3 is connected to the base 7 through the cooperation of the insert plate 73 and the socket 30. When the steel casing 3 is subsequently pulled out, it can smoothly detach upwards under the pulling force, allowing the insert plate 73 to slide out of the socket 30, while the base 7, its positioning post 74, and the sheath 6 remain permanently inside the pile hole. Through the above arrangement, the base 7 can achieve the positioning connection between the steel casing 3 and the sheath 6. The base 7, through its guiding cooperation with the steel sleeve 2, ensures the lowering accuracy of the composite casing structure. At the same time, the lifting lug 75 simplifies the lifting process and improves construction efficiency.

[0042] The second waterproof layer 41 is wrapped around the outside of all the sheaths 6, forming a cylindrical structure with the sheaths 6 as the framework. A connection is provided between the second waterproof layer 41 and the sheaths 6, specifically using hot-melt gaskets or adhesive tape to fix the inner wall of the second waterproof layer 41 to the outer wall of the sheaths 6 in a point-like or strip-like manner. This connection provides radial constraint to the second waterproof layer 41, allowing it to withstand the frictional force generated by the relative movement between the inner wall of the steel sleeve 2 and the second waterproof layer 41 during the rotation and extraction of the steel sleeve 2 without displacement. Furthermore, after the steel sleeve 2 is extracted, this connection continues to function, ensuring that the second waterproof layer 41 maintains its preset cylindrical shape even without the support of the steel sleeve 2, thus providing a stable forming space for the subsequent filling and grouting of the crushed stone layer 5.

[0043] After the crushed stone layer 5 fills the circumferential gap between the steel casing 3 and the second waterproof layer 41, the outer side of the second waterproof layer 41 directly contacts the inner wall of the steel sleeve 2. When the steel sleeve 2 is rotated out, the inner wall of the steel sleeve 2 slides relative to the outer side of the second waterproof layer 41. Since the second waterproof layer 41 is fixed to the casing 6 by the connecting measures, and the casing 6 is located inside the crushed stone layer 5, the second waterproof layer 41 can effectively resist the sliding friction force, ensuring that it is not twisted or torn. After the steel sleeve 2 is completely pulled out, the second waterproof layer 41 maintains its cylindrical structure under the joint support of the connecting measures and the casing 6, so that the annular space between the first waterproof layer 4 and the second waterproof layer 41 remains uniform, creating conditions for the subsequent injection of cement grout to form a crushed stone concrete composite layer.

[0044] When the steel casing 3 is pulled out, since the insertion plate 73 and the socket 30 are detachable, the steel casing 3 can be smoothly pulled out upward under the action of the pulling force, so that the insertion plate 73 slides out of the socket 30, while the base 7 and its positioning post 74, the sleeve 6 and the second waterproof layer 41 fixed by the connection measures are permanently left at the bottom of the hole.

[0045] The sheath 6 is made of multiple sections of steel pipe. The length of each section is determined according to the processing, transportation and hoisting capacity, preferably 3m to 5m. The thickness of the sheath 6 is 3 to 5mm, so that there will be no problem of local buckling. A through groove 61 is pre-cut on the side wall of each section of the sheath 6 along its length direction. The width of the through groove 61 is adapted to the thickness of the connection between the insert plate 73 and the sling support 81, so as to ensure that the connection between the sling support 81 and the steel casing 3 can slide smoothly in the through groove 61 without jamming.

[0046] On the prefabrication site, the base 7 is first placed on the platform, with the lower surface of the base plate 72 fitting against the platform, and the inner cylinder 71 vertically upward. The first section of the sheath 6 is then inserted from above onto the positioning post 74, with its bottom end contacting the upper surface of the base plate 72. The circumferential position of the sheath 6 is adjusted so that the through groove 61 is properly inserted into the insert plate 73. Subsequently, the second section of the sheath 6 is attached above the first section, aligned with the through groove 61, and welded to fix it at the joint between the two sections of the sheath 6. The above steps are repeated, extending the sheath section by section upward until the total length of the sheath 6 reaches the design value (equal to the length of the second waterproof layer 41). The welding connection between adjacent sections of the sheath 6 can be spot welded. After the sheath 6 is wrapped by the crushed stone layer 5 inside the pile hole, there is no risk of buckling.

[0047] To ensure a tight fit between the sheath 6 and the steel casing 3 during the overall hoisting and lowering process, and to prevent gaps between the through groove 61 and the outer wall of the steel casing 3, multiple steel strand binding straps are used to temporarily tighten all sheaths 6 to the outer wall of the steel casing 3 at intervals of 1.0m to 1.5m along the height direction of the sheath 6 during all sheath 6 assembly processes or after all assembly is completed. These steel strand binding straps are retained after the composite casing structure is lowered into the steel sleeve 2 and are not removed.

[0048] It should be noted that the steel strand binding straps, as a structural measure, play the following roles throughout the construction process: First, during the hoisting and lowering phase of the composite casing structure, the steel strand binding straps ensure that the sheath 6 and the steel casing 3 remain tightly fitted. Second, after the steel sleeve 2 is completely pulled out, the sheath 6 loses the radial constraint of the steel sleeve 2. At this time, the steel strand binding straps continue to play a tightening role, firmly binding each section of the sheath 6 around the outer wall of the steel casing 3, preventing the extra-long sheath 6 from bending, deforming, or falling apart due to its own weight or slight disturbance of the surrounding strata. When cement grout is subsequently poured into the space between the first waterproof layer 4 and the second waterproof layer 41, the steel strand binding straps will be completely wrapped by the cement grout. After the cement grout has fully set, the steel strand binding straps and the crushed stone concrete composite layer are solidified into one, becoming part of the permanent structure, further enhancing the integrity and stability of the sheath 6.

[0049] To further prevent any potential gravel or dust from seeping into the sheath 6 through the microscopic gap between the channel 61 and the outer wall of the steel casing 3 during construction, waterproof tape (preferably fiber-reinforced PVC tape or cloth tape) can be applied from the outside along the entire length of the channel 61 after the sheath 6 is assembled and temporarily fixed. The width of the tape should be greater than the width of the channel 61 (e.g., 10mm to 20mm wider), ensuring that both sides are adhered to the outer wall of the sheath 6. Since the opening of the channel 61 is already tightly attached to the outer wall of the steel casing 3 under the tension of the steel strand binding straps, pebble particles (minimum diameter 5mm) in the gravel layer 5 cannot enter the channel 61; the tape serves only as an optional auxiliary sealing layer. When the steel casing 3 is pulled out, relative sliding occurs between the steel casing 3 and the sheath 6, and the tape will be naturally torn off by the connection of the sling support 81 or the outer wall of the steel casing 3, without creating additional resistance to the pulling process.

[0050] All the above processing, assembly, and temporary fixing operations are completed on the ground, with ample operating space and a simple and reliable process. After the composite casing structure is hoisted into the hole, the outer side of the casing 6 is radially constrained by the inner wall of the steel sleeve 2, while the inner side is supported by the outer wall of the steel casing 3. Furthermore, the subsequent filling of the crushed stone layer 5 exerts uniform circumferential pressure on the casing 6, ensuring that the casing 6 maintains a stable, upright posture. After the steel sleeve 2 is removed, the casing 6, as the inner steel reinforcement within the crushed stone layer 5, is not removed.

[0051] In another technical solution, the sheath 6 and the second waterproof layer 41 are fixedly connected by a plurality of hot melt gaskets 62. One side of the hot melt gasket 62 is fixedly connected to the inner wall of the second waterproof layer 41, and the other side of the hot melt gasket 62 is fixedly connected to the outer wall of the sheath 6.

[0052] The hot-melt gasket 62 can be made of a thermoplastic material compatible with the material of the second waterproof layer 41, such as modified PVC or EVA material of the same type as PVC waterproof membrane, to ensure that the two can fully fuse and form a strong connection during hot-melt welding. The hot-melt gasket 62 can be circular, strip-shaped, elliptical, or rectangular, and a through hole can be opened in its center to increase the welding area and shear strength. During processing, one side of the hot-melt gasket 62 is first fixedly connected to the outer wall of the sheath 6. The fixing method can be injection molding during factory prefabrication or bonding with high-strength adhesive to ensure that the connection between the sheath 6 and the hot-melt gasket 62 does not detach during subsequent construction. Subsequently, at the construction site or prefabrication site, the inner wall of the second waterproof layer 41 is hot-melted and welded to the other side of the hot-melt gasket 62: the surface of the hot-melt gasket 62 and the inner wall of the second waterproof layer 41 are locally heated to a molten state using a hot air gun, and pressure is applied to fuse the two into one, forming a strong connection point with molecular-level fusion after cooling.

[0053] The hot-melt gaskets 62 are spaced apart along the length of the sheath 6. The vertical spacing can be determined by considering the thickness of the second waterproof layer 41, the tensile strength of the PVC roll, and the frictional resistance when the steel sleeve 2 is pulled out, and is usually 300-500 mm. At the same time, at least one hot-melt gasket 62 is provided between each sheath 6 and the second waterproof layer 41 along the circumference of the sheath 6, and the hot-melt gaskets 62 on each sheath 6 are evenly distributed in the circumference to ensure that the constraint force on the second waterproof layer 41 is evenly transmitted. The advantages of this connection method are as follows: Firstly, the hot-melt gasket 62 reliably fixes the second waterproof layer 41 to the sheath 6, enabling the second waterproof layer 41 to resist the frictional resistance between the inner wall of the steel sleeve 2 and the outer side of the second waterproof layer 41 during the removal of the steel sleeve 2, thus preventing the second waterproof layer 41 from being torn or curled due to friction. Secondly, after the steel sleeve 2 is removed, this connection continues to function, maintaining the second waterproof layer 41 in a stable cylindrical shape, providing a good forming boundary for the subsequent filling of the crushed stone layer 5 and the injection of cement grout. In addition, since the hot-melt gasket 62 only connects the second waterproof layer 41 to the sheath 6 and not to the steel sleeve 3, when the steel sleeve 3 is removed, the second waterproof layer 41 can remain in the hole along with the sheath 6, unaffected by the removal of the steel sleeve 3, thus achieving the permanent retention of the second waterproof layer 41 and the recyclable separation of the steel sleeve 3.

[0054] In another technical solution, the lower part of the steel casing 3 is provided with a plurality of sling supports 81. The sling supports 81 are adapted to the inner wall of the sheath 6 and can slide vertically within the sheath 6. The sheath 6 has a through groove 61 along its length, which penetrates the side wall of the sheath 6. The insert plate 73 is disposed between the inner cylinder 71 and the positioning post 74 and passes through the through groove 61. When the sheath 6 is fitted onto the positioning post 74, the opening of the through groove 61 contacts the outer wall of the steel casing 3. The connection between the sling support 81 and the steel casing 3 passes through the through groove 61. The sling support 81 is connected to the bottom end of the steel sling 8, and the cable of the steel sling 8 is threaded inside the sheath 6.

[0055] After the sheath 6 is fitted onto the positioning post 74, its circumferential rotation is subject to dual constraints. First, the insert plate 73 horizontally passes through the through groove 61. The insert plate 73, as part of the base 7, forms a rigid whole with the positioning post 74, the inner cylinder 71, and the bottom plate 72. When the sheath 6 is fitted onto the positioning post 74, the insert plate 73 is embedded in the through groove 61. Because the width of the insert plate 73 matches that of the through groove 61, the sheath 6 cannot rotate circumferentially under the constraint of the insert plate 73. Second, the connection between the sling support 81 and the steel casing 3 also passes through the through groove 61. This connection is fixedly connected to the steel casing 3. When the steel casing 3 is stationary or rising in the hole, this connection is also embedded in the through groove 61, further restricting the circumferential rotation of the sheath 6 relative to the steel casing 3. Through the aforementioned dual constraints, the sheath 6 maintains the accurate orientation of its through groove 61 toward the outer wall of the steel casing 3 throughout the entire construction process, preventing the smooth passage of the insert plate 73 and the sling support 81 connection from being affected by the deflection of the sheath 6. As an auxiliary measure, the positioning post 74 can have a polygonal cross-section, which matches the polygonal hole in the inner wall of the sheath 6, thereby further ensuring the circumferential positioning of the sheath 6.

[0056] After the sheath 6 is installed, the opening of the through groove 61 is in close contact with the outer wall of the steel casing 3, forming a continuous barrier. This effectively prevents the pebble particles in the subsequent filling crushed stone layer 5 from entering the interior of the sheath 6 through the through groove 61, ensuring that the interior space of the sheath 6 remains unobstructed and providing an unobstructed passage for the installation and sliding of the steel sling 8.

[0057] The sling support 81 adopts a cylindrical structure, and its outer diameter is adapted to the inner diameter of the sheath 6, allowing the sling support 81 to slide smoothly within the sheath 6. The upper end of the sling support 81 is fixedly connected to the lower outer wall of the steel casing 3, and the connection method can be welding or bolting. The lower end of the sling support 81 is provided with a cable-passing hole. After the bottom end of the steel sling 8 passes through the cable-passing hole, it is fixedly connected to the sling support 81 by compression anchoring or welding. Specifically, the bottom end of the steel sling 8 can be inserted into the cylindrical body of the sling support 81, and the end of the steel sling 8 can be compressed into the sling support 81 by a hydraulic extrusion machine, or an anchoring plate can be set at the bottom of the sling support 81, and the steel sling 8 can be passed through and anchored with clamps to ensure reliable connection. The cable body of the steel sling 8 is led upward from the sling support 81, passes through the internal cavity of the sheath 6, and extends along the length of the sheath 6 to the top of the steel casing 3.

[0058] When the steel casing 3 is pulled out, the steel sling 8 applies an upward traction force to the bottom of the steel casing 3 through the sling support 81, and the sling support 81 slides upward within the sheath 6. Since the connection between the insert plate 73 and the sling support 81 is always located within the through groove 61, and the contact surface between the through groove 61 and the outer wall of the steel casing 3 acts as a guide, the steel casing 3 remains vertically stable during its ascent, avoiding additional resistance due to tilting. After the steel casing 3 is completely pulled out, the sling support 81 rises along with the steel casing 3 and detaches from the sheath 6, while the sheath 6 becomes part of the permanent structure. Through the above arrangement, the structure in which the connection between the insert plate 73 and the sling support 81 is embedded in the through groove 61 achieves both anti-rotation positioning of the sheath 6 and reliable traction of the steel casing 3 by the steel sling 8. Simultaneously, the tight fit between the through groove 61 and the outer wall of the steel casing 3 effectively prevents the intrusion of gravel, further improving the smoothness and reliability of the pipe pulling operation.

[0059] In another technical solution, the bottom of the first waterproof layer 4 is disposed in the annular gap between the steel casing 3 and the inner cylinder 71; the bottom of the second waterproof layer 41 bypasses the lower edge of the bottom plate 72 and folds upward to form an overlapping seal with the first waterproof layer 4.

[0060] When the first waterproof layer 4 is laid on the inner wall of the steel casing 3, its bottom extends downward to the bottom end of the steel casing 3 and enters the annular gap between the steel casing 3 and the inner cylinder 71. The bottom of the first waterproof layer 4 is pressed and fixed within this gap by the inner wall of the steel casing 3 and the outer wall of the inner cylinder 71, forming the first sealing boundary. When the second waterproof layer 41 is laid on the outer wall of the steel casing 3, its bottom extends downward beyond the bottom end of the steel casing 3. After the bottom of the second waterproof layer 41 passes around the lower edge of the bottom plate 72, it folds upward and overlaps with the first waterproof layer 4 in the bottom area of ​​the steel casing 3.

[0061] In another technical solution, the crushed stone layer 5 is filled with pebbles, which are continuously graded with a particle size of 5mm to 25mm. Specifically, particles with a particle size of 5mm to 10mm account for 25% to 35% of the mass, particles with a particle size of 10mm to 20mm account for 40% to 55%, and particles with a particle size of 20mm to 25mm account for 10% to 20%. The pebbles are round or sub-round, with no more than 5% of needle-like or flaky particles, no more than 1% mud content, and no more than 0.5% mud lump content. After filling, the continuously graded pebbles form a dense granular structure with good interlocking between particles. This effectively transmits and homogenizes external formation pressure and provides uniform penetration channels for cement grout during subsequent grouting, ensuring that the grout fully fills the gaps between particles. The round or sub-rounded particle shape significantly reduces frictional damage to the second waterproof layer 41 during filling and prevents sharp edges from scratching the waterproof membrane. Strict control over mud content and mud lump content ensures the cleanliness of the crushed stone layer 5, preventing the formation of weak interlayers after grouting, thus ensuring the overall strength and durability of the crushed stone concrete composite layer. Through the above gradation and quality requirements, the crushed stone layer 5 can stably perform its functions of positioning, wall protection, and forming a composite protective layer with cement grout during both construction and service periods.

[0062] In another technical solution, the steel sling 8 is configured to satisfy the following: in, n For the number of steel slings, The allowable tensile force of a single steel cable G For the self-weight of the steel casing, For the upward pull-out force of the full-rotation casing drilling rig on the steel casing, The total friction force inside the steel casing, The total friction force on the outside of the steel casing, The coefficient of friction between the steel casing and the first waterproof layer. The coefficient of friction between the steel casing and the gravel layer, The effective lateral pressure of concrete at initial setting, D For the diameter of the steel casing, h For pile length, a The pressure transmission reduction factor for the gravel layer is 0.2 to 0.5. The static lateral pressure coefficient of the gravel layer, It is a heavily gravelly layer.

[0063] Permissible tensile force of steel slings The safety factor is determined based on the selected steel strand specifications and is typically 0.4 to 0.5 times the breaking strength of the steel strand; the self-weight of the steel casing. G The upward pull-out force is determined based on the diameter, wall thickness, length, and steel density of the steel casing. Based on the clamping force and friction coefficient of the equipment, it is estimated to be 50-100 kN; the inner friction coefficient Since a release agent or lubricating grease can be pre-applied between the first waterproof layer 4 and the inner wall of the steel casing 3, with a value of 0.08 to 0.12; the outer friction coefficient Friction between the outer wall of the steel casing and the crushed stone, with a value of 0.45–0.55; effective lateral pressure at initial setting of concrete. The pressure is determined based on the concrete mix proportions and initial setting time, typically ranging from 0.01 to 0.03 MPa; the diameter D of the steel casing and the pile length h are determined according to the design drawings; the static lateral pressure coefficient of the crushed stone layer... The value is determined based on the internal friction angle of the crushed stone, generally taken as 0.35–0.45; the unit weight of the crushed stone layer. Take 17-19 kN / m³.

[0064] Pressure transmission reduction factor of crushed stone layer a The principle behind using a value of 0.2 to 0.5 is as follows: The crushed stone layer, as a thin granular body filling the space between the steel casing and the second impermeable layer, is typically only 10 to 20 cm thick, much smaller than the pile diameter and length. During the extraction of the steel casing, the lateral pressure on the crushed stone layer mainly originates from the static earth pressure generated by its own weight. However, due to the limited thickness of the crushed stone layer and its close contact with the flexible second impermeable layer, the pressure is significantly attenuated during radial transmission, making it difficult to reach the full value of the theoretical static earth pressure. Furthermore, the interlocking effect between crushed stone particles cannot form a complete pressure arch under thin-layer conditions, and some pressure is absorbed by the deformation and rearrangement of the crushed stone layer. Simultaneously, the crushed stone layer will experience localized loosening due to the rise of the casing during steel casing extraction, further reducing pressure transmission efficiency. Therefore, a reduction factor is introduced. a To reflect the pressure transmission characteristics of thin-layer granular materials under actual working conditions, a value ranging from 0.2 to 0.5 is used. This value, verified through engineering practice, ensures the reliability of the calculation results while avoiding overly conservative design. This value considers multiple factors such as the thickness of the crushed stone layer, particle size distribution, density, and construction dynamics. Users can select the value within this range according to specific engineering conditions; the upper limit is used when the crushed stone layer thickness is large, the gradation is good, and the compaction degree is high, and vice versa. The above parameter values ​​are used in conjunction with… a The proper selection of coefficients can ensure that the number and load-bearing capacity of steel slings meet the mechanical requirements for the smooth extraction of steel casings, thus ensuring construction safety and avoiding waste caused by excessive configuration.

[0065] In another technical solution, a pile foundation operation platform for karst areas is proposed, which is applied to the aforementioned pile foundation construction method in karst areas, including: A base frame 91 is placed on the ground; an upper frame 92 is positioned above the base frame 91; a hydraulic lifting mechanism 93 is connected between the base frame 91 and the upper frame 92, used to drive the upper frame 92 to rise and fall relative to the base frame 91; a slewing mechanism 94 is installed on the upper frame 92, used to clamp the outer wall of the steel sleeve 2 or steel protective sleeve 3, and can drive the clamped steel sleeve 2 or steel protective sleeve 3 to rotate; a prestressed support 95 is installed on the upper frame 92, used to fix the upper end of the steel sling 8 and tighten the steel sling 8.

[0066] Specifically, the pile foundation operating platform is assembled on the construction site. The bottom frame 91 is welded from structural steel and fixed to the ground with anchor bolts or counterweights, providing a stable foundation for the entire pile foundation operating platform. The upper frame 92 adopts a box girder structure, the dimensions of which match the bottom frame 91, and is connected to the bottom frame 91 via a hydraulic lifting mechanism 93. The hydraulic lifting mechanism 93 is typically composed of multi-stage hydraulic cylinders, and cylinders with a stroke of 1 to 3 meters can be selected according to construction needs to achieve the lifting and lowering adjustment of the upper frame 92. The slewing mechanism 94 is installed in the center of the upper frame 92. Its clamping system is hydraulically driven, and the grippers can open and close to accommodate the outer diameter of the steel sleeve 2 or steel casing 3. It is equipped with a slewing motor to drive the clamped steel sleeve 2 or steel casing 3 to rotate forward or backward. The prestressed support 95 is installed on the edge or side of the upper frame 92, and its position corresponds to the lead-out point of the steel cable 8. The support is equipped with a through-type jack or wedge-shaped anchoring device to fix the upper end of the steel cable 8 and apply pre-tension.

[0067] When the slewing mechanism 94 interferes with the vertical position of the steel sling 8, a guide wheel structure 96 can be added to the upper frame 92. The guide wheel structure 96 includes one or more sets of fixed pulleys, which change the direction of the steel sling 8 so that it bypasses the outer edge of the slewing mechanism 94 and extends upward to the prestressed support 95, thereby avoiding any possible direct contact between the steel sling 8 and the slewing mechanism 94 and ensuring that they do not interfere with each other during operation. The groove surface of the guide wheel structure 96 is hardened to reduce the frictional wear of the steel sling 8.

[0068] During the extraction of the steel casing 3, the operating platform works in coordination according to the following steps: First, the slewing mechanism 94 clamps the outer wall of the steel casing 3, and the prestressed support 95 is activated, applying pre-tension to the upper end of the steel sling 8, putting the steel sling 8 in a taut state. At this time, the bottom end of the steel sling 8 is connected to the lower part of the steel casing 3 through the sling support 81, forming an upward traction force on the bottom of the steel casing 3. Subsequently, the hydraulic lifting mechanism 93 starts to operate, driving the upper frame 92, along with the slewing mechanism 94 and the prestressed support 95, to rise together. During the rising process, the slewing mechanism 94 drives the top of the steel casing 3 to move upward, while the prestressed support 95 continuously applies traction force to the bottom of the steel casing 3 through the steel sling 8, achieving coordinated extraction from both the top and bottom. The rising speed of the hydraulic lifting mechanism 93 and the tension of the prestressed support 95 are coordinated and adjusted by the control system to ensure that the casing is subjected to stable force during the rising process. In another technical solution, a temporary fixing device is also included, which is installed on the bottom frame 91 or the ground to temporarily fix the steel casing 3 when the slewing mechanism 94 releases the steel casing 3 and the prestressed support 95 releases the steel sling 8. The temporary fixing device can take the form of a hydraulic clamping clamp, a mechanical claw, or a wedge locking mechanism, and is installed on the upper surface of the bottom frame 91 or on a pre-set support on the ground, with its clamping center aligned with the axis of the steel casing 3. During intermittent pipe pulling operations, after the hydraulic lifting mechanism 93 completes one upward stroke, the upper frame 92 needs to be lowered and reset for the next clamping and pulling. At this time, the slewing mechanism 94 needs to release the steel casing 3, and the prestressed support 95 also needs to release the steel sling 8 so that the upper frame 92 can descend smoothly. In this state, the steel casing 3 remains in the hole solely due to its own weight. Even if there is some frictional resistance between the steel casing 3 and the surrounding medium (first waterproof layer 4, gravel layer 5, etc.), this resistance may not be sufficient to completely offset its own weight or external disturbances, posing a risk that the steel casing 3 may slip down due to gravity. The temporary fixing device is activated at this stage, and its clamping or locking mechanism engages to temporarily hold the steel casing 3 from the outside, maintaining its current height and preventing accidental slippage of the steel casing 3 during the release of the rotating mechanism 94 and the prestressed support 95.

[0069] After the upper frame 92 completes its descent and reset, the slewing mechanism 94 re-clamps the steel casing 3, and the prestressed support 95 re-tensions the steel slings 8, the temporary fixing device is released, relieving the constraint on the steel casing 3 and allowing the pipe extraction operation to proceed to the next cycle. Through this setup, the temporary fixing device effectively solves the problem of steel casing slippage that may occur during intermittent pipe extraction due to the loosening of the clamping and traction devices, ensuring the continuity and safety of the extraction operation. Especially for the recovery of steel casings from deep, long piles or large-diameter piles, this device provides reliable fall protection, further enhancing the stability and controllability of the construction process.

[0070] It should be noted that although the steps are described in a specific order above, this does not mean that they must be performed in that order. In fact, some of these steps can be executed concurrently, or even in a different order, as long as the required functionality is achieved. The number of devices and processing scale described herein are for simplification of the invention; applications, modifications, and variations of this invention will be readily apparent to those skilled in the art.

[0071] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for constructing pile foundations in karst areas, characterized in that, Includes the following steps: S1: Drive the full-rotation casing drilling rig to rotate the steel sleeve (2) to cut the karst strata (1) and drill downwards until the bedrock (10). During the drilling process, the soil and broken rock inside the steel sleeve (2) are removed simultaneously. S2. A steel casing (3) is prefabricated on the ground. A first waterproof layer (4) is laid on the inner wall of the steel casing (3), and a second waterproof layer (41) is laid on the outside of the steel casing (3). The first waterproof layer (4) and the second waterproof layer (41) are connected at the bottom. The steel casing (3), the first waterproof layer (4), and the second waterproof layer (41) form a composite casing structure. One end of a steel sling (8) is connected to the outer wall at the lower part of the steel casing (3), and the other end of the steel sling (8) extends from between the steel casing (3) and the second waterproof layer (41). S3. The composite protective sleeve structure is hoisted as a whole and placed in the center inside the steel sleeve (2); S4. Graded crushed stone is filled into the circumferential gap between the steel casing (3) and the second waterproof layer (41) to form a crushed stone layer (5); S5. After the crushed stone layer (5) is filled and compacted, start the full-rotation casing drilling rig to rotate and pull out the steel sleeve (2); S6. Place the pile reinforcement cage (31) in the space enclosed by the first waterproof layer (4) and pour the pile body concrete into the space. S7. When the concrete of the pile body reaches the initial setting state and has not yet undergone volume expansion due to heat of hydration, the steel casing (3) is pulled out of the hole by the steel sling (8) and the lifting equipment working together. The first water-proof layer (4), the second water-proof layer (41) and the crushed stone layer (5) between them are all left in the hole. S8. Cement grout is injected into the space between the first waterproof layer (4) and the second waterproof layer (41), and the cement grout and the crushed stone layer (5) are solidified to form a crushed stone concrete composite layer.

2. The pile foundation construction method in karst areas as described in claim 1, characterized in that, Both the first waterproof layer (4) and the second waterproof layer (41) are made of PVC polymer waterproof membrane. The PVC base layer of the first waterproof layer (4) is in contact with the inner wall of the steel casing (3). The anti-seepage crystallization layer of the first waterproof layer (4) faces the inside of the pile hole and is used to form an anchor bond with the pile concrete. The PVC base layer of the second waterproof layer (41) faces the inner wall of the steel sleeve (2). The anti-seepage crystallization layer of the second waterproof layer (41) faces the inside and is used to form an anchor bond with the crushed stone concrete composite layer.

3. The pile foundation construction method in karst areas as described in claim 1, characterized in that: The bottom of the steel casing (3) is provided with a base (7), the base (7) includes an inner cylinder (71), a bottom plate (72) and several insert plates (73); The outer diameter of the inner cylinder (71) is smaller than the inner diameter of the steel casing (3). The bottom of the steel casing (3) is provided with several vertical insertion ports (30). The insertion plate (73) is horizontally protruding from the outer wall of the inner cylinder (71). The insertion plate (73) and the insertion ports (30) are inserted and matched one by one. The bottom plate (72) is annular, and it is horizontally protruding from the bottom outer edge of the inner cylinder (71) and extends to the outside of the steel casing (3). Several positioning posts (74) are vertically arranged on the bottom plate (72), and all positioning posts (74) are spaced apart on the outside of the steel casing (3). The positioning post (74) is fitted with a protective sleeve (6), the bottom end of the protective sleeve (6) is connected to the base plate (72), and the length of the protective sleeve (6) is equal to the length of the second waterproof layer (41), which is wrapped around the outside of all the protective sleeves (6).

4. The pile foundation construction method in karst areas as described in claim 3, characterized in that, The sheath (6) and the second waterproof layer (41) are fixedly connected by a plurality of hot melt pads (62). One side of the hot melt pad (62) is fixedly connected to the inner wall of the second waterproof layer (41), and the other side of the hot melt pad (62) is fixedly connected to the outer wall of the sheath (6).

5. The pile foundation construction method in karst areas as described in claim 3, characterized in that, The lower part of the steel casing (3) is provided with several sling supports (81). The sling supports (81) are adapted to the inner wall of the sheath (6) and can slide vertically inside the sheath (6). The sheath (6) has a through groove (61) along its length direction. The through groove (61) penetrates the side wall of the sheath (6). The insert plate (73) is disposed between the inner cylinder (71) and the positioning post (74) and passes through the through groove (61). When the sheath (6) is fitted onto the positioning post (74), the opening of the through groove (61) contacts the outer wall of the steel casing (3). The connection between the sling support (81) and the steel casing (3) passes through the through groove (61), the sling support (81) is connected to the bottom end of the steel sling (8), and the steel sling (8) is threaded inside the sheath (6).

6. The pile foundation construction method in karst areas as described in claim 3, characterized in that, The bottom of the first waterproof layer (4) is located in the annular gap between the steel casing (3) and the inner cylinder (71); the bottom of the second waterproof layer (41) wraps around the lower edge of the bottom plate (72) and folds upward to form an overlap seal with the first waterproof layer (4).

7. The pile foundation construction method in karst areas as described in claim 1, characterized in that, The crushed stone layer (5) is filled with pebbles, which are continuously graded with a particle size of 5mm to 25mm. Among them, the mass percentage of particles with a particle size of 5mm to 10mm is 25% to 35%, the mass percentage of particles with a particle size of 10mm to 20mm is 40% to 55%, and the mass percentage of particles with a particle size of 20mm to 25mm is 10% to 20%. The shape of the pebbles is round or sub-round, the content of needle-shaped particles is not greater than 5%, the mud content is not greater than 1%, and the mud lump content is not greater than 0.5%.

8. The pile foundation construction method in karst areas as described in claim 5, characterized in that, The steel sling (8) is configured to meet the following requirements: in, n For the number of steel slings, The allowable tensile force of a single steel sling G For the self-weight of the steel casing, For the upward pull-out force of the full-rotation casing drilling rig on the steel casing, For the total friction force inside the steel casing, The total friction force on the outside of the steel casing, The coefficient of friction between the steel casing and the first waterproof layer. The coefficient of friction between the steel casing and the gravel layer, The effective lateral pressure of concrete at initial setting, D For the diameter of the steel casing, h For pile length, a The pressure transmission reduction factor for the gravel layer is 0.2 to 0.

5. The static lateral pressure coefficient of the gravel layer, It is a heavily gravelly layer.

9. A pile foundation operation platform for karst areas, applied to the pile foundation construction method for karst areas as described in any one of claims 1 to 8, characterized in that, include: The base frame (91) is placed on the ground; The upper frame (92) is disposed above the lower frame (91); A hydraulic lifting mechanism (93) is connected between the bottom frame (91) and the upper frame (92) for driving the upper frame (92) to rise and fall relative to the bottom frame (91); A rotary mechanism (94) is installed on the upper frame (92) for clamping the outer wall of the steel sleeve (2) or the steel protective sleeve (3) and can drive the clamped steel sleeve (2) or the steel protective sleeve (3) to rotate. A prestressed support (95) is installed on the upper frame (92) to fix the upper end of the steel sling (8) and tighten the steel sling (8).

10. The karst area pile foundation operation platform as described in claim 9, characterized in that, It also includes a temporary fixing device, which is set on the bottom frame (91) or the ground, for temporarily fixing the steel casing (3) when the slewing mechanism (94) releases the steel casing (3) and the prestressed support (95) releases the steel sling (8).

Images