Pile foundation underpinning construction process suitable for karst geology

Abstract

The application relates to the technical field of pile foundation underpinning, in particular to a pile foundation underpinning construction process suitable for karst geology, which comprises the following steps: pile cutting and reinforcement body construction: when a shield passes, a pile foundation underpinning adopts a pile grinding mode, a shield opening condition is reserved, a pre-reinforcement body is set in advance, the reinforcement body is grouted and reinforced by using a ground sleeve valve pipe, and a strong weathered stratum is obtained; foundation pit support construction: a steel sheet pile is used for reinforcement on the side of the road close to the foundation pit, a steel waist beam and a newly-built underpinning beam are arranged in the interval surrounded by the steel sheet pile, and the two long sides of the steel waist beam are respectively supported and reinforced through steel pipes; cover plate construction: a steel pipe pile is arranged on the periphery of the corner part of the foundation pit, a cover plate is arranged on the foundation pit, the cover plate and the foundation pit are simultaneously connected through a plurality of steel pipe piles, and the part of the foundation pit provided with the cover plate completely covers the steel sheet pile. The pile foundation underpinning can be realized in the construction scene of karst geology, meanwhile, the underpinning structure can be stably installed, and deformation control is within a safe range.

Description

Technical Field

[0001] This application relates to the technical field of pile foundation underpinning, and in particular to a pile foundation underpinning construction process suitable for karst geology. Background Technology

[0002] Pile foundation underpinning is a process of underpinning foundations using pile foundations. In other words, a new underpinning structure is constructed to support the columns, piles, or foundations that need to be underpinned, and the load borne by them is transferred to the newly constructed underpinning structure.

[0003] When carrying out underground pile foundation replacement projects, the project is constrained by surrounding buildings, pipelines, and geology, and the impact of construction on the surrounding environment, including noise and vibration, must also be considered. This is especially true when dealing with underground projects in karst geology, where karst development is intense, cavities vary in size and distribution, and their shapes and scales are diverse. Karst cavities are often poorly filled, frequently unfilled, or the filling material is weak and easily eroded by water flow. Some cavities contain gravel mixed with recently collapsed material, making them unstable and detrimental to track stability. This significantly impacts tunnel excavation and can easily lead to ground collapse, ground subsidence, and tunnel collapse.

[0004] Currently, the pile foundation replacement structure has limited adaptability to uneven settlement of new and old piles. Before the replacement pile is cut, the replacement pile has friction with the outside of the tunnel, and the soil pressure of the soil near the karst on the tunnel meets the anti-buoyancy design requirements of the original structure. After the replacement pile is cut, under the influence of adverse geological conditions, the newly constructed replacement structure deforms or overturns due to the loosening of the soil near the karst. The soil pressure of the soil near the karst on the tunnel cannot meet the anti-buoyancy design of the new structure. Summary of the Invention

[0005] In order to achieve pile foundation replacement in karst geological construction scenarios, while ensuring stable installation of the replacement structure and controlling deformation within a safe range, this application provides a pile foundation replacement construction process suitable for karst geological conditions.

[0006] This application provides a pile foundation underpinning construction technology suitable for karst geology, which adopts the following technical solution: A pile foundation underpinning construction technique suitable for karst geology includes the following construction steps: Construction of pile reinforcement body: The pile foundation replacement adopts the pile grinding method when the shield tunnel passes through, the conditions for shield tunnel opening are reserved, the pre-reinforced body is set in advance, and the reinforcement body is reinforced by grouting through ground sleeve valve pipe to obtain a strongly weathered stratum; Foundation pit support construction: Steel sheet piles are used to reinforce the side of the foundation pit adjacent to the road. Steel waist beams and newly built support beams are set in the area enclosed by the steel sheet piles. The two long sides and corners of the steel waist beams are reinforced by steel pipes. Sheet plate construction: Several steel pipe piles are set around the corner of the foundation pit, with two adjacent steel pipe piles being vertically parallel. A sheet plate is set on the foundation pit, and the sheet plate is connected to the foundation pit by several steel pipe piles at the same time. The part of the foundation pit with the sheet plate completely covers the steel sheet piles.

[0007] By adopting the above-mentioned technical solutions and using the grinding pile method, the pile foundation can be precisely treated when the shield tunnel passes through, creating conditions for the subsequent shield tunneling. Especially in strongly weathered strata, the use of ground sleeve valve pipes for grouting reinforcement can effectively improve the bearing capacity and stability of the strata, thereby strengthening the strata and providing a solid foundation for pile foundation replacement. The use of steel sheet piles to reinforce the foundation pit near the road side effectively reduces soil collapse and displacement during the foundation pit excavation process, improving construction safety. In this application, the setting of steel waist beams and newly built replacement beams, combined with steel pipe support reinforcement, forms a stable support system, significantly enhancing the overall stability and bearing capacity of the foundation pit. Furthermore, the vertical parallel setting of the steel pipe piles provides stable support points for the cover plate, ensuring the installation accuracy and stability of the cover plate. The cover plate completely covers the steel sheet pile part, which can effectively protect the construction environment inside the foundation pit. At the same time, it provides a path for vehicles to travel in the construction site, minimizing the adverse impact on the surrounding traffic and environment.

[0008] Compared with the prior art, this application strengthens the foundation before carrying out pile foundation replacement construction through the above-mentioned processing steps, which can better adapt to underground engineering construction in karst geology and has stronger applicability. It can effectively overcome the soil pressure of the soil near the karst on the underpass tunnel, and avoid deformation or overturning of the newly constructed replacement structure. It can realize pile foundation replacement in karst geological construction scenarios, while ensuring stable installation of the replacement structure and controlling deformation within a safe range.

[0009] Preferably, the construction of the pile-cutting reinforcement also includes the following steps: Hole formation: Holes are drilled on the working ground using a drilling rig, and bentonite slurry is used for wall protection during the hole formation process; Grouting pipes: Solid pipes are used in non-reinforced areas and above the top of the karst cave, while perforated pipes are used in reinforced areas and within the karst cave. A cap is added to the bottom of the grouting pipe. The grouting pipe is lowered to the bottom of the grouting body or the bottom of the karst cave, and an air vent is provided. Grouting slurry preparation; Grouting: First, water injection is carried out, and the grouting pipeline is checked and the grout absorption capacity of the stratum is determined.

[0010] By adopting the above technical solution, drilling rigs are used to drill holes on the working ground, and bentonite grout is used for wall protection, making the drilling process more stable and the holes more complete. In this application, solid pipes are used in non-reinforced areas and above the top of the karst cave. The solid pipes are used to ensure that the grout does not leak into the non-reinforced areas, while perforated pipes are used in the reinforced areas and within the karst cave. The perforated pipes can better disperse the grout to the areas that need reinforcement, achieving uniform and effective reinforcement. Specialized preparation of grout helps to adjust the composition and properties of the grout according to specific geological conditions and engineering needs, thereby ensuring the best grouting effect and further enhancing the strength and stability of the reinforced body. In addition, water injection can be used to check whether the grouting pipeline is unobstructed and to determine the grout absorption capacity of the stratum, providing an important reference for subsequent grouting operations. This ensures the smooth progress of the grouting process and reduces problems such as uneven grouting or blockage caused by differences in the grout absorption capacity of the stratum.

[0011] Preferably, the grouting process also includes the following construction steps: Retreating segmented grouting: A grouting core tube is installed inside the grouting pipe. Grouting is carried out from bottom to top in the grouting pipe, and the grouting pipe is divided into several grouting sections. After the first grouting section is completed, the grouting core tube is lifted up to carry out the grouting construction of the second grouting section. When the length of the grouting core tube pulled out is greater than the length of one pipe section, grouting is stopped, the pulled-out part of the grouting core tube and the joint are removed, the joint is connected to the grouting core tube that has not been pulled out, and grouting continues. This process is repeated until the grouting is completed.

[0012] By adopting the above technical solution, segmented grouting can ensure that each part of the stratum is fully and uniformly reinforced, so as to avoid problems such as uneven reinforcement or omissions in traditional grouting methods. At the same time, with the lifting and removal of the grouting core pipe, the grouting process can flexibly adapt to different stratum conditions, improving the flexibility and efficiency of construction.

[0013] Preferably, the grouting process also includes the following construction steps: Repeated grouting: The time interval between two grouting sessions is 6 to 10 hours, and the number of grouting sessions is no less than 3. Grouting is stopped when the final hole pressure is met, and the hole is sealed with cement mortar.

[0014] By adopting the above technical solution, setting the time interval between two grouting sessions to 6 to 10 hours and ensuring that the number of grouting sessions is not less than 3, it helps to improve the density and reinforcement effect of the grouting. Furthermore, repeated grouting can fully fill the voids in the formation, enhance the integrity and stability of the reinforced body. When the final hole pressure is met, grouting is stopped and the hole is sealed with cement mortar. This operation further ensures the reliability and durability of the grouting quality.

[0015] Preferably, the construction steps in the grouting pipe include: Before the grouting pipe is lowered, it is filled with water. At the same time, the upper end of the grouting pipe is exposed at the orifice and covered with a cap. After the grouting pipe is lowered, cement mortar is filled between the bottom end of the grouting pipe and the orifice wall for reinforcement.

[0016] By adopting the above technical solution, the grouting pipe is filled with water before being lowered. On the one hand, this helps to check the sealing and unobstructedness of the grouting pipe, ensuring the smooth progress of the subsequent grouting process. On the other hand, the clean water in the grouting pipe can use gravity to counteract the buoyancy of the casing material, thus making it easier and smoother to lower the grouting pipe.

[0017] The upper end of the grouting pipe is exposed at the orifice and covered with a cap. This prevents debris from entering the grouting pipe. The combined effect of the upper and lower caps can effectively reduce grout leakage from both ends of the grouting pipe and ensure that the grouting material can be fully and evenly filled into the target area, improving the grouting effect. At the same time, it helps to control the grouting pressure and speed, achieving precise control of pressure and speed during the grouting process, and ensuring the safety and stability of the grouting process.

[0018] Filling the gap between the bottom of the grouting pipe and the hole wall with cement mortar for reinforcement helps to enhance the stability and fixation of the grouting pipe, so as to prevent displacement or leakage of the grouting pipe during the grouting process.

[0019] Preferably, the construction of the cover plate includes the following steps: The cover plate construction is carried out after the steel sheet piles and steel pipe piles within its coverage area are completed. After the ground is broken, excavation is carried out, the cushion layer is poured, the reinforcing bars are tied, the side formwork is installed, and then the concrete is poured. During the binding of the reinforcing bars, the steel sheet piles and steel pipe piles are respectively provided with reserved reinforcing bars, and the reserved reinforcing bars are fixedly connected to the main reinforcing bars of the cover plate.

[0020] By adopting the above technical solution, through a series of operations such as breaking the ground, excavation, pouring the foundation layer, tying the reinforcing bars, installing the side formwork and pouring concrete, a complete and reliable cover plate structure is formed, which helps to enhance the connection strength and integrity between the cover plate and the pile foundation, and reduce the displacement or damage of the cover plate during use.

[0021] Preferably, the construction steps in the foundation pit support construction include: Multiple sheet piles are taken and fixed one by one on the working ground using a vibratory pile driver, with the inclination of each sheet pile not exceeding 2%; The two adjacent sheet piles are interlocked, and when they are joined, the straightness deviation between them is controlled within 0.1%, forming a screen-like screen wall.

[0022] By adopting the above technical solution, vibratory pile drivers are used to fix steel sheet piles one by one on the working ground, and the inclination of each steel sheet pile is controlled within 2%. The strict control of the inclination ensures the tight interlocking between the steel sheet piles, enhances the integrity and stability of the support structure, and helps to improve the stability and accuracy of the steel sheet piles. When two adjacent steel sheet piles are interlocked, the straightness deviation during the connection is controlled within 0.1%, forming a screen-like screen wall. Compared with a single steel sheet pile, the screen wall has good resistance to lateral displacement and can effectively prevent the leakage of soil and water, further improving its support effect.

[0023] Preferably, the construction steps in the foundation pit support construction include: Based on the tilt direction, length, and verticality of the screen wall, the construction sequence of the screen wall includes forward sequence, reverse sequence, reciprocating sequence, split sequence, neutralization sequence, and composite sequence.

[0024] By adopting the above technical solutions, different construction sequences can be flexibly used, such as forward sequence, reverse sequence, reciprocating sequence, split sequence, neutral sequence, and composite sequence, according to the tilt direction, length, and verticality of the screen wall. The diverse selection of construction sequences makes the construction process more flexible and efficient, and can adapt to various complex construction site environments. A reasonable construction sequence arrangement helps to reduce stress concentration and deformation during construction, ensuring the overall stability and safety of the screen wall.

[0025] Preferably, the construction of the cover plate includes the following steps: There are several reserved steel bars, each including a connecting part and an inclined extension part, with the inclined extension part fixed to the top of the connecting part. When the steel pipe pile is connected to the cover plate, several of the reserved steel bars are evenly distributed along the inner wall of the steel pipe pile, so that the inclined extension is inclined downward from the outside to the connection part. The connection part is welded to the inner wall of the steel pipe pile along its entire length. The cover plate completely covers the inclined extension until it is fixedly connected to the top of the steel pipe pile. When the sheet pile is connected to the cover plate, several of the reserved reinforcing bars are set on the inner wall of the sheet pile, so that the inclined extension is inclined downward from the outside to the connection. The connection is welded to the inner wall of the sheet pile along its length. The cover plate completely covers the inclined extension until it is fixedly connected to the top of the sheet pile.

[0026] By adopting the above technical solution, the inclined extension is inclined downward from the outside to the connection, which can provide a larger contact area, which helps to enhance the connection strength and stability between the reserved steel bar and the cover plate, and also helps to disperse stress and reduce stress concentration at the connection point. Several pre-reserved reinforcing bars are evenly distributed along the inner wall of the steel pipe pile and are firmly connected to the inner wall of the pipe through continuous welding. This helps to provide the integrity between the cover plate and the steel pipe pile, effectively transfer the load, and improve the bearing capacity of the structure. At the same time, the cover plate completely covers the inclined extension until it is fixedly connected to the top of the steel pipe pile, which further enhances the reliability and stability of the connection. The special structure of the inclined extension and connection, as well as the continuous welding method, significantly improve the connection strength and stability between the sheet pile and the cover plate, effectively resisting horizontal and vertical loads and ensuring the safety of the foundation pit support structure.

[0027] Preferably, the construction steps in the foundation pit support construction include: A triangular brace is provided between the steel waist beam and the steel sheet pile. The steel waist beam and the steel sheet pile are respectively welded to the triangular brace. Plain concrete is backfilled between the steel waist beam and the steel sheet pile.

[0028] By adopting the above technical solution, the triangular brace, as a simple and effective reinforcement component, can effectively resist horizontal forces to prevent deformation or displacement of the support structure. The steel wainscoting and steel sheet piles are welded to the triangular brace to form a firm connection between the three, which can effectively transfer loads and improve the stability of the overall structure. Plain concrete can fill the gaps, provide additional support, and help reduce groundwater infiltration. By backfilling plain concrete between the steel wainscoting and the steel sheet piles, the integrity and stability of the support structure can be further enhanced, ensuring the safe construction of the foundation pit.

[0029] In summary, this application includes at least one of the following beneficial technical effects: 1. This application, by strengthening the foundation before carrying out pile foundation replacement construction, can better adapt to underground engineering construction in karst geology, and has greater applicability. It can effectively overcome the soil pressure of the soil near the karst on the underpass tunnel, and avoid deformation or overturning of the newly constructed replacement structure. It can realize pile foundation replacement in karst geological construction scenarios, while ensuring stable installation of the replacement structure and controlling deformation within a safe range. 2. Strict control of the inclination of the sheet piles ensures a tight interlocking between them, enhancing the integrity and stability of the support structure and helping to improve the stability and precision of the sheet piles. The interlocking of two adjacent sheet piles forms a screen-like barrier wall. Compared with a single sheet pile, the barrier wall has good resistance to lateral displacement and can effectively prevent the leakage of soil and water, further improving its support effect. 3. Segmented grouting ensures that each part of the stratum is fully and uniformly reinforced, avoiding problems such as uneven reinforcement or omissions that occur with traditional grouting methods. At the same time, with the lifting and removal of the grouting core pipe, the grouting process can flexibly adapt to different stratum conditions, improving the flexibility and efficiency of construction. Attached Figure Description

[0030] Figure 1 This is a flowchart of the punching pile construction process in the embodiments of this application.

[0031] Figure 2 This is a plan view of the pile foundation underpinning in the first case of the embodiments of this application.

[0032] Figure 3 This is a plan view of the pile foundation underpinning in the second case of the embodiments of this application.

[0033] Figure 4 This is a plan view of the pile foundation underpinning in the third case of the embodiments of this application.

[0034] Figure 5 This is a schematic diagram of sheet pile splicing in an embodiment of this application.

[0035] Figure 6 This is a schematic diagram of the gusset plate arrangement in an embodiment of this application.

[0036] Figure 7 This is a detailed drawing of the connection between the steel pipe pile and the cover plate in an embodiment of this application.

[0037] Figure 8 This is a detailed drawing of the connection between the sheet pile and the cover plate in the embodiments of this application.

[0038] Figure 9 yes Figure 8 The main view.

[0039] Figure 10 This is a detailed drawing of the nodes between the sheet piles, wainscoting, and supports in the embodiments of this application.

[0040] Figure 11 This is a detailed drawing of the concrete filling between the sheet pile and the wainscoting in the embodiment of this application.

[0041] Figure 12 This is a diagram showing the arrangement of jacks for the pre-jacking platform in the embodiments of this application.

[0042] Explanation of reference numerals in the attached drawings: 1. Steel pipe pile; 2. Steel sheet pile; 3. Steel groin beam; 4. Support beam; 5. Cover plate; 6. Steel pipe support; 7. Thick steel plate; 8. Horizontal channel steel; 9. Water supply pipe; 10. Reserved reinforcing bar; 101. Connection part; 102. Inclined extension part; 11. Reinforcing gusset plate; 12. Guide frame; 13. Triangular brace; 14. Hanging bar; 15. Plain concrete; 16. Pre-jacking foundation; 17. Jack; 18. Steel pipe pad. Detailed Implementation

[0043] The following is in conjunction with the appendix Figure 1-12 This application will be described in further detail.

[0044] This application discloses a pile foundation underpinning construction process applicable to karst geology.

[0045] A pile foundation underpinning construction technique suitable for karst geology includes the following construction steps: Pipeline relocation: The pile foundation replacement construction in this application is surrounded by commercial and residential buildings. There is an immigration border inspection station on one side of the airport expressway and a residential area on the other side. In order to strictly control the adverse effects of noise and vibration generated during construction on people, the construction site is relatively narrow. The pile foundation construction method of vertical shaft drilling or perforation is adopted.

[0046] We need to thoroughly understand the geological and hydrological data of the pile foundation locations, determine the location of the karst caves, roughly estimate their extent, size, and filling conditions, and formulate appropriate construction plans and technical support measures based on the results of advance drilling of each pile.

[0047] Before construction, a site survey must be conducted. Any overhead power lines, underground cables, water supply and drainage pipes, gas pipelines, or other facilities on the site that obstruct construction or affect safe operation must be removed, relocated, or properly handled before construction can begin.

[0048] Drainage ditches are set up around the site, and the performance of construction machinery must meet the design requirements for pile driving. Drilling piles should only be carried out after the foundation pit has been excavated to the design depth.

[0049] Cave treatment: Investigate the groundwater type, groundwater level, and groundwater recharge and discharge at the construction site. Since the pile foundation replacement construction site in this application is karst geology, the risk of karst ground collapse under stress is relatively high. Therefore, before construction, it is necessary to fully investigate the karst morphology, karst distribution pattern, characteristics of filling materials, cave stability, presence of toxic gases, and distribution of toxic gases, and then make corresponding plans based on the actual investigation.

[0050] Construction of pile reinforcement body: The pile foundation replacement adopts the pile grinding method when the shield tunnel passes through, and the conditions for shield tunnel opening are reserved. The pre-reinforced body is set in advance, and the reinforcement body is reinforced by grouting through ground sleeve valve pipe to obtain a strongly weathered stratum.

[0051] Specifically, this application provides a preferred embodiment, which requires the reinforcement height to be 2 meters above the tunnel top and 2 meters below the tunnel bottom of the strongly weathered strata, the reinforcement width to be 2 meters outside the tunnel outline, and the reinforcement length to be 20.4 meters. Since the strata at the bottom of the foundation pit are sand or cohesive soil during the foundation pit excavation stage, in order to reinforce the foundation after excavation, the grouting reinforcement body is expanded to the area from the foundation elevation after excavation to 2 meters below the tunnel bottom. The reinforcement body construction is carried out before the drilling pile and foundation pit construction, and the pipeline situation is investigated in advance.

[0052] The shield tunneling pile reinforcement body is reinforced by grouting with sleeve valve pipes installed in ground drilling. The drilling spacing is 1 meter × 1 meter, the bottom burial depth of the reinforcement body is 23 meters to 24 meters, and the drilling depth reaches the bottom elevation of the karst cave according to the requirements of karst cave treatment. The grouting reinforcement area is a rectangular area extending 2 meters outward from the tunnel perimeter, and the grouting body is 6 meters long.

[0053] In this application, the construction steps for pile reinforcement are further included: Hole Formation: Holes are drilled on the working ground using a drilling rig. During the hole formation process, bentonite slurry is used to protect the hole wall, making the drilling process more stable and the hole more complete.

[0054] Grouting pipes: Solid pipes are used in non-reinforced areas and above the top of the karst cave. Perforated pipes are used in reinforced areas and within the karst cave. A cap is added to the bottom of the grouting pipe. The grouting pipe is lowered to the bottom of the grouting body or the bottom of the karst cave and is equipped with vent holes. There is at least one vent hole in each cave and additional vent holes are added when the spacing exceeds 4 meters.

[0055] In this application, solid pipes are used in unreinforced areas and above the top of the karst cave. The solid pipe portion ensures that the grouting fluid does not leak into the unreinforced areas. In the reinforced areas and within the karst cave, perforated pipes are used. The perforated portion of the pipes better disperses the grouting fluid to the areas requiring reinforcement, achieving uniform and effective reinforcement. After connecting the solid and perforated pipes as required, the pipes are filled with clean water to check the sealing performance. The grouting pipes are then lowered along the borehole to the design elevation, with the upper end protruding approximately 40 cm above the ground. A cap is then placed over the pipe to prevent debris from entering.

[0056] To ensure successful pipe lowering, the pipe should be filled with clean water. This helps check the sealing and unobstructed flow of the grouting pipe, ensuring the smooth progress of the subsequent grouting process. Furthermore, the water helps counteract the buoyancy of the casing material. After lowering the pipe, cement mortar should be filled and used to reinforce the gap between the borehole walls and the ground within 1-2 meters. The combined effect of the upper and lower caps effectively reduces grout leakage from both ends of the grouting pipe and ensures that the grouting material is fully and evenly filled into the target area, improving the grouting effect. Simultaneously, it helps control the grouting pressure and speed, enabling precise control of pressure and speed during the grouting process, ensuring the safety and stability of the grouting process.

[0057] During the excavation stage, the strata at the bottom of the foundation pit are sand or cohesive soil. To reinforce the foundation after excavation, cement mortar is filled between the bottom of the grouting pipe and the hole wall. The grouting reinforcement is extended to a range of 2 meters below the tunnel bottom after excavation. This operation helps to enhance the stability and fixation of the grouting pipe, so as to prevent displacement or leakage of the grouting pipe during the grouting process.

[0058] Grouting slurry preparation: Estimate the volume of pre-mixed slurry based on the volume of the grouting body and the area and height of the karst cave. Calculate the required amount of cement and water according to the water-cement ratio. Add the calculated amount of water to the mixer while stirring, and add the calculated amount of cement. After stirring evenly, pour the mixture into the slurry storage tank for later use.

[0059] Grouting: Water injection is performed first to check whether the grouting pipeline is unobstructed, to check the grouting pipeline and to judge the grout absorption capacity of the stratum. This provides an important reference for subsequent grouting operations, ensuring the smooth progress of the grouting process and reducing problems such as uneven grouting or blockage caused by differences in the grout absorption capacity of the stratum.

[0060] This application provides a preferred embodiment, in which the grouting construction specifically includes the following steps: Retreating segmented grouting: A grouting core tube is installed inside the grouting pipe. Grouting is carried out from bottom to top in the grouting pipe, with each grouting segment being 0.5 meters long. The grouting pipe is divided into several grouting segments. After the first grouting segment is completed, the grouting core tube is pulled up to carry out the grouting construction of the second grouting segment. When the length of the grouting core tube pulled out is greater than the length of one pipe section, grouting is stopped, the pulled-out part of the grouting core tube and the joint are removed, the joint is connected to the grouting core tube that has not been pulled out, and grouting continues. This process is repeated until the grouting is completed.

[0061] Segmented grouting ensures that each part of the stratum is fully and uniformly reinforced, avoiding problems such as uneven reinforcement or omissions that occur with traditional grouting methods. At the same time, with the lifting and removal of the grouting core tube, the grouting process can flexibly adapt to different stratum conditions, improving the flexibility and efficiency of construction.

[0062] If grouting needs to be paused during the grouting process, the cement grout pipe must first be removed and placed in a bucket of clean water. At the same time, the grouting core pipe should be raised 0.4 meters, clean water should be injected into the hole, and then grouting should be stopped. This will keep the pipeline unobstructed and ensure that the grouting is not affected by water injection.

[0063] For dual-liquid grouting, when grouting needs to be paused or before each grouting is completed, switch to single-liquid grouting for a few minutes. After removing the grouting core tube, use a steel pipe to flush the grouting tube from bottom to top with clean water. For single-liquid grouting, flush directly after stopping the grouting and cover the top of the tube with a cap.

[0064] Repeated grouting: The time interval between two grouting sessions is 6 to 10 hours, and the number of grouting sessions is no less than 3. This helps to improve the density and reinforcement effect of the grouting. In addition, repeated grouting can fully fill the voids in the formation, enhance the integrity and stability of the reinforced body. Grouting is stopped when the final borehole pressure is met, and the borehole is sealed with cement mortar, which can further ensure the reliability and durability of the grouting quality.

[0065] During the grouting process, maintain a grouting work record sheet, documenting the grouting status of the grouting holes and the operation time of each grouting step. Analyze and improve the grouting operation continuously throughout the process; and meticulously record the actual hole location, hole depth, underground materials within the hole, water inflow, etc. If discrepancies are found with the geological report, take corrective measures.

[0066] Drilled piles: Due to limited clearance at the construction site, rotary drilling rigs are difficult to use for hole formation. Therefore, pile driving machines are used for hole formation. (Refer to...) Figure 1 The construction steps of bored piles include: setting out the pile location, drilling and detecting underground pipelines, installing the casing, positioning and centering the drilling rig, drilling, removing drill cuttings, hole acceptance, first hole cleaning, installing the reinforcing steel cage, moving the drilling rig and lowering the guide pipe, second hole cleaning, pouring underwater concrete, and recovering construction tools.

[0067] Specifically, during the construction process of impact drilling, the geological conditions are first understood, the pile positions are determined, the site is leveled, and the geological conditions of each pile foundation are checked by drilling one pile at each pile position before construction. Based on the results of advance drilling for each pile, appropriate construction plans and technical support measures are formulated. According to the site conditions, interception, drainage and protection measures are constructed on the surface of the pile foundation construction area; and the soil on the top slope of the pile is cleaned and the load is reduced.

[0068] In this application, before drilling, a steel plate casing is installed at the borehole opening to fix the pile position and prevent the borehole opening from collapsing. An overflow outlet is opened at the upper edge of the casing. A stiffening rib is welded on the upper, lower and middle outer sides of the casing to ensure its rigidity and reduce deformation. At the same time, the casing is installed above the construction ground. The gap between the casing and the borehole wall is filled with clay to prevent water leakage. The deviation between the center of the casing and the center of the pile position is ≤20mm. The casing is placed vertically. After the drilling rig is in place, the flatness of the drilling rig and the elevation of the top surface of the steel casing are checked and recorded so that the drilling depth can be verified after the hole is formed.

[0069] Furthermore, different pile positions on the same foundation cannot be constructed continuously. The strength of the previous pile foundation must reach more than 70% of the design strength before the next pile can be constructed. This is to reduce the shearing damage to the already poured concrete piles caused by impact drilling. During this process, by adopting appropriate pile spacing, hole collapse and diameter reduction are prevented, and the hole formation of the concrete-poured piles relies on mud to achieve a balanced state.

[0070] At the start of drilling, maintain a low impact height to form a solid mud skin at the cutting edge of the casing. After drilling to the preset height at the cutting edge of the casing, gradually increase the drilling speed according to the soil conditions, keeping the mud specific gravity at 1.4. When drilling in clay layers, increase the impact height and pump flow rate to keep the mud specific gravity at 1.25. When just entering the rock strata, reduce the impact height, and after the rock surface is leveled, restore the normal impact height, keeping the mud specific gravity at 1.35. In bedrock layers, use low-impact or intermittent impact. When impacting in bedrock, use a long stroke, increase the impact frequency, and increase the impact energy.

[0071] This application utilizes the continuous rotation of the impact head to change its impact position at the bottom of the hole, thus preventing a quincunx-shaped hole bottom or hole deviation. A hole inspection is conducted after each drilling depth, and inspections are performed before changing the drill bit or at areas prone to hole shrinkage. Furthermore, to ensure the verticality of the hole, the drilling rig's support area is enlarged to stabilize it. During construction and before lowering the reinforcing cage, the hole diameter and inclination are checked.

[0072] Specifically, during the impact drilling process, a short stroke is used when first entering the rock strata. If the rock surface and the drill bit are found to be inclined, the backfill rubble is corrected. The impact speed is slower as the hole approaches the bottom. After the borehole reaches the designed depth, the first cleaning is carried out. The cleaning is carried out according to the principle of cleaning the slag first and then the mud. That is, the mud specific gravity is not reduced first, and it is ensured to be above 1.3. The slag particles in the hole are removed into the sedimentation tank. During the cleaning process, the drill bit is used to stir the bottom of the hole to ensure that no slag settles at the bottom of the hole.

[0073] In this application, when arranging the sonic logging pipes, it is required that three sonic logging pipes be arranged for each underpinning pile in an equilateral triangle. The sonic logging pipes are made of metal and are fixed to the reinforcing cage by binding or welding. When binding or welding, the pipes are kept parallel to each other. For piles with segmented reinforcing cages, the sonic logging pipes are consistent with the reinforcing cages and are connected by segmented welding. Finally, the lower end of the sonic logging pipe is sealed and the upper end is capped to ensure that there are no foreign objects inside the pipe. The connection of the sonic logging pipes is smooth and the pipe opening is higher than the top surface of the concrete.

[0074] In this application, the impact drilling method described above lays a solid foundation for the construction of cast-in-place concrete piles, making it less prone to subsequent issues such as hole collapse, diameter reduction, pile hole deviation, and insufficient pile tip reachability, thereby effectively improving the quality of concrete pouring and the bearing capacity of the pile.

[0075] Foundation pit support construction: The foundation pit is reinforced with steel sheet piles 2 on the side adjacent to the road. The steel sheet piles 2 referred to in this application are Larsen IV type steel sheet piles 2. Steel waist beams 3 and newly built support beams 4 are set in the area enclosed by the steel sheet piles 2. The two long sides of the steel waist beams 3 are reinforced by steel pipe supports 6 on opposite sides and corners respectively.

[0076] In this application, the pile foundation replacement construction is divided into the following three types according to the construction scenario: The first type, referring to Figure 2 The foundation pit is an irregularly shaped foundation pit. The side of the foundation pit adjacent to the road is reinforced with steel sheet piles 2, so that the steel sheet piles 2 completely surround the perimeter of the foundation pit. Within the area enclosed by the steel sheet piles 2, steel waist beams 3 and newly built support beams 4 are set up. The newly built support beams 4 are distributed inside the steel waist beams 3. At the same time, the two long sides of the steel waist beams 3 are reinforced by steel pipe supports 6 on opposite sides and corners.

[0077] According to the construction progress, a cover plate 5 is installed on the top of the foundation pit. The cover plate 5 is a concrete cover plate 5, which can effectively protect the construction environment inside the foundation pit. At the same time, it provides a path for vehicles to drive in the construction site, minimizing the adverse impact on the surrounding traffic and environment.

[0078] The second method, refer to Figure 3 Unlike the first type of pile foundation replacement construction scenario, the sheet piles 2 do not completely surround the perimeter of the foundation pit; instead, a construction gap is reserved. During the fabrication of this construction gap, the horizontal channel steel 8 is supported as excavation progresses, and both ends of the gap are welded and fixed to the adjacent sheet piles 2, ensuring a vertical fit between them and guaranteeing the stability of the connection between the horizontal channel steel 8 and the sheet piles 2.

[0079] Based on the reinforcement of the two long sides and corners of the steel waist beam 3 by steel pipe supports 6, a thick steel plate 7 is added at the corner of the steel waist beam 3. The thick steel plate 7 is a right-angled triangular steel plate, and the two right-angled sides of the thick steel plate 7 are welded to the two sides of the corner of the steel waist beam 3. This application does not specifically limit the specific number of steel pipe supports 6 and thick steel plates 7. During installation, multiple thick steel plates 7 can be set vertically at the same corner of the steel waist beam 3 according to the actual situation to further reinforce it.

[0080] The third type, refer to Figure 4 The difference between this and the first type of pile foundation replacement construction scenario is that the foundation pit is a regular-shaped foundation pit. When excavating the foundation pit, it is always set symmetrically along its long side central axis. The corners of the foundation pit are chamfered to form a triangular structure, so that the foundation pit has good compressive strength. At this time, it is not necessary to add steel pipe supports 6 to the corners of the steel waist beam 3 for reinforcement. However, several steel pipe supports 6 can be added to the opposite sides of the two long sides of the steel waist beam 3 to keep the steel waist beam 3 structurally stable.

[0081] If a water supply pipe 9 needs to be introduced around the foundation pit, the water supply pipe 9 must not penetrate the foundation pit. It needs to be set around the steel sheet pile 2. The cover plate 5 is installed on the top of the foundation pit on the side away from the water supply pipe 9 to prevent vehicles or people from accidentally entering the installation area of ​​the water supply pipe 9 and to ensure traffic and construction safety.

[0082] Reference Figure 5 and Figure 6 The specific construction steps for installing steel sheet piles 2 during foundation pit support construction are as follows: Take multiple sheet piles 2. Before driving the piles, inspect each sheet pile 2 and remove any sheet piles 2 with rusted or severely deformed interlocking joints. The unqualified ones can only be used after repair. At the same time, apply grease to the interlocking joints of the sheet piles 2 to facilitate driving and pulling out. Fix the sheet piles 2 one by one on the working ground using a vibratory pile driver. The inclination of each sheet pile 2 should not exceed 2%. If the inclination is too large and cannot be adjusted by pulling, pull it out and drive it again. Interlock two adjacent sheet piles 2. When connecting them, the straightness deviation between the two should be controlled within 0.1% to form a screen-like screen wall.

[0083] More specifically, refer to Figure 5 and Figure 6 To ensure the verticality of the sheet piles 2 during the splicing process, the sheet piles 2 are driven using a screen-type method, with a double-layer guide frame 12 for guidance. The splicing operation is carried out when the top of the first sheet pile 2 is located between the upper and lower crossbeams of the guide frame 12. During splicing, the splicing of the two sheet piles 2 at both ends of each row of pile foundations is completed first, and after driving them into the ground for a certain length, the middle part is constructed in sequence.

[0084] A set of sheet piles 2 at both ends of the screen wall are driven to the design elevation or a certain depth, and the verticality is strictly controlled. They are fixed to the steel waist beam 3 by electric welding. Then, in the middle, one-third or one-half of the sheet pile height is driven in sequence.

[0085] This application utilizes a screen-type driving method, which reduces the likelihood of sheet piles buckling, twisting, tilting, and unevenness on the wall surface. It achieves high driving accuracy and facilitates closure. Compared to a single steel sheet pile, the screen wall has excellent resistance to lateral displacement and can effectively prevent soil and water leakage, further enhancing its support effect.

[0086] Furthermore, based on the tilt direction, length, and verticality of the screen wall, the construction sequence of the screen wall includes forward sequence, reverse sequence, reciprocating sequence, split sequence, neutral sequence, and composite sequence. The driving sequence directly affects the verticality, displacement, axial expansion and contraction, unevenness of the sheet pile wall, and driving efficiency of the sheet piles. Therefore, in this application, when the sheet piles already driven at both ends of the screen wall are tilted in the opposite direction, a forward sequence is used; conversely, a reverse sequence is used; when the sheet piles at both ends of the screen wall are kept vertical, a reciprocating sequence is used; and when the sheet pile wall is very long, a composite sequence is used.

[0087] By flexibly adopting different construction sequences and offering diverse choices, the construction process becomes more flexible and efficient, adapting to various complex construction site environments. A reasonable construction sequence arrangement helps reduce stress concentration and deformation during construction, ensuring the overall stability and safety of the screen wall.

[0088] Reference Figure 6When two adjacent sheet piles 2 are joined together, the joint is welded on both sides, and the interlocking area is avoided from being welded. After the sheet piles 2 are welded, the weld slag and spatter around the weld must be removed, and the weld should be ground smooth with a grinding wheel. The surface quality of the weld should be checked to see if it meets the acceptance quality requirements. After the joint welding meets the requirements, the reinforcing plate 11 is welded. For each joint, two reinforcing plates are used, one on the front and one on the back. After the four sides of the reinforcing plate are fully welded, the weld slag and spatter around the weld are removed, and the surface quality of the weld is checked again to see if it meets the acceptance quality requirements.

[0089] Construction of cover plate 5: Several steel pipe piles 1 are set around the corner of the foundation pit. Two adjacent steel pipe piles 1 are vertically parallel. Cover plate 5 is set on the foundation pit. Cover plate 5 is connected to the foundation pit by several steel pipe piles 1 at the same time. The part of the foundation pit with cover plate 5 completely covers the steel sheet piles 2.

[0090] Specifically, the construction of the cover plate 5 is carried out after the steel sheet piles 2 and steel pipe piles 1 within its coverage area are completed. After the ground is broken, the excavation is carried out, the cushion layer is poured and the reinforcing bars are tied, the side formwork is installed and the concrete is poured. During the binding of the reinforcing bars, the steel sheet piles 2 and steel pipe piles 1 are respectively provided with reserved reinforcing bars 10, and the reserved reinforcing bars 10 are fixedly connected to the main reinforcing bars of the cover plate 5.

[0091] After a series of operations, including breaking the ground, excavation, pouring the foundation layer, tying the reinforcing bars, installing the side formwork, and pouring concrete, a complete and reliable cover plate 5 structure is formed. This helps to enhance the connection strength and integrity between the cover plate 5 and the pile foundation, and reduces the displacement or damage of the cover plate 5 during use.

[0092] More specifically, there are several pre-reserved steel bars 10, each pre-reserved steel bar 10 including a connecting portion 101 and an inclined extension portion 102, the inclined extension portion 102 being fixed to the top of the connecting portion 101.

[0093] Reference Figure 7 When the steel pipe pile 1 is connected to the cover plate 5, several reserved steel bars 10 are evenly distributed along the inner wall of the steel pipe pile 1, so that the inclined extension 102 is inclined downward from the outside to the connection 101. The connection 101 is welded to the inner wall of the steel pipe pile 1 along its length. The cover plate 5 completely covers the inclined extension 102 until it is fixedly connected to the top of the steel pipe pile 1. When connecting sheet pile 2 to cover plate 5, refer to Figure 8 and Figure 9 Several pre-reserved reinforcing bars 10 are set on the inner wall of the sheet pile 2, so that the inclined extension 102 is inclined downward from the outside to the connecting part 101. The connecting part 101 is welded to the inner wall of the sheet pile 2 along its length. The cover plate 5 completely covers the inclined extension 102 until it is fixedly connected to the top of the sheet pile 2.

[0094] In this application, the inclined extension 102 is inclined downward from the outside to the connection 101, which can provide a larger contact area, help to enhance the connection strength and stability between the reserved steel bar 10 and the cover plate 5, and also help to disperse stress and reduce stress concentration at the connection point. Several reserved steel bars 10 are evenly distributed along the inner wall of the steel pipe pile 1 and are firmly connected to the inner wall of the pipe by continuous welding, which helps to provide the integrity between the cover plate 5 and the steel pipe pile 1, effectively transfer the load, and improve the bearing capacity of the structure. At the same time, the cover plate 5 completely covers the inclined extension 102 until it is fixedly connected to the top of the steel pipe pile 1, which further enhances the reliability and stability of the connection. The special structure of the inclined extension 102 and the connection 101, as well as the continuous welding method, significantly improve the connection strength and stability between the steel sheet pile 2 and the cover plate 5, which can effectively resist horizontal and vertical loads and ensure the safety of the foundation pit support structure.

[0095] In addition, refer to Figure 10 and Figure 11 In the construction of the foundation pit support, a triangular brace 13 is set between the steel waist beam 3 and the steel sheet pile 2. The steel waist beam 3 and the steel sheet pile 2 are welded to the triangular brace 13 respectively. Plain concrete 15 is backfilled between the steel waist beam 3 and the steel sheet pile 2.

[0096] Specifically, a suspension rod 14 is connected between the steel girders 3 and the steel sheet piles 2. The angle between the suspension rod 14 and the horizontal line is 30°-60°, and the included angle shown in the attached drawings of this application is 45°. The two ends of the suspension rod 14 are welded to the steel girders 3 and the steel sheet piles 2 respectively. This application can provide multiple suspension rods 14 along the vertical direction of the steel sheet piles 2 to further improve the connection stability between the steel girders 3 and the steel sheet piles 2.

[0097] The triangular brace 13 has good structural stability and can effectively resist horizontal forces to prevent deformation or displacement of the support structure. The steel wainscoting 3 and steel sheet pile 2 are welded to the triangular brace 13 to form a firm connection between the three, which can effectively transfer loads and improve the stability of the overall structure. Plain concrete 15 can fill the gaps, provide additional support, and help reduce groundwater infiltration. By backfilling plain concrete 15 between the steel wainscoting 3 and the steel sheet pile 2, the integrity and stability of the support structure can be further enhanced, ensuring the safe construction of the foundation pit.

[0098] Construction of the replacement beam and pre-supported foundation: The formwork for the pre-supported foundation 16 and the replacement beam adopts brick formwork. In this application, the selection and thickness of the brick formwork are determined according to the depth of the side formwork, so that the brick formwork can better withstand the lateral pressure of the outer soil after it is built, and prevent slope slippage or collapse due to surface water erosion.

[0099] Further, the interface and rebar installation are carried out. In this process, the pile to be replaced is first laid out and marked to determine the roughening position. After the cement slurry on the surface of the original pile is roughened, the concrete surface is chiseled into an uneven surface, an interface agent is applied, it is cleaned and fully moistened. Then the pile to be replaced is laid out and the rebar hole positions are marked. Holes are drilled using a drilling tool. After the rebar on the same side is installed, the holes on other sides are drilled. This process is repeated until all the anchor bars are anchored.

[0100] When installing rebar on piles, the work should be carried out in batches, with each batch of rebar installation not exceeding two holes in any horizontal section of the pile. The next batch of rebar installation should only begin after the adhesive from the previous batch has cured. After the holes are cleaned, they are filled with adhesive, and then the rebar is inserted.

[0101] In this application, when installing rebar, it is required that the original concrete of the anchoring part of the rebar must not have local defects, the adhesive used for rebar installation is a modified epoxy and modified vinyl ester adhesive, the length of the rebar is greater than the depth of the anchor hole, the positional deviation is controlled within the set range, the old and new concrete are connected by rebar installation, the concrete surface needs to be roughened and an interface treatment agent is applied.

[0102] Furthermore, during the pouring of the replacement beam, it is crucial to ensure the precision of the steel reinforcement used in the construction process. The reinforcement should be straight without local bending, and the surface should be free of cracks, peeling, or breakage after forming. If any breaks occur, they must be welded for fixation. Cement mortar is used to form concrete protective layer spacers, which are then tied to the bottom of the tie plate with wire before pouring the replacement beam. The concrete for the replacement beam is waterproof concrete, poured using a concrete pump, with the slump controlled within the preset range. The concrete is vibrated in layers using an immersion vibrator until the surface shows a layer of slurry and no longer settles.

[0103] Pre-top: Reference Figure 12 During the pre-lifting of the replacement beam, a gap is reserved under the replacement beam at the top of the new replacement pile. The jacks 17 with self-locking function are used to apply pre-pressure to the beam to achieve controllable force between the pile and the beam. In this application, three jacks 17 are used for pre-lifting, arranged in a ring at 120° intervals on the top of the replacement pile. Steel pipe blocks 18 are placed between adjacent jacks 17 as safety devices. After the steel pipe blocks 18 are placed, steel wedges are inserted into the gaps to make them tightly connected to the bottom of the replacement beam. At the same time, the pre-lifting loading adopts the principle of graded loading. The load increment of each grade is 10% of the upper limit of the jack 17 loading value, and the secondary load must be added only after the structure is stable.

[0104] During the construction of the replacement beam and the pre-topped foundation 16, steel plates are pre-embedded at the corresponding positions of the jacks 17. After the replacement beam is completed and reaches 85% of its design strength and the bored piles reach the preset age, the brick formwork between the bottom of the replacement beam and the pre-topped foundation 16 is removed.

[0105] In this application, during the underpinning construction, the pre-jacking measures can effectively eliminate the adverse effects of the deformation of the new underpinning pile on the underpinning system, prevent the settlement of the top of the new underpinning pile from causing the settlement of the original bridge pile, and achieve active underpinning.

[0106] Piling sealing: After the pre-jacking is completed, while keeping the pre-jacking force stable, weld the reserved steel bars in the pile core, add steel spacers, and pour the pile core concrete. When the pile core concrete reaches 90%, remove jack 17, weld the reserved steel bars on the outside, and fill the gap between the pre-jacking pile cap 16 and the supporting beam with micro-expansion concrete. Then, grout the gap between the sealing concrete and the bottom hole of the supporting beam through the pre-embedded grouting pipe.

[0107] Pile cutting: After the pre-topping is completed and the micro-expansion concrete at the pile end is poured, the pile cutting operation pit is excavated on both sides of the replacement beam next to the original pile. When the concrete of the replacement pile cap reaches 100%, the pile is cut in the pit using tools such as wire saw. During the pile cutting process, the settlement of the broken pile column and the slippage of the interface between the old and new concrete are continuously monitored.

[0108] Backfilling: The backfilling process adopts layered and uniform compaction to avoid uneven stress on the structure. For areas where backfilling is difficult, micro-expansion cement grout is injected for backfilling.

[0109] The above are all preferred embodiments of this application. These embodiments are merely explanations of this application and are not intended to limit the scope of protection of this application. Therefore, all equivalent changes made in accordance with the structure, shape, and principle of this application should be covered within the scope of protection of this application.

Claims

1. A pile foundation underpinning construction technology suitable for karst geology, characterized in that, The construction steps include the following: Construction of pile reinforcement body: The pile foundation replacement adopts the pile grinding method when the shield tunnel passes through, the conditions for shield tunnel opening are reserved, the pre-reinforced body is set in advance, and the reinforcement body is reinforced by grouting through ground sleeve valve pipe to obtain a strongly weathered stratum; Foundation pit support construction: Steel sheet piles (2) are used to reinforce the side of the foundation pit adjacent to the road. Steel waist beams (3) and newly built support beams (4) are set in the area enclosed by the steel sheet piles (2). The two long sides and corners of the steel waist beams (3) are reinforced by steel pipe supports (6). Construction of cover plate (5): Several steel pipe piles (1) are set around the corner of the foundation pit. Two adjacent steel pipe piles (1) are vertically parallel. A cover plate (5) is set on the foundation pit. The cover plate (5) is connected to the foundation pit by several steel pipe piles (1) at the same time. The part of the foundation pit with the cover plate (5) completely covers the steel sheet piles (2). The construction of the cover plate (5) specifically includes the following steps: The construction of the cover plate (5) is carried out after the steel sheet piles (2) and steel pipe piles (1) within its coverage area are completed. After the ground is broken, the excavation is carried out, the cushion layer is poured and the reinforcing bars are tied, the side formwork is installed and the concrete is poured. Among them, when tying the reinforcing bars, the steel sheet piles (2) and steel pipe piles (1) are respectively provided with reserved reinforcing bars (10), and the reserved reinforcing bars (10) are fixedly connected to the main reinforcing bars of the cover plate (5). The specific construction steps in foundation pit support construction include: Take multiple sheet piles (2), and fix the sheet piles (2) one by one on the working ground using a vibratory pile driver. The inclination of each sheet pile (2) shall not exceed 2%. The two adjacent sheet piles (2) are interlocked with each other. When they are joined, the straightness deviation between them is controlled within 0.1%, forming a screen-like screen wall. The specific construction steps in foundation pit support construction include: Based on the tilt direction, length, and verticality of the screen wall, the construction sequence of the screen wall includes forward sequence, reverse sequence, reciprocating sequence, split sequence, neutralization sequence, and composite sequence. The construction of the cover plate (5) specifically includes the following steps: There are several reserved steel bars (10), each reserved steel bar (10) including a connecting part (101) and an inclined extension part (102), the inclined extension part (102) being fixed to the top of the connecting part (101); When the steel pipe pile (1) is connected to the cover plate (5), several of the reserved steel bars (10) are evenly distributed along the inner wall of the steel pipe pile (1), so that the inclined extension (102) is inclined downward from the outside to the connection part (101), the connection part (101) is welded to the inner wall of the steel pipe pile (1) along the whole length, and the cover plate (5) completely covers the inclined extension (102) until it is fixedly connected to the top of the steel pipe pile (1); When the sheet pile (2) is connected to the cover plate (5), several of the reserved steel bars (10) are set on the inner wall of the sheet pile (2), so that the inclined extension (102) is inclined downward from the outside to the connecting part (101). The connecting part (101) is welded to the inner wall of the sheet pile (2) along its length. The cover plate (5) completely covers the inclined extension (102) until it is fixedly connected to the top of the sheet pile (2). The specific construction steps in foundation pit support construction include: A triangular brace (13) is provided between the steel waist beam (3) and the steel sheet pile (2). The steel waist beam (3) and the steel sheet pile (2) are respectively welded to the triangular brace (13). Plain concrete (15) is backfilled between the steel waist beam (3) and the steel sheet pile (2).

2. The pile foundation underpinning construction technology suitable for karst geology according to claim 1, characterized in that, The construction of pile-cutting reinforcement also includes specific construction steps: Hole formation: Holes are drilled on the working ground using a drilling rig, and bentonite slurry is used for wall protection during the hole formation process; Grouting pipes: Solid pipes are used in non-reinforced areas and above the top of the karst cave, while perforated pipes are used in reinforced areas and within the karst cave. A cap is added to the bottom of the grouting pipe. The grouting pipe is lowered to the bottom of the grouting body or the bottom of the karst cave, and an air vent is provided. Grouting slurry preparation; Grouting: First, water injection is carried out, and the grouting pipeline is checked and the grout absorption capacity of the stratum is determined.

3. The pile foundation replacement construction technology suitable for karst geology according to claim 2, characterized in that, Grouting also includes specific construction steps: Retreating segmented grouting: A grouting core tube is installed inside the grouting pipe. Grouting is carried out from bottom to top in the grouting pipe, and the grouting pipe is divided into several grouting sections. After the first grouting section is completed, the grouting core tube is lifted up to carry out the grouting construction of the second grouting section. When the length of the grouting core tube pulled out is greater than the length of one pipe section, grouting is stopped, the pulled-out part of the grouting core tube and the joint are removed, the joint is connected to the grouting core tube that has not been pulled out, and grouting continues. This process is repeated until the grouting is completed.

4. The pile foundation replacement construction technology suitable for karst geology according to claim 3, characterized in that, Grouting also includes specific construction steps: Repeated grouting: The time interval between two grouting sessions is 6 to 10 hours, and the number of grouting sessions is no less than 3. Grouting is stopped when the final hole pressure is met, and the hole is sealed with cement mortar.

5. A pile foundation underpinning construction technology suitable for karst geology according to any one of claims 2-4, characterized in that, The specific construction steps for the grouting pipe are as follows: Before the grouting pipe is lowered, it is filled with water. At the same time, the upper end of the grouting pipe is exposed at the orifice and covered with a cap. After the grouting pipe is lowered, cement mortar is filled between the bottom end of the grouting pipe and the orifice wall for reinforcement.

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