Pile foundation underpinning construction technology suitable for karst geology

Abstract

The invention 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 reinforcement body construction: adopting a pile grinding mode when a shield passes through the pile foundation underpinning, reserving a shield cabin opening condition, arranging a pre-reinforcement body in advance, and performing grouting reinforcement on the reinforcement body by adopting a ground sleeve valve pipe; a strong weathering stratum is obtained; the side, close to the road, of the foundation pit is reinforced through steel sheet piles, a steel waist beam and a newly-built underpinning beam are arranged in the interval defined by the steel sheet piles, and the opposite side faces and corners of the two long edges of the steel waist beam are reinforced through steel pipe supports; steel pipe piles are arranged on the peripheries of the corners of the foundation pit, a cover plate is arranged on the foundation pit, the cover plate and the foundation pit are connected through the steel pipe piles at the same time, and the part, provided with the cover plate, of the foundation pit completely covers the steel plate piles. Pile foundation underpinning can be achieved under the construction scene of karst geology, meanwhile, it is guaranteed that the underpinning structure is stably installed, and deformation is controlled within the safe range.

Description

Technical Field

[0001] The present 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 Art

[0002] Pile underpinning is a process of replacing foundations using pile foundations, that is, using a newly constructed underpinning structure to support the columns, piles or foundations that need to be replaced, and transferring the load they bear to the newly constructed underpinning structure.

[0003] When carrying out underground pile foundation replacement projects, we are restricted by the buildings, pipelines, and geology around the work site, and we also consider the impact of the construction area on the surrounding environment noise and vibration. Especially when facing underground projects with karst geology, due to the strong development of karst, the size of the cave entrances is different, the distribution is poor, the shapes and scales are different, the karst caves are not well filled, and most of them are not filled. The filling materials in the filling state are weak in engineering properties and are easily eroded by water flow. The local cave fillings are mixed with gravel, which are recent collapse materials. Most of them are unstable caves, which are very unfavorable to the stability of the line and have a great impact on tunnel excavation. They are easy to cause ground collapse, ground subsidence, and tunnel collapse.

[0004] At present, the adaptability of the pile foundation underpinning structure to the uneven settlement of the new and old piles is limited. Before the underpinning piles are cut, the underpinning piles have friction on the outside of the tunnel passing through, and the soil pressure of the soil near the karst on the tunnel passing through meets the anti-floating design requirements of the original structure. After the underpinning piles are cut, under the influence of unfavorable geology, the newly constructed underpinning structure is deformed or overturned due to the loosening of the soil near the karst, and the soil pressure of the soil near the karst on the tunnel passing through cannot meet the anti-floating design of the new structure. Summary of the invention

[0005] In order to realize pile foundation underpinning in the construction scenario of karst geology, while ensuring stable installation of the underpinning structure and controlling deformation within a safe range, the present application provides a pile foundation underpinning construction process suitable for karst geology.

[0006] The present application provides a pile foundation underpinning construction process suitable for karst geology, which adopts the following technical solution: A pile foundation underpinning construction process suitable for karst geology includes the following construction steps: Construction of pile cutting and reinforcement: When the shield passes through, the pile foundation underpinning adopts the method of grinding piles, reserves the conditions for the shield opening, sets up the pre-reinforcement bodies in advance, and uses the sleeve valve pipe on the ground to carry out grouting reinforcement to obtain the strongly weathered stratum; Foundation pit support construction: Steel sheet piles are used to reinforce the foundation pit near the road. Steel waist beams and newly built underpinning beams are set in the area surrounded by the steel sheet piles. The two long sides of the steel waist beam are supported and reinforced by steel pipes on the opposite sides and corners. Cover plate construction: Several steel pipe piles are set at the outer corners of the foundation pit. Two adjacent steel pipe piles are vertically parallel. A cover plate is set on the foundation pit. The cover plate and the foundation pit are connected simultaneously by several steel pipe piles. The part of the foundation pit where the cover plate is set completely covers the steel sheet piles.

[0007] By adopting the above-mentioned technical scheme and the pile grinding method, the pile foundation can be accurately processed when the shield passes through, creating conditions for the subsequent shield opening, 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 stratum, thereby achieving the purpose of strengthening the stratum and providing a solid foundation for pile foundation replacement. The steel sheet piles are used to reinforce the foundation pit adjacent to the road side, which effectively reduces the collapse and displacement of the soil during the excavation of the foundation pit and improves the construction safety. In this application, the setting of steel waist beams and newly built replacement beams, combined with steel pipe support reinforcement, form a stable support system, which significantly enhances the overall stability and bearing capacity of the foundation pit. In addition, the vertical parallel setting of the steel pipe piles provides a stable support point 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 in the foundation pit. At the same time, a path is provided in the construction site for vehicles to travel, minimizing the adverse effects on the surrounding traffic and environment.

[0008] Compared with the prior art, the present application, through the above-mentioned processing steps, first strengthens the foundation and then carries out pile foundation underpinning construction, which can better adapt to the underground engineering construction of karst geology and has stronger applicability, thereby being able to effectively overcome the earth pressure of the soil near the karst on the underpass, and avoiding deformation or rollover of the newly constructed underpinning structure. It can realize pile foundation underpinning in the construction scenario of karst geology, while ensuring the stable installation of the underpinning structure and controlling the deformation within a safe range.

[0009] Preferably, in the construction of the pile reinforcement body, the following construction steps are specifically included: Drilling: Drill holes on the working ground with a drilling rig, and use bentonite slurry to protect the wall during the drilling process; Lower grouting pipe: solid pipes are used in non-reinforced areas and above the cave roof, and flower pipes are used in reinforced areas and within the cave. A cover is added to the bottom of the grouting pipe, and the grouting pipe is lowered to the bottom of the grouting body or the bottom of the cave, and an exhaust hole is set; Grouting slurry preparation; Grouting: First carry out water injection treatment, check the grouting pipeline and judge the grouting absorption capacity of the formation.

[0010] By adopting the above technical scheme, a drilling rig is used to drill holes on the working ground, and bentonite slurry is used to protect the wall, so that the drilling process is more stable and the hole is more complete. In the present application, solid pipes are used in non-reinforced areas and above the top plate of the cave. The solid pipe part is used to ensure that the grouting liquid will not leak into the non-reinforced area, while flower pipes are used in the reinforced area and within the scope of the cave. The flower pipe part can better disperse the grouting liquid to the area that needs to be reinforced, so as to achieve uniform and effective reinforcement; the special preparation of grouting slurry is helpful to adjust the composition and performance of the slurry according to specific geological conditions and engineering requirements, so as to ensure that the grouting effect is optimal, thereby helping to further enhance the strength and stability of the reinforced body; in addition, by performing water injection treatment, it can be used to check whether the grouting pipeline is unobstructed, and the grouting capacity of the formation can be judged, which provides an important reference for subsequent grouting operations, and can ensure the smooth progress of the grouting process, and reduce problems such as uneven grouting or blockage caused by differences in the grouting capacity of the formation.

[0011] Preferably, the grouting process further includes the following construction steps: Retreat segmented grouting: A grouting core tube is arranged in the grouting pipe, and grouting is carried out from the bottom of the hole to the top in the grouting pipe, and the grouting pipe is divided into several grouting sections. After the first grouting section is grouted, the grouting core tube is lifted up to carry out grouting construction of the second grouting section. When the pulled-out length of the grouting core tube is greater than the length of one section of the pipe, the 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 the grouting is continued, and 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 evenly 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 tube, the grouting process can flexibly adapt to different stratum conditions, thereby improving the flexibility and efficiency of construction.

[0013] Preferably, the grouting process further includes the following construction steps: Repeat grouting: The time interval between two groutings is 6 to 10 hours, and the number of groutings is not less than 3 times. Stop grouting when the final hole pressure is met, and seal the hole with cement mortar.

[0014] By adopting the above technical solution, the time interval between two groutings is set to 6 to 10 hours, and the number of groutings is ensured to be no less than 3 times, which helps to improve the density of the grouting and the reinforcement effect. In addition, repeated grouting can fully fill the voids in the formation, enhance the integrity and stability of the reinforced body, stop grouting when the final hole pressure is met, and seal the hole with cement mortar. This operation further ensures the reliability and durability of the grouting quality.

[0015] Preferably, in the lower grouting pipe, the construction steps specifically include: The grouting pipe is filled with water before being lowered, and the upper end of the grouting pipe exposes the hole and is covered with a cover. After the grouting pipe is lowered, cement mortar is filled between the bottom end of the grouting pipe and the hole wall for reinforcement.

[0016] By adopting the above technical solution, the grouting pipe is filled with water before being lowered. On the one hand, it is helpful to check the sealing and patency of the grouting pipe to ensure the smooth progress of the subsequent grouting process; on the other hand, the clean water in the grouting pipe can use gravity to offset the buoyancy of the casing material, so that the grouting pipe can be lowered more smoothly and easily.

[0017] The hole at the upper end of the grouting pipe is exposed and covered with an upper blind cover, which can prevent debris from entering the grouting pipe. The cooperation of the upper blind cover and the lower blind cover can effectively reduce the leakage of grouting from both ends of the grouting pipe, and can ensure that the grouting material can be fully and evenly filled into the target area, thereby improving the grouting effect. At the same time, it helps to control the pressure and speed of the grouting, realize fine control of the pressure and speed during the grouting process, and ensure the safety and stability of the grouting process.

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

[0019] Preferably, in the construction of the cover plate, the construction steps are specifically included: The cover slab construction is carried out after the steel sheet piles and steel pipe piles in its coverage area are completed. Excavation is carried out after the ground is broken, and the steel bars are tied after the cushion layer is poured. Concrete is poured after the side formwork is installed. When tying the steel bars, reserved steel bars are respectively arranged inside the steel sheet piles and steel pipe piles, and the reserved steel bars are fixedly connected to the main bars of the cover slab.

[0020] By adopting the above technical solution, after a series of operations such as breaking the ground, excavating, pouring the cushion layer, tying steel bars, installing side formwork and pouring concrete, a cover plate structure with complete structure and reliable strength 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, in the foundation pit support construction, the construction steps are specifically included: Take a plurality of steel sheet piles and fix them one by one on the working ground by a vibrating pile driver, wherein the slope of each steel sheet pile does not exceed 2%; Two adjacent steel sheet piles are interlocked with each other, and when they are butt-jointed, the straightness deviation between the two is controlled within 0.1%, forming a screen-like screen wall.

[0022] By adopting the above technical solution, the steel sheet piles are fixed one by one on the working ground using a vibrating pile driver, and the slope of each steel sheet pile is controlled within 2%. The strict control of the slope ensures the tight fit between the steel sheet piles, enhances the integrity and stability of the support structure, and helps to improve the stability and precision of the steel sheet piles; two adjacent steel sheet piles are interlocked with each other, and the straightness deviation during docking is controlled within 0.1%, forming a screen-like screen wall. Compared with a single steel sheet pile, the screen wall has good anti-lateral displacement ability, can effectively avoid the leakage of soil and moisture, and further improves its support effect.

[0023] Preferably, in the foundation pit support construction, the construction steps are specifically included: According to the inclination direction, length and vertical condition of the screen wall, the construction sequence of the screen wall includes forward sequence, reverse sequence, reciprocating sequence, center-dividing sequence, neutralizing sequence and compound sequence.

[0024] By adopting the above technical scheme, different construction sequences such as forward sequence, reverse sequence, reciprocating sequence, center-division sequence, neutralization sequence and compound sequence are flexibly adopted according to the inclination direction, length and vertical condition of the screen wall. The diversified construction sequence selection makes the construction process more flexible and efficient, and can adapt to various complex construction site environments. Reasonable construction sequence arrangement helps to reduce stress concentration and deformation during the construction process, and ensure the overall stability and safety of the screen wall.

[0025] Preferably, in the construction of the cover plate, the construction steps are specifically included: There are several reserved steel bars, each of which includes a connecting portion and an inclined extension portion, wherein the inclined extension portion is fixed to the top of the connecting portion; When the steel pipe pile is connected to the cover plate, the plurality of reserved steel bars are evenly distributed along the inner wall of the steel pipe pile, so that the inclined extension portion is inclined downward from the outside to the connection portion, the connection portion is welded to the inner wall of the steel pipe pile through the length, and the cover plate completely covers the inclined extension portion until it is fixedly connected to the top of the steel pipe pile; When the steel sheet pile is connected to the cover plate, several of the reserved steel bars are arranged on the inner wall of the steel sheet pile, so that the inclined extension part is inclined downward from the outside to the connecting part, the connecting part and the inner wall of the steel sheet pile are welded through the length, and the cover plate completely covers the inclined extension part until it is fixedly connected to the top of the steel sheet pile.

[0026] By adopting the above technical solution, the inclined extension part is inclined downward from the outside to the connection part, which can provide a larger contact area, help to enhance the connection strength and stability between the reserved steel bars and the cover plate, and also help to disperse stress and reduce stress concentration at the connection point; Several reserved steel bars are evenly distributed along the inner wall of the steel pipe pile and firmly connected to the inner wall of the pipe through full-length welding, which helps to provide 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, further enhancing the reliability and stability of the connection; The special structure of the inclined extension and connection parts, as well as the full-length welding method, have significantly improved the connection strength and stability between the steel sheet piles and the cover plate, and can effectively resist horizontal and vertical loads, ensuring the safety of the foundation pit support structure.

[0027] Preferably, in the foundation pit support construction, the construction steps are specifically included: A triangular support is arranged between the steel waist beam and the steel sheet piles, the steel waist beam and the steel sheet piles are respectively welded to the triangular support, and plain concrete is backfilled between the steel waist beam and the steel sheet piles.

[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 waist beam and steel sheet piles are welded to the triangular brace respectively, so that the three are firmly connected, which can effectively transfer the load 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 waist beam and the steel sheet piles, the integrity and stability of the support structure can be further enhanced to ensure the safe progress of the foundation pit construction.

[0029] In summary, the present application includes at least one of the following beneficial technical effects: 1. This application can better adapt to the construction of underground projects in karst geology by strengthening the foundation first and then carrying out pile foundation underpinning construction. It has stronger applicability, and can effectively overcome the earth pressure of the soil near the karst on the underpass tunnel, avoiding deformation or rollover of the newly constructed underpinning structure, and can realize pile foundation underpinning in the construction scene of karst geology, while ensuring the stable installation of the underpinning structure and controlling the deformation within a safe range; 2. Strict control of the inclination of the steel sheet piles ensures the close fit 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; two adjacent steel sheet piles are interlocked to form a screen-like screen wall. Compared with a single steel sheet pile, the screen wall has good anti-lateral displacement ability, can effectively avoid soil and water leakage, and further improves its support effect; 3. Segmented grouting can ensure that each part of the stratum is fully and evenly 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 tube, the grouting process can flexibly adapt to different stratum conditions, thereby improving the flexibility and efficiency of construction. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a process flow chart of punching piles in the embodiment of the present application.

[0031] Figure 2 It is a plan view of pile foundation underpinning in the first case in the embodiment of the present application.

[0032] Figure 3 It is a plan view of pile foundation underpinning in the second case in the embodiment of the present application.

[0033] Figure 4 It is a plan view of pile foundation underpinning in the third case in the embodiment of the present application.

[0034] Figure 5 It is a schematic diagram of steel sheet pile connection in an embodiment of the present application.

[0035] Figure 6 It is a schematic diagram of the arrangement of the fixing plates in the embodiment of the present application.

[0036] Figure 7 This is a large-scale drawing of the connection between the steel pipe pile and the cover plate in the embodiment of the present application.

[0037] Figure 8 This is a large-scale drawing of the connection between the steel sheet piles and the cover plate in the embodiment of the present application.

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

[0039] Fig.10 This is a large-scale drawing of the nodes between the steel sheet piles, waist beams, and supports in the embodiment of the present application.

[0040] Fig.11 This is a large-scale drawing of the concrete filled between the steel sheet piles and the waist beam in the embodiment of the present application.

[0041] Fig.12 It is a layout diagram of the pre-jacking platform jack in the embodiment of the present application.

[0042] Explanation of the reference numerals in the drawings: 1. Steel pipe piles; 2. Steel sheet piles; 3. Steel waist 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 steel bars; 101. Connection part; 102. Inclined extension part; 11. Reinforced tie plate; 12. Guide frame; 13. Triangle support; 14. Hanger; 15. Plain concrete; 16. Pre-jacking base; 17. Jack; 18. Steel pipe block. DETAILED DESCRIPTION

[0043] The following is combined with Figure 1-12 This application is described in further detail.

[0044] The embodiment of the present application discloses a pile foundation underpinning construction process suitable for karst geology.

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

[0046] Understand the geological and hydrological data of the pile foundation in detail, grasp the location of the cave, and roughly infer its scope, size and filling condition, and formulate appropriate construction plans and technical support measures based on the results of advance drilling of each pile.

[0047] Before construction, the site must be surveyed. If there are overhead wires, underground cables, water supply and drainage pipes, gas pipelines and other facilities on the site that hinder construction or affect safe operations, they must be cleared, relocated or properly handled before construction can begin.

[0048] Drainage ditches are set up around the site, and the performance of the construction machinery must meet the design requirements for pile formation. Bored pile construction can only be carried out after the foundation pit is dug to the designed 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, there is a high risk of karst ground collapse under the action of force. Therefore, before construction, it is necessary to fully investigate the karst morphology, karst distribution pattern, filling material characteristics, cave stability, whether there are toxic gases, and the distribution of toxic gases, and then make corresponding plans based on the actual survey conditions.

[0050] Construction of pile cutting and reinforcement bodies: Pile foundation underpinning adopts the method of pile grinding when the shield passes through, reserves the conditions for shield opening, sets up pre-reinforcement bodies in advance, and uses ground sleeve valve pipes for grouting reinforcement to obtain strongly weathered strata.

[0051] Specifically, the present application provides a preferred implementation method. The design requires the reinforcement height to be 2 meters above the tunnel top and 2 meters below the tunnel bottom for strongly weathered strata. The reinforcement width is 2 meters outside the outer contour of the tunnel. The reinforcement length is 20.4 meters. Due to the excavation stage of the foundation pit, the strata at the bottom of the foundation pit are sand layers or clay layers. In order to reinforce the base after excavation, the grouting reinforcement body is expanded to the base elevation after excavation to 2 meters below the tunnel bottom. The reinforcement body construction is carried out before the bored piles and foundation pit construction, and the pipeline conditions are explored in advance.

[0052] The shield pile cutting reinforcement body is reinforced by grouting by installing sleeve valve pipes through ground drilling. The drilling spacing is 1m×1m, the bottom surface of the reinforcement body is buried at a depth of 23m~24m, and the drilling depth reaches the cave bottom elevation according to the requirements of cave treatment. The grouting reinforcement range is a rectangular range extending 2m around the tunnel, and the grouting body is 6m long.

[0053] In this application, the construction of the pile reinforcement body specifically includes the following construction steps: Drilling: Drill holes on the working ground with a drilling rig. Bentonite slurry is used to protect the wall during the drilling process, making the drilling process more stable and the hole more complete.

[0054] Lower grouting pipe: solid pipe is used in non-reinforced area and the part above the cave top plate, and decorative pipe is used in reinforced area and cave. A cover is added to the bottom of the grouting pipe. The grouting pipe is lowered to the bottom of the grouting body or the cave bottom, and exhaust holes are set. There is at least one exhaust hole in each cave, and exhaust holes are added if the spacing exceeds 4 meters.

[0055] In this application, solid pipes are used in non-reinforced areas and above the cave roof. The solid pipe part is used to ensure that the grouting liquid will not leak into the non-reinforced area, while flower pipes are used in the reinforced area and the cave range. The flower pipe part can better disperse the grouting liquid to the area that needs to be reinforced, achieving uniform and effective reinforcement. After connecting the solid and flower pipes according to the requirements, fill the pipe with clean water to check the sealing performance, and lower the grouting pipe along the drill hole to the designed elevation, with the upper end exposed about 40 cm above the ground, and cover it with a cover to prevent debris from entering the grouting pipe.

[0056] To ensure smooth pipe lowering, the pipe should be filled with clean water, which helps to check the sealing and patency of the grouting pipe, ensure the smooth progress of the subsequent grouting process, and can offset the buoyancy of the casing material with gravity. After the pipe is lowered, cement mortar is filled in the gap between the hole wall within 1 to 2 meters close to the ground for reinforcement. The cooperation of the upper and lower caps can effectively reduce the leakage of grouting 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 pressure and speed of grouting, realize the fine control of pressure and speed during the grouting process, and ensure the safety and stability of the grouting process.

[0057] During the foundation pit excavation stage, the stratum at the bottom of the foundation pit is a sand layer or a clay layer. In order to reinforce the base after excavation, cement mortar is filled between the bottom end of the grouting pipe and the hole wall for reinforcement, and the grouting reinforcement body is expanded to the base elevation after excavation to 2 meters below the tunnel bottom. This operation helps to enhance the stability and fixing effect of the grouting pipe to prevent displacement or leakage of the grouting pipe during the grouting process.

[0058] Preparation of grouting slurry: Estimate the volume of the pre-prepared slurry according to the volume of the grouting body and the area and height of the cave, calculate the required amount of cement and water according to the water-cement ratio, add the calculated amount of water into the mixer, add the calculated amount of cement while stirring, stir evenly and pour into the slurry storage tank for use.

[0059] Grouting: Water injection is first carried out to check whether the grouting pipeline is unobstructed, check the grouting pipeline and judge the grouting absorption capacity of the formation, provide important reference for subsequent grouting operations, ensure the smooth progress of the grouting process, and reduce problems such as uneven grouting or blockage caused by differences in the grouting absorption capacity of the formation.

[0060] The present application provides a preferred implementation mode, wherein the grouting construction specifically comprises the following steps: Reverse segmented grouting: A grouting core tube is arranged in the grouting pipe, and grouting is carried out from the bottom of the hole to the top in the grouting pipe. Each grouting section is 0.5 meters long, and the grouting pipe is divided into several grouting sections. After the first grouting section is filled, the grouting core tube is lifted up to carry out grouting construction of the second grouting section. When the pulled-out length of the grouting core tube is greater than the length of one section of the pipe, the 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 the grouting is continued. This process is repeated until the grouting is completed.

[0061] Segmented grouting can ensure that each part of the stratum is fully and evenly reinforced, avoiding problems such as uneven reinforcement or omissions in 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] During the grouting process, if it is necessary to pause the grouting, the cement slurry pipe must be taken out and placed in a clean water bucket, and at the same time the grouting core pipe must be lifted by 0.4 meters. After injecting clean water into the hole, the grouting can be stopped. This will keep the pipeline unobstructed and ensure that the grouting is not affected by water injection.

[0063] For double-liquid grouting, when it is necessary to pause grouting or before each grouting is completed, use single-liquid grouting for minutes. After taking out the grouting core tube, insert a steel pipe into the bottom of the grouting tube and flush the grouting tube from bottom to top with clean water. For single-liquid grouting, flush it directly after stopping the grouting and cover the top of the tube.

[0064] Repeated grouting: The time interval between two groutings is 6 to 10 hours, and the number of groutings is not less than 3 times. This helps to improve the density of grouting and the reinforcement effect. In addition, repeated grouting can fully fill the voids in the formation, enhance the integrity and stability of the reinforced body, stop grouting when the final hole pressure is met, and seal the hole with cement mortar, which can further ensure the reliability and durability of the grouting quality.

[0065] During the grouting process, make a grouting work record sheet, including the grouting work status of the grouting holes and the grouting process time. Analyze and improve the grouting work at any time during the grouting process; and carefully record the actual hole position, hole depth, underground objects in the hole, water gushing, etc. When it is inconsistent with the geological report, take measures to correct it.

[0066] Punching cast-in-place piles: Due to the limited clearance height of the construction site, it is difficult to use a rotary drilling rig to drill holes. A pile driver is used in the drilling process. Figure 1 The construction steps of bored piles include: laying out the pile position, exploring holes and testing underground pipelines, burying casings, positioning and centering the drill rig, drilling, scooping up drill cuttings, hole completion acceptance, first hole cleaning, installing steel skeleton, shifting the drill rig to lower the guide tube, second hole cleaning, pouring underwater concrete, and recycling construction tools.

[0067] Specifically, during the impact drilling construction process, we first understand the geological conditions, determine the pile position, and level the site. Before each pile foundation construction, a "one pile, one drill" geological condition check is completed at the pile position, and appropriate construction plans and technical support measures are formulated based on the advance drilling results of each pile. Based on the on-site conditions, surface interception, drainage and protection measures are built on the pile foundation construction area; the slope soil on the top of the pile is cleaned and deloaded.

[0068] In this application, before punching, a steel plate casing is buried at the hole mouth to fix the pile position and prevent the hole mouth from collapsing. A grout overflow port is opened at the upper edge of the casing, and 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 buried above the construction ground, and the gap between the casing and the hole 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 records are kept to verify the drilling depth after drilling.

[0069] Furthermore, different pile positions on the same cap cannot be constructed continuously. The strength of the previous pile foundation must reach more than 70% of the design strength before the next pile construction can be carried out to reduce the shear damage of the cast-in-place concrete piles caused by impact drilling. In this process, by adopting an appropriate pile spacing to prevent hole collapse and diameter shrinkage, the concrete cast-in-place pile hole can rely on the mud to reach a balanced state.

[0070] At the beginning of punching, maintain a low impact height to form a solid mud wall at the foot of the casing. After punching to the preset height under the foot of the casing, gradually increase the drilling speed according to the soil conditions, and control the mud density at 1.4; when punching in the clay layer, increase the impact height, increase the pump volume, and control the mud density at 1.25; when just entering the rock layer, lower the impact height, and restore the normal impact height after the rock surface is flattened, and control the mud density at 1.35; at the bedrock level, use low hammer impact or intermittent impact; when impacting in the bedrock, use a long stroke to speed up the impact frequency and increase the impact energy.

[0071] The present application continuously rotates the impact head to change the impact position of the punch at the bottom of the hole to avoid a plum blossom-shaped hole bottom or a slanted hole. After drilling a certain depth, a hole inspection is performed. Before replacing the drill bit or at places where the hole is prone to shrinkage, a hole inspection is performed. In addition, in order to ensure the verticality of the hole, the support area of ​​the drilling rig is expanded to make the drilling rig stable. During the construction process and after the hole is formed, the diameter and inclination of the pile hole are checked before lowering the steel cage.

[0072] Specifically, during the impact drilling process, a small stroke is used when just entering the rock formation. If it is found that the rock surface and the drill bit are inclined, the backfill stone is corrected. The closer to the bottom of the hole, the slower the impact speed. The first hole cleaning is carried out after the hole reaches the designed depth. The hole cleaning is carried out according to the principle of clearing the slag first and then the slurry. That is, the mud density is not reduced first, and the mud density is ensured to be above 1.3. The slag particles in the hole are removed into the sedimentation tank. During the slag cleaning process, the drill bit is used to stir the bottom of the hole to ensure that there is no sediment at the bottom of the hole.

[0073] In the present application, when arranging the sonic detection tubes, it is required that three sonic detection tubes be arranged for each underpinning pile in an equilateral triangle arrangement. The sonic detection tubes are made of metal tubes and are fixed to the steel cage by bundling or welding. The tubes are kept parallel during bundling or welding. The piles are connected to the steel cage in sections, and the sonic detection tubes are consistent with the steel cage and are connected by section welding. Finally, the lower end of the sonic detection tube is closed and the upper end is covered to ensure that there is no foreign matter in the tube. The connection of the sonic detection tube is smooth and the tube mouth is higher than the top surface of the concrete.

[0074] In the present application, the impact drilling is performed through the above-mentioned construction steps, which can lay a good foundation for the construction of concrete bored piles, making it less likely for the subsequent collapse of the hole, shrinkage of the diameter, deviation of the pile hole, and failure of the pile end to reach the pile, thereby effectively improving the quality of the concrete pouring of the pile body and the bearing capacity of the pile.

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

[0076] In this application, pile foundation underpinning construction is divided into the following three types according to the construction scenario: The first one, refer to Figure 2 The foundation pit is an irregular shaped foundation pit. The side of the foundation pit close to the road is reinforced with steel sheet piles 2 so that the steel sheet piles 2 completely surround the entire periphery of the foundation pit. A steel waist beam 3 and a newly built underpinning beam 4 are arranged in the area surrounded by the steel sheet piles 2. The newly built underpinning beam 4 is distributed inside the steel waist beam 3. At the same time, the two long sides of the steel waist beam 3 are reinforced with steel pipe supports 6 on the opposite sides and corners.

[0077] According to the progress of construction, a cover plate 5 is arranged on the top of the foundation pit. The cover plate 5 is a concrete cover plate 5, which can effectively protect the construction environment in the foundation pit. At the same time, a path is provided in the construction site for vehicles to travel, thereby minimizing the adverse effects on the surrounding traffic and environment.

[0078] The second one, refer to Figure 3 , which is different from the first pile foundation replacement construction scenario in that the steel sheet piles 2 do not surround the entire foundation pit, but reserve a construction gap. During the processing of the construction gap, the two ends of the gap are welded and fixed to the steel sheet piles 2 close to it in a way that the horizontal channel steel 8 is supported as the excavation progresses, ensuring that the two are vertically close to each other and the connection stability between the horizontal channel steel 8 and the steel sheet piles 2 is ensured.

[0079] On the basis that the two long sides of the steel waist beam 3 are reinforced by steel pipe supports 6 respectively on the opposite sides and corners, thick steel plates 7 are added at the corners of the steel waist beam 3. The thick steel plates 7 are right-angled triangle steel plates, so that the two right-angled sides of the thick steel plates 7 are respectively welded to the two side faces of the corners of the steel waist beam 3. The present 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 vertically arranged at the same corner of the steel waist beam 3 according to actual conditions to play a role in further reinforcement.

[0080] The third one, refer to Figure 4 The difference from the first pile foundation replacement construction scenario is that the foundation pit is a regular-shaped foundation pit. When excavating the foundation pit, the foundation pit is always symmetrically arranged along the central axis of its long side, and the corners of the foundation pit are chamfered to form a triangular structure, so that the foundation pit has good compressive resistance. At this time, there is no need to add steel pipe supports 6 to the corners of the steel waist beam 3 for reinforcement, but 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 it is necessary to introduce a water supply pipe 9 around the foundation pit, the water supply pipe 9 cannot penetrate the foundation pit and needs to be installed on the periphery of the steel sheet pile 2. The cover plate 5 is installed on the top of the side of the foundation pit away from the water supply pipe 9 to prevent vehicles or people from mistakenly entering the installation area of ​​the water supply pipe 9, thereby ensuring driving and construction safety.

[0082] Reference Figure 5 and Figure 6 In the foundation pit support construction, the specific construction steps for installing steel sheet piles 2 are: Take a plurality of steel sheet piles 2, and before piling, check the steel sheet piles 2 one by one, remove the steel sheet piles 2 with rust or serious deformation at the connecting locks, and use the unqualified ones only after repair. At the same time, apply grease inside the locks of the steel sheet piles 2 to facilitate driving in and out, and fix the steel sheet piles 2 one by one on the working ground by a vibrating pile driver. The slope of each steel sheet pile 2 does not exceed 2%. When the deflection is too large to be adjusted by the alignment method, pull it up and hit it again. The two adjacent steel sheet piles 2 are interlocked with each other. When docking, the straightness deviation of the two is controlled within 0.1%, forming a screen-like screen wall.

[0083] More specifically, refer to Figure 5 and Figure 6 In order to ensure the verticality of the steel sheet piles 2 during the extension process, the steel sheet piles 2 are driven in using a screen-type method, and are guided by a double-layer guide frame 12. When the top of the first steel sheet pile 2 is located between the upper and lower beams of the guide frame 12, the pile connection operation is carried out. When connecting the piles, the two steel sheet piles 2 at both ends of each row of pile foundations are first extended, and after they are driven in for a certain length, the middle part is constructed in sequence.

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

[0085] The present application adopts the screen-type driving method, which makes it difficult for the sheet piles to bend, twist, tilt, or have uneven wall surfaces. The driving accuracy is high and it is easy to achieve closure. Compared with a single steel sheet pile 2, the screen wall has good anti-lateral displacement capability, can effectively avoid soil and moisture leakage, and further improves its support effect.

[0086] In addition, according to the inclination direction, length and vertical condition of the screen wall, the construction sequence of the screen wall includes forward sequence, reverse sequence, reciprocating sequence, center-division sequence, neutralization sequence and composite sequence. The injection sequence has a direct impact on the verticality, displacement, expansion and contraction in the axial direction, convexity and concavity of the sheet pile wall and the efficiency of pile driving. Therefore, in the present application, when the sheet piles installed at both ends of the screen wall are reversely inclined, the forward sequence is used for injection; otherwise, the reverse sequence is used for injection; when the sheet piles at both ends of the screen wall are kept in a vertical condition, the reciprocating sequence is used for injection; when the length of the sheet pile wall is very long, the composite sequence is used for injection.

[0087] By flexibly adopting different construction sequences and diversified construction sequence options, the construction process becomes more flexible and efficient, and can adapt to various complex construction site environments. Reasonable construction sequence arrangement helps to reduce stress concentration and deformation during the construction process, and ensure the overall stability and safety of the screen wall.

[0088] Reference Figure 6, two adjacent steel sheet piles 2 are interlocked with each other. When butt jointing, the butt joints of the two are welded on both sides, and the lock joints are avoided and not welded. After the welding of the steel sheet piles 2 is completed, the welded weld must be knocked clean of welding slag and splashes around the weld, and polished with a grinder to check whether the weld surface quality meets the acceptance quality requirements. After the joint welding meets the requirements, the reinforcing plate 11 is welded. For each butt joint, two reinforcing plates are used, one on the front and one on the back. After the four sides of the reinforcing plates are fully welded, the welding slag and splashes around the weld must be knocked clean, and then the weld surface quality is checked to see if it meets the acceptance quality requirements.

[0089] Construction of cover plate 5: Several steel pipe piles 1 are arranged at the outer corners of the foundation pit, and two adjacent steel pipe piles 1 are vertically parallel. A cover plate 5 is arranged on the foundation pit, and the cover plate 5 and the foundation pit are connected simultaneously through several steel pipe piles 1. The part of the foundation pit where the cover plate 5 is arranged 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 in its coverage area are completed. Excavation is carried out after the ground is broken, and the steel bars are tied after the cushion layer is poured. After the side formwork is installed, concrete is poured. When tying the steel bars, reserved steel bars 10 are respectively arranged inside the steel sheet piles 2 and steel pipe piles 1, and the reserved steel bars 10 are fixedly connected to the main bars of the cover plate 5.

[0091] After a series of operations such as breaking the ground, excavating, pouring the cushion layer, tying steel bars, installing side formwork and pouring concrete, a cover plate 5 structure with complete structure and reliable strength is formed, which helps to enhance the connection strength and integrity between the cover plate 5 and the pile foundation and reduce the displacement or damage of the cover plate 5 during use.

[0092] More specifically, there are several reserved steel bars 10 , and the reserved steel bars 10 include a connecting portion 101 and an inclined extending portion 102 , and the inclined extending portion 102 is fixed at the top end 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 part 102 is inclined downward from the outside to the connecting part 101, and the connecting part 101 is welded to the inner wall of the steel pipe pile 1 through the length, and the cover plate 5 completely covers the inclined extension part 102 until it is fixedly connected to the top of the steel pipe pile 1; When connecting the steel sheet pile 2 and the cover plate 5, refer to Figure 8 and Fig. 9 Several reserved steel bars 10 are arranged on the inner wall of the steel sheet pile 2, so that the inclined extension part 102 is inclined downward from the outside to the connecting part 101, and the connecting part 101 is welded to the inner wall of the steel sheet pile 2 through the length, and the cover plate 5 completely covers the inclined extension part 102 until it is fixedly connected to the top of the steel sheet pile 2.

[0094] In the present application, the inclined extension portion 102 is inclined downward from the outside to the connecting portion 101, which can provide a larger contact area, help to enhance the connection strength and stability between the reserved steel bars 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 through-length welding, which helps to provide integrity between the cover plate 5 and the steel pipe pile 1, effectively transfer loads, and improve the bearing capacity of the structure. At the same time, the cover plate 5 completely covers the inclined extension portion 102 until it is fixedly connected to the top of the steel pipe pile 1, further enhancing the reliability and stability of the connection; the special structure of the inclined extension portion 102 and the connecting portion 101, and the through-length welding method, have significantly improved the connection strength and stability between the steel sheet pile 2 and the cover plate 5, can effectively resist horizontal loads and vertical loads, and ensure the safety of the foundation pit support structure.

[0095] In addition, refer to Fig.10 and Fig.11 In the foundation pit support construction, a triangular support 13 is arranged 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 support 13 respectively, and plain concrete 15 is backfilled between the steel waist beam 3 and the steel sheet pile 2.

[0096] Specifically, a hanger 14 is connected between the steel waist beam 3 and the steel sheet pile 2. The angle between the hanger 14 and the horizontal line is 30°-60°, and the angle shown in the drawings of the present application is 45°. The two ends of the hanger 14 are welded to the steel waist beam 3 and the steel sheet pile 2 respectively. The present application can be provided with multiple hangers 14 along the vertical direction of the steel sheet pile 2 to further improve the connection stability between the steel waist beam 3 and the steel sheet pile 2.

[0097] The triangular support 13 has good structural stability and can effectively resist horizontal forces to prevent deformation or displacement of the supporting structure. The steel waist beam 3 and the steel sheet piles 2 are respectively welded to the triangular support 13 to ensure a firm connection between the three. This can effectively transfer the load and improve the stability of the overall structure. The plain concrete 15 can fill the gaps, provide additional support, and help reduce groundwater infiltration. By backfilling the plain concrete 15 between the steel waist beam 3 and the steel sheet piles 2, the integrity and stability of the supporting structure can be further enhanced to ensure the safe progress of the foundation pit construction.

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

[0099] Furthermore, the interface is processed and the reinforcement is planted. During the process, the replaced piles are first marked and the roughening position is determined. After the cement slurry on the surface of the original pile is roughened, the concrete surface is chiseled into an uneven surface, the interface agent is applied, the surface is cleaned and fully moistened, and then the replaced piles are marked and the positions of the reinforcement holes are marked. The holes are drilled with a drill bit, and after the reinforcement on the same side is planted, other surfaces are drilled, and the cycle is repeated until all the anchor bars are anchored.

[0100] When constructing the rebar on the pile, the work should be done in batches, and each batch of rebar should not exceed two holes in any horizontal section of the pile, and the next batch of rebar should be constructed after the previous batch of rebar glue solidifies. After the holes are cleaned, fill the holes with rebar glue and then insert the steel bars.

[0101] In this application, when planting rebar, it is required that the original component concrete at the anchoring position of the planting rebar must not have local defects, the adhesive used for planting rebar shall be modified epoxy and modified vinyl ester adhesives, the planting rebar length shall be greater than the anchor hole depth, the position deviation shall be controlled within the set range, the new and old concrete shall be connected by planting rebar, the concrete surface shall be roughened, and the interface treatment agent shall be applied.

[0102] Furthermore, during the pouring of the underpinning beam, ensure that the steel bar size used in the construction process is accurate, the steel bar should be straight without local bending, and there should be no cracks, scales or breaks on the surface after forming. If there is a break, it must be welded and fixed, and the concrete protective layer pad is formed with cement mortar. Before pouring the underpinning beam, it is tied with wire at the bottom of the tie plate. The concrete of the underpinning beam is waterproof concrete, which is poured by concrete pumping, and the slump is controlled within the preset range. The concrete is vibrated in layers using an inserted vibrator until the concrete appears on the surface and no longer sinks.

[0103] Pre-top: Refer to Fig.12 When pre-lifting the replacement beam, a gap is reserved under the replacement beam on the top of the new pile, and a jack 17 with a self-locking function is used to apply pre-pressure to the beam to achieve a controllable force between the pile and the beam. In this application, three jacks 17 are used for pre-lifting. They are arranged in a ring at a spacing of 120° on the top of the replacement pile. In addition, steel pipe pads 18 are arranged between adjacent jacks 17 as a safety device. After the steel pipe pads 18 are placed, steel wedges are inserted into the gap to make them tightly connected to the bottom of the replacement beam. At the same time, the pre-lifting loading of the replacement adopts the principle of graded loading. The load increment of each level is 10% of the upper limit of the loading of the jack 17, and the secondary load is increased each time after the structure is stable.

[0104] During the construction of the underpinning beam and the pre-topped pedestal 16, steel plates are pre-buried at the corresponding positions of the jacks 17. After the underpinning beam is completed and reaches 85% of the design strength and the bored piles reach the preset age, the brick membrane between the bottom of the underpinning beam and the pre-topped pedestal 16 is chiseled off.

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

[0106] Sealing the pile: After the pre-jacking is completed, keep the pre-jacking force stable, weld the reserved steel bars in the pile core, add steel pads, and pour the pile core concrete. When the pile core concrete reaches 90%, remove the jack 17, weld the outer reserved steel bars, and fill the space between the pre-jacking pedestal 16 and the underpinning beam with micro-expansion concrete, and grout the gap between the pile sealing concrete and the bottom of the underpinning beam through the pre-buried grouting pipe.

[0107] Pile cutting: After the pre-jacking is completed and the micro-expansion concrete at the pile end is poured, a foundation pit for pile cutting is excavated on both sides of the replacement beam next to the original pile. When the concrete of the replacement base reaches 100%, the pile is cut in the foundation pit using tools such as rope saws. During the pile cutting process, the settlement of the broken pile column and the slippage of the interface between the new and old concrete are continuously observed.

[0108] Backfill: The filling process adopts layered and uniform compaction to avoid uneven stress on the structure. For areas where backfill is difficult, pressurized micro-expansive cement slurry is used for backfilling.

[0109] The above are all preferred embodiments of the present application. The embodiments are only explanations of the present application and are not intended to limit the protection scope of the present application. Therefore, all equivalent changes made based on the structure, shape, and principle of the present application should be included in the protection scope of the present application.

Claims

1. A pile foundation underpinning construction process suitable for karst geology, characterized in that: The construction steps include: Construction of pile cutting and reinforcement: When the shield passes through, the pile foundation underpinning adopts the method of grinding piles, reserves the conditions for the shield opening, sets up the pre-reinforcement bodies in advance, and uses the sleeve valve pipe on the ground to carry out grouting reinforcement to obtain the strongly weathered stratum; Foundation pit support construction: The foundation pit is reinforced on the road side by steel sheet piles (2), a steel waist beam (3) and a newly built underpinning beam (4) are arranged in the area surrounded by the steel sheet piles (2), and the two long sides of the steel waist beam (3) are reinforced at the opposite sides and corners by steel pipe supports (6); Construction of the cover plate (5): Several steel pipe piles (1) are arranged at the periphery of the corners of the foundation pit, and two adjacent steel pipe piles (1) are vertically parallel. A cover plate (5) is arranged on the foundation pit, and the cover plate (5) and the foundation pit are simultaneously connected through several steel pipe piles (1). The portion of the foundation pit where the cover plate (5) is arranged completely covers the steel sheet piles (2).

2. The pile foundation underpinning construction process applicable to karst geology according to claim 1 is characterized in that: In the construction of pile reinforcement, the specific construction steps include: Drilling: Drill holes on the working ground with a drilling rig, and use bentonite slurry to protect the wall during the drilling process; Lower grouting pipe: solid pipes are used in non-reinforced areas and above the cave roof, and flower pipes are used in reinforced areas and within the cave. A cover is added to the bottom of the grouting pipe, and the grouting pipe is lowered to the bottom of the grouting body or the bottom of the cave, and an exhaust hole is set; Grouting slurry preparation; Grouting: First carry out water injection treatment, check the grouting pipeline and judge the grouting absorption capacity of the formation.

3. The pile foundation underpinning construction process applicable to karst geology according to claim 2 is characterized in that: In grouting, the specific construction steps also include: Retreat segmented grouting: A grouting core tube is arranged in the grouting pipe, and grouting is carried out from the bottom of the hole to the top in the grouting pipe, and the grouting pipe is divided into several grouting sections. After the first grouting section is grouted, the grouting core tube is lifted up to carry out grouting construction of the second grouting section. When the pulled-out length of the grouting core tube is greater than the length of one section of the pipe, the 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 the grouting is continued, and this process is repeated until the grouting is completed.

4. The pile foundation underpinning construction process applicable to karst geology according to claim 3 is characterized in that: In grouting, the specific construction steps also include: Repeat grouting: The time interval between two groutings is 6 to 10 hours, and the number of groutings is not less than 3 times. Stop grouting when the final hole pressure is met, and seal the hole with cement mortar.

5. A pile foundation underpinning construction process suitable for karst geology according to any one of claims 2 to 4, characterized in that: In the lower grouting pipe, the specific construction steps include: The grouting pipe is filled with water before being lowered, and the upper end of the grouting pipe exposes the hole and is covered with a cover. After the grouting pipe is lowered, cement mortar is filled between the bottom end of the grouting pipe and the hole wall for reinforcement.

6. The pile foundation underpinning construction process applicable to karst geology according to claim 1, characterized in that: The construction of the cover plate (5) specifically includes the following construction steps: The construction of the cover plate (5) is carried out after the steel sheet piles (2) and steel pipe piles (1) within the coverage area thereof are completed. The ground is broken and excavation is carried out. After the cushion layer is poured, the steel bars are tied. After the side formwork is installed, concrete is poured. When the steel bars are tied, reserved steel bars (10) are respectively arranged inside the steel sheet piles (2) and the steel pipe piles (1). The reserved steel bars (10) are fixedly connected to the main bars of the cover plate (5).

7. The pile foundation underpinning construction process applicable to karst geology according to claim 6, characterized in that: In the foundation pit support construction, the specific construction steps include: Taking a plurality of steel sheet piles (2), fixing the steel sheet piles (2) one by one on the working ground by using a vibrating pile driver, wherein the slope of each steel sheet pile (2) does not exceed 2%; Two adjacent steel sheet piles (2) are interlocked with each other, and when butt-jointed, the straightness deviation between the two is controlled within 0.1%, thereby forming a screen-like screen wall.

8. The pile foundation underpinning construction process applicable to karst geology according to claim 7, characterized in that: In the foundation pit support construction, the specific construction steps include: According to the inclination direction, length and vertical condition of the screen wall, the construction sequence of the screen wall includes forward sequence, reverse sequence, reciprocating sequence, center-dividing sequence, neutralizing sequence and compound sequence.

9. A pile foundation underpinning construction process suitable for karst geology according to any one of claims 6 to 8, characterized in that: The construction of the cover plate (5) specifically includes the following construction steps: There are a plurality of reserved steel bars (10), each of which comprises a connecting portion (101) and an inclined extension portion (102), wherein the inclined extension portion (102) is fixed to the top end of the connecting portion (101); When the steel pipe pile (1) is connected to the cover plate (5), the plurality of reserved steel bars (10) are evenly distributed along the inner wall of the steel pipe pile (1), so that the inclined extension portion (102) is arranged to be inclined downward from the outside to the connecting portion (101), the connecting portion (101) and the inner wall of the steel pipe pile (1) are welded through the length, and the cover plate (5) completely covers the inclined extension portion (102) until it is fixedly connected to the top of the steel pipe pile (1); When the steel sheet pile (2) is connected to the cover plate (5), the plurality of reserved steel bars (10) are arranged on the inner wall of the steel sheet pile (2), so that the inclined extension portion (102) is arranged to be inclined downward from the outside to the connection portion (101), the connection portion (101) and the inner wall of the steel sheet pile (2) are welded throughout the length, and the cover plate (5) completely covers the inclined extension portion (102) until it is fixedly connected to the top of the steel sheet pile (2).

10. The pile foundation underpinning construction process applicable to karst geology according to claim 9, characterized in that: In the foundation pit support construction, the specific construction steps include: A triangular support (13) is provided between the steel waist beam (3) and the steel sheet piles (2); the steel waist beam (3) and the steel sheet piles (2) are respectively welded to the triangular support (13); and plain concrete (15) is backfilled between the steel waist beam (3) and the steel sheet piles (2).

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