A method for sectional installation of a tunnel pile foundation reinforcement cage under ultra-low clearance conditions

Abstract

The application discloses a segmented installation method of a tunnel pile foundation steel reinforcement cage under an ultra-low clearance condition and relates to the technical field of tunnel pile foundation construction. The segmented installation method of the tunnel pile foundation steel reinforcement cage under the ultra-low clearance condition comprises the following steps: first, a light steel reinforcement framework containing only stirrups and reinforcing hoops is prepared and is lowered to the bottom of a hole in sections; then, a plurality of longitudinal main reinforcements are sent into the hole from the hole opening and are sunk to the bottom of the hole under the dead weight, and a pile end anchoring body is formed by grouting; a corrugated pipe is sleeved on the top end of the lower main reinforcement which needs to be lengthened to form an annular cavity, the bottom end of the upper main reinforcement is processed into an expanded end head, the expanded end head is inserted into the corrugated pipe after the corrugated pipe is filled with anchoring slurry, and slurry anchoring splicing is completed, and after all the main reinforcements are lengthened in turn, the pile body concrete is poured through a guide pipe. The application changes the mechanical connection at the hole opening into the anchoring of the flow state material in the hole, and the clearance required for construction only needs to meet the passing of a single component, so that the segmented installation of the steel reinforcement cage under the ultra-low clearance condition is facilitated.

Description

Technical Field

[0001] This application belongs to the field of tunnel pile foundation construction technology, specifically relating to a method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions. Background Technology

[0002] When constructing bored piles in pilot tunnels of subway stations, existing shield tunnels, or tunnels with small clearances, the working clearance is limited by the existing structural roof slab or support system. Conventional whole-frame hoisting of reinforcing cages cannot be implemented due to insufficient vertical space, necessitating segmented installation. Currently, the common practice is to divide the reinforcing cage into short sections, using small hoisting equipment to lift each section into the borehole opening. At the opening, the main reinforcing bars are manually connected with straight threaded sleeves or welded to extend them, and crossbeams or positioning clamps are used for temporary fixation at the opening.

[0003] In actual construction, the above process has the following shortcomings: (1) The connection operation at the borehole opening requires vertical space. Under extremely limited clearance conditions, in order to leave space for connection, the length of a single cage section can only be further compressed, resulting in a multiple increase in the number of connection joints and a longer installation time; (2) Manually completing the alignment of the main reinforcement and tightening of the sleeve in a narrow space makes it difficult to control the tightening torque evenly. Some joints may be under-tightened or misaligned due to operational deviations, resulting in local weak connection areas; (3) The alignment deviations caused by multiple joint connections accumulate segment by segment and are not easy to correct in subsequent procedures, which has an adverse effect on the verticality of the pile. In addition, the borehole positioning clamps and guide frames themselves occupy a certain height and are mostly fixed geometric dimensions, which have limited adaptability to different pile diameters and clearance conditions, and do not change the process logic that mechanical connection must be completed at the borehole opening.

[0004] Based on this, this application proposes a method for segmented installation of steel cages for tunnel pile foundations under ultra-low clearance conditions. Summary of the Invention

[0005] This application provides a method for segmented installation of steel cages for tunnel pile foundations under ultra-low clearance conditions. It aims to solve the problems of limited clearance height in existing tunnels or pilot tunnels, which lead to compression of single cage length and increase in the number of joints due to the occupation of vertical space by the orifice connection operation; as well as the problems of difficulty in uniformly controlling the tightening torque when manually connecting straight threaded sleeves, the possibility of connection defects in individual joints, and the accumulation of alignment deviations caused by multiple joint connections, which affect the verticality of the pile.

[0006] The technical solution to the technical problem addressed in this application is as follows:

[0007] According to one aspect of this application, a method for segmented installation of steel reinforcement cages for tunnel pile foundations under ultra-low headroom conditions is provided, comprising the following steps:

[0008] S1. Fabricate a steel reinforcement cage consisting only of stirrups and reinforcing stirrups tied or welded together, without longitudinal main reinforcement. The steel reinforcement cage is then hoisted into the pile hole in the tunnel in sections. After fixing two adjacent sections of the steel reinforcement cage at the hole opening, it is lowered to the bottom of the hole section by section.

[0009] The skeleton does not contain longitudinal main reinforcement. Its self-weight is only composed of stirrups and reinforcing stirrups. The weight of a single section is light. The operating height required to complete the segmented connection at the borehole opening is small. The connection action is mainly completed on the side of the pile hole. It does not occupy the vertical clearance above the borehole opening. This allows the skeleton to be placed only if the height of a single section of the skeleton is sufficient to pass through under the limited clearance.

[0010] S2. Insert multiple longitudinal main reinforcement bars one by one from the opening of the pile hole and lower them to the bottom of the hole.

[0011] The longitudinal main reinforcement bars are inserted into the hole one by one. The clearance required when inserting a single main reinforcement bar is the height required for it to pass through the hole opening along its own length. It sinks to the predetermined position at the bottom of the hole by its own weight.

[0012] S3. Inject the first anchoring grout into the bottom of the hole through the grouting pipe. The first anchoring grout covers the bottom end of the main reinforcement that has been lowered and solidifies to form the pile end anchor body.

[0013] The first anchoring grout fills the gap between the bottom ends of each main reinforcement bar at the bottom of the hole and bonds the bottom ends of each scattered main reinforcement bar into one piece. After curing, it forms an enlarged head type anchoring section at the bottom of the pile, providing bottom embedment for the pile body.

[0014] S4. A corrugated pipe is fitted onto the top of the main reinforcing bar that needs to be extended. The lower end of the corrugated pipe is sealed and fixed to the outer wall of the main reinforcing bar, while the upper end is open. The inner wall of the corrugated pipe and the outer wall of the main reinforcing bar form an annular cavity.

[0015] This annular cavity serves as a continuation chamber for subsequently containing anchoring grout and the expanded diameter end of the upper main reinforcement. The bellows is pre-inserted into the hole along with the lower main reinforcement, and the cavity structure required for continuation is positioned at the depth within the hole, without occupying the operating space above the hole opening.

[0016] S5. Process the bottom end of the upper main reinforcement bar to be connected into an enlarged diameter end.

[0017] S6. After filling the annular cavity of the corrugated pipe with the second anchoring grout through the grouting pipe, insert the enlarged end of the upper main reinforcement into the corrugated pipe, so that the second anchoring grout fills the gap between the enlarged end and the inner wall of the corrugated pipe, thus completing the splicing of a single main reinforcement.

[0018] After the second anchoring grout solidifies, it forms a force-transmitting transition section between the enlarged end and the inner wall of the bellows, combining chemical bonding and mechanical embedding, thus connecting the upper and lower main reinforcement sections into a whole within the hole. The entire splicing operation is completed at the depth within the hole, and the grouting and insertion process does not occupy the vertical space directly above the hole opening.

[0019] S7. Repeat S5 and S6 to extend all the main reinforcement bars in the pile hole to the design elevation segment by segment.

[0020] Each segment is connected independently. The connection positioning is guided by the structural fit between the corrugated pipe opening and the expanded diameter end. The centering deviation of each segment is not passed on and accumulated from segment to segment.

[0021] S8. Pile body concrete is poured into the pile hole from the bottom upward through a guide pipe.

[0022] The concrete of the pile body covers the steel reinforcement cage, all main reinforcement bars and all joints that have been inserted into the hole from bottom to top, forming a continuous reinforced concrete force transmission structure.

[0023] Furthermore, before S1, there is a formation pretreatment step: after drilling to the designed depth, the drill rod is withdrawn, and at the same time, an anti-water dispersion micro-expansion formation modification slurry is injected into the bottom of the hole and the hole wall through the central channel of the drill rod.

[0024] The formation-modified grout forms a consolidation layer on the borehole wall surface, which helps to enhance the stability of the borehole wall. At the same time, the grout penetrates into the weak layer and fissures at the bottom of the borehole, displacing the free water in the gaps between the sediments. After solidification, it produces micro-expansion and compacts the surrounding medium, forming an improved bearing layer at the bottom of the borehole, providing stable formation conditions for the subsequent implantation of the main reinforcement and the pile end anchor.

[0025] Furthermore, the formation modification slurry is composed of sulfoaluminate cement, ordinary silicate cement, Class I fly ash, silica fume, bentonite, hydroxypropyl methylcellulose, anhydrite powder, expanding agent, retarder, early strength activator and water, with a water-cement ratio of 0.35 to 0.38.

[0026] Sulfoaluminate cement provides early strength, ordinary silicate cement ensures later strength development, Class I fly ash and silica fume fill pores and participate in hydration reaction, bentonite imparts thixotropic properties and suspension stability to the grout, hydroxypropyl methylcellulose thickens upon contact with water so that the grout is not easily diluted by groundwater after injection, anhydrite powder and expansion agent work together to produce micro-expansion, and retarder and early strength activator work together to regulate setting time to match the grouting construction window.

[0027] Furthermore, the first anchoring grout is composed of P·O52.5R silicate cement, ultrafine fly ash microspheres, silica fume, expanding agent, polycarboxylate superplasticizer, retarder and water, with a water-cement ratio of 0.26 to 0.30.

[0028] P·O52.5R silicate cement provides high early strength, enabling the anchor body to acquire sufficient strength in a short time; the dual filling of ultrafine fly ash microspheres and silica fume gives the grout high fluidity and low bleeding rate, which is beneficial for filling the irregular gaps between the bottom ends of each main reinforcement in the hole by self-leveling; the expansion agent generates micro-expansion to compensate for the shrinkage during the curing process of the grout, and enhances the interfacial contact between the grout and the surface of the main reinforcement and the stratum at the bottom of the hole; the polycarboxylate superplasticizer ensures workability at a low water-cement ratio; the retarder extends the working time to adapt to continuous grouting operations.

[0029] Furthermore, the second anchoring grout is composed of P·O52.5R silicate cement, metakaolin, S95 grade slag powder, anhydrite powder, polycarboxylate superplasticizer, retarder and water, with a water-cement ratio of 0.25 to 0.28.

[0030] Metakaolin reacts with calcium hydroxide in cement hydration products to form hydrated calcium aluminate and hydrated calcium silicate gels. S95 grade slag powder exerts its potential hydraulic properties under alkaline activation conditions. The two work together to improve the interfacial bonding strength between the grout and the reinforcing steel and to densify the microstructure. Anhydrite powder provides sulfate ions, which participate in the formation of ettringite and produce micro-expansion, avoiding shrinkage gaps in the corrugated pipe cavity after the grout solidifies.

[0031] Furthermore, the corrugated pipe in S4 is a metal corrugated pipe with a length of 30 to 50 times the diameter of the main reinforcement and an inner diameter of 2.0 to 4.0 times the diameter of the main reinforcement. The lower end of the corrugated pipe is welded to the outer wall of the main reinforcement through an annular steel plate, and the outer wall of the corrugated pipe is wrapped with spiral stirrups.

[0032] The length of the corrugated pipe is 30 to 50 times the diameter of the main reinforcement bar to provide sufficient bonding length for the anchorage section; the inner diameter is 2.0 to 4.0 times the diameter of the main reinforcement bar to reserve insertion space for the enlarged end and ensure that the thickness of the annular gap meets the requirements for the grout to play a bonding role.

[0033] The lower end is sealed by welding a ring-shaped steel plate, forming a grouting container with a sealed bottom and an open top. During grouting, the grout fills the cavity from the bottom upwards and drives the air out from the bottom upwards.

[0034] The spiral stirrups wrapped around the outer wall apply radial restraint to the corrugated pipe, limiting the radial deformation of the grout material under shear expansion and maintaining the lateral stiffness of the anchorage section.

[0035] Furthermore, the enlarged end in S5 is a tapered or wedge-shaped enlarged end with a rough surface or threads, and the maximum outer diameter is 80% to 95% of the inner diameter of the bellows. The length of the enlarged section is 5 to 15 times the diameter of the main rib.

[0036] The tapered or wedge-shaped end of the expanded diameter serves as a guide and center when inserted into the grout-filled cavity of the corrugated pipe; the rough surface or threaded grooves increase the mechanical interlocking area between the end surface and the cured grout, improving the interfacial shear resistance; the maximum outer diameter is 80% to 95% of the inner diameter of the corrugated pipe, leaving a grout-filled gap between the expanded diameter end and the inner wall of the corrugated pipe; the length of the expanded diameter section is 5 to 15 times the diameter of the main reinforcement, providing a bond length that meets the force transmission requirements of the reinforcement in the anchorage section, and the force transmission safety reserve of the anchorage section is greater than the yield strength of the reinforcement.

[0037] Furthermore, S6 specifically includes: first inserting the grouting hose into the bottom of the annular cavity of the corrugated pipe, starting the grouting pump while grouting and slowly lifting the grouting hose, and after the second anchoring grout overflows from the upper port of the corrugated pipe, pulling out the grouting hose, and inserting the enlarged end of the upper main reinforcement into the corrugated pipe before the second anchoring grout initially sets.

[0038] After the grouting hose is inserted into the bottom of the cavity, grout is injected from bottom to top, so that the grout gradually fills the cavity from the bottom and drives the air in the cavity upward, avoiding the formation of filling defects due to residual air at the end.

[0039] While grouting, slowly raise the grouting hose to keep the pipe opening below the grouted surface to prevent grout interruption and air pockets caused by lifting the pipe too quickly; when the grout overflows evenly from the upper port, it indicates that the cavity is fully filled; after pulling out the grouting hose, insert the enlarged end within the time window before the grout initially sets, and take advantage of the grout still being in a flowing state to further fill the rough texture and the tiny space of the annular gap on the surface of the enlarged end. After the grout sets, a dense bonding and anchoring layer is formed between the enlarged end and the corrugated pipe.

[0040] Furthermore, in S2, before each main reinforcement bar is lowered, a frustum-shaped cement ball is pre-cast at the bottom of the main reinforcement bar. The diameter of the cement ball is larger than the diameter of the main reinforcement bar, and the height is 30cm to 50cm. Specifically, S2 involves sending the main reinforcement bar with the cement ball into the pile hole from the hole opening, lowering it to the bottom of the hole, and embedding the cement ball into the bottom stratum of the hole. In S3, the injection volume of the first anchoring grout is such that the top surface of the pile end anchor body exceeds the upper surface of all the cement balls by 0.8m to 1.5m.

[0041] The prefabricated frustoconical cement clump at the bottom of the main reinforcement bar has a diameter larger than that of the main reinforcement bar, increasing the contact area between the bottom of the main reinforcement bar and the soil at the bottom of the hole. When the main reinforcement bar sinks under its own weight, the cement clump embeds into the soil at the bottom of the hole, forming a mechanical anchoring point, so that each scattered main reinforcement bar maintains a relatively stable and independent position before grouting. After the first anchoring grout is injected, it covers the entire cement clump and extends upwards by 0.8m to 1.5m, bonding the bottom of each scattered main reinforcement bar and the cement clump together. The pile end anchor body transfers the axial load to the improved soil at the bottom of the hole through the enlarged head. The diameter of the enlarged head is larger than that of the pile body, increasing the end bearing area.

[0042] Furthermore, S8 specifically includes: pouring pile body concrete from the bottom of the hole upwards through a guide pipe, wherein the bottom end of the guide pipe is buried at a depth of not less than 2m below the concrete surface during the pouring process, and the pile body concrete replaces the mud in the hole from bottom to top until it is poured to the designed pile top elevation.

[0043] The tremie pipe is used to start pouring concrete from the bottom of the hole. The first batch of concrete forms a sealing layer at the bottom of the hole and gradually rises upward. The bottom-up pouring method ensures that the top surface of the concrete rises evenly and smoothly replaces and discharges the mud, avoiding the mud from being trapped in the concrete. The bottom end of the tremie pipe is buried at a depth of not less than 2m below the concrete surface, so that the newly injected concrete is always discharged inside the already poured concrete and does not come into direct contact with the mud, maintaining the density and continuity of the pile concrete. During the concrete rising process, the steel reinforcement cage, all main reinforcements and all joints in the hole are covered into a whole, forming a continuous force transmission structure between the pile body and each anchorage section.

[0044] Compared with existing technologies, the segmented installation method for tunnel pile foundation reinforcement cages under ultra-low headroom conditions described in this application has the following advantages:

[0045] (1) In this application, the steel cage is divided into a lightweight stirrup skeleton without main reinforcement and an independent post-implanted main reinforcement. The skeleton is lowered after being connected in sections at the borehole opening. After the main reinforcement sinks to the bottom by its own weight, it forms an anchor body through grouting at the bottom of the borehole. The main reinforcement is spliced ​​in sections using an in-hole grout anchoring method. That is, after filling the corrugated pipe that has been pre-fixed at the top of the lowered main reinforcement with anchoring grout, the enlarged end of the upper section of the main reinforcement is inserted. The splicing operation is completed in the borehole, eliminating the need to reserve vertical operating space above the borehole opening for mechanical connection. The required clearance for construction is compressed to the height of only a single section of skeleton or a single main reinforcement. Thus, under the condition of limited clearance, the problem of compressing the length of a single section of cage due to reserved connection space and causing the number of joints to increase exponentially is avoided.

[0046] (2) In this application, the annular gap between the corrugated pipe and the expanded diameter end is filled with anchoring grout modified with metakaolin and slag powder. After hardening, it forms a force-transmitting transition section with the combined effect of chemical bonding and mechanical embedding. This connection method controls the fullness of filling and bonding strength by controlling the grout ratio and grouting process, replacing the manual tightening of the sleeve. The connection quality of each joint is guaranteed by the material properties and grouting process, which helps to reduce connection defects caused by uneven tightening torque or misalignment.

[0047] (3) In this application, the main reinforcement is spliced ​​by segmented grouting and anchoring. Each splice uses the upper end of the corrugated pipe as the insertion guide. The enlarged end of the upper main reinforcement slides into the full grout cavity along the pipe opening with its own weight and manual assistance. The positioning is mainly based on the centering of the structure itself, without relying on external clamps for repeated adjustments. Each splice segment is completed independently, and the centering deviation is not transmitted and accumulated segment by segment, which helps to improve the adverse effect of multiple joint connections on the verticality of the pile. At the same time, the pile concrete is poured from the bottom of the hole upward through the tremie pipe, forming an integral force system with the hardened grouting and anchoring connection segment, resulting in good continuity of the pile structure. Attached Figure Description

[0048] Figure 1 This is a schematic diagram of the installation process for a segmented installation method of a tunnel pile foundation reinforcement cage under ultra-low headroom conditions, as described in this application.

[0049] Figure 2 This is a flowchart of the in-hole grout anchor splicing operation in this application. Detailed Implementation

[0050] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application. Unless otherwise defined, the technical or scientific terms used herein should have the ordinary meaning understood by those skilled in the art to which this disclosure pertains. The terms "upper," "lower," "left," "right," "front," and "back" used in the specification and claims of this patent application are only used to indicate relative positional relationships. When the absolute position of the described object changes, the relative positional relationship also changes accordingly. Words such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Any aspects not detailed in this application are well-known to those skilled in the art.

[0051] like Figure 1 and 2 As shown in the figure, this application discloses a method for segmented installation of steel cages for tunnel pile foundations under ultra-low clearance conditions, which is used to complete the installation of steel cages for deep bored piles under working conditions where the clearance below the existing tunnel roof or the support structure of the mined tunnel is limited.

[0052] This method deconstructs the traditional monolithic steel cage into a lightweight stirrup skeleton and post-implanted longitudinal main bars. The segmented splicing of the main bars does not rely on mechanical connections at the borehole opening. Instead, it completes the grouting and anchoring splicing in the borehole by inserting the enlarged diameter end after injecting anchoring grout into a pre-set corrugated pipe cavity. This reduces the clearance required for construction to a height where only a single component needs to pass.

[0053] The following is a detailed explanation of the specific implementation steps.

[0054] The optional formation pretreatment steps are as follows:

[0055] Before fabricating and lowering the reinforcing steel cage, a ground pretreatment step can be optionally added. This step specifically involves: after drilling to the designed depth, withdrawing the drill rod, and simultaneously injecting a water-resistant, micro-expansion ground modification slurry into the bottom and wall of the hole through the central channel of the drill rod.

[0056] The mixing ratio of the formation modification grout (per 1 m³) is as follows: 300 kg to 350 kg of sulfoaluminate cement, 100 kg to 120 kg of P·O42.5 ordinary Portland cement, 80 kg to 100 kg of Grade I fly ash, 25 kg to 35 kg of silica fume (SiO2 content ≥92%, specific surface area ≥20000 m² / kg), 40 kg to 60 kg of bentonite (montmorillonite content ≥85%), 1.0 kg to 1.5 kg of hydroxypropyl methylcellulose (viscosity 200000 mPa·s), 40 kg to 55 kg of anhydrite powder (CaSO4 content ≥90%, fineness 400 mesh), 25 kg to 35 kg of calcium sulfoaluminate expansive agent, 0.4 kg to 0.8 kg of sodium gluconate retarder, 0.3 kg to 0.5 kg of lithium carbonate early strength activator, and water usage controlled by a water-cement ratio of 0.35 to 0.38. The grouting pressure is 0.5 MPa to 1.5 MPa. After the grout initially sets, it forms a stable borehole wall and improves the borehole bottom.

[0057] The installation steps are as follows:

[0058] Step S1: Create and lower the steel reinforcement cage without main reinforcement.

[0059] Based on the designed pile diameter and length, a steel reinforcement cage consisting only of spiral stirrups and multiple circular reinforcing stirrups is prefabricated at the processing site. The reinforcing stirrups are made of HRB400 steel bars with a diameter of 12mm to 20mm, and the spiral stirrups are made of HPB300 steel bars with a diameter of 8mm to 12mm, with spacing meeting design requirements. The cage does not contain any longitudinal main reinforcement. The cage is segmented according to the allowable clearance height of the tunnel, with each segment being 2.5m to 3.5m long, ensuring vertical passage through the confined space above the borehole opening during hoisting.

[0060] A hand-operated hoist or mini electric hoist is installed above the pile hole to move the first section of the frame horizontally into the hole, then vertically, and hoist it into the hole. When the first section of the frame is lowered to within 0.3m above the hole opening, a crossbeam is used to temporarily hold it at the hole opening. The second section of the frame is then hoisted up, and the reinforcing hoops of the two sections are aligned and fixedly connected at the hole opening. The fixing connection can be achieved by spot welding, metal clip connection, or lap welding of adjacent reinforcing hoops. The crossbeam is then removed, and the frame is lowered further until the top of the second section of the frame reaches near the hole opening again. This operation is repeated until the bottom of the frame reaches the design elevation.

[0061] Taking a pile diameter of 1.0m, stirrup diameter of 8mm and spacing of 200mm, reinforcing stirrup diameter of 14mm and spacing of 2000mm, and single cage length of 3m as an example, the weight of a single cage section is 35kg to 40kg. The connection is completed by manpower or simple tools, without occupying the core clearance above the borehole.

[0062] Step S2: Insert the bottom main reinforcement bars one by one.

[0063] The longitudinal main reinforcement bars are processed in sections according to the pile length. The main reinforcement bars use HRB400 steel bars with a diameter of 25mm to 36mm. A frustum-shaped cement block is pre-cast at the bottom of the first section of main reinforcement bar. The cement block is formed using rapid-hardening sulfoaluminate cement mortar with a water-cement ratio of 0.30 to 0.35. A pre-drilled hole is left in the middle for inserting the main reinforcement bar. After inserting the bottom end of the main reinforcement bar, it is concentrically aligned and cured until the compressive strength is not less than 20MPa. The bottom diameter of the frustum of the cement block is 60mm to 80mm, and the height is 300mm to 500mm, forming an enlarged end at the end of the main reinforcement bar.

[0064] The main reinforcement bars with cement lumps are horizontally lifted to one side of the borehole opening, gradually tilted to make them vertical, and then inserted into the borehole. Using their own weight, the main reinforcement bars fall along the positioning space reserved in the frame until the cement lumps are embedded to a certain depth in the bottom stratum of the borehole, forming initial embedment. Following the principle of symmetry, all the main reinforcement bars in the bottom section of the pile are lowered into place one by one.

[0065] If wet hole construction with mud slurry is used, the pressure on the hole wall is balanced by maintaining the mud density. If necessary, short-term pressure is applied at the hole opening or the main reinforcement is slightly rotated to help it be in place. Overall, the main method is still sinking by its own weight.

[0066] Step S3: Grouting at the bottom of the hole forms the pile end anchor body.

[0067] After all the bottom main reinforcement bars are inserted, a grouting hose is lowered along the gap next to one of the main reinforcement bars to a position 0.2m from the bottom of the hole. The pre-prepared first anchoring grout is then injected using a grouting pump.

[0068] The mixing ratio of the first anchoring grout (per 1 m³): 900 kg to 1000 kg of P·O52.5R silicate cement, 80 kg to 120 kg of ultrafine fly ash microspheres (specific surface area ≥ 600 m² / kg), 25 kg to 35 kg of silica fume (SiO2 content ≥ 92%, specific surface area ≥ 20000 m² / kg), 70 kg to 90 kg of calcium sulfoaluminate expansive agent, 3 kg to 5 kg of polycarboxylate superplasticizer powder, 0.6 kg to 1.0 kg of sodium gluconate retarder, and the water dosage is controlled at a water-cement ratio of 0.26 to 0.30, with a final flowability ≥ 320 mm.

[0069] The grouting volume is calculated based on filling the gaps between all cement lumps and wrapping upwards to a certain height, so that the top surface of the pile end anchor body is 0.8m to 1.5m higher than the upper surface of each cement lump, forming an enlarged head anchoring section at the pile bottom. The grout's 1-day compressive strength is ≥30MPa, and its bond strength with the reinforcing steel is ≥15MPa.

[0070] Step S4: Process the corrugated pipe connection node at the top of the lower main reinforcement.

[0071] In the processing plant, a section of corrugated metal tubing is pre-inserted onto the top of the lower section of the main reinforcing bar that needs to be extended. The corrugated tubing is made of galvanized cold-rolled sheet steel, spirally corrugated, with threaded protrusions on the inner wall. The length of the corrugated tubing is 30 to 50 times the diameter of the main reinforcing bar; when the diameter of the main reinforcing bar is 32mm, the length of the corrugated tubing is 1200mm to 1600mm. The inner diameter of the corrugated tubing is 2.0 to 4.0 times the diameter of the main reinforcing bar, with an inner diameter of 96mm. The lower end of the corrugated tubing is fully welded to the outer wall of the main reinforcing bar using a ring-shaped steel plate to form a sealed bottom, while the upper end remains open. Spiral stirrups, with a diameter of 6mm to 8mm and a spacing of 50mm to 100mm, are wound evenly around the outer wall of the corrugated tubing to enhance radial restraint.

[0072] When the lower main reinforcement is hoisted in, the corrugated pipe is fixed at its top and then lowered into the hole to the predetermined depth.

[0073] Step S5: Process the enlarged diameter end of the upper main reinforcement.

[0074] The bottom end of the upper main reinforcement section to be connected is machined into an enlarged diameter end. The enlarged diameter end is made into a tapered or wedge-shaped enlarged head by forging or welding prefabricated parts, with a rough surface finish or rolled thread-like engravings. The maximum outer diameter of the enlarged head is 80% to 95% of the inner diameter of the bellows; when the inner diameter of the bellows is 96mm, the outer diameter of the enlarged head is 80mm to 90mm. The length of the enlarged section is 5 to 15 times the diameter of the main reinforcement, ranging from 200mm to 400mm.

[0075] Step S6: Grouting and anchoring operation inside the hole.

[0076] Once the main reinforcement bar is inserted into the hole and the corrugated pipe is near the hole opening, proceed with the continuation operation. Insert a 6mm outer diameter grouting hose along the hole wall into the bottom of the annular cavity of the corrugated pipe. Start the grouting pump and inject the pre-prepared second anchoring grout at low pressure. While grouting, slowly raise the grouting hose, ensuring the hose opening is always below the grout surface and close to the bottom of the cavity. When the grout overflows evenly from the upper end of the corrugated pipe, stop grouting and remove the grouting hose.

[0077] The mix proportions of the second anchoring grout (per 1 m³): 950 kg to 1050 kg of P·O52.5R silicate cement, 100 kg to 130 kg of metakaolin (Al2O3+SiO2 content ≥90%, fineness 1250 mesh), 80 kg to 110 kg of S95 grade slag powder (specific surface area ≥400 m² / kg), 30 kg to 40 kg of anhydrite powder (CaSO4 content ≥90%, fineness 400 mesh), 3 kg to 4 kg of polycarboxylate superplasticizer powder, 0.3 kg to 0.5 kg of sodium gluconate retarder, water usage controlled by a water-cement ratio of 0.25 to 0.28, fluidity ≥350 mm, and initial setting time 45 min to 60 min.

[0078] After the pipe is removed, before the grout initially sets, slowly insert the enlarged end of the upper main reinforcement section into the center of the corrugated pipe. The weight of the main reinforcement and manual intervention will force the enlarged end into the grout-filled cavity, and excess grout will drain from the pipe opening. The insertion depth is controlled by pre-marked positions on the main reinforcement, ensuring the enlarged section is completely submerged in the corrugated pipe. Maintain the main reinforcement position stable until the grout initially sets; the 28-day bond strength is ≥15MPa. For a 32mm diameter main reinforcement section with a 300mm enlarged section, the anchorage force can reach over 750kN.

[0079] Step S7: Lengthen all main reinforcement bars segment by segment.

[0080] Repeat steps S5 and S6, and extend all the main reinforcement bars segment by segment to an appropriate length above the designed pile top elevation according to the pile length. Each splice is an independent grout anchor connection, and is carried out sequentially in the depth of the hole without occupying the vertical space directly above the hole opening.

[0081] Step S8: Pile body concrete pouring.

[0082] After all main reinforcement bars are spliced, concrete is poured using the tremie pipe method. The tremie pipe has an inner diameter of 250mm to 300mm, and its bottom is initially 0.3m from the bottom of the hole. During the pouring process, the bottom of the tremie pipe is always buried at least 2m below the surface of the poured concrete. The pile body concrete uses C40 underwater concrete with a slump of 200mm ± 20mm, displacing the mud in the hole from bottom to top until the designed pile top elevation is reached. During the pouring process, the pile body concrete encapsulates the reinforcement cage and all splices, forming an integral load-bearing system.

[0083] Example 1:

[0084] An anti-uplift pile was constructed in the pilot tunnel of a subway station. The pile diameter was 1000mm, the pile length was 35m, the net height of the pilot tunnel was 3.8m, and the main reinforcement consisted of 32 HRB400 steel bars with a diameter of 32mm.

[0085] In this embodiment, a formation pretreatment step is added before step S1: after drilling to the designed depth, the drill rod is withdrawn, and at the same time, an anti-water dispersion micro-expansion formation modification slurry is injected into the bottom of the hole and the hole wall through the central channel of the drill rod.

[0086] The mix proportions of the formation modification grout (per 1 m³): 320 kg sulfoaluminate cement, 110 kg P·O42.5 ordinary Portland cement, 90 kg Grade I fly ash, 30 kg silica fume, 50 kg bentonite, 1.2 kg hydroxypropyl methylcellulose (viscosity 200000 mPa·s), 48 kg anhydrite powder, 30 kg calcium sulfoaluminate expansive agent, 0.6 kg sodium gluconate, 0.4 kg lithium carbonate, water-cement ratio 0.36. Grouting pressure: 0.8 MPa. After initial setting, the grout will stabilize the borehole wall and improve the borehole bottom.

[0087] Step S1: The skeleton is divided into 13 sections, each 2.8m long, and spot welded together at the opening.

[0088] Step S2: The bottom section of the main reinforcement is 12m long. A frustum-shaped cement block with a height of 400mm and a bottom diameter of 70mm is poured at the bottom end. The main reinforcement is then inserted one by one and embedded into the bottom of the pre-treated hole.

[0089] Step S3: Inject 2.5m³ of the first anchoring grout, with an anchoring height of 1.2m at the pile end.

[0090] Steps S4 to S7: The corrugated pipe is 1200mm long and has an inner diameter of 96mm. The bottom is sealed with a ring-shaped steel plate, and the outer wall is wrapped with spiral stirrups. The expanded end is tapered with a maximum outer diameter of 88mm and a 300mm long rolled thread. During splicing, the pipe is lifted while grouting and inserted before initial setting. All main reinforcement bars are spliced ​​twice to reach the design length.

[0091] Step S8: Use a duct to pour C40 underwater concrete. During the pouring process, the bottom end of the duct should be buried at a depth of not less than 2m below the concrete surface until the designed pile top elevation is reached, and the pile is formed.

[0092] Example 2:

[0093] Rock-socketed piles with a diameter of 800mm and a length of 28m were constructed at the bottom of a shield tunnel, with a net height of 4.2m inside the tunnel. The main reinforcement consisted of 20 HRB400 steel bars with a diameter of 28mm. The method of this invention was used for construction after adding a ground pretreatment step.

[0094] The corrugated pipe is 1120mm long and has an inner diameter of 84mm. The lower end is sealed with a ring-shaped steel plate, and the outer wall is wrapped with spiral reinforcement. The expanded end is tapered with rolled threads, 300mm long, and has a maximum outer diameter of 78mm. The skeleton is fixed at the borehole opening using metal clips. The mixing ratio of the first and second anchoring grouts is the same as in Example 1; the injection volume of the first anchoring grout and the height of the anchor body at the pile end are adjusted accordingly for a pile diameter of 800mm and a pile length of 28m; the remaining steps are the same as in Example 1.

[0095] Example 3

[0096] Piles were erected inside a subway tunnel section. The piles have a diameter of 1200mm and a length of 30m, with a clearance of 3.5m inside the tunnel. The main reinforcement consists of 28 HRB400 steel bars with a diameter of 36mm. No ground pretreatment was performed; the method of this invention was used directly during construction.

[0097] The main reinforcement section is 10m long at the bottom. The corrugated pipe is 1800mm long with an inner diameter of 108mm. The bottom is sealed with a ring-shaped steel plate, and the outer wall is wrapped with spiral stirrups. The enlarged end is wedge-shaped with a maximum outer diameter of 100mm, a rough surface finish, and a length of 400mm. The skeleton is fixed at the borehole opening by lap welding of adjacent reinforcing stirrups. The mixing ratio of the first anchoring grout and the second anchoring grout is the same as in Example 1; the injection volume of the first anchoring grout and the height of the pile end anchor are adjusted accordingly for a pile diameter of 1200mm and a pile length of 30m; the remaining steps are the same as in Example 1.

[0098] Comparative Example 1

[0099] Similar to Example 1, this project employed a conventional segmented hoisting method with straight threaded sleeve connections at the borehole opening. The reinforcing cage was fabricated as a single 3m section, including main reinforcement and stirrups, with each section weighing 700kg. Sections were hoisted into the borehole opening one by one using a low-headroom truck-mounted crane. Manual alignment of the main reinforcement in each section was performed at the opening, and the connection was tightened using straight threaded sleeves, requiring a total of 11 joints. Each joint took 25 minutes, with a total installation time of 10 hours.

[0100] Comparative Example 2

[0101] Similar to Example 1, a flexible steel strand cage scheme was adopted, replacing the main reinforcing bars with high-strength steel strands. The entire cage was compressed, hoisted in, and then unfolded. The steel strands were anchored using extrusion anchors and corrugated tubes.

[0102] The test results are compared in detail below:

[0103] The piles formed in Examples 1, 2, 3, Comparative Example 1, and Comparative Example 2 were subjected to low-strain reflected wave method integrity testing, well caliper verticality measurement, single pile vertical compressive static load test, and core sampling splitting tensile strength test at the joints. The results are shown in the table below.

[0104] Low strain integrity The wave velocity was uniform, and no obvious defects were observed. The wave velocity was uniform, and no obvious defects were observed. The wave velocity was uniform, and no obvious defects were observed. Slight impedance changes at some joints The waveform showed no obvious abnormalities. Verticality deviation 0.35% 0.28% 0.41% 0.93% 0.45% Percentage increase in single pile bearing capacity compared to design value 18% 15% 13% 6% 11% Joint core splitting tensile strength Greater than the corresponding value of the pile body concrete Greater than the corresponding value of the pile body concrete Greater than the corresponding value of the pile body concrete The strength of the local joint is lower than that of the pile concrete. The anchorage section strength meets the requirements.

[0105] As can be seen from the table above, the verticality deviation of the method of the present invention is reduced from 0.93% of the conventional method to the range of 0.28% to 0.41%, the increase in single pile bearing capacity compared with the design value is increased from 6% to the range of 13% to 18%, and the joint quality is stable.

[0106] In Comparative Example 1, due to uneven control of the manual tightening torque at the borehole opening, micro-gaps were found in some joints as verified by core drilling, requiring post-grouting reinforcement treatment.

[0107] The bending stiffness of the pile body in Comparative Example 2 differs from that of a conventional steel cage, and the design unit requires additional full-scale tests for verification before it can be adopted.

Claims

1. A method for segmented installation of steel reinforcement cages for tunnel pile foundations under ultra-low clearance conditions, characterized in that, Includes the following steps: S1. Fabricate a steel reinforcement cage consisting only of stirrups and reinforcing stirrups tied or welded together, without longitudinal main reinforcement. Hoist the steel reinforcement cage into the pile hole in the tunnel in sections. After fixing two adjacent sections of the steel reinforcement cage at the hole opening, lower them section by section to the bottom of the hole. S2. Insert multiple longitudinal main reinforcement bars one by one from the opening of the pile hole and lower them to the bottom of the hole; S3. Inject the first anchoring grout into the bottom of the hole through the grouting pipe. The first anchoring grout covers the bottom end of the entire main reinforcement that has been lowered and solidifies to form the pile end anchor body. S4. A corrugated pipe is fitted onto the top of the main reinforcement that needs to be extended. The lower end of the corrugated pipe is closed and fixed to the outer wall of the main reinforcement, while the upper end is open. The inner wall of the corrugated pipe and the outer wall of the main reinforcement form an annular cavity. S5. Process the bottom end of the upper main reinforcement bar to be connected into an enlarged diameter end; S6. After filling the annular cavity of the corrugated pipe with the second anchoring grout through the grouting pipe, insert the enlarged end of the upper main reinforcement into the corrugated pipe, so that the second anchoring grout fills the gap between the enlarged end and the inner wall of the corrugated pipe, thus completing the splicing of a single main reinforcement. S7. Repeat S5 and S6 to extend all the main reinforcement bars in the pile hole to the design elevation segment by segment. S8. Pile body concrete is poured into the pile hole from the bottom upward through a guide pipe.

2. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 1, characterized in that, Before step S1, a formation pretreatment step is also included: after drilling to the designed depth, the drill rod is withdrawn, and at the same time, an anti-water dispersion micro-expansion formation modification slurry is injected into the bottom of the hole and the hole wall through the central channel of the drill rod.

3. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 2, characterized in that, The formation modification slurry is composed of sulfoaluminate cement, ordinary silicate cement, Class I fly ash, silica fume, bentonite, hydroxypropyl methylcellulose, anhydrite powder, expanding agent, retarder, early strength activator and water, with a water-cement ratio of 0.35 to 0.

38.

4. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 1, characterized in that, The first anchoring grout is composed of P·O52.5R silicate cement, ultrafine fly ash microspheres, silica fume, expansion agent, polycarboxylate superplasticizer, retarder and water, with a water-cement ratio of 0.26 to 0.

30.

5. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 1, characterized in that, The second anchoring grout is composed of P·O52.5R silicate cement, metakaolin, S95 grade slag powder, anhydrite powder, polycarboxylate superplasticizer, retarder and water, with a water-cement ratio of 0.25 to 0.

28.

6. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 1, characterized in that, The corrugated pipe in step S4 is a metal corrugated pipe with a length of 30 to 50 times the diameter of the main reinforcement and an inner diameter of 2.0 to 4.0 times the diameter of the main reinforcement. The lower end of the corrugated pipe is welded to the outer wall of the main reinforcement through an annular steel plate, and the outer wall of the corrugated pipe is wrapped with spiral stirrups.

7. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 6, characterized in that, In step S5, the enlarged end is a tapered or wedge-shaped enlarged head with a rough surface or threads. The maximum outer diameter is 80% to 95% of the inner diameter of the bellows, and the length of the enlarged section is 5 to 15 times the diameter of the main rib.

8. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 6, characterized in that, Step S6 specifically includes: first inserting the grouting hose into the bottom of the annular cavity of the corrugated pipe, starting the grouting pump and slowly lifting the grouting hose while grouting, and after the second anchoring grout overflows from the upper port of the corrugated pipe, pulling out the grouting hose, and inserting the enlarged end of the upper main reinforcement into the corrugated pipe before the second anchoring grout initially sets.

9. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 1, characterized in that, In step S2, before each main reinforcement bar is lowered, a frustum-shaped cement ball is pre-cast at the bottom of the main reinforcement bar. The diameter of the cement ball is larger than the diameter of the main reinforcement bar, and the height is 30cm to 50cm. Specifically, S2 involves sending the main reinforcement bar with the cement ball into the pile hole from the hole opening, lowering it to the bottom of the hole, and embedding the cement ball into the bottom stratum of the hole. In step S3, the injection volume of the first anchoring grout is such that the top surface of the pile end anchor body exceeds the upper surface of all the cement balls by 0.8m to 1.5m.

10. The method for segmented installation of tunnel pile foundation reinforcement cages under ultra-low clearance conditions according to claim 1, characterized in that, Step S8 specifically includes: pouring pile body concrete from the bottom of the hole upwards through a guide pipe. During the pouring process, the bottom end of the guide pipe is buried at a depth of not less than 2m below the concrete surface. The pile body concrete replaces the mud in the hole from bottom to top until it is poured to the designed pile top elevation.

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