Railway trunk line bridge pile foundation underpinning structure and construction method thereof

Abstract

The application discloses an operation railway trunk bridge wooden pile foundation underpinning structure and a construction method thereof, which comprises a wooden pile foundation, an enlarged foundation is cast on the top of the wooden pile foundation, a newly-built pile foundation is arranged outside the enlarged foundation, and the newly-built pile foundation is a drilled pile or a manually-dug hole pile; a planted bar is arranged around the enlarged foundation, a prestressed hole is formed in the enlarged foundation, a ring frame beam is cast between the outside of the enlarged foundation and the top of the newly-built pile foundation, and the ring frame beam is anchoringly connected with the planted bar; a prestressed bar is arranged in the prestressed hole, anchor boxes are arranged at the two ends of the prestressed hole, anchor backing plates are arranged in the anchor boxes, the end of the prestressed bar is tensioned and fixed with the anchor backing plate, and cement slurry is injected into the prestressed hole, so as to form an integral structure. The application is used for improving the bearing capacity of the bridge foundation, guaranteeing the normal operation of the railway trunk, changing the original stress system of the bridge through the stress cooperation of the new and old structures, and realizing the underpinning and reinforcement of the wooden pile foundation.

Description

Technical Field

[0001] This application relates to the field of railway bridge engineering reinforcement technology, and in particular to a timber pile foundation replacement structure for operating railway trunk line bridges and its construction method. Background Technology

[0002] In the past, many railway trunk line bridges in various regions of China used a foundation structure of wooden piles and spread foundations. This type of foundation structure has a simple construction process and low material cost, which is in line with the engineering construction needs and technical capabilities at the time. However, in the long-term operation of many decades, due to the combined effects of the natural environment and human activities, its structural defects have gradually become prominent, becoming a major safety hazard for the operation of railway trunk lines.

[0003] In terms of the natural environment, bridge pile foundations are often located in areas with complex hydrological environments such as rivers and riverbanks. Long-term immersion in water makes the piles susceptible to problems such as wood corrosion, insect infestation, and carbonization, leading to a reduction in pile cross-section and a significant decrease in load-bearing capacity. Simultaneously, long-term scouring by river water and the impact of floodwaters during the rainy season cause soil erosion around the bridge foundation, gradually exposing the spread foundation and pile foundation, resulting in insufficient foundation depth and a sharp decrease in anti-sliding and anti-overturning stability. In terms of human activities, artificial sand mining and river regulation operations around bridges directly damage the surrounding topography, exacerbating foundation exposure and soil erosion, further worsening the stress state of the foundation.

[0004] The continued development of the above problems will lead to defects in bridge structures such as foundation settlement, pier tilting, and beam cracking, resulting in a significant decline in safety performance. In severe cases, it may even cause bridge collapse and railway line operation interruption, causing not only huge economic losses, but also serious impacts on the normal operation of railway passenger and freight transport, threatening the lives and property of the people.

[0005] Regarding the aforementioned safety hazards of bridge pile foundations, existing technologies primarily employ passive protection measures, which are insufficient to fundamentally address the problem. Firstly, railway authorities and local governments jointly issue bans on sand mining and assign personnel for patrols and supervision, attempting to prevent damage to the surrounding terrain from the source. However, such administrative control measures are limited by factors such as the scope of supervision and enforcement strength, making comprehensive and thorough control difficult, and illegal sand mining still occurs frequently. Secondly, constructing silt-trapping dams and revetments downstream of railway bridges alters the direction and speed of water flow, preventing river erosion that could alter the surrounding terrain and expose the foundation. However, these measures only provide external protection for the bridge foundation and cannot improve the corrosion and damage of the pile foundation itself, nor can they enhance its load-bearing capacity.

[0006] After long-term corrosion, the mechanical properties of bridge timber pile foundations deteriorate significantly. Furthermore, under the continuous vibration and repeated loading of heavy-load railway trains, the bond strength between the pile and the soil continuously decreases. Even if the surrounding terrain remains unchanged, there is still a significant risk of uneven foundation settlement, structural cracking, and even collapse. Existing passive protection measures can only maintain the original state of the surrounding terrain and are insufficient to address the inherent defects of the bridge timber pile foundations themselves, making it difficult to guarantee the long-term safe use of operating railway bridges. Therefore, a replacement structure and construction method capable of actively reinforcing bridge foundations and reducing or even eliminating safety hazards is urgently needed in the field of railway bridge reinforcement technology. Summary of the Invention

[0007] The main purpose of this application is to provide a replacement structure for the wooden pile foundation of an operating railway trunk bridge and its construction method, aiming to solve the technical problem that the existing technology can only provide passive protection for the wooden pile foundation of railway bridges, which poses a significant safety hazard.

[0008] To achieve the above objectives, this application provides a timber pile foundation underpinning structure for operating railway trunk bridges, comprising a timber pile foundation, an enlarged foundation poured on top of the timber pile foundation, and a newly constructed pile foundation set on the outside of the enlarged foundation, the newly constructed pile foundation being a bored pile or a manually excavated bored pile; rebar is installed on the four walls of the enlarged foundation, prestressed ducts are opened inside the enlarged foundation, a ring frame beam is poured between the outside of the enlarged foundation and the top of the newly constructed pile foundation, the ring frame beam being anchored to the rebar; prestressed tendons are inserted into the prestressed ducts, anchor boxes are set at both ends of the prestressed ducts, anchor plates are set inside the anchor boxes, the ends of the prestressed tendons are tensioned and fixed to the anchor plates, and cement grout is poured into the prestressed ducts.

[0009] Optionally, a cushion layer is poured around the pile head of the newly constructed pile foundation. The cushion layer is located at the bottom of the ring beam, and a layer of graded sand and gravel is set in the foundation pit outside the ring beam.

[0010] Optionally, the subbase is a concrete subbase, and the graded sand and gravel layer is constructed by layered compaction, with each layer having a backfill thickness of no more than 300mm and a compaction coefficient of no less than 0.95.

[0011] Optionally, the rebars are arranged in a matrix around the outside of the enlarged foundation, with a rebar spacing of 500mm×500mm. Two layers of prestressed ducts are provided on each wall of the enlarged foundation, with a spacing of not less than 500mm between the prestressed duct layers in the longitudinal and transverse directions.

[0012] Optionally, the newly constructed pile foundation may use rock-socketed piles or friction piles, and the ring frame beam may be a reinforced concrete structure.

[0013] Optionally, a corrugated pipe is pre-embedded between the prestressed ducts and the anchor box within the ring beam range, and the corrugated pipe is seamlessly connected to the prestressed ducts.

[0014] Optionally, the wall surface of the expanded foundation may be roughened to a depth of not less than 5 mm.

[0015] Based on the aforementioned timber pile foundation underpinning structure for operating railway trunk line bridges, this application also provides a corresponding construction method, including the following steps: S1: Complete the site survey, equipment debugging and material preparation before construction, clarify the bridge foundation structural parameters and geological environment of the construction area, excavate the foundation pit on the outside of the enlarged foundation and clean the wall and bottom surfaces, and do a good job of railway operation and construction protection. S2: New pile foundations are constructed by rotary drilling or manual excavation. The type and depth of the pile foundation are determined according to the geological environment. The pile head is broken and the pile head reinforcement is straightened. A cushion layer is poured around the pile head and at the bottom of the ring beam. S3: Roughen the walls around the enlarged foundation and clean the slag. Install the anchoring bars at a spacing of 500mm×500mm. Use high-pressure water hydraulic demolition to open the prestressed ducts. Set two layers on each wall with a layer spacing of not less than 500mm. S4: Tie the steel reinforcement cage of the ring beam, connect it with the anchoring of the rebar, weld and fix it with the pile head steel reinforcement of the newly constructed pile foundation, install anchor boxes and anchor plates at both ends of the prestressed duct, pre-embed corrugated pipes and connect them to the prestressed duct, install and reinforce the ring beam formwork, and carry out water curing after pouring the ring beam concrete. S5: Conduct strength tests on newly constructed pile foundations and ring frame beams. After meeting the specifications, insert prestressed tendons into the prestressed ducts and tension and anchor them in stages. After tensioning, inject cement grout within 48 hours. After grouting is compacted, seal the anchors with fine stone concrete. S6: Conduct an overall quality acceptance inspection of the underpinning structure. After the inspection is passed, use graded sand and gravel to backfill the foundation pit in layers and compact it. Backfill to the original ground elevation to complete the construction of the underpinning structure.

[0016] Optionally, in step S5, the conditions for the newly constructed pile foundation and the ring frame beam to meet the specifications are: the concrete strength of the ring frame beam must reach more than 90% of the design strength and the concrete age must be greater than 7 days.

[0017] Optionally, in step S6, the backfilling of the foundation pit is carried out in layers using a small compactor. The thickness of each layer of graded sand and gravel backfilling is controlled at 200-300mm, and a compaction test is completed after each layer of backfilling. The next layer of backfilling is carried out only after the test is qualified.

[0018] The beneficial effects that this application can achieve are as follows: This application involves arranging new piles deep into hard rock or with sufficient friction in a ring around the outside of the enlarged foundation of the timber pile foundation. This is combined with rebar installation to achieve anchorage between the ring beam and the enlarged foundation, and welding of the pile head reinforcement to achieve a fixed connection between the ring beam and the new piles. Then, through prestressing tendons tensioning in the prestressed ducts and cement grouting, the ring beam and the enlarged foundation form a robust, integrated load-bearing structure. Replacing the timber pile foundation with new piles creates a stable support system, solving the problem of insufficient load-bearing capacity caused by the easy corrosion and exposure of timber pile foundations. Furthermore, the integrated and coordinated load-bearing design of the new and old structures effectively disperses and transfers the load on the bridge superstructure, significantly improving the overall load-bearing capacity and structural stability of the bridge foundation for operating railway trunk lines. Simultaneously, the tight connection of each component and the reasonable stress distribution allow it to adapt to the heavy-load and vibration-prone operating environment of railways, ensuring the long-term safe use of the bridge. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn to scale.

[0020] Figure 1 A schematic diagram of the construction process for a construction method of a timber pile foundation underpinning structure for an operating railway trunk bridge, provided as an embodiment of this application; Figure 2 A schematic diagram showing the arrangement of the enlarged foundation and the ring frame beam provided for an embodiment of this application; Figure 3 A schematic diagram of the rebar arrangement provided in an embodiment of this application; Figure 4 A schematic diagram of the arrangement of prestressed ducts provided in an embodiment of this application; Figure 5 This is a schematic diagram of the arrangement of rebar and prestressed ducts provided in an embodiment of this application.

[0021] The realization of the purpose, functional features and advantages of this application will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0022] 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 a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0023] It should be noted that all directional indicators (such as up, down, left, right, front, back, etc.) in the embodiments of this application are only used to explain the relative positional relationship and movement of each component in a certain specific posture (as shown in the figure). If the specific posture changes, the directional indicator will also change accordingly.

[0024] In this application, unless otherwise expressly specified and limited, the terms "connection," "fixed," etc., should be interpreted broadly. For example, "fixed" can mean a fixed connection, a detachable connection, or an integral part; it can mean a mechanical connection or an electrical connection; it can mean a direct connection or an indirect connection through an intermediate medium; it can mean the internal communication of two components or the interaction between two components, unless otherwise expressly limited. Those skilled in the art can understand the specific meaning of the above terms in this application according to the specific circumstances.

[0025] Furthermore, if the embodiments of this application involve descriptions such as "first" or "second," these descriptions are for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined with "first" or "second" may explicitly or implicitly include at least one of those features. Additionally, the meaning of "and / or" throughout the text includes three parallel solutions; for example, "A and / or B" includes solution A, solution B, or a solution where both A and B are satisfied simultaneously. Furthermore, the technical solutions of the various embodiments can be combined with each other, but this must be based on the ability of those skilled in the art to implement them. When the combination of technical solutions is contradictory or impossible to implement, it should be considered that such a combination of technical solutions does not exist and is not within the scope of protection claimed in this application.

[0026] Please see Figures 1-5 This embodiment provides a timber pile foundation underpinning structure for an operating railway trunk bridge, including a timber pile foundation, an enlarged foundation poured on top of the timber pile foundation, and a newly constructed pile foundation set on the outside of the enlarged foundation. The newly constructed pile foundation is a bored pile or a manually excavated pile. Rebar is installed on the four walls of the enlarged foundation, and prestressed ducts are opened inside. A ring frame beam is poured between the outer side and the top of the newly constructed pile foundation, and the ring frame beam is anchored to the rebar. Prestressed tendons are inserted into the prestressed ducts, and anchor boxes are set at both ends of the prestressed ducts. Anchor plates are set inside the anchor boxes, and the ends of the prestressed tendons are tensioned and fixed to the anchor plates. Cement grout is poured into the prestressed ducts.

[0027] In this embodiment, the vertical load transmitted by the main bridge structure first acts on the spread foundation. The spread foundation divides the load into two parts for transmission: one part is transmitted to the timber pile foundation through its tight connection with the timber pile foundation, where the timber pile foundation bears part of the load; the other part is transmitted to the ring beam through the rebar and prestressing tendons. The ring beam evenly distributes the load to the newly constructed pile foundations around it, and the newly constructed pile foundations transmit the load to the lower bearing layer, thereby achieving the underpinning and reinforcement of the timber pile foundation, effectively sharing the load of the timber pile foundation, reducing its settlement, and solving the problem of insufficient bearing capacity of the timber pile foundation. At the same time, by setting prestressing tendons and applying prestress inside the spread foundation, the shrinkage and bending deformation of the spread foundation can be effectively restrained, enhancing the crack resistance and stiffness of the spread foundation. The cement grout in the prestressing ducts provides reliable anti-corrosion protection for the prestressing tendons, extending the service life of the underpinning structure, ensuring that the underpinning structure can work stably for a long time, and meeting the long-term operational requirements of the operating railway trunk line bridge.

[0028] As an optional implementation, a cushion layer is poured at the pile head of the newly constructed pile foundation. The cushion layer is located at the bottom of the ring frame beam, and a layer of graded sand and gravel is set in the foundation pit outside the ring frame beam.

[0029] In this embodiment, the subbase is made of C20 plain concrete with a thickness of 100mm. Its surface is leveled and vibrated to provide a flat and solid support surface for the casting of the ring frame beam. The graded sand and gravel layer covers the entire construction pit and is backfilled to be level with the original ground level. The density is ensured by layered compaction, which effectively prevents the subsequent settlement of the backfill area of ​​the pit from generating additional stress on the underpinning structure.

[0030] As an optional implementation method, the subbase is a concrete subbase, and the graded sand and gravel layer adopts a layered compaction construction method, with each layer having a backfill thickness of no more than 300mm and a compaction coefficient of no less than 0.95.

[0031] In this embodiment, the pile heads and the bottom of the foundation pit are leveled and compacted before the concrete cushion layer is poured. During pouring, a plate vibrator is used to compact the concrete. The concrete is cured for no less than 3 days to ensure that there is no sand or hollowing. The graded sand and gravel are selected from a continuous graded mixture of coarse stone, medium sand and fine sand in a ratio of 4:3:3. Each layer is compacted with a small tamping machine. After the compaction coefficient is tested by the ring cutter method and meets the standard, the next layer is backfilled to ensure the overall density and stability of the graded sand and gravel layer.

[0032] As an optional implementation method, the rebars are arranged in a matrix around the enlarged foundation with a spacing of 500mm×500mm. Two layers of prestressed ducts are provided on each wall of the enlarged foundation, and the spacing between the prestressed duct layers in the longitudinal and transverse directions is not less than 500mm.

[0033] In this embodiment, the rebars are arranged in an equally spaced matrix to ensure that the anchoring force is evenly distributed on the wall of the enlarged foundation; the prestressed ducts are arranged in two layers with a reasonable layer spacing to ensure that the force on each part of the enlarged foundation is balanced after prestressing, preventing local concrete from cracking and breaking due to excessive tension stress. At the same time, the two layers of prestressed ducts can improve the connection tightness between the ring beam and the enlarged foundation.

[0034] As an optional implementation method, the newly constructed pile foundation adopts rock-socketed piles or friction piles, and the ring frame beam is a reinforced concrete structure.

[0035] In this embodiment, when the geological strata contain moderately or slightly weathered hard rock, rock-socketed piles are selected for the newly constructed pile foundations, with the pile tip embedded in the hard rock for no less than 1m, relying on the contact force at the pile tip for support; when the geological strata do not contain obvious hard rock, friction piles are selected, bearing the load through the side friction resistance between the pile body and the strata, adapting to the construction requirements of different geological conditions; the prestressing tendons are selected from high-strength, low-relaxation steel strands with a nominal diameter of 15.24mm and a tensile strength of no less than 1860MPa, meeting the prestressing requirements of heavy-load railway bridges; the steel reinforcement cage of the ring frame beam consists of HRB400 grade longitudinal main rib steel bars and HRB335 grade transverse stirrups, with a stirrup spacing of 200mm, which greatly improves the structural strength and deformation resistance of the ring frame beam.

[0036] As an optional implementation, a corrugated pipe is pre-embedded between the prestressed ducts and the anchor box within the ring frame beam range, and the corrugated pipe is seamlessly connected to the prestressed ducts.

[0037] In this embodiment, the corrugated pipe is made of metal, with an inner diameter slightly larger than the diameter of the prestressing duct. One end is sealed with sealing tape around the end of the prestressing duct, and the other end extends into the anchor box and fits against the anchor plate. This can prevent the prestressing tendons from getting stuck when they are threaded through, and can also form a closed channel when the cement grout is poured to prevent the cement grout from leaking, ensuring that the duct is filled without gaps and improving the durability of the prestressing system.

[0038] As an optional implementation, the wall surface of the expanded foundation is roughened to a depth of not less than 5 mm.

[0039] In this embodiment, a pneumatic chisel is used to chisel the walls around the enlarged foundation, completely breaking the surface laitance layer and forming an uneven, rough surface. Then, high-pressure water washing combined with hard brush cleaning removes the surface slag and dust until fresh concrete aggregate is exposed, effectively improving the bonding force between the old and new concrete and preventing interlayer separation between the ring beam and the enlarged foundation.

[0040] As an optional implementation method, based on the above-mentioned timber pile foundation replacement structure for operating railway trunk line bridges, this application also provides a corresponding construction method, which includes the following steps: S1: Complete the site survey, equipment debugging and material preparation before construction, clarify the original foundation structural parameters of the bridge and the geological environment of the construction area, excavate the foundation pit on the outside of the enlarged foundation and clean the wall and bottom surfaces, and do a good job of railway operation and construction protection. S2: New pile foundations are constructed by rotary drilling or manual excavation. The type and depth of the pile foundation are determined according to the geological environment. The pile head is broken and the pile head reinforcement is straightened. A cushion layer is poured on top of the pile head. S3: Roughen the walls around the enlarged foundation and clean the slag. Install the reinforcing bars at a spacing of 500mm×500mm. Use high-pressure water hydraulic demolition to open the prestressed ducts. Set two layers on each wall with a layer spacing of not less than 500mm. S4: Tie the steel reinforcement cage of the ring beam, connect it with the anchoring of the rebar, weld and fix it with the pile head steel reinforcement of the newly constructed pile foundation, install anchor boxes and anchor plates at both ends of the prestressed duct, pre-embed corrugated pipes and connect them to the prestressed duct, install and reinforce the ring beam formwork, and carry out water curing after pouring the ring beam concrete. S5: Conduct strength tests on newly constructed pile foundations and ring frame beams. After meeting the specifications, insert prestressed tendons into the prestressed ducts and tension and anchor them in stages. After tensioning, inject cement grout within 48 hours. After grouting is compacted, seal the anchors with fine stone concrete. S6: Conduct an overall quality acceptance inspection of the underpinning structure. After the inspection is passed, use graded sand and gravel to backfill and compact the foundation pit, backfilling to the original ground elevation to complete the construction of the underpinning structure.

[0041] In this implementation, the construction steps are carried out in a strictly sequential manner according to the logical order of "foundation construction - original structure treatment - connection structure construction - prestressed construction - acceptance and backfilling". Each process is closely linked and standardized, employing a low-disturbance construction workflow throughout to minimize the impact on railway operations. The foundation construction stage mainly involves drilling, pouring, and treating the pile heads of the newly constructed piles. Low-noise equipment is used, and construction periods are rationally planned to avoid peak railway operating hours. The original structure treatment focuses on roughening, cleaning, and installing rebar on the enlarged foundation walls, as well as opening prestressed ducts. Strict control of construction intensity is maintained during operation to avoid disturbing the existing wooden pile foundation. The core of the connection structure construction is the binding, pouring, and curing of the ring frame beam to ensure reliable connection with the rebar and newly constructed piles. Prestressed construction strictly follows the graded tensioning, grouting, and anchor sealing process, precisely controlling various parameters. Clear quality control points are set at each stage, and the construction process, material quality, and structural dimensions are inspected throughout the entire process. Any non-conformities are rectified promptly to comprehensively ensure the quality of the replacement structure construction, balancing construction safety and railway operational safety.

[0042] As an optional implementation method, in step S5, the conditions for the newly constructed pile foundation and the ring frame beam to meet the specifications are: the concrete strength of the ring frame beam must reach more than 90% of the design strength and the concrete age must be greater than 7 days.

[0043] In this embodiment, the design strength of the ring frame beam is C40, which means that the measured concrete strength must be no less than 36MPa, and the curing period must be more than 7 days from the date of completion of pouring before prestressing tensioning can be carried out. If the strength does not meet the standard or the curing period is insufficient, moist curing must continue until the dual requirements are met to ensure that the ring frame beam has sufficient structural strength and stiffness to withstand the stress generated by prestressing tensioning and prevent quality problems such as tensioning cracks and deformation stress loss.

[0044] As an optional implementation method, in step S6, the foundation pit backfill is compacted in layers using a small tamping machine. The thickness of each layer of graded sand and gravel backfill is controlled at 200-300mm, and a compaction test is completed after each layer of backfill. The next layer of backfill is carried out only after the test is qualified.

[0045] In this embodiment, the thickness of the graded sand and gravel backfill is strictly controlled to ensure that the small compactor can compact each layer of mixture to the required level, avoiding excessive backfill that would result in insufficient compaction. After each layer of backfill is compacted, the compaction coefficient is tested using the ring cutter method. Only when the compaction coefficient is not less than 0.95 can the next layer of backfill be carried out, ensuring that the density of the entire graded sand and gravel backfill layer is uniform and consistent, effectively preventing later settlement and ensuring the coordinated stress performance of the underpinning structure and the foundation.

[0046] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A railway main line bridge timber pile foundation underpinning structure, characterized in that: Including timber pile foundations, An enlarged foundation is poured on top of the wooden pile foundation, and a new pile foundation is set on the outside of the enlarged foundation. The new pile foundation is a bored pile or a manually excavated pile. The enlarged foundation is equipped with reinforcing bars on its four sides, and prestressed ducts are opened inside the enlarged foundation. A ring frame beam is poured between the outer side of the enlarged foundation and the top of the newly constructed pile foundation, and the ring frame beam is anchored to the reinforcing bars. Prestressing tendons are inserted into the prestressing duct, anchor boxes are provided at both ends of the prestressing duct, anchor plates are provided inside the anchor boxes, the ends of the prestressing tendons are tensioned and fixed to the anchor plates, and cement grout is injected into the prestressing duct.

2. The jacking structure of claim 1, wherein: A cushion layer is poured around the pile head of the newly constructed pile foundation, and the cushion layer is located at the bottom of the ring frame beam.

3. A railway main line bridge timber pile foundation underpinning structure according to claim 2, characterized in that The foundation layer is a concrete foundation layer, and the graded sand and gravel layer is constructed by layered compaction, with each layer having a backfill thickness of no more than 300mm and a compaction coefficient of no less than 0.

95.

4. The timber pile foundation underpinning structure for an operating railway trunk line bridge according to claim 1, characterized in that: The rebars are arranged in a matrix around the outside of the enlarged foundation, with a rebar spacing of 500mm×500mm. The prestressed ducts are provided in two layers on each wall of the enlarged foundation, with a spacing of not less than 500mm between the prestressed duct layers in the longitudinal and transverse directions.

5. The timber pile foundation underpinning structure for an operating railway trunk line bridge according to claim 1, characterized in that: The newly constructed pile foundation adopts rock-socketed piles or friction piles, and the ring frame beam is a reinforced concrete structure.

6. The timber pile foundation underpinning structure for an operating railway trunk line bridge according to claim 1, characterized in that: A corrugated pipe is pre-embedded between the prestressed duct and the anchor box within the ring frame beam range, and the corrugated pipe is seamlessly connected to the prestressed duct.

7. A timber pile foundation underpinning structure for an operating railway trunk line bridge according to claim 1, characterized in that: The wall surface of the enlarged foundation is roughened to a depth of not less than 5 mm.

8. A construction method for a timber pile foundation underpinning structure for an operating railway trunk bridge as described in any one of claims 1-7, characterized in that: The construction method includes the following steps: Complete on-site surveys, equipment debugging and material preparation before construction, clarify the structural parameters of the bridge foundation and the geological environment of the construction area, excavate the foundation pit on the outside of the enlarged foundation and clean the wall and bottom surfaces, and do a good job of railway operation and construction protection. New pile foundations are constructed by rotary drilling. The type and depth of the pile foundation are determined according to the geological environment. The pile head is broken and the pile head reinforcement is straightened. A cushion layer is poured on top of the pile head. Roughen the walls around the enlarged foundation and remove the slag. Install the reinforcing bars at a spacing of 500mm×500mm. Use high-pressure water hydraulic demolition to open the prestressed ducts. Set two layers on each wall with a layer spacing of not less than 100mm. Tie the steel reinforcement cage of the ring frame beam, connect it with the anchoring of the rebar, weld it to the pile head steel reinforcement of the newly constructed pile foundation, install anchor boxes and anchor plates at both ends of the prestressed duct, pre-embed corrugated pipes and connect them to the prestressed duct, install and reinforce the ring frame beam formwork, and carry out water curing after pouring the ring frame beam concrete. Strength testing was conducted on the newly constructed pile foundations and ring frame beams. After meeting the specifications, prestressed tendons were inserted into the prestressed ducts and tensioned and anchored in stages. Cement grout was injected within 48 hours after tensioning, and fine stone concrete was used to seal the anchors after the grouting was compacted. The overall quality of the underpinning structure is inspected and accepted. After the inspection is passed, graded sand and gravel are used to backfill the foundation pit in layers and compact it. The backfill is then completed to the original ground elevation, thus completing the construction of the underpinning structure.

9. The construction method according to claim 8, characterized in that: The conditions under which the newly constructed pile foundations and ring frame beams meet the specifications are as follows: The concrete strength of the ring frame beam must reach more than 90% of the design strength and the concrete age must be greater than 7 days.

10. The construction method according to claim 8, characterized in that: The foundation pit backfilling is carried out by compacting in layers with a small tamping machine. The thickness of each layer of graded sand and gravel backfilling is controlled at 200-300mm. Each layer of backfilling is compacted and tested, and the next layer of backfilling is carried out only after the test is qualified.

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