Construction method for reducing existing railway pier cap

Abstract

The application discloses a kind of construction methods for reducing existing railway pier cap, comprising the following steps: S1, in the range of existing pile foundation, construction new pile foundation is added;Construction newly-built cap;S2, construction first-pouring part of lengthening pier body;Construction support system on newly-built cap;S3, according to design requirements, simultaneously apply jacking force of jacking equipment to support system to tighten existing cap;S4, cut off the connection between existing pile foundation and existing cap, then simultaneously apply jacking force of jacking equipment, remove the stress of support system;S5, remove support system and existing pile body above the top surface of newly-built cap, cut off the excess part of existing cap;S6, on the cut-off surface of existing pier body and excess part of existing cap, plant reinforcement, construction outer package post-cast part of lengthening pier body.The application uses existing cap, existing pile foundation and newly-built cap with reduced new pile foundation, lengthening pier body to carry out two stress system conversions, instead of the construction of temporary portal pier, reduce engineering quantity and construction difficulty.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering technology, and in particular to a construction method for reducing the height of existing railway bridge pier caps. Background Technology

[0002] With the rapid development of bridge construction in my country, bridge defects have frequently appeared during construction and use, making the treatment of these defects increasingly prominent. As my country's economy develops and transportation becomes more advanced, railways inevitably intersect with rivers. Due to climate change and economic development, river erosion and sand mining activities have led to riverbed erosion or the construction of new roads. Originally, bridge piers were buried below the riverbed, but riverbed erosion and new road construction have exposed the piers and pile foundations. Exposed pile foundations not only affect the aesthetics of the bridge but, more importantly, pose significant safety hazards, seriously affecting the safe use of the bridge. Necessary treatment is required to restore the bridge to a normal, safe operating condition. Therefore, it is necessary to lower the existing railway bridge piers and bury them below the riverbed to minimize the impact on railway operations and save on investment.

[0003] The existing technology for modifying exposed pier caps involves first installing reinforcing bars laterally on the pier base, constructing foundations on both sides of the existing railway pier foundation to form a transverse portal pier that lifts the bridge beam, constructing permanent pile foundations, removing the existing pier cap, constructing a lowered new pier cap, extending the pier body, and finally dismantling the temporary portal pier, thus completing the pier cap lowering. This existing method utilizes temporary portal piers, removing and rebuilding the exposed pier cap. This method has a long construction period, requires rebuilding the pile foundations, and during the extended reconstruction period, the existing railway cannot operate, resulting in significant social and economic impacts, wasting resources and greatly increasing costs. Furthermore, due to construction and site constraints, adding pile foundations is often very difficult, rendering the project infeasible. Summary of the Invention

[0004] The purpose of this invention is to overcome the above-mentioned deficiencies in the prior art and provide a construction method for reducing the pier cap of existing railway bridges. This is a solution that reduces investment and minimizes the impact on existing railways. The idea is to eliminate the construction of temporary portal piers and use the existing pier cap, existing pile foundations, and newly added pile foundations after the pier cap is lowered to extend the pier body and perform two stress system conversions.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0006] A construction method for reducing the height of existing railway bridge pier caps includes the following steps:

[0007] Step S1: Construct new pile foundations within the existing pile foundation area under the existing pile cap; the top surface of the new pile foundations is flush with the bottom surface of the new pile cap; construct the new pile cap on top of the new pile foundations;

[0008] Step S2: Construct the pre-cast portion of the extended pier body on the newly built foundation. The extended pier body is the pier body between the existing pier body and the newly built foundation. The top surface of the pre-cast portion is connected to the bottom surface of the existing foundation. Construct a support system on the newly built foundation. The support system is equipped with jacking equipment.

[0009] Step S3: Apply the jacking force of the lifting equipment synchronously according to the design requirements until the support system presses against the existing bearing platform;

[0010] Step S4: After the pre-poured concrete reaches the design strength, the connection between the existing pile foundation and the existing abutment is cut off, and then the jacking force of the jacking equipment is applied again synchronously to relieve the stress on the support system.

[0011] Step S5: Remove the support system and the existing pile body above the top surface of the newly built pile cap, and cut off the excess part of the existing pile cap;

[0012] Step S6: Reinforcing bars are installed on the cut surfaces of the existing pier body and the existing abutment, and the outer post-cast portion of the extended pier body is constructed. The outer post-cast portion is set on the outer surface of the pre-cast portion, the existing abutment, and part of the existing pier body, thus completing the reduction of the existing railway bridge pier abutment.

[0013] The technical solution of this invention involves constructing new pile foundations within the existing pile foundation area to form a new pile group foundation. Then, a new pile cap and a pre-cast section are constructed. A temporary support system is installed on the pre-cast section. Two system conversions are then performed. When the jacking equipment presses the existing pile cap tightly, the temporary support system and the pre-cast section can withstand the load on the existing pile cap, thus completing the first system conversion. Next, the connection between the existing pile foundation and the existing pile cap is severed, transferring the load on the existing pile cap to the pre-cast section, thus completing the second system conversion. Finally, the excess portion of the existing pile cap is removed, and the extended pier body is constructed with a newly cast reinforced concrete core. This creates a rigid connection between the extended pier body and the existing pier body, achieving the goal of reducing the existing railway bridge pier pile cap.

[0014] As a preferred embodiment of the present invention, before construction, protective railings are installed at the construction site, the construction site is leveled, and the original concrete of the existing foundation layer is removed and leveled.

[0015] As a preferred embodiment of the present invention, in step S1, the specific method for constructing the new pile foundation is as follows: first, construct protective piles, excavate the new pile cap foundation pit, apply sealing concrete to the base of the foundation pit, install rebar on the existing pile foundation, tie the new pile cap rebar, and then pour concrete and cure it.

[0016] As a preferred embodiment of the present invention, the specific method for constructing the newly added pile cap in step S1 is as follows: first, construct protective piles, excavate and seal the bottom, then install reinforcing bars on the existing pile foundation and the newly added pile foundation, tie the reinforcing bars of the newly added pile cap, and then pour and cure concrete. When constructing the newly added pile cap, it is necessary to pre-embed the reinforcing bars for the pre-cast pier body and the outer post-cast portion.

[0017] As a preferred embodiment of the present invention, in step S1, the specific method for constructing the pre-cast portion of the extended pier body is as follows: reinforcing bars are installed on the bottom surface of the existing foundation, grouting pipes are installed, the reinforcing bars of the pre-cast portion are tied, the casting formwork of the pre-cast portion is installed, micro-expansion concrete is poured, and then cured. It should be noted that the bottom surface of the existing foundation should be roughened and cleaned before reinforcing bar installation. When tying the reinforcing bars of the pre-cast portion, attention should be paid to reserving stirrups for the outer concrete of the subsequent cast portion and embedded steel plates for welding with the inclined support steel pipes. More preferably, embedded steel pipes are set during the pouring of the pre-cast concrete. These embedded steel pipes serve as vents during concrete pouring and as grouting pipes for later use, ensuring that the newly poured concrete of the pre-cast portion is tightly bonded to the existing foundation ground.

[0018] As a preferred embodiment of the present invention, the support system includes a supporting steel pipe and a steel plate, the bottom end of the supporting steel pipe is connected to the newly built pier, and the lifting device is disposed between the supporting steel pipe and the steel plate.

[0019] As a more preferred embodiment of the present invention, the support system further includes horizontal braces and diagonal braces; a plurality of horizontal braces are provided in the vertical direction of the supporting steel pipe, and both ends of the plurality of horizontal braces are respectively connected to the adjacent supporting steel pipes in the vertical direction; both ends of the plurality of diagonal braces are respectively connected to the adjacent supporting steel pipes in the vertical direction and are connected to the adjacent horizontal braces.

[0020] As a preferred embodiment of the present invention, in step S4, the specific method for cutting off the connection between the existing pile foundation and the existing abutment is as follows: The tops of the existing pile foundations are symmetrically and successively cut off to disconnect the connection between the existing pile foundations and the existing abutment, thereby transferring the load on the existing abutment to the pre-cast portion. After relieving the stress on the supporting steel columns of the support system and releasing the jacking force of the lifting equipment, all the load on the existing abutment is uniformly transferred to the pre-cast portion of the extended pier. Speed ​​limits are enforced during the disconnection of the connection between the existing pile foundations and the existing abutment.

[0021] As a preferred embodiment of the present invention, in step S1, after the strength of the pre-cast concrete reaches more than 90% of the design strength and the curing time is not less than 10 days, the jacking force of the jacking equipment is applied synchronously according to the design requirements until the supporting steel column is firmly against the bottom surface of the existing foundation, and then the jacking force of the jacking equipment is released. Furthermore, when dismantling the existing pile body above the top surface of the new foundation, a working support is erected using steel pipes. The working support undergoes foundation treatment first, and the construction unit needs to design the support to ensure safety.

[0022] As a preferred embodiment of the present invention, in step S5, the excess part of the existing pier cap is removed by cutting it out symmetrically from the outside to the inside and from both sides simultaneously. The removal of the existing pier cap is done by pallet cutting. Necessary safety measures must be taken during the cutting process to prevent vibration and ensure the safety of the existing pier body and the pre-cast part.

[0023] The post-cast outer portion is located on the outer surface of the pre-cast portion, the outer surface of the existing pile cap after the excess portion has been removed, and part of the outer surface of the existing pier body. Specifically, the post-cast outer portion is located on the outer surface of the bottom end of the existing pier body, and the length of the post-cast outer portion covering the bottom end of the existing pier body is 200-500 cm. The post-cast outer portion is made of cast-in-place concrete, and the thickness of the post-cast outer portion is designed based on factors such as the allowable bearing capacity and linear stiffness of a single pile. Before the construction of the post-cast outer portion, the cut surface of the existing pile cap and part of the existing pier body are roughened, cleaned, and then rebar is installed. To ensure a consistent appearance, the concrete used for the post-cast outer portion must be from the same batch, and the top of the post-cast outer portion is rounded at the junction with the existing pier body.

[0024] As a preferred embodiment of the present invention, in step S6, after the construction of the external post-cast portion is completed, the foundation pit is backfilled, the riverbank is protected, and the river channel is leveled.

[0025] As a preferred embodiment of the present invention, during the construction process, deformation and displacement stress monitoring elements are arranged on the existing pier top, the existing abutment, and the newly added pile foundation to detect stress and strain conditions and ensure the safety of construction.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1. The construction method of this invention involves constructing new pile foundations within the existing pile foundation area to form a new pile group foundation. This allows the existing pile foundations and the newly constructed pile foundations to work together effectively to meet the stress requirements of the renovated bridge piers. Through two system transformations, the load on the existing pier cap is transferred to the newly constructed pier cap, thereby reducing the load on the existing railway bridge pier cap. This invention utilizes the existing pier cap, existing pile foundations, and the newly constructed pile foundations and extended pier body of the lowered pier cap to perform two stress system transformations. The newly poured reinforced concrete outer layer forms a rigid connection between the extended pier body and the existing pier body, replacing the construction of temporary portal piers. This reduces the amount of work and the difficulty of construction, and significantly minimizes the impact on railway operations.

[0028] 2. The method of the present invention can be widely applied to existing railway bridge piers that require lowering of the pier cap, improving economy and safety. Under the condition of repairing the bridge with guaranteed quality and quantity, normal traffic can be restored more quickly, avoiding inconvenience caused by traffic interruption, minimizing social impact, greatly improving social benefits, and achieving good economic benefits. Attached image description:

[0029] Figure 1 This is a schematic diagram showing the arrangement of the newly added pile foundations in this invention;

[0030] Figure 2 This is a front view of the newly added pile foundation in this invention;

[0031] Figure 3 The front view of the newly added support platform in this invention;

[0032] Figure 4 This is a front view of the pre-cast portion of the present invention;

[0033] Figure 5 This is a front view of the support system of the present invention;

[0034] Figure 6 This is a side view of the support system of the present invention;

[0035] Figure 7 This is a front view of the outer post-cast portion of the present invention;

[0036] Figure 8 This is a side view of the post-cast portion of the outer casing of the present invention;

[0037] Markings in the diagram: 1-Ground plane, 2-Existing pier body, 3-Existing pile cap, 4-Existing pile foundation, 5-Cast-in-place part, 6-Outer post-cast part, 7-New pile cap, 8-New pile foundation, 9-Supporting steel pipe, 10-Steel plate. Detailed Implementation

[0038] The present invention will be further described in detail below with reference to experimental examples and specific embodiments. However, this should not be construed as limiting the scope of the above-mentioned subject matter of the present invention to the following embodiments; all technologies implemented based on the content of the present invention fall within the scope of the present invention.

[0039] Example 1

[0040] Taking a railway in Southwest China as an example, the railway passes through a river with multiple bridge piers. The tops of the piers are the existing railway structure, the bottoms are existing foundations, the tops of the foundations are the existing pier bodies, and the bottoms are existing pile foundations. Due to severe illegal sand mining in the river section near the bridge site, the riverbed is unstable, causing frequent swings in the thalweg and severe downcutting of the main channel. This has resulted in severe overall scouring of the bridge site, exposing the existing foundations, which are now above ground level, and the existing pile bodies are exposed up to 10 meters above the ground, threatening railway safety. It is necessary to lower the foundations below the downcut riverbed.

[0041] Therefore, this implementation case provides a construction method for reducing the height of existing railway bridge pier caps, including the following steps:

[0042] Step S1: Construct new pile foundations within the existing pile foundation area under the existing pile cap; the top surface of the new pile foundations is flush with the bottom surface of the new pile cap; construct the new pile cap on top of the new pile foundations.

[0043] Step S2: Construct the pre-cast portion of the extended pier body on the new foundation. The extended pier body is the pier body between the existing pier body and the new foundation. The top surface of the pre-cast portion is connected to the bottom surface of the existing foundation. Construct a support system on the new foundation. The support system is equipped with jacking equipment.

[0044] Step S3: Apply the jacking force of the jacking equipment synchronously according to the design requirements until the support system tightens the existing bearing platform;

[0045] Step S4: After pouring a portion of the concrete to the design strength, cut off the connection between the existing pile foundation and the existing pile cap, and then apply the jacking force of the jacking equipment again to relieve the stress on the support system.

[0046] Step S5: Remove the support system and the existing pile body above the top surface of the new pile cap, and cut off the excess part of the existing pile cap;

[0047] Step S6: Install reinforcing bars on the cut surfaces of the excess parts of the existing pier body and existing abutment, construct the extended outer cast-in-place portion of the pier body, and complete the lowering of the existing railway bridge pier abutment.

[0048] Before construction, protective railings are installed at the construction site, the site is leveled, and the existing concrete cushion layer at the bottom of the existing foundation is removed and leveled. In this embodiment, the foundation construction uses steel sheet piles for protective excavation, C20 concrete for bottom sealing, and a sealing thickness of 50mm. In some embodiments, for construction in water, the existing pile foundation is reinforced with a steel casing extending 14m, with a steel casing wall thickness of 10mm.

[0049] The specific method for constructing new pile foundations is as follows: within the existing pile foundation area, new pile foundations are constructed using drilling and grouting methods according to the arrangement of the new pile foundations. In this embodiment of the invention, the project requires the lowering of the pier caps for 6 bridge piers. Taking pier No. 24 as an example, the pier has 8 existing pile foundations with a diameter of 1.8m, and 6 new pile foundations with a diameter of 1.8m. The new pile foundations are distributed inside the existing pile foundations, such as... Figure 1 , Figure 2 As shown.

[0050] The specific method for constructing the new pile cap is as follows: First, construct protective piles, excavate the foundation pit for the new pile cap, pour bottom sealing concrete at the base of the pit, install reinforcing bars on the existing pile foundation and the new pile foundation, tie the reinforcing bars for the new pile cap, then pour concrete, and cure it. The construction of the new pile cap is as follows: Figure 3 As shown. When constructing a new pier cap, it is necessary to pre-embed the reinforcing steel bars for the pre-cast pier body and the outer casing of the post-cast portion. During the construction of the new pier cap, attention should be paid to reserving a 1600×1600mm platform at the support point of the support system. During the construction process, the existing railway line will be subject to speed restrictions for 5 days after the concrete pouring of the new pier cap.

[0051] The specific method for constructing the pre-cast section of the extended pier body is as follows: reinforcing bars are installed on the bottom surface of the existing pier cap, grouting pipes are installed, the reinforcing bars for the pre-cast section are tied, the formwork for the pre-cast section is installed, micro-expansion concrete is poured, and curing is performed. The construction of the pre-cast section is as follows: Figure 4 As shown. It is important to note that before constructing the first section, the existing foundation surface should be roughened and cleaned before installing the reinforcing bars. When tying the reinforcing bars for the first section, care should be taken to reserve stirrups for wrapping the concrete of the subsequent sections and to embed steel plates for connecting with the support system's supporting steel pipes. More preferably, embedded steel pipes should be installed during the pouring of the first section of concrete. These embedded steel pipes serve as vents during concrete pouring and as grouting conduits for later application, ensuring a tight bond between the newly poured concrete and the existing foundation surface. During construction, speed limits will be imposed on existing railway lines for five days after the first section of concrete is poured.

[0052] The support system includes supporting steel pipes and steel plates. Multiple supporting steel pipes are installed, with newly constructed foundations connecting the bottom ends of each pipe. Each supporting steel pipe has a steel plate at its top, and lifting equipment is mounted on the steel plate. Figure 5 , 6 As shown. The lifting equipment is a self-locking hydraulic jack.

[0053] The support system also includes horizontal braces and diagonal braces; multiple horizontal braces are provided in the vertical direction of the supporting steel pipe, and the two ends of the multiple horizontal braces are respectively connected to the adjacent supporting steel pipes in the vertical direction; the two ends of the multiple diagonal braces are respectively connected to the adjacent supporting steel pipes of the steel pipe column in the vertical direction, and are connected to the adjacent horizontal braces.

[0054] The specific method for severing the connection between the existing pile foundation and the existing pile cap is as follows: The tops of the existing pile foundations are symmetrically and sequentially cut to disconnect the connection between the existing pile foundation and the existing pile cap, transferring the load on the existing pile cap to the pre-cast portion. After relieving the stress on the supporting steel columns of the support system and releasing the jacking force of the lifting equipment, all the load on the existing pile cap is evenly transferred to the pre-cast portion of the extended pier. Speed ​​limits are imposed on traffic during the severing of the connection between the existing pile foundation and the existing pile cap. Specifically, the existing pile foundation is manually and symmetrically cut 0.5m below the existing pile cap to detach it from the existing pile cap. Monitoring and measurement are strengthened during the cutting process. If the stress and strain exceed the design value when cutting the existing pile foundation, the construction unit immediately fills the cutting seam with wedges and takes timely action.

[0055] After the initial concrete pour reaches at least 90% of its design strength and has been cured for at least 10 days, apply the jacking force of the lifting equipment simultaneously as required by the design until the supporting steel columns are firmly pressed against the bottom surface of the existing foundation, and then release the jacking force. Speed ​​limits must be enforced on the line during the jacking process.

[0056] When demolishing the existing piles above the top surface of the new foundation, steel pipes are used to erect working supports. The foundation must be treated before the working supports are erected. The construction unit must design the supports to ensure safety.

[0057] The removal of excess portions of the existing foundation cap will be carried out symmetrically in sections, from the outside in, simultaneously from both sides. The removal of the existing foundation cap will employ a coiled cutting method, and necessary safety measures must be taken during the cutting process to prevent vibration and ensure the safety of the existing pier body and the pre-cast sections. The removal of the existing foundation cap will be carried out sequentially, with each section weighing no more than 10 tons. It should be noted that, based on the dimensions of the pre-cast sections, any portion of the existing foundation cap that extends beyond the pre-cast sections is considered excess.

[0058] like Figure 7 , 8As shown, the post-cast outer portion is placed on the outer surface of the pre-cast portion, the outer surface of the existing pile cap after the excess portion has been removed, and the outer surface of part of the existing pier body. Specifically, the post-cast outer portion is placed on the outer surface of the bottom end of the existing pier body, and the length of the post-cast outer portion covering the bottom end of the existing pier body is 200-500cm. Specifically, in this embodiment, the length of the post-cast outer portion covering the bottom end of the existing pier body is 300cm. The post-cast outer portion is prepared using cast-in-place concrete, and the thickness of the post-cast outer portion is designed based on factors such as the allowable bearing capacity and linear stiffness of a single pile. Before the construction of the post-cast outer portion, the cut surface of the existing pile cap and part of the existing pier body are roughened, cleaned, and then rebar is installed. To ensure a consistent appearance, the concrete for the post-cast outer portion must be from the same batch, and the top of the post-cast outer portion is joined to the existing pier body with a rounded corner. In this embodiment, the cast-in-place portion extending the pier body uses micro-expansion C40 concrete, the post-cast outer portion uses C40 concrete, and the rebar in the post-cast outer portion is HRB400.

[0059] After the construction of the outsourced post-cast section is completed, the foundation pit is backfilled, the riverbank is protected, and the river channel is leveled.

[0060] During construction, deformation and displacement stress monitoring elements are installed on existing pier tops, existing pile caps, and newly added pile foundations to detect stress and strain conditions and ensure construction safety. The distance L between the existing pile body sensor measuring points and the bottom of the lower pile cap should be less than twice the pile diameter (3.6m). The pier body sensor measuring points are vertically arranged in the center of the pre-cast pier body. During construction, the sensors and cables should be protected, and care should be taken to avoid damage to the sensors caused by vibration.

[0061] To verify the safety and feasibility of this construction plan, a stress simulation analysis was conducted on the bridge piers and pile foundations during the design process. Both the bridge piers and pile foundations met the stress requirements, and the stress components are as follows:

[0062] (1) Calculation of pile foundation bearing capacity

[0063] Table 1. Stress parameters of pier No. 24

[0064]

[0065] (2) Calculation of stress on existing foundation

[0066] When the calculated height of the existing pier cap is taken as 2.5m, the results of the punching shear and oblique section shear resistance checks of the pier cap are shown in Table 2 below:

[0067]

[0068] The calculated height of the existing foundation is taken as 3.5m. The bending resistance check results of the foundation are shown in Table 3 below:

[0069]

[0070] The calculated height of the existing foundation is taken as 3.5m. Under the most unfavorable load combination, the tensile stress at the bottom of the foundation is 2.1MPa.

[0071] (3) Calculation of stress on existing pile foundations

[0072] The eight existing piles with a diameter of 1.80m were under small eccentric compression under all working conditions, bearing an axial force of 5482.4kN, a bending moment of 510.1kN·m, and a maximum concrete compressive stress of 2.7MPa.

[0073] (4) Calculation of stress and deformation of the supporting steel pipe

[0074] The supporting steel pipe is designed with a diameter of 1000mm, a wall thickness of 24mm, and is made of Q235 steel.

[0075] Assuming the entire load is borne by the supporting steel pipe, the vertical force borne by a single supporting steel pipe is 43063.4 kN, and the stress is 97.5 MPa. Its compressive deformation is 4.53 mm.

[0076] (5) Calculation of stress and deformation of the precast part of the extended pier body

[0077] Assuming the load is borne entirely by the pre-cast section, the vertical force it bears is 43063.4 kN, the concrete compressive stress is 2.56 MPa, and the compression deformation is 0.13 mm.

[0078] Assuming the core pier does not bear vertical pressure, and calculated as a pure bending member, the maximum bending moment that the pre-cast section can withstand in the transverse direction is 11750 kN·m, and the maximum bending moment that it can withstand in the longitudinal direction is 10450 kN·m.

[0079] The technical solution of this invention involves constructing new pile foundations within the existing pile foundation area to form a new pile group foundation. Then, a new pile cap and a pre-cast pier section are constructed. A temporary support system is installed on the new pile cap. Two system conversions are then performed. When the jacking equipment presses the existing pile cap tightly, the temporary support system and the pre-cast section can withstand the load on the existing pile cap, thus completing the first system conversion. Next, the connection between the existing pile foundation and the existing pile cap is severed, transferring the load on the existing pile cap to the pre-cast section, thus completing the second system conversion. Finally, the excess portion of the existing pile cap is removed, and the extended pier section is constructed, achieving the goal of reducing the height of the existing railway bridge pier pile cap. This solution accelerates construction, minimizes the impact on railway bridge traffic, reduces railway operation risks, and saves investment. For a single pier of a double-track railway bridge, it saves approximately 5 million yuan in investment and approximately 3 months of construction time.

[0080] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A construction method for reducing the height of existing railway bridge pier caps, characterized in that, Includes the following steps: Step S1: Construct new pile foundations within the existing area under the existing pile cap; the top surface of the new pile foundations is flush with the bottom surface of the new pile cap; construct the new pile cap on the top of the new pile foundations; when constructing the new pile caps, pre-embed the reinforcing bars for the pre-cast pier body and the outer post-cast portion; and arrange deformation and displacement stress monitoring elements on the top of the existing pile foundation piers, the existing pile caps, and the new pile foundations. Step S2: Construct the pre-cast portion of the extended pier body on the newly built foundation. The extended pier body is the pier body between the existing pier body and the newly built foundation. The top surface of the pre-cast portion is connected to the bottom surface of the existing foundation. Construct a support system on the newly built foundation. The support system is equipped with jacking equipment. Step S3: Apply the jacking force of the lifting equipment synchronously according to the design requirements until the support system presses against the existing bearing platform; Step S4: After the pre-poured concrete reaches the design strength, the connection between the existing pile foundation and the existing abutment is cut off. Then, the jacking force of the jacking equipment is applied synchronously again to relieve the stress on the support system. The specific method for cutting off the connection between the existing pile foundation and the existing abutment is to symmetrically and successively cut off the top of the existing pile foundation. Step S5: Remove the support system and the existing pile foundation above the top surface of the newly built pile cap, and cut off the excess part of the existing pile cap. The excess part of the existing pile cap is cut off in sections from the outside to the inside and from both sides simultaneously in a symmetrical manner. The existing pile cap is cut off by a coiled cutting method. Step S6: Reinforcing bars are installed on the cut surfaces of the existing pier body and the existing abutment, and the outer post-cast portion of the extended pier body is constructed. The outer post-cast portion is set on the outer surface of the pre-cast portion, the existing abutment, and part of the existing pier body, thus completing the reduction of the existing railway bridge pier abutment.

2. The construction method for reducing the foundation of existing railway bridge piers according to claim 1, characterized in that, Before construction, protective railings were installed at the construction site, the site was leveled, and the original concrete layer at the bottom of the existing foundation was removed and leveled.

3. The construction method for reducing the foundation of existing railway bridge piers according to claim 1, characterized in that, In step S1, the specific method for constructing the new foundation is as follows: first, construct protective piles, excavate the foundation pit of the new foundation, apply sealing concrete to the bottom of the foundation pit, install rebar on the existing pile foundation, tie the steel bars of the new foundation, and then pour concrete and cure it.

4. The construction method for reducing the foundation of existing railway bridge piers according to claim 1, characterized in that, In step S2, the specific method for constructing the pre-cast portion of the extended pier body is as follows: reinforcing bars are planted on the bottom surface of the existing pier cap, grouting pipes are installed, reinforcing bars of the pre-cast portion are tied, the casting formwork of the pre-cast portion is installed, micro-expansion concrete is poured, and then cured.

5. The construction method for reducing the foundation of existing railway bridge piers according to claim 1, characterized in that, The support system includes a supporting steel pipe and a steel plate. The bottom end of the supporting steel pipe is connected to the newly built pier, and the lifting device is installed between the supporting steel pipe and the steel plate.

6. The construction method for reducing the foundation of existing railway bridge piers according to claim 5, characterized in that, The support system also includes horizontal braces and diagonal braces; multiple horizontal braces are provided in the vertical direction of the supporting steel pipe, and the two ends of the multiple horizontal braces are respectively connected to the adjacent supporting steel pipes in the vertical direction; the two ends of the multiple diagonal braces are respectively connected to the adjacent steel pipe columns in the vertical direction, and are connected to the adjacent horizontal braces.

7. The construction method for reducing the foundation of existing railway bridge piers according to claim 6, characterized in that, In step S1, after the strength of the pre-poured concrete reaches more than 90% of the design strength and the curing time is not less than 10 days, the jacking force of the jacking equipment is applied synchronously according to the design requirements until the supporting steel pipe is pressed against the bottom surface of the existing foundation, and then the jacking force of the jacking equipment is released.

8. The construction method for reducing the foundation of existing railway bridge piers according to any one of claims 1-7, characterized in that, In step S6, after the construction of the external post-cast portion is completed, the foundation pit is backfilled, the riverbank is protected, and the river channel is leveled.

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