A backfill assembly type protection structure for existing bridge piers and a construction method thereof

Abstract

The application relates to a backfill assembly type protection structure of an existing bridge pier and a construction method thereof. The construction method comprises the following steps: constructing an independent foundation around the existing bridge pier, wherein the independent foundation comprises a ring-shaped bearing platform which is isolated from the existing stress system; assembling prefabricated components on the ring-shaped bearing platform in stages to form an assembly type protection cylinder which covers the bridge pier, and a gap is reserved between the protection cylinder and the ring-shaped bearing platform; after single-layer segments are assembled, aligned and mechanically fastened, node leakage prevention plugging work is performed; after the solidification strength of the closed part reaches the standard, internal buffer sand is filled in the gap, and external backfill soil is layered on the outside of the protection cylinder, the filling of the two is basically consistent in vertical elevation, and the filling of the two is alternately performed synchronously. In the structure of the application, adjacent prefabricated components are connected through lateral staggered joint seams on the side surfaces, and are connected through vertical joint seams in the upper and lower layers. The joints and the staggered joint seams are filled with sealing layers. The application effectively resists lateral shear force of high eccentric load backfill, blocks additional stress transmission and prevents sand leakage in a limited space.

Description

Technical Field

[0001] This invention relates to the field of bridge engineering and foundation pit backfilling construction technology, and in particular to a prefabricated protective structure for backfilling existing bridge piers and its construction method. Background Technology

[0002] With the continuous advancement of urbanization, construction land resources are becoming increasingly scarce. Urban construction often requires backfilling of areas with significant elevation differences, such as valleys and slopes. When backfilling is carried out in the area of ​​existing urban bridges (especially high-pier bridges with strict deformation control), direct backfilling will result in enormous backfill pressure acting directly on the bridge piers, leading to increased lateral loads, excessive lateral deformation, and cracks exceeding the standard width. In particular, for high-pier continuous rigid frame bridges, direct backfilling will reduce the actual calculated pier height, causing a sharp increase in pier top stiffness and bending moment, ultimately altering the stress state of the entire bridge and seriously endangering the structural safety of existing bridges.

[0003] To achieve physical stress isolation, several protection schemes have been proposed in the prior art. For example, CN222120293U discloses a backfill protection structure for high piers of existing urban rail bridges, which constructs a macroscopic composite frame combining independent pile foundations, abutments, casing bodies, and internal sand filling to distribute the load. However, the construction process of traditional cast-in-place protection devices is cumbersome and time-consuming; while when prefabricated structures are adopted to improve efficiency (such as the conventional assembly concept disclosed in CN111472265A, which involves mass-producing prefabricated protection segments in a factory and then circumferentially splicing and bolting them on site), many serious structural and procedural defects are still exposed when facing the extremely harsh engineering boundary conditions of ultra-deep, asymmetrical, and high-eccentric load backfilling of existing high pier protection zones.

[0004] Specifically, due to the enormous inward compressive lateral pressure generated by the backfill soil in deep foundation pits, conventional flat overlaps or simple assembly joints, in the intermediate construction state before being fully compacted by the gravity of the upper layer, are prone to excessive cantilever bending moments, leading to shear cracking of the joints or inward bursting and instability of the overall structure. Asymmetrical backfill loads can also be transmitted downwards, causing eccentric bending failure of the bottom independent pile foundation. In addition, the high-frequency vibrations generated by large road rollers during compaction around the perimeter can easily cause horizontal misalignment of the newly assembled casing segments. At the same time, the extremely narrow confined space between the casing and the existing bridge piers presents severe challenges to groundwater infiltration and corrosion prevention, as well as the filling of granular fine sand. In the absence of specific joint leak-proof structures, if the inner and outer filling materials do not achieve dynamic pressure balance, or if the timing of sand and soil filling is not strictly controlled before the anti-corrosion sealing layer of the gaps reaches sufficient curing strength, catastrophic engineering consequences such as groundwater backflow eroding fasteners, large amounts of sand leakage from unsealed joints, and pipe wall rupture under pressure can easily occur.

[0005] Furthermore, on the one hand, there are differences in understanding among those skilled in the art; on the other hand, the applicant studied a large number of documents and patents when making this invention, but due to space limitations, not all details and contents were listed in detail. However, this does not mean that the present invention does not possess the features of these prior art. On the contrary, the present invention already possesses all the features of the prior art, and the applicant reserves the right to add relevant prior art to the background art. Summary of the Invention

[0006] In view of the shortcomings of the existing technology, the present invention provides an existing bridge pier backfill prefabricated protection structure and its construction method to solve at least some of the above-mentioned technical problems.

[0007] This invention discloses a construction method for backfilling within the protection zone of existing bridge piers, comprising the following steps: S1. Construct independent foundations around the existing piers. The independent foundations include annular pile caps that surround the existing piers and are isolated from the existing load-bearing system. S2. Above the ring-shaped pier, prefabricated components are assembled step by step to form a prefabricated casing that covers the existing pier. A gap is maintained between the prefabricated casing and the outer wall of the existing pier. S3. After the single-layer segment of the prefabricated casing is assembled, aligned, and mechanically fastened, perform node leak prevention and sealing operations to seal the assembly gaps and fastening operation surfaces of the prefabricated components. S4. After the curing strength of the sealed area reaches the preset requirements, fill the gap between the prefabricated casing and the existing pier with internal buffer sand, and backfill the outer side of the prefabricated casing with layered external backfill soil; the filling of internal buffer sand and the layered backfilling of external backfill soil should be basically consistent in vertical elevation and carried out synchronously and alternately. Repeat steps S2 to S4, using the sequence of sealing first and then filling synchronously after the strength meets the standard, until the entire structure is filled to the design elevation.

[0008] To address the technical problems of the immense lateral earth pressure during backfilling of deep foundation pits, which can easily lead to lateral deformation and damage to existing bridge piers, and the long construction cycle and potential disturbance to the original bridge caused by traditional cast-in-place protective structures, this invention proposes a construction method for backfilling within the existing pier protection zone. This method breaks away from the conventional approach of simply using rigid compressive strength in protective frames. It combines cast-in-place foundations with factory-prefabricated standard components, using a cast-in-place annular cap at the bottom to block the vertical force transmission path, ensuring that the backfill load is completely bypassed from the existing load-bearing system. Prefabricated components are then assembled in stages above the cap to form a prefabricated casing that covers the pier, avoiding direct contact with the pier and eliminating the need for cumbersome on-site scaffolding erection. More importantly, this invention reconstructs the causal chain of operations within a confined space. After single-layer segment assembly and mechanical fastening, priority is given to sealing the joints and working surfaces to prevent leakage. Once the curing strength reaches the required level, internal buffer sand and external backfill soil are then filled alternately and synchronously. This timing sequence completely cuts off the outward loss path of subsequent backfill sand and cleverly utilizes the elastic support and buffering effect of the internal buffer sand from the inside out to achieve a dynamic balance with the inward pressure applied by the external backfill soil. This "sealing before filling, internal and external balance" construction mechanism overcomes the mechanical defects of conventional prefabricated casings, which are easily squeezed inward by unilateral soil pressure and become unstable in the intermediate construction state when they are not fully compacted by the upper layer. From the perspective of process evolution, it endows the protection system with extremely high stress robustness.

[0009] According to a preferred embodiment, the isolated foundation further includes isolated pile foundations; step S1 includes: Independent pile foundations are constructed using a skip-pile excavation process. The bottom elevation of the independent pile foundation is set to be lower than the bottom elevation of the existing pile foundation of the existing bridge pier, and the top elevation of the independent pile foundation is set to be higher than the top elevation of the existing abutment of the existing bridge pier. A ring-shaped pile cap is poured on top of the independent pile foundation. The support of the independent pile foundation is used to maintain a vertical clearance between the bottom surface of the ring-shaped pile cap and the top surface of the existing pile cap, which is physically isolated.

[0010] In the construction of the foundation of this invention, the construction of independent pile foundations through a skip-pile excavation process, the configuration of specific upper and lower elevations, and the pre-reserved vertical clearance at the bottom of the pier cap effectively isolates the new foundation from the ground disturbance and stress superposition of the original bridge substructure. Specifically, the skip-pile excavation effectively suppresses the concentrated disturbance of stress release in the strata surrounding the existing piers caused by continuous drilling operations; the pile bottom elevation of the independent pile foundation is lowered below the existing pile base surface, which, from a geotechnical perspective, causes the stress-affected bubbles of the new pile group to directly cross the load-bearing zone of the original bridge pile foundation, and directly dissipates the massive backfill load intercepted and transmitted downward by the prefabricated casing into a deeper bearing layer, avoiding the overlap of vertical stress bubbles between the new and old pile foundations at the same elevation. Furthermore, relying on the support of the independent pile foundation's higher top elevation than the existing pier cap, a physical isolation vertical gap is naturally formed between the bottom surface of the cast-in-place annular pier cap and the existing pier cap. This vertical gap serves as a buffer zone for settlement deformation, ensuring that when the annular pile cap experiences minor settlement due to the compaction load of the thick backfill soil above and its own weight, its bottom surface can never touch or exert pressure on the existing pile cap, thus achieving absolute blockage of the vertical load transfer path at the lower boundary.

[0011] According to a preferred embodiment, in step S4, when the internal buffer sand is filled and the external backfill soil is backfilled in layers, the top surface height of the filling is controlled to be lower than the top surface of the currently spliced ​​single-layer segment. A safe space is reserved between the top surface height and the top surface of the single-layer segment, which is not to be filled, as a buffer section to prevent cantilever bending moment cracking and a construction platform for subsequent assembly.

[0012] To address the challenges of shear cracking at the joints of newly assembled segments due to earth pressure during backfilling and the scarcity of high-altitude work surfaces, this invention creatively introduces a buffer mechanism with reserved safety space during the synchronous backfilling process. From the perspective of the dynamic evolution of prefabricated structures, when a single-layer segment has just been assembled and has not yet borne the weight of the preceding component, its upper part is essentially in a cantilevered, structurally weak state. This invention strictly constrains the height of the top fill surface to be lower than the top surface of the current segment, intentionally leaving a safety section unfilled at the upper end of the pipe wall. This ensures that the resultant force of the external lateral earth pressure always lies within the lower pipe wall section, which is stably held by the inner and outer fill materials. This space significantly shortens the lever arm of the inward overturning moment generated by the earth pressure, blocking the mechanical transmission path that would cause the joint to be overturned and torn inward. At the same time, the top surface of the filler under the reserved height difference is transformed into a physical operating platform for the precise hoisting and positioning of the next level components and for construction workers to enter the narrow cavity to tighten bolts, thus achieving logical consistency between structural mechanical defense measures and the construction of the on-site construction work surface.

[0013] According to a preferred embodiment, in step S4, the layered backfilling operation of the external backfill soil is carried out symmetrically around the periphery of the prefabricated casing, and the backfill height difference between each direction is controlled not to exceed a preset height difference threshold. Furthermore, a control boundary for mechanical operations is defined around the perimeter of the prefabricated casing. Within this control boundary, the operation of large machinery is restricted to prevent damage to the not-yet-fully-cured sealed area caused by the weight of the machinery and shock waves.

[0014] To prevent eccentric bending failure of the underlying foundation due to the transmission of eccentric load from external backfill soil, and to prevent early joint sealing damage caused by seismic waves from heavy compaction machinery, this invention establishes a comprehensive load prevention mechanism of "symmetrical advancement on the periphery and strict control of the work boundary." During the outer backfilling stage, the backfill height difference between all directions around the casing is strictly controlled to not exceed a set threshold. This ensures that the lateral earth pressure vector around the closed annular casing is always in a symmetrical equilibrium state with a resultant force approximately zero, thus preventing bending and shear instability of the casing wall due to huge local eccentric pressure and effectively cutting off the eccentric load failure path of asymmetrical bending moment transmitted along the casing wall to the underlying independent pile foundation. Based on this, a mechanical operation control boundary is delineated around the casing, and the operation of large machinery is restricted, establishing a spatial isolation zone based on the principles of vibration wave propagation and dynamic load superposition. This measure avoids excessive additional lateral compressive force generated by the huge weight of the heavy compaction equipment close to the cylinder wall, while eliminating fatigue shear damage caused by high-frequency mechanical vibration waves to the sealing layer at the joint of the segments that has not yet been fully cured, ensuring that the joint water-stopping and grouting system can complete hydration and consolidation in a static and undisturbed environment.

[0015] According to a preferred embodiment, after the entire fill is completed to the design elevation, the construction method further includes: Control the overall fill height and continue filling until it is higher than the design elevation to allow for settlement. A top sealing cover is installed on the top surface of the highest segment of the prefabricated casing to cover the gap opening, and the joints of the top sealing cover are grouted to create a waterproof barrier.

[0016] This invention addresses the technical problems of surface water accumulation and seepage erosion of internal sand filling caused by late-stage consolidation of deep backfill, from a macro-lifecycle perspective, through a combination of finishing processes including pre-reserved settlement allowance during ultra-high filling, top cover sealing, and joint grouting. Based on the long-term creep and pore water dissipation consolidation mechanism of granular backfill, deep backfill sites inevitably experience gradual foundation settlement after completion. The ultra-high filling action implemented in this invention intervenes in advance and compensates for soil shrinkage and settlement throughout the entire lifecycle, ensuring smooth connection of the site top surface to the surface drainage system even at the final settlement state, preventing the formation of low-lying water-filled basins. Subsequently, a sealing cover is laid flat on top of the casing and rigid grouting is applied, completely transforming the open physical gap between the casing and the pier into an absolutely sealed underground independent cavity. This rigid water-stopping cap cuts off the hydraulic channels through which atmospheric precipitation, surface runoff, and capillary water seep into the casing, preventing the internal buffer sand from being hollowed out and lost due to long-term seepage and erosion. It also prevents the formation of destructive hydrostatic pressure due to a sudden increase in water accumulation in the narrow cavity, providing lifelong constant humidity and pressure sealing protection for the existing bridge piers.

[0017] This invention also discloses a prefabricated protective structure for backfilling existing bridge piers, applied in the aforementioned construction method, comprising: Independent foundations include an annular abutment set around the existing pier. The annular abutment has a hollow area in the middle that avoids the existing pier. With the support of the bottom independent foundation, a vertical clearance that is physically isolated is maintained between the bottom surface of the annular abutment and the top surface of the existing abutment of the existing pier. The prefabricated casing is erected vertically above the annular pier and forms a rigid anchorage with the annular pier. The prefabricated casing is assembled from multi-segment prefabricated components to form a closed annular structure, and a physical gap is reserved between the inner wall of the prefabricated component and the outer wall of the existing pier to accommodate the internal buffer sand. In this process, adjacent prefabricated components are assembled and connected on the side by horizontal staggered joints, and the upper and lower prefabricated components are staggered and interlocked by vertical joints; the component wall at the horizontal staggered joint is provided with an inwardly recessed operating groove.

[0018] This invention provides a prefabricated protective structure for backfilling existing bridge piers. As the core physical carrier for implementing the aforementioned dynamic backfilling construction method, it addresses the technical problems of cumbersome traditional cast-in-place protection procedures and the tendency for joints of conventional assembled cylinders to shift and tear under high pressure in deep foundation pits. It reconstructs the system isolation and the microscopic force transmission interface of the assembly nodes. In terms of macroscopic structure, a centrally hollow annular bearing platform with pre-reserved physical gaps at the bottom, together with a rigidly rooted closed annular casing above it, forms an outer rigid armor independent of the original bridge load-bearing system. This armor completely intercepts the enormous lateral thrust generated by the deep backfill soil and dissipates it downwards along the pipe wall into the ground. The internal physical gaps completely isolate the stress transmission between the old and new structures. In terms of the micro-component connection structure, the transverse staggered joints on the sides of adjacent precast components directly push and interlock through stepped geometric steps, transforming external horizontal shear force into local cross-sectional bearing pressure and constructing a solid geometric constraint boundary against horizontal inward slippage. Meanwhile, the vertical butt joints between upper and lower components rely on a tongue-and-groove staggered nesting form, automatically triggering a three-dimensional circumferential locking mechanism after the components are laterally closed, effectively limiting the omnidirectional sliding freedom of the upper segment in the horizontal plane. Combined with the inwardly recessed, fully enclosed operating groove filled with micro-expansion concrete, the mechanical fasteners are concealed and rigidly sealed inside the pipe wall, thus endowing the precast cylinder with ultra-high shear closure stiffness equivalent to a cast-in-place monolithic structure under extremely harsh backfill boundary conditions.

[0019] According to a preferred embodiment, the annular support is provided with a closed annular misaligned support boss at the position corresponding to the installation section of the prefabricated casing; the thickness of the misaligned support boss is greater than the conventional wall thickness of the prefabricated casing. The top surface of the staggered protrusion of the pier cap matches the shape of the bottom of the first prefabricated casing to form a vertical staggered interlocking. The staggered protrusion of the pier cap is provided with a pre-embedded steel sleeve with a vertical opening, which is used to vertically grout and connect with the reinforcing bars inside the first prefabricated casing.

[0020] In the design of the lower rooting node of the protective structure of this invention, the technical problem of easy breakage and shear misalignment of the prefabricated cylindrical cantilever root under the huge overturning bending moment is overcome by the thickened structure of the staggered boss of the bearing platform, the stepped staggered surface matching, and the coordinated structure of the internal pre-embedded steel sleeve vertical grouting connection. According to the internal force distribution law of the cantilever bending member, the bottom edge of the casing erected on the bearing platform is the maximum peak section of the superposition of the bending moment and horizontal shear force of the whole cylinder. This invention breaks the conventional equal wall thickness design and explicitly configures the thickness of the upwardly protruding staggered boss of the bearing platform to be greater than the conventional wall thickness of the casing. By using local physical solid thickening, the cross-sectional bending stiffness and shear area of ​​the root interface are directly improved. In terms of the fixed connection force transmission mechanism, the stepped misalignment on the top surface of the staggered boss of the bearing platform and the staggered bottom edge of the first section of the casing form a vertically misaligned nested interlock, which first locks the degree of freedom of the first section of the casing to slide along the horizontal section. Then, in conjunction with the vertical steel sleeve pre-embedded inside the boss, the high-strength vertical steel bars inside the first section of the casing are inserted into the sleeve and high-strength grouting is performed. Relying on the ultra-high gripping friction force generated between the hardened grout and the steel bars and the inner wall of the sleeve, the rigid fixed connection node is perfectly reproduced on the prefabricated assembly section. This three-in-one rooting structure of "geometric thickening, staggered interlocking, and sleeve grouting" builds a bottom fixed support boundary with extremely high anti-overturning stability for the entire prefabricated cantilever shell.

[0021] According to a preferred embodiment, the cross-section of the precast component has a straight line segment and a circular arc segment transitioning to a concentric circular arc segment, and the transverse staggered joint is arranged near a quarter span of the cross-section of the precast component; and a pre-embedded reinforcing steel plate is embedded in the turning area inside the stepped surface of the transverse staggered joint.

[0022] To address the micromechanical defects of precast components under deep burial and lateral pressure, such as stress concentration cracking at right-angle corners, weak shear resistance at section joints, and compressive collapse at stepped corners, this invention deeply optimizes the cross-sectional morphology and internal reinforcement network of the components. In controlling the geometric deformation of the closed frame structure, the component cross-section abandons conventional right-angle splicing, adopting a smooth transition design between straight segments and concentric circular arc segments. This eliminates the right-angle tip effect from a topological perspective, allowing external lateral earth pressure to be uniformly transmitted and dissipated along the smooth circular arc surface when flowing through the corner area, preventing the corner concrete from collapsing due to stress concentration. Regarding joint location and reinforcement of weak sections, this invention utilizes the bending moment envelope diagram of the closed frame under uniformly distributed loads, precisely arranging the transverse staggered joints in the zero-moment inflection point region near the quarter-span of the section, cleverly avoiding the maximum bending sections at the mid-span and corners, ensuring that the joint section mainly bears pure shear force. To address the localized stiffness weakening gap in transverse misaligned joints where the cross-sectional dimensions are halved due to step chamfering and the reinforcing bars cannot be continuously connected, this invention embeds a reinforcing steel plate within the geometric transition zone of the stepped surface. Utilizing the excellent shear stiffness and compressive ductility of the steel plate, it strongly constrains and shares the concentrated shear stress borne at the step corner, eliminating the potential for shear-induced joint breakage from within the cross-section.

[0023] According to a preferred embodiment, the operating slot includes an outer operating slot and an inner operating slot respectively opened on the outer wall and inner wall of the precast component. The precast components have pre-embedded channels for high-strength bolts to pass through. The diameter of the pre-embedded channels is larger than the outer diameter of the high-strength bolts to provide tolerance for perforation. The fastening nuts that are fastened to the ends of the high-strength bolts are deeply recessed and enclosed in the operating slot.

[0024] This invention solves the interference problem of difficulty in perfectly aligning holes during high-altitude hoisting of large precast components and the technical challenge of dragging backfill soil with exposed fasteners by using double operating slots on the inner and outer walls of the components, a through-hole fit with a diameter larger than the outer diameter of the high-strength bolt, and a deeply recessed and hidden fastening nut structure. Considering the unavoidable spatial orientation and geometric tolerances when hoisting and positioning ultra-heavy precast components in the harsh working environment of deep foundation pits, this invention configures the diameter of the reserved hole to be significantly larger than the outer diameter of the bolt, releasing ample radial geometric tolerance within the cavity. This ensures that even if adjacent components experience slight vertical misalignment or skewed installation, the high-strength bolt can still smoothly penetrate the entire hole to complete the locking, greatly improving the assembly error tolerance rate during on-site construction. Regarding the optimization of the fastening node morphology, operating slots are recessed inward on the inner and outer pipe walls on both sides of the transverse joint. This not only provides the necessary operating space for construction personnel to tighten the nuts by reaching into the pipe wall section, but more importantly, it ensures that the locked nuts and washers are deeply recessed and hidden within the component's solid wall thickness. This non-standard construction prevents any metal protrusions from intruding into the outer soil-facing side or the inner backwater-facing side of the casing. This provides a smooth base for the continuous application of the subsequent outer waterproof layer and completely eliminates the huge vertical negative skin friction generated by the soil particles shearing and pulling on the exposed screw heads when the thick backfill soil around the casing undergoes natural consolidation and settlement. This protects the casing structure from downward dragging and tearing damage.

[0025] According to a preferred embodiment, the assembled surfaces of the horizontal staggered joints and the vertical butt joints are coated with a rigid penetrating crystalline waterproof layer, and a flexible sealant is provided at the joints near the inner wall of the component. The sealing layer inside the operating slot is made of micro-expansion concrete, which offsets shrinkage deformation and makes the surface of the slot flush and dense with the inner and outer walls of the precast component after sealing.

[0026] Addressing the technical challenges of capillary seepage in prefabricated joints buried in groundwater environments and the corrosion and failure of internal fasteners due to the drying shrinkage and detachment of ordinary sealing mortar, this invention constructs a multi-layered joint waterproofing and grouting system that combines rigidity and flexibility in the joints and features micro-expansion self-locking at the groove openings. At the microscopic water-stopping mechanism of the joint interface, the cement-based penetrating crystalline waterproofing material pre-coated on the misaligned surface of the components, upon encountering groundwater infiltration, generates active needle-like crystals that actively penetrate and spread into the capillary pores of the component surface, sealing the channels. This spontaneously evolves into a rigid water-blocking interface with self-healing capabilities at the assembly section. Combined with the flexible sealant embedded on the back side, relying on the excellent elastic deformation capability of the polymer material, it perfectly compensates for the shortcomings of the rigid crystalline layer in resisting brittle micro-cracks caused by construction rolling shock waves and bridge live load micro-vibrations, forming a double-layered joint waterproofing zone that addresses both internal and external issues. For the three-dimensional sealing of the operating groove, this invention abandons ordinary sealing mortar and uses micro-expansion concrete as the sealing layer. Relying on the microscopic volume expansion stress generated by the material in the early stage of hydration and hardening, it strongly counteracts the shrinkage deformation of the concrete itself, causing the newly poured sealing layer to push outwards tightly and bite into the inner wall of the original operating groove. This tightly seals the high-strength bolts and nuts inside into a completely oxygen-free and water-free, dense, and rigid body, making the surface of the sealed groove and the component pipe wall a perfect whole. This fundamentally ensures the lifelong corrosion resistance and durability of the core connector in a confined space buried for a long time. Attached Figure Description

[0027] Figure 1 This is a schematic front view cross-section of the high pier backfill protection structure provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the construction process of the lower foundation of the high pier backfill protection structure provided in an embodiment of the present invention; Figure 3 This is a schematic diagram of the synchronous filling process of internal buffer sand and external backfill soil provided in an embodiment of the present invention; Figure 4 This is a schematic diagram of the plan layout of the independent foundation (ring cap and independent pile foundation) under the high pier backfill protection structure provided in the embodiment of the present invention; Figure 5 This is a schematic diagram of the cross-section of the assembled casing ring and the planar structure of a single U-shaped prefabricated component provided in the embodiment of the present invention; Figure 6 This is a three-dimensional isometric evolution diagram of the overall construction process of the high pier backfill protection structure provided in the embodiment of the present invention; Figure 7 This is a cross-sectional schematic diagram of the transverse staggered assembly of the U-shaped prefabricated component and the distribution of the inner and outer operating slots provided in the embodiment of the present invention; Figure 8This is a schematic diagram of the elevation and partial cross-sectional structure of the U-shaped prefabricated component provided in an embodiment of the present invention; Figure 9 This is a partial cross-sectional schematic diagram of the transverse assembly node of the prefabricated casing (including bolt fastening and leak-proof sealing structure) provided in an embodiment of the present invention; Figure 10 This is a partial cross-sectional schematic diagram of the rigid rooting node between the first assembled casing and the annular bearing platform provided in an embodiment of the present invention.

[0028] List of reference numerals 100: Existing pier; 101: Existing pile cap; 102: Existing pile foundation; 200: Independent pile foundation; 210: Circular pile cap; 211: Hollow area of ​​pile cap; 212: Staggered protrusion of pile cap; 213: Embedded steel sleeve; 300: Prefabricated casing; 310: U-shaped precast component; 311: Straight section of component; 312: Circular section of component; 313: Inner wall of component; 314: Outer wall of component; 315: Vertical reinforcement; 320: Top closed cover plate; 400: transverse staggered joint; 410: high-strength bolt; 411: reserved hole; 412: fastening nut; 420: outer operating groove; 421: inner operating groove; 500: vertical butt joint; 501: top closed boss; 502: bottom butt staggered joint; 600: penetrating crystalline waterproof layer; 610: flexible sealant; 620: sealing layer; 700: internal buffer sand; 710: external backfill soil. Detailed Implementation

[0029] The following is a detailed explanation with reference to the accompanying drawings.

[0030] This implementation method takes avoiding additional earth pressure and construction loads on the existing pier 100 as the design baseline. To ensure that the original bridge's stress system remains unchanged, a superimposed high pier backfill protection structure is set around the existing pier 100. This protection structure adopts a cast-in-place and precast assembly combination structure, thereby forming a load dissipation path independent of the existing pile foundation 102 and the existing abutment 101. Specifically, as follows... Figure 1 As shown, the protective structure has an independent foundation at its bottom, consisting of cast-in-place independent piles 200 and an annular pile cap 210. The annular pile cap 210 has a hollow area in the center that avoids the existing pier 100, so that the backfilling operation does not have direct force transmission contact with the existing protected system. Furthermore, the protective structure also includes a prefabricated casing 300, which is constructed by cross-section assembly and segmental hoisting. Specifically, it consists of prefabricated reinforced concrete U-shaped components symmetrically installed to form a closed annular structure, vertically erected above the annular pile cap 210. The prefabricated casing 300 is arranged circumferentially around the existing pier 100 to form a covering, and a physical gap consisting of internal buffer sand 700 is provided between the inner wall 313 of the component and the outer wall of the existing pier 100.

[0031] The external backfill soil 710 mainly contacts the outer wall 314 of the prefabricated casing 300 and the top surface of the annular abutment 210 to avoid direct force transmission with the existing pier 100. The prefabricated casing 300 intercepts lateral earth pressure and transmits internal forces downward to the annular abutment 210, ultimately guiding them into the deep bearing layer through independent pile foundations 200. Based on this structure, the force transmission path is physically blocked, bypassing and dispersing the backfill soil pressure and construction load outside the existing bridge structure. This provides a basis for avoiding direct earth pressure acting on the existing pier 100, which would lead to increased lateral load and deformation, and excessive cracking. It also avoids the problem of increased pier top stiffness and bending moment caused by the reduction in the actual calculated pier height in high-pier continuous rigid frame bridges, preventing changes in the overall bridge stress state and ensuring the safety of the existing bridge structure throughout the entire process.

[0032] To ensure the structural relationships can be reproduced and to achieve physical stress isolation, such as Figure 2 As shown, in this embodiment, the bottom of the protective structure adopts a pile group foundation as the lower independent foundation system, which specifically includes independent pile foundations 200 constructed by cast-in-place and an annular pile cap 210 set above the independent pile foundations 200. The independent pile foundations 200 are arranged circumferentially around the existing pile foundations 102, and the minimum pile spacing requirement is met between adjacent independent pile foundations 200. In the hole-forming process, the independent pile foundations 200 can be selected as rotary drilling piles or manually excavated piles according to geological conditions, and it is preferred to use a skip-pile excavation process to reduce the disturbance of the hole-forming operation to the existing load-bearing system. Furthermore, in order to break through the original load-bearing influence bubble and form a load dissipation path independent of the original bridge foundation, the bottom elevation of the independent pile foundations 200 is preferably configured to be a certain depth lower than the bottom elevation of the existing pile foundations 102. For example, this depth difference can be set to 5 meters or more. Meanwhile, the top elevation of the independent pile foundation 200 is configured to be higher than the top elevation of the existing pile cap 101, optionally with the difference controlled between 0.2 meters and 0.5 meters.

[0033] like Figure 4As shown, the annular pile cap 210, cast above the independent pile foundation 200, can be configured as a hexagonal or octagonal solid depending on the array arrangement of the pile foundations. The height of the annular pile cap 210 is determined based on the height of the subsequent backfill, and its vertical dimension can be exemplarily controlled between 2 and 3 meters. A hollow pile cap area 211 is defined in the middle of the annular pile cap 210, which serves as a space to accommodate the existing pier 100. A lateral clearance is maintained between its inner boundary and the outer wall of the existing pier 100, thereby achieving lateral force transmission interruption. In addition, a vertical clearance is maintained between the bottom surface of the annular pile cap 210 and the top surface of the existing pile cap 101, which is physically isolated. The elevation of its bottom surface is determined based on the pile top elevation of the independent pile foundation 200, and is preferably located 0.2 to 0.5 meters above the top surface of the existing pile cap 101. The vertical gap with a certain height forms a settlement and deformation buffer zone, which helps the bottom surface of the ring-shaped pile cap 210 to avoid touching or pressing the existing pile cap 101 when it experiences slight settlement under its own weight and subsequent external backfill compaction load, thereby effectively blocking the transmission path of additional vertical stress.

[0034] like Figure 10 As shown, to resist the maximum overturning moment of the foundation generated by unilateral backfill and the maximum earth pressure borne at the bottom, this embodiment configures a rigid rooting node on the annular foundation 210. Specifically, along the periphery of the hollow area 211 of the annular foundation 210, corresponding to the installation section of the prefabricated casing 300, a closed annular foundation misalignment boss 212 is provided, protruding upwards. To increase the root stiffness, the solid thickness of the foundation misalignment boss 212 is configured to be greater than the conventional wall thickness of the prefabricated casing 300, forming a thickened structure. In terms of the assembly interface morphology, the top surface of the foundation misalignment boss 212 has a stepped misalignment design, which matches the shape of the bottom mating misalignment 502 of the first prefabricated casing 300, so as to achieve misaligned interlocking when the two are vertically mated.

[0035] Furthermore, to ensure a stable and reliable connection at the cantilever root, the internal structure of the misaligned boss 212 of the bearing platform avoids ordinary flat lap joints. Instead, it is equipped with pre-embedded steel sleeves 213 with vertical openings. By inserting the vertical reinforcing bars 315 inside the first prefabricated casing 300 into the pre-embedded steel sleeves 213 and performing high-strength grouting, a vertical grouting connection structure is physically executed and formed. In addition, the side wall of the misaligned boss 212 of the bearing platform is also reserved with high-strength mechanical bolt holes for lateral fastening and their corresponding closed slots. Through the synergistic effect of the locally thickened misaligned boss 212 of the bearing platform, the grouting action performed based on the vertical pre-embedded steel sleeves 213, and the stepped misaligned interface, a high-strength rigid anchoring of the first prefabricated casing 300 and the annular bearing platform 210 is achieved, thereby constructing a high-resistance overturning bottom boundary condition independent of the existing load-bearing system.

[0036] To isolate the backfill soil pressure and provide internal safety space, the main body of the prefabricated casing 300 in this embodiment adopts an assembly structure, with each individual splicing unit being a precast reinforced concrete component. Combined with... Figure 5 As shown, to adapt to the underground burial environment, this single-unit splicing arrangement is specifically configured as a U-shaped precast component 310 produced in a prefabrication plant using impermeable concrete. In terms of assembly morphology, each standard casing segment is constructed by moving and symmetrically installing two U-shaped precast components 310 of identical size and structure towards each other, ultimately forming a complete closed ring structure. The vertical height of a single prefabricated casing 300 can be optionally configured to be 2.0 meters to 3.0 meters, and its cross-sectional wall thickness is determined based on the earth pressure generated by the final backfill height.

[0037] Furthermore, the U-shaped precast component 310 is smoothly composed of a straight component segment 311 and a component arc segment 312 located at the corner. To avoid destructive stress concentration at the corner of the casing due to the pressure of the deep backfill soil, the component arc segment 312 is preferably configured as a concentric arc segment transition. For example, its inner wall chamfer radius can be set to R1000, and its outer wall chamfer radius can be set to R1800, thereby eliminating the geometric abrupt change in the case of right-angle splicing. At the same time, combined with the physical requirements of the lower limit of the working space of manually excavated piles, after the U-shaped precast component 310 is assembled, an appropriate planar clearance distance is maintained between its inner wall 313 and the outer wall of the existing pier 100. This clearance distance is preferably reserved at about 0.8 meters. The direct purpose of designing this clearance limit within the above range is to provide a physically feasible operating surface limit for workers to descend into the casing and operate fastening tools to tighten the internal nuts. In addition, the annular physical gap also simultaneously defines the cavity for the subsequent filling of internal buffer sand 700.

[0038] like Figure 7 As shown, to resist the lateral shear force of the inward compression of the external backfill soil and optimize the overall stress distribution, two adjacent U-shaped precast components 310 are assembled and connected on the side by a stepped transverse staggered joint 400. The transverse staggered joint 400 is preferably located near a quarter span of the cross-section of the U-shaped precast component 310 to avoid the maximum bending moment zone of the structure. This stepped interlocking section can convert the external shear force into local bearing pressure of the cross-section through the direct jacking of the physical geometric steps, thus structurally constructing a constraint boundary against horizontal inward slippage. Considering that the cross-sectional dimensions are relatively small and the internal steel bars are not easy to bend within the transverse staggered joint 400, in order to compensate for the weakening of local strength, embedded reinforcing steel plates are embedded in the internal cross-section of the stepped surface of the transverse staggered joint 400, especially in the internal turning area.

[0039] like Figure 9As shown, in terms of fastening, this embodiment uses high-strength bolts 410 and matching washers that traverse the transverse misaligned joint 400 for tying and locking. The interior of the U-shaped prefabricated component 310 has pre-embedded holes 411 along the horizontal direction for the high-strength bolts 410 to pass through. Considering the perforation tolerance and installation tolerance required for hoisting large prefabricated components on site, the diameter of the pre-embedded holes 411 is explicitly configured to be larger than the outer diameter of the high-strength bolts 410. As a preferred example of dimensional fit, the high-strength bolts 410 can be M30, and the corresponding diameter of the pre-embedded holes 411 can be set to Φ36mm. Along the vertical extension direction of the transverse misaligned joint 400, the pre-embedded holes 411 and their corresponding high-strength bolts 410 are arranged in a multi-row, spaced array, with the vertical spacing between adjacent rows exemplarily configured as 600mm.

[0040] Furthermore, to achieve concealed anchoring and control surface flatness, the U-shaped precast component 310 has an outer operating groove 420 and an inner operating groove 421 recessed inward at corresponding positions on the outer wall 314 and inner wall 313 near the transverse misalignment joint 400. The fastening nuts 412 are not pre-embedded in the factory; instead, after the U-shaped precast component 310 is hoisted and assembled on-site, construction personnel insert the fastening nuts 412 into both ends of the high-strength bolts 410 through the outer operating groove 420 and inner operating groove 421, and perform a double-sided tightening action. Based on the spatial configuration of the operating grooves, the fastening nuts 412 are deeply embedded within the component wall thickness after tightening, avoiding protrusion from the inner and outer surfaces of the casing, thus providing an unobstructed, flat interface for subsequent waterproofing layer application, concrete sealing, and settlement of the outer backfill soil.

[0041] To prevent horizontal misalignment of the casing segments due to vibrations from the adjacent compaction machinery, this embodiment incorporates a vertical butt joint 500 at the junction of the upper and lower U-shaped precast components 310 to prevent misalignment. Specifically, as follows... Figure 8 As shown, the wall thickness section of the U-shaped prefabricated component 310 is designed to be non-straight in the vertical direction. Its top surface is provided with an upwardly protruding top closed boss 501, and its bottom surface is provided with a matching and inwardly recessed bottom mating joint 502.

[0042] During on-site assembly, after the lower U-shaped precast component 310 is horizontally assembled into a closed ring, its top closed boss 501 is then spliced ​​to form a continuous closed ring. Subsequently, the bottom misalignment joint 502 of the lower edge of the upper U-shaped precast component 310 is vertically aligned and nested with the continuous closed ring at the top of the lower layer. When the upper U-shaped precast component 310 is also horizontally assembled and closed, its bottom misalignment joint 502 also forms a continuous closed state. Relying on the all-round three-dimensional nesting form after the upper and lower components are double-looped and closed, the vertical joint 500 forms a self-locking condition after assembly, so that the tongue and groove of the upper and lower layers naturally form a three-dimensional circumferential locking mechanism. This structural feature effectively restricts the sliding freedom of the upper segment in all directions in the horizontal plane, so that it can mainly rely on the self-weight of the components and the geometry of the tongue and groove to effectively resist the mechanical shock waves generated by the operation of large road rollers in the surrounding area, ensuring the overall assembly and operation stability of the casing.

[0043] To address the groundwater environment and achieve long-term corrosion and water-blocking protection for joints and mechanical fasteners, this embodiment constructs a three-dimensional joint corrosion prevention and grout sealing system at the assembly nodes of the prefabricated casing 300. Specifically, before assembly, the assembly surfaces of the transverse misaligned joint 400 and the vertical butt joint 500 are coated with a penetrating crystalline waterproof layer 600. This penetrating crystalline waterproof layer 600 is preferably made of a cement-based penetrating crystalline waterproof material to form a rigid water-blocking interface. Furthermore, on the backwater side of the joint, i.e., the side near the inner wall 313 of the component close to the existing pier 100, a flexible sealant 610 is applied along the direction of the joint extension.

[0044] After the single-segment hoisting and alignment of the prefabricated casing 300, construction workers lower themselves between the prefabricated casing 300 and the existing pier 100 (the planar clearance of the annular gap at this location is preferably reserved to be about 0.8 meters), and operate and tighten the internal fastening nuts 412. For example, the high-strength bolts used with the fastening nuts 412 can be M30 specification. After the tightening operation is completed, it is advisable to follow the procedure of sealing first and then filling, and perform the node leak-proof sealing operation. In specific operation, concrete with a grade higher than that of the U-shaped precast component 310 body (e.g., higher grade micro-expansion concrete) is used as the sealing layer 620, which fills and seals the outer operating slots 420 and inner operating slots 421 opened on the outer wall 314 and inner wall 313 of the component, wrapping the mechanical fasteners such as the fastening nuts 412. Utilizing the micro-expansion characteristics of this concrete, the surface of the sealed slots is flush and dense with the inner and outer walls of the U-shaped precast component 310.

[0045] After the sealing layer 620 is poured, backfilling should not be carried out immediately. During construction, it is necessary to wait for its sealing strength to reach the design requirements to ensure that the sealing material is fully cured and forms a rigid anti-corrosion body. After the strength meets the requirements, the filling of the internal buffer sand 700 and the layered compaction of the external backfill soil 710 can be started, and the two should be carried out synchronously and alternately. The core purpose of adopting this sequence of sealing first and then filling sand is to avoid filling sand directly when the node is not sealed, effectively cutting off the leakage path of the internal buffer sand 700, and also preventing the sealing layer 620, which is not fully cured, from cracking and failing due to premature lateral pressure. Based on this leakage prevention and sand filling mechanism, the overall construction process adopts a reciprocating cycle of progressively raising and assembling the prefabricated casing 300, sealing and tightening the node, and synchronously and alternately filling the internal and external fillers until the whole is filled to the design elevation.

[0046] To establish a dynamic equilibrium mechanism for the stress system, this implementation method controls the height difference between the inner and outer fill materials and the reserved working surface during the synchronous backfilling process. Specifically, as follows: Figure 3 As shown, the filling of the internal buffer sand 700 inside the prefabricated casing 300 and the layered backfilling of the external backfill soil 710 outside the casing can be carried out simultaneously at the same vertical elevation. The direct technical purpose of this synchronous filling method, where the inner and outer heights are basically the same, is to utilize the injection of the internal buffer sand 700 to provide an elastic buffering effect, pushing the U-shaped prefabricated component 310 from the inside out. This effectively resists the lateral pressure exerted inward by the external backfill soil 710, preventing the closed-loop structure of the prefabricated casing 300 from becoming unstable or damaged due to inward compression by unilateral soil pressure.

[0047] Regarding elevation control, when carrying out each layer of backfill and sand filling operations, the top surface height of the backfill should avoid being flush with the top surface of the current casing segment. Preferably, a safety space that will not be filled is reserved between the top surface of the backfill and the closed boss 501 at the top of the current segment. As a preferred parameter configuration, the height of this reserved safety space can be selected to be between 1.0 meter and 1.2 meters.

[0048] The core purpose of reserving a safety space is to consider both construction feasibility and structural mechanical protection. On the one hand, this reserved height directly provides a stable working platform for construction personnel to stand on the top surface of the filling material to align and assemble the next segment and tighten the high-strength bolts 410.

[0049] On the other hand, the reserved unfilled pipe wall area protects the top free end of the current casing segment from direct lateral earth pressure. In the intermediate state where the joint is not yet vertically compressed by the weight of the upper component, this reserved space acts as a buffer against cantilever bending moment, physically preventing the joint from being pushed inward and ensuring structural stability during the heightening assembly process.

[0050] To ensure that the prefabricated casing 300 is always under uniform axial compression and to avoid eccentric load failure, the backfilling of the outer backfill soil 710 should be carried out in layers and symmetrically. Specifically, during the outer backfilling process, the height difference of the backfill soil in each direction around the prefabricated casing 300 is effectively controlled. As a preferred parameter configuration, the height difference of the backfill soil in each direction is controlled to not exceed a preset height difference threshold, which can be selected from 1.0 meter to 1.5 meters. The technical purpose of controlling the height difference to achieve symmetrical advancement is to prevent the outer backfill soil 710 from generating unbalanced lateral thrust around the annular structure. This effectively avoids bending and shear failure of the prefabricated casing 300 itself due to eccentric pressure, and also blocks the downward transmission of asymmetrical bending moment, preventing the independent pile foundation 200 at the bottom from eccentric bending failure due to eccentric load.

[0051] Furthermore, during the backfilling operation of the external backfill soil 710, a control boundary for mechanical operations is demarcated around the perimeter of the prefabricated casing 300. For example, this control boundary can be set as an area within 3 meters of the perimeter of the prefabricated casing 300. To protect the newly assembled joint nodes, the operation of large rollers should be avoided within this control boundary area. The main technical purpose of implementing this equipment restriction is to prevent the additional lateral pressure caused by the weight of large machinery, while also preventing mechanical shock waves from causing cracking or failure of the not-yet-fully-cured sealing layer 620 at the joint.

[0052] To ensure the compaction of the backfill within the control boundary, it is advisable to use small machinery or manual operation as alternatives within this range to safely complete the layered compaction and densification of the external backfill 710.

[0053] like Figure 6 As shown, to complete the final closure of the load-bearing system and establish a waterproof barrier on the surface, the overall construction requires repeating the preceding component assembly, node sealing, and simultaneous internal and external filling procedures step by step until the overall filling elevation meets the overfill control conditions. Specifically, the cyclic filling operation needs to continue until the overall filling height reaches a certain height above the design elevation. As a preferred parameter example, this overfill height can be set at approximately 1 meter. The technical purpose of performing this overfill action is to reserve settlement margin for the deep fill soil during later natural consolidation, ensuring that after settlement stabilization, the top surface of the fill can still be at or slightly above the final surface design elevation, preventing the formation of low-lying water accumulation areas on the surface.

[0054] Once the overall filling height reaches the aforementioned overfill control conditions, a top sealing cover 320 is installed on the top surface of the highest segment of the prefabricated casing 300. As a specific component selection, the top sealing cover 320 can be a precast concrete ring slab or a closed steel plate, which is laid flat on the top surface of the highest segment of the casing to cover the annular opening.

[0055] After the top sealing cover 320 is installed, cement mortar is preferably used to grout the joints between it and the outer wall of the existing pier 100 and the inner wall of the prefabricated casing 300, thereby constructing a rigid waterproof barrier. The installation of the top sealing cover 320, supplemented by cement mortar grouting, aims to effectively seal the internal cavity between the inner wall of the casing and the existing pier 100, thus effectively preventing the infiltration of surface capillary water and runoff into the internal gaps and avoiding adverse consequences such as fine sand loss or increased water pressure caused by water intrusion.

[0056] It should be noted that the specific embodiments described above are exemplary. Those skilled in the art can devise various solutions inspired by the disclosure of this invention, and these solutions all fall within the scope of this invention and its protection. Those skilled in the art should understand that this specification and its accompanying drawings are illustrative and not intended to limit the scope of the claims. The scope of protection of this invention is defined by the claims and their equivalents. This specification contains multiple inventive concepts; terms such as "preferredly," "according to a preferred embodiment," or "optionally" indicate that the corresponding paragraph discloses an independent concept. The applicant reserves the right to file divisional applications based on each inventive concept.

Claims

1. A construction method for backfilling within the protection zone of existing bridge piers, characterized in that, Includes the following steps: S1. Construct an independent foundation around the existing pier (100), the independent foundation including an annular pier (210) surrounding the existing pier (100) and isolated from the existing load-bearing system. S2. Above the annular pier (210), prefabricated components are assembled step by step to form an assembled protective casing (300) covering the existing pier (100). A gap is maintained between the assembled protective casing (300) and the outer wall of the existing pier (100). S3. After the single-layer segment of the prefabricated casing (300) is assembled, aligned and mechanically fastened, a node leak-proof sealing operation is performed to seal the assembly gaps and fastening operation surfaces of the prefabricated components. S4. After the curing strength of the sealed area reaches the preset requirement, the gap between the prefabricated casing (300) and the existing pier (100) is filled with internal buffer sand (700), and the external backfill soil (710) is backfilled in layers on the outside of the prefabricated casing (300); wherein, the filling of the internal buffer sand (700) and the layered backfilling of the external backfill soil (710) are basically consistent in vertical elevation and are carried out synchronously and alternately. Repeat steps S2 to S4, using the sequence of sealing first and then filling synchronously after the strength meets the standard, until the entire structure is filled to the design elevation.

2. The construction method for backfilling within the protection zone of existing bridge piers according to claim 1, characterized in that, The independent foundation also includes independent pile foundations (200); step S1 includes: The independent pile foundation (200) is constructed using a skip-pile excavation process. The bottom elevation of the independent pile foundation (200) is configured to be lower than the bottom elevation of the existing pile foundation (102) of the existing bridge pier (100), and the top elevation of the independent pile foundation (200) is configured to be higher than the top elevation of the existing pier cap (101) of the existing bridge pier (100). The annular pile cap (210) is poured on top of the independent pile foundation (200). With the support of the independent pile foundation (200), a vertical clearance is maintained between the bottom surface of the annular pile cap (210) and the top surface of the existing pile cap (101) in a physically isolated manner.

3. The construction method for backfilling within the protection zone of existing bridge piers according to claim 1 or 2, characterized in that, In step S4, when the inner buffer sand (700) is filled and the outer backfill soil (710) is backfilled in layers, the top surface height of the filling is controlled to be lower than the top surface of the currently spliced ​​single-layer segment. A safe space is reserved between the top surface height and the top surface of the single-layer segment, which is not to be filled, as a buffer section to prevent cantilever moment cracking and a construction platform for subsequent assembly.

4. The construction method for backfilling within the protection zone of an existing bridge pier according to any one of claims 1 to 3, characterized in that, In step S4, the layered backfilling operation of the external backfill soil (710) is carried out symmetrically around the periphery of the prefabricated casing (300), and the backfill height difference between each direction is controlled not to exceed the preset height difference threshold. Furthermore, a mechanical operation control boundary is defined around the periphery of the assembled casing (300), restricting the operation of large machinery within the control boundary to avoid damage to the not yet fully cured sealed area caused by the weight of the large machinery and shock waves.

5. The construction method for backfilling within the protection zone of an existing bridge pier according to any one of claims 1 to 4, characterized in that, After the entire fill is completed to the design elevation, the construction method further includes: Control the overall fill height and continue filling until it is higher than the design elevation to allow for settlement. A top sealing cover (320) is installed on the top surface of the highest segment of the assembled casing (300) to cover the opening of the gap, and the joint of the top sealing cover (320) is grouted to build a waterproof barrier.

6. A prefabricated protective structure for backfilling existing bridge piers, applied in the construction method described in any one of claims 1 to 5, characterized in that, include: An independent foundation includes an annular pier (210) set around an existing pier (100). The annular pier (210) has a hollow area (211) in the middle that avoids the existing pier (100). With the support of the bottom independent foundation, a vertical clearance that is physically isolated is maintained between the bottom surface of the annular pier (210) and the top surface of the existing pier (101) of the existing pier (100). The prefabricated casing (300) is vertically erected above the annular pier (210) and forms a rigid anchorage with the annular pier (210). The prefabricated casing (300) is assembled from multi-layer segmental prefabricated components to form a closed annular structure. A physical gap is maintained between the inner wall of the prefabricated component and the outer wall of the existing pier (100) to accommodate the internal buffer sand (700). The adjacent prefabricated components are assembled and connected on the side by a horizontal staggered joint (400), and the upper and lower layers of prefabricated components are staggered and interlocked by a vertical butt joint (500); an operating groove is provided on the component wall surface at the horizontal staggered joint (400) with an inward recess.

7. The prefabricated protective structure for backfilling existing bridge piers according to claim 6, characterized in that, The annular support (210) is provided with a closed annular support staggered boss (212) that protrudes upward at the position corresponding to the installation section of the prefabricated casing (300); the thickness of the support staggered boss (212) is greater than the conventional wall thickness of the prefabricated casing (300). The top surface of the misaligned boss (212) of the bearing platform matches the shape of the bottom of the first prefabricated casing (300) to form a vertical misaligned engagement, and a pre-embedded steel sleeve (213) with a vertical opening is provided in the misaligned boss (212) of the bearing platform for vertical grouting connection with the steel bars inside the first prefabricated casing (300).

8. The prefabricated protective structure for backfilling existing bridge piers according to claim 6 or 7, characterized in that, The precast component has a cross section with a straight line segment and a circular arc segment transitioning to a concentric circular arc segment. The transverse staggered joint (400) is arranged near a quarter span of the cross section of the precast component. Furthermore, a pre-embedded reinforcing steel plate is embedded in the turning area inside the stepped surface of the transverse staggered joint (400).

9. The prefabricated protective structure for backfilling existing bridge piers according to any one of claims 6 to 8, characterized in that, The operating slots include an outer operating slot (420) and an inner operating slot (421) respectively opened on the outer wall (314) and inner wall (313) of the precast component. The prefabricated component has a reserved hole (411) for a high-strength bolt (410) to pass through. The diameter of the reserved hole (411) is larger than the outer diameter of the high-strength bolt (410) to provide perforation tolerance. The fastening nut (412) fastened to the end of the high-strength bolt (410) is deeply recessed and wrapped in the operating slot.

10. The prefabricated protective structure for backfilling existing bridge piers according to any one of claims 6 to 9, characterized in that, The surfaces of the horizontal misaligned joint (400) and the vertical butt joint (500) are coated with a rigid penetrating crystalline waterproof layer (600), and a flexible sealant (610) is provided at the joint near the inner wall (313) of the component. The sealing layer (620) in the operating slot is made of micro-expansion concrete, which counteracts shrinkage deformation and makes the surface of the sealed slot flush and dense with the inner and outer walls of the precast component.

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