A method for pre-assembling and sinking a multi-layered coffering purlin cross brace of an ultra-deep water steel pipe pile cofferdam

Abstract

The application discloses a kind of ultra-deep water steel pipe pile cofferdam multilayered coffering purlin cross bracing pre-assembly sinking construction method, steps include: inserting and driving steel pipe pile closure and keeping internal and external water level balance;Coffering purlin is segmented pre-assembled on water platform, end is reserved step-shaped interface;Adjustable hook bracket is hung on the top of steel pipe pile to form annular temporary assembly platform, and coffering purlin cross bracing frame is assembled layer by layer;Each layer coffering purlin cross bracing frame is placed from bottom layer to upper layer layer by layer to design elevation and fixed by pile foundation steel casing as force column hanging system;After all supports are in place, underwater excavation and bottom sealing concrete pouring are carried out under water level balance condition;After bottom sealing reaches standard, water is pumped out in stages, and support system and bottom sealing are stressed cooperatively;After draining, pile cap and pier column are constructed without water.The application adopts reverse construction method of first assembling support and then pumping water, eliminates construction stage support overload, is suitable for water depth of more than 20m, deep silt layer condition, and construction is safe, quality is controllable.

Description

Technical Field

[0001] This invention relates to the field of bridge deep-water foundation construction technology, specifically to a method for the pre-assembly and sinking of multi-layer waler cross bracing in ultra-deep-water steel pipe pile cofferdams, which is particularly suitable for the construction of interlocking steel pipe pile cofferdams in geological conditions where the water depth is more than 20m and the riverbed is a thick layer of silt. Background Technology

[0002] In deep-water foundation construction of bridges, interlocking steel pipe pile cofferdams are a commonly used water-retaining structure, widely applied in the construction of underwater pier caps for bridges spanning rivers and seas. The basic principle is as follows: steel pipe piles with PC interlocking are driven along the perimeter of the pier cap to form a closed cofferdam structure. Waler and cross bracing systems are installed inside the cofferdam to resist external water and soil pressure. Then, the water inside the cofferdam is pumped out for pier and column construction.

[0003] Currently, the construction of deep-water steel pipe pile cofferdam walers and internal supports generally adopts a top-down, layer-by-layer dewatering and installation method. The specific procedure is as follows: after the steel pipe piles are joined, water is first pumped to approximately 0.5m below the first support, and the first waler and cross bracing are installed above the water; then, water is pumped to below the second support, and the second waler and cross bracing are installed; this process continues until all supports are installed. The specifications of the walers and cross bracing are usually designed based on the hydrostatic pressure borne by each layer under the final dewatering condition. Since water pressure increases linearly with depth, the bottom support experiences the greatest stress; therefore, a "weak at the top, strong at the bottom" configuration is adopted.

[0004] However, the aforementioned existing technologies still have the following shortcomings:

[0005] (1) The supports were severely overloaded during the construction phase, posing a safety hazard. In the traditional construction process of pumping water and installing layer by layer, each support, after being installed and before the next support is installed, needs to independently bear all the water and soil pressure of that phase. Taking five supports as an example, the first support, after installation, needs to bear all the water pressure from the water surface to the elevation of the second support alone. The load it receives during the construction phase far exceeds the value to be shared under the final design state. This results in the upper support bearing a "peak construction load" that it should not bear during the construction process, and the design specifications of the upper support are precisely the weakest, making it extremely prone to deformation or even instability.

[0006] (2) The design concept of "weak at the top and strong at the bottom" is seriously out of touch with the actual construction. The load calculation is only performed according to the final state in the design stage, without fully considering the dynamic stress path during the construction process, resulting in inconsistency between the design conditions and the construction conditions. This contradiction between "static design and dynamic construction" leads to structural defects in the support system configured according to the final water pressure during the construction process.

[0007] (3) Underwater installation is difficult and the quality is hard to guarantee. The lower walers and cross braces need to be installed underwater or in deep water. The welding quality is affected by factors such as water depth and current, and it is difficult to guarantee. In addition, diving operations have high safety risks.

[0008] (4) The construction difficulty is further increased under the condition of deep silt layer. Under the geological conditions of ultra-deep water level and riverbed with deep silt layer, the stability of cofferdam after steel pipe pile is poor, silt is easy to flow into the cofferdam, and the deformation of cofferdam is difficult to control during the process of pumping water layer by layer, which further aggravates the stress risk of the support system.

[0009] (5) The overall transportation and hoisting of large support frames are difficult. The existing technology, such as the method of overall assembly and synchronous lowering adopted by Chinese invention patent CN115748767B, requires the entire support frame to be assembled in the rear field and then transported to the site for hoisting. For large cofferdams (tens of meters in diameter), the transportation and hoisting of the overall frame is extremely difficult, the requirements for hoisting equipment are high, and the attitude control during the overall lowering process is difficult.

[0010] (6) Existing adjustable corbel platform technology has not yet solved the problem of flexibility in layer-by-layer assembly within the cofferdam. For example, although the temporary corbel of Chinese utility model patent CN210369003U can be clamped onto the steel casing, it is mainly used for the final support of the waler, rather than as an assembly platform, and does not have the function of being easy to dismantle or flip to make way for the lowering passage.

[0011] This invention is proposed to address the shortcomings of the prior art. Summary of the Invention

[0012] This invention addresses the shortcomings of the prior art by providing a method for the pre-assembly and sinking construction of multi-layered waler cross bracing for ultra-deep-water steel pipe pile cofferdams. The objective of this invention is to solve the following technical problems:

[0013] (1) Change the traditional process of “draining water layer by layer and installing water layer by layer” and adopt the construction method of “installing the support system first and then pumping water and loading” so that all the supports of each layer are in place before pumping water, and avoid the upper support being overloaded during the construction stage.

[0014] (2) The installation of walers and cross braces was changed from underwater operation to pre-assembly on the water and then lowered in layers, which improved the installation accuracy and construction quality and reduced safety risks.

[0015] (3) To solve the problem of difficult overall transportation and hoisting of large support frames, the weight of a single hoisting is reduced by prefabrication in sections and assembly within the cofferdam.

[0016] (4) In view of the geological conditions of deep silt layer, the steel pipe piles that have been driven are used as guides and the steel casing of the pile foundation is used as the load-bearing structure to ensure the accuracy and stability of the support frame being lowered.

[0017] (5) Provides a safe and efficient construction method for interlocking steel pipe pile cofferdams suitable for geological conditions of ultra-deep water level (above 20m) and thick silt layer (large thickness, fluid plastic).

[0018] The core concept of this invention is as follows: after the steel pipe piles of the cofferdam are closed, the water is not pumped out immediately. Instead, when the water levels inside and outside the cofferdam are balanced, the steel pipes of each waler and cross brace are pre-assembled into an integral frame on the water surface. Then, the pre-extended pile foundation steel casing is used as the load-bearing structure. The pre-assembled waler and cross brace frame is lowered layer by layer to the design depth and fixed through a hoisting system. The cofferdam is pumped out after all the supports are in place.

[0019] In the above technical concept, all walers and cross bracing steel pipes are in place before pumping, so that the support system can be stressed by each layer during pumping, which is consistent with the design conditions. At the same time, the specific process route of "segmented prefabrication → layer-by-layer assembly in the cofferdam → layer-by-layer lowering" solves the problem of difficult overall transportation and hoisting of large support frames. Furthermore, the temporary assembly platform composed of multiple adjustable hook brackets enables precise assembly on the water surface, avoiding the quality and safety problems of underwater assembly. The combination of the above-mentioned technical features is not a simple superposition: Maintaining water level balance inside and outside the cofferdam eliminates water pressure differences, prevents cofferdam deformation during construction, and provides a stable environment for subsequent support installation; segmented prefabrication solves the problem of transporting large frames, but introduces new challenges such as difficult assembly and positioning within the cofferdam; while the temporary assembly platform formed by adjustable hook brackets effectively solves the assembly station problem. During assembly, the weight of each segment is controllable, and the stepped interface provides high interface alignment efficiency and improved alignment accuracy. Furthermore, the temporary assembly platform can be used in conjunction with the layered lowering process to provide a lowering channel; layered lowering avoids excessive overall lowering weight, facilitates attitude control, solves the problem of excessive overall hoisting weight, and uses the existing steel pipe pile sidewalls as a guide to ensure high lowering accuracy, eliminating the need for additional guiding devices, simplifying construction, ensuring verticality, and providing good safety. These features synergize and complement each other, together forming a complete technical solution suitable for ultra-deep water silt layers.

[0020] To achieve the above objectives, the present invention adopts the following technical solution:

[0021] This invention provides a method for the pre-assembly and sinking construction of multi-layer waler cross bracing for ultra-deepwater steel pipe pile cofferdams, comprising the following steps:

[0022] S1. Drive interlocking steel pipe piles around the perimeter of the pier cap until closure, forming a closed cofferdam. The water level inside the cofferdam is the same as that outside, maintaining the balance of water pressure inside and outside.

[0023] S2. Above the water surface, pre-assemble each waler and cross brace steel pipe into a closed waler and cross brace frame.

[0024] S3. Using the pre-extended pile foundation steel casing as the load-bearing structure of the hanging system, the pre-assembled waler cross bracing frames are lowered layer by layer to their respective design elevations and fixed through the hanging system; the lowering sequence is from the bottom waler cross bracing frame upwards layer by layer.

[0025] S4. After all the waler cross bracing frames are lowered and fixed, the underwater foundation pit is excavated to the bottom sealing design elevation under the condition of water level balance inside and outside the cofferdam, and the bottom sealing concrete is poured underwater to form the bottom sealing water-proof layer.

[0026] S5. After the bottom sealing concrete reaches the design strength, the water in the cofferdam is pumped out in stages. The external water pressure is jointly borne by the waler cross bracing frame and the bottom sealing waterproof layer that have been put in place.

[0027] S6. After the water inside the cofferdam is drained, the foundation and piers are constructed under waterless conditions.

[0028] Furthermore, in step S2, the pre-assembly of each waler is specifically as follows: on the previous water-based steel structure platform, each waler is divided into several standard segments for segmental pre-assembly, and each standard segment has a stepped interface reserved at its end; then the pre-assembled standard segments are hoisted into the cofferdam and assembled layer by layer into a closed waler ring frame inside the cofferdam.

[0029] In step S2, the design purpose and technical effect of the stepped interface are as follows: Since the waler segments cannot be continuously welded along their entire length during pre-assembly on the steel structure platform above water, the connection needs to be completed inside the cofferdam. With the stepped interface, adjacent standard segments can achieve rapid alignment and engagement of the upper and lower flange plates and web plates during assembly inside the cofferdam, reducing the time and difficulty of alignment on water. At the same time, compared to a flat, straight joint structure, the stepped joint structure has a larger welding contact area and better force transmission performance, ensuring the integrity of the frame.

[0030] Furthermore, the standard segments of the upper walers are welded together using double-I-beams, while the standard segments of the lower walers are welded together using triple-I-beams.

[0031] Furthermore, the stepped interface is a Z-shaped stepped staggered joint structure; when adjacent standard segments are joined, the upper and lower flanges and the web are aligned before welding.

[0032] Preferably, the step height is 1 / 3 to 1 / 2 of the height of the steel section.

[0033] Furthermore, the specific method of assembling the cofferdam is as follows: adjustable hook brackets extending into the inner side of the cofferdam are respectively hung on the top of several steel pipe piles. Multiple adjustable hook brackets distributed around the inner wall of the cofferdam together form a ring-shaped temporary assembly platform; standard segments of the waler are assembled segment by segment on the temporary assembly platform, welded into a closed waler ring frame, and the cross bracing steel pipes are welded and fixed to the waler ring frame to form a single-layer complete waler cross bracing frame.

[0034] In step S2, the annular temporary assembly platform serves as the assembly station for the waler cross bracing frame. The elevation of the temporary assembly platform is above the water surface, which facilitates personnel operation and component hoisting.

[0035] Furthermore, the adjustable hook bracket consists of a steel plate guide rail, a height adjustment bolt, and a horizontal support bracket. The steel plate guide rail is arranged vertically, and its upper part is provided with a hook portion that snaps into the top of the steel pipe pile. The height adjustment bolt is a vertically arranged screw rod, which is rotatably connected to the steel plate guide rail. The horizontal support bracket is slidably connected to the steel plate guide rail vertically. Nuts are connected to the upper and lower ends of the screw rod. The upper end of the screw rod passes through the upper end ear plate of the steel plate guide rail, and a nut connected to the upper end of the screw rod is located above the upper end ear plate of the steel plate guide rail. The lower end of the screw rod passes through the horizontal support bracket, and another nut connected to the lower end of the screw rod is located below the horizontal support bracket. By rotating the nut located above the height adjustment bolt, the horizontal support bracket can be raised or lowered, thereby achieving precise adjustment of the elevation of the temporary assembly platform.

[0036] The purpose and technical effect of using hanging-type rather than welded brackets are as follows: welded brackets are difficult to remove after installation and will obstruct the lowering passage of the waler cross bracing frame; while the hanging-type adjustable hook brackets can be quickly removed or flipped to avoid obstruction after each layer of waler cross bracing frame is assembled and before lowering, so as to ensure unobstructed lowering passage. This achieves the effect of one bracket platform supporting the assembly of multiple layers of waler cross bracing frames layer by layer, which greatly reduces the amount of brackets used and the workload of installation and dismantling.

[0037] Furthermore, in step S3, the hoisting system includes an extended pile foundation steel casing, a lowering crossbeam installed on top of the pile foundation steel casing, a through-hole jack installed on the lowering crossbeam, and a vertically arranged precision-rolled threaded steel rod; the upper end of the precision-rolled threaded steel rod passes through the lowering crossbeam and connects to the through-hole of the through-hole jack, and the lower end of the precision-rolled threaded steel rod connects to a preset hoisting point on the waler cross bracing frame; through the step-by-step reciprocating motion of the through-hole jack, the waler cross bracing frame is smoothly lowered along the inner wall of the cofferdam to the design elevation.

[0038] In step S3, the purpose and technical effect of using the pile foundation steel casing as the load-bearing column instead of setting up lifting points on the cofferdam steel pipe piles are as follows: the pile foundation steel casing is a permanent structure with a large diameter, high rigidity, and strong bearing capacity, and its position is located inside the cofferdam, so it is not affected by the deformation of the cofferdam; at the same time, using the existing pile foundation steel casing as the load-bearing structure eliminates the need to set up additional lifting towers, saving on the cost of the measure.

[0039] Specifically, before lowering the waler cross bracing frame after each layer is assembled, remove the adjustable hook bracket at that location or flip the adjustable hook bracket to avoid it.

[0040] Furthermore, during the lowering process of the waler cross bracing frame, the inner wall of the cofferdam is used as a guide. After being lowered to the design elevation, the waler cross bracing frame is fixed to the side wall of the steel pipe pile by underwater welding brackets.

[0041] The purpose and technical effect of using the inner wall of the cofferdam as a guide are as follows: the steel pipe piles forming the cofferdam are evenly distributed around the perimeter of the pier cap, so that the inner wall of the cofferdam forms a natural guide surface, eliminating the need for additional guide devices; at the same time, the verticality of the steel pipe piles is controlled during the driving process, which can be used as a benchmark to ensure the horizontality and verticality of the waler cross bracing frame when it is lowered.

[0042] Furthermore, in step S4, the bottom sealing concrete is poured under the condition of water level balance inside and outside the cofferdam.

[0043] In step S4, the purpose and technical effect of pouring the bottom sealing concrete under the condition of internal and external water level balance is to avoid the risk of base heave and piping caused by the difference in pressure head at the bottom when water is pumped out first and then sealed; at the same time, the bottom sealing concrete is poured under static water conditions, which has good fluidity and is not easy to segregate, which is conducive to ensuring the quality of bottom sealing.

[0044] Furthermore, in step S5, when pumping water to the position of each waler cross brace frame, pumping is paused to check the fit between the waler cross brace frame and the side wall of the steel pipe pile. If there is a gap, concrete is poured or steel pads are installed between the waler cross brace frame and the steel pipe pile to ensure tight force transmission.

[0045] In step S5, a combination of staged pumping and layer-by-layer force transmission inspection is adopted to ensure that each layer of support is evenly stressed, eliminate gaps, and achieve the effect of structural synergy and high safety redundancy.

[0046] Furthermore, in step S6, after the foundation and pier construction is completed, the waler cross bracing frames are dismantled in a bottom-up order, and then the steel pipe piles are pulled out. Since the pier is poured section by section from bottom to top during construction, and water is released and refilled section by section after demolding to balance the water pressure, the lower waler cross bracing frames can be dismantled layer by layer from bottom to top.

[0047] This invention provides a method for the pre-assembly and sinking of multi-layered waler cross bracing in ultra-deep-water steel pipe pile cofferdams. Breaking away from the traditional "dewatering first, then installing supports" construction path dependency, it creatively adopts a reverse construction method of "installing supports first, then dewatering." Through a complete process route of "segmented prefabrication → assembly of adjustable hook bracket platforms → layered lowering of the suspension system," it achieves safe and efficient construction of steel pipe pile cofferdams in ultra-deep water and deep silt layers. Through the mutual cooperation and synergy of various technical means, the stress path of the support system is consistent with the design conditions, fundamentally solving the safety hazard of support overload during construction. Compared with existing technologies, this invention has the following beneficial effects:

[0048] (1) Eliminating overload of supports during construction and significantly improving construction safety. This invention changes the traditional process of pumping water and installing it layer by layer, so that all supports are in place before pumping, and each layer of supports is stressed together from the beginning of pumping, fundamentally avoiding the problem of the upper support bearing the "peak construction load" during the construction stage. The actual stress of the support system is consistent with the design conditions, and the structure is safe and reliable.

[0049] (2) Sealing the bottom before pumping out water significantly improves the stability of the base. Compared with the traditional method of pumping out water before sealing the bottom, this invention pours the sealing concrete under the condition of balanced internal and external water pressure, avoiding the risk of base heave and piping caused by the pressure difference at the bottom after pumping out water; the sealing concrete is pumped out after it reaches the strength, and the reliable sealing waterproof layer is used as the base plate to resist the buoyancy of water.

[0050] (3) The support and the bottom sealing work together to bear the force, making the structure more reasonable. After the water is pumped out, the external water pressure is borne by the waler cross bracing system and the bottom slab formed by the bottom sealing concrete. The force path is clear and the structure has high redundancy.

[0051] (4) The amount of underwater work is greatly reduced, and the safety risks are significantly reduced. Only divers are needed to check the quality of the base and bottom sealing. The main operations such as waler assembly and welding are all completed on the water surface, avoiding the welding quality problems of underwater assembly and the safety risks of diving operations.

[0052] (5) Solved the problem of transporting and hoisting large support frames. Through the two-stage assembly process of “segmented prefabrication + assembly within the cofferdam”, the large frame is decomposed into standard segments for transportation, which reduces the weight of a single hoisting and the equipment requirements, and improves the operability of construction.

[0053] (6) The adjustable hook bracket platform is flexible and efficient to operate. It is hung on the top of the steel pipe pile, and the elevation is adjustable. It can be quickly dismantled or flipped to avoid obstacles when lowering. One platform can support the assembly of multiple layers of waler cross bracing frames, and has the dual functions of assembly station and lowering channel. It has a simple structure and is easy to install and dismantle.

[0054] (7) Layered lowering improves installation accuracy. Compared with overall lowering, layered lowering has a smaller single weight and easier posture control. Furthermore, the sidewalls of the steel pipe piles are used as guides to ensure the levelness and concentricity of the waler cross bracing frames of each layer.

[0055] (8) Create completely dry working conditions without water. After pumping out the water, the cofferdam is completely drained, and the subsequent construction of the abutment and pier is carried out in a dry environment with no water and visible dryness. The quality of steel reinforcement binding, formwork installation and concrete pouring is completely controllable, avoiding the quality risks of underwater pouring or working with water.

[0056] (9) It is particularly suitable for ultra-deep water levels (above 20m) and thick silt layers. The deformation of the cofferdam is controlled by the balance of internal and external water pressure throughout the process, which effectively solves the technical problem of cofferdam instability caused by pumping water layer by layer in the silt layer. Attached Figure Description

[0057] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0058] Figure 1 This is a schematic diagram showing the distribution of the steel structure platform on water according to one embodiment of the present invention;

[0059] Figure 2 This is a schematic diagram showing the distribution of the cofferdam and adjustable hook brackets according to one embodiment of the present invention;

[0060] Figure 3 This is a schematic diagram of the adjustable hook bracket according to one embodiment of the present invention;

[0061] Figure 4 This is a schematic diagram of the waler cross brace frame according to one embodiment of the present invention;

[0062] Figure 5 This is a schematic diagram of the phased splicing of the first and second waler cross bracing frames according to one embodiment of the present invention;

[0063] Figure 6 A schematic diagram of the phased splicing of the third, fourth, and fifth waler cross bracing frames according to one embodiment of the present invention;

[0064] Figure 7 A side view of one embodiment of the invention when the waler cross brace frame is lowered;

[0065] Figure 8 This is a top view of one embodiment of the invention when the waler cross brace frame is lowered;

[0066] Figure 9 This is a schematic diagram of one embodiment of the invention after the five waler cross bracing frames have been lowered and fixed.

[0067] Reference numerals: 1. Waterborne steel structure platform; 2. Cofferdam; 21. Steel pipe pile; 3. Waler cross bracing frame; 31. Waler ring frame; 311. Long side waler; 312. Short side waler; 32. Cross bracing steel pipe; 33. Standard segment; 331. Stepped interface; 4. Adjustable hook bracket; 41. Steel plate guide rail; 42. Hook part; 43. Height adjustment bolt; 44. Horizontal support bracket; 45. Nut; 46. Upper end ear plate; 5. Suspension system; 51. Pile foundation steel casing; 52. Lowered crossbeam; 53. Through-hole jack; 54. Precision rolled threaded steel hanger; 6. Bottom sealing waterproof layer; 7. Foundation. Detailed Implementation

[0068] The technical solution of the present invention will be clearly and completely described below through detailed embodiments and in conjunction with the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0069] Example 1: Construction of underwater pier foundation for a cross-river bridge

[0070] This embodiment uses the construction of a pier foundation in water for a cross-river bridge to illustrate the construction method of pre-assembly and sinking of multi-layer waler cross bracing for ultra-deep water steel pipe pile cofferdams.

[0071] Project Overview

[0072] The main pier abutment of a cross-river bridge is located in the middle of the river, at a depth of approximately 22m. The riverbed geology, from top to bottom, consists of: ① a permeable sand layer (approximately 8m thick, medium-dense, with a high mud content); ② a completely weathered granite layer (approximately 5m thick, relatively impermeable); and ③ a weakly weathered granite layer (impermeable). The abutment is designed as a rectangle, with plan dimensions of 32m × 18m, and the bottom elevation of the abutment is 5m below the riverbed. Cofferdam 2 adopts a lock-lock steel pipe pile cofferdam structure, with plan dimensions of 36m × 22m, and is internally supported by a five-waler cross bracing frame 3.

[0073] Main materials description

[0074] Steel pipe pile 21: Spiral welded steel pipe with a diameter of 820mm and a thickness of 14mm, PC Larsen lock, material Q345B;

[0075] Waler cross bracing frame 3: The waler ring frame 31 of the first and second waler cross bracing frames 3 is double-jointed 2×I45c I-beams; the waler ring frame 31 of the third to fifth waler cross bracing frames 3 is triple-jointed 3×HW700×300×13×24 steel; the cross bracing steel pipe 32 of the first to third waler cross bracing frames 3 is Φ609×12mm steel pipe; the cross bracing steel pipe 32 of the fourth and fifth waler cross bracing frames 3 is Φ800×16mm steel pipe;

[0076] Fine-rolled threaded steel hanger 54: Fine-rolled threaded steel with a diameter of 32mm and a standard tensile strength of 930MPa;

[0077] Bottom sealing layer 6: C25 underwater concrete is used as the bottom sealing concrete, with a designed thickness of 3.5m;

[0078] Post-pouring filling material: C30 micro-expansion concrete.

[0079] Specific construction process

[0080] I. Construction Preparation and Steel Pipe Pile Driving

[0081] (1) Construct a construction trestle bridge and a steel structure platform 1 on the water at the bridge site. The top elevation of the platform is 2m above the construction water level (i.e., +5.0m elevation, and the construction water level is +3.0m). The steel structure platform 1 on the water adopts a steel pipe pile foundation + steel beam structure, and the platform area meets the operation requirements of steel pipe pile driving equipment and hoisting equipment.

[0082] (2) Based on the design drawings and reference Figure 1 The positions of the cofferdam steel pipe piles were accurately laid out on the trestle bridge and the steel structure platform 1 over the water, with the planar positioning error controlled within ±20mm. For example... Figure 2 and Figure 8 As shown, locking steel pipe piles 21 are arranged around the perimeter of the foundation 7.

[0083] (3) Positioning using a guide frame. The guide frame is a double-layer structure, with both the upper and lower guide frames being rectangular frames welded from steel profiles, fixed to the steel structure platform 1 on the water via positioning piles. Steel pipe piles 21 are inserted along the guide frame, and a crawler crane (rated lifting capacity 80t) is used in conjunction with a hydraulic vibratory hammer (excitation force 2000kN) to vibrate and lock the steel pipe piles 21 one by one. During the vibratory insertion of the steel pipe piles 21, the verticality deviation is controlled to be no more than 1%, and adjacent steel pipe piles 21 are interlocked using PC Larsen locking mechanisms.

[0084] (4) The steel pipe pile 21 needs to be driven through the upper permeable sand layer (about 8m thick) and into the lower completely weathered granite layer by 1m to 3m. The actual driving depth is 2.5m into the completely weathered rock layer to ensure the formation of an effective closed water-proof structure. After each steel pipe pile 21 is driven, the engagement quality of the locking is tested by ultrasonic method.

[0085] (5) After all the steel pipe piles 21 are driven and closed, the water level inside the cofferdam 2 is kept consistent with the outside (+3.0m elevation) to maintain complete balance of water pressure inside and outside. Water level observation gauges are set up inside and outside the cofferdam 2, and the water level is observed once in the morning and once in the evening every day to ensure that the difference between the water level inside and outside does not exceed 5cm.

[0086] II. Segmented prefabrication and assembly within the cofferdam of the waler cross bracing frame

[0087] (I) Main platform segmented pre-assembly

[0088] (1) The water-based steel structure platform 1, which was erected during the early bridge pile foundation construction, was used as the main prefabrication site.

[0089] (2) Divide each waler into several standard segments 33 according to the design drawings. (Reference) Figures 4-6 Based on the perimeter of cofferdam 2, each waler is divided into 8 standard segments 33 (3 segments on each long side and 1 segment on each short side), with the weight of a single segment controlled within 5t, to facilitate transportation and hoisting.

[0090] Each standard segment 33 has a pre-reserved Z-shaped stepped interface 331 at its end. Taking HW700 steel as an example, the step height is 350mm (half the height of the steel section), the step width is 200mm, and the upper and lower flanges and web plates are staggered at the step, forming an interlocking joint when adjacent segments are joined. The step surface is machined to a surface roughness Ra≤25μm to ensure welding quality.

[0091] The standard segments 33 of the first and second waler ring frames 31 are assembled and welded from double-jointed 2×I45c I-beams. At the stepped interfaces 331 at both ends of the segments, the steps of the two I-beams are aligned.

[0092] The standard segments 33 of the third to fifth waler ring frames 31 are welded together using three 3×HW700 steel sections. At the stepped interfaces 331 at both ends of the segments, the steps of the three steel sections are in the same position.

[0093] The cross bracing steel pipes 32 (Φ609×12mm and Φ800×16mm) are precisely cut to the designed length, and the ends of the cross bracing steel pipes 32 are welded with connecting node plates.

[0094] (3) After all standard segments 33 are pre-assembled, they are uniformly numbered (numbering rule: D1-01 represents the first segment of the first purlin, and so on), and stored on the water platform for later use.

[0095] (ii) Adjustable hook brackets are hung on the top of the steel pipe piles of the cofferdam as temporary assembly platforms.

[0096] refer to Figure 2 After the steel pipe piles 21 forming the cofferdam 2 are joined together, adjustable hook brackets 4 are hung on the top of the steel pipe piles 21 (above the normal water level). Figure 3 As shown, the adjustable hook bracket 4 consists of a steel plate guide rail 41, a height adjustment bolt 43, and a horizontal support bracket 44. The steel plate guide rail 41 is arranged vertically, and its upper part is provided with a hook part 42 that is snapped onto the top of the steel pipe pile 21. The height adjustment bolt 43 is a vertically arranged screw rod that is rotatably connected to the steel plate guide rail 41. The horizontal support bracket 44 is slidably connected to the steel plate guide rail 41 vertically, and its side wall is in sliding contact with the steel plate guide rail 41. Nuts 45 are connected to the upper and lower ends of the screw rod, respectively. The upper end of the screw rod passes through the upper end ear plate 46 of the steel plate guide rail 41, and one nut 45 connected to the upper end of the screw rod is located above the upper end ear plate 46 of the steel plate guide rail 41. The lower end of the screw rod passes through the horizontal support bracket 44, and another nut 45 connected to the lower end of the screw rod is located below the horizontal support bracket 44. The top surface of the horizontal support bracket 44 is the working support surface. Multiple adjustable hook brackets 4, distributed around the inner wall of the cofferdam 2, together form a ring-shaped temporary assembly platform, serving as the assembly station for the waler cross bracing frame 3. This temporary assembly platform is positioned above the water surface, facilitating personnel operation and component hoisting. By rotating a nut 45 located above the height adjustment bolt 43, the horizontal support bracket 44 can be raised or lowered, thereby achieving precise adjustment of the temporary assembly platform's elevation.

[0097] (iii) Assemble the waler cross bracing frame layer by layer on the temporary assembly platform.

[0098] refer to Figure 8 Using a truck crane (rated lifting capacity 25t), pre-numbered standard segments 33 of the waler are lifted from the steel structure platform 1 on the water into the cofferdam 2. These segments are then assembled, aligned, and welded together to form a closed waler ring frame 31. Next, the corresponding layer of cross bracing steel pipes 32 are hoisted into place, and the end node plates are initially connected to the node plates on the waler ring frame 31 using high-strength bolts. This connection is then reinforced by welding, forming a complete single-layer waler cross bracing frame 3. Figure 4 As shown.

[0099] refer to Figure 7 and Figure 9 The assembly sequence is as follows: First, assemble the fifth waler horizontal brace frame 3 at the bottom layer. After completion, pre-set lifting points at the four corners of this layer's waler horizontal brace frame 3. After the fifth waler horizontal brace frame 3 is installed, continue assembling the fourth waler horizontal brace frame 3 above in the same manner, and pre-set lifting points at the four corners of the fourth waler horizontal brace frame 3. Similarly, after the fourth waler horizontal brace frame 3 is lowered and fixed, complete the assembly of the other layers' waler horizontal brace frames 3 in sequence. After all waler horizontal brace frames 3 are assembled, lowered, and fixed, as shown... Figure 9 As shown.

[0100] 3. The layering of the waler cross bracing frame is extended to the design depth.

[0101] (I) Decentralization System Construction

[0102] refer to Figure 7 and Figure 8 Four pre-extended steel casings 51 (2.0m in diameter, 20mm in wall thickness) within the pile cap 7 area are used as the load-bearing columns of the suspension system 5. After the pile foundation construction is completed, the steel casings 51 are kept at a certain height above the water surface (top elevation +6.0m). The top surface of the steel casings 51 is raised to the +6.5m elevation by extending the pipe sections (with the same diameter as the steel casings 51 and welded together).

[0103] A lowered crossbeam 52 is installed on top of the steel casing 51 of the pile foundation. The lowered crossbeam 52 is welded from double I56b I-beams, with a length of 3.5m and a height of 560mm. The lowered crossbeam 52 is fixed to the top of the steel casing 51 of the pile foundation by clamps and anchor bolts.

[0104] A walking-type through-hole jack 53 (rated lifting capacity 100t, stroke 200mm) is installed on the lowered crossbeam 52. The through-hole diameter of the through-hole jack 53 is 40mm, which is matched with the precision rolled threaded steel rod 54 (diameter 32mm).

[0105] The upper end of the threaded steel rod 54 passes through the through hole of the lowering beam 52 and the through-hole jack 53, and is clamped by the clamp of the through-hole jack 53; the lower end of the threaded steel rod 54 is connected to the preset lifting point on the waler cross brace frame 3 through a connector (using a JLM-32 type connector). This assembles a complete hanging system 5. Through the step-by-step reciprocating motion of the through-hole jack 53, the waler cross brace frame 3 can be smoothly lowered along the inner wall of the cofferdam 2 to the design elevation.

[0106] (ii) Lower and fix the bottommost (fifth) waler cross bracing frame.

[0107] refer to Figure 8 After the fifth set of three waler cross bracing frames is assembled and passes inspection, the lower end of the installed threaded steel rod 54 is connected to the preset lifting point on the waler cross bracing frame 3, and the upper end of the threaded steel rod 54 passes through the through hole of the through-hole jack 53 and is clamped by the clamp. The four through-hole jacks 53 are activated (working synchronously at all four corners) to slightly lift the waler cross bracing frame 3 (lifting by approximately 20mm), detaching it from the temporary assembly platform. The adjustable hook brackets 4 at this location are removed (the hook parts 42 of the adjustable hook brackets 4 are removed from the top of the steel pipe pile 21) to clear the lowering path. The waler cross bracing frame 3 is smoothly lowered along the inner wall of the cofferdam 2 using the step-by-step reciprocating motion of the through-hole jacks 53 (each stroke 200mm, cyclical motion).

[0108] During the lowering process, the inner wall of cofferdam 2 was used as a guide. The lowering was paused every 2 meters, and divers checked the lowering posture of the waler cross bracing frame 3 underwater to ensure it was level and not tilted. The design elevation of the fifth waler cross bracing frame 3 was 0.5m above the bottom of the pier (i.e., 4.5m below the riverbed, elevation -4.5m). Figure 7 As shown, after being lowered to this elevation, with the assistance of divers, the waler cross bracing frame 3 and the side wall of the steel pipe pile 21 are fixed by underwater welded brackets. The underwater brackets are triangular stiffened plate structures.

[0109] (iii) Lower the fourth purlin cross bracing frame to the first purlin layer by layer.

[0110] After the fifth waler cross brace frame 3 is lowered and fixed, the threaded steel rod 54 is raised and connected to the preset lifting point of the fourth waler cross brace frame 3. The through-hole jack 53 is started to lift slightly (about 15mm), and each adjustable hook bracket 4 is removed. The fourth waler cross brace frame 3 is lowered to the design elevation (elevation -1.5m) and fixed.

[0111] like Figure 9 As shown, repeat the above operation, successively lowering the third waler cross bracing frame 3 (elevation +0.5m), the second waler cross bracing frame 3 (elevation +2.0m), and the first waler cross bracing frame 3 (elevation +3.2m). At this point, all five waler cross bracing frames 3 are in place at the underwater design depth, the water levels inside and outside the cofferdam 2 remain balanced (+3.0m), and the support system is not under stress.

[0112] IV. Excavation of underwater foundation pit within the cofferdam

[0113] (1) After all the waler cross bracing frames 3 are lowered and fixed, the water level inside the cofferdam 2 is kept consistent with the external waterway to maintain the balance of internal and external water pressure.

[0114] (2) Using a long-arm excavator in conjunction with a sand pump, underwater excavation was carried out from the working platform at the top of the cofferdam 2. The excavation was carried out in layers, with each layer excavated to a depth of 1.0m, until the silt and sand layers in the cofferdam 2 were removed to a position 5m below the riverbed (i.e., elevation -5.0m, the design elevation of the bottom of the sealing concrete).

[0115] (3) During the excavation process, divers will check the base elevation and flatness underwater. The test will be conducted once after each layer is excavated to ensure that the base elevation error does not exceed ±100mm, the flatness does not exceed ±50mm, and there are no debris or floating mud.

[0116] V. Underwater Bottom Sealing Concrete Pouring

[0117] (1) Under the condition that the internal and external water levels are completely balanced, the working platform erected on the top of the steel casing 51 of the pile foundation is used to arrange the multi-point pouring pipes. A total of 8 pipes (diameter Φ300mm) are arranged evenly on the plane of the cofferdam 2, with the spacing between the pipes not exceeding 4m and the bottom of the pipes 20-30cm from the base surface.

[0118] (2) The underwater concrete pouring process is adopted, and the C25 bottom sealing concrete is poured to the design top surface elevation (i.e., the bottom elevation of the foundation) using the tremie method. During the pouring process, divers check the flow radius and flatness of the concrete underwater to ensure that the bottom sealing concrete is of uniform thickness, continuous and free of mud.

[0119] (3) After the bottom sealing concrete is poured, it shall be cured for 7 days. During this period, the water level inside and outside the cofferdam 2 shall be kept in balance. The design thickness of the bottom sealing concrete shall meet the requirements for anti-buoyancy and water-stopping, forming the bottom sealing and water-proof layer 6 at the bottom of the cofferdam 2.

[0120] VI. Cofferdam Dewatering and Load Transfer

[0121] (1) After the bottom sealing concrete reaches the design strength, the water in the cofferdam 2 is pumped out in stages.

[0122] (2) During the pumping process, the external water pressure is gradually transmitted to the support system (the waler cross bracing frame 3 of each layer) and the bottom sealing waterproof layer 6. Since all the supports were in place before the pumping, and the bottom sealing concrete has formed the bottom sealing waterproof layer 6 as the bottom consolidation support, the waler support system and the bottom sealing concrete work together to resist the external water pressure and the buoyancy of the groundwater.

[0123] (3) When pumping water to the position of each waler, stop pumping and check the fit between the waler and the steel pipe pile 21. If there is a gap, pour C30 micro-expansion concrete (using duct injection method, injection pressure 0.3~0.5MPa) or set steel pads (steel plates with a thickness of 2~20mm, selected according to the size of the gap) between the waler cross bracing frame 3 and the steel pipe pile 21 to ensure tight and reliable force transmission.

[0124] (4) Continue pumping water until the water in cofferdam 2 is completely drained. Then carry out the plugging operation of cofferdam 2: divers insert geotextile strips or inject quick-setting plugging material (such as polyurethane grouting material) along the PC plug leakage point on the outside of cofferdam 2 to seal the plug leakage point and ensure that the cofferdam 2 is in a waterless state.

[0125] VII. Construction of Pier Caps and Columns under Waterless Conditions

[0126] (1) After the cofferdam 2 is completely drained and plugged, clean the surface of the bottom sealing concrete, remove the over-poured part, and form a clean and dry construction work surface.

[0127] (2) Under completely dry conditions, the reinforcement binding, formwork installation and concrete pouring of the foundation 7 were carried out.

[0128] (3) After the concrete curing of the pier cap 7 is completed, the construction of the pier reinforcement, formwork and concrete pouring will continue until the pier is constructed to the design elevation (above the first waler cross bracing frame of the cofferdam top).

[0129] 8. Cofferdam Removal

[0130] After the pier construction is completed and the pier concrete reaches the design strength, the waler cross bracing frames 3 are dismantled in a bottom-up order. Then, the steel pipe piles 21 are pulled out one by one using a vibratory hammer. After all the steel pipe piles 21 are pulled out, the cofferdam 2 is dismantled.

[0131] The above embodiments are merely illustrative of the concept and technical solution of the present invention and are not intended to limit the present invention. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in the present invention should still be covered by the claims of the present invention.

[0132] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for pre-assembling and sinking multi-layer waler cross bracing of an ultra-deep-water steel pipe pile cofferdam, characterized in that... Includes the following steps: S1. Drive interlocking steel pipe piles around the perimeter of the pier cap until closure, forming a closed cofferdam. The water level inside the cofferdam is the same as that outside, maintaining the balance of water pressure inside and outside. S2. Above the water surface, pre-assemble each waler and cross brace steel pipe into a closed waler and cross brace frame; S3. Using the pre-extended pile foundation steel casing as the load-bearing structure of the hanging system, the pre-assembled waler cross bracing frames are lowered layer by layer to their respective design elevations and fixed through the hanging system; the lowering sequence is from the bottom waler cross bracing frame upwards layer by layer. S4. After all the waler cross bracing frames are lowered and fixed, the underwater foundation pit is excavated to the bottom sealing design elevation under the condition of water level balance inside and outside the cofferdam, and the bottom sealing concrete is poured underwater to form the bottom sealing water-proof layer. S5. After the bottom sealing concrete reaches the design strength, the water in the cofferdam is pumped out in stages. The external water pressure is jointly borne by the waler cross bracing frame and the bottom sealing waterproof layer that have been put in place. S6. After the water inside the cofferdam is drained, the foundation and piers are constructed under waterless conditions.

2. The construction method for pre-assembly and sinking of multi-layer waler cross bracing in ultra-deepwater steel pipe pile cofferdams according to claim 1, characterized in that, In step S2, the pre-assembly of each waler is specifically as follows: on the previous water-based steel structure platform, each waler is divided into several standard segments for segmental pre-assembly, and each standard segment has a stepped interface reserved at its end; then the pre-assembled standard segments are hoisted into the cofferdam and assembled layer by layer into a closed waler ring frame inside the cofferdam.

3. The construction method for pre-assembly and sinking of multi-layer waler cross bracing in ultra-deepwater steel pipe pile cofferdams according to claim 2, characterized in that, The standard segments of the upper walers are welded together using double-I-beams, while the standard segments of the lower walers are welded together using triple-I-beams.

4. The construction method for pre-assembly and sinking of multi-layer waler cross bracing in ultra-deepwater steel pipe pile cofferdams according to claim 3, characterized in that, The stepped interface is a Z-shaped stepped staggered joint structure; when adjacent standard segments are joined, the upper and lower flanges and the web are aligned before welding.

5. The construction method for pre-assembly and sinking of multi-layer waler cross bracing in ultra-deepwater steel pipe pile cofferdams according to claim 2, characterized in that, The specific assembly method within the cofferdam is as follows: adjustable hook brackets extending into the inner side of the cofferdam are hung on the top of several steel pipe piles. Multiple adjustable hook brackets distributed around the inner wall of the cofferdam together form a ring-shaped temporary assembly platform. On the temporary assembly platform, standard segments of the waler are assembled section by section, welded into a closed waler ring frame, and the cross bracing steel pipes are welded and fixed to the waler ring frame to form a single-layer complete waler cross bracing frame.

6. The construction method for pre-assembly and sinking of multi-layer waler cross bracing in ultra-deepwater steel pipe pile cofferdams according to claim 5, characterized in that, The adjustable hook bracket consists of a steel plate guide rail, a height adjustment bolt, and a horizontal support bracket. The steel plate guide rail is arranged vertically, and the upper part of the steel plate guide rail is provided with a hook part that is snapped to the top of the steel pipe pile. The height adjustment bolt is a vertically arranged screw rod that is rotatably connected to the steel plate guide rail. The horizontal support bracket is slidably connected to the steel plate guide rail vertically. Nuts are connected to the upper and lower ends of the screw rod. The upper end of the screw rod passes through the upper end ear plate of the steel plate guide rail, and a nut connected to the upper end of the screw rod is located above the upper end ear plate of the steel plate guide rail. The lower end of the screw rod passes through the horizontal support bracket, and another nut connected to the lower end of the screw rod is located below the horizontal support bracket.

7. The method for pre-assembling and sinking multi-layer waler cross bracing of ultra-deepwater steel pipe pile cofferdam according to claim 1, characterized in that, In step S3, the hoisting system includes an extended pile foundation steel casing, a lowering crossbeam installed on top of the pile foundation steel casing, a through-hole jack installed on the lowering crossbeam, and a vertically arranged precision-rolled threaded steel rod; the upper end of the precision-rolled threaded steel rod passes through the lowering crossbeam and connects to the through-hole of the through-hole jack, and the lower end of the precision-rolled threaded steel rod connects to a preset hoisting point on the waler cross bracing frame; through the step-by-step reciprocating motion of the through-hole jack, the waler cross bracing frame is smoothly lowered along the inner wall of the cofferdam to the design elevation.

8. The method for pre-assembling and sinking multi-layer waler cross bracing of ultra-deepwater steel pipe pile cofferdam according to claim 7, characterized in that, During the lowering process of the waler cross bracing frame, the inner wall of the cofferdam is used as a guide. After being lowered to the design elevation, the waler cross bracing frame is fixed to the side wall of the steel pipe pile by underwater welding brackets.

9. The construction method for pre-assembly and sinking of multi-layer waler cross bracing in ultra-deepwater steel pipe pile cofferdams according to claim 1, characterized in that, In step S4, the bottom sealing concrete is poured under the condition of water level balance inside and outside the cofferdam; in step S5, pumping is stopped when the water reaches the position of each waler cross brace frame in stages, and the fit between the waler cross brace frame and the side wall of the steel pipe pile is checked. If there is a gap, concrete is poured or steel pads are set between the waler cross brace frame and the steel pipe pile to ensure tight force transmission.

10. The method for pre-assembling and sinking multi-layer waler cross bracing of ultra-deepwater steel pipe pile cofferdam according to claim 1, characterized in that, In step S6, after the foundation and pier construction is completed, the waler cross bracing frames are removed in a bottom-up order, and then the steel pipe piles are pulled out.

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