N+1 steel sheet pile and steel pipe pile combined cofferdam supporting structure and construction method thereof

Abstract

The invention provides an N + 1 steel sheet pile and steel pipe pile combined cofferdam supporting structure and a construction method thereof. The N + 1 steel sheet pile and steel pipe pile combined cofferdam supporting structure comprises a cofferdam area, peripheral pipe piles, a supporting plate set, a connecting mechanism, an inner supporting layer, a filling layer and a reinforcing mechanism. The pipe pile is made of a seamless steel pipe with an optimized alloy ratio, and the height H of the pipe pile and the height h of the supporting plate set meet the condition that H / h is larger than or equal to 1.25 and smaller than or equal to 1.5. The supporting and covering effects are guaranteed through height proportion optimization, tight rigid connection of all components is achieved through the connecting mechanism, loads can be effectively dispersed, deformation can be resisted, and instability is avoided; the seamless steel pipe has high strength, high toughness and high durability, meets the bearing requirement of the deep foundation pit, and meanwhile forms a reliable water stop barrier to guarantee construction safety. The structure layout is reasonable, construction is controllable, quality is easy to guarantee, stability, safety and economical efficiency are considered, the operation and maintenance cost is reduced, the structure is reliable, adaptability is wide, and popularization in large-span bridge main pier deep foundation pit engineering is facilitated.

Description

Technical Field

[0001] This application belongs to the field of large-scale building support technology, specifically relating to an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure and its construction method. Background Technology

[0002] In existing technologies, cofferdam support structures play a crucial role in the construction of main pier caps for long-span bridges. They provide a safe and stable working environment for the construction of large-scale, deeply embedded pile foundation caps, ensuring the smooth progress of excavation and subsequent cap pouring, and directly impacting the construction quality, safety, and overall schedule of bridge foundation engineering. With the increasing maturity of bridge construction technology, long-span bridges are experiencing rapid development. Main pier cap construction, as a deep foundation pit project exceeding a certain scale and posing significant risks, places higher demands on the reliability and adaptability of cofferdam support structures. However, the materials and structural forms of commonly used cofferdam support structures on the market currently have some shortcomings.

[0003] In existing technologies, steel pipe pile cofferdams, while having a large self-weight and high rigidity, are suitable for construction scenarios with deep foundation pits. They also require fewer internal supports, exhibit minimal deflection, and provide excellent water-stopping effects, effectively resisting soil pressure around deep foundation pits and ensuring construction safety. However, in practical applications, they suffer from high construction difficulty, high costs, and long construction periods, significantly increasing investment in bridge foundation engineering and hindering project efficiency. Steel sheet pile cofferdams, as a commonly used support structure for bridge foundation construction, offer advantages such as light weight, high construction efficiency, low cost, and easy quality control, reducing construction difficulty and costs. However, due to their lower rigidity, they are only suitable for shallower foundation pit support scenarios and cannot meet the stringent requirements for support rigidity and stability in deep foundation pit construction of large-span bridge piers. In deep foundation pit environments, they are prone to significant deformation, making it difficult to guarantee construction safety.

[0004] Therefore, in order to comprehensively improve the adaptability of cofferdam support structures to the construction of deep foundation pits for main piers of long-span bridges, and to take into account the requirements of support stiffness, water-stopping effect, construction efficiency and cost control, it is now urgent to make improvements to provide a cofferdam support structure that has both excellent support performance and economic practicality, thereby improving the construction safety, stability and economy of bridge foundation engineering. Summary of the Invention

[0005] In order to address the technical problem that existing cofferdam support structures are difficult to adapt to the construction requirements of deep foundation pits for main piers of long-span bridges, and that the strength of pipe piles and sheet piles cannot meet the stringent requirements for support stiffness and stability of deep foundation pits for main piers of long-span bridges due to limitations in the performance of traditional support steel, this application proposes an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure.

[0006] To address the technical problems raised in this application, this application also provides a construction method for an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure.

[0007] This application adopts the following scheme: an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure, including a cofferdam area preset on a reference plane, a plurality of pipe piles disposed on the outer periphery of the cofferdam area, a support plate group disposed between two adjacent pipe piles, a first connecting mechanism disposed between the pipe piles and the support plate group, an inner support layer disposed on the periphery of the cofferdam area, a filling layer disposed between the inner support layer, the pipe piles, and the support plate group, and a reinforcing mechanism disposed on the support plate group. The pipe piles are made of seamless steel pipes, the height of the pipe piles is defined as H, and the height of the support plate group is defined as h. The relationship between H and h is: 1.25≤H / h≤1.5.

[0008] In some feasible embodiments, the support plate assembly includes a plurality of sheet piles disposed between two adjacent pipe piles, and a second connecting mechanism disposed between two adjacent sheet piles.

[0009] In some feasible embodiments, the second connecting mechanism includes second connecting ears disposed at both ends of the sheet pile, the sheet pile having an isosceles trapezoidal cross-sectional shape, and two adjacent sheet piles being staggered along the outer periphery of the cofferdam area, the sheet pile and the second connecting ears being integrally formed.

[0010] In some feasible embodiments, the second connecting ear includes a second horizontal portion disposed at one end of the sheet pile, a second inclined portion disposed at an inclination on the second horizontal portion, and a second connecting groove formed by the second horizontal portion and the second inclined portion together. When two adjacent sheet piles are connected to each other, the second inclined portion can be matched and inserted into the second connecting groove.

[0011] In some feasible embodiments, the first connecting mechanism includes a first connecting ear disposed on both sides of the pipe pile. The first connecting ear includes a first horizontal portion disposed at one end of the pipe pile, a first inclined portion disposed on the first horizontal portion, and a first connecting groove formed by the first horizontal portion and the first inclined portion. When the support plate is disposed between two adjacent pipe piles, the first inclined portion can be matched and inserted into the second connecting groove.

[0012] In some feasible embodiments, the reinforcing mechanism includes a plurality of inclined reinforcing members disposed between the corners of the inner support layer, and a horizontal reinforcing member disposed between two adjacent inclined reinforcing members.

[0013] In some feasible embodiments, the seamless steel pipe is composed of the following components by mass percentage: carbon (C) 0.12-0.16, silicon (Si) 0.30-0.40, phosphorus (P) ≤0.025, sulfur (S) ≤0.020, aluminum (Al) 0.02-0.04, vanadium (V) 0.08-0.10, niobium (Nb) 0.03-0.05, titanium (Ti) 0.01-0.03, rare earth elements (RE) 0.01-0.02, iron (Fe) balance, and unavoidable impurities, the total amount of unavoidable impurities being ≤0.1%.

[0014] The rare earth elements are obtained by combining praseodymium (Pr) and yttrium (Y) in a mass ratio of 1:1.

[0015] In some feasible embodiments, the method for preparing the seamless steel pipe includes the following steps:

[0016] Step 101: The alloy combination in the preset ratio is put into the smelting furnace and melted, refined and vacuum degassed in sequence at 1600℃-1650℃ to obtain the first melt.

[0017] Step 102. Transfer the first melt to the arc continuous casting machine, and cast the first melt into a rough billet under the conditions of casting temperature of 1520℃-1540℃, crystallizer vibration frequency of 300-350 times / minute and crystallizer amplitude of 3mm-5mm.

[0018] Step 103. Transfer the billet to a walking beam furnace and heat it to 1200℃-1220℃. Then transfer the heated billet to the Mannesmann skew rolling mill and pierce it at 1180℃-1220℃ to obtain the primary seamless steel pipe.

[0019] Step 104. Transfer the primary seamless steel pipe to the PQF continuous rolling mill and roll it at 1050℃-1150℃. After rolling, the total wall reduction rate is 60%-70%, which yields the secondary seamless steel pipe.

[0020] Step 105. Transfer the grade II seamless steel pipe to the tension reducing machine. Under the condition of reducing temperature of 900℃-950℃, the grade II seamless steel pipe is formed to the preset size specification to obtain the grade III seamless steel pipe.

[0021] Step 106. Transfer the grade III seamless steel pipe to a quenching furnace and quench it at 900℃. After quenching, transfer the grade III seamless steel pipe to a tempering furnace and temper it at 580℃-620℃ for 90min-120min. Then cool it to room temperature to obtain the finished seamless steel pipe.

[0022] In some feasible embodiments, the seamless steel pipe has a yield strength of 430MPa-470MPa, a tensile strength of 620MPa-655MPa, and an elongation at break of 20%-23%.

[0023] To address the technical problems raised in this application, this application also provides a construction method for an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure, which is used to construct the aforementioned cofferdam support structure and includes the following steps:

[0024] Step 201. Hoist the pipe piles to the preset positions on the reference surface according to the preset spacing, and use a theodolite to detect the verticality of the pipe piles relative to the reference surface in real time;

[0025] Step 202. Use a vibratory pile driver to drive the pipe pile into the reference plane at a speed of 0.5m / min-1m / min. During the pile driving process, use a theodolite to detect the verticality of the pipe pile relative to the reference plane in real time.

[0026] Step 203. Hoist the sheet pile to the space between two adjacent pipe piles according to the preset installation position, and achieve precise positioning and connection between the pipe pile and the sheet pile through the first connecting mechanism. After installation, weld and reinforce the connection between the two.

[0027] Step 204. Place the inner support layer within the pre-set cofferdam area according to the pre-set installation position;

[0028] Step 205. Place the inclined reinforcement and the horizontal reinforcement on the inner support layer according to the preset installation position.

[0029] Compared with the prior art, this application has the following beneficial effects:

[0030] This application provides an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure and its construction method, including a cofferdam area, outer peripheral pipe piles, support plate assembly, connecting mechanism, inner support layer, filling layer, and reinforcing mechanism. The pipe piles are made of seamless steel pipes with optimized alloy ratio, and the height H of the pipe piles and the height h of the support plate assembly satisfy 1.25≤H / h≤1.5. This application ensures the support coverage effect through height ratio optimization, and achieves tight and rigid connection of each component through the connecting mechanism, which can effectively distribute load, resist deformation, and avoid instability. The seamless steel pipes have high strength, high toughness, and high durability, adapting to the bearing requirements of deep foundation pits, while forming a reliable water-stop barrier to ensure construction safety. This application has a reasonable structural layout, controllable construction, and easy quality assurance, taking into account stability, safety, and economy, reducing operation and maintenance costs, and has reliable structure and wide adaptability, making it easy to promote in deep foundation pit projects for main piers of large-span bridges. Attached Figure Description

[0031] Figure 1 This is a structural schematic diagram of an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to this application;

[0032] Figure 2 This application Figure 1 A magnified view of a section at point A in the middle;

[0033] Figure 3 This is a structural schematic diagram of an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure under the service state of this application;

[0034] Figure 4 This is a front view of an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to this application;

[0035] Figure 5 This is a top view of an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to this application;

[0036] Figure 6 This application Figure 5 A magnified view of a section at point B. Detailed Implementation

[0037] Combination Figures 1 to 6 The content shown further illustrates the technical solution proposed in this application. This application adopts the following technical solution: an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure, comprising a cofferdam area pre-set on a reference plane, multiple pipe piles 1 disposed on the outer periphery of the cofferdam area, a support plate group 2 disposed between two adjacent pipe piles 1, a first connecting mechanism 3 disposed between the pipe piles 1 and the support plate group 2, an inner support layer 4 disposed on the outer periphery of the cofferdam area, a filling layer disposed between the inner support layer 4, the pipe piles 1, and the support plate group 2, and a reinforcing mechanism 5 disposed on the support plate group 2. The material of the pipe piles 1 is seamless steel pipe. The height of the pipe piles 1 is defined as H, and the height of the support plate group 2 is defined as h. The relationship between H and h is: 1.25 ≤ H / h ≤ 1.5.

[0038] For example, the values ​​of H / h are 1.25, 1.3, 1.4, and 1.5.

[0039] Preferably, the value of H / h is 1.25. When H / h < 1.25, the support coverage of the pipe pile to the support plate assembly is insufficient, which is prone to local stress concentration; when H / h > 1.5, the redundant height of the pipe pile increases the construction cost and affects the overall stability.

[0040] In this embodiment, the support plate group 2 includes a plurality of sheet piles 20 disposed between two adjacent pipe piles 1, and a second connecting mechanism 21 disposed between two adjacent sheet piles 20.

[0041] In actual implementation, the material of sheet piles is the same as that of pipe piles.

[0042] In this embodiment, the second connecting mechanism 21 includes second connecting ears 210 disposed at both ends of the sheet pile 20. The cross-sectional shape of the sheet pile 20 is an isosceles trapezoid. Two adjacent sheet piles 20 are staggered along the outer periphery of the cofferdam area. The sheet pile 20 and the second connecting ears 210 are integrally formed.

[0043] In this embodiment, the second connecting ear 210 includes a second horizontal part 211 disposed at one end of the sheet pile 20, a second inclined part 212 disposed on the second horizontal part 211, and a second connecting groove 213 formed by the second horizontal part 211 and the second inclined part 212. When two adjacent sheet piles 20 are connected to each other, the second inclined part 212 can be matched and inserted into the second connecting groove 213.

[0044] In this embodiment, the first connecting mechanism 3 includes a first connecting ear 30 disposed on both sides of the pipe pile 1. The first connecting ear 30 includes a first horizontal part 300 disposed at one end of the pipe pile 1, a first inclined part 301 disposed on the first horizontal part 300, and a first connecting groove 302 formed by the first horizontal part 300 and the first inclined part 301. When the support plate assembly 2 is disposed between two adjacent pipe piles 1, the first inclined part 301 can be matched and inserted into the second connecting groove 213.

[0045] In this embodiment, the reinforcing mechanism 5 includes a plurality of inclined reinforcing members 50 disposed between the four corners of the inner support layer, and a horizontal reinforcing member 51 disposed between two adjacent inclined reinforcing members 50.

[0046] In actual implementation, the cofferdam area is first demarcated on the construction reference surface to clarify the layout range of the support structure. Seamless steel pipe piles 1 are fixed along the outer perimeter of the cofferdam area at preset intervals. At the same time, the ratio of the pipe pile height H to the subsequent support plate group height h is strictly controlled to be between 1.25 ≤ H / h ≤ 1.5 (preferably 1.25). This ratio design ensures that the pipe piles provide effective support and coverage for the support plate group, avoiding stress imbalance due to excessive height of the support plate group or support gaps due to insufficient height. Support plate groups 2 are installed between two adjacent pipe piles 1. The support plate group 2 is composed of multiple sheet piles 20 spliced ​​together by a second connecting mechanism 21. The sheet pile cross-section adopts an isosceles trapezoidal design, and adjacent sheet piles are staggered along the outer perimeter of the cofferdam. With the second horizontal part 211, the second inclined part 212, and the second connecting groove 213 of the second connecting mechanism, the second inclined part of the adjacent sheet piles can be accurately inserted into the second connecting groove of the other, realizing a tight splicing between the sheet piles. At the same time, the one-piece molding design improves the connection strength. The first connecting mechanism 3 completes the coordinated fixing of the pipe pile and the support plate assembly. The first connecting ears 30 on both sides of the pipe pile, through the structural design of the first horizontal part 300, the first inclined part 301 and the first connecting groove 302, enable the first inclined part to be matched and inserted into the second connecting groove 213 at the end of the support plate assembly, forming a rigid connection between the pipe pile and the sheet pile, and constructing a complete peripheral support closed loop. An inner support layer 4 is arranged around the perimeter of the cofferdam area. The gap between the inner support layer and the outer pipe pile and support plate assembly is filled by the filling layer, which improves the overall sealing and stability of the support structure. Finally, a reinforcing mechanism 5 is installed on the support plate assembly. The corners of the inner support layer are connected by inclined reinforcing members 50, and the adjacent inclined reinforcing members are connected by horizontal reinforcing members 51, forming an internal three-dimensional reinforcement network.

[0047] During the excavation of the foundation pit, loads such as the surrounding soil pressure and groundwater pressure are first applied to the closed-loop support system composed of the outer pipe piles 1 and the support plate assembly 2. The pipe piles, as the core load-bearing components, bear the main load thanks to the high rigidity of their seamless steel pipes, and transfer part of the load to the support plate assembly through the first connecting mechanism. The support plate assembly, through splicing, forms an integral structure that distributes the load. The isosceles trapezoidal sheet pile cross-section design enhances bending resistance and prevents localized deformation. The inner support layer 4 is tightly fitted to the outer support structure through a filling layer, further dispersing the load transferred from the outer perimeter to the entire inner support network. The three-dimensional reinforcement structure formed by the inclined reinforcing member 50 and the horizontal reinforcing member 51 of the strengthening mechanism effectively resists the lateral deflection deformation of the support structure, preventing structural instability due to excessive localized stress. The tight splicing of the pipe piles and the support plate assembly, the precise fit of the connecting mechanism, and the sealing effect of the filling layer together form an effective water-stopping barrier, preventing groundwater from seeping into the foundation pit and ensuring a safe construction environment.

[0048] Example 2

[0049] (1) The method for preparing seamless steel pipes includes the following steps:

[0050] Step 101: Put each alloy component shown in Table 1 into a smelting furnace, melt at 1625℃ for 90 min, refine for 45 min, and degas under vacuum for 30 min to obtain the first melt.

[0051] During the smelting process, the carbon content, phosphorus content, and sulfur content are strictly controlled;

[0052] Step 102. Transfer the first melt to the arc continuous casting machine, and cast the first melt into a rough billet under the conditions of casting temperature of 1530℃, crystallizer vibration frequency of 350 times / minute and crystallizer amplitude of 5mm.

[0053] Step 103. Transfer the billet to a walking beam furnace and heat it to 1200℃-1220℃. Then transfer the heated billet to the Mannesmann skew rolling mill and pierce it at 1180℃-1220℃ to obtain the primary seamless steel pipe.

[0054] The heating process of the walking beam furnace includes a preheating section, a heating section, and a soaking section.

[0055] The temperature of the preheating section is 700℃, and the holding time is 60-90 minutes;

[0056] The temperature of the heating section is 1210℃, and the holding time is 90min-120min;

[0057] The temperature of the heat spreader is 1220℃, and the holding time is 30-40 minutes.

[0058] Step 104. Transfer the primary seamless steel pipe to the PQF continuous rolling mill and roll it at 1100℃. After rolling, the total wall reduction rate is 65%, which yields the secondary seamless steel pipe.

[0059] Step 105. Transfer the grade II seamless steel pipe to the tension reducing machine. Under the condition of reducing temperature of 950℃, the grade II seamless steel pipe is formed to the preset size specification to obtain the grade III seamless steel pipe.

[0060] Step 106. Transfer the grade III seamless steel pipe to the quenching furnace and quench it at 900℃. After quenching, transfer the grade III seamless steel pipe to the tempering furnace and temper it at 600℃ for 100 minutes. Then cool it to room temperature to obtain the finished seamless steel pipe.

[0061] The quenching process includes a heating section and a soaking section. The quenching medium is high-pressure water with a water pressure of 12MPa and a water temperature of 25℃.

[0062] The temperature of the heating section is 920℃, and the holding time is 100min-150min;

[0063] The temperature of the heat spreader is 900℃, and the heat preservation time is calculated based on the pipe wall thickness (1.5 min per millimeter of wall thickness).

[0064] (2) Construction method of N+1 steel sheet pile and steel pipe pile combined cofferdam support structure, including the following steps:

[0065] Step 201. Hoist the pipe piles to the preset positions on the reference surface according to the preset spacing, and use a theodolite to detect the verticality of the pipe piles relative to the reference surface in real time;

[0066] The reference plane undergoes preprocessing, which includes the following steps:

[0067] Site survey: A ground-penetrating radar detector was used to survey the cofferdam area and the surrounding 3m range to determine the underground pipelines (pipeline burial depth and material), the thickness of the soft soil layer (error ≤ 0.5m), and the groundwater level (measurement accuracy ± 5cm).

[0068] Site clearing: Remove weeds, gravel, construction waste, and other obstacles from the cofferdam area; level the site using a grader, controlling the flatness within ±10cm; for soft soil areas, use the replacement method, with graded sand and gravel (particle size 5mm-30mm), replacement thickness ≥50cm, compacted in layers (each layer 20cm thick), compaction degree ≥95%;

[0069] Step 202. Use a vibratory pile driver to drive the pipe pile into the reference plane at a speed of 0.5m / min-1m / min. During the pile driving process, use a theodolite to monitor the verticality of the pipe pile relative to the reference plane in real time to ensure that the verticality deviation of the pipe pile is ≤0.5%.

[0070] Step 203. Hoist the sheet pile to the space between two adjacent pipe piles according to the preset installation position, and achieve precise positioning and connection between the pipe pile and the sheet pile through the first connecting mechanism. After installation, weld and reinforce the connection between the two. Specifically, connect the pipe pile and the support plate assembly through the first connecting mechanism. The first connecting ears on both sides of the pipe pile are precisely aligned with the second connecting ears of the support plate assembly. The first inclined part is matched and inserted into the second connecting groove. The gap between the two parts is ≤1mm (measured with a feeler gauge). Weld and fix it using an electric welding machine. The welding method is double-sided fillet welding. The weld leg height is 8mm-10mm. There are two welding layers. The welding current for the first layer is 180A-200A, and the welding current for the second layer is 200A-220A. After welding, remove the weld slag and check for defects such as porosity and slag inclusion.

[0071] Step 204. Place the inner support layer in the pre-set cofferdam area according to the pre-set installation position. The elevation error of the inner support layer is ≤ ±3cm. The inner support layer is made of steel (H-beam, model H400×200×8×12) and spliced ​​into a closed frame according to the shape of the cofferdam.

[0072] Step 205. Weld the inclined reinforcement and the horizontal reinforcement to the inner support layer according to the preset installation position.

[0073] Comparative Example 1

[0074] The difference between Comparative Example 1 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of Si in the alloy composition is adjusted (0%, 0.2%, 0.5%), while the other components and processes remain unchanged.

[0075] Comparative Example 2

[0076] The difference between Comparative Example 2 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of Mn in the alloy composition is adjusted (0%, 1.2%, 1.5%), while the other components and processes remain unchanged.

[0077] Comparative Example 3

[0078] The difference between Comparative Example 3 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of Al in the alloy composition is adjusted (0%, 0.015%, 0.04%), while the other components and processes remain unchanged.

[0079] Comparative Example 4

[0080] The difference between Comparative Example 4 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of V in the alloy composition is adjusted (0%, 0.06%, 0.1%), while the other components and processes remain unchanged.

[0081] Comparative Example 5

[0082] The difference between Comparative Example 5 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of Nb in the alloy composition is adjusted (0%, 0.03%, 0.05%), while the other components and processes remain unchanged.

[0083] Comparative Example 6

[0084] The difference between Comparative Example 6 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of Ti in the alloy composition is adjusted (0%, 0.01%, 0.03%), while the other components and processes remain unchanged.

[0085] Comparative Example 7

[0086] The difference between Comparative Example 7 and Example 1 is that the alloy composition of the seamless steel pipe is adjusted. Specifically, the mass percentage of RE in the alloy composition is adjusted (0%, 0.01%, 0.02%), while the other components and processes remain unchanged. The RE is obtained by compounding praseodymium (Pr) and yttrium (Y) in a mass ratio of 1:1.

[0087] Table 1. Components of Example 1 and Comparative Examples 1-7

[0088]

[0089] Continued from Table 1

[0090]

[0091] The seamless steel pipes prepared in Example 1 and Comparative Examples 1-7 were subjected to the following tests:

[0092] Test 1: The tensile strength, low-temperature impact resistance, and yield strength of the seamless steel pipe were measured according to GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test method at room temperature" and GB / T229-2020 "Metallic materials - Charpy impact test method".

[0093] Test 2: Referring to GB / T10125-2021 "Artificial Atmosphere Corrosion Test - Salt Spray Test", the corrosion resistance of seamless steel pipes was measured, and the test results are shown in Table 2 below.

[0094] Table 2 Results of Tests 1-2

[0095]

[0096] As shown in Table 2, in Comparative Example 1, Si significantly improves the strength and corrosion resistance of seamless steel pipes. As the Si content increases from 0 to 0.50%, the yield strength increases from 380 MPa to 470 MPa, the tensile strength increases from 580 MPa to 670 MPa, and the corrosion rate decreases from 0.018 mm / a to 0.010 mm / a. However, it has little effect on toughness (elongation and impact energy). That is, the core role of Si is solid solution strengthening and improving oxidation corrosion resistance, which is suitable for the strength and durability requirements of cofferdam support.

[0097] In Comparative Example 2, Mn is a powerful strengthening and grain-refining element. Without Mn, the steel pipe has the lowest strength (Rel=360MPa, Rm=550MPa). As the Mn content increases to 1.50%, the strength increases significantly (Rel=480MPa, Rm=680MPa). At the same time, the low-temperature toughness and corrosion resistance are also significantly improved (impact energy increases from 35J to 43J, and corrosion rate decreases from 0.020mm / a to 0.011mm / a). This shows that Mn can comprehensively improve the mechanical properties and corrosion resistance of steel pipes and is a key component to ensure the bearing capacity of cofferdam support.

[0098] In Comparative Example 3, the core role of Al is to refine the grains and improve toughness. Without Al, the elongation is only 18% and the impact energy is 32J. As the Al content increases to 0.04%, the elongation increases to 23% and the impact energy increases to 45J. At the same time, the strength also increases slightly. This shows that Al can effectively balance the strength and toughness of the steel pipe, avoid the steel pipe from becoming brittle due to high strength, and improve the impact resistance and deformation resistance of the cofferdam support.

[0099] In Comparative Example 4, V is the key element for microalloying reinforcement, which can significantly improve the strength of the steel pipe. Without V, Rel=410MPa, and with V=0.12%, Rel=475MPa. At the same time, the corrosion resistance is also significantly improved (the corrosion rate decreases from 0.015mm / a to 0.010mm / a), while having little effect on toughness. This indicates that V can strengthen the steel pipe and improve its load-bearing stability without reducing toughness.

[0100] In Comparative Example 5, Nb can refine the austenite grains and improve the deformation resistance of the steel pipe. Without Nb, the elongation is only 20%, while with Nb=0.05%, the elongation increases to 22%, and the strength also increases from 420MPa to 465MPa. Corrosion resistance is also improved simultaneously, indicating that Nb can optimize the comprehensive mechanical properties of the steel pipe by refining the grains and enhance its anti-settlement ability in deep foundation pit support.

[0101] In Comparative Example 6, Ti can fix the nitrogen element in the steel, improve welding performance, and enhance toughness and corrosion resistance. Without Ti, the impact energy is only 35J, while with Ti=0.03%, the impact energy increases to 44J, and the corrosion rate decreases from 0.017mm / a to 0.012mm / a. This shows that Ti can improve the welding reliability and durability of steel pipes, making it suitable for welding steel pipes and other components in cofferdam support.

[0102] In Comparative Example 7, RE can modify impurities in steel and reduce the harm of harmful impurities. Without RE, the corrosion rate is as high as 0.019 mm / a and the impact energy is only 34 J. With RE=0.02%, the corrosion rate drops to 0.011 mm / a and the impact energy increases to 43 J. This shows that RE can significantly improve the corrosion resistance and toughness of steel pipes and extend their service life in underwater humid environments.

[0103] In summary, the preferred formulation of seamless steel pipes (Si=0.35%, Mn=1.35%, Al=0.03%, V=0.09%, Nb=0.04%, Ti=0.02%, RE=0.015%) achieves the best balance between mechanical properties and corrosion resistance. The synergistic effect of each beneficial component ensures that the seamless steel pipes possess the high strength, high toughness, and high durability required for cofferdam support.

[0104] This application provides an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure and a matching seamless steel pipe preparation and construction method, which includes a cofferdam area pre-set on a reference surface, pipe piles set on the outer periphery of the cofferdam area, a support plate assembly between adjacent pipe piles and a first connection mechanism between the two, an inner support layer on the periphery of the cofferdam area, a filling layer between the inner support layer and the pipe piles and the support plate assembly, and a reinforcing mechanism on the support plate assembly; the pipe piles are made of seamless steel pipes with a specific preferred formula, and the height H of the pipe piles and the height h of the support plate assembly satisfy the ratio relationship of 1.25≤H / h≤1.5. This application optimizes the height ratio of pipe piles to support plate assemblies, ensuring effective support and coverage of the support plate assemblies by the pipe piles, thus preventing stress imbalance or gaps in the support plate assemblies. Through the precise matching design of the first and second connecting mechanisms, a tight, rigid connection is achieved between the pipe piles, support plate assemblies, and sheet piles, constructing a complete closed-loop peripheral support system. This, combined with the three-dimensional reinforcement network formed by the inner support layer, filling layer, and reinforcing mechanisms, effectively disperses the pressure from the surrounding soil and groundwater, resists lateral deflection, and prevents structural instability. The seamless steel pipes used for the pipe piles, processed with optimized alloy components and precise manufacturing processes, possess high strength, high toughness, and high durability, meeting the stringent load-bearing requirements of deep foundation pit support. Simultaneously, the tight splicing of each structure and the sealing effect of the filling layer form a reliable water-stopping barrier, preventing groundwater infiltration and ensuring a safe construction environment. The overall structure of this application is reasonable, the construction process is highly controllable, the quality is easy to guarantee, and it can take into account the stability, safety and economy of the support, reduce the operation and maintenance cost of the project, and has the advantages of reliable structure, wide adaptability and controllable implementation cost. It is easy to promote and implement in engineering scenarios such as deep foundation pits of main piers of long-span bridges.

[0105] The embodiments provided by the present invention have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of the present invention. The descriptions of the embodiments above are merely for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make various improvements and modifications to the present invention without departing from its principles, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A combined N+1 sheet pile and steel pipe pile cofferdam support structure, characterized in that, The system includes a cofferdam area pre-set on a reference surface, multiple pipe piles (1) located around the periphery of the cofferdam area, a support plate group (2) located between two adjacent pipe piles (1), a first connecting mechanism (3) located between the pipe piles (1) and the support plate group (2), an inner support layer (4) located around the periphery of the cofferdam area, a filling layer located between the inner support layer (4), the pipe piles (1), and the support plate group (2), and a reinforcing mechanism (5) located on the support plate group (2). The pipe piles (1) are made of seamless steel pipes. The height of the pipe piles (1) is defined as H, and the height of the support plate group (2) is defined as h. The H and the h satisfy the following relationship: 1.25≤H / h≤1.

5.

2. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 1, characterized in that, The support plate assembly (2) includes a plurality of sheet piles (20) disposed between two adjacent pipe piles (1), and a second connecting mechanism (21) disposed between two adjacent sheet piles (20).

3. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 2, characterized in that, The second connecting mechanism (21) includes a second connecting lug (210) at both ends of the sheet pile (20). The sheet pile (20) has an isosceles trapezoidal cross section. Two adjacent sheet piles (20) are staggered along the outer periphery of the cofferdam area. The sheet pile (20) and the second connecting lug (210) are integrally formed.

4. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 3, characterized in that, The second connecting ear (210) includes a second horizontal part (211) disposed at one end of the sheet pile (20), a second inclined part (212) disposed on the second horizontal part (211), and a second connecting groove (213) formed by the second horizontal part (211) and the second inclined part (212). When two adjacent sheet piles (20) are connected to each other, the second inclined part (212) can be matched and inserted into the second connecting groove (213).

5. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 4, characterized in that, The first connecting mechanism (3) includes a first connecting ear (30) on both sides of the pipe pile (1). The first connecting ear (30) includes a first horizontal part (300) at one end of the pipe pile (1), a first inclined part (301) on the first horizontal part (300), and a first connecting groove (302) formed by the first horizontal part (300) and the first inclined part (301). When the support plate group (2) is located between two adjacent pipe piles (1), the first inclined part (301) can be matched and inserted into the second connecting groove (213).

6. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 1, characterized in that, The reinforcing mechanism (5) includes a plurality of inclined reinforcing members (50) disposed between the corners of the inner support layer (4), and a horizontal reinforcing member (51) disposed between two adjacent inclined reinforcing members (50).

7. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 1, characterized in that, The seamless steel pipe, by mass percentage, is composed of the following components: carbon (C) 0.12-0.16, silicon (Si) 0.30-0.40, phosphorus (P) ≤0.025, sulfur (S) ≤0.020, aluminum (Al) 0.02-0.04, vanadium (V) 0.08-0.10, niobium (Nb) 0.03-0.05, titanium (Ti) 0.01-0.03, rare earth elements (RE) 0.01-0.02, iron (Fe) balance, and unavoidable impurities, the total amount of unavoidable impurities being ≤0.1%. The rare earth elements are obtained by combining praseodymium (Pr) and yttrium (Y) in a mass ratio of 1:

1.

8. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 1, characterized in that, The method for preparing the seamless steel pipe includes the following steps: Step 101: The alloy combination in the preset ratio is put into the smelting furnace and melted, refined and vacuum degassed in sequence at 1600℃-1650℃ to obtain the first melt. Step 102. Transfer the first melt to the arc continuous casting machine, and cast the first melt into a rough billet under the conditions of casting temperature of 1520℃-1540℃, crystallizer vibration frequency of 300-350 times / minute and crystallizer amplitude of 3mm-5mm. Step 103. Transfer the billet to a walking beam furnace and heat it to 1200℃-1220℃. Then transfer the heated billet to the Mannesmann skew rolling mill and pierce it at 1180℃-1220℃ to obtain the primary seamless steel pipe. Step 104. Transfer the primary seamless steel pipe to the PQF continuous rolling mill and roll it at 1050℃-1150℃. After rolling, the total wall reduction rate is 60%-70%, which yields the secondary seamless steel pipe. Step 105. Transfer the grade II seamless steel pipe to the tension reducing machine. Under the condition of reducing temperature of 900℃-950℃, the grade II seamless steel pipe is formed to the preset size specification to obtain the grade III seamless steel pipe. Step 106. Transfer the grade III seamless steel pipe to a quenching furnace and quench it at 900℃. After quenching, transfer the grade III seamless steel pipe to a tempering furnace and temper it at 580℃-620℃ for 90min-120min. Then cool it to room temperature to obtain the finished seamless steel pipe.

9. The N+1 steel sheet pile and steel pipe pile combined cofferdam support structure according to claim 8, characterized in that, The seamless steel pipe has a yield strength of 430MPa-470MPa, a tensile strength of 620MPa-655MPa, and an elongation at break of 20%-23%.

10. A construction method for an N+1 steel sheet pile and steel pipe pile combined cofferdam support structure, characterized in that, It is used for the construction of the cofferdam support structure according to any one of claims 1-9, and includes the following steps: Step 201. Hoist the pipe piles to the preset positions on the reference surface according to the preset spacing, and use a theodolite to detect the verticality of the pipe piles relative to the reference surface in real time; Step 202. Use a vibratory pile driver to drive the pipe pile into the reference plane at a speed of 0.5m / min-1m / min. During the pile driving process, use a theodolite to detect the verticality of the pipe pile relative to the reference plane in real time. Step 203. Hoist the sheet pile to the space between two adjacent pipe piles according to the preset installation position, and achieve precise positioning and connection between the pipe pile and the sheet pile through the first connecting mechanism. After installation, weld and reinforce the connection between the two. Step 204. Place the inner support layer within the pre-set cofferdam area according to the pre-set installation position; Step 205. Place the inclined reinforcement and the horizontal reinforcement on the inner support layer according to the preset installation position.

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