A cast-in-situ construction method for the joint of prefabricated immersed tubes in a semi-submerged floating state

Abstract

The present invention provides a method for casting the connection seam of a prefabricated immersed tube in a semi-submerged floating state, which is applicable to the field of prefabricated immersed tube construction. The present invention includes construction preparation, installation of a connection seam steel grid, casting of a connection seam construction section, first synchronous flipping of immersed tubes, casting of a new connection seam construction section, second synchronous flipping of immersed tubes, and repeated casting and flipping. The short-sized prefabricated immersed tubes are transported in batches to a calm sea area with a low sea condition level, and the immersed tubes to be connected are temporarily rigidly connected using a steel grid to avoid the shaking caused by wind and waves causing cracks in the cast concrete of the connection seam. A water retaining groove is used to isolate the construction surface of the immersed tube connection seam floating on one side of the water surface to avoid the seawater from washing away the newly cast concrete. A ballast water tank, a crane ship, and a tugboat are used to provide a capsizing moment to complete the flipping of the immersed tube. The connection seam cast-in-place construction is completed in a semi-submerged floating state, which solves the problem that the connection seam cast-in-place construction of short-sized immersed tubes cannot be performed on large ships when the size of the port immersed tube loading site is limited.

Description

Technical Field

[0001] The present invention relates to the field of precast immersed tube construction, and particularly to a cast-in-place construction method for the connection joint of precast immersed tubes in a semi-submerged floating state. Background Art

[0002] During the construction of immersed tube tunnels, the immersed tube tunnels are connected by two sections of immersed tubes through hydraulic caulking underwater. The GINA waterstop and OMEGA waterstop are respectively used to form the first waterproof layer and the second waterproof layer at the connection joint. Due to the high cost of these two waterstops, when the length of the immersed tube is long enough, the number of connection joints required for a tunnel is small. At this time, the underwater connection method can be accepted. However, during the construction of some immersed tube tunnels, due to the location of the precast immersed tube processing plant and the size limitation of the port immersed tube loading site, the length of the precast immersed tube is short. If the hydraulic caulking method is continued, one to two times more waterstop connection joints will be added additionally. It is obviously inappropriate to use this method for underwater connection.

[0003] Due to the excessive weight of the immersed tube, it needs to be transported onto the ship through a special trolley track for transportation, and it is not suitable for land transportation and hoisting and splicing. At the same time, due to the size limitation of the port immersed tube loading site, multiple sections of immersed tubes cannot be transported to a large semi-submersible barge, and the cast-in-place construction of the connection joints of multiple sections of immersed tubes cannot be carried out on the large semi-submersible barge. It is necessary to barge multiple sections of immersed tubes in batches to a calm sea area and carry out the connection joint construction in a semi-submerged floating state at sea. Therefore, a method for constructing the connection joints of precast immersed tubes in a semi-submerged floating state at sea is proposed. Summary of the Invention

[0004] The purpose of the present invention is to solve the problem that due to the size limitation of the precast immersed tube processing plant and the port immersed tube loading site, the short-sized precast immersed tubes cannot be cast in place for the connection joints on a large semi-submersible barge, and a cast-in-place construction method for the connection joints of precast immersed tubes in a semi-submerged floating state is proposed, which can be widely applied to the field of precast immersed tube construction.

[0005] To achieve the above purpose, the present invention provides the following technical solutions:

[0006] S1. Construction preparation;

[0007] The construction preparation includes obtaining the sea area meteorological conditions around the construction site of the undersea tunnel, selecting the calmest sea area with a sea state level not greater than n as the cast-in-place construction site for the connection joint, and the sea state level of the construction site is not greater than n within the next T 1 days, and then transporting two semi-submerged precast immersed tubes to be connected to the construction site. The length, width and height of the two immersed tubes are all L, B, H, and the weight is M 2 , and the initial ballast water volume in the ballast water tank of each immersed tube is V 0, meanwhile, the height of the immersed tube floating on the sea surface is greater than 1.5 times the wave height in sea state n. Take the side of the immersed tube floating on the water surface as the current construction section, remove the protective sleeve and rust the steel bars in the construction section, and then carry out butt joint;

[0008] S2. Installation of the steel grid for the joint;

[0009] The installation of the steel grid for the joint includes arranging the steel grid around the outer circle of the joint. The width of the joint is b, and the steel grid extends l in the length direction of the two immersed tubes. The length direction of the immersed tube is the same as the length direction of the ship. Then the length of the steel grid in the length direction of the immersed tube is 2l + b, and the steel grid is rigidly connected to the embedded parts on the immersed tube;

[0010] S3. Pouring of the construction section of the joint;

[0011] The pouring of the construction section of the joint includes isolating the working surface of the joint from the sea water by using a water retaining groove in the construction section. The surface of the water retaining groove in contact with the sea water needs a waterproof and anti-corrosion coating. Both ends of the water retaining groove are bolted to the steel grid, and the bottom surface of the water retaining groove is not less than the threshold h below the lower surface of the top of the immersed tube 1 , the height of the water retaining groove does not exceed the upper surface of the top of the immersed tube. During the erection process of the formwork and support system in the construction section, use inclined struts to transfer the load of the bottom formwork of the construction section to the end seal door, and reinforce and jack the end seal door on the dry side inside the immersed tube. After the erection of the formwork and support system is completed, carry out concrete pouring and curing;

[0012] S4. The first synchronous turnover of the immersed tube;

[0013] The first synchronous turnover of the immersed tube includes, after 7 days of curing of the construction section, releasing the restraint of the mooring points on the left side of the two immersed tubes, and injecting water into the ballast tank on the left side at a speed not exceeding Q. The injection volume satisfies the following formula,

[0014] V 1 ≤2(LBH - M 2 / ρ 1 - V 0 ) (1)

[0015] In the formula, V 1 - The maximum injection volume, m 3 , L, B, H - The length, width and height of a single immersed tube, m, M 2 - The weight of a single immersed tube, kg, ρ 1 - The density of sea water, kg / m 3; While injecting water, release the mooring cable wrapped around the right mooring point, so that the immersed tube is synchronously tilted to the left, fill the ballast water tank with water, and after the immersed tube is stable, the left side of each section of the immersed tube is pressed down by the pressure arm of the ship, and the right side is pulled up by the crane ship with a hoisting cable, so that the immersed tube is further turned over in the tilted state, and then the water in the ballast water tank is discharged at a speed not exceeding Q, until the water in the ballast water tank is completely discharged, the immersed tube completes a 90° turn, and the mooring cable of the right mooring point is tightened;

[0016] S5, pouring of new construction section of connecting joint;

[0017] The pouring of the new construction section of the joint includes determining the nearest T 2 Is there a continuous T 1 During the period of time when the sea condition level is not greater than level n, if it exists, continue to stay at the construction site; if it does not exist, tow it to a surrounding sea area that meets the requirements as a new construction site; take the side that floats on the water after synchronous flipping as the new construction section, and then repeat step S3;

[0018] S6, the immersed tube is turned over synchronously for the second time;

[0019] The second synchronous turning of the immersed tube includes filling all ballast water tanks in the immersed tube with a total volume of V 3 water, so that the height of the submerged tube floating on the water surface does not exceed the threshold h 2 , then the total volume V 3 Determined according to the following formula:

[0020] V 3 ≥2[LH(Bh 2 )-M 2 / ρ 1 ] (2)

[0021] Where V 3 -Total water injection volume, m 3 , L, B, H-length, width and height of a single immersed tube, m, h 2 -Threshold of the height of the submerged tube floating on the water surface, m, M 2 -Weight of a single immersed tube, kg, ρ 1 -Density of seawater, kg / m 3 ; The total volume V 3 Evenly distribute to each ballast water tank, install a traction rope at the rigid connection point of the upper right angle steel grid of the immersed tube, the traction rope bypasses the top of the immersed tube and is connected to the tugboat on the left side of the immersed tube, install a traction rope at the rigid connection point of the lower left angle steel grid of the immersed tube, the traction rope bypasses the bottom of the immersed tube and is connected to the tugboat on the right side of the immersed tube, and the distance between the left tugboat and the immersed tube is not less than h 2+5m, pull the towing cable forward to the left, and the distance between the right tugboat and the immersed tube shall be not less than B - h 3 +5m, where h 3 is the actual height of the immersed tube floating on the water surface. The right tugboat pulls the towing cable forward to the right to complete the second 90° flip of the immersed tube;

[0022] S7. Repeat the pouring and flipping;

[0023] The repeated pouring and flipping includes repeating step S5 to complete the concrete pouring and curing of the new construction section, then repeating step S4 to complete the third synchronous flip. After the flip, repeat step S5 to complete the concrete pouring and curing of the new construction section, and finally repeat step S6 to flip the immersed tube into the initial state, that is, the state where the ballast water tank is on the bottom surface of the immersed tube. After the concrete of the last pour has completed 28-day age curing, remove the steel grid and transport the assembled immersed tube to the seabed tunnel construction site.

[0024] Further, in the above step S4, the pressure on the pressure arm and the pulling force of the crane ship are determined according to the external moment required for the overturning of the immersed tube M t ,

[0025]

[0026] M t ≥M r,max (4)

[0027] In the formula, i - the i-th crane ship, n - the total number of crane ships, F - the pressure on the pressure arm and the pulling force of the crane ship, N, e - the lever arm from the acting point of the pressure and pulling force to the center of gravity of the immersed tube, m, M r,max - the maximum restoring moment, N·m; the maximum restoring moment M r,max is determined according to the transverse lever arm GZ between the center of gravity and the center of buoyancy within the range of the inclination angle θ from 0 to 90°,

[0028] M r =GZ×M 2 g (5)

[0029] In the formula, M r - the restoring moment, N·m.

[0030] Further, in the above step S6, the height threshold h of the immersed tube floating on the water surface 2 is 5m.

[0031] The beneficial effects of the present invention are as follows: By transporting short-sized precast immersed tubes in batches to a calm sea area with a low sea state level, using a steel grid frame to temporarily rigidly connect the immersed tubes to be connected, avoiding cracks in the cast-in-place concrete of the connection seam caused by the swaying caused by wind and waves, using a water retaining groove to isolate the construction surface of the connection seam of the immersed tube floating on one side of the water surface, avoiding the newly cast concrete being washed away by seawater, using ballast water tanks, a crane ship and a tugboat to provide an overturning moment to complete the overturning of the immersed tube, and performing the in-situ construction of the connection seam of the other side of the immersed tube. The in-situ construction of the connection seam of the immersed tube is completed in a semi-submerged floating state, solving the problem that the in-situ construction of the connection seam of multiple short-sized precast immersed tubes cannot be carried out on a large submerged barge under the condition of the size limitation of the loading site of the port immersed tube. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 is a flowchart of the method for in-situ casting of the connection seam of precast immersed tubes in a semi-submerged floating state according to the present invention;

[0033] Figure 2 is a front view schematic diagram of the connection seam of the immersed tube according to the present invention;

[0034] Figure 3 is a side view schematic diagram of the connection seam of the immersed tube according to the present invention;

[0035] Figure 4 is a schematic diagram of the first synchronous overturning of the in-situ casting construction of the connection seam of the immersed tube according to the present invention;

[0036] Figure 5 is a schematic diagram of the second synchronous overturning of the in-situ casting construction of the connection seam of the immersed tube according to the present invention;

[0037] Reference numerals in the drawings: 1 - immersed tube, 2 - steel grid frame, 3 - ballast water tank, 4 - water retaining groove, 5 - sea level, 6 - end seal door, 7 - lifting cable, 8 - pressure arm, 9 - towing cable, 10 - towing cable. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The following is a detailed description of the specific embodiments of the present invention with reference to the accompanying drawings. It should be understood that the specific embodiments given here are only for explaining and interpreting the present invention and cannot be used to limit the present invention. It should be noted that many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention may have other embodiments and variations, and therefore, the protection scope of the present invention is not limited by the specific embodiments disclosed below.

[0039] Example 1 relies on the main tunnel of a certain project on the Shenzhen-Yanjiang Expressway. The length of the immersed tube section is 5664 m. There are two-way eight lanes on both sides of the immersed tube tunnel. The standard section length of the immersed tube segment is 65 m, the width is 43.4 m, the height is 11.4 m, and the total weight is about 29250 t. The width of the connection seam is 3 m. The method for in-situ casting of the connection seam of precast immersed tubes in a semi-submerged floating state is as follows:

[0040] S1. Construction preparation;

[0041] Obtain the marine meteorological conditions around the construction site of the subsea tunnel, select the calmest sea area with a sea state level not greater than 3 as the cast-in-place construction site for the connection joint, and ensure that the sea state level of the construction site is not greater than 3 within the next 8 days, so as to ensure that there are no large waves during the concrete casting and initial curing stages. Then transport the two semi-submerged precast immersed tubes 1 to be connected to the construction site. The length, width and height of each of the two immersed tubes 1 are L = 65m, B = 43.4m, H = 11.4m, and the weight is M 2＝ 2.925×10 7 kg. The initial ballast water volume in the ballast water tank 3 of each immersed tube 1 is 0. At the same time, the height of the immersed tube 1 floating on the sea surface is greater than 1.5 times the wave height of sea state n. In this embodiment, it is 2m. Take the side of the immersed tube 1 floating on the water surface as the current construction section, remove the protective sleeve and rust the steel bars of the construction section, and then carry out butt joint;

[0042] S2. Installation of the steel grid of the connection joint;

[0043] Use the steel grid 2 to arrange around the outer circle of the connection joint. The width of the connection joint is 3m. The steel grid 2 extends 6m in the length direction of the two immersed tubes 1. The length direction of the immersed tube 1 is the same as the length direction of the ship. Then the length of the steel grid 2 in the length direction of the immersed tube is 15m. The steel grid 2 is rigidly connected to the embedded parts on the immersed tube 1 to ensure the synchronous movement of the two immersed tubes 1 and avoid cracking during the concrete curing process of the connection joint caused by relative dislocation;

[0044] S3. Pouring of the construction section of the connection joint;

[0045] Use the water retaining groove 4 to isolate the working surface of the connection joint from the sea water in the construction section. The water retaining groove 4 is made of steel plate, and the side in contact with the sea water needs a waterproof and anti-corrosion coating. The two ends of the water retaining groove 4 are bolted to the steel grid 2. The bottom surface of the water retaining groove 4 is not less than 2m below the lower surface of the immersed tube top to ensure that there is appropriate operating space on the lower surface of the immersed tube top. The height of the water retaining groove 4 does not exceed the upper surface of the immersed tube top. During the erection of the formwork and support system in the construction section, use inclined struts to transfer the load of the bottom formwork of the construction section to the end seal door 6, and use the end seal door 6 as the main load-bearing member of the support system. The water retaining groove 4 is mainly used to isolate sea water and does not participate in the load-bearing of the support system. The end seal door 6 is reinforced and jacked on the side without water inside the immersed tube 1 to avoid gaps at the sealing place of the end seal door 6 and cause water seepage. After the formwork and support system are erected, carry out concrete pouring and curing;

[0046] S4. The first synchronous turnover of the immersed tube;

[0047] The first synchronous turnover of the immersed tube includes that after 7 days of curing of the construction section, the mooring point constraints on the left side of two sections of the immersed tube 1 are released, and water is injected into the left ballast tank 3 at a speed not exceeding 1 m 3 / s, and the injection volume satisfies the following formula,

[0048] V 1 ≤2(LBH - M 2 / ρ 1 - V 0 ) = 8068.8 m 3 (1)

[0049] In the formula, V 1 - Maximum injection volume, m 3 , L, B, H - The length, width and height of a single - section immersed tube, m, M 2 - The weight of a single - section immersed tube, kg, ρ 1 - Seawater density, kg / m 3 ; In this embodiment, each section of the immersed tube 1 can be injected with 3000 m 3 on one side. Therefore, when the ballast tanks 3 of two sections of the immersed tube 1 are filled with water, V 1 = 6000 m 3 ; While injecting water, the mooring cables wound around the right mooring point are released, so that the immersed tube 1 tilts synchronously to the left. After the ballast tank 3 is filled with water, when the immersed tube 1 is stable, the left side of each section of the immersed tube 1 is pressed down by the pressure arm 8 of the ship, and the right side is pulled up by the lifting cable 7 of the crane ship. The pressure of the pressure arm 8 and the pulling force of the crane ship are determined according to the external moment required for the overturning of the immersed tube M t ,

[0050]

[0051] M t ≥M r,max (3)

[0052] In the formula, i - The i - th crane ship, n - The total number of crane ships, F - The pressure of the pressure arm and the pulling force of the crane ship, N, e - The lever arm from the action point of the pressure and the pulling force to the centroid of the immersed tube, m, M r,max - The maximum restoring moment, N·m; The maximum restoring moment M r,max is determined according to the transverse lever arm GZ between the centroid and the buoyancy center within the range of the inclination angle θ from 0 to 90°,

[0053] M r = GZ×M 2 g (4)

[0054] In the formula, M r- Restoring moment, N·m; with the centroid of the immersed tube cross-section as the origin of coordinates, the positive x-axis is in the rightward direction along the height of the immersed tube, the positive y-axis is in the upward direction along the width of the immersed tube, and the angle between the x-axis and the sea level is the inclination angle θ. The coordinates of the center of gravity of the immersed tube (x G , y G ) can be obtained according to the following formula:

[0055]

[0056] In the formula, M i - The mass of the i-th component of the immersed tube, x i - The abscissa of the i-th component of the immersed tube, y i - The ordinate of the i-th component of the immersed tube; The lifting ship slowly twists the immersed tube synchronously. Without loss of generality, assume that the gravity and buoyancy of the immersed tube are approximately equal at any time, that is, the drainage volume is constant. The area S 1 immersed in the water of the immersed tube cross-section is constant and is

[0057] S 1 ≥M 2 / (ρ 1 L) + V 1 / 2L = 478.8m 2 (7)

[0058] It is known that the immersed area is a right trapezoid, and the height of the trapezoid is the height of the immersed tube. According to the immersed area S 1 and the inclination angle θ, the upper base a and the lower base b of the immersed area are obtained, and then the coordinates of each corner point of the right trapezoid are obtained. According to the centroid formula of the right trapezoid, the coordinates of the center of buoyancy (x b , y b ) at the inclination angle θ are obtained. The coordinates in the coordinate system with the rightward direction along the height of the immersed tube as the positive x-axis are converted into the coordinates in the coordinate system with the rightward direction along the sea level as the positive x-axis. Then the coordinates of the center of gravity of the immersed tube (x’ G , y’ G ) and the coordinates of the center of buoyancy (x’ b , y’ b ) are

[0059] x' G = x G cosθ - y G sinθ (8)

[0060] y' G = y G cosθ + x G sinθ (9)

[0061] x' b = x b cosθ - y b sinθ (10)

[0062] y' b = y b cosθ + x b sinθ (11)

[0063] The transverse lever arm GZ is determined according to the following formula

[0064] GZ = |x G - x b | (12)

[0065] Find the maximum transverse lever arm GZ according to the relationship between the transverse lever arm GZ and the inclination angle θ max , and obtain the estimated value of the maximum restoring moment M r,max of 5.85×10 5 kN·m; On the left side of each immersed tube 1, 2 ships provide a pressure of not less than 800t, and on the right side, 2 crane ships provide a tension of not less than 800t to provide the external moment M t is approximately 6.4×10 5 kN·m, so that the immersed tube 1 further flips in the inclined state, and then the water in the ballast tank 3 is discharged at a speed not exceeding 1m 3 / s until the water in the ballast tank 3 is completely discharged, and the immersed tube 1 completes a 90° flip, and tightens the mooring cable at the right mooring point;

[0066] S5. Pour the new construction section of the joint;

[0067] Judge whether there is a time period with a sea state level not greater than 3 for 8 consecutive days in the recent half month at the construction site. If so, continue at the construction site. If not, tow it to a surrounding sea area that meets the requirements as a new construction site; In this embodiment, after the first synchronous flip, it is still at the original construction site; In this embodiment, after the first synchronous flip, it is still at the original construction site; Take the side floating on the water surface after the synchronous flip as the new construction section, and then repeat step S3;

[0068] S6. Second synchronous flip of the immersed tube;

[0069] The second synchronous flip of the immersed tube 1 includes injecting water with a total volume of V 3 into all the ballast tanks 3 in the immersed tube 1, so that the height of the immersed tube 1 floating on the water surface does not exceed 5m, then the total volume V 3 is determined according to the following formula

[0070] V 3 ≥2[LH(B - h 2 ) - M 2 / ρ 1 = 658.8m3 (13)

[0071] Wherein, V 3 - Total volume of water injection, m 3 , L, B, H - Length, width and height of a single - section immersed tube, m, h 2 - Threshold height of the immersed tube floating on the water surface, m, M 2 - Weight of a single - section immersed tube, kg, ρ 1 - Seawater density, kg / m 3 ; The total volume V 3 is evenly distributed to each ballast tank 3. A towing cable 9 is installed at the rigid connection point of the steel grid at the upper - right corner of the immersed tube 1. The towing cable 9 bypasses the top of the immersed tube 1 and is connected to the tugboat on the left side of the immersed tube 1. A towing cable 10 is installed at the rigid connection point of the steel grid at the lower - left corner of the immersed tube 1. The towing cable 10 bypasses the bottom of the immersed tube 1 and is connected to the tugboat on the right side of the immersed tube 1. The left - hand tugboat is 20 m away from the immersed tube 1 and pulls the towing cable 9 forward to the left. The right - hand tugboat is 50 m away from the immersed tube 1, and this distance needs to satisfy not less than B - h 3 + 5 m, where h 3 is the actual height of the immersed tube 1 floating on the water surface. The right - hand tugboat pulls the towing cable 10 forward to the right. Since the metacenter of the immersed tube 1 is below the center of gravity in the capsized state and is in an unstable state, only the tugboats are evenly arranged with four tugboats on each side of the two sections of the immersed tube 1, a total of 8 tugboats. The towing force of the tugboats is not less than 100 t to complete the second 90° flip of the immersed tube 1;

[0072] S7. Repeat pouring and flipping;

[0073] The repeated pouring and flipping includes repeating step S5 to complete the concrete pouring and curing of the new construction section, then repeating step S4 to complete the third synchronous flip. After flipping, repeat step S5 to complete the concrete pouring and curing of the new construction section, and finally repeat step S6 to flip the immersed tube 1 into the initial state, that is, the state where the ballast tank 3 is on the bottom surface of the immersed tube 1. After the concrete of the last pour has completed 28 - day age - period curing, the steel grid 2 is removed and the assembled immersed tube 1 is transported to the submarine tunnel construction site.

[0074] In summary, the in - situ casting construction method for the connection joints of precast immersed tubes in the semi - submerged floating state of the present invention has the characteristics of being convenient for construction and saving construction costs in the field of precast immersed tube construction.

[0075] It should be understood that the above - mentioned embodiments are one or more embodiments of the present invention. Based on the present invention, there are many other embodiments and their deformations; when ordinary technicians in this industry do not make pioneering innovations, the deformations and modifications made through the present invention all fall within the protection scope of the present invention.

Claims

1. A method for casting in situ the connection seam of a semi-submerged prefabricated immersed tube, characterized in that The specific steps include: S1. Construction preparation; The construction preparation includes obtaining the meteorological conditions of the sea area surrounding the construction site of the submarine tunnel, selecting the calmest sea area with a sea condition level not greater than level n as the construction site for the cast-in-place connection joint, and the sea condition level of the construction site is not greater than level n in the next T1 days, and then transporting two sections of semi-submerged prefabricated immersed tubes to be connected to the construction site, the length, width and height of the two sections of the immersed tubes are L, B and H, and the weight is M2. The initial ballast water volume in the ballast water tank of each section of the immersed tube is V0, and at the same time, the height of the immersed tube floating above the sea level is greater than 1.5 times the wave height of the sea condition level n, and the side of the immersed tube floating on the water surface is taken as the current construction section, and the steel bars of the construction section are removed from the protective cover, rust-removed, and then connected; S2, installation of connecting seam steel grid; The installation of the connecting seam steel grid comprises arranging a steel grid around the outer circle of the connecting seam, the width of the connecting seam is b, the steel grid extends l in the length direction of the two sections of the immersed tube, the length direction of the immersed tube is consistent with the length direction of the ship, then the length of the steel grid in the length direction of the immersed tube is 2l+b, and the steel grid is rigidly connected to the embedded parts on the immersed tube; S3, pouring of the connection joint construction section; The construction section of the connection joint is poured, including the use of a water retaining groove in the construction section to isolate the working surface of the connection joint from seawater, the side of the water retaining groove in contact with seawater requires a waterproof and anti-corrosion coating, the two ends of the water retaining groove are connected to the steel grid by bolts, the bottom surface of the water retaining groove is lower than the lower surface of the top of the immersed tube by at least a threshold value h1, the height of the water retaining groove does not exceed the upper surface of the top of the immersed tube, during the erection of the formwork and support system of the construction section, the bottom formwork load of the construction section is transferred to the end sealing door by an inclined brace, the end sealing door is reinforced on the waterless side inside the immersed tube, and after the erection of the formwork and support system is completed, concrete pouring and curing are carried out; S4, the immersed tube is turned synchronously for the first time; The first synchronous turning of the immersed tube includes releasing the mooring point constraints on the left side of the two sections of the immersed tube after the construction section has been cured for 7 days, and filling the ballast water tank on the left side with water at a speed not exceeding Q, and the water filling volume satisfies the following formula: V1≤2(LBH-M2 / ρ1-V0) (1) Where, V1-maximum water injection volume, m 3 , L, B, H-length, width and height of a single section of immersed tube, m, M2-weight of a single section of immersed tube, kg, ρ1-density of seawater, kg / m 3 ; While injecting water, release the mooring cable wrapped around the right mooring point, so that the immersed tube is synchronously tilted to the left, fill the ballast water tank with water, and after the immersed tube is stable, the left side of each section of the immersed tube is pressed down by the pressure arm of the ship, and the right side is pulled up by the crane ship with a hoisting cable, so that the immersed tube is further turned over in the tilted state, and then the water in the ballast water tank is discharged at a speed not exceeding Q, until the water in the ballast water tank is completely discharged, the immersed tube completes a 90° turn, and the mooring cable of the right mooring point is tightened; S5, pouring of new construction section of connecting joint; The pouring of the new construction section of the connection seam includes judging whether there is a period of time in the construction site within the last T2 days in which the sea condition level is not greater than level n for T1 consecutive days, if so, continuing at the construction site, if not, towing to a surrounding sea area that meets the requirements as a new construction site; taking the side that floats on the water after synchronous flipping as the new construction section, and then repeating step S3; S6, the immersed tube is turned over synchronously for the second time; The second synchronous turning over of the submerged tube includes injecting water with a total volume V3 into all ballast water tanks in the submerged tube so that the height of the submerged tube floating on the water surface does not exceed the threshold value h2. The total volume V3 is determined according to the following formula: V3≥2[LH(B-h2)-M2 / ρ1] (2) Where, V3-total volume of water injection, m 3 , L, B, H-length, width and height of a single section of the submerged tube, m, h2-height threshold of the submerged tube floating on the water surface, m, M2-weight of a single section of the submerged tube, kg, ρ1-seawater density, kg / m 3 The total volume V3 is evenly distributed to each ballast water tank. A traction cable is installed at the rigid connection point of the upper right angle steel grid of the immersed tube. The traction cable bypasses the top of the immersed tube and is connected to the tugboat on the left side of the immersed tube. A traction cable is installed at the rigid connection point of the lower left angle steel grid of the immersed tube. The traction cable bypasses the bottom of the immersed tube and is connected to the tugboat on the right side of the immersed tube. The left tugboat is at a distance of not less than h2+5m from the immersed tube and moves forward to the left to pull the traction cable. The right tugboat is at a distance of not less than B-h3+5m from the immersed tube, where h3 is the actual height of the immersed tube floating on the water surface. The right tugboat moves forward to the right to pull the traction cable to complete the second 90° flip of the immersed tube. S7, repeat pouring and turning; The repeated pouring and flipping include repeating step S5 to complete the concrete pouring and curing of the new construction section, then repeating step S4 to complete the third synchronous flipping, repeating step S5 after flipping to complete the concrete pouring and curing of the new construction section, and finally repeating step S6 to flip the immersed tube to its initial state, that is, the state of the ballast water tank on the bottom surface of the immersed tube. After the last poured concrete completes 28 days of curing, the steel grid is dismantled and the assembled immersed tube is transported to the submarine tunnel construction site.

2. The method for casting in situ the connection seam of a semi-submerged prefabricated immersed tube according to claim 1 is characterized in that: In step S4, the pressure arm pressure and the pulling force of the crane vessel are determined according to the external moment M required for the submerged tube to capsize. t Sure, M t ≥M r,max (4) Where, i is the i-th crane ship, n is the total number of crane ships, F is the pressure of the pressure arm and the tension of the crane ship, N, e is the force arm from the pressure and tension point to the center of gravity of the submerged tube, m, M r,max - Maximum restoring moment, N·m; the maximum restoring moment M r,max Determined by the lateral force arm GZ between the center of gravity and the center of buoyancy within the range of the inclination angle θ of 0 to 90°, M r =GZ×M2g (5) Where M r - Restoring torque, N·m.

3. The method for casting in situ the connection seam of a semi-submerged prefabricated immersed tube according to claim 1 is characterized in that: In step S6, the height threshold h2 of the submerged tube floating on the water surface is 5m.

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