Construction method for back-pressure placement of double-wall steel cofferdam into compacted silt

Abstract

The present application discloses a construction method for back-pressure placement of a double-wall steel cofferdam into compacted silt, including the following steps: constructing a trestle bridge:

Description

Technical Field The present invention relates to the field of placement construction of double-wall steel cofferdams, and in particular to a construction method for back-pressure placement of a double-wall steel cofferdam into compacted silt. Background Art Double-wall steel cofferdam is a steel structure composed of inner and outer steel shells, and several sections of vertical square steel and several layers of circular horizontal trusses located in an interlayer between the inner and outer steel shells. Tire vertical square steel and the horizontal trusses are welded to the imrer and outer steel shells to reinforce the double-wall steel cofferdam. In order to make a bridge pier in a river, the double-wall steel cofferdam is usually mounted in position, and then the water inside the double-wall steel cofferdam is pumped out to block the water through the double-wall steel cofferdam to form a space for fabricating the bridge pier. The existing construction method for the placement of the double-wall steel cofferdam usually includes firstly constructing a trestle bridge connecting the shore and the mounting position of the double-wall steel cofferdam, then transporting blocks manufactured by the factory for fabricating the double-wall steel cofferdam to the construction area on the shore for assembling, welding the connecting parts of the blocks, then lifting the double-wall steel cofferdam to a transport ship through a gantry crane, then starting the transport ship to transport the double-wall steel cofferdam to a position close to the mounting position, then lifting the double-wall steel cofferdam to the mounting position through a floating crane, and finally pouring cement concrete into the interlayer of the double-wall steel cofferdam through a concrete mixer truck until the double-wall steel cofferdam is placed in position. However, in the existing construction method for the placement of tire double-wall steel cofferdam, when the double-wall steel cofferdam is placed into compacted silt, due to the high density’ and viscosity of the compacted silt, it is difficult for the double-wall steel cofferdam to break through the compacted silt. As a result, it is difficult to place and mount the double-wall steel cofferdam in position into the compacted silt under its own gravity and the gravity of the cement concrete poured into the interlayer of the double-wall steel cofferdam, thus greatly reducing the efficiency of placing tire double-wall steel cofferdam into the compacted silt. Therefore, there is still room for improvement in this method. Summary of the Invention In order to improve the efficiency of placing a double-wall steel cofferdam into compacted silt, the present application provides a construction method for back-pressure placement of a double-wall steel cofferdam into compacted silt. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt provided in the present application adopts the following technical solution: A construction method for back-pressure placement of a double-wall steel cofferdam into compacted silt, including the following steps: step SI: constructing a trestle bridge; step S2: inserting and driving pile casings; step S3: fabricating the double-wall steel cofferdam; step S4: transporting the double-wall steel cofferdam to a position close to a mounting position; step S5: lifting the double-wall steel cofferdam; step S6: placing the double-wall steel cofferdam to a bed; step S7: mounting brackets to the pile casings, where one ends of the brackets are fixed to side walls of the pile casings and the other ends are located above the double-wall steel cofferdam; and step S8: mounting fixed rods for preventing the double-wall steel cofferdam from moving in a direction perpendicular to a horizontal plane to the pile casings, where one ends of the fixed rods are fixedly comiected with the side walls of the pile casings and the other ends are abutted against an inner side of the double-wall steel cofferdam, then placing jacks between the brackets and the double-wall steel cofferdam, using tire jacks to increase the pressure between the brackets and the double-wall steel cofferdam, and placing the double-wall steel cofferdam in position. By adopting the above technical solution, the brackets are fixed to the pile casings, the pile casings are used as the support points of the brackets, and then the pressure between the brackets and the double-wall steel cofferdam is increased through the jacks, thus increasing the pressure between the double-wall steel cofferdam and the compacted silt, making the double-wall steel cofferdam break through the compacted silt rmder the pressure of the jacks, allowing the double-wall steel cofferdam to be placed in position into the compacted silt, reducing the situations of water backflow from the bottom of the double-wall steel cofferdam and shaking of the double-wall steel cofferdam under the impact of water during subsequent construction due to the difficulty in placing the double-wall steel cofferdam in position due to the obstruction of the compacted silt, and improving the stability of the double-wall steel cofferdam. At the same time, by fixedly connecting one ends of the fixed rods with the side walls of the pile casings, and abutting the other ends against the steel shell on the inner side of the double-wall steel cofferdam, when the jacks apply back pressure to the double-wall steel cofferdam to break through the compacted silt and make it placed, the fixed rods restrict the double-wall steel cofferdam to place it along the direction perpendicular to the horizontal plane into the compacted silt, thus reducing the situation that the double-wall steel cofferdam tilts since the double-wall steel cofferdam easily tilts towards positions with smaller viscosity in the compacted silt due to the different viscosities of the compacted silt at different positions in the process that the jacks apply acting force to the double-wall steel cofferdam to drive and place it into the compacted silt, and reducing the work of readjusting the tilt of the double-wall steel cofferdam since the double-wall steel cofferdam tilts when the jacks apply back pressure to place the double-wall steel cofferdam. Alternatively, at least one bracket is fixed to each pile casing, and each bracket is provided with an independent jack. By adopting the above technical solution, at least one bracket is mounted on each pile casing, each pile casing is used as a support point for the jack through the bracket, and then the reaction force of the jack against the double-wall steel cofferdam is dispersed on each pile casing through the bracket, so that all pile casings share the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, thus reducing the situation that the pile casings get loose relative to the compacted silt under the reaction force when the jacks apply back pressure to the double-wall steel cofferdam since a single pile casing bears the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, and maintaining the stability of the pile casings in the process that tire jacks apply back pressure to the double-wall steel cofferdam. At the same time, by mounting at least one bracket on each pile casing and making each bracket provided with an independent jack, the acting force of the jacks is dispersed at multiple points of the double-wall steel cofferdam, thus reducing the situation that the stressed point of the double-wall steel cofferdam is deformed under the acting force of the jacks since a single point of the double-wall steel cofferdam is subjected to the acting force of the jacks, and reducing the need for repair due to deformation of the double-wall steel cofferdam under the acting force of the jacks. Simultaneously distributing the jacks to multiple points of the double-wall steel cofferdam makes it easier for the double-wall steel cofferdam to break through the compacted silt under the acting force at multiple points, thus improving the efficiency of placing the double-wall steel cofferdam in position into the compacted silt. Alternatively, in step S7, firstly through holes for the brackets to pass through are formed in the side walls of the pile casings, then the brackets are inserted into tire through holes, and then the brackets are fixed to the side walls of tire pile casings. By adopting the above technical solution, the brackets themselves have rigidity, so that the connections between the brackets and between the brackets and tire pile casings share the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, thus reducing the situation that the connections between the brackets and the pile casings are easily fractured under the reaction force when the jacks apply back pressure to double-wall steel cofferdam since the connections between the brackets and the pile casings independently bear the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, reducing the need for re-fixation due to fracture at the connections between the brackets and the pile casings, and improving the stability of the connections between the brackets and the pile casings. Alternatively, in step S7, each bracket is composed of several sections of partitioned steel; after each through hole is formed in the pile casing, firstly the partitioned steel is assembled to two ends of the through hole along a length direction, then the partitioned steel is placed at a middle position of the through hole along the length direction until the partitioned steel is assembled into the bracket, and finally connections between the partitioned steel and tire pile casing and between the partitioned steel and the partitioned steel are fixed. By adopting the above technical solution, the weight of the entire bracket is large and the several sections of partitioned steel are sequentially mounted to the pile casings, thus reducing the situation that it is not easy to align the brackets to the through holes since the shape of the through holes is the same as the shape of the side surfaces of the ends of the brackets along the length direction and the weight of the brackets are large in the process of inserting and extending the entire brackets into the through holes, and improving the efficiency of mounting the brackets to the pile casings. At the same time, by making each bracket composed of several sections of partitioned steel, the number of people required for transporting and mounting the brackets is reduced since it is easier to handle the partitioned steel than the brackets, reducing the situation that hurt is caused due to mutual collision between working personnel since the top space of the interlayer of the double-wall steel cofferdam in (he process of handling and mounting the brackets, and improving the construction safely. Alternatively, in step S7, after the brackets are mounted to the pile casings, diagonal struts are used to fixedly connect the ends of the brackets away from the pile casings with the side walls of the pile casings. By adopting the above technical solution, the acting force applied by the jacks to the brackets is distributed to the connections between the diagonal stmts and the pile casings and the connections between the brackets and the pile casings through the diagonal stmts, so that the connections between the diagonal stmts and the pile casings and the connections between the brackets and the pile casings share the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, thus reducing the situation that the connections between the brackets and the pile casings are fractured due to the large reaction force when the jacks apply back pressure to the double-wall steel cofferdam. At the same time, the diagonal stmts are used to support the ends of the brackets far from the pile casings, thus improving the rigidity of the brackets and reducing the situation that the brackets are fractured under the reaction force when the jacks apply back pressure to the double-wall steel cofferdam. Alternatively, in step S7, after the brackets and the diagonal struts are mounted to the pile casings, triangular plates for reinforcing the brackets and the diagonal stmts are fixed to connections between the brackets, the pile casings and the diagonal stmts. By adopting the above technical solution, triangle has stability. By fixing the triangular plates at the connections between the brackets, the pile casings and the diagonal stmts, the stability of the connections between the brackets, the pile casings and the diagonal stmts can be improved, thus reducing the situation that the connections between the brackets, the pile casings and the diagonal stmts are easily fractured due to the large reaction force when the jacks apply back pressure to the double-wall steel cofferdam, and improving the stability of the bracket, the diagonal stmt and the pile casing. Alternatively, in step S7, firstly an iron net is mounted to a top of the double-wall steel cofferdam, then through holes are formed in the side walls of the pile casings, and finally the brackets are mounted. By adopting the above technical solution, the iron net is mounted to the top of the double-wall steel cofferdam, so that the working personnel can walk on the top of the interlayer of the double-wall steel cofferdam, helping the working personnel to carry out the construction. At the same time, since the weight of the iron net is lighter than that of the steel plate, it can be mounted more easily. Moreover, since the rigidity of the iron net is higher than that of the plastic plate, it is beneficial for improving the stability of the working personnel during walking. Alternatively, in step S7, after the brackets, diagonal stmts and triangular plates are mounted, steel platforms are mounted on the top of the double-wall steel cofferdam, the steel platforms are fixed to the top of the double-wall steel cofferdam, and steel plates are located directly below the brackets. By adopting the above technical solution, when the jacks need to be placed, the jacks are placed on the steel platforms, so that the jacks can transmit the acting force when the jacks apply back pressure to the double-wall steel cofferdam to the double-wall steel cofferdam through the steel platforms, thus reducing the situation that it is difficult to place the jacks since there is an interlayer in the double-wall steel cofferdam. To sum up. the present application has at least one of the following beneficial technical effects: 1. The brackets are fixed to the pile casings, the fixed rods for preventing the double-wall steel cofferdam from moving in the direction perpendicular to the horizontal plane are arranged, and tire pressure between the brackets and the double-wall steel cofferdam is increased through the jacks, thus making the double-wall steel cofferdam break through the compacted silt under the pressure of the jacks, allowing the double-wall steel cofferdam to be placed in position into the compacted silt, reducing the situations of water backflow from the bottom of the double-wall steel cofferdam and shaking of the double-wall steel cofferdam under the impact of water during subsequent construction due to the difficulty in placing the double-wall steel cofferdam in position due to the obstruction of the compacted silt, and improving the stability' of the double-wall steel cofferdam. At the same time, by fixedly connecting one ends of the fixed rods with the side walls of the pile casings, and abutting the other ends against the steel shell on the inner side of the double-wall steel cofferdam, when the jacks apply back pressure to the double-wall steel cofferdam to break through the compacted silt and make it placed, the fixed rods restrict the double-wall steel cofferdam to place it along the direction perpendicular to the horizontal plane into the compacted silt, thus reducing the situation that the double-wall steel cofferdam tilts since the double-wall steel cofferdam easily tilts towards positions with smaller viscosity in the compacted silt due to the different viscosities of the compacted silt at different positions in the process that the jacks apply acting force to tire double-wall steel cofferdam to drive and place it into the compacted silt, and reducing the work of readjusting the tilt of the double-wall steel cofferdam since the double-wall steel cofferdam tilts when the jacks apply back pressure to place the double-wall steel cofferdam. 2. At least one bracket is fixed on each pile casing, each bracket is provided with an independent jack, and the brackets run through the pile casings and the diagonal struts to fixedly connect the ends of the brackets away from the pile casings with the side walls of the pile casings, so that all pile casings share the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, thus reducing the situation that the pile casings get loose relative to tire compacted silt under the reaction force when tire jacks apply back pressure to the double-wall steel cofferdam since a single pile casing bears the reaction force when the jacks apply back pressure to the double-wall steel cofferdam, and maintaining the stability of the pile casings in the process that the jacks apply back pressure to the double-wall steel cofferdam. At the same time, the brackets run through the pile casings and the diagonal struts improve the rigidity of the brackets, thus reducing the situation that the connections between the brackets and the pile casings are fractured due to the large reaction force when the jacks apply back pressure to the double-wall steel cofferdam. Brief Description of the Drawings FIG. 1 is a schematic diagram of an overall structure of a double-wall steel cofferdam. FIG. 2 is an enlarged view of position A in FIG. 1. Description of reference signs: 11-iron net; 12-guard fence; 2-steel platform; 3-jack; 4-bracket; 5-diagonal strut; 6-triangular plate; 7-working platform; 8-pile casing; 81-square steel; 9-fixed rod. Detailed Description of the Invention The present application will be further described below in detail with reference to FIGS. 1-2. An embodiment of the present application discloses a construction method for back-pressure placement of a double-wall steel cofferdam into compacted silt. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt includes the following steps: In step SI, a steel casing is inserted and driven through a hydraulic vibration hammer to fabricate a support frame of a trestle bridge according to drawings, and steel plates, square steel 81 and I-shaped steel are welded to fabricate the trestle bridge. One end of the trestle bridge is communicated with the shore, and the other end is located near a mounting position of a double-wall steel cofferdam. In step S2, pile casings 8 are inserted and driven at the mounting position of the double-wall steel cofferdam through hydraulic vibration hammers according to drawings. In this embodiment, four pile casing 8 are provided. Then, a concrete mixer truck is driven through the trestle bridge to a position close to the pile casings 8, and then cement concrete is poured into the pile casings 8 through the concrete mixer track. In step S3, double-wall steel cofferdams are intensively fabricated in a factory according to drawings. In this embodiment, the double-wall steel cofferdam is in the shape of a square cylinder, the double-wall steel cofferdam is obliquely arranged, an inner frame of the double-wall steel cofferdam is obliquely arranged towards an outer frame, and the double-wall steel cofferdam is divided into four blocks according to four side walls. In step S4, the blocks in step S3 are transported to a construction site on the shore through a transport vehicle. The blocks are lifted for assembling through a gantry crane. Connections between the blocks are welded through electric welding to form a double-wall steel cofferdam. Then, the double-wall steel cofferdam is transported to a position close to the mounting position through a transport ship. The specific operation steps are as follows: In step S41, the blocks in step S3 are transported to a construction site on the shore through a transport vehicle. Steel wire ropes are threaded into horizontal trusses. Then, a hook of a gantry crane hooks lifting rings at two ends of the steel wire rope. The blocks are lifted to a fixing frame through the gantry crane. The blocks are fixed through tire fixing frame. Then, the blocks are sequentially mounted to the fixing frame through the gantry' crane to assemble the blocks on the fixing frame. In step S42, connections between the blocks are welded through electric welding to fabricate a double-wall steel coiferdam. In step S43, the double-wall steel cofferdam is lifted to a transport ship through the gantry crane. In step S44, the transport ship is started to transport the double-wall steel cofferdam to a position close to the mounting position. In step S5. the double-wall steel cofferdam is lifted to the mounting position through a floating crane. The specific operation steps are as follows: In step S51, the steel wire ropes are threaded into the horizontal trusses. Then, a lifting hook of the floating crane hooks the lifting rings at the two ends of the steel wire ropes. Four steel wire ropes are provided. The four steel wire ropes are respectively threaded into the horizontal trusses at the four comers of the double-wall steel cofferdam. The lifting rings of the four steel wire ropes are all hooked by the same lifting hook of the floating crane. In step S52, the floating crane is started to lift the double-wall steel cofferdam to a position 10cm away from the transport ship, and then it is suspended in air for trial lifting for 30min. In step S53, the transport ship is moved away from the mounting position. In step S54, the floating crane is operated to lift the double-wall steel cofferdam to a position 20cm above a water surface at the mounting position, and then it is suspended in air and stood for 30min. In step S55. the floating crane is operated and adjusted until the double-wall steel cofferdam is aligned to the mounting position. Then, the floating crane is operated to lower the double-wall steel cofferdam. In step S6, the concrete mixer truck is driven to the end of the trestle bridge close to the double-wall steel cofferdam. Then, cement concrete is poured into an interlayer of the double-wall steel cofferdam through the concrete mixer truck until tire double-wall steel cofferdam is placed to a bed. Then, the concrete mixer truck and the floating crane are removed. In step S7. firstly, an iron net 11 is mounted to a top of the interlayer of the double-wall steel cofferdam. Then, a working platform 7 for mounting brackets 4 is fabricated. Then, the brackets 4 and diagonal stmts 5 are mounted to the pile casings 8. The specific operation steps are as follows: In step S71, firefly, an iron net 11 is cut through gas cutting according to drawings. The iron net 11 is laid on the top of the interlayer of the double-wall steel cofferdam. Then, two sides of the iron net 11 along the length direction are respectively welded to the tops of the inner and outer steel shells of the double-wall steel cofferdam through electric welding. Then, a guard fence 12 is welded to the tops of the inner and outer steel shells of the double-wall steel cofferdam through electric welding. Then, floor beams for building a working platform 7 is fabricated by cutting a steel plate through gas cutting. The floor beams are placed to the top of the inner steel shell of the double-wall steel cofferdam. One ends of the floor beams are abutted against the top of the inner steel shell of the double-wall steel cofferdam. The other ends are abutted against the side walls of adjacent pile casings 8. Then, two ends of each floor beam are spot welded to the top of the inner steel shell of the double-wall steel cofferdam and the side wall of the pile casing 8 respectively through electric welding. Then, the two ends of each floor beam are fully welded to the top of the inner steel shell of the double-wall steel cofferdam and the side wall of the pile casing 8 respectively through electric welding. Then, the floor beams are assembled into the working platform 7. Finally, connections between the floor beams are fixedly welded through electric welding. In this embodiment, the working platform 7 is in the shape of a square frame, and the pile casings 8 are located within the working platform 7. In step S72, through holes are formed in each pile casing 8 through gas cutting according to drawings. In this embodiment, the through holes are rectangular in shape, their positions are higher than the highest point of the double-wall steel cofferdam, two through holes are formed, the two through holes face to each other, and one through hole is located in the side of the pile casing 8 close to the steel shell of the double-wall steel cofferdam. In step S73, I-shaped steel is cut through gas cutting according to drawings to fabricate partitioned steel for fabricating the brackets 4. In this embodiment, three sections of partitioned steel are provided. A crane is driven to the end of the trestle bridge close to the double-wall steel cofferdam. Then, the partitioned steel is inserted into the through holes for assembling. Then, two sections of partitioned steels are respectively mounted to two ends of the through hole along the length direction. Then, one section of partitioned steel is mounted to a middle position of the through hole along the length direction. Finally, connections between the partitioned steel and the pile casing 8 and between the partitioned steel and the partitioned steel are welded through electric welding to fabricate the bracket 4. In step S74, using the bracket 4 as a standing point, square steel 81 for working personnel to stand when mounting diagonal struts is welded to positions close to the connections between diagonal struts 5 and the pile casing 8 according to drawings. I-shaped steel is cut through gas cutting to fabricate diagonal struts 5. In this embodiment, two diagonal struts 5 are mounted on each bracket 4. In step S75, the crane is driven to the end of the trestle bridge close to the double-wall steel cofferdam. The diagonal struts 5 are lifted to the bracket 4 through the crane. The working personnel cooperate with the crane to abut the two ends of tire diagonal struts 5 against the end of the bracket 4 away from the pile casing 8 and the side wall of the pile casing 8 higher than the bracket 4 respectively. Then, abutting positions between the diagonal struts 5 and the bracket 4 and abutting positions between the bracket 4 and the pile casing 8 are welded through electric welding. In step S76. triangular plates 6 are cut from steel plates through gas cutting. Then, the triangular plates 6 are welded to the connections between the brackets 4, the pile casings 8 and the diagonal struts 5. In step S77, I-shaped steel is cut through gas cutting to fabricate steel platforms 2. In this embodiment, four steel platforms 2 are provided. The crane is driven to the end of the trestle bridge close to the double-wall steel cofferdam. The four steel platforms 2 are respectively lifted to positions directly below the four brackets 4 through the crane. The steel platforms 2 are adjusted until two ends of the steel platforms 2 along the length direction are respectively abutted against the tops of the inner and outer steel shells of the double-wall steel cofferdam. Then, the steel platforms 2 are welded to the steel shells of the double-wall steel cofferdam through electric welding. In step S8, the pressure between the brackets (4) and the double-wall steel cofferdam is increased through jacks (3) to place the double-wall steel cofferdam under back pressure. In this embodiment, four jacks (3) are provided. The specific operation steps are as follows: In step S81, rectangular steel is cut through gas cutting to fabricate fixed rods (9). In this embodiment, eight fixed rods (9) are provided, two fixed rods (9) are fixed to each pile casing (8), and the length directions of the fixed rods (9) fixed to the same pile casing (8) are perpendicular to each other. The fixed rods (9) are mounted to the pile casings (8). One ends of the fixed rods (9) are abutted against the side walls of the pile casings (8), and the other ends are abutted against the inner steel shell of the double-wall steel cofferdam. Then, abutting positions between the fixed rock (9) and the side walls of the pile casings (8) are welded through electric welding. In step S82. four jacks (3) are respectively placed at the tops of the four steel platforms (2). In step S83, four working persomrel respectively operate the four jacks (3) until the jacks (3) jack to the side of the brackets (4) close to the jacks (3). In step S84, the placement of the double-wall steel cofferdam is checked. The double-wall steel cofferdam is adjusted by operating the jacks (3) until a top plane of the double-wall steel cofferdam is in parallel with a horizontal plane. In step S85. the four jacks (3) are simultaneously operated until the double-wall steel cofferdam is placed in position. In step S86. the jacks (3) are retracted. What are described above are just exemplary embodiments of the present application, which, however, do not limit the scope of protection of the present application. Therefore, any equivalent changes made according to the structure, shape, and principle of the present application should be included in the scope of protection of the present application.

Claims

1. A construction method for back-pressure placement of a double-wall steel cofferdam into compacted silt, characterized by comprising the following steps:step SI: constructing a trestle bridge;step S2: inserting and driving pile casings (8);step S3: fabricating the double-wall steel cofferdam;step S4: transporting the double-wall steel cofferdam to a position close to a mounting position;step S5: lifting the double-wall steel cofferdam:step S6: placing the double-wall steel cofferdam to a bed;step S7: mounting brackets (4) to the pile casings (8), wherein one ends of the brackets (4) are fixed to side walls of the pile casings (8) and the other ends are located above the double-wall steel cofferdam; andstep S8: mounting fixed rods (9) for preventing the double-wall steel cofferdam from moving in a direction perpendicular to a horizontal plane to the pile casings (8). wherein one ends of the fixed rods (9) are fixedly connected with the side walls of the pile casings (8) and the other ends are abutted against an inner side of the double-wall steel cofferdam, then placing jacks (3) between the brackets (4) and the double-wall steel cofferdam, using the jacks (3) to increase the pressure between the brackets (4) and the double-wall steel cofferdam, and placing the double-wall steel cofferdam in position.

2. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 1, characterized in that at least one bracket (4) is fixed to each pile casing (8), and each bracket (4) is provided with an independent jack (3).

3. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 2, characterized in that in step S7, firstly through holes for the brackets (4) to pass through are formed in the side walls of the pile casings (8), then the brackets (4) are inserted into the through holes, and then the brackets (4) are fixed to the side walls of the pile casings (8).

4. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 3, characterized in that in step S7, each bracket (4) is composed of several sections of partitioned steel; after each through hole is formed in the pilecasing (8), firstly the partitioned steel is assembled to two ends of the through hole along a length direction, then the partitioned steel is placed at a middle position of the through hole along the length direction until the partitioned steel is assembled into the bracket (4), and finally connections between the partitioned steel and the pile casing (8) and between the partitioned steel and the partitioned steel are fixed.

5. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 4, characterized in that in step S7, after the brackets (4) are mounted to the pile casings (8), diagonal stmts (5) are used to fixedly coimect the ends of the brackets (4) away from the pile casings (8) with the side walls of the pile casings (8).

6. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 5, characterized in that in step S7, after the brackets (4) and the diagonal struts (5) are mounted to the pile casings (8), triangular plates (6) for reinforcing the brackets (4) and the diagonal struts (5) are fixed to coimections between the brackets (4), the pile casings (8) and the diagonal struts (5).

7. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 1, characterized in that in step S7, firstly an iron net (11) is mounted to a top of the double-wall steel cofferdam, then through holes are formed in the side walls of tire pile casings (8), and finally the brackets (4) are mounted.

8. The construction method for back-pressure placement of the double-wall steel cofferdam into compacted silt according to claim 7, characterized in that in step S7, after the brackets (4), diagonal struts (5) and triangular plates (6) are mounted, steel platforms (2) are mounted on the top of the double-wall steel cofferdam, the steel platforms (2) are fixed to the top of the double-wall steel cofferdam, and steel plates are located directly below the brackets (4).14

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