Double-column type step open caisson suitable for long-span bridge and supporting structure

Abstract

The invention discloses a double-column type step open caisson suitable for a long-span bridge and a supporting structure, and relates to the field of bridge open caisson foundations. The double-column type step open caisson suitable for the long-span bridge comprises a step assembly and a scouring reduction assembly; the open caisson lower step is provided with a plurality of axial through type well holes, the double-column upper step extends upwards from the top face of the open caisson lower step, the interior of the double-column upper step is hollow, the double-column upper step is communicated with the well holes of the open caisson lower step, and the double-column upper step comprises two protruding column bodies which are arranged in a spaced mode; the first flow guide units are arranged at the bottoms of the outer sides of the two protruding columns, the second flow guide unit is located at the bottom between the two protruding columns, each first flow guide unit comprises a sharp end used for shearing water flow, and concave curved surfaces used for guiding the sheared water flow are arranged on the two sides of each second flow guide unit. The self weight can be controlled, consumables are reduced, water flow scouring on the upstream side can be improved, and sediment accumulation on the downstream side can be reduced.

Description

Technical Field

[0001] The invention relates to the field of bridge caisson foundations, and in particular to a double-column step caisson and a supporting structure suitable for large-span bridges. Background Art

[0002] As the foundation of a bridge, the deadweight of the caisson accounts for a large proportion of the bridge load, especially when the bearing layer of the base is weak, the deadweight of the caisson accounts for more than 50% of the bridge load. Nowadays, the span of bridges is increasing and the conditions for bridge construction are becoming more complex. Therefore, in order to meet higher construction requirements and make bridge structures safe and reliable, the requirements for scour reduction and bearing capacity of caisson foundations are also increasing. In order to meet the bearing requirements of large-span bridges, the following measures are often taken when designing caisson foundations: (1) expanding the cross-sectional area of ​​the caisson structure to use a larger base area of ​​the caisson structure to bear the load of large-span bridges; (2) increasing the burial depth of the caisson structure to make the bearing capacity per unit area of ​​the caisson base stronger.

[0003] In past engineering practices, both expanding the cross-sectional area of ​​the caisson and increasing the burial depth of the caisson have been widely used and the technology is relatively mature. However, both of the above methods will significantly increase the cost of bridge construction, especially after the cross-sectional area of ​​the caisson structure is expanded, which also means that the caisson structure will be subject to greater water flow scouring force. At the same time, after the caisson structure sinks, it will be scoured by water flow for a long time, forming scour pits at the front end of the caisson structure and siltation at the rear end of the caisson structure. This imbalance of local scouring and silting at the front and back will increase the risk of dismantling the external cofferdam on the top surface of the step, and may even lead to uneven settlement during the operation period, causing safety hazards. Summary of the invention

[0004] The purpose of the present application is to provide a double-column stepped caisson and supporting structure suitable for large-span bridges, so as to solve the technical problem that the cross-section of the caisson structure in the large-span bridge in the prior art is increased accordingly, which not only leads to increased construction costs, but also the larger cross-section of the caisson structure will form scour pits at the front end and siltation at the rear end after being impacted by water flow, resulting in uneven settlement and causing safety hazards.

[0005] In a first aspect, the present application provides a double-column stepped caisson suitable for a long-span bridge, comprising:

[0006] A step assembly, the step assembly is used to be arranged in the direction of water flow, and comprises a caisson lower step having a plurality of axial through-type well holes, and a double-column upper step extending upward from the top surface of the caisson lower step, which is hollow inside and connected with the well holes of the caisson lower step, and the double-column upper step comprises two protruding columns spaced apart along the length direction of the caisson lower step;

[0007] The scour reduction brush assembly includes a first guide unit arranged at the bottom of the outer sides of the two protruding columns and a second guide unit located at the bottom between the two protruding columns, the first guide unit includes a sharp end for shearing water flow, the sharp ends of the two first guide units are respectively used to face the water front side and the water back side, and the second guide unit has concave curved surfaces on both sides for guiding the sheared water flow.

[0008] In one embodiment, both the water-facing side and the water-receiving side of the lower step of the caisson are provided with arc end surface flow ends.

[0009] In one embodiment, a caisson bottom sealing area is provided at the bottom of the lower step of the caisson.

[0010] In one embodiment, a step cover plate for sealing the well hole is provided at the top of the lower step of the caisson.

[0011] In one embodiment, a top pedestal for connecting to the structure above the bridge is provided at the top of the steps on the double columns.

[0012] In one embodiment, the horizontal cross-sectional area of ​​the bottom of the top platform is larger than the horizontal cross-sectional area of ​​the protruding column.

[0013] In one embodiment, the first guide unit includes two guide plates that are oppositely disposed, and one side of the two guide plates are separated from each other, and the other side is gathered to form a sharp end.

[0014] In one embodiment, the second flow guiding unit includes two flow guiding plates arranged opposite to each other.

[0015] In one embodiment, the guide plate has a certain curvature and the curved surfaces of the two guide plates are close to each other to form a concave curved surface.

[0016] In a second aspect, an embodiment of the present application provides a bridge support structure, which includes the above-mentioned double-column stepped caisson, in which a bridge tower for supporting the bridge is provided on the top of the upper steps of the double columns in the double-column stepped caisson.

[0017] The beneficial effects brought by the technical solution provided in the embodiments of the present application include:

[0018] 1. The double-column step caisson provided in the present application, which is suitable for long-span bridges, has a lower step for supporting the upper steps of the double columns. Compared with the traditional method of simply expanding the cross-sectional area of ​​a single caisson, this type of structure has better economic benefits while meeting the stability requirement.

[0019] 2. The cross-sectional size of the upper step of the caisson in the double-column step caisson suitable for large-span bridges provided by the present application is smaller than that of the lower step of the caisson. On the one hand, it can reduce the amount of materials used and save capital investment. On the other hand, the reduction in water blocking area brought about by the double-column structure can effectively reduce scouring. When the water flows through the rear column, it will accelerate the scouring of the rear end again due to the reduction in the water-passing cross-section. In addition, the step structure formed by the upper step of the double column and the lower step of the caisson can consume the energy of the water flow in the form of shear when the water flows through the double-column step caisson in the present application.

[0020] 3. The double-column stepped caisson provided in the present application and suitable for large-span bridges is provided with a first guide unit and a second guide unit, wherein the first guide unit at the front end effectively suppresses the formation of downcutting water flow and can effectively reduce the scouring of the riverbed by the horseshoe vortex, and the second guide unit and the first guide unit at the rear end can shear and guide the water flow, further reducing siltation at the rear end. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0022] Figure 1 A schematic diagram of a double-column stepped caisson structure suitable for a long-span bridge provided in an embodiment of the present application;

[0023] Figure 2 A schematic diagram of another implementation of a protruding column in a double-column stepped caisson suitable for a long-span bridge provided in an embodiment of the present application;

[0024] Figure 3 A cross-sectional view of a double-column stepped caisson suitable for a long-span bridge provided in an embodiment of the present application;

[0025] Figure 4 A top view of a double-column stepped caisson suitable for a long-span bridge provided in an embodiment of the present application;

[0026] Figure 5 A schematic diagram of a bridge support structure provided in an embodiment of the present application.

[0027] In the figure: 1. lower step of caisson; 101. flow end of arc end face; 102. step cover; 2. upper step of double columns; 201. protruding column; 3. first guide unit; 301. guide plate; 4. second guide unit; 401. guide plate; 5. bottom sealing area of ​​caisson; 6. top pedestal; 7. bridge tower. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the solution of the present application, the technical solution in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0029] On the first aspect, an embodiment of the present application provides a double-column stepped caisson suitable for large-span bridges, which can solve the technical problems in the prior art that the cross-section of the caisson structure in large-span bridges is increased, resulting in increased construction costs, and the large cross-section of the caisson structure will form scour pits at the front end and siltation at the rear end after being impacted by water flow for a long time, resulting in uneven settlement and causing safety hazards.

[0030] The double-column step caisson in the present application includes a step assembly and an scour reduction assembly, wherein the step assembly is used to form the basic structure of the stepped caisson in the present application and can significantly reduce the water-blocking area on the upper part of the foundation. The step assembly is buried in the riverbed to provide basic support for the bridge, and the upper steps of the double columns break through the riverbed and reach above the water surface, significantly reducing the water-blocking area of ​​the foundation and effectively improving the foundation scour situation. At the same time, when the water flow passes through the rear end of the step assembly, it can be accelerated again based on the structural effect of the scour reduction assembly to reduce the siltation at the rear end of the double-column step caisson. The scour reduction assembly is arranged on the step assembly, and the double-column step caisson in the present application is arranged along the flow direction as a whole. The scour reduction assembly can first contact the water flow, shear and guide the water flow, prevent it from accumulating on the periphery of the step assembly, reduce siltation, and further improve the foundation scour and siltation condition.

[0031] Specifically, Figure 1 A schematic diagram of a double-column stepped caisson structure suitable for a long-span bridge provided in an embodiment of the present application is shown in FIG. Figure 1 As shown, the step assembly in the present application includes a caisson lower step 1 having a plurality of axial through-type wellbores, and a double-column upper step 2 extending upward from the top surface of the caisson lower step 1 and having a hollow interior and communicating with the wellbores of the caisson lower step 1. The double-column upper step 2 includes two protruding columns 201 spaced apart.

[0032] The lower step 1 of the caisson adopts a hollow metal shell with concrete poured inside. It has a number of well holes that run through the lower step 1 of the caisson. This structure can not only greatly reduce the dead weight of the double-column step caisson, but also, during the sinking process, the well holes can serve as a channel for discharging mud and sand, thereby facilitating the double-column step caisson to sink to the designed depth. In a possible implementation manner of the present application, the cross-section of the lower step 1 of the caisson is in the shape of a long strip. When assembled, the lower step 1 of the caisson is arranged as a whole transverse to the bridge to conform to the direction of the water flow. The length of the long side of the lower step 1 of the caisson is not less than the width of the bridge, and the wide side of the lower step 1 of the caisson faces the direction of the water flow to reduce the scouring force of the water flow.

[0033] The two protruding columns 201 in the double-column upper step 2 are spaced apart along the length direction of the caisson lower step 1 and are fixedly connected to the caisson lower step 1. The material and manufacturing method of the double-column upper step 2 are consistent with those of the caisson lower step 1, and the top of the double-column upper step 2 extends vertically upward and protrudes a certain height from the surface of the caisson lower step 1 to form a step shape.

[0034] As an optional embodiment, Figure 2 A schematic diagram of another implementation of a protruding column 201 in a double-column stepped caisson suitable for a long-span bridge provided in an embodiment of the present application is shown in FIG. Figure 2 As shown, the cross section of the protruding column 201 can be a circle, a semicircle or any other shape, as long as the outer side of the protruding column 201 has a curved surface, which is not limited in the present application.

[0035] It should be noted here that the "outer side" in this article refers to the side where the two protruding columns 201 are away from each other, that is, the water-facing side and the water-receiving side, and the "inner side" refers to the side where the two protruding columns 201 are close to each other, which will not be repeated in the subsequent text.

[0036] Furthermore, the scour reduction assembly includes a first guide unit 3 disposed at the outer bottom of the two protruding columns 201 and a second guide unit 4 located at the bottom between the two protruding columns 201. The first guide unit 3 is open at the top and includes a sharp end for shearing the water flow. The sharp ends of the two first guide units 3 are respectively used to face the water-facing side and the water-receiving side. The second guide unit 4 is open at the top and has concave curved surfaces on both sides for guiding the sheared water flow.

[0037] A group of first guide units 3 are arranged on each protruding column 201, one end of the first guide unit 3 is fixedly connected to the outer side of the protruding column 201, and the other end forms a sharp end and protrudes toward the direction of the water flow. The lower step 1 of the caisson is arranged along the flow direction. Based on the structural setting, the two protruding columns 201 are also arranged front and back along the water flow direction, so that the sharp ends of the two first guide units 3 are respectively facing the upstream side and the downstream side. When the water flow drives the sediment to pass through the first guide unit 3 located on the upstream side, the first guide unit 3 will first separate the water flow to reduce the direct scouring of the protruding column 201 provided with the first guide unit 3, and the first guide unit 3 located on the downstream side can be used to assist in guiding the sheared water flow and reduce the accumulation of sediment at the rear end of the caisson.

[0038] The second guide unit 4 is arranged between the two protruding columns 201, and the length direction of the second guide unit 4 is also arranged along the water flow direction. The sheared water flow will pass through the concave curved surfaces on both sides of the second guide unit 4 respectively. Since the water-passing cross-section is reduced, the water flow is accelerated, so that the water flow produces scouring. At the same time, the concave curved surface can use the power of the flowing water to guide the water flow to the protruding column 201 at the rear end, and then conduct secondary guidance through the first guide unit 3 on the double-column upper step 2 at the rear end, and use the scouring force to further reduce the accumulation of sediment at the rear end.

[0039] The lower step 1 of the caisson and the upper step 2 of the double columns can control the basic deadweight of the double-column step caisson in the present application. Compared with the traditional method of simply expanding the cross-sectional area of ​​a single caisson, it has better stability and supporting force. At the same time, the setting of the upper step 2 of the double columns can not only reduce the overall material consumption of the structure and save capital investment, but also the step-shaped structure of the lower step 1 of the caisson. When water flows through the double-column step caisson in the present application, the energy of the water flow is initially consumed by the step structure in the form of shear. The cross-section of the upper step 2 of the double columns is relatively small, which can also reduce the impact of the water flow on the double-column step caisson as a whole in the present application. In combination with the first guide unit 3 and the second guide unit 4, the water flow can be further sheared and guided. At the same time, the sheared water flow can speed up its flow rate due to the reduction of the water cross-section, thereby generating a greater scouring force and further avoiding the accumulation of silt. The structures complement and interact with each other.

[0040] Furthermore, Figure 3 A cross-sectional view of a double-column stepped caisson suitable for a long-span bridge provided in an embodiment of the present application, such as Figure 3As shown, both the water-facing side and the water-receiving side of the lower step 1 of the caisson are provided with arc end surface flow ends 101. Combined with the above description, the narrower side of the lower step 1 of the caisson faces the direction of water flow, and the arc end surface flow ends 101 are arranged on the narrower sides of the lower step 1 of the caisson, so as to face the water-facing side and the water-receiving side. Based on this structure, the resistance at both ends when the water flows through the lower step 1 of the caisson can be reduced, and the stress caused by the water flow load can be more evenly distributed, thereby reducing the water flow scouring force on the whole double-column step caisson in the present application.

[0041] For further information, see Figure 3 A caisson bottom sealing area 5 is provided at the bottom of the lower step 1 of the caisson, and the caisson bottom sealing area 5 covers the entire bottom surface of the lower step 1 of the caisson, and is used to form a solid bottom sealing structure by pouring concrete. This bottom sealing structure not only significantly increases the force-bearing area and overall deadweight of the bottom surface of the lower step 1 of the caisson, enabling it to better disperse and withstand loads from above and the sides, thereby improving its overall stability, but also greatly enhances the anti-overturning ability of the double-column step caisson in the present application. Even under the action of external loads such as water flow, the double-column step caisson can remain stable and will not tilt or collapse due to uneven force.

[0042] At the same time, as an optional embodiment, the bottom surface of the lower step 1 of the caisson is divided into several pouring areas. When performing the bottom sealing operation, the caisson bottom sealing area 5 can be poured in different areas. The staff can adopt appropriate pouring strategies and equipment according to the characteristics and needs of different areas, so as to improve work efficiency. By pouring in different areas, it can be ensured that the concrete pouring in each area is more uniform. This helps to form a solid bottom sealing structure, improve the overall stability and bearing capacity, and at the same time, the partitioned pouring of concrete also helps quality control. The staff can monitor and manage the pouring process of each area more carefully to ensure that the quality and performance of the concrete meet the design requirements.

[0043] For further information, see Figure 3 A step cover plate 102 for sealing the well hole is provided on the top of the lower step 1 of the caisson to prevent mud and sand from entering the well hole. The cross section of the step cover plate 102 is consistent with the cross section of the lower step 1 of the caisson and a through hole is reserved for the protruding column 201 to pass through so as to tightly cover the lower step 1 of the caisson.

[0044] For further information, see Figure 3 The top of the double-column upper step 2 is provided with a top pedestal 6 for connecting to the structure above the bridge, and the horizontal cross-sectional area of ​​the bottom of the top pedestal 6 is larger than the horizontal cross-sectional area of ​​the protruding column 201, so that the top pedestal 6 completely covers the top of the double-column upper step 2. The top pedestal 6 with a larger cross-sectional area can increase the overall stability of the top pedestal 6 and improve the bearing capacity, prevent shaking or collapse caused by uneven force, and can more effectively distribute and transfer the load, so that the overall force of the top pedestal 6 is more uniform.

[0045] Furthermore, Figure 4 A top view of a double-column stepped caisson suitable for a long-span bridge provided in an embodiment of the present application, such as Figure 4 As shown, the first guide unit 3 includes two guide plates 301 that are oppositely arranged, and one side of the two guide plates 301 is separated from each other, and the other side is gathered to form a sharp end, the two guide plates 301 are longitudinally arranged along the height direction of the protruding column 201, and the one side of the two guide plates 301 that is separated from each other is respectively connected to the outer wall of one of the protruding columns 201, and the other side of the two guide plates 301 is gathered to form a sharp end, which is used to face the upstream side and the downstream side respectively.

[0046] For further information, see Figure 4 The second guide unit 4 includes two oppositely arranged guide plates 401. The guide plates 401 have a certain curvature and the curved surfaces of the two guide plates 401 are close to each other to form a concave curved surface. The two side edges of the guide plates 401 are respectively connected to the inner outer walls of the two protruding columns 201. This design helps to guide the sheared water flow to flow along a specific path and reduce the accumulation of sediment at the rear end of the caisson.

[0047] When constructing the double-column step caisson in the present application, first construct the caisson lower step 1 and the double-column upper step 2 in sequence according to the design dimensions, then sink the completed caisson lower step 1 and the double-column upper step 2 to the bottom, and after sinking to the bottom, perform concrete bottom sealing and step cover 102 installation, then construct the top pedestal 6, and install the first guide unit 3 and the second guide unit 4.

[0048] The double-column step caisson in the present application can form a step structure by setting a lower step 1 of the caisson and a double-column upper step 2 with a smaller cross-section than the lower step 1 of the caisson. While controlling the deadweight of the double-column step caisson, reducing the overall material usage of the structure, and saving capital investment, the water-blocking cross-section of the double-column upper step 2 is greatly reduced, which can effectively reduce scouring, and the water flow will be accelerated again when passing through the rear end column, reducing the sedimentation at the rear end of the caisson. In addition, when the water flows through the lower step 1 of the caisson, the water flow is initially consumed by the step structure in the form of shear, further reducing the impact of the water flow on the foundation. By setting the first diversion unit 3 and the second diversion unit 4, the water flow can be sheared and guided, and the flow velocity and flow direction of the sheared and guided water flow are stable, which can control the scouring pit around the caisson to be flatter and avoid sediment accumulation at the rear end.

[0049] In a second aspect, an embodiment of the present application further provides a bridge support structure, and the bridge support structure includes the above-mentioned double-column stepped caisson, Figure 5A schematic diagram of a bridge support structure provided for an embodiment of the present application, as shown in Figure 5, is provided at the top of the double-column upper step 2 in the double-column step caisson, that is, there are two bridge towers 7 for supporting the bridge, that is, the bridge towers 7 are respectively connected to the top pedestals 6 on the tops of the two protruding columns 201, and the bridge towers 7 are mainly used to support the bridge deck structure and are responsible for transferring these loads to the foundation. They disperse the loads underground through a solid basic structure to ensure the stability and safety of the bridge. Combined with the above description, the above-mentioned double-column step caisson is arranged transversely to the bridge, and the two bridge towers 7 are respectively supported on both sides of the wide side of the bridge.

[0050] In the description of the present application, it should be noted that the terms "upper", "lower", etc. indicate orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present application. Unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be an indirect connection through an intermediate medium, or it can be a connection between the two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0051] It should be noted that, in this application, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the presence of other identical elements in the process, method, article or device including the element.

[0052] The above description is only a specific implementation of the present application, so that those skilled in the art can understand or implement the present application. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest range consistent with the principles and novel features applied for herein.

Claims

1. A double-column stepped caisson suitable for long-span bridges, characterized in that: include: A step assembly, the step assembly is used for being arranged in the direction of water flow, and comprises a caisson lower step (1) having a plurality of axial through-type well holes, and a double-column upper step (2) extending upward from the top surface of the caisson lower step (1) and having a hollow interior and being connected to the well holes of the caisson lower step (1), wherein the double-column upper step (2) comprises two protruding columns (201) arranged at intervals along the length direction of the caisson lower step (1); A scour reduction brush assembly, comprising a first guide unit (3) disposed at the bottom of the outer sides of two protruding columns (201) and a second guide unit (4) located at the bottom between the two protruding columns (201), wherein the first guide unit (3) comprises a sharp end for shearing water flow, the sharp ends of the two first guide units (3) are respectively used to face the water front side and the water back side, and the second guide unit (4) is provided with concave curved surfaces on both sides for guiding the sheared water flow.

2. A double-column stepped caisson suitable for a long-span bridge as claimed in claim 1, characterized in that: The water-facing side and the water-receiving side of the lower step (1) of the caisson are both provided with arc end surface flow-through ends (101).

3. A double-column stepped caisson suitable for a long-span bridge as claimed in claim 2, characterized in that: A caisson bottom sealing area (5) is provided at the bottom of the caisson lower step (1).

4. A double-column stepped caisson suitable for a long-span bridge as claimed in claim 3, characterized in that: A step cover plate (102) for sealing the well hole is provided on the top of the lower step (1) of the caisson.

5. The double-column stepped caisson suitable for a long-span bridge as claimed in claim 1, characterized in that: The top of the double-column upper step (2) is provided with a top bearing platform (6) for connecting to the upper structure of the bridge.

6. A double-column stepped caisson suitable for a long-span bridge as claimed in claim 5, characterized in that: The horizontal cross-sectional area of ​​the bottom of the top support platform (6) is larger than the horizontal cross-sectional area of ​​the protruding column (201).

7. The double-column stepped caisson suitable for a long-span bridge as claimed in claim 1, characterized in that: The first guide unit (3) comprises two guide plates (301) arranged opposite to each other, and one side of the two guide plates (301) are separated from each other, and the other side is gathered to form a sharp end.

8. The double-column stepped caisson suitable for a long-span bridge as claimed in claim 1, characterized in that: The second flow guide unit (4) comprises two flow guide plates (401) arranged opposite to each other.

9. A double-column stepped caisson suitable for a long-span bridge as claimed in claim 8, characterized in that: The guide plate (401) has a certain curvature and the curved surfaces of the two guide plates (401) are close to each other to form a concave curved surface.

10. A bridge support structure, characterized in that: The bridge support structure comprises a double-column stepped caisson as claimed in any one of claims 1 to 9, wherein a bridge tower (7) for supporting the bridge is provided at the top of the upper steps (2) of the double columns in the double-column stepped caisson.

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