How to remove existing foundations

Abstract

To provide an existing foundation removal method and an existing foundation removal system capable of surely removing an existing hollow foundation underwater in a short period of time.SOLUTION: An existing foundation removal system 30 comprises a full circumference rotary excavator 13 fitted with a standard casing pipe 11, a large-diameter casing pipe 10 connected to the tip of the standard casing pipe 11, and an injection rod 15 capable of injecting fluid into the casing pipe 10. To remove a caisson 2, the casing pipe 10 is placed in the ground 1 around the caisson 2 to below a lower edge 27 of the caisson 2. After the improved ground 16 is formed inside the casing pipe 10 under the caisson 2, water is drained from the inside of the casing pipe 10, and filling material 8 is taken out from the inside of the caisson 2. Next, a suspension member 18 is fixed inside the caisson 2, and the caisson 2 is pulled out from the ground 1 while the muddy water 23 is injected between the casing pipe 10 and the caisson 2 to give buoyancy to the caisson 2.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] The present invention relates to a method for removing an existing foundation and a system for removing an existing foundation. [Background technology]

[0002] Recently, various infrastructure facilities have been updated, but it is necessary to remove the underwater foundations of bridges installed over rivers and sea areas. Conventionally, for example, caisson foundations were removed by installing a temporary cofferdam such as a double cofferdam around the foundation, drying up the foundation to expose the top of the foundation, and then drilling cylindrical holes in the foundation in succession using the all-casing method. In addition, for pile foundations, there was a method in which the soil inside the pile foundation was drained, multiple small holes were formed in the part of the pile foundation deeper than the bottom of the water, and then the head of the pile foundation was blocked and compressed air was supplied inside while the pile foundation was pulled out (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 6457871 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the conventional method for removing caisson foundations has problems such as leaving cutting blocks inside the casing pipe and noise caused by crushing on-site. In addition, the method for removing pile foundations requires equipment to supply compressed air.

[0005] The present invention has been made in consideration of the above-mentioned problems, and its object is to provide a method and system for removing an existing foundation that can reliably remove an existing underwater hollow foundation in a short period of time. [Means for solving the problem]

[0006] In order to achieve the above-mentioned object, the first invention is a method for removing an existing hollow foundation underwater, comprising the steps of: a) placing a casing pipe larger than the diameter of the foundation in the ground surrounding the foundation, below the lower end of the foundation; b) improving the ground inside the casing pipe in the ground below the foundation; c) pumping water from inside the casing pipe and extracting at least one of soil material and water from inside the foundation; d) fixing a hanging member inside the foundation; and e) pulling out the foundation from the ground while applying buoyancy to the foundation by injecting or flowing a fluid between the casing pipe and the foundation.

[0007] In the first invention, the inside of the casing pipe is improved and a fluid is injected or poured between the casing pipe and the foundation, thereby greatly reducing the frictional resistance around the foundation. Also, the foundation can be pulled out while being given buoyancy. Therefore, the foundation can be reliably removed in a short period of time. Also, there is no need to crush the foundation on site, which makes it possible to avoid noise problems on site.

[0008] In the first invention, for example, the foundation has a top plate, and includes a step f of drilling holes in the top plate before the step c. In this case, it is preferable that the steps a and f are performed using the same full-circle rotary drilling machine, the step f drills holes in the top plate with a standard casing pipe having a diameter smaller than that of the foundation, and the step a connects the casing pipe to the standard casing pipe and places it on the outer periphery of the foundation.

[0009] If the top plate is drilled before step c, step c of extracting soil material and water from inside the foundation and step d of fixing the hoisting member inside the foundation can be easily performed. Also, by connecting a casing pipe with a larger diameter than the foundation to a standard casing pipe with a smaller diameter than the foundation, step a and step f can be performed using the same full-circle rotary excavator.

[0010] The foundation may have a bottom slab, and step d may be a step of attaching the hanging member to the bottom slab. In this case, for example, the foundation may be a pneumatic caisson, the bottom slab may have a shaft hole, the hanging member may be disposed so as to close the shaft hole via a water-stopping member, and at least one fixing member for fixing the hanging member to the bottom slab may be joined to the filling concrete filled in the shaft hole.

[0011] If the suspension member is attached to the bottom slab, the suspension member can be easily fixed inside the foundation. Also, if the foundation is a pneumatic caisson, the suspension member can be arranged so as to block the shaft hole via the water-stopping member, thereby ensuring water-stopping performance inside the foundation.

[0012] The second invention is a method for removing an existing hollow foundation underwater, comprising the steps of: a step a) of placing a casing pipe larger than the diameter of the foundation in the ground surrounding the foundation, below the lower end of the foundation; a step d) of fixing a hanging member inside the foundation and ensuring watertightness inside the foundation; and a step e) of injecting or allowing a fluid to flow between the casing pipe and the foundation to impart buoyancy to the foundation, and pulling out the foundation from the ground while adjusting the amount of water inside the foundation.

[0013] In the second invention, by injecting or flowing a fluid between the casing pipe and the foundation, the frictional resistance around the foundation can be significantly reduced. Also, by adjusting the amount of water inside the foundation that ensures watertightness, the foundation can be pulled out while being given an appropriate amount of buoyancy. Therefore, the foundation can be reliably removed in a short period of time.

[0014] In the second invention, for example, the foundation is a monopile foundation, the upper end of the monopile foundation is exposed above the water, and the hanging member can be inserted inside by reducing its diameter to be smaller than the diameter of the upper end of the monopile foundation, and the hanging member is expanded inside the monopile foundation and fixed near the lower end of the monopile foundation so as to ensure watertightness inside the monopile foundation. This allows the hanging member to be inserted from the upper end of the monopile foundation and fixed near the lower end while ensuring watertightness.

[0015] In the first and second aspects of the present invention, in step e, the fluid is preferably a fluid having a higher specific gravity than water, which can impart a larger buoyancy to the foundation than when water is used.

[0016] In addition, in the step e, it is preferable to gradually raise the liquid level of the fluid within a range not exceeding the upper end of the foundation while pulling the foundation upward, thereby making it possible to maintain the buoyancy applied to the foundation while preventing the fluid from flowing into the foundation.

[0017] Furthermore, in the step e, the foundation may be pulled up while being cut on the water, thereby making it possible to pull up the foundation while reducing its weight. Effect of the Invention

[0020] According to the present invention, it is possible to provide a method and system for removing an existing foundation that can reliably remove an existing underwater hollow foundation in a short period of time. [Brief description of the drawings]

[0021] [Figure 1] A diagram showing how to remove caisson 2. [Diagram 2] A diagram showing how to remove caisson 2. [Diagram 3] A diagram showing how to remove caisson 2. [Figure 4] A diagram showing how to remove caisson 2. [Diagram 5]A diagram showing how to remove caisson 2. [Figure 6] A diagram showing how to remove caisson 2. [Figure 7] A diagram showing the state in which a hanging member 18a is fixed to a caisson 2a. [Figure 8] A diagram showing how to remove monopile 2b. [Figure 9] A diagram showing how to remove monopile 2b. [Figure 10] A diagram showing how to remove monopile 2b. [Figure 11] A diagram showing how to remove monopile 2b. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, a first embodiment of the present invention will be described in detail with reference to the drawings. Figures 1 to 6 are diagrams showing a method for removing a caisson 2. Figure 1(a) is a diagram showing a caisson 2 to be removed, and is a vertical cross-sectional view of ground 1.

[0023] As shown in Figure 1(a), the caisson 2 is an existing hollow foundation underwater, which supports a bridge pier 9. The caisson 2 is a pneumatic caisson with a horizontal cross section that is, for example, approximately circular, and is composed of a top concrete slab 3, side wall concrete 4, bottom concrete slab 5, cutting edge portion 6, etc. In the caisson 2, a shaft hole 19 in the bottom concrete slab 5 and cutting edge portion 6 are filled with filler concrete 7. Furthermore, the space surrounded by the top concrete slab 3, side wall concrete 4, and bottom concrete slab 5 is filled with filler material 8. The filler material 8 is soil material such as filler sand, or water.

[0024] FIG. 1(b) is a diagram showing the process of placing the casing pipe 10. To remove the caisson 2 from the ground 1, as shown in FIG. 1(b), a casing pipe 10 having a size (large diameter) larger than the diameter of the caisson 2 is connected to a standard casing pipe 11 of a full-circle rotary excavator 13 using a connecting material 12. The connecting material 12 is a plate-like or rod-like member that can reliably transmit rotational force from the standard casing pipe 11 to the casing pipe 10. Then, the full-circle rotary excavator 13 is used to rotate and press the casing pipe 10 into the ground 1 to drill a hole in the ground 1 below the lower end 27 of the caisson 2, and the casing pipe 10 is placed in the ground 1 on the outer periphery of the caisson 2. It is desirable that the lower end of the casing pipe 10 is located about 2 m below the lower end 27 of the caisson 2, and the upper end is exposed about 2 m above the water surface.

[0025] Fig. 2(a) is a diagram showing the process of improving the ground 1 below the casing pipe 10. After the casing pipe 10 is placed on the ground 1, the full-circle rotary excavator 13, the standard casing pipe 11, and the connecting material 12 are removed. Then, as shown in Fig. 2(a), an injection rod 15 attached to a ground improvement construction machine 14 is used to drill a hole in the ground 1 between the casing pipe 10 and the caisson 2, so that the tip 31 reaches the bottom of the caisson 2, and the ground inside the casing pipe 10 is improved to form the improved ground 16. At this time, the injection rod 15 is moved to multiple locations along the inner surface of the casing pipe 10, and ground improvement is performed so that the vicinity of the lower end of the casing pipe 10 is blocked by the improved ground 16.

[0026] Figure 2(b) is a diagram showing the process of drilling holes in the top plate concrete 3. Once the improved ground 16 has been formed, the ground improvement construction machine 14 and injection rod 15 are removed. Then, as shown in Figure 2(b), the inside of the casing pipe 10 is drained, and the top plate concrete 3 is drilled using a full-circle rotary excavator 13 and a standard casing pipe 11. The standard casing pipe 11 has a smaller diameter than the caisson 2, and the top plate concrete 3 is cut with a casing bit (not shown) attached to the end of the standard casing pipe 11.

[0027] Figure 3(a) is a diagram showing the process of removing the top plate concrete 3. After drilling the top plate concrete 3, the top plate concrete 3 inside the standard casing pipe 11 is removed together with the pier 9 as shown in Figure 3(a). The top plate concrete 3 and the pier 9 are lifted and removed by a crane ship or the like (not shown).

[0028] Figure 3(b) is a diagram showing the process of removing the fill material 8 from inside the caisson 2. After the top slab concrete 3 and the like have been removed, the fill material 8 is removed from inside the caisson 2. For example, if the fill material 8 is filled sand, it is excavated and removed using a hammer grab 17 as shown in Figure 3(b). If the fill material 8 is water, the water is drained from inside the caisson 2 using a pump or the like.

[0029] Figure 4(a) is a diagram showing the process of fixing the hanging member 18 inside the caisson 2, Figure 4(b) is an enlarged view of range A in Figure 4(a), and Figure 4(c) is a schematic diagram of the hanging member 18. After the filler material 8 has been removed from inside the caisson 2, the hanging member 18, which is made of a standard casing pipe 11 with a water-stopping steel plate welded to the tip, is attached inside the caisson 2 as shown in Figure 4(a).

[0030] The tip of the hanging member 18 is, for example, a disk-shaped iron plate as shown in Fig. 4(c), and has multiple fixing holes 22. The hanging member 18 is arranged so as to block the shaft hole 19 via a water-stopping member 20 as shown in Fig. 4(b), and is fixed inside the caisson 2 by attaching fixing members 21, which are anchor bolts that also serve as hanging fittings, to the fixing holes 22. At least one of the fixing members 21 is joined to the filling concrete 7, and the rest is joined to the base slab concrete 5. As the inside of the caisson 2 has been dried up, the hanging member 18 can be installed by manual work in the air.

[0031] Fig. 5(a) is a diagram showing the process of replacing the ground 1 around the caisson 2 with muddy water 23. After the hanging member 18 is fixed to the caisson 2, the muddy water 23 is stirred and injected between the casing pipe 10 and the caisson 2 by the injection rod 15 of the ground improvement construction machine 14 as shown in Fig. 5(a). As a result, the ground 1 between the casing pipe 10 and the caisson 2 is replaced with the muddy water 23, reducing the frictional resistance around the caisson 2. The muddy water 23 is a fluid with a higher specific gravity than the surrounding water or seawater, and replacing the ground 1 with the muddy water 23 imparts buoyancy to the caisson 2.

[0032] Here, as shown in Fig. 5(a), the existing foundation removal system 30 comprises a full-circle rotary excavator 13 to which a standard casing pipe 11 is attached, a casing pipe 10 having a larger diameter than the standard casing pipe 11, and an injection rod 15 arranged along the inner surface of the casing pipe 10 and capable of injecting a fluid such as mud water 23 into the inside of the casing pipe 10. The casing pipe 10 can be connected to the tip of the standard casing pipe 11 as shown in Fig. 1(b).

[0033] Figure 5(b) is a diagram showing the process of cutting the edge of the bottom of the caisson 2. Once the surroundings of the caisson 2 have been replaced with muddy water 23, the caisson 2 and standard casing pipe 11 are pulled up by a full-circle rotary excavator 13 while taking advantage of the buoyancy of the muddy water 23, as shown in Figure 5(b). This cuts the adhesion between the bottom of the caisson 2 and the ground 1. During the pulling up process, a rotational force is also applied as indicated by the arrows, if necessary.

[0034] FIG. 6(a) is a diagram showing the process of pulling out the caisson 2. After the bottom of the caisson 2 is cut off, the full-circle rotary excavator 13 is removed. Then, while giving buoyancy to the caisson 2 by the muddy water 23, the standard casing pipe 11 and the caisson 2 are lifted by a sling 24 using a crane ship or the like (not shown) and pulled out from the ground 1 as shown in FIG. 6(a). While the caisson 2 is being pulled up, the buoyancy by the muddy water 23 is maintained by gradually raising the liquid level 25 of the muddy water 23 within a range not exceeding the upper end 26 of the caisson 2 while injecting a fluid such as water, seawater or muddy water into the casing pipe 10. The muddy water 23 is a fluid that has a higher specific gravity than water even when diluted with water or seawater.

[0035] Figure 6(b) is a diagram showing the process of backfilling the ground 1. After the caisson 2 is pulled out and removed, backfilling sand 28 is poured into the casing pipe 10 as shown in Figure 6(b), and backfilling is performed up to the surface of the ground 1. In addition, the muddy water 23 in the casing pipe 10 is replaced with water or seawater. Thereafter, the casing pipe 10 is pulled out by the full-circle rotary excavator 13.

[0036] Thus, according to the first embodiment, by forming the improved ground 16 inside the casing pipe 10 and injecting muddy water 23, which has a higher specific gravity than water or seawater, between the casing pipe 10 and the caisson 2, it is possible to significantly reduce the frictional resistance around the caisson 2. In addition, the muddy water 23 can impart buoyancy to the caisson 2 while it is being pulled out. Therefore, the caisson 2 can be reliably removed in a short period of time, and the freedom of spatial development within the ground 1 can be increased.

[0037] In the first embodiment, the hanging member 18 is positioned so as to block the shaft hole 19 via the water-stopping member 20, and at least one fixing member 21 is joined to the backfill concrete 7 filled in the shaft hole 19, so that the hanging member 18 can be fixed to the base slab concrete 5 while ensuring water-stopping properties inside the caisson 2.

[0038] In the first embodiment, by using the existing foundation removal system 30, the process of placing a large diameter casing pipe 10 in the ground 1 surrounding the caisson 2 and the process of drilling a hole in the top plate concrete 3 of the caisson 2 can be carried out by the same 360° rotating excavator 13.

[0039] In the first embodiment, the pneumatic caisson 2 is the object to be removed, but an open caisson, monopile, etc. may also be the object to be removed.

[0040] The open-type caisson 2a is constructed by sinking the sidewall concrete 4a into the ground 1 and pouring the bottom concrete 5a at the bottom as shown in FIG. 7. In the open-type caisson 2a, the hanging member 18a is fixed instead of the hanging member 18 in the process shown in FIG. 4. As shown in FIG. 7, the tip of the hanging member 18a consists of a disk part 181 arranged along the bottom concrete 5a and a cylindrical part 184 arranged along the sidewall concrete 4a. The hanging member 18a is attached to the bottom concrete 5a by connecting at least one of the fixing members 21a to the bottom concrete 5a and connecting the remaining fixing members 21a to the sidewall concrete 4a. When the watertightness of the caisson 2a is ensured, the hanging member 18a is directly fixed inside the caisson 2a as shown in FIG. 7(a). If the watertightness of the caisson 2a is not ensured, the hanging member 18a is fixed inside the caisson 2a via a watertight member 20a as shown in FIG. 7(b).

[0041] Next, a second embodiment will be described. In the second embodiment, differences from the first embodiment will be described, and the same configurations will be denoted by the same reference numerals in the drawings and the like, and description thereof will be omitted.

[0042] 8 to 11 are diagrams showing a method for removing a monopile foundation. In the second embodiment, a steel monopile 2b is removed using an existing foundation removal system 30 (FIG. 11(a)) similar to that in the first embodiment.

[0043] Fig. 8(a) is a diagram showing the monopile 2b to be removed, and is a vertical cross-sectional view of the ground 1. As shown in Fig. 8(a), the monopile 2b is an existing hollow foundation used in offshore wind power generation facilities and the like. The lower end 27a of the monopile 2b is placed in the ground 1 at the bottom of the water, and the upper end 26a is exposed above the water.

[0044] Fig. 8(b) is a diagram showing a process of placing the casing pipe 10. To remove the monopile 2b from the ground 1, as shown in Fig. 8(b), a casing pipe 10 having a larger diameter than the monopile 2b is used to drill a hole in the ground 1 below the lower end 27a of the monopile 2b, and the casing pipe 10 is placed in the ground 1 on the outer periphery of the monopile 2b.

[0045] Fig. 9(a) is a diagram showing the process of excavating the ground 1 within the monopile 2b. After the casing pipe 10 is placed in the ground 1, the ground 1 within the monopile 2b is excavated and removed by a hammer grab 17 as shown in Fig. 9(a).

[0046] Fig. 9(b) is a diagram showing the process of fixing the hanging member 18b inside the monopile 2b. After the ground 1 inside the monopile 2b is excavated, the hanging member 18b is welded to the tip of the standard casing pipe 11, and the hanging member 18b is attached to the inside of the monopile 2b as shown in Fig. 9(b).

[0047] Fig. 10(a) is an enlarged view of range B in Fig. 9(b), and Fig. 10(b) and Fig. 10(c) are schematic views of the hanging member 18b. As shown in Fig. 10, the hanging member 18b has a disk portion 181 and an opening / closing portion 182 connected to the outer edge of the disk portion 181 by a hinge portion 183. When the opening / closing portion 182 is closed as shown in Fig. 10(b), the hanging member 18b contracts in diameter to a diameter smaller than that of the upper end 26a of the monopile 2b, and when the opening / closing portion 182 is opened as shown in Fig. 10(c), the hanging member 18b expands in diameter to approximately the same as that near the lower end 27a of the monopile 2b.

[0048] The suspension member 18b is inserted from the upper end 26a of the monopile 2b in a contracted state, and is fixed in an expanded state near the lower end of the monopile 2b (for example, above the submarine cable extraction hole of the monopile 2b) as shown in Fig. 10(a). Water is sealed between the suspension member 18b and the monopile 2b by pressing a water-stopping member 20b such as a packing rubber against the inner peripheral surface of the monopile 2b. Since the monopile 2b is filled with water, the suspension member 18b has an opening / closing part 182 which can be expanded in diameter by mechanical remote control, and is fixed into the monopile 2b by unmanned underwater operation.

[0049] Figure 11(a) is a diagram showing the process of replacing the ground 1 around the monopile 2b with muddy water 23. After the hanging member 18b is fixed to the monopile 2b, the muddy water 23 is stirred and injected between the casing pipe 10 and the monopile 2b by the injection rod 15 of the ground improvement construction machine 14 as shown in Figure 11(a). As a result, the ground 1 between the casing pipe 10 and the monopile 2b is replaced with the muddy water 23, and the frictional resistance around the monopile 2b is reduced.

[0050] Fig. 11(b) is a diagram showing the process of pulling out the monopile 2b. After replacing the surroundings of the monopile 2b with muddy water 23, the monopile 2b and the standard casing pipe 11 are pulled up by a full-circle rotary excavator 13 while maintaining the buoyancy of the muddy water 23, and the monopile 2b and the standard casing pipe 11 are hoisted up by a crane (not shown) as shown in Fig. 11(a) and pulled out from the ground 1. Since the steel monopile 2b floats completely due to the buoyancy of the muddy water 23 when the inside is dried up, the amount of water inside the monopile 2b is adjusted while the monopile 2b is being hoisted to balance the weight of the monopile 2b including the water inside and the buoyancy of the muddy water 23, and an appropriate amount of buoyancy is given to the monopile 2b. In the second embodiment, seawater may flow into between the casing pipe 10 and the monopile 2b from the lower end side of the casing pipe 10, and the muddy water 23 diluted with seawater is also a fluid with a higher specific gravity than water.

[0051] After the monopile 2b is pulled out and removed, backfill sand is poured into the casing pipe 10 to backfill the ground 1 up to the surface, and the muddy water 23 in the casing pipe 10 is replaced with seawater as necessary. Thereafter, the casing pipe 10 is pulled out by the full-circle rotary excavator 13.

[0052] Thus, according to the second embodiment, by injecting muddy water 23 between the casing pipe 10 and the monopile 2b, the frictional resistance around the monopile 2b can be significantly reduced. In addition, by fixing the suspension member 18b to the monopile 2b and adjusting the amount of water inside while ensuring watertightness, the monopile 2b can be pulled out while providing an appropriate amount of buoyancy. Therefore, the monopile 2b can be reliably removed in a short period of time, and the degree of freedom in developing the space in the ground 1 can be increased. Note that, in the case of removing the monopile 2b, the suspension member may be fixed to the monopile by ground improvement and drying up the inside of the monopile 2b, as in the case of the caisson.

[0053] In the first and second embodiments, the caissons 2, 2a and the monopile 2b may be pulled up while being cut on the water when they are pulled out from the ground 1. The caissons 2, 2a and the monopile 2b may be pulled out to a predetermined position and left in the water without being removed in their entirety.

[0054] Although the preferred embodiment of the present invention has been described above with reference to the accompanying drawings, the present invention is not limited to such an example. It is clear that a person skilled in the art can come up with various modified or altered examples within the scope of the technical ideas disclosed in this application, and it is understood that such examples also naturally belong to the technical scope of the present invention. [Explanation of symbols]

[0055] 1……ground 2, 2a…Caisson 2b………Monopile 3....Top plate concrete 4, 4a………Side wall concrete 5, 5a……Base plate concrete 6……Blade mouth part 7………Concrete filling 8………Filling material 9...Pier 10...Casing pipe 11...Standard casing pipe 12……Connecting material 13...……Full-circle rotary excavator 14……Soil improvement construction machine 15....Injection rod 16……improved ground 17...Hammer grab 18, 18a, 18b... Suspension members 19....Shaft hole 20, 20a, 20b...Water-stopping member 21, 21a...Fixing member 22……Fixing hole 23……muddy water 24……hanging ring 25……Liquid level 26, 26a……Top end 27, 27a……lower end 28………Sand 30………Existing foundation removal system 31……Tip 181...Disc section 182………Opening and closing section 183...Hinge section 184... Cylindrical part

Claims

1. A method for removing an existing hollow foundation underwater, comprising the steps of: A step a) of placing a casing pipe having a size larger than the diameter of the foundation in the ground around the foundation to below the lower end of the foundation; A step b of improving the ground below the foundation and inside the casing pipe; A step c of pumping water from inside the casing pipe and removing at least one of soil material and water from inside the foundation; A step d of fixing a hanging member inside the foundation; A step e of injecting or flowing a fluid between the casing pipe and the foundation to impart buoyancy to the foundation while pulling out the foundation from the ground; A method for removing an existing foundation, comprising:

2. The foundation has a top plate, 2. The method for removing an existing foundation according to claim 1, further comprising a step f of drilling holes in the top plate prior to the step c.

3. The steps a and f are carried out using the same full-circle rotary excavator, The step f includes drilling a hole in the top plate with a standard casing pipe having a diameter smaller than that of the foundation; 3. The method for removing an existing foundation according to claim 2, wherein said step (a) comprises connecting said casing pipe to said standard casing pipe and arranging said casing pipe around the outer periphery of said foundation.

4. The foundation has a base plate, 4. The method for removing an existing foundation according to claim 1, wherein the step d is a step of attaching the hanging members to the bottom slab.

5. The foundation is a pneumatic caisson, The bottom plate has a shaft hole formed therein, the suspending member is disposed so as to close the shaft hole via a water-stopping member, The method for removing an existing foundation according to claim 4, characterized in that at least one fixing member for fixing the hanging member and the bottom slab is joined to the backfill concrete filled in the shaft hole.

6. A method for removing an existing hollow foundation underwater, comprising the steps of: A step a) of placing a casing pipe having a size larger than the diameter of the foundation in the ground around the foundation to below the lower end of the foundation; A step d of fixing a hanging member to the inside of the foundation and ensuring water-tightness inside the foundation; A process e of injecting or flowing a fluid between the casing pipe and the foundation to impart buoyancy to the foundation and pulling out the foundation from the ground while adjusting the amount of water inside the foundation; A method for removing an existing foundation, comprising:

7. The foundation is a monopile foundation, The upper end of the monopile foundation is exposed above water, A method for removing an existing foundation as described in claim 6, characterized in that the hanging member can be inserted inside the monopile foundation by reducing its diameter to be smaller than the diameter of the upper end of the monopile foundation, and the hanging member is expanded inside the monopile foundation and fixed near the lower end of the monopile foundation so as to ensure watertightness inside the monopile foundation.

8. 8. The method for removing an existing foundation according to claim 1, wherein in step e, the fluid is a fluid having a specific gravity greater than that of water.

9. A method for removing an existing foundation as described in any one of claims 1 to 8, characterized in that in step e, while the foundation is pulled upward, the liquid level of the fluid is gradually raised within a range not exceeding the upper end of the foundation.

10. The method for removing an existing foundation according to claim 9, characterized in that in the step (e), the foundation is pulled upward while being cut on the water.

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