Scour prevention method for pile foundations

Abstract

To provide a scouring prevention construction method of a pile foundation allowing installation of scouring prevention measures in low cost and a short period without using a large amount of debris, blocks, and bagged bodies around a bottom part of the pile foundation settled in a water bottom.SOLUTION: A scouring prevention construction method for this pile foundation comprises the steps of: installing a cylindrical body 11 comprising sheet material 20 having an inner diameter larger than an outer diameter of a pile so as to surround a pile 10 installed in a water bottom and having elasticity; reducing a diameter of an upper end part of the cylindrical body so as to contact the upper end part of the cylindrical body with the pile over the whole periphery by sinking the cylindrical body in water; and installing the sheet material at least in a range corresponding to an inner diameter of the cylindrical body around the pile on a water bottom surface.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for preventing scouring of pile foundations. [Background technology]

[0002] Conventionally, when scouring occurs at the base of an underwater structure, where sediment on the seabed is washed away by the effects of water currents and waves, there is a risk that the base may become loose, so measures to prevent scouring have been required. Commonly known measures to prevent scouring include, for example, placing rubble, blocks, bags, etc. on the seabed (rub net method, bag-type foot protection method, etc.).

[0003] As an example of the above-mentioned bag-like body, Patent Document 1 discloses a bag-like body produced by putting a filling material such as broken stones into the bag-like body with a cover sheet attached and then closing the opening of the cover sheet (Fig. 8). Also, Patent Document 2 discloses a civil engineering mat (abstract) which is laid on the seabed to prevent scouring of soil and sand, and which is made of a water-permeable multi-layer fabric with an apparent density of 1.4 to 1.8 woven using resin-coated fiber yarns with an apparent density of 1.4 to 2.0 as warp and weft, the resin-coated fiber yarns having a core yarn such as para-aramid fiber coated with a resin containing a high specific gravity component such as fine iron oxide powder. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 11-241325 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-239330 Summary of the Invention [Problem to be solved by the invention]

[0005] The installation of conventional rubble, blocks, or bags on the seabed, especially in deep ocean conditions such as the pile foundations of offshore wind power plants, places a heavy burden on the construction work and increases construction costs. Furthermore, the pile foundations of offshore wind power plants have larger pile diameters than the general pile specifications used in port construction, which means a wider area needs to be protected against scouring, requiring a larger quantity of stone and increasing costs. Furthermore, the installation of rubble or blocks raises concerns about bottom sediment leaching, which may require separate leaching prevention work using crushed stone, leading to increased construction costs.

[0006] In view of the problems of the conventional technology as described above, the present invention aims to provide a method for preventing scouring of pile foundations, which enables scouring prevention measures to be installed around the bottom of a pile foundation driven into the bottom of the water at low cost and in a short construction period without using large amounts of stone, blocks, or bags. [Means for solving the problem]

[0007] The first pile foundation scouring prevention method for achieving the above-mentioned objectives involves placing a cylindrical body made of flexible sheet material and having an inner diameter larger than the outer diameter of the pile around a pile driven into the bottom of the water so that it surrounds the pile, sinking the cylindrical body underwater, reducing the diameter of the upper end of the cylindrical body using an underwater construction machine so that the upper end of the cylindrical body contacts the pile around its entire circumference, and laying the sheet material on the bottom of the water around the pile within an area at least equivalent to the inner diameter of the cylindrical body.

[0008] According to this method for preventing scouring of pile foundations, a cylindrical body made of sheet material with an inner diameter larger than the outer diameter of the pile is placed around the pile and submerged underwater, the upper end of the cylindrical body is reduced in diameter, and the sheet material is laid on the water bottom around the pile within an area at least equivalent to the inner diameter of the cylindrical body.This allows scouring prevention work by laying sheet material to be installed in a short construction period, and also eliminates the need to use large amounts of rubble, blocks, or bags as in the past, allowing scouring prevention measures to be implemented at low cost.

[0009] In the above-mentioned first pile foundation scouring prevention method, it is preferable to provide a plurality of weights at predetermined intervals around the circumferential direction of the lower end of the cylindrical body, and to reduce the diameter of the upper end of the cylindrical body while the cylindrical body is sinking in the water or after the plurality of weights have reached the bottom of the water.

[0010] In addition, the cylindrical body can be installed by hanging the rectangular sheet material from above the water into the water and using a surface machine to deform it into a cylindrical shape so that it surrounds the pile.

[0011] A second pile foundation scouring prevention method for achieving the above-mentioned object involves installing a cylindrical body made of flexible sheet material and having an inner diameter larger than the outer diameter of the pile around a pile driven into the bottom of the water, and providing a plurality of submersible drainage pumps at predetermined intervals around the circumferential direction of the lower end of the cylindrical body with their drainage outlets facing the outer peripheral surface of the pile. The cylindrical body is then lowered into the water, and while the cylindrical body is lowering, the plurality of submersible drainage pumps are operated to spray water toward the outer peripheral surface of the pile, causing the plurality of submersible drainage pumps to move away from the pile due to the reaction force, thereby expanding the diameter of the cylindrical body at the lower end. The upper end of the cylindrical body is then reduced in diameter by an underwater construction machine so that the upper end is in contact with the pile around its entire circumference, and the sheet material is laid on the bottom surface around the pile within a range at least equivalent to the inner diameter of the cylindrical body.

[0012] According to this method for preventing scouring of pile foundations, a cylindrical body made of sheet material with an inner diameter larger than the outer diameter of the pile is placed around the pile and lowered into water, and multiple submersible drainage pumps attached to the lower end of the cylindrical body are operated, causing each submersible drainage pump to move away from the pile due to the reaction force generated by the spray of water toward the outer surface of the pile, thereby expanding the diameter of the cylindrical body at the lower end and reducing the diameter at the upper end of the cylindrical body, and laying sheet material around the pile on the waterbed within an area equivalent to at least the inner diameter of the cylindrical body.This allows scouring prevention work by laying sheet material to be installed in a short construction period, and also eliminates the need to use large amounts of rubble, blocks, or bags as in the past, making scouring prevention work possible at low cost.

[0013] In the above-mentioned second pile foundation scouring prevention method, it is preferable that the upper end of the cylindrical body is reduced in diameter by an underwater construction machine before the operation of the multiple underwater drainage pumps so that the upper vertical end of the cylindrical body approaches the pile around its entire circumference.

[0014] It is also preferable that the plurality of submersible drainage pumps are detached and recovered after the cylindrical body has reached the bottom of the water.

[0015] In the above-mentioned first and second pile foundation scouring prevention construction methods, it is preferable that the cylindrical body has a flexible ring-shaped member at its upper end, and the upper end of the cylindrical body is reduced in diameter by pulling the ring-shaped member in an approximately horizontal direction using the underwater construction machine.

[0016] It is also preferable that the outer periphery of the sheet material be positioned on the bottom of the water by an underwater construction machine.

[0017] It is also preferable that after the sheet material is laid around the pile, a pressing member is installed near the outer periphery of the sheet material.

[0018] The cylindrical body preferably has a cylindrical shape or a truncated cone shape.

[0019] The sheet material is preferably made of a geotextile material or a rubber material. [Effects of the Invention]

[0020] According to the present invention, a method for preventing scouring of pile foundations can be provided that enables scouring prevention measures to be installed around the bottom of pile foundations driven into the bottom of the water at low cost and in a short construction period without using large amounts of stone, blocks, or bags. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a flowchart illustrating each step of a scouring prevention method for a pile foundation according to a first embodiment. [Figure 2]FIG. 2 is a diagram illustrating the main steps (a) to (c) of the steps in FIG. 1. [Figure 3] FIG. 3 is a top view of the cylindrical body of FIG. 2(a) viewed from above. [Figure 4] FIG. 3 is a top view schematically showing a state in which a pressing member is installed near the outer periphery of the sheet material laid around the pile in FIG. 2(c). [Figure 5] FIG. 2(a) is a schematic diagram for explaining a method for manufacturing and installing the cylindrical body in the construction water area. [Figure 6] 10 is a flowchart illustrating each step of a scour prevention method for a pile foundation according to a second embodiment. [Figure 7] FIG. 7 is a diagram illustrating the main steps (a) and (b) of the steps in FIG. 6. [Figure 8] 7(a) and 7(b) are front and side views showing the submersible drainage pump, pump mounting jig, automatic hook, and power cable and wire of FIG. 7(a). [Figure 9] 1A is a side view showing the cylindrical shape of the cylindrical body in the first and second embodiments, and FIG. 1B is a side view showing the truncated cone shape. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Fig. 1 is a flowchart illustrating each step of a scouring prevention method for pile foundations according to a first embodiment. Fig. 2 is a diagram schematically illustrating main steps (a) to (c) of the steps in Fig. 1. Fig. 3 is a top view of the cylindrical body of Fig. 2(a) viewed from above. Fig. 4 is a top view schematically illustrating a state in which a pressing member is installed near the outer periphery of the sheet material laid around the pile of Fig. 2(c). Fig. 5 is a schematic diagram illustrating a method for fabricating and installing the cylindrical body of Fig. 2(a) in a construction water area.

[0023] First Embodiment Steps S01 to S07 of the scour prevention method for pile foundations according to the first embodiment will be described with reference to Figures 1 to 4. First, as shown in Figure 2(a), a cylindrical body 11 is installed from above a pile 10 driven into the bottom of the water so as to surround the pile 10 (S01). The cylindrical body 11 has a plurality of floats 12 attached at equal intervals in the circumferential direction at its upper end 11a, and a plurality of weights 13 attached at equal intervals in the circumferential direction at its lower end 11b.

[0024] As shown in FIG. 3, a bag 14 made of a sheet material and formed into a circumferentially elongated cylindrical frame is provided at the upper end 11a of the cylindrical body 11. A flexible wire 15 (shown by a dashed line in FIG. 2) is housed in the bag 14 in a ring shape, and the upper end 11a of the cylindrical body 11 is held in a circular shape by the ring-shaped wire 15. The wire 15 has annular pulling portions 15a and 15b protruding from the bag 14 at positions 180 degrees opposite each other in the circumferential direction. The annular pulling portions 15a and 15b are engaged with hook portions or manipulators 30A1 and 30B1 of underwater construction machines 30A and 30B, such as ROVs or underwater drones, and the underwater construction machines 30A and 30B pull the pulling portions 15a and 15b, thereby reducing the diameter of the wire 15 and squeezing the cylindrical body 11 at the upper end 11a until it comes into contact with the outer circumferential surface of the pile 10.

[0025] It should be noted that a linear elastic member made of, for example, steel wire or thin wood may be provided in a ring shape at the lower end 11b of the cylindrical body 11. By providing the linear elastic member in a ring shape on the outer periphery of the lower end 11b of the cylindrical body 11, the elastic member will try to stretch within its range when the sheet material 20 is laid on the water bottom, thereby expanding the sheet material 20. Therefore, the material used for the lower end 11b of the cylindrical body 11 does not have to be steel wire or thin wood, as long as it is a material that can provide this effect.

[0026] The cylindrical body 11 is formed into a cylindrical shape from a geotextile sheet material 20 made of a nonwoven or woven fiber material or a plastic sheet material. The inner diameter D of the cylindrical body 11 is larger than the outer diameter d of the pile 10. The vertical length of the cylindrical body 11 is set based on the area where scouring is to be prevented, taking into account the bottom sediment near the water bottom around the target pile, the magnitude of hydrodynamic force, and the pile diameter, and is set to a length at least equivalent to the inner diameter D of the cylindrical body 11. The cylindrical body 11 can be fabricated in a roughly cylindrical shape on land, transported to the construction waters, and installed as shown in Figure 2(a).

[0027] Next, the cylindrical body 11 arranged around the pile 10 sinks in the water in a downward direction V toward the bottom of the water due to the weight of the multiple weights 13, resisting the buoyancy of the multiple floats 12, as shown in Figure 2(a) (S02).

[0028] Next, after the cylindrical body 11 sinks in the water and hits the bottom of the water as shown in Figure 2(b) (S03), the pulling sections 15a and 15b of the wire 15 are pulled in the horizontal directions H and H' by the underwater construction machines 30A and 30B as shown in Figure 3, thereby reducing the diameter of the wire 15 and reducing the diameter of the upper end section 11a of the cylindrical body 11 (S04). As a result, as shown in Figure 2(c), the upper end section 11a of the cylindrical body 11 comes into close contact with the outer peripheral surface of the lower section of the pile 10. It is desirable to provide transmitters to the pulling sections 15a and 15b so that the underwater construction machines 30A and 30B can easily detect the positions of the pulling sections 15a and 15b.

[0029] 2(c), it is desirable that the water depth position of the upper end 11a of the cylindrical body 11 after the diameter is reduced be closer to the water bottom, so it is preferable that the underwater construction machines 30A, 30B start reducing the diameter of the upper end 11a of the cylindrical body 11 after the lower end 11b of the cylindrical body 11 has reached the water bottom and it is confirmed by a depth gauge attached to the upper end 11a that the upper end 11a has reached a predetermined water depth. Note that the diameter reduction operation of the upper end 11a of the cylindrical body 11 may be started before the cylindrical body 11 reaches the water bottom, but in this case, the diameter reduction is completed after it is confirmed that the upper end 11a of the cylindrical body 11 has reached a predetermined water depth.

[0030] 2(c), after the wire 15 has been reduced in diameter, it is preferable that the wire 15 be secured by the tensioning portions 15a and 15b so as not to slacken, for example, by using an underwater construction machine to hook the tensioning portions 15a and 15b onto hooks (not shown) provided on the outer periphery of the pile 10 and secure them in place. Alternatively, if multiple hooks are installed facing downward at intervals around the periphery of the pile 10 near the bottom of the water, and the wire 15 is reduced in diameter below the hooks, the wire 15 will be captured by the hooks even if it loosens.

[0031] Furthermore, one of the tensioning parts 15a, 15b may be omitted and tensioning may be performed at one location, but as shown in Figure 2, by arranging the underwater construction machines 30A, 30B point-symmetrically about the center of the pile 10 and pulling equally in opposite directions like squeezing a purse, it is easier to shape the upper end part 11a of the cylindrical body 11. Note that in Figure 3, the tensioning parts 15a, 15b are provided at two locations around the circumference of the wire 15, but this is not limited to this, and tensioning parts may be provided at three or four locations evenly around the circumference depending on the diameter of the cylindrical body 11, and three or four underwater construction machines may be used.

[0032] 2(c), the position of the sheet material 20 is adjusted on the water bottom by the underwater construction machines 30A and 30B around the outer periphery of the sheet material 20 constituting the cylindrical body 11 (S05). In this way, the sheet material 20 is laid around the pile 10 within a range equivalent to at least the inner diameter of the cylindrical body 11 (S06). The underwater construction machine 30A has a gripping part (not shown), which can pinch and grasp the outer periphery of the sheet material 20 to adjust the position.

[0033] 2(c) shows underwater construction machines 30A and 30B, but other underwater construction machines may be added and their positions adjusted. The underwater construction machine that can be used in this embodiment is an ROV (Remotely Operated Vehicle), a known underwater robot that can move freely on the water surface and underwater, is equipped with an underwater camera, and has underwater work capabilities, or an underwater drone, and can perform various underwater works by remote control while visually checking images from the underwater camera.

[0034] Next, as shown in Figure 4, pressing members 31 such as crushed stone, blocks, bags, etc. are poured and installed near the outer periphery 20a of the sheet material 20 on the water bottom (S07). As a result, the sheet material 20 laid around the pile 10 is pressed down by the pressing members 31 at and near the outer periphery 20a, preventing the sheet material 20 from floating up due to the water current. Furthermore, since pressing members are not installed on the entire surface of the sheet material 20, the amount of pressing members 31 installed can be smaller than in the past.

[0035] 1 to 4, a cylindrical body 11 made of sheet material 20 and arranged around a pile 10 and having an inner diameter larger than the outer diameter d of the pile 10 is submerged in water, the upper end 11a of the cylindrical body 11 is reduced in diameter, the outer periphery of the cylindrical body 11 is positioned on the water bottom, and the sheet material 20 is laid around the pile within an area corresponding to at least the inner diameter D of the cylindrical body 11. This allows for a short construction period for scour prevention work by laying sheet material, and also eliminates the need for the large amounts of rubble, blocks, or bags used in the past, allowing for low-cost scour prevention measures. Furthermore, by holding down the sheet material 20 with a holding member 31 near the outer periphery 20a, the stability of the sheet material 20 is ensured, and the water bottom surface around the pile 10 can be reliably covered with the sheet material 20. As described above, by laying the sheet material 20, the water flow velocity acting on the bottom surface due to the influence of water flow and waves can be reduced, and the sheet material 20 can prevent the outflow of soil and sand from the bottom surface, thereby providing measures to prevent scouring of the pile foundation.

[0036] Furthermore, even if the holding member 31 cannot be installed immediately after the sheet material 20 is laid due to weather or other reasons, the sheet material 20 covers the bottom surface of the water, which has the effect of reducing the flow velocity acting on the bottom surface of the water, so it is expected to be effective as a measure to mitigate scouring until the holding member 31 is installed.

[0037] Referring to FIG. 5, a method for fabricating and installing the cylindrical body shown in FIG. 2(a) on the top of a pile will be described. As shown in FIG. 5, a diver temporarily fastens one end 21a of a rectangular sheet material 20 to the outer periphery of the pile 10 near the water surface S with temporary fasteners 22. Next, a work vessel SP (or an underwater construction machine) grabs the other end 21b of the sheet material 20 and makes a large turn around the pile 10. The sheet material 20 encounters water resistance and expands into a cylindrical shape. After one turn, the diver connects the ends of the sheet material 20 at multiple locations along the depth direction, connecting one end 21a and the other end 21b of the temporary fasteners 22, completing the cylindrical shape. Next, the sheet material 20 is slowly detached from the temporary fasteners 22 on the outer periphery of the pile 10, completing the installation step S01 of the cylindrical body 11, as shown in FIG. 2(a). Subsequently, the waterbed installation steps S02 to S07 are performed. By attaching a wire-shaped or other elastic member to the lower end of the sheet material 20, when the sheet material 20 is formed into a cylindrical shape and the ends are connected together, the elastic properties of the elastic member can maintain the ring-shaped expanded state at the lower end 11b of the cylindrical body 11. Also, instead of the temporary fastening portion 22, a spacing member having approximately the same distance as the distance between the inner peripheral surface of the cylindrical body 11 and the outer peripheral surface of the pile 10 may be suspended in water and protruded from the outer peripheral surface of the pile 10, and the sheet material 20 may be made to go around the pile 10 in a rather large circle to install the cylindrical body 11.

[0038] Second Embodiment Fig. 6 is a flowchart illustrating each step of the method for preventing scouring of a pile foundation according to the second embodiment. Fig. 7 is a diagram schematically illustrating the main steps (a) and (b) of the steps in Fig. 6. Fig. 8 is a front view (a) and a side view (b) showing the submersible drainage pump, pump mounting jig, automatic hook, and power cable / wire of Fig. 7(a).

[0039] In this embodiment, as shown in Figure 7(a), multiple submersible drainage pumps 16 are provided at the lower end 11b of the cylindrical body 11 instead of a sinker, at equal intervals in the circumferential direction so that each drainage port 16a (Figure 7(b)) faces the outer circumferential surface of the pile 10. The upper end 11a of the cylindrical body 11 has the same configuration as in Figure 3, and the diameter of the cylindrical body 11 can be reduced by pulling the wire 15 at the tensioning sections 15a, 15b using underwater construction machines 30A, 30B. Note that although a float is omitted, it may be provided at the upper end 11a.

[0040] As shown in Figures 8(a) and 8(b), a pump mounting jig 17 is attached to the lower end 11b of the cylindrical body 11, and a submersible drainage pump 16 is attached and fixed to the pump mounting jig 17 via a known automatic hook 23. An integrated power cable and wire 18 is also attached to the pump mounting jig 17. When the cylindrical body 11 hits the bottom, the automatic hook 23 activates and automatically detaches the submersible drainage pump 16 from the pump mounting jig 17, after which the submersible drainage pump 16 can be retrieved by the power cable and wire 18. The power cable and wire 18 is held in place by a retaining ring member 19 attached to the upper outer periphery of the pile 10, as shown in Figure 7(b), to ensure stability during construction.

[0041] As shown in Figure 7(a), a cylindrical body 11 is placed around the pile 10 (S11). Next, the cylindrical body 11 sinks underwater in a downward direction V toward the bottom of the water due to its own weight (including the weight of the pump mounting jig 17 and the automatic hook 23) including the multiple submersible drainage pumps 16 (S12). Because the multiple submersible drainage pumps 16 and the like also function as weight sinkers, the weight sinker 13 in Figure 2(a) can be omitted, although a weight may be added if necessary.

[0042] Next, during the sinking of the cylindrical body 11, as shown in FIG. 3, the tensioning portions 15a, 15b of the wire 15 are pulled in the horizontal directions H, H' by the underwater construction machines 30A, 30B to reduce the diameter of the upper end portion 11a of the cylindrical body 11 (S13), and the upper end portion 11a of the cylindrical body 11 is brought closer to the outer peripheral surface of the pile 10 as shown in FIG. 7(b). In this case, the distance w between the upper end portion 11a of the cylindrical body 11 and the outer peripheral surface of the pile 10 is set to, for example, about 25 cm, and the cylindrical body 11 is not reduced to the point where it comes into contact with the outer peripheral surface of the pile 10 in order to allow the cylindrical body 11 to sink smoothly. Note that the above-mentioned diameter reduction step S13 may be performed in the cylindrical body 11 placement step S11.

[0043] Next, the multiple submersible drainage pumps 16 are operated (S14), and as shown in Figure 7(b), the reaction force generated by uniformly spraying water from each drain outlet 16a of the multiple submersible drainage pumps 16 in radial directions u, u' toward the outer surface of the pile 10 and the force of the water flow returning from the outer surface of the pile 10 cause each submersible drainage pump 16 to move horizontally outward h, h' away from the pile 10, causing the cylindrical body 11 to expand in diameter at the lower end 11b relative to the reduced diameter at the upper end 11a.

[0044] It is preferable that the operation step S14 of the submersible drainage pump 16 is performed near the bottom of the water before the pump hits the bottom in the next step S15. For this reason, by attaching a water pressure gauge to the pump mounting jig 17 or the like, the depth of the submersible drainage pump 16 can be determined.

[0045] Next, when the cylindrical body 11 reaches the bottom of the water with its lower end 11b in an expanded diameter state (S15), as shown in Figure 7(b), the operation of the multiple submersible drainage pumps 16 is stopped, and each submersible drainage pump 16 is detached from the pump mounting jig 17 by releasing the automatic hook 23 (S16).

[0046] Next, if it is necessary to further press the upper end 11a against the outer peripheral surface of the pile 10, the pulling portions 15a, 15b of the wire 15 may be further pulled in the horizontal directions H, H' by the underwater construction machines 30A, 30B, as shown in Figure 3, thereby reducing the diameter of the upper end 11a of the cylindrical body 11 (S17) as shown by the dashed line in Figure 6.

[0047] The submersible drainage pumps 16 that have been detached from the pump mounting jig 17 as described above are recovered by pulling up the power cables and wires 18 shown in Figures 8(a) and 8(b) (S18). The power cables and wires 18 of the submersible pumps 16 are held by a retaining ring member 19 installed near the water surface on the outer periphery of the pile 10, and can therefore be easily recovered.

[0048] Next, the position of the sheet material 20 is adjusted around the outer periphery by the underwater construction machine 30A (S19), and the sheet material 20 is laid around the pile 10 within a range corresponding to at least the inner diameter of the cylindrical body 11 (S20).

[0049] Next, as in Fig. 4, pressing members 31 such as crushed stones, blocks, bags, etc. are poured and placed on the bottom of the water near the outer periphery 20a of the sheet material 20 (S21). This allows the sheet material 20 laid around the pile 10 to be pressed down by the pressing members 31 at and near the outer periphery 20a.

[0050] According to the second embodiment shown in Figures 6 to 8, a cylindrical body 11 made of sheet material 20 with an inner diameter larger than the outer diameter of the pile 10 is submerged in water. The submersible drainage pumps 16 attached to the lower end 11b of the cylindrical body 11 are operated to spray water toward the outer periphery of the pile 10, causing each submersible drainage pump 16 to move away from the pile 10 due to the reaction force. This causes the diameter of the cylindrical body 11 to expand at the lower end 11b and the diameter of the upper end 11a of the cylindrical body 11 to contract. This allows the sheet material 20 to be laid around the pile 10 within an area corresponding to at least the inner diameter of the cylindrical body 11 on the water bottom. This allows for the installation of sheet material-based scour prevention measures in a short construction period. Furthermore, the use of large amounts of rubble, blocks, or bags as in the conventional method is unnecessary, resulting in low-cost scouring prevention measures. Furthermore, the sheet material 20 is held down by a holding member 31 near the outer periphery 20a, ensuring the stability of the sheet material 20, ensuring reliable coverage of the water bottom around the pile 10 with the sheet material 20. As described above, by laying the sheet material 20, the water flow velocity acting on the bottom surface due to the influence of water flow and waves can be reduced, and the sheet material 20 can prevent the outflow of soil and sand from the bottom surface, thereby providing measures to prevent scouring of the pile foundation.

[0051] The shape of the cylindrical body 11 will be described with reference to FIG. 9. In the first and second embodiments, the cylindrical body 11 has a cylindrical shape with the upper end 11a and the lower end 11b having approximately the same inner diameter, as shown in FIG. 9(a). However, as shown in FIG. 9(b), the cylindrical body 11 may have a truncated cone shape in which the inner diameter of the upper end 11c is smaller than the inner diameter of the lower end 11d. When the cylindrical body 11 has the truncated cone shape shown in FIG. 9(b), the diameter of the upper end 11c can be made closer to the outer diameter of the pile 10, making it easier to reduce the diameter of the cylindrical body 11. Furthermore, by adjusting the inner diameter D1 of the lower end 11d, the laying range n of the sheet material 20 around the pile 10 can be adjusted. For example, by making the inner diameter D1 of the lower end 11d in FIG. 9(b) larger than the inner diameter D of the lower end 11b in FIG. 9(a), the laying range n of the sheet material 20 in FIG. 9(b) can be made larger than the laying range m in FIG. 9(a).

[0052] 1 and 6, the state of narrowing of the upper end 11a of the cylindrical body 11 in steps S04, S13, and S17 can be visually confirmed using images from an underwater camera mounted on the underwater construction machine. Furthermore, the outer peripheral position of the sheet material 20 in steps S05 and S19 can also be visually confirmed using images from the underwater camera of the underwater construction machine. Depending on the depth, it may be desirable to use a light provided on the underwater construction machine for confirmation.

[0053] Furthermore, when implementing the above-mentioned pile foundation scour prevention method, it is advisable to check the wave forecast and forecast the wave height around the construction site and carry out the work during a period when there will be sufficient days with wave heights below the work limit. If unexpected high waves are expected to hit, the ship should evacuate with the sheet material temporarily installed on the water bottom, and construction should resume at a later date. Furthermore, adjusting the position of the outer periphery of the sheet material 20 on the water bottom is susceptible to the influence of tidal currents, and positioning using underwater construction machinery is not easy. Therefore, it is preferable to carry out the work on a day (neap tide) or at a time (high tide or low tide) when the tidal current is relatively weak.

[0054] Although the embodiments of the present invention have been described above, the present invention is not limited to these, and various modifications are possible within the scope of the technical concept of the present invention. For example, in this embodiment, the sheet material 20 of the cylindrical body 11 is made of geotextile, but in this case, selecting a fine-meshed material such as woven fabric can reliably achieve scouring prevention effects. The sheet material 20 may also be made of a malleable rubber material, in which case it can accommodate unevenness such as irregularities and reefs on the water bottom within the area where the sheet material 20 is laid.

[0055] 8(a)(b), instead of the automatic hook 23, the submersible drainage pump 16 may be attached directly to the pump mounting jig 17, and a release mechanism may be provided that automatically detaches the pump mounting jig 17 from the sheet material 20 with a predetermined force, so that the submersible drainage pump 16 and the pump mounting jig 17 will detach from the sheet material 20 together with the pump mounting jig 17 when the combined force of the reaction force when the submersible drainage pump 16 sprays water and the force of the water flow returning from the outer surface of the pile reaches its maximum and the lower end 11b of the cylindrical body 11 expands to its maximum diameter. [Industrial Applicability]

[0056] According to the pile foundation scouring prevention method of the present invention, sheet material can be easily laid around the piles even if the pile foundation is located at great depths, so that, for example, the monopile foundation of an offshore wind power generation facility installed at great depths can be prevented from being scouring the bottom of the water. [Explanation of symbols]

[0057] 10 stakes 11 Cylindrical body 11a Upper end 11b Bottom end 12 Float 13 weight 15 wire 15a, 15b tension section 16 Submersible drainage pump 16a Drain 17 Pump mounting jig 18 Power Cables and Wires 20 Sheet material 20a Outer circumference 23 Automatic Hook 30A,30B Underwater construction machinery 31 Holding member

Claims

1. A cylindrical body made of a flexible sheet material and having an inner diameter larger than the outer diameter of the pile is installed on the pile driven into the bottom of the water so as to surround the pile, Submerging the cylindrical body in water; The upper end of the cylindrical body is reduced in diameter by an underwater construction machine so that the upper end of the cylindrical body contacts the pile over the entire circumference, A method for preventing scouring of a pile foundation, in which the sheet material is laid on the bottom surface of the water around the pile within a range equivalent to at least the inner diameter of the cylindrical body.

2. A method for preventing scouring of pile foundations as described in claim 1, in which multiple weights are provided at predetermined intervals around the circumferential direction of the lower end of the cylindrical body, and the diameter of the upper end of the cylindrical body is reduced while the cylindrical body sinks in the water or after the multiple weights reach the bottom of the water.

3. A method for preventing scouring of pile foundations as described in claim 1 or 2, in which the rectangular sheet material is hung from above the water into the water and deformed into a cylindrical shape so as to surround the pile using an underwater machine, thereby installing the cylindrical body.

4. A cylindrical body made of a flexible sheet material and having an inner diameter larger than the outer diameter of the pile is installed on the pile driven into the bottom of the water so as to surround the pile, A plurality of submersible drainage pumps are provided at predetermined intervals around the circumferential direction of the lower end of the cylindrical body, with their drainage outlets facing the outer circumferential surface of the pile, Submerging the cylindrical body in water; During the sinking of the cylindrical body, the submersible drainage pumps are operated to spray water toward the outer peripheral surface of the pile, and a reaction force causes the submersible drainage pumps to move away from the pile, thereby expanding the diameter of the cylindrical body at the lower end, The upper end of the cylindrical body is reduced in diameter by an underwater construction machine so that the upper end of the cylindrical body contacts the pile over the entire circumference, A method for preventing scouring of a pile foundation, in which the sheet material is laid on the bottom surface of the water around the pile within a range equivalent to at least the inner diameter of the cylindrical body.

5. A method for preventing scouring of pile foundations as described in claim 4, wherein the upper end of the cylindrical body is reduced in diameter by an underwater construction machine so that the vertical upper end of the cylindrical body approaches the pile around its entire circumference before the operation of the multiple underwater drainage pumps.

6. 6. A method for preventing scouring of a pile foundation according to claim 4 or 5, wherein the plurality of submersible drainage pumps are recovered after the cylindrical body has reached the bottom of the water.

7. the cylindrical body has a flexible ring-shaped member at its upper end; 7. A method for preventing scouring of a pile foundation according to claim 1, wherein the diameter of the upper end of the cylindrical body is reduced by pulling the ring-shaped member in a substantially horizontal direction using the underwater construction machine.

8. 8. A method for preventing scouring of a pile foundation according to claim 1, wherein the position of the outer periphery of the sheet material on the bottom surface of the water is adjusted by an underwater construction machine.

9. 9. The method for preventing scouring of a pile foundation according to claim 1, further comprising the step of: laying the sheet material around the pile; and then installing a holding member near the outer periphery of the sheet material.

10. The method for preventing scouring of a pile foundation according to any one of claims 1 to 9, wherein the cylindrical body has a cylindrical shape or a truncated cone shape.

11. 11. The method for preventing scouring of a pile foundation according to claim 1, wherein the sheet material is made of a geotextile material or a rubber material.

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