Method for creating underwater banking structure

Abstract

To provide a method for creating an underwater banking structure which can increase a sediment receiving capacity without performing separate work aiming at depression creation for receiving sediment, when a banking structure for preventing sediment from flowing into a route is created in the water.SOLUTION: In the vicinity of a banking creation scheduled position, a conventional ground at a sea bottom and a modifier are stirred and mixed at an original position and modified soil is produced, the modified soil is moved to a creation scheduled position and banking is created. When the modified soil is produced, a casing 21 having an agitation blade therein is pressed into the sea bottom, the conventional ground is taken in the casing 21, the modifier is sent into the casing 21, and the conventional ground taken into the casing 21 and the modifier sent into the casing 21 are agitated and mixed by the agitation blade. A gap remaining after the modified soil produced in the casing 21 is moved is left without being backfilled, and is made to function as a recess capable of receiving the sediment.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for constructing an underwater embankment structure to prevent sediment from flowing into a waterway. [Background technology]

[0002] A modifier made from steelmaking slag has been proposed. This modifier has properties such as increasing strength and solidifying when mixed with highly water-containing clayey soil, and so its application to landfill materials and materials for construction of shallow areas and tidal flats as a technology for effectively utilizing clayey soil such as the seabed is being considered. Patent Document 1 proposes a technology for constructing submerged breakwaters on the seabed or other water bottom using a modifier made from steelmaking slag, with the aim of preventing sediment from flowing into waterways. This technology makes it possible to effectively utilize "dredged clayey soil," which has traditionally been disposed of as waste, as a material for constructing submerged breakwaters.

[0003] Also, Patent Document 2 proposes a method of constructing an excavation near submerged breakwaters installed on both sides of the channel to receive sediment that may fill the channel. In this method, excavations are formed at appropriate intervals along the sides of the channel, and submerged breakwaters are installed on the channel side of the edges of the excavations to deposit sediment stirred up by waves inside the excavations, preventing sediment from accumulating in the channel, and dredging the sediment accumulated in the excavations at appropriate intervals. According to the method of Patent Document 2, the deposition of sediment in the channel is prevented by installing submerged breakwaters and excavations on the sides of the channel, and dredging of sediment accumulated in the excavations on the sides of the channel can be performed without blocking the channel, and does not affect the navigation of ships. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2017-190667 A (Fig. 14) [Patent Document 2] Patent No. 4041903 (Figure 1) Summary of the Invention [Problem to be solved by the invention]

[0005] Even if a submerged breakwater is constructed on the seabed using the technology of Patent Document 1, sediment will flow in and accumulate from rivers and areas around the waterway, and if left untreated, the seabed will become shallower, hindering ship navigation and impairing the function of the waterway. Therefore, in order to ensure the water depth of the waterway and maintain its function, it is necessary to remove the accumulated sediment, and frequent dredging has been indispensable. Therefore, even if a submerged breakwater is constructed using the technology of Patent Document 1, a new means has been demanded that can reduce the frequency of dredging to maintain the function of the waterway.

[0006] Also, apart from constructing the submerged breakwater using the technology of Patent Document 1, it was considered to carry out additional work using the method of Patent Document 2 to create a new depression (excavation) on the seabed to receive the inflowing sediment. However, if a depression to receive the sediment is to be created in another location (for example, near the submerged breakwater that has already been constructed) after the submerged breakwater is constructed, the submerged breakwater construction work and the depression creation work are required separately, and additional vessels such as grab dredgers and soil transporters are required to create the depression, resulting in double work and increased construction costs.

[0007] In view of the above-mentioned problems, the object of the present invention is to provide a method for constructing underwater embankment structures which makes it possible to increase the volume of sediment that can be received when constructing underwater embankment structures to prevent sediment from flowing into a waterway, without carrying out separate construction work to create a depression for receiving the sediment. [Means for solving the problem]

[0008] The above-mentioned object is achieved by a method for constructing an underwater embankment structure to prevent sediment from flowing into a waterway, the method including the steps of: producing modified soil by stirring and mixing existing ground on the bottom of the water with modified material in situ near the planned underwater embankment construction location; moving the modified soil toward the planned underwater embankment construction location so that a depression for receiving sediment is formed after the movement of the modified soil; and constructing an embankment at the planned underwater embankment construction location using the modified soil.

[0009] The above-mentioned method for constructing an underwater embankment structure may further include a step of producing modified soil by stirring and mixing the existing ground below the planned embankment construction site and the modified material in situ before constructing the embankment.

[0010] In addition, in the above-mentioned method for constructing an underwater embankment structure, the process of producing modified soil includes the steps of pressing a casing equipped with a mixing means on the inside into the existing ground to take in the existing ground into the casing, sending the modifying material toward the inside of the casing pressed into the existing ground, and mixing the existing ground taken into the casing during the pressing process and the modifying material sent into the casing by the mixing means.

[0011] In addition, in the above-mentioned method for constructing an underwater embankment structure, when moving the produced modified soil, the modified soil obtained by mixing the existing ground and the modifying material is contained in the casing, and the casing is pulled out from the existing ground and moved to above the planned embankment construction location.

[0012] The modifier used in the above-mentioned method for constructing an underwater embankment structure may be, for example, a material that hardens upon reaction with clayey soil, a specific example of which is steel slag. Effect of the Invention

[0013] According to the construction of a water-bottom embankment structure according to the present invention, the following excellent effects can be achieved. In this application, specific examples of the "bottom of the water" include the bottom of the ocean, the bottom of a river, and the bottom of a lake. In addition, specific examples of the "ground at the bottom of the water" include the bottom of the ocean, the bottom of a river, and the bottom of a lake. In the following description of the present invention, the seabed will be taken as an example of the bottom of the water, and the seabed ground will be taken as an example of the bottom ground of the water.

[0014] According to the method for constructing an underwater embankment structure of the present invention, the gaps in the seabed (the voids left by the casing being pulled out) formed in the process of constructing the embankment structure are used as depressions capable of receiving sediment, so that in one construction work, both the underwater embankment structure for preventing sediment from flowing into the channel and the depression for receiving sediment can be simultaneously constructed. Therefore, according to the present invention, there is no need for grab dredgers or soil transport ships to carry out separate construction work for the purpose of constructing a depression for receiving sediment, and there is no need to spend time and effort on increasing the capacity to receive sediment.

[0015] Furthermore, according to the present invention, the existing ground (clay soil) on the seabed can be effectively utilized as a material for constructing embankment structures, eliminating the need to dispose of it.

[0016] In addition, when the ground on which the embankment structure of the present invention is to be constructed is weak, the stability of the embankment structure can be increased and subsidence or collapse can be prevented by producing modified soil in advance below the planned location for embankment construction. Furthermore, in the case where a rise occurs when producing modified soil by mixing the existing ground and the modifying material in situ, the effect of raising the top of the embankment structure is also achieved.

[0017] Furthermore, in the present invention, modified soil is produced within a casing by mixing the existing seabed ground (clay soil) with the modifying material, and the modified soil is then transported while contained within the casing to the planned construction location of the embankment structure, so that no pollution is caused in the sea during the construction process of the embankment structure.

[0018] The present invention can also be used, for example, when constructing a submerged breakwater (a breakwater to prevent sediment from flowing into a seaway) to prevent sediment from flowing into the seaway. In this case, the existing ground on the seaway side or outside of the breakwater and the modifying material are mixed in situ inside the casing to produce modified soil. As a result, a void (a depression that can receive sediment) can be formed in the seabed ground on the seaway side or outside of the breakwater to be constructed. When improving soil is produced using the existing ground on the "channel side" of the sediment inflow prevention dike, the gaps formed during the process when the casing is removed can function as depressions to prevent the accumulation of sediment in the channel. On the other hand, when improving soil is produced using the existing ground "outside" the seaway sediment inflow prevention dike, the gaps left when the casing is removed during the process increase the capacity of the seaway sediment inflow prevention dike to receive inflowing sediment, thereby reducing the frequency of dredging.

[0019] In addition, in the present invention, for example, a material that reacts with clayey soil and hardens is used as the modifier. A specific example of such a material is a material that causes a hydration reaction, for example, a calcia modifier. The calcia modifier is a material that uses converter steelmaking slag, a type of steel slag, as a raw material, and has its components controlled and its particle size adjusted. Therefore, by using the "calcia modifier" as a modifier, it is possible to effectively utilize the steelmaking slag. [Brief description of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic diagram showing an overview of a first embodiment of a method for constructing an underwater embankment structure. [Diagram 2] FIG. 2 is a schematic diagram showing the effect of the method for constructing the underwater embankment structure shown in FIG. 1. [Diagram 3] FIG. 13 is a schematic diagram showing an overview of a second embodiment of a method for constructing an underwater embankment structure. [Figure 4] FIG. 4 is a schematic diagram showing the effect of the method for constructing an underwater embankment structure shown in FIG. 3. [Diagram 5]FIG. 1A is a side view showing a work boat (ground improvement boat) and an embankment structure construction device used in implementing the present invention, and FIG. 1B is an enlarged side view showing the state of construction of an embankment using the embankment structure construction device. [Figure 6] 6 is an enlarged side view showing the specific configuration of the stirring and mixing section of the embankment structure construction device shown in Figure 5, where Figure 6(a) shows the grab at the lower end of the casing in an open state, and Figure 6(b) shows the grab in a closed state. [Figure 7] FIG. 1 is a process diagram showing a first embodiment of a method for constructing an underwater embankment structure. [Figure 8] FIG. 11 is a process diagram showing a second embodiment of a method for constructing an underwater embankment structure. [Figure 9] 11 is a process diagram showing a second embodiment of the method for constructing an underwater embankment structure, showing a continuation of FIG. 8. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] (First embodiment of method for constructing embankment structure) First, an outline of a first embodiment of the present invention will be described with reference to Fig. 1 and Fig. 2. Specific steps of the first embodiment will be described later with reference to Fig. 7.

[0022] The method for constructing an undersea embankment structure is carried out using an embankment structure construction device 5 equipped with a casing 21, a grab 25, etc. The specific configuration of the embankment structure construction device 5 will be described later with reference to Figs.

[0023] In constructing an embankment structure, first, as shown in Fig. 1(a), an embankment structure construction device 5 is positioned on the seabed a predetermined distance away from the planned embankment construction position (for example, near the planned embankment construction position), and a casing 21 of the embankment structure construction device 5 is pressed into the seabed ground. During the process of pressing the casing 21, the existing ground (clay soil) on the seabed is taken into the casing 21.

[0024] Next, the existing ground taken into the casing 21 of the embankment structure construction device 5 and the modifying material poured into the casing 21 via the pipeline 9 are stirred and mixed in situ to produce modified soil. In this application, "in situ" means the location where the existing ground was taken into the casing 21 (i.e. the location where the taken-in existing ground was located), and the production of modified soil is carried out in such an in situ location.

[0025] The modified soil is made by mixing the existing seabed soil with the modified material. The modified soil produced inside the casing will be used in the construction of embankments in a later process. The constituent materials of the modifier to be mixed with the existing ground are not particularly limited, and may be steel slag, cement, cement-based solidification material, quicklime, lime-based solidification material, etc. A specific example of a usable modifier is a material that develops strength through a hydration reaction over time, such as calcia modifier. Calcia modifier is a material that uses converter steelmaking slag, a type of steel slag, as a raw material, and has its components and particle size adjusted. Therefore, by using calcia modifier as a modifier, steelmaking slag can be effectively utilized.

[0026] Next, as shown in Fig. 1(b), the modified soil, which is made by mixing the existing ground and the modified material, is moved to the planned embankment construction position. At that time, the grab 25 of the embankment structure construction device 5 is closed to confine the modified soil inside the casing 21, and the casing is pulled out from the seabed and moved to the specified position.

[0027] 1(c), the produced modified soil is discharged from the casing 21 at the planned embankment construction position to construct the embankment. In this embodiment, the embankment constructed in this manner functions as an embankment structure.

[0028] After the produced modified soil is moved, a gap formed by the extraction of the casing 21 remains on the seabed, but this gap is left as it is without being backfilled and functions as a depression capable of receiving soil and sand, as shown in Figure 1(c). In other words, the gap where the casing was extracted that remains after the modified soil is moved functions as a pocket for receiving soil and sand.

[0029] In constructing the embankment, the process of producing modified soil in situ on the seabed and the process of moving the modified soil to raise the height of the embankment may be repeated several times, thereby increasing the volume of the void left by removing the casing, and further increasing the soil receiving capacity.

[0030] The method for constructing an embankment structure according to the first embodiment described above is used when constructing a submerged breakwater (sewerage channel sediment inflow prevention breakwater) that prevents sediment from flowing into a sea channel, as illustrated in Fig. 2(a) and (b). In this case, the existing ground on the sea channel side or outside of the sea channel sediment inflow prevention breakwater and the modifying material are mixed in situ inside the casing to produce modified soil. As a result, a void (a depression that can receive sediment) is formed in the seabed ground on the sea channel side or outside of the sea channel sediment inflow prevention breakwater to be constructed. As shown in Figure 2(a), when improving soil is produced using the existing ground on the "channel side" of the sediment prevention dike, the gap formed during the process when the casing is removed can function as a depression to prevent sediment from entering the channel. On the other hand, as shown in Figure 2(b), when improving soil is produced using the existing ground "outside" the sediment inflow prevention dike, the depression formed during the process when the casing is removed increases the capacity of the dike to receive inflowing sediment, thereby reducing the frequency of dredging to remove accumulated sediment.

[0031] (Second embodiment of the method for constructing an embankment structure) Next, an outline of a second embodiment of the present invention will be described with reference to Figures 3 and 4. Specific steps of the second embodiment will be described later with reference to Figures 8 and 9.

[0032] In the second embodiment, before constructing the embankment, an underground part is constructed below the planned location for the embankment. That is, first, as shown in Figures 3(a) and (b), an underground part that will be the foundation of the embankment is constructed, and then, as shown in Figures 3(c) and (d), the embankment is constructed on top of the underground part. The combination of the underground part and the embankment constructed on top of it functions as an embankment structure. This will be explained in detail below.

[0033] First, as shown in Fig. 3(a), the embankment structure construction device 5 is positioned at the planned embankment construction location, and the casing 21 of the embankment structure construction device 5 is pressed into the seabed ground. In the process of pressing the casing 21, the existing ground (clay soil) on the seabed is taken into the casing 21. Then, the existing ground taken into the casing and the modifying material introduced into the casing 21 through the pipeline 9 are stirred and mixed inside the casing 21 to produce modified soil. Next, the grab 25 at the bottom of the casing is opened, and the thoroughly mixed and remediated soil is backfilled in the original location (where the existing ground was taken in) and left there. In other words, the existing ground is replaced with the remediated soil. This completes a part of the underground area that will become the foundation for constructing the embankment.

[0034] The above steps are repeated several times to create an underground section made of modified soil, as shown in FIG. 3(b).

[0035] Next, the embankment structure construction device 5 is positioned on the seabed a predetermined distance away from the planned embankment construction position (for example, near the planned embankment construction position), and the casing 21 of the embankment structure construction device is pressed into the seabed ground. During the process of pressing the casing 21, the existing ground (clay soil) on the seabed is taken into the casing 21. Next, the existing ground taken into the casing 21 of the embankment structure construction device 5 and the modifying material introduced into the casing 21 via the pipeline 9 are stirred and mixed in situ to produce modified soil.

[0036] Next, as shown in Fig. 3(c), the modified soil produced at a location away from the planned embankment construction location is moved to the planned embankment construction location. At that time, the grab 25 of the embankment structure construction device 5 is closed to confine the modified soil within the casing 21, and the casing is pulled out from the seabed and moved to a predetermined location.

[0037] Then, as shown in Fig. 3(d), the modified soil produced in the casing 21 is discharged from the casing 21 above the already constructed underground part to construct an embankment. In this embodiment, the combination of the underground part and the embankment constructed thereon functions as an embankment structure.

[0038] After the modified soil produced at a location away from the planned embankment construction site is moved, a gap left on the seabed by the extraction of the casing 21 is left, but this gap is left as it is without being backfilled, as shown in Figure 3(d), and functions as a depression that can receive soil and sand. In other words, the gap where the casing was extracted that remains after the modified soil is moved functions as a pocket to receive soil and sand. In constructing the embankment, the process of producing modified soil in situ on the seabed and the process of moving the modified soil to raise the height of the embankment may be repeated several times, thereby increasing the volume of the void left by removing the casing, and further increasing the soil receiving capacity.

[0039] The method for constructing an embankment structure according to the second embodiment described above is used when constructing a submerged breakwater (sewerage channel sediment inflow prevention breakwater) that prevents sediment from flowing into a sea channel, as illustrated in Fig. 4(a) and (b). In this case, the existing ground on the sea channel side or outside of the sea channel sediment inflow prevention breakwater and the modifying material are mixed in situ inside the casing to produce modified soil. As a result, a void (a depression that can receive sediment) is formed in the seabed ground on the sea channel side or outside of the sea channel sediment inflow prevention breakwater to be constructed. As shown in Figure 4(a), when improving soil is produced using the existing ground on the "channel side" of the sediment prevention dike, the gap formed during the process when the casing is removed can function as a depression to prevent sediment from entering the channel. On the other hand, as shown in Figure 4(b), when improving soil is produced using the existing ground "outside" the sediment inflow prevention dike, the depression formed during the process when the casing is removed increases the capacity of the dike to receive inflowing sediment, thereby reducing the frequency of dredging to remove accumulated sediment.

[0040] (Work boat) Next, the configuration of a work boat (ground improvement boat) used in carrying out the method for constructing an embankment structure will be described with reference to Figs.

[0041] As shown in Figure 5, the work boat 1 used to carry out the embankment structure construction method mainly has a leader 3 erected on the bow side of the work boat 1, and an embankment structure construction device 5 that is freely movable up and down along this leader 3.

[0042] The configuration of the embankment structure constructing device 5 provided on the work vessel will be described in detail below.

[0043] (Embankment structure creation equipment) The embankment structure construction device 5 is a device configured to produce modified soil by stirring and mixing existing ground (clay soil) on the seabed with a modifying material in situ, and to be able to discharge the produced modified soil at a predetermined position in the sea.

[0044] As shown in FIG. 5, the embankment structure construction device 5 is provided so as to be able to move up and down along the leader 3. Also, as shown in FIGS. 5 and 6, A stirring and mixing unit 7 (stirring and mixing means) including a casing 21, a stirring blade 23, and a glove 25; A pipe 9 including a flow path for feeding the modifying material into the casing 21; A material inlet 13 for injecting a modifier into the inner pipe 11 of the pipe 9; an auger motor 15 which is a rotary drive device; It has a rod (not shown) for transmitting the power of the auger motor 15 to the stirring blades 23 of the stirring and mixing section 7.

[0045] The pipeline 9 has an outer pipe 10 which functions as a support member for the casing 21, and an inner pipe 11 which is a material supply path provided so as to be movable up and down inside the outer pipe 10. The upper end side of the inner pipe 11 which the pipeline 9 has communicates with the material inlet 13, and the lower end side thereof communicates with the inside of the casing 21 of the stirring and mixing section 7.

[0046] 6, the inner pipe 11 of the pipeline 9 has a modifying agent outlet 12 (not shown) above (directly above) the agitating blade 23, and functions as a flow path for sending the modifying agent toward the agitating blade 23 in the casing 21. The inner pipe 11, which is a material supply path, is provided so that its outlet 12 follows the rising and falling movement of the agitating blade 23 in the casing 21. In other words, the agitating blade 23 and the inner pipe 11 are configured to be able to rise and fall together.

[0047] The inner pipe 11 also serves as an air flow path for sending compressed air into the casing 21. By sending compressed air from an air compressor (not shown) into the casing 21 through the inner pipe 11, it is possible to use the air pressure to discharge seawater from inside the casing 21, which is submerged in the sea, and to maintain the state in which seawater has been discharged from inside the casing. The embankment structure construction device 5 is equipped with an internal casing pressure control device that reduces the air pressure inside the casing 21 by exhausting the air inside the casing 21 to the outside of the casing. By using this internal casing pressure control device, the air pressure inside the casing 21 (the pressure of the compressed air sent into the casing 21) can be controlled.

[0048] The rotational power output from the auger motor 15 is transmitted to the stirring blades 23 provided in the casing 21 of the stirring and mixing section 7 via a rod (not shown).

[0049] As shown in Fig. 5(a), this auger motor 15 is connected to the leader 3 so as to be able to rise and fall freely, and its upper end is connected to a wire rope 16. By operating the wire rope 16 in the up and down direction, the entire embankment structure construction device 5 including the auger motor 15 rises and falls along the leader 3.

[0050] As shown in Figs. 5 and 6, the stirring and mixing section 7 provided at the lower end of the pipe 9 mainly has the following functions: A sealable casing 21 with an openable grab 25; The mixing blade 23 is provided in the casing 21 so as to be capable of ascending and descending.

[0051] The casing 21 is a box-shaped container-like component (rectangular casing) that is generally rectangular in plan view, and is configured so that the opening at the bottom can be opened and closed. The lower end of the outer pipe 10 of the pipeline 9 is connected to the top of the casing 21, and the modifier introduced from the material introduction port 13 is fed into the casing 21 through the inner pipe 11. Compressed air sent by an air compressor (not shown) is also fed into the casing 21 through the inner pipe 11. When the stirring and mixing section 7 is submerged in seawater, the seawater can be discharged from inside the casing 21 by the pressure of the compressed air. That is, a space into which the seawater is discharged by air pressure can be secured within the casing 21.

[0052] The lower end of a rod (not shown) connected to the auger motor 15 is connected to an agitator blade 23 provided inside the casing 21. The agitator blade 23 is driven to rotate by power transmitted via the rod, and is provided so as to be movable up and down inside the casing 21.

[0053] Also, an openable grab 25 is provided at the tip (lower end) of the casing 21. The combination of the casing 21 and the grab 25 can be said to constitute one casing as a whole. When the grab 25 is opened, the bottom of the casing 21 opens, and when the grab 25 is closed, the bottom of the casing 21 closes and is sealed. In other words, the grab 25 not only serves as an intake means for taking in the existing ground (clay soil) on the seabed into the casing, but also serves as a lid for opening and closing the bottom of the casing (a lid for sealing the bottom of the casing 21).

[0054] Such a grab 25 has, for example, a hydraulic cylinder as a means for opening and closing the grab. 6(a) shows the grab 25 in an open state. In this state, the bottom of the casing 21 is open. Through a series of operations when the grab 25 is closed from the open state, the existing ground on the seabed can be taken into the inside of the grab 25 and the casing 21. The closed state of the grab 25 is shown in Fig. 6(b). In this state, the bottom of the casing 21 is closed and the casing 21 is sealed. When the grab 25 is closed after the existing ground is taken into the casing 21, the casing 21 is sealed and the existing ground is trapped inside the casing 21.

[0055] The stirring blade 23, which is a stirring and mixing means, is provided in the casing 21 so as to be rotatable and movable up and down. The main role of the mixing blades 23 is to mix the existing seabed ground taken into the casing 21 by the grab 25 with the modifier introduced into the casing 21 through the inner pipe 11 of the pipeline 9 . The stirring blades 23 also play a role in loosening (loosening) the existing soil that has been taken into the casing 21 before the above-mentioned stirring and mixing is performed.

[0056] (Specific steps of the embankment structure construction method of the first embodiment) Next, specific steps of the first embodiment of the method for constructing an embankment structure shown in Fig. 1 will be described with reference to Fig. 7. Note that steps a to i described below correspond to Fig. 7(a) to (i).

[0057] Process a With the grab 25 open, compressed air is sent into the casing 21 through the inner pipe 11 of the pipeline 9 to drain the seawater from the casing and make the inside of the casing hollow. That is, the air pressure ensures a space without seawater inside the casing 21. Thereafter, the air pressure keeps the casing 21 from letting seawater in.

[0058] Process b The tip of the casing 21, i.e., the tip of the mixing section 7, is pressed against the top of the existing ground (clay soil) at the planned location for embankment construction. Then, the hydraulically driven grab 25 at the tip of the casing 21 is used to take the existing ground on the seabed into the casing 21.

[0059] Process c The grab 25 at the tip of the casing 21 is repeatedly opened and closed to draw the existing ground into the casing 21. In this embodiment, the embankment structure construction device 5 is equipped with a casing pressure control device that reduces the air pressure inside the casing 21 by exhausting the air inside the casing 21 to the outside of the casing. By using this casing pressure control device, the existing ground can be taken into the casing 21 without being hindered by the air pressure inside the casing 21.

[0060] Process d Once the required amount of existing ground has been taken into the casing 21, the grab 25 at the end of the casing is closed. This seals the casing 21, and the existing ground taken into the casing 21 is isolated from the seawater and ground surrounding the casing.

[0061] Process e The agitator blades 23 in the casing 21 are lowered, and the existing ground taken into the casing 21 is loosened by the rotation and up-down movement of the agitator blades 23. This loosens the existing ground in the casing 21, making it possible to uniformly mix the existing ground and the modifier in the subsequent step (step g).

[0062] process f With the bottom (grab 25) of the casing 21 kept closed, a predetermined amount of modifier is introduced into the casing 21 through the inner pipe 11 of the pipeline 9.

[0063] Process g After the modifying material is poured into the casing 21, the agitating blades 23 inside the casing are rotated and moved up and down inside the casing 21 to mix and stir the "existing ground taken into the casing 21" and the "modifying material poured into the casing 21." By mixing and stirring these inside the casing 21, "modified soil" is produced.

[0064] Process h Next, the modified soil, which is a mixture of the existing ground and the modified material, is moved to the planned embankment construction location. At this time, the grab 25 of the embankment structure construction device 5 is closed to trap the modified soil inside the casing 21, and the casing 21 is pulled out from the seabed and moved to the specified location. The hole where the casing 21 is pulled out after the modified soil has been moved functions as a soil receiving pocket.

[0065] Process i Next, the bottom (grab 25) of the casing 21 is opened at the planned position for constructing the embankment, and the produced modified soil is discharged by dropping it from the casing 21, to construct an embankment made of the modified soil. When discharging the modified soil from the casing 21, if the modified soil does not fall well after the grab 25 is opened or if it is desired to forcibly discharge the modified soil, compressed air may be sent into the casing 21 as an auxiliary. Also, the stirring blades 23 may be lowered without rotating to push the modified soil out of the casing 21.

[0066] In this embodiment, the embankment constructed through the above-mentioned steps functions as an embankment structure.

[0067] (Specific steps of the embankment structure construction method of the second embodiment) Next, specific steps of the second embodiment of the method for constructing an embankment structure shown in Fig. 3 will be described with reference to Fig. 8 and Fig. 9. Steps A to H described below correspond to Fig. 8(a) to (h). Steps I to Q correspond to Fig. 9(I) to (Q).

[0068] In this embodiment, an underground part that will be the foundation for constructing the embankment is constructed by carrying out steps A to H, and the embankment is constructed on top of the underground part by carrying out steps I to Q. The combination of the underground part and the embankment constructed on top of it functions as an embankment structure.

[0069] Process A With the grab 25 open, compressed air is sent into the casing 21 through the inner pipe 11 of the pipeline 9 to drain the seawater from the casing and make the inside of the casing hollow. That is, the air pressure ensures a space without seawater inside the casing 21. Thereafter, the air pressure keeps the casing 21 from letting seawater in.

[0070] Process B The tip of the casing 21, i.e., the tip of the mixing section 7, is pressed against the top of the existing ground (clay soil) at the planned location for embankment construction. Then, the hydraulically driven grab 25 at the tip of the casing 21 is used to take the existing ground on the seabed into the casing 21.

[0071] Process C The grab 25 at the tip of the casing 21 is repeatedly opened and closed to draw the existing ground into the casing 21.

[0072] Process D Once the required amount of existing ground has been taken into the casing 21, the grab 25 at the end of the casing is closed. This seals the casing 21, and the existing ground taken into the casing 21 is isolated from the seawater and ground surrounding the casing.

[0073] Process E The agitator blades 23 in the casing 21 are lowered, and the existing ground taken into the casing 21 is loosened by the rotation and up-down movement of the agitator blades 23. This loosens the existing ground in the casing 21, making it possible to uniformly mix the existing ground and the modifier in the subsequent process (process G).

[0074] Process F With the bottom (grab 25) of the casing 21 kept closed, a predetermined amount of modifier is introduced into the casing 21 through the inner pipe 11 of the pipeline 9.

[0075] Process G After the modifying material is poured into the casing 21, the agitating blades 23 inside the casing are rotated and moved up and down inside the casing 21 to mix and stir the "existing ground taken into the casing 21" and the "modifying material poured into the casing 21." By mixing and stirring these inside the casing 21, "modified soil" is produced.

[0076] Process H Next, the bottom (grab 25) of the casing 21 is opened, and the thoroughly mixed and remediated soil is backfilled and left in the original location (where the existing ground was taken). In other words, the existing ground is replaced with the remediated soil. This completes a part of the underground area that will become the foundation for constructing the embankment.

[0077] Repeat steps A to H The above-mentioned steps A to H are repeated at different positions, and finally, the grab 25 is crushed by the casing 21 in the closed state to adjust the top height, and an underground portion made of modified soil is created.

[0078] Through these steps, the underground section that will serve as the foundation for the embankment has been prepared. Next, the underwater embankment structure construction device is moved to the planned embankment construction position, and the following steps I to Q are carried out to construct an embankment on the underground portion.

[0079] Process I With the grab 25 open, compressed air is sent into the casing 21 through the pipeline 9 to drain the seawater from the casing and make the inside of the casing hollow. That is, the air pressure ensures a space without seawater inside the casing 21. Thereafter, the air pressure keeps the casing 21 from letting seawater in.

[0080] Process J The tip of the casing 21, i.e., the tip of the mixing and stirring section 7, is pressed against the top of the existing ground (for example, the ground near the planned embankment construction site) a predetermined distance away from the planned embankment construction site. Then, the hydraulically driven grab 25 at the tip of the casing 21 is used to take the existing ground on the seabed into the casing.

[0081] Process K The grab 25 at the tip of the casing 21 is repeatedly opened and closed to draw the existing ground into the casing 21.

[0082] Process L Once the required amount of existing ground has been taken into the casing 21, the grab 25 (taking-in means) at the tip of the casing is closed. This seals the casing 21, and the existing ground taken into the casing 21 is isolated from the seawater and ground surrounding the casing.

[0083] Process M The agitator blades 23 in the casing 21 are lowered, and the existing ground taken into the casing 21 is loosened by the rotation and up-down movement of the agitator blades 23. This loosens the existing ground in the casing 21, making it possible to uniformly mix the existing ground and the modifier in the subsequent process (process O).

[0084] Process N With the bottom (grab 25) of the casing 21 kept closed, a predetermined amount of modifier is introduced into the casing 21 through the inner pipe 11 of the pipeline 9.

[0085] Process O After the modifying material is poured into the casing 21, the agitating blades 23 inside the casing are rotated and moved up and down inside the casing 21 to mix and stir the "existing ground taken into the casing 21" and the "modifying material poured into the casing 21." By mixing and stirring these inside the casing 21, "modified soil" is produced.

[0086] Process P Next, the modified soil, which is a mixture of the existing ground and the modified material, is moved to the planned embankment construction location. At this time, the grab of the embankment structure construction device is closed to trap the modified soil inside the casing 21, and the casing is pulled out from the seabed and moved to the specified location. The hole where the casing 21 is pulled out after the modified soil has been moved functions as a soil receiving pocket.

[0087] Process Q Next, the bottom (grab 25) of the casing 21 is opened at the planned position for constructing the embankment, and the produced modified soil is discharged from the casing to construct the embankment made of the modified soil.

[0088] In this embodiment, the combination of the underground portion constructed through the above-mentioned steps and the embankment constructed on top of it functions as an embankment structure.

[0089] Finally, the above-mentioned embodiment is an example of the present invention described in the claims, and the present invention is not necessarily limited to this embodiment. In addition, the application of the present invention is not limited to the improvement of seabed ground, but can also be used for the improvement of riverbeds and lakebeds. [Explanation of symbols]

[0090] 1. Work boat (ground improvement boat) 3. Leader 5 Embankment structure creation equipment 7 Stirring and mixing section 9 conduit 10 outer tube 11 Inner tube 12 Outlet 13 Material input port 15 Auger motor 16 Wire Rope 21 Casing 23 Stirring blade (stirring and mixing means) 25 Grab

Claims

1. A method for constructing an underwater embankment structure to prevent sediment from flowing into a waterway, comprising the steps of: A process for producing modified soil by mixing the existing ground and the modified material in situ near the planned underwater embankment construction location; A step of moving the produced modified soil toward the planned embankment construction position so that a depression for receiving soil and sand is formed after the movement of the produced modified soil; A step of constructing an embankment at the planned embankment construction position using the produced modified soil; A method for constructing an underwater embankment structure, comprising:

2. The method for constructing an underwater embankment structure as described in claim 1, further comprising a step of producing modified soil by stirring and mixing the existing ground and modified material in situ below the planned embankment construction location before constructing the embankment.

3. The step of producing the modified soil includes: A step of pressing a casing having an agitation and mixing means therein into the existing ground to take in the existing ground into the casing; sending the modifying material toward the inside of the casing injected into the existing ground; a step of stirring and mixing the conventional ground taken into the casing during the injection process and the modifier sent into the casing by the stirring and mixing means; 3. A method for constructing an underwater embankment structure according to claim 1 or 2, further comprising:

4. When moving the produced modified soil, the modified soil obtained by mixing the existing ground and the modifying material is contained in the casing, and the casing is pulled out from the existing ground and moved to above the planned position for embankment construction.

4. A method for constructing an underwater embankment structure according to claim 3.

5. The modifier is a material that reacts with clay and hardens.

3. A method for constructing an underwater embankment structure according to claim 1 or 2.

6. The modifier is steel slag; 6. A method for constructing an underwater embankment structure according to claim 5.

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