JP2022013448A - Backfilling method for deep-dig underwater depression - Google Patents

Abstract

To provide a backfilling method for an underwater depression of a water bottom which can effectively use viscous soil-based dredge soil.SOLUTION: Construction for backfilling a deep-dig underwater depression of the sea bottom installs viscous soil-based dredge soil to the vicinity of a bottom surface of the underwater depression, and forms a viscous soil layer upward from the bottom surface of the underwater depression, and then further forms a modified soil layer at the original position so as to cover the top of the viscous soil layer by using a modifier in the surface layer part of the viscous soil layer formed in the underwater depression.SELECTED DRAWING: Figure 7

Description

The present invention relates to a method of backfilling a depression on the seabed such as a deep digging trace of the seabed.

Anoxic water and hydrogen sulfide, which adversely affect living organisms, tend to stay in depressions such as deep digging traces on the seabed, which may cause blue tides and worsen the marine environment. Therefore, for the purpose of improving the marine environment, backfilling work of depressions such as deep digging traces on the seabed has been carried out conventionally.

When backfilling depressions such as deep seabed traces, "sandy dredged soil" is generally used as the backfill material in order to reduce turbidity during construction and reduce the risk of fluid mud rolling up after backfilling. Is often used.

In conventional backfilling using sandy dredged soil, for example,
・ Free fall from a water depth (for example, about 15m) where tremie can be thrown,
・ A large sealed grab can be used for free fall from a water depth (for example, a water depth of about 25 m).
・ Install a large mud pressure pump on the work boat and place it underwater from the discharge pipe.
The method such as is adopted.

In recent years, a method of preventing fluid mud from rolling up after backfilling has been proposed by forming a modified soil in which steelmaking slag is mixed with highly hydrous cohesive soil as a covering layer (see, for example, Patent Document 1).

Japanese Patent No. 5573692

Sandy dredged soil is used as a standard material for each application as sea sand. On the other hand, cohesive soil-based dredged soil, which is generated in large quantities in various dredging works, has been stably treated and carried out by disposal or by adding a large amount of solidifying material.

Therefore, the inventors of the present application have changed the "cohesive dredged soil", which was conventionally disposed of at a large cost, to the seabed from the viewpoint of effective use of resources for the purpose of building a sound material-cycle society. It was decided to examine whether it could be effectively used as a backfill material in the backfilling work of the depression.

However, since the cohesive soil-based dredged soil is soft and has a high water content, the cohesive soil-based dredged soil is simply poured into the pockmark by the above-mentioned conventional backfilling method. There is a risk that pollution will occur due to the rolling up of dredged soil, causing environmental damage such as damaging the surrounding fishing grounds.

In addition, in the backfilling of depressions in deep water (about 20 m or deeper) or strong tidal currents, cohesive dredged soil diffuses during the backfilling work in free fall, causing turbidity. If dredged soil is placed near the bottom of the depression by pumping, the turbidity generated on the bottom will not easily diffuse out of the depression at the beginning, but as the construction progresses, the turbidity will be suppressed by the topography of the depression. The effect is reduced.

If the modified dredged soil can be formed as a covering layer as in Patent Document 1, it can be expected to prevent the dredged soil from rolling up after backfilling. However, since this reformed dredged soil is supposed to be a method that utilizes a tremie pipe or a bucket of a grab ship, it will be used for backfilling depressions in deep water (about 20 m or deeper) and strong tidal currents. I couldn't.

Therefore, in view of the above-mentioned problems, an object of the present invention is to fill a depression in the bottom of the water, which can effectively utilize "viscous soil-based dredged soil" as a backfill material even in a deep water region or a strong tidal current region. It is to provide a return method.

The purpose of the above is to cast a cohesive soil-based dredged soil near the bottom surface of the depression so as to backfill the depression at the bottom of the water, and to form a cohesive soil layer from the bottom surface of the depression toward the top, and a cohesive soil layer. It is achieved by the step of forming the modified soil layer in the in-situ so as to cover the top of the cohesive soil layer by using the modifying material on the surface layer portion of the above, and the method of backfilling the depression of the water bottom including.

Further, as the reforming material used in the backfilling method, for example, a material that reacts with cohesive soil-based dredged soil and hardens can be used. Specifically, steel slag can be used as the modifier.

Further, the backfilling method preferably further includes a step of forming a sand-covering material layer made of a sand-covering material in the depression so as to cover the modified soil layer.

In the step of forming the sand covering material layer in the depression, it is preferable to use natural sand as the sand covering material.

An excellent effect is achieved by using the method according to the present invention for backfilling a submerged depression such as a deep digging trace of the seabed.
Specific examples of the "water bottom" in the present invention include seabeds, riverbeds, and lake bottoms. Specific examples of the "underwater ground" include seabed ground, riverbed ground, and lake bottom ground.
Hereinafter, in the description of the present invention, the seabed will be mentioned as an example of the seabed, and the seabed will be mentioned as an example of the seabed ground.

In the present invention, first, a cohesive soil-based dredged soil is cast near the bottom surface of the depression to form a cohesive soil layer from the bottom surface of the depression toward the upper part. Next, a reforming material is used on the surface layer of the cohesive soil layer formed in the depression to further form the modified soil layer in the in-situ so as to cover the top of the cohesive soil layer.
If "cohesive soil-based dredged soil" is simply used as a backfill material, the cohesive soil-based dredged soil put into the depression will flow out under the influence of tidal currents and cause seabed pollution. In the present invention, dredged soil is directly placed near the bottom surface of the depression by pumping using a casing pipe or the like, and a modified soil layer is formed in the original position so as to cover the cohesive soil layer. .. Since the modified soil covers the top of the cohesive soil layer and functions like a lid, the cohesive soil layer below it is not affected by tidal currents, etc. Therefore, even if cohesive soil-based dredged soil is used, the seabed Does not cause pollution. In addition, by forming the modified soil layer in the original position, the risk of turbidity due to the rolling up of the cohesive soil-based dredged soil is smaller than that of dropping the modified soil with a tremie pipe or the like. Especially in the backfilling of depressions in deep water, a modified soil layer can be formed efficiently and reliably.
Therefore, according to the present invention, the "viscous soil-based dredged soil" that has been disposed of in the past can be effectively utilized as a backfill material in the backfilling work of the depression. In addition, the in-situ formation of the modified soil layer by using the surface layer portion of the cohesive soil layer and the reforming material is also a more effective utilization of the dredged soil of the cohesive soil system. Moreover, the cohesive dredged soil used for backfilling does not cause seafloor pollution. Therefore, according to the present invention, effective utilization of cohesive soil-based dredged soil and improvement of the deteriorated seabed environment can be realized at the same time.
Further, according to the present invention, the "viscous soil-based dredged soil" that has been disposed of in the past can be effectively used as a backfill material as it is without stabilizing treatment, so that the backfilling work can be carried out at low cost. Is possible.

Further, in the present invention, for example, a material that cures by reacting with cohesive soil-based dredged soil is used as the modifier.
By using such a material, a hardened modified soil layer is formed, and as a result, the dredged soil of the cohesive soil system directly under the soil can be hardly covered, and it becomes possible to surely prevent the seafloor pollution. ..
In addition, by forming a hard modified soil layer between the dredged soil of the cohesive soil system that was put into the pockmark of the seabed and the sand covering material layer that forms the surface layer of the backfill, the sand covering material becomes cohesive soil. It can prevent it from being buried in the dredged soil. In other words, by forming a hard modified soil layer on the cohesive soil layer, the state where the sand covering material is exposed at the top of the backfilled depression is surely maintained, and the growth and habitat of marine organisms are maintained. Restoration can be attempted.

Further, in the present invention, a material that causes a hydration reaction, for example, a calcia modifier is used as the modifier.
The calcia modifier is a material whose composition is controlled and the particle size is adjusted using a converter-type steelmaking slag, which is a kind of steel slag, as a raw material. Therefore, by using the "calcia modifier" as the modifier, the steelmaking slag can be effectively utilized.

Further, in the present invention, a sand-covering material layer made of a sand-covering material is formed in the deep moat depression so as to further cover the modified soil layer formed on the cohesive soil layer. As the sand covering material, for example, natural sand can be used.
By forming a sand-covering material layer made of such natural sand on the modified soil layer and exposing the sand-covering material on the seabed after backfilling (the top of the backfilled depression), marine organisms It is possible to restore the growth and habitat of the seaweed.
In addition, even when procuring natural sand from mountains or islands, in the present invention, only natural sand is used for the surface layer (not the entire backfill), so a small amount of natural sand is procured, and natural sand is used. Does not cause environmental damage to the supplier.

It is a side view (a) and a front view (b) which show an example of the work ship (ground improvement ship) used for carrying out this invention. 2 (a) is an enlarged side view showing the bow side of the work boat shown in FIG. 1 (a), and FIG. 2 (b) is a side view showing a single unit of the stirring / mixing device shown in FIG. 2 (a). 2 (c) is a side view showing a single unit of the material input device shown in FIG. 2 (a). 3 (a) is a front enlarged view mainly showing the stirring and mixing device provided in the work boat shown in FIG. 1 (b), and FIG. 3 (b) shows a single unit of the stirring and mixing device shown in FIG. 3 (a). It is a front view which shows. 4 (a) is a front enlarged view mainly showing the material loading device provided in the work boat shown in FIG. 1 (b), and FIG. 4 (b) shows a single unit of the material loading device shown in FIG. 4 (a). It is a front view which shows. FIG. 5A is an enlarged side view showing a specific configuration of the stirring / mixing device included in the work vessel shown in FIG. 1, and FIG. 5A shows a state in which the grab (taking-in means) at the tip of the casing is open. 5 (b) shows a state in which the grab is closed. FIG. 6A is an enlarged front view showing a specific configuration of the stirring / mixing device included in the work boat shown in FIG. 1, and FIG. 6A shows a state in which the stirring / mixing blade in the casing is raised, and FIG. 6B is shown. ) Indicates a state in which the stirring and mixing blade in the casing is lowered. It is a figure which shows the main process of the backfilling method of a deep moat depression. It is one step of the backfilling method of the deep moat depression shown in FIG. 7, and is the figure which shows the specific procedure of the modified soil layer formation step (FIG. 7 (d)).

(Working ship)
First, the configuration of a work ship (ground improvement ship) used for carrying out the method according to the present invention will be described with reference to FIGS. 1 to 6.

First, the work vessel 1 used for carrying out the present invention has an appearance as shown in FIG.
Further, as shown in the detailed views of FIGS. 2 to 6, mainly
・ The leader 3 erected on the bow side of the work vessel 1 and
A stirring and mixing device 5 (first backfilling device) provided so as to be able to move up and down along the reader 3 and
-Has a material loading device 7 (second backfilling device) provided so as to be able to move up and down along the reader 3.

In the work of backfilling the deep moat depression on the seabed, backfilling is carried out using either the stirring mixing device 5 or the material charging device 7. It is also possible to carry out the entire backfilling process only with the stirring / mixing device 5 without using the material charging device 7.

In the backfilling of the deep moat depression on the seabed using the stirring and mixing device 5 and the material charging device 7, in addition to the cohesive soil-based dredged soil, a modifier and a sand covering material are used as the backfilling material. Hereinafter, the cohesive soil-based dredged soil used for backfilling is simply abbreviated as "backfilling cohesive soil" as necessary.

The material of the "modifying material" is not particularly limited, and may be any material having a soil quality improving effect such as steel slag, cement, cement-based solidifying material, quicklime, and lime-based solidifying material.
Specific examples of available modifiers include materials that develop strength through hydration reactions over time, such as Calcia modifiers. The calcia modifier is a material whose composition is controlled and the particle size is adjusted using a converter-type steelmaking slag, which is a kind of steel slag, as a raw material. Therefore, by using the "calcia modifier" as the improving material, the steelmaking slag can be effectively utilized.

Specific examples of the "sand covering material" include natural sand and the like.

Hereinafter, each configuration of the stirring / mixing device 5 and the material charging device 7 included in the work boat will be described in detail.

(Stirring mixer / first backfilling device)
The stirring and mixing device 5 mainly functions as a "stirring and mixing device" that stirs and mixes the backfilled cohesive soil put into the deep moat depression on the seabed and the reforming material, and also various types from the work boat to the deep moat depression on the seabed. It is a device equipped with a function as a "material loading device" for loading materials (backfilling viscous soil, reforming material, sand covering material, etc.).

What are the "various materials" that are thrown into the deep moat depression on the seabed using the stirring and mixing device 5?
-When using the stirring and mixing device 5 in the process of forming a cohesive soil layer made of cohesive soil-based dredged soil in a deep moat depression (see FIGS. 7 (b) and 7 (c)), "cohesive soil-based dredged soil" Corresponds to the material,
-In the step of forming a modified soil layer in a deep moat depression so as to cover the top of the cohesive soil layer (see FIGS. 7 (d) and 7 (e)), when the stirring and mixing device 5 is used, the "modified material" is used. Corresponds to the material
-When the stirring and mixing device 5 is used in the step of forming the sand covering material layer in the deep moat depression (see FIGS. 7 (f) and 7 (g)), the "sand covering material" corresponds to the material.

The stirring / mixing device 5 is provided so as to be able to move up and down along the reader 3.
As shown in FIGS. 2, 3, 5, 6, 6 and the like.
A stirring / mixing section 29 provided with a casing 32 and a stirring / mixing blade 35,
A pipeline 21 including a flow path for guiding various materials into the casing 32, and
A material input port 23 for charging materials into the pipeline 21 and
・ Auger motor 25, which is a rotary drive device,
A rod 27 for transmitting the power of the motor to the stirring / mixing blade 35 (stirring / mixing means) in the stirring / mixing unit 29 is provided.

The upper end side of the pipeline 21 communicates with the material input port 23, and the lower end side thereof communicates with the casing 32 of the stirring and mixing unit 29.

The pipeline 21 also serves as an air flow path for sending compressed air into the casing 32. By sending compressed air into the casing 32 from an air compressor (not shown) via the conduit 21, it is possible to discharge seawater from the casing 32 in a state of being submerged in the sea by the air pressure. It is possible to maintain the state in which seawater is discharged from the inside of the casing. The stirring / mixing device 5 includes a casing pressure control device that reduces the air pressure inside the casing by exhausting the air inside the casing 32 to the outside of the casing. By using this casing pressure control device, it is possible to control the air pressure inside the casing 32 (the pressure of the compressed air sent into the casing 32).

The rotational power output from the auger motor 25 is transmitted to the stirring / mixing blade 35 provided in the casing 32 of the stirring / mixing unit 29 via the rod 27 (see FIG. 6).

As shown in FIG. 2A, the auger motor 25 is vertically connected to the reader 3 and its upper end is connected to the wire rope 26. By operating the wire rope 26 in the vertical direction, the entire stirring / mixing device 5 including the auger motor 25 moves up and down along the reader 3.

As shown in FIGS. 5 and 6, the stirring / mixing section 29 provided at the lower end of the pipeline 21 is mainly composed of the stirring / mixing section 29.
A hermetically sealable casing 32 equipped with an openable grab 31 (capture means),
It has a stirring / mixing blade 35 (stirring / mixing means) provided so as to be able to move up and down in the casing 32.

The casing 32 is a box-shaped container-shaped component (rectangular casing) having a substantially rectangular shape in a plan view, and is configured so that the opening at the bottom can be opened and closed. The lower ends of the pair of pipelines 21 are connected to the upper part of the casing 32, and various materials input from the material input port 23 are guided into the casing 32 via the pipeline 21. Further, compressed air pumped from an air compressor (not shown) is sent into the casing 32 via the pipeline 21. When the stirring / mixing unit 29 is submerged in seawater, the pressure of the compressed air allows the seawater to be discharged from the inside of the casing 32. That is, it is possible to secure a space in the casing 32 where seawater is discharged by air pressure.

The lower end of the rod 27 connected to the auger motor 25 is connected to the stirring mixing blade 35 provided in the casing 32. The stirring and mixing blade 35 is rotationally driven by the power transmitted via the rod 27, and is provided so as to be able to move up and down in the casing 32 as shown in FIG.

Further, an opening / closing type grab 31 (taking-in means) is provided at the tip end (lower end) of the casing 32. It can be said that the combination of the casing 32 and the grab 31 constitutes one casing as a whole. When the grab 31 is opened, the bottom of the casing 32 is opened, and when the grab 31 is closed, the bottom of the casing 32 is closed and sealed. That is, the grab 31 serves not only as a means for taking the backfilled cohesive soil into the casing, but also as a lid for opening and closing the bottom of the casing (a lid for sealing the bottom of the casing 32).

Such a grab 31 (taking-in means) has, for example, a hydraulic cylinder as a means for opening / closing operation.
FIG. 5A shows a state in which the grab 31 is open. In this state, the bottom of the casing 32 is open. Then, through a series of operations when the grab 31 is closed from the open state, the backfilled cohesive soil that has been charged can be taken into the inside of the grab 31 and the casing 32.
The state in which the grab 31 is closed is shown in FIG. 5 (b). In this state, the bottom of the casing 32 is closed and the casing 32 is sealed. When the grab 31 is closed after the charged backfilled cohesive soil is taken into the casing 32, the casing 32 is sealed and the charged backfilled cohesive soil is confined in the casing 32.

The stirring / mixing blade 35 (stirring / mixing means) is rotatable and is provided so as to be able to move up and down in the casing 32 as shown in FIG.
The stirring and mixing blade 35 is mainly used.
Backfilling cohesive soil taken into the casing 32 by the grab 31 (take-in means),
The modifier, which is charged into the casing 32 via the pipeline 21.
It plays the role of stirring and mixing.
Further, the stirring / mixing blade 35 also plays a role of dehumidifying the backfilled cohesive soil taken into the casing 32 before performing the stirring / mixing described above.

The stirring / mixing device 5 having the above configuration is
-Steps of forming a cohesive soil layer consisting of cohesive soil-based dredged soil in a deep moat depression (Fig. 7 (b) (c)),
-Steps of forming a modified soil layer in a deep moat depression so as to cover the top of the cohesive soil layer (Fig. 7 (d) (e)),
-Step of forming a sand-covered lumber layer in a deep moat depression (Fig. 7 (f) (g)),
It can be used in each process of.

In the modified soil layer forming step (see FIGS. 7 (d) and 7 (e)), the stirring and mixing device 5 is used. In this step, all the configurations included in the stirring / mixing device 5 are used. That is, in the modified soil layer forming step, the stirring and mixing device 5 functions as a "material charging device" for charging the backfilled viscous soil from the work boat toward the deep moat depression, and the backfill viscosity charged into the deep moat depression. The stirring / mixing device 5 functions as a “stirring / mixing device” that stirs and mixes the soil and the modifier.

Therefore, the layer thickness of the modified soil layer formed so as to cover the top of the cohesive soil layer in the in-situ position is substantially equal to the size of the stirring and mixing unit 29.

In the cohesive soil layer forming step (see FIGS. 7 (b) and (c)) and the sand covering material layer forming step (see FIGS. 7 (f) and 7 (g)), either one of the stirring and mixing device 5 and the material charging device 7 is used. Select and use. When the stirring and mixing device 5 is used in the cohesive soil layer forming step and the sand covering material layer forming step, the configuration for stirring and mixing such as the auger motor 25 and the stirring and mixing blade 35 does not function, and the work boat to the seabed The stirring and mixing device 5 functions as a "material charging device" for charging various materials (modifying material, sand covering material, etc.) toward the deep moat depression. Therefore, in the cohesive soil layer forming step and the sand covering material layer forming step, the stirring and mixing section 29 provided with the casing 32 and the stirring and mixing blade 35 simply guides various materials toward the deep moat depression, similarly to the pipeline 21. Functions as a casing.

(Material input device / second backfilling device)
The material charging device 7 is provided so as to be able to move up and down along the leader 3, and plays a role of loading various materials (backfilling clay, sand covering material, etc.) from the work boat toward the deep moat depression on the seabed. ..

In addition, what is "various materials" that are input to the deep moat depression on the seabed using the material input device 7?
In the process of forming a cohesive soil layer consisting of cohesive soil-based dredged soil in a deep moat depression, when the material input device 7 is used, "cohesive soil-based dredged soil" corresponds to the material.
When the material input device 7 is used in the step of forming the sand covering material layer in the deep moat depression, the "sand covering material" corresponds to the material.

The material input device 7 is as shown in FIGS. 2 (c) and 4 as shown in FIG.
・ A substantially pipe-shaped chute 41 that guides the material toward the depression on the seabed,
A material input port 43 for charging material into the chute 41,
It has a chute steady rest metal fitting 45 provided so as to be slidable with respect to the reader 3 (FIG. 2 (c)).

The upper end side of the chute 41 is connected to the wire rope 46. By operating the wire rope 46 in the vertical direction, the entire material loading device 7 including the chute 41 moves up and down along the reader 3. At that time, the chute steady rest metal 45 is guided by the reader 3 so that the chute 41 does not swing.

As shown in FIG. 4, the lower end side of the chute 41 has a diameter expanded in front view, and the upper end side thereof branches in a substantially Y shape. A material input port 43 is provided at each of the upper ends of the chute 41 branched in a substantially Y shape.

(How to backfill the Fukahori depression)
Next, a method of backfilling the deep moat depression on the seabed using a work boat equipped with the above-mentioned stirring / mixing device 5 and material charging device 7 will be described.
The construction procedure will be specifically described with reference to FIG. 7.
The steps a to g described below correspond to FIGS. 7 (a) to 7 (g).

Step a
FIG. 7A schematically shows a deep moat depression on the seabed before backfilling.
A work vessel equipped with the above-mentioned stirring / mixing device 5 and material charging device 7 is moved directly above the deep moat depression, which is the target of backfilling.

Step b
Next, using the material charging device 7 (second backfilling device), the cohesive soil-based dredged soil is charged from the work boat toward the deep moat depression on the seabed.
In this step, the stirring / mixing device 5 (first backfilling device) may be used instead of the material charging device 7, or both devices may be used in combination. As described above, the stirring / mixing device 5 also has a function as a “material charging device” for charging various materials from the work boat toward the deep moat depression on the seabed. It is also possible to throw the dredged soil toward the deep moat depression.

Step c
We will continue to add cohesive dredged soil so that the deep moat depression on the seabed will be backfilled. At that time, care should be taken so that the cohesive soil thrown into the depression has a substantially uniform height, and care is taken not to generate pollution in the process of pouring the cohesive soil.
Then, when the top end of the cohesive soil thrown into the depression reaches a predetermined height almost uniformly, the formation of the cohesive soil layer made of the dredged soil of the cohesive soil system is completed.

Step d
Next, using the stirring and mixing device 5 (first backfilling device), a modified soil layer made of modified soil is formed in the deep moat depression so as to cover the cohesive soil layer formed in steps b and c. .. Specifically, using the surface layer of the cohesive soil layer formed in steps b and c and the reforming material newly introduced from the work boat, it is modified in the in-situ (at the place where the cohesive soil layer is formed). Form a soil layer. The procedure for forming the modified soil layer using the cohesive soil and the modifier will be described later with reference to FIG.
The modifier used in this step is a material that hardens by reacting with a cohesive soil-based dredged soil, and the strength of the material increases with time. Specific examples of the modified material include steel slag.

Step e
The procedure of the above step d is repeated at different locations, and the modified soil layer is formed in the deep moat depression so that the top of the cohesive soil layer is uniformly covered by the modified soil layer.

Step f
Next, using the material charging device 7 (second backfilling device), a sand-covering material layer made of sand-covering material is formed in the deep moat depression so as to cover the modified soil layer. As the sand covering material, for example, natural sand can be used.
In this step, the stirring / mixing device 5 (first backfilling device) may be used instead of the material charging device 7, or both devices may be used in combination. As described above, the stirring / mixing device 5 also has a function as a “material charging device” for charging various materials from the work boat toward the deep moat depression on the seabed. Therefore, the stirring / mixing device 5 is used to cover the sand covering material. It is also possible to throw in toward the Fukahori depression.

Process g
The procedure of the above step f is repeated at different locations, and the modified soil layer is formed in the deep moat depression so that the entire top of the modified soil layer is uniformly covered by the modified soil layer.

(Specific procedure for in-situ formation of modified soil layer)
Next, a specific procedure of the modified soil layer forming step (FIG. 7 (d)) using the stirring and mixing device 5, which is one step in the above-mentioned backfilling method of the deep moat depression, will be described.

Based on FIG. 8, the procedure for forming the modified soil layer using the stirring / mixing device 5 (first backfilling device) will be specifically described.
The following procedures 1 to 8 correspond to FIGS. 8 (1) to 8 (8).

Step 1
Compressed air is sent into the casing 32 through the pipeline 21, the seawater in the casing is drained, and the inside of the casing is made hollow. That is, the air pressure secures a space without seawater in the casing 32. After that, the state in which seawater does not flow into the casing 32 is maintained due to the action of air pressure.

Step 2
The tip of the casing 32, that is, the tip of the stirring / mixing portion 29 is pressed against the top end of the charged cohesive soil (cohesive soil layer formed in FIGS. 7 (b) and 7 (c)). Then, the top end portion of the cohesive soil layer is taken into the casing by using the hydraulically driven grab 31 (taking-in means) at the tip of the casing 32.

Step 3
The grab 31 (taking-in means) at the tip of the casing 32 is repeatedly opened and closed, and the top end portion of the cohesive soil layer is taken into the casing 32.
In the present embodiment, the stirring / mixing device 5 includes a casing pressure control device that reduces the air pressure inside the casing by exhausting the air inside the casing 32 to the outside of the casing. By using this casing pressure control device, the top end portion of the cohesive soil layer can be taken into the casing 32 without being hindered by the air pressure in the casing 32.

Step 4
After the required amount of cohesive soil is taken into the casing 32, the grab 31 (taking-in means) at the tip of the casing is closed. As a result, the casing 32 is sealed, and the cohesive soil taken into the casing 32 is isolated from the seawater and cohesive soil around the casing.

Step 5
The stirring / mixing blade 35 in the casing 32 is lowered, and the cohesive soil taken in the casing 32 is demudged by the rotation of the stirring / mixing blade 35. As a result, the cohesive soil in the casing 32 is loosened, and the cohesive soil and the reforming material can be uniformly stirred and mixed in a later step (procedure 7).

Step 6
With the bottom portion (grab 31) of the casing 32 closed, a predetermined amount of the modifier is charged into the casing 32 via the pipeline 21.

Step 7
After charging the modifier into the casing 32, the stirring and mixing blade 35 in the casing is rotated and moved up and down in the casing 32 to "fill the cohesive soil taken into the casing 32" and "charge into the casing 32". Stir and mix with the modified material. By stirring and mixing these in the casing 32, "modified soil" is produced.

Step 8
Subsequently, the bottom portion (grab 31) of the casing 32 is opened, and the modified soil that has been sufficiently stirred and mixed is backfilled in the original place (the place where the cohesive soil is taken in). Through the above procedure, the in-situ formation of the modified soil layer is completed at a part of the top end of the cohesive soil layer.

Finally, the embodiments described above are examples of the present invention described in the claims, and the embodiments of the present invention are not necessarily limited thereto. Further, the use of the present invention is not limited to the improvement of the seabed ground, but can also be used for the ground improvement of the riverbed and the ground improvement of the lake bottom.

1 Work ship (ground improvement ship)
3 Leader 5 Stirring and mixing device (first backfilling device)
7 Material input device (second backfilling device)
21 Pipeline 23 Material input port 25 Auger motor (rotation drive device)
26 Wire rope 27 Rod 29 Stirring mixing part 31 Grab (taking-in means)
32 Casing 35 Stirring and mixing blade (stirring and mixing means)
41 Chute 43 Material input port 45 Chute steady rest hardware 46 Wire rope

Claims (5)

A step of placing a cohesive soil-based dredged soil near the bottom surface of the depression so as to fill up the depression at the bottom of the water, and forming a cohesive soil layer from the bottom surface of the depression toward the top.
A step of forming a modified soil layer in an in-situ so as to cover the surface layer of the cohesive soil layer by using a modifying material on the surface layer portion of the cohesive soil layer.
A method of backfilling a depression in the bottom of the water, characterized by including. The method for backfilling a depression at the bottom of the water according to claim 1, wherein the modified material is a material that hardens by reacting with a cohesive soil-based dredged soil. The method for backfilling a depression at the bottom of the water according to claim 2, wherein the reforming material is steel slag. The bottom of the water according to any one of claims 1 to 3, further comprising a step of forming a sand-covering material layer made of a sand-covering material in the depression so as to cover the modified soil layer. How to backfill a depression. In the step of forming the sand covering material layer in the depression
The method for backfilling a depression at the bottom of the water according to claim 4, wherein natural sand is used as the sand covering material.

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