Consolidation settlement method for subaqueous ground

Abstract

To ensure necessary water depth in a sea route or an anchorage, and to reduce the cost and labor involved in doing so.SOLUTION: A plurality of strainer pipes 11 having a strainer portion at a lower end and a supply and exhaust pipe 14 are buried in the underwater ground G. The underground water level is lowered by suctioning underground water from the plurality of strainer pipes 11 through the strainer portions. An unsaturated zone is formed in the ground around the plurality of strainer pipes 11. By vacuum suctioning the air in the unsaturated zone through the supply and exhaust pipe 14, a negative pressure zone is formed in the ground around the plurality of strainer pipes 11. Thereafter, the air is sent to the negative pressure zone through the supply and exhaust pipe 14 to cause cavitation of moisture present in gaps between countless soil particles in the underwater ground G, thereby causing the moisture to implode, and a shock wave to generate. Water is removed from the gaps between countless soil particles, and the underwater ground G is consolidated and settled by water pressure and atmospheric pressure.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a consolidation settlement method for underwater ground.

Background Art

[0002] Conventionally, in order to secure navigation channels and berths in rivers, harbors, etc., dredging work has been carried out by dredging vessels such as grab ships and pump ships (see, for example, Patent Document 1). For example, in the case of a grab ship, a grab bucket suspended from a crane mounted on the hull is lowered to the seabed, the sediment on the seabed is picked up, and a dredging operation is performed to load the sediment onto a soil transport ship that has come alongside the grab ship. In addition, the sediment loaded on the soil transport ship is transported to a sediment disposal site (landfill) and unloaded on land.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] By the way, the sediment generated by the dredging work is transported to the sediment disposal site by the soil transport ship as described above. However, there are many cases where there is no sediment disposal site or it is difficult to newly create a sediment disposal site. In such cases, the sediment is improved at a cost and recycled. In addition, when the dredging work is carried out, there are cases where navigation regulation over a wide area is required or measures to reduce the environmental load due to pollution are necessary, resulting in a problem of high cost. Furthermore, in the dredging work, construction errors occur with respect to the planned dredging height and the normal line, so over-digging work is required, and there is the trouble of having to dredge extra sediment compared to the pure soil volume. Moreover, in the case of thin-layer dredging, it is often the case that the over-digged sediment volume exceeds the pure soil volume, which has a great impact on the cost.

[0005] The present invention has been made in view of the above circumstances, and an object thereof is to secure a necessary water depth in a waterway or a berth and to reduce the costs and labor involved therein.

Means for Solving the Problems

[0006] In order to solve the above problems, the invention according to claim 1 is a method for consolidating and subsiding a subaqueous ground, which causes cavitation in the moisture existing in the gaps between innumerable soil particles in the subaqueous ground to explosively compress the moisture and generate shock waves, removes the moisture from the gaps between the innumerable soil particles, and consolidates and subsides the subaqueous ground by water pressure and atmospheric pressure, wherein a plurality of strainer pipes each having a strainer portion at a lower end and an air supply and exhaust pipe are buried in the subaqueous ground, the groundwater level is lowered by sucking the groundwater in the ground through the strainer portions from the plurality of strainer pipes, and an unsaturated zone is formed in the ground around the plurality of strainer pipes, the air in the unsaturated zone is vacuum-sucked through the air supply and exhaust pipe to form a negative pressure zone in the ground around the plurality of strainer pipes, and then air is sent through the air supply and exhaust pipe to the negative pressure zone to cause cavitation in the moisture existing in the gaps between the soil particles.

[0007] The invention according to claim 2 is the method for consolidating and subsiding a subaqueous ground according to claim 1, characterized in that the periphery of the strainer portion in the plurality of strainer pipes buried in the ground of the subaqueous ground is in a state where the permeability coefficient is higher the closer it is to the strainer portion.

[0008] The invention according to claim 3 is the method for consolidating and subsiding a subaqueous ground according to claim 1 or 2, characterized in that a water stop cap layer for suppressing the inflow of water from underwater is formed at a height position near the assumed consolidation and subsidence level of the subaqueous ground among the peripheries of the plurality of strainer pipes buried in the ground of the subaqueous ground.

[0009] The invention according to claim 4 is the method for consolidating and subsiding the submarine ground according to any one of claims 1 to 3, wherein In the step of burying the strainer pipe from above the water surface toward the submarine ground, an auxiliary steel pipe is connected to the upper end portion of the strainer pipe, and the upper end portion of the auxiliary steel pipe is positioned above the water surface. After the strainer pipe is buried, the auxiliary steel pipe is removed, and an upper lid with a drain port is attached to the upper end portion of the strainer pipe to enable drainage of the water in the ground. The air supply and exhaust pipe buried together with the plurality of strainer pipes and a cavitation generator including a vacuum pump are connected by a connecting pipe.

[0010] The invention according to claim 5 is the method for consolidating and subsiding the submarine ground according to any one of claims 1 to 4, wherein A cavitation generator including a vacuum pump connected to the air supply and exhaust pipe is installed on the ground to manage the generation of cavitation.

[0011] The invention according to claim 6 is the method for consolidating and subsiding the submarine ground according to any one of claims 1 to 5, wherein When the lower layer located under the upper layer constituting the bottom of the submarine ground is a permeable layer, the strainer portion in the strainer pipe is positioned in the lower layer which is the permeable layer, and when the lower layer is an impermeable layer, the strainer portion is positioned at the bottom of the upper layer which is the impermeable layer.

[0012] The invention according to claim 7 is the method for consolidating and subsiding the submarine ground according to any one of claims 1 to 6, wherein A pressure influence blocking hole for blocking the influence of the pressure by vacuum suction through the air supply and exhaust pipe is formed between the location where the air supply and exhaust pipe is buried in the submarine ground and the location where it is desired to block the influence of the pressure by vacuum suction through the air supply and exhaust pipe.

[0013] The invention according to claim 8 is the method for consolidating settlement of underwater ground according to any one of claims 1 to 7, wherein the number and intervals of the plurality of strainer pipes are derived from the target amount of consolidation settlement, the physical properties of the upper layer constituting the underwater bottom of the underwater ground, the layer thickness of the upper layer, the consolidation characteristics of the upper layer, the water permeability of the lower layer located under the upper layer, and the state of the pressure applied to the lower layer.

[0014] The invention according to claim 9 is the method for consolidating settlement of underwater ground according to any one of claims 1 to 8, wherein before constructing a structure built along the coast, the underwater ground is consolidated and settled.

[0015] The invention according to claim 10 is the method for consolidating settlement of underwater ground according to any one of claims 1 to 9, wherein the plurality of strainer pipes and the air supply and exhaust pipes are buried from a work barge.

[0016] The invention according to claim 11 is the method for consolidating settlement of underwater ground according to any one of claims 1 to 10, wherein a dynamic compaction method of dropping a hammer onto the underwater bottom of the underwater ground to compact the underwater ground is used in combination.

[0017] The invention according to claim 12 is the method for consolidating settlement of underwater ground according to any one of claims 1 to 11, wherein the amount of consolidation settlement of the underwater ground is measured using a distance measuring sensor.

Advantages of the Invention

[0018] According to the present invention, it is possible to secure the necessary water depth in a waterway or a berth, and at the same time, it is possible to reduce the costs and labor involved.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Mode for Carrying Out the Invention

[0020] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, although various technically preferable limitations are imposed on the embodiments described below for carrying out the present invention, the technical scope of the present invention is not limited to the following embodiments and illustrated examples.

[0021] In FIGS. 2 to 4, reference numeral W represents the sea (water), reference numeral B represents the seabed (bottom of the water), reference numeral G represents the seabed ground (bottom ground), reference numeral G1 represents the upper layer of the seabed ground G, reference numeral G2 represents the lower layer of the seabed ground G, reference numeral S represents a ship, and reference numeral 10 represents a ground settlement device. In the present embodiment, the seabed ground G is consolidated and settled by the ground settlement device 10 to secure a predetermined water depth required in a waterway or a berth. Note that, as the seabed ground to be consolidated and settled, in the present embodiment, the ground of the sea is taken as an example for explanation, but it is not limited thereto, and the ground of a river or a lake may be the target of consolidation settlement.

[0022] When consolidating and settling the seabed ground G, first, a casing 5 (which may also be referred to as a guide pipe) for burying a strainer pipe 11 described later is penetrated into the seabed ground G. The penetration of the casing 5 is performed using a workboat 1 equipped with a workbench as shown in FIG. 1, for example. The workboat 1 shown in FIG. 1 is a SEP (Self-Elevating Platform) workboat, and a plurality of legs 2 can be landed on the seabed ground G, and the hull can be jacked up, lifted from the water surface, and made self-supporting. In this embodiment, an SEP barge is used, but the present invention is not limited to this, and a crane barge or a normal barge may be used. Also, the working barge 1 used may be one or two or more.

[0023] An overhanging stay 3 that projects above the water from the edge is provided at the edge of the working barge 1, and a casing driver 4 is installed on the overhanging stay 3. Note that an opening may be formed in the working platform, and the casing driver 4 may be installed at the portion where the opening is formed. The casing driver 4 is a device that holds the casing 5 for ground excavation and rotates the casing 5 to penetrate the seabed ground G. When two working barges 1 are used, the casing driver 4 is installed on the overhanging stay 3 provided across the two working barges 1.

[0024] The casing 5 is set on the casing driver 4 by a crawler crane 6 placed on the working platform of the working barge 1. Then, the casing 5 is penetrated into the seabed ground G to a predetermined depth by the casing driver 4. Also, the upper end of the casing 5 protrudes above the sea surface (casing driver 4) so that sea water does not directly enter.

[0025] In this embodiment, the depth at which the casing 5 is penetrated is up to the boundary between the clay layer of the upper layer G1 and the lower layer G2 of the seabed ground G, but the present invention is not limited to this, and in some cases, it may be up to the middle part of the upper layer G1 or may be penetrated to the lower layer G2. Note that the lower layer G2 of the seabed ground G includes a sand layer, a gravel layer, a rock layer, and the like. Incidentally, the depth at which the casing 5 is penetrated is also the depth at which the strainer pipe 11 is buried, and the depth is determined based on conditions such as whether the lower layer G2 of the seabed ground G is a pervious layer or an impervious layer, and conditions such as the layer thickness of the upper layer G1 of the seabed ground G. If the lower layer G2 of the seabed ground G is a pervious layer, the penetration depth of the casing 5 (the burial depth of the strainer pipe 11) is set to reach the lower layer G2. If the lower layer G2 of the seabed ground G is an impervious layer, the penetration depth of the casing 5 (the burial depth of the strainer pipe 11) is set to reach the bottom of the upper layer G1.

[0026] The sediment in the casing 5 penetrated to a predetermined depth of the seabed ground G is excavated and discharged by the hammer grab 7 suspended by the crawler crane 6. The discharged soil is transported to a soil disposal site (landfill) by a soil transport ship and landed.

[0027] When the casing 5 is penetrated into the seabed ground G and the sediment is discharged, the ground settlement device 10 is installed around the seabed ground G to be consolidated and settled. The ground settlement device 10 includes a plurality of strainer pipes 11, a drainage pump 12, a drainage pipe 13, an air supply and exhaust pipe 14, and a cavitation generator 15. Also, the casing 5 is removed from the seabed ground G after the installation of the ground settlement device 10.

[0028] The strainer pipe 11 is a pipe body in which the drainage pump 12 and the drainage pipe 13 are housed, has a permeable strainer portion at the lower end, and is inserted into the inside of the casing 5 from which the sediment has been discharged. A filter 11a is provided on the outer periphery of the strainer portion. In FIG. 2, the strainer pipe 11 and the filter 11a are partially cut away to show the inside of the strainer pipe 11 and the filter 11a (the structure of the drainage pump 12, the drainage pipe 13, and the strainer portion).

[0029] The strainer pipe 11 is inserted into the casing 5 from the work barge 1 while connecting the auxiliary steel pipe 110 to the upper end. The auxiliary steel pipe 110 is added as needed. Before the removal of the casing 5, the lower end of the strainer pipe 11 is arranged at the height of the lower end of the casing 5, and the upper end connected to the upper end of the strainer pipe 11 is arranged at the height of the upper end of the casing 5 and protrudes above the sea surface. That is, before the removal of the casing 5, sea water does not directly enter the strainer pipe 11. Note that the connection between the auxiliary steel pipe 110 and the strainer pipe 11 and the connection between the plurality of auxiliary steel pipes 110 are made by flange joints. Also, before the compaction settlement work is carried out after the embedding of the plurality of strainer pipes 11 is completed, the casing 5 has been removed, and the auxiliary steel pipe 110 is also removed from the upper end of the strainer pipe 11. At this time, the upper end of the strainer pipe 11 is arranged at the height of the seabed B.

[0030] The drainage pump 12 is arranged on the inner side of the strainer part of the strainer pipe 11, and the drain pipe 13 is arranged inside the strainer pipe 11. The lower end of the drain pipe 13 is connected to the drainage pump 12, and the upper end is arranged at the height of the seabed B. Therefore, during the compaction settlement work, the groundwater in the seabed ground G that has entered the inside of the strainer part through the filter 11a is sucked by the drainage pump 12 and discharged into the sea W as drainage D through the drain pipe 13 (see Figure 3).

[0031] The air supply and exhaust pipe 14 is inserted into the casing 5 together with the strainer pipe 11. That is, it is arranged between the outer peripheral surface of the strainer pipe 11 and the inner peripheral surface of the casing 5. In this embodiment, a plurality (a pair) of air supply and exhaust pipes 14 are inserted into the casing 5. The lower end of this air supply and exhaust pipe 14 is arranged at the height of the lower end of the filter 11a in the strainer pipe 11, and the upper end is arranged at the height of the seabed B. Furthermore, a connecting pipe 14a for connecting the intake and exhaust pipe 14 and the cavitation generator 15 is connected to the upper end of the intake and exhaust pipe 14.

[0032] Among the gaps formed between the outer peripheral surface of the strainer pipe 11 and the inner peripheral surface of the casing 5, a water-stop cap layer 16 composed of bentonite pellets, a hardening agent, a hardening accelerator, etc. is formed at a height position near the assumed consolidation settlement level E of the seabed ground G (slightly below the consolidation settlement level E). That is, considering that the seabed B of the seabed ground G subsides due to the consolidation settlement work, a water-stop cap layer 16 is formed at a height position near the assumed consolidation settlement level E (the height position of the seabed B after the consolidation settlement work) to suppress the inflow of seawater into the gap formed between the outer peripheral surface of the strainer pipe 11 and the inner peripheral surface of the casing 5. More specifically, considering the amount of consolidation settlement, bentonite pellets are placed at a position spaced below the seabed B, and chemicals such as a hardening agent are filled directly below them to form the water-stop cap layer 16.

[0033] Also, among the gaps formed between the outer peripheral surface of the strainer pipe 11 and the inner peripheral surface of the casing 5, the gaps where the water-stop cap layer 16 is not formed are filled with a filter material (for example, pea gravel). That is, the filter material is filled both above and below the water-stop cap layer 16. The intake and exhaust pipe 14 will be held by the water-stop cap layer 16 and the filter material filled between the outer peripheral surface of the strainer pipe 11 and the inner peripheral surface of the casing 5.

[0034] Around the strainer part of the strainer pipe 11, a region with a high permeability coefficient (coarse mesh) including a filter material located below the water-stop cap layer 16 is formed. The closer the region is to the strainer part, the higher the apparent permeability coefficient becomes, the strainer part (filter 11a) is less likely to be clogged, and water can be sucked more quickly and reliably the closer it is to the strainer part. To achieve such a state, a large amount of water is pumped around the strainer section while a large amount of air is sent, and the operations of sucking the water and air simultaneously or alternately are repeated. As a result, the vicinity of the strainer section is agitated, and coarser particles gather closer to the strainer section. Such operations can be performed both before and after the removal of the casing 5.

[0035] In this embodiment, it is assumed that the casing 5 is removed after a water-stop cap layer 16 is formed inside the casing 5. However, the water-stop cap layer 16 may be formed after the casing 5 is removed. By removing the casing 5, the strainer pipe 11 and the air supply and exhaust pipe 14 are in a state of being buried in the seabed ground G.

[0036] Once the strainer pipe 11 and the air supply and exhaust pipe 14 are in a state of being buried in the seabed ground G, the auxiliary steel pipe 110 is removed from the upper end of the strainer pipe 11. Such an operation will be carried out by a diver in the sea. Further, after removing the auxiliary steel pipe 110, an upper lid with a drain port (not shown) is attached to the upper end of the strainer pipe 11 to enable drainage of the groundwater. That is, the drain port of the upper lid will be connected to the above-mentioned drain pipe 13. An electromagnetic flowmeter is attached to the drain port, and the drainage D can be managed. Furthermore, the air supply and exhaust pipe 14 buried together with the plurality of strainer pipes 11 and the cavitation generator 15 including a vacuum pump are connected by a connecting pipe 14a.

[0037] The cavitation generator 15 is a device for causing a cavitation phenomenon that compresses moisture by adjusting air pressure and generates shock waves with respect to the moisture in the seabed ground G, and a connecting pipe 14a connected to the air supply and exhaust pipe 14 is connected thereto. As a configuration that enables such a cavitation phenomenon to occur, the cavitation generator 15 has, although not shown, an adjustment tank, a vacuum pump, a cooling water tank, an air inlet valve, and a control unit.

[0038] The adjustment tank is formed in a hollow shape and is normally filled with only air. A tubular suction part that communicates the inside and the outside is integrally provided on the tank wall, and a suction pipe of a vacuum pump is connected to the suction part. Such an adjustment tank is in a vacuum state or a state close to vacuum when the cavitation phenomenon occurs, and thus should have at least a strength capable of withstanding atmospheric pressure and high airtightness. And a connecting pipe 14a connected to the air supply and exhaust pipe 14 is connected to the adjustment tank.

[0039] In this embodiment, a water-sealed Elmo type vacuum pump is adopted as the vacuum pump. The water-sealed Elmo type vacuum pump has a fan built in the casing (housing), and the casing is provided with a suction port and a discharge port. The cylindrical fan has its fan center eccentrically incorporated by about 20 to 30 mm with respect to the center of the cylindrical casing, and a suction pipe is connected to the suction port. This suction pipe is connected to the suction part of the adjustment tank. And such a water-sealed Elmo type vacuum pump vacuum-sucks air (or water vapor) from the inside of the adjustment tank into the casing through the suction port via the suction pipe by the eccentric rolling rotation of the fan with respect to the casing, and discharges it from the discharge port. Furthermore, water is enclosed inside the casing. That is, a circulation water channel is connected to the bottom of the casing, and the tip of this circulation water channel is introduced into the circulating water filled in the above-mentioned cooling water tank with a large capacity and excellent heat dissipation. Therefore, due to the eccentric rolling rotation of the fan with respect to the casing, the circulating water in the cooling water tank is vacuum-sucked from the circulation water channel, and the moisture in the air is discharged from the discharge port. Note that the type of the vacuum pump is not limited to the water-sealed Elmo type vacuum pump, and other vacuum pumps may also be used.

[0040] The air inlet valve is an electromagnetic valve (also referred to as an electronic valve) for sending air into the inside of the above-mentioned adjustment tank, and is integrally provided on the tank wall of the adjustment tank. It includes a tubular inlet that allows air to flow in by connecting the inside and outside of the adjustment tank, and a valve body that opens and closes the inlet.

[0041] The control unit is communicably connected to the air inlet valve and can transmit a control signal for controlling the opening and closing operation of the valve body in the air inlet valve at a preset timing. By controlling the opening and closing operation of the valve body and operating the valve body to open the inlet, external air can be sent into the adjustment tank, and by operating the valve body to close the inlet, air can be prevented from flowing into the adjustment tank. Under the control of such a control unit, the degree of vacuum inside the adjustment tank that is vacuum-sucked by the vacuum pump can be adjusted or released to the atmosphere. The timing for controlling the opening and closing operation of the valve body can be set in various patterns. In this embodiment, a pattern is adopted in which the valve body is operated to open the inlet when a predetermined time has elapsed, and then the valve body is operated to close the inlet when another predetermined time has elapsed, and this operation is repeated. More specifically, for several minutes (30 minutes in this embodiment), the inside of the adjustment tank is vacuum-sucked to a high vacuum by the vacuum pump, then for several minutes (5 minutes in this embodiment), the valve body of the air inlet valve is opened, and after those several minutes have elapsed, the valve body is closed. The opening and closing of the valve body are controlled at such a timing. And such a process is repeated for a predetermined period (several months in this embodiment). However, the timing for controlling the opening and closing operation of the valve body is not limited to this and can be appropriately changed without departing from the spirit of the present invention. In this embodiment, when the vacuum pressure by the vacuum pump is set to a high vacuum by closing the valve body of the air inlet valve, it is set to -0.08 MPa. However, the vacuum pressure is not limited to this and can be appropriately changed within the range of the capacity of the vacuum pump. That is, the vacuum pressure can be appropriately changed without departing from the spirit of the present invention.

[0042] Note that the vacuum pump and the control unit may operate individually, but they may also be connected to an integrated control device (not shown) so that the vacuum pump and the control unit can be comprehensively controlled. For example, the control unit controls the opening and closing operation of the valve body in the air inlet valve at the preset timing as described above. However, if the operation of the vacuum pump is out of sync, effective vacuum suction cannot be performed. Therefore, if the integrated control device can comprehensively control the vacuum pump and the control unit, the operation of the vacuum pump and the control of the air inlet valve by the control unit can be performed in a timely manner. That is, the integrated control device functions as a controller in the cavitation generator 15.

[0043] Such an integrated control device is composed of a general-purpose computer (such as a personal computer, a tablet terminal, etc.) equipped with a CPU, a ROM, a RAM, etc., and is communicably connected to the cavitation generator 15. More specifically, the ON / OFF switch of the vacuum pump is controlled by the integrated control device, and the transmission of the control signal of the control unit is controlled.

[0044] The cavitation generator 15 configured as described above can vacuum-suck the inside of the adjustment tank at a high vacuum (P = about -0.85 to -0.95 MPa) by a water-sealed Elmo type vacuum pump, and expand the moisture in the seabed ground G into low-temperature water vapor 1700 times through the air supply and exhaust pipe 14 and the connecting pipe 14a. In addition, since the water enclosed inside the vacuum pump is used as circulating water for cooling, even when sucking with low-temperature high steam (cloud), it is possible to achieve a vacuum up to about Q = 50 (l / min) without freezing and without the need for heating. That is, since a water-sealed Elmo type vacuum pump is connected to the adjustment tank and the circulating water in the cooling water tank is connected to the vacuum pump via a circulating water channel, by driving the vacuum pump to vacuum-suck the inside of the adjustment tank at room temperature, the moisture at the target site can be sucked with low-temperature water vapor.

[0045] The cavitation generating device 15 is installed on land (coast) near the seabed ground G where consolidation settlement occurs, enabling the consolidation settlement work to be managed on the ground. The connecting pipe 14a is connected to the air supply and exhaust pipe 14 after the removal of the casing 5 from such ground facilities (cavitation generating device 15).

[0046] Note that when cavitation is caused in the moisture existing in the countless gaps between soil particles in the seabed ground G by the cavitation generating device 15, the seabed ground G will consolidate and settle, and the seabed B will descend. However, for example, existing quay walls (referring to structures such as mooring facilities at port piers in a harbor) should not be affected. Therefore, a pressure influence cutoff hole 17 for cutting off the influence of the pressure by vacuum suction through the air supply and exhaust pipe 14 is formed between the location where the air supply and exhaust pipe 14 is buried in the seabed ground G and the location where the influence of the pressure by vacuum suction through the air supply and exhaust pipe 14 is desired to be blocked. That is, an air hole is formed, such as by providing an air supply pipe, between the location where the air supply and exhaust pipe 14 is buried in the seabed ground G and the location where the influence of the pressure by vacuum suction through the air supply and exhaust pipe 14 is desired to be blocked. An air curtain is formed around the pressure influence cutoff hole 17 by the compressed air pressure of a compressor (not shown) to create an unsaturated zone and form a highly water-tight area. Thereby, the influence on the location where the influence of the pressure by vacuum suction through the air supply and exhaust pipe 14 is desired to be blocked can be blocked.

[0047] When performing consolidation settlement of the seabed ground G by the ground settlement device 10, first, as shown in FIG. 3, a plurality of auxiliary steel pipes 110 are removed from the upper end of the strainer pipe 11, and an upper lid with a drain opening is attached to the upper end of the strainer pipe 11 to enable drainage from the drain pipe 13 inside the strainer pipe 11. Furthermore, a connecting pipe 14a is also connected to the upper end of the air supply and exhaust pipe 14 to connect the cavitation generating device 15, which is ground equipment, and the air supply and exhaust pipe 14. Also, an air curtain is formed around the pressure influence cutoff hole 17 to form a highly water-tight area between the plurality of strainer pipes 11 and the air supply and exhaust pipe 14, and the coastal land where the cavitation generating device 15, which is ground equipment, is installed.

[0048] Then, water in the ground is sucked through the strainer part from a plurality of strainer pipes 11 with an upper lid having a drain port and closed. Thereby, the groundwater level of the seabed ground G is lowered, and an unsaturated zone is formed in the ground around the plurality of strainer pipes 11. Note that, among the surroundings of the strainer pipe 11 and the air supply and exhaust pipe 14, the side closer to the seabed B is blocked by the water stop cap layer 16, so seawater hardly infiltrates around the strainer pipe 11 and the air supply and exhaust pipe 14. Further, since the water that seeps into the ground from the seabed B is also sucked by the plurality of strainer pipes 11 and discharged into the sea W, it is easy to maintain the state in which the unsaturated zone is formed in the seabed ground G.

[0049] When an unsaturated zone is formed in the ground, the air in the unsaturated zone is vacuum-sucked through the air supply and exhaust pipe 14 to form a negative pressure zone in the ground around the plurality of strainer pipes 11. Thereafter, by sending air through the air supply and exhaust pipe 14 to the negative pressure zone, cavitation is caused in the moisture existing in the innumerable gaps between soil particles in the seabed ground G. That is, when the ground is in a vacuum state, the moisture boils and turns into vapor, and at that time, cavities (air bubbles) are generated. At that time, by introducing air from the outside of the vacuum state, bubble collapse, that is, cavitation occurs. That is, cavitation occurs by sending air through the air supply and exhaust pipe 14 to the negative pressure zone. Cavitation refers to a phenomenon in which, immediately after the air bubbles implode, they collapse and generate a shock wave toward the outside. The shock wave is comparable to a pressure of 1000 to 10000 atmospheres per micron in the case of water, and can cause erosion even in the case of metal. There are gaps between soil particles that make up the seabed ground G, through which air, gas, and water can penetrate. Such a structure (soil particle skeleton) is called a single-grain structure, a honeycomb structure, a cotton-wool structure, etc. When extracting moisture from the gaps between soil particles, conventionally, a vacuum pump or a pumping pump has been used, or a compressor has been used to send air into the ground to assist the suction by the vacuum pump to control the groundwater level. However, in this embodiment, cavitation is caused during this process so that the effect of vacuum suction can be further promoted. Furthermore, the compaction effect of the ground is exerted by shock waves, enabling the strengthening of the ground. In short, by repeatedly performing the method of putting the seabed ground G into a vacuum state by a vacuum pump and then sending air from that state, cavitation can be caused, moisture can be removed from the innumerable gaps between soil particles in the seabed ground G, and the seabed ground G can be effectively dried (dehydrated) to perform ground improvement.

[0050] The step of forming a negative pressure zone in the ground around the plurality of strainer pipes 11 by vacuum suction through the air supply and exhaust pipe 14 as described above, and the step of then sending air through the air supply and exhaust pipe 14 to the negative pressure zone are repeatedly performed over a predetermined period. Here, although the specific number of the predetermined period is not limited, for example, it is assumed to refer to "about several months". Note that "several months" represents a time sense longer than several days or several weeks and shorter than several years. In the Practical Japanese Expression Dictionary, "several months" is stated as "a certain number of months. Several months. It often refers to about 4 to 6 months or so."

[0051] When these steps are repeatedly performed over a predetermined period, the seabed ground G is consolidated and subsided by water pressure and atmospheric pressure, and the position of the seabed B can be lowered. That is, since the water depth becomes deeper, the required water depth in the waterway or berth can be ensured. The measurement of the consolidation settlement amount of the seabed ground G is performed by a ranging sensor using a sensing technology such as an acoustic depth finder or an underwater laser so as not to interfere with the navigation of the ship S. In addition, although consolidation settlement of the seabed ground G can be achieved by water pressure and atmospheric pressure, in order to more surely cause consolidation settlement, a dynamic compaction method may be used in combination, in which a weight is dropped onto the seabed B of the seabed ground G to compact the seabed ground G. The dropping of the weight is, for example, to be performed from the work support vessel 1.

[0052] When the seabed B (ground surface) sinks to the consolidation settlement level E due to consolidation settlement, the strainer pipe 11 and the air supply and exhaust pipe 14 will protrude from the lowered seabed B (ground surface) accordingly. Since the protruding strainer pipe 11 and air supply and exhaust pipe 14 may interfere with the navigation of the ship S, it is necessary to process the strainer pipe 11 and the air supply and exhaust pipe 14. As methods for processing the strainer pipe 11 and the air supply and exhaust pipe 14, there are a method of completely pulling out the strainer pipe 11 and the air supply and exhaust pipe 14, and a method of cutting only the portion protruding from the seabed B (ground surface) when leaving the strainer pipe 11 and the air supply and exhaust pipe 14 buried. When leaving the strainer pipe 11 and the air supply and exhaust pipe 14, a lid may be provided after cutting the portion protruding from the seabed B (ground surface).

[0053] In addition, when it is necessary to construct a structure such as a quay on the coastal land, it is preferable to perform consolidation settlement of the seabed ground G before the construction of the structure so as not to be affected by the pressure due to vacuum suction through the air supply and exhaust pipe 14. When consolidation settlement of the seabed ground G must be performed after the construction of the structure, a highly water-tight area is surely formed between the structure and the plurality of strainer pipes 11 and air supply and exhaust pipes 14 by the pressure influence blocking holes 17 so that the structure is not affected by the pressure due to vacuum suction through the air supply and exhaust pipe 14.

[0054] In addition, after consolidating and settling the seabed ground G to ensure the required water depth in a waterway or anchorage, dredging work is to be regularly performed to maintain the water depth.

[0055] 〔Example〕 Figure 4 is a schematic diagram for explaining a specific example of consolidation settlement of the subaqueous ground. The lower layer G2 of the seabed ground G is a rock layer, the upper layer G1 is a clay layer with a layer thickness of 30 meters, and the water depth before consolidation settlement is 9.5 meters. Note that the strainer pipe 11 is not buried in the lower layer G2. The number and interval of the plurality of strainer pipes 11 buried in such a seabed ground G are derived from the target consolidation settlement amount, the physical properties of the upper layer G1 constituting the seabed B of the seabed ground G, the layer thickness of the upper layer G1, the consolidation characteristics of the upper layer G1, the permeability of the lower layer G2 located under the upper layer G1, and the confined state of the lower layer G2. The physical properties of the upper layer G1, the layer thickness of the upper layer G1, the consolidation characteristics of the upper layer G1, the permeability of the lower layer G2, and the confined state of the lower layer G2 can be determined by boring surveys. The target consolidation settlement amount is appropriately determined by calculating the numerical value of how much the target location of consolidation settlement can be consolidated.

[0056] When calculating the numerical value of how much the target location of consolidation settlement can be consolidated, for example, in the case of the liquid limit of a general clay layer (upper layer G1), the calculation is as follows. <Liquid limit of clay layer> Assume WL (liquid limit) = 100%, 80%, From Cc (compression index) = 0.009(WL - 10), Cc = 0.81, 0.72. <Void ratio at the center of the clay layer> Assume e0 = 1.50. <Effective stress of the clay layer> Assume the unit volume weight of clay γsat = 15.0 KN / m 3 Then, the effective stress at the center of the clay layer is calculated as P0 = (15.0 - 10.0) × 15 = 75 KN / m 2 It is calculated. Negative pressure of -85 KN / m by the strainer pipe 11 and the air supply and exhaust pipe 14 2 The effective stress of the clay layer due to the load is calculated as P1 = 75 + 85 = 160 KN / m 2 It is calculated. Under the above-mentioned preconditions, when Cc = 0.81, the consolidation settlement amount is calculated as follows in Equation (1).

[0057]

Equation

[0058] Also, when Cc = 0.72, the consolidation settlement amount is calculated as follows in Equation (2).

[0059]

Equation

[0060] That is, the consolidation settlement amount when Cc = 0.81 is 3.18 meters, and the consolidation settlement amount when Cc = 0.72 is 2.82 meters. Therefore, when Cc = 0.81, the thickness of the upper layer G1 is 26.82 meters and the water depth is 12.68 meters. Also, when Cc = 0.72, the thickness of the upper layer G1 is 27.18 meters and the water depth is 12.32 meters. As a result, it becomes clear how much the target location of consolidation settlement can be consolidated and settled. Based on this, the target consolidation settlement amount can be determined. From the above, the number and interval of the plurality of strainer pipes 11 buried in the seabed ground G can be derived.

[0061] According to this embodiment, by sucking the underground water from a plurality of strainer pipes 11 buried in the seabed ground G through the strainer section, the groundwater level is lowered, an unsaturated zone is formed in the ground around the plurality of strainer pipes 11, and the air in the unsaturated zone is vacuum-sucked through the air supply and exhaust pipe 14 buried in the seabed ground G, thereby forming a negative pressure zone in the ground around the plurality of strainer pipes 11. Then, by sending air through the air supply and exhaust pipe 14 to the negative pressure zone, cavitation is caused in the moisture existing in the countless gaps between the soil particles in the seabed ground G. Therefore, the impact force due to cavitation is applied to extract the moisture from the gaps between the soil particles, and the seabed ground G can be dehydrated and dried quickly and surely. Such seabed ground G will be consolidated and subsided by water pressure and atmospheric pressure, so the position of the seabed B can be lowered, and as a result, the water depth becomes deeper, ensuring the required water depth in the shipping lane and anchorage. Furthermore, the impact force due to cavitation acts to exert a consolidating effect on the seabed ground G, enabling the strengthening of the seabed ground G. And since the seabed ground G can be consolidated and subsided without performing dredging work as in the prior art, the costs and labor involved in the treatment of earth and sand including over-excavated soil, route regulation in the surrounding sea area, and measures to reduce the environmental load due to pollution can be reduced.

[0062] In addition, since the periphery of the strainer section in the plurality of strainer pipes 11 buried in the ground of the seabed ground G is in a state where the permeability coefficient is higher the closer it is to the strainer section, the strainer section (filter 11a) is less likely to be clogged, and water can be sucked more quickly and surely the closer it is to the strainer section.

[0063] In addition, a water stop cap layer 16 that suppresses the inflow of water from the sea W is formed at a height position near the assumed consolidation settlement level E of the seabed ground G among the areas around the plurality of strainer pipes 11 buried in the ground of the seabed ground G. Therefore, it becomes difficult for seawater to penetrate around the plurality of strainer pipes 11. Furthermore, since the water seeping into the ground from the seabed B is also sucked by the plurality of strainer pipes 11 and discharged into the sea W, it is easy to maintain the state in which an unsaturated zone is formed in the seabed ground G.

[0064] Also, in the step of burying the strainer pipe 11 from the sea surface toward the seabed ground G, an auxiliary steel pipe 110 is connected to the upper end portion of the strainer pipe 11, and the upper end portion of the auxiliary steel pipe 110 is positioned above the sea surface. Therefore, sea water does not directly enter the inside of the strainer pipe 11. As a result, the work of arranging a drainage pump 12 and a drain pipe 13 inside the strainer pipe 11 can be easily performed. Furthermore, after the strainer pipe 11 is buried, the auxiliary steel pipe 110 is removed, and a lid with a drain port is attached to the upper end portion of the strainer pipe 11 to enable drainage of underground water. Therefore, it is possible to form an unsaturated zone in the ground around the strainer pipe 11. Moreover, an air supply and exhaust pipe 14 buried together with a plurality of strainer pipes 11 and a cavitation generator 15 including a vacuum pump are connected by a connecting pipe. Therefore, it is possible to cause cavitation in the moisture existing in the innumerable gaps between soil particles in the seabed ground G.

[0065] Also, since a cavitation generator 15 including a vacuum pump connected to the air supply and exhaust pipe 14 is installed on the ground to manage the occurrence of cavitation, it is much easier to manage the occurrence of cavitation compared to the case where cavitation occurs on the seabed B.

[0066] Also, when the lower layer G2 located below the upper layer G1 constituting the seabed B of the seabed ground G is a permeable layer, the strainer portion of the strainer pipe 11 is positioned in the lower layer G2 which is the permeable layer. When the lower layer G2 is an impermeable layer, the strainer portion is positioned at the bottom of the upper layer G1 which is the impermeable layer. Therefore, depending on whether the lower layer G2 is a permeable layer or an impermeable layer, the depth at which the strainer pipe 11 is buried can be determined, and the depth of the seabed ground G to be consolidated and settled can be changed according to the situation.

[0067] Further, between the location in the seabed ground G where the air supply and exhaust pipe 14 is buried and the location where the influence of the pressure due to the vacuum suction through the air supply and exhaust pipe 14 is desired to be blocked (for example, the location where the cavitation generator 15 is installed), a pressure influence blocking hole 17 for blocking the influence of the pressure due to the vacuum suction through the air supply and exhaust pipe 14 is formed. Thus, an air curtain is formed around the pressure influence blocking hole 17 to create an unsaturated zone, and a highly water-tight area can be formed, blocking the influence on the location where the influence of the pressure due to the vacuum suction through the air supply and exhaust pipe 14 is desired to be blocked.

[0068] Further, the number and intervals of the plurality of strainer pipes 11 are derived from the target consolidation settlement amount, the physical properties of the upper layer G1 constituting the seabed B of the seabed ground G, the layer thickness of the upper layer G1, the consolidation characteristics of the upper layer G1, the water permeability of the lower layer G2 located below the upper layer G1, and the confined state of the lower layer G2. Therefore, numerical values considering the state of the seabed ground G where the plurality of strainer pipes 11 are to be buried, the navigation of the ship S, and its use as a berth are calculated, and based on this, the seabed ground G can be consolidated and settled.

[0069] Further, when it is necessary to construct a structure such as a quay wall on the coastal land, before the construction of the structure, the seabed ground G is consolidated and settled, so that the influence of the pressure due to the vacuum suction through the air supply and exhaust pipe 14 can be prevented from affecting the structure constructed on the coastal land.

[0070] Further, since the plurality of strainer pipes 11 and the air supply and exhaust pipe 14 are buried from the work barge 1, the costs and labor for constructing facilities for burying the plurality of strainer pipes 11 and the air supply and exhaust pipe 14 at sea can be omitted.

[0071] Further, since a dynamic compaction method in which a weight is dropped onto the seabed B of the seabed ground G to compact the seabed ground G is used in combination, the compaction effect of the seabed ground G can be increased, and the seabed ground G can be more reliably consolidated and settled.

[0072] In addition, since the consolidation settlement amount of the seabed ground G is measured using a distance measuring sensor, unlike the case of measuring the consolidation settlement amount using an object for distance measurement such as a settlement plate, the consolidation settlement work of the seabed ground G can be carried out without interfering with the navigation of the ship S.

Explanation of Signs

[0073] W Water (sea) B Seabed (seafloor) G Seabed ground (seafloor ground) G1 Upper layer (clay layer) G2 Lower layer (sand layer, gravel layer, rock layer) S Ship D Drainage E Consolidation settlement level 1 Work barge 2 Legs 3 Cantilever 4 Casing driver 5 Casing 6 Crawler crane 7 Hammer grab 10 Ground settlement device 11 Strainer pipe 11a Filter 110 Auxiliary steel pipe 12 Drainage pump 13 Drain pipe 14 Air supply and exhaust pipe 14a Connecting pipe 15 Cavitation generator 16 Water stop cap layer 17 Pressure influence cut-off hole

Claims

1. A consolidation settlement method for a submarine ground, which causes cavitation in the moisture existing in the gaps between innumerable soil particles in the submarine ground to explosively compress the moisture and generate shock waves, removes the moisture from the gaps between the innumerable soil particles, and consolidates and settles the submarine ground by water pressure and atmospheric pressure, comprising: burying a plurality of strainer pipes each having a strainer portion at a lower end and an air supply / discharge pipe in the submarine ground; lowering the groundwater level by sucking the groundwater in the ground through the strainer portions from the plurality of strainer pipes, and forming an unsaturated zone in the ground around the plurality of strainer pipes; forming a negative pressure zone in the ground around the plurality of strainer pipes by vacuum suction of the air in the unsaturated zone through the air supply / discharge pipe, and then causing cavitation in the moisture existing in the gaps between the soil particles by sending air through the air supply / discharge pipe into the negative pressure zone.

2. The consolidation settlement method for a submarine ground according to claim 1, characterized in that the periphery of the strainer portion in the plurality of strainer pipes buried in the ground of the submarine ground is in a state where the permeability coefficient is higher closer to the strainer portion.

3. The consolidation settlement method for a submarine ground according to claim 1 or 2, characterized in that a water stop cap layer for suppressing the inflow of water from underwater is formed at a height position near the assumed consolidation settlement level of the submarine ground among the periphery of the plurality of strainer pipes buried in the ground of the submarine ground.

4. In the step of burying the strainer pipes from above the water surface toward the submarine ground, an auxiliary steel pipe is connected to the upper end of the strainer pipe, and the upper end of the auxiliary steel pipe is positioned above the water surface; after burying the strainer pipes, the auxiliary steel pipe is removed, and an upper cover with a drain port is attached to the upper end of the strainer pipe to enable drainage of the groundwater in the ground; The consolidation settlement method for a submarine ground according to any one of claims 1 to 3, characterized in that the air supply / discharge pipe buried together with the plurality of strainer pipes and a cavitation generating device including a vacuum pump are connected by a connecting pipe.

5. The consolidation settlement method for a submarine ground according to any one of claims 1 to 4, characterized in that a cavitation generating device including a vacuum pump connected to the air supply / discharge pipe is installed on the ground to manage the generation of cavitation.

6. When the lower layer located below the upper layer constituting the seabed of the seabed ground is a permeable layer, the strainer portion in the strainer pipe is positioned in the lower layer which is the permeable layer; when the lower layer is an impermeable layer, the strainer portion is positioned at the bottom of the upper layer which is the impermeable layer. The consolidation settlement method for seabed ground according to any one of claims 1 to 5, characterized in that.

7. A pressure influence cutoff hole for cutting off the influence of the pressure by vacuum suction through the air supply and exhaust pipe is formed between the location where the air supply and exhaust pipe is buried in the seabed ground and the location where it is desired to cut off the influence of the pressure by vacuum suction through the air supply and exhaust pipe. The consolidation settlement method for seabed ground according to any one of claims 1 to 6, characterized in that.

8. The number and interval of the plurality of strainer pipes are derived from the target consolidation settlement amount, the physical properties of the upper layer constituting the seabed of the seabed ground, the layer thickness of the upper layer, the consolidation characteristics of the upper layer, the permeability of the lower layer located below the upper layer, and the confined state of the lower layer. The consolidation settlement method for seabed ground according to any one of claims 1 to 7, characterized in that.

9. Before the construction of the structure built along the coast, the seabed ground is consolidated and settled. The consolidation settlement method for seabed ground according to any one of claims 1 to 8, characterized in that.

10. The plurality of strainer pipes and the air supply and exhaust pipe are buried from a work barge. The consolidation settlement method for seabed ground according to any one of claims 1 to 9, characterized in that.

11. A dynamic compaction method in which a hammer is dropped onto the seabed of the seabed ground to compact the seabed ground is used in combination. The consolidation settlement method for seabed ground according to any one of claims 1 to 10, characterized in that.

12. The consolidation settlement amount of the seabed ground is measured using a distance measuring sensor. The consolidation settlement method for seabed ground according to any one of claims 1 to 11, characterized in that.

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