Rapid vacuum consolidation method for improving bottom sediment, airtight loading box, sand capping construction method, and sand capping installation box

Abstract

The transformation of seabed sediments into organic sludge is a serious problem, both in improving the water environment of enclosed sea areas through sediment improvement and in river dredging as a flood control measure. Japan's organic sludge disposal sites are controlled facilities, and their remaining capacity is becoming critical. Constructing new disposal sites requires enormous project costs and construction time. [Solution] The solution of this invention involves forming a subsoil improvement layer on the seabed by consolidating the surface layer of the organic sludge using a rapid vacuum consolidation method, thereby increasing its density and strength, and stabilizing it on the seabed. Substances that cause water pollution in the subsoil are forcibly dissolved by consolidation wastewater to form an impermeable improved soil layer, which then seals in the organic sludge below. Meanwhile, the wastewater containing pollutants is properly treated. By forming an even more effective impermeable improved soil layer instead of permeable sand capping, a sustainable and healthy aquatic environment is restored without the need for sand capping materials or the generation of dredged soil.

Description

Technical Field

[0001] The present invention relates to sediment improvement of muddy sediment in rivers and coastal waters. More specifically, it relates to sediment improvement for flood control measures that do not carry out organic sludge removal and for water environment improvement measures in closed sea areas.

Background Art

[0002] In recent years, due to the intensification of rainfall possibly caused by climate change, large-scale inundation damage has occurred in wide areas of rivers. Due to heavy rain, the deposition of sediment in rivers has progressed and the flood risk has a tendency to increase. River dredging as a flood control measure has the advantage of surely lowering the water level and not requiring the acquisition of new land such as for floodways or widening the river width. However, urban rivers in the lower reaches of rivers are often filled with high-concentration nutrient-rich organic mud that has become muddy and deposited due to sediment, sewage, etc. flowing in from the land over many years. And the muddification of organic mud can be seen to spread from the estuary area to the coastal waters. Also, in recent years, sediment pollution by heavy metals and dioxins has become a problem.

[0003] In closed sea areas, although sewerage systems are being developed, water environment improvement has not progressed so much. The reasons are as follows. The red tide plankton generated in large quantities due to eutrophication eventually dies and sinks, becoming so-called sludge and depositing on the seabed. During warm climate periods, bacteria actively decompose the organic matter in the sludge, and in the process, a large amount of dissolved oxygen in the water is consumed and the bottom layer water becomes oxygen-deficient. As a result, benthic organisms such as bivalves and clams living on the seabed die and the purification function by organisms is lost, and their remains become a cause of new pollution. And nutrients elute from the sludge into seawater, promoting the growth of red tide plankton. It is considered that the deterioration of water quality and sediment cannot escape from a series of negative spirals in which the deterioration of water quality and sediment causes a decrease in the biological purification function and an increase in the pollution load, and thereby the water quality and sediment further deteriorate. Fundamental countermeasures are to prevent the inflow of organic matter and nutrients from the land, while sediment improvement of muddied organic sludge is essential.

[0004] Current methods for improving sediment bottom are broadly classified into dredging and capping. Dredging is a method of removing bottom sediment and is a fundamental solution. In recent years, in order to minimize the amount of waste to be disposed of, grab buckets for thin-layer dredging limited to the surface layer containing harmful substances have been developed. However, even with these measures, securing final disposal sites for dredged soil remains a major challenge. Capping is a method of covering the bottom sediment by spreading sand on top of it. Capping traps the sludge-like bottom sediment and suppresses the leaching of nutrients. However, from the perspective of reducing the cost of public works and conserving resources of capping materials, there is a trend towards thin-layer capping, where the thickness of the capping sand is thinner than before.

[0005] In Japan, dredged soil is generally classified as construction waste. However, dredged sludge containing organic matter is treated differently. Organic dredged sludge cannot be accepted in stable-type landfills and must be disposed of in controlled-type landfills. However, the remaining capacity of controlled-type landfills is already strained. Furthermore, in recent years, contamination of bottom sediment by heavy metals and dioxins has become a problem. This contaminated soil must be disposed of in containment-type landfills, further exacerbating the strain on remaining capacity.

[0006] Vacuum consolidation has traditionally been widely used as a ground improvement method on land, but its use is also progressing in seabeds and other areas. From an environmental perspective, a key feature of vacuum consolidation is that it can increase the density and strength of the seabed ground through consolidation settlement without causing water pollution in situ. If water depth can be maintained solely through consolidation settlement, it becomes a water depth maintenance method that does not generate dredged soil. To clarify the novelty and inventiveness of this invention, the progress of vacuum consolidation methods for riverbeds, seabeds, etc., will be described below. Vacuum consolidation methods for seabed ground improvement maintain airtightness of the loaded ground surface by some means, and use atmospheric pressure and water pressure for loading. A promising method for this is the airtight loading box.

[0007] One dredging method that utilizes vacuum pressure is the vacuum consolidation dredging method. This method combines the processes of vacuum consolidation and dredging into a single process. This method uses an airtight loading box. The airtight loading box serves as both a ground consolidation load and a dredging bucket. The structure of the airtight loading box is a box-shaped structure with an open bottom, and an airtight gas-water separation tank and a box tower are attached to the central part of the upper exterior surface of the box. In addition, a thin vacuum tank that communicates with the airtight gas-water separation tank is provided on the ceiling surface inside the box, and directly below it is divided into multiple compartments (divided spaces) by a box partition with a drain function, and a permeable lid is provided on the upper surface of each compartment. The structure of this airtight loading box remained basically unchanged until the airtight loading box with vacuum boiling consolidation specifications of the present invention appeared. In contrast, dredging with a grab bucket involves scooping up soil and sand underwater with a bucket. In the case of cohesive soil, the cohesive soil is disturbed and its strength decreases, and the dredged soil becomes artificially soft soil with a high water content due to the mixing with seawater. Furthermore, measures to prevent pollution diffusion are necessary during dredging. (See Patent Document 1)

[0008] In 2019, a high-vacuum consolidation system was invented in which the pressure reduction below the airtight sheet approaches -100kPa. Conventional vacuum consolidation systems typically achieve a pressure reduction of around -70 to -80kPa. The aforementioned high-vacuum consolidation system is so named because it uses a high vacuum that causes the pore water in the ground to boil. Once boiling occurs, maintaining a high vacuum pressure within a range that does not cause boiling allows for rapid and efficient consolidation settlement of clayey ground. This phenomenon of rapid consolidation settlement due to vacuum (reduced pressure) boiling is called vacuum boiling consolidation. (See Patent Document 2)

[0009] As is well known, water evaporates even without boiling. Water vapor, in particular, evaporates very actively in a vacuum. When water changes into a gas (water vapor), its volume increases dramatically by approximately 1700 times. Therefore, the generation of water vapor acts as an extremely large pressure increase, inverse to the effect of reduced pressure. Conventional vacuum consolidation systems have overlooked this negative effect of water vapor pressure. For this reason, the pressure reduction limit of conventional vacuum consolidation systems is around 70-80 kPa. The solution to this water vapor pressure problem is the high-vacuum consolidation system. The vacuum-related equipment of a high-vacuum consolidation system, which enables vacuum boiling consolidation, consists of a reinforced vacuum tank (high-vacuum storage tank) and a cold trap, installed in that order between the gas-water separation vacuum tank and the vacuum pump. The role of the cold trap is to dry the air by capturing the water vapor generated in the vacuum path as frost (ice), thereby maximizing the function of the vacuum pump. The reinforced vacuum tank is a tank that stores a large amount of high vacuum pressure and is used to rapidly create a high vacuum from a gas-water separation tank at atmospheric pressure. (See Patent Document 2)

[0010] In 2021, a vacuum boiling consolidation method was invented that clarified the mechanism of vacuum boiling consolidation, enabling quantitative design and construction management. (See Patent Document 3) The principle of vacuum boiling consolidation lies in the newly confirmed fine bubble effect. Details of the fine bubble effect in vacuum boiling consolidation are discussed based on experiments in a paper titled "Next-Generation Vacuum Consolidation Method - Vacuum Boiling Consolidation Method and Blue Carbon Towards a Decarbonized Society." (See Non-Patent Document 1)

[0011] To elucidate the mechanism of vacuum boiling consolidation, a vacuum boiling consolidation testing apparatus was developed to boil clay samples. Vacuum boiling consolidation tests and weight consolidation tests were conducted using this apparatus, and the results were compared. As a result, a remarkable consolidation-promoting effect was confirmed when sufficient boiling time was given to generate the required amount of fine bubbles in the clay sample. It was revealed that the required amount of fine bubbles is determined by the cation exchange capacity of the viscous soil, etc., and the amount of fine bubbles is determined by the boiling time. Therefore, in the design and construction management of the vacuum boiling consolidation method, the amount of fine bubbles is set based on the cation exchange capacity of the viscous soil, etc., and the vacuum consolidation test, and the effective boiling time that generates a corresponding amount of fine bubbles is incorporated separately into the vacuum consolidation process. (See Patent Document 3) (See Non-Patent Document 1)

[0012] Furthermore, the airtight loading casing described in Patent Document 3 has a rigid floating body attached to almost the entire upper surface of the casing. As a result, the raising and lowering of the casing can be freely performed by exchanging air and water in the internal space of the floating body using a diving control device. In addition, a seabed horizontal movement device is attached to the casing. This device consists of multiple horizontal telescopic beams with vertical telescopic spuds fixed to their tips, attached to the outer edges of the upper surface of the casing in two directions. Horizontal movement of the casing in the forward, backward, left, and right directions on the seabed can be freely performed by the diving control device and the horizontal movement control device for the horizontal telescopic beams, and these control devices are characterized by their ability to freely perform diving and seabed horizontal movement.

[0013] The consolidation system described in Patent Document 4 is a next-generation high-vacuum consolidation system. Its specification includes the following description: The actual depressurization of a normal vacuum consolidation system is -70 to -80 kPa. The time required for this depressurization is several tens of days. Next, the high-vacuum consolidation system described in Patent Document 2, which boils the ground, depressurizes to over -99 kPa (absolute pressure: 2.0 kPa). The high-vacuum storage tank of this high-vacuum consolidation system has a function to increase the depressurization rate. Even so, the time required for depressurization is expected to be several days. Originally, this high-vacuum consolidation system was not intended for rapid depressurization. Rapid depressurization can be achieved by preparing a large-capacity high-vacuum storage tank and corresponding vacuum equipment, etc. However, this is not cost-effective.

[0014] The description of the high-vacuum consolidation system in Patent Document 4 is insufficient and does not accurately convey its content. The high-vacuum consolidation system achieves rapid consolidation by reducing the external pressure to the saturated vapor pressure, which is the boiling point of the pore water in the ground, causing the ground to boil. If the external pressure were only atmospheric pressure, there would be no problem. However, the external pressure of actual saturated ground is atmospheric pressure plus water pressure. Simply reducing the pressure to the saturated vapor pressure at the boiling point will not cause the ground to boil. However, it has been experimentally confirmed that boiling can be induced by rapidly reducing the pressure, even with the added water pressure. If a high vacuum sufficient to boil the ground is stored in a high-vacuum storage tank, releasing it all at once will result in a rapid depressurization. In this case, the volume of the high-vacuum storage tank needs to be several times larger than that of the gas-water separation tank. Furthermore, the vacuum device needs to have the capacity to maintain the high vacuum of the gas-water separation tank for the required time. This is considered not cost-effective. (See Patent Document 4)

[0015] The key feature of the next-generation high-vacuum consolidation system lies in a vacuum device called a pressure-increasing / decreasing tank. This tank consists of an outer cylinder and a piston forming a sealed tank. The piston's up-and-down movement increases or decreases the tank's volume between a minimum (close to zero) and a maximum. The sealing of the gap around the moving piston is maintained by using a cylindrical piston, with its outer surface enclosed by a cylindrical seal. The principle is simple, based on Boyle's Law. Therefore, increasing the capacity of the vacuum device is considered easy. (See Patent Document 4)

[0016] Furthermore, the consolidation effect due to the vacuum boiling consolidation phenomenon is an entirely new field, and unresolved issues remain. Non-patent document 1 describes an experiment to confirm whether the vacuum boiling consolidation method can be applied to actual seabed ground, using a fine bubble (microbubble, ultrafine bubble) observation device to verify the depressurized (vacuum) boiling phenomenon under water pressure at a depth of 10 m. The outline of the experiment is as follows: The observation pipe of this device is a transparent acrylic resin pipe with a height of 11.0 m and a diameter of 3.5 mm. A 0.5 m space is left at the top of the pipe, and it is filled with water to a height of 10.5 m and sealed. Then, a vacuum is drawn from the top end with a vacuum pump and observation was performed. The water used is tap water. The water temperature in the experiment was 17.5°C, and its saturated vapor pressure was 2.0 kPa. The depressurization was set to -99.8 kPa (absolute pressure: 1.5 kPa), which is slightly higher than the saturated vapor pressure. The experiment involved rapid depressurization. Microbubbles are visible because they cause cloudiness. Ultrafine bubbles are invisible to the naked eye and can only be identified with a green laser pointer. Microbubbles were not observed, but the linear trajectory of the laser pointer was clearly visible. However, this alone is insufficient to determine whether the phenomenon was due to ultrafine bubbles, as laser pointers also react to impurities in tap water. A notable finding of this experiment is the phenomenon of countless millibubbles adhering to the entire inner surface of the observation pipe over time. These millibubbles, adhering to the pipe, formed throughout the entire pipe over time. This indicates boiling. Atmospheric pressure was reduced to 1.5 kPa, and the external pressure at a water depth of 10.5 m at the bottom of the observation pipe was 104.5 kPa. The boiling point at this external pressure is typically around 101°C. It is presumed that ultrafine bubbles were generated at a water temperature of 17.5°C, adhering to the pipe and merging to grow into millibubbles. This is a previously unknown phenomenon. Boiling at 17.5°C is a fact. However, the reason why ultrafine bubbles were generated remains unclear. (See Non-Patent Literature 1)

[0017] An experiment was conducted to confirm the depressurized (vacuum) boiling phenomenon under water pressure at a depth of 10m using pure water. The experimental conditions other than the use of pure water were the same as in Non-Patent Document 1. The experiment began with a rapid depressurization, and immediately after the start, fine bubbles visible to the naked eye were actively generated from around 70cm depth in the upper layer of the observation pipe. As they rose, millibubbles grew into centibubbles and burst at the water surface. Microbubbles were not observed in the lower layer below 70cm depth, but ultrafine bubbles could be clearly seen with a laser pointer. Bubbles generated by normal boiling almost stopped after about 25 minutes. Millibubbles adhering to the pipe appeared from around 18 minutes after the start of the experiment and increased in density over time. Here, as the force of normal boiling decreased below 70cm depth, the water temperature decreased. This is due to the heat of vaporization. The vacuum pressure remained constant at 1.5kPa, but boiling stopped due to the decrease in water temperature. The boiling point at a saturated vapor pressure of 1.5kPa is 13.0℃. The water temperature drops by 4.5°C due to the heat of vaporization. However, this temperature drop is only observed at depths of 70 cm or less; the water temperature at deeper depths remains unchanged at 17.5°C. The generation of ultrafine bubbles remains active, and the density of millibubbles adhering to the pipe continues to increase. The heat of vaporization due to the generation of ultrafine bubbles appears to be negligible.

[0018] Non-Patent Document 1 summarizes the results of vacuum boiling consolidation tests in the following three points: 1. In vacuum consolidation tests, when the temperature of the pore water in a clay sample is reduced to the saturated vapor pressure at its boiling point, it boils, generating fine bubbles and rapidly consolidating. 2. Vacuum boiling consolidation is rapidly accelerated because the surfactant action of the fine bubbles temporarily disrupts the electrical equilibrium of the clay structure, facilitating the rearrangement of pores. 3. The amount of fine bubbles required to accelerate soil consolidation varies depending on the type of clay (soil). The amount of fine bubbles depends on the boiling time, and it is stated that clays with a particularly large cation exchange capacity require longer boiling times. This is an experimental fact. (See Non-Patent Document 1)

[0019] In the vacuum boiling and consolidation test, the fine bubble effect could not be confirmed in organic sludge. The reason given for this is that the boiling time of 1 hour was insufficient for organic sludge, which has a large cation exchange capacity. However, a clay sample whose main clay mineral is kaolinite showed an astonishing effect in just 2 minutes. In comparison, the boiling time for organic sludge is approximately 50 times longer, requiring 100 minutes. It is unnatural that there was almost no boiling effect (fine bubble effect) after 60 minutes of boiling. The clay sample was small, with a diameter of 60 mm and a height of 10 mm. It is presumed that the temperature of the interstitial water in the clay sample decreased due to the heat of vaporization, causing the boiling to stop. A clay sample that was fine after 10 minutes of boiling showed an astonishing effect. Boiling continued for about 10 minutes. However, judging from the experiment to confirm the boiling phenomenon under reduced pressure (vacuum) again under a water pressure of 10 m depth, boiling in the vacuum boiling and consolidation test must have stopped at least 25 minutes earlier. In other words, the insufficient boiling time was due to the boiling stopping due to the heat of vaporization. This is a problem that can be solved once the cause is clarified. (See non-patent document 1) [Prior art documents] [Patent Documents]

[0020] [Patent Document 1] Patent No. 6582361 [Patent Document 2] Patent No. 7198406 [Patent Document 3] Patent No. 7325701 [Patent Document 4] Japanese Patent Application No. 2022-111443 [Non-patent literature]

[0021] [Non-Patent Document 1] Masayoshi Kondo et al. / Next-generation vacuum consolidation method "Vacuum boiling consolidation method" and blue carbon towards a decarbonized society / Proceedings of the 14th Symposium on Environmental Geotechnical Engineering / Japanese Geotechnical Society, September 2021 [Overview of the project]

Problems to be Solved by the Invention

[0022] The organic sludgeification of sediment has become a serious problem in sediment improvement for the improvement of the water environment in closed sea areas and also in river dredging as a flood control measure. The final disposal sites for organic sludge in our country are management-type final disposal sites, and their remaining capacities are in a tight situation. The construction of new final disposal sites requires huge project costs and construction periods. The problem to be solved by the present invention is how to realize measures for improving the water environment in closed sea areas and flood control measures for rivers without transporting the organic sludgeified sediment. In achieving this, the characteristics of the vacuum consolidation method are utilized to the maximum extent. The characteristics of the vacuum consolidation method from an environmental perspective are that the density and strength of the seabed ground can be increased by consolidation settlement without generating water pollution in-situ. On the other hand, a long consolidation time is required.

[0023] First, there is a sand covering method among the sediment improvement methods that do not carry out organic sludge. Here, let's consider again the principle of improving the water environment and the growth environment of benthic animals by the sand covering method. By covering the organic sludge on the seabed or the like with sand or the like, the organic sludge is contained and direct contact between seawater and the organic sludge is prevented. As a result, the nutrients that have returned from the organic sludge to the seawater are suppressed, and the consumption of dissolved oxygen in the seawater by the decomposition of organic substances on the seabed by bacteria is suppressed. In this way, it is intended to keep the water quality and ecosystem of the sea area in good condition. Now, usually sand is used as the sand covering material. If we want to prevent direct contact between seawater and organic sludge, impermeable clay is better than permeable sand. If we want to prevent the loss of the sand covering material, gravel heavier than sand is better. The conventional sand covering method has been to throw sand from the sea surface and level it at the seabed. Since the diffusion of pollution occurs when clay is dropped into the sea, it was not suitable as a sand covering material. The principle of the sand covering method is that if it can prevent direct contact between seawater and organic sludge, without diffusing pollution, and without the sand covering material flowing out, the sand covering material does not have to be sand. That is, using sand as the sand covering material is a measure against pollution diffusion. Sand with good water permeability does not necessarily have a good sand covering function for the pollution generation layer. The first problem of the present invention is the development of a sediment improvement method that does not carry out organic sludge and does not carry in a sand covering material. That is, it is the development of a technology to regenerate the sediment, which is organic sludge, into a stable improved soil layer having a sand covering function exceeding that of sand covering by the rapid vacuum consolidation method.

[0024] Sediment improvement is not only to prevent direct contact between seawater and organic sludge. It also contributes to improving the habitat environment of benthic organisms. Measures for the growth environment are attached to Problem 1. Also, the organic sludge ground always has a floating mud layer, large or small. When the sand covering material is put in during the construction of the sand covering, the floating mud rises, the pollution diffuses, and a large amount of nutrients and the like elute. Few of the conventional sand covering methods were intended for this countermeasure against muddy sediment. Countermeasures against floating mud are attached to Problem 1 of the present invention.

[0025] This invention maximizes the use of vacuum consolidation methods. This involves using organic sludge as a sand cover material for bottom sediment, or ground (bottom sediment) settlement to maintain water depth without generating dredged soil. Ground settlement and ground improvement by consolidation require a long time. The rapid vacuum consolidation method of this invention requires an airtight loading box and a work vessel, resulting in higher capital investment compared to land-based vacuum consolidation methods. Even if an improved soil layer with high sand cover function is regenerated, if the consolidation time is several months, as with land-based vacuum consolidation methods, it is not cost-effective. The second challenge of this invention is to dramatically shorten the consolidation time and reduce construction costs. This involves developing a rapid vacuum consolidation method and an airtight loading box that make this possible.

[0026] The vacuum boiling consolidation method described in Patent Document 2 exhibits a larger consolidation settlement amount and a higher consolidation rate compared to conventional vacuum consolidation methods. However, Patent Document 4 states that the rapid depressurization using the high-vacuum consolidation system described in Patent Document 2 is not cost-effective. However, the vacuum boiling consolidation method still has room for improvement. Water evaporates actively in a vacuum. Patent Document 2 collects the generated water vapor as frost (ice) in a cold trap. The generation of water vapor is unavoidable. However, it is possible to reduce the generation itself. If the generation of water vapor can be drastically reduced, the cost-effectiveness will be drastically improved. Drastically reducing water vapor is related to Problem 2. Furthermore, Non-Patent Document 1 stated that the fine bubble effect did not occur in organic sludge because the boiling time was insufficient. This insufficient boiling time was due to the boiling stopping due to the heat of vaporization. Addressing this heat of vaporization is also related to Problem 2.

[0027] In vacuum consolidation methods for the seabed and other areas, airtight loading boxes are used to maintain airtightness on the loading surface. The interior of these boxes is divided into partitioned spaces by partitions with drainage functions. The height of the box's interior, i.e., the height of the partitioned spaces, determines the consolidation depth. Here, the partitions with drainage functions in the airtight loading box are, for example, those with cylindrical drainage sheets attached to the entire inner surface of the partitions in the partitioned spaces. The role of the partitions is to increase the rigidity of the airtight loading box and to serve as a mounting surface for the drainage sheets. Since the partitions are partitions with drainage functions on two sides, the horizontal drainage distance of this box is half the partition spacing. A promising method for shortening the consolidation time is to shorten the consolidation drainage distance. Consolidation time is shortened inversely proportional to the square of the drainage distance according to Terzaghi's theory. Therefore, the shorter the drainage distance, the shorter the consolidation time. However, when the box structure is pulled out from the bottom sediment (seabed ground), the proportion of the adhesive force of the compacted sediment increases as the distance between the partitions decreases, and the likelihood of the sediment being pulled out increases dramatically. This conflicting problem is Problem 3 of the present invention, and it is the biggest problem. Problem 3 is essentially one with Problem 2, which is the dramatic reduction of the consolidation time.

[0028] Flood control measures that ensure sufficient water depth without generating dredged soil involve ground (bottom sediment) settlement using the vacuum consolidation method. Conventional airtight loading boxes are used in vacuum consolidation dredging methods intended for dredging, and their height is approximately 1m. If ground settlement is 1m, then an airtight loading box height of approximately 10m is required. Problem 3, the co-uplift of the ground, becomes more pronounced. For example, if the horizontal cross-sectional area of ​​the divided space is 10cm x 10cm, it is significantly excessive in terms of rigidity. Also, drain sheets are consumables and need to be replaced. If the box height is 1m, it is possible to manufacture a box divided by a partition wall and replace the drain sheet. However, if the box height is 10m, it becomes impractical to manufacture. The method for manufacturing airtight loading boxes with a large box height is attached to Problem 3. Note that ground settlement using the vacuum consolidation method is a one-time method for ensuring sufficient water depth. River sedimentation is a constant process. However, with the current progress in sewage systems, the inflow of organic matter and nutrients from land is minimal, and it can be said that sludge formation of sediment (bottom sediment) is virtually nonexistent. Therefore, controlled landfills are not necessary.

[0029] Bottom sediment improvement not only prevents the leaching of substances related to water pollution, but also improves the habitat for benthic organisms. The fourth objective of this invention is to create appropriate capping sand on the upper surface of an improved layer with a capping function, using a capping method, which is necessary for improving the environment for benthic plants and animals. This involves developing a capping method that reduces construction costs by accurately and quickly creating the density and thickness of the capping sand, and developing a capping installation box that makes this possible. Means for solving the problem:

[0030] The problem that this invention aims to solve is how to implement measures to improve the water environment in enclosed sea areas without transporting organic sludge, and similarly, how to implement flood control measures for rivers. In achieving this, the advantages of the vacuum consolidation method are maximized. The advantage of the vacuum consolidation method is that it increases the density and strength of the bottom sediment through consolidation settlement using atmospheric pressure and water pressure as the load, without generating pollution. The disadvantage is that the consolidation time is long, and this accounts for the majority of the construction period. This is the conventional understanding of the vacuum consolidation method. The vacuum consolidation method forcibly discharges the pore water of the ground using vacuum pressure. This is obvious. However, this obvious action produces an extraordinary effect. That is, consolidation dissolves pollutants into the pore water and forcibly discharges them together with the pore water. Pollutants remaining in the bottom sediment will not be discharged unless a vacuum pressure or load exceeding the history vacuum pressure is applied, and will be permanently stored. This is a newly noteworthy major feature of the vacuum consolidation method of this invention and is the means of solving problem 1 of this invention.

[0031] The solution to problem 1 of the present invention is to transform organic sludge (seabed or riverbed ground) into an impermeable and highly barrier-functional improved soil layer by a rapid vacuum consolidation method. The structure of the airtight loading box of the present invention is basically the same as that of a conventional open-bottom steel box. The main difference is the structure of the horizontal drain. (See below) A drain sheet that also serves as a screen and acts as a horizontal drain is attached to the ceiling surface of the airtight loading box. A gas-water separation tank to which the horizontal drain is connected is installed on the top surface of the box, and this gas-water separation tank is connected to a vacuum device and a compressed air device by their respective dedicated routes. In addition, the inside of this box is divided into partitioned spaces separated by partitions with drain functions. By applying vacuum pressure and water pressure to this airtight loading box, the surface sediment is consolidated and settled, and the density and strength of the surface sediment are increased to form an improved soil layer, which is then stabilized on the seabed. In parallel, substances related to water pollution of the sediment are forcibly discharged by consolidated drainage. This process regenerates the soil into an impermeable, highly effective barrier layer. Meanwhile, wastewater containing leached pollutants is separated into water and pollutants for water treatment. The stable, impermeable soil layer then contains the underlying pollution-generating layer. As a result, the sediment improvement method of this invention is characterized by its ability to regenerate a healthy water environment sustainably without requiring capping materials or generating dredged soil.

[0032] Wastewater containing high concentrations of pollutants leached during consolidation is properly treated. For example, if the pollutants are nutrients, one method of treating them involves mixing them with an inorganic treatment agent, separating the nutrients from the water, and then filtering. Furthermore, a floating sludge layer always exists above organic sludge. In conventional sand capping methods, this floating sludge was stirred up during construction, spreading the pollution. In this method, this floating sludge is consolidated and incorporated into the improved soil layer.

[0033] The sediment improvement method of the present invention is not a cover with permeable sand, but a stable, impermeable improved soil layer with increased density and strength in the surface layer of the sediment. A stable improved soil layer has two elements of stability. One is that the sediment is stably fixed to the seabed. In other words, the improved soil layer does not flow and wash away due to the various currents that occur on the seabed. Covering with sand is significantly more stable than sediment that has become sludge-like. However, an improved soil layer can be made even more stable. The other element of stability is the containment of the underlying pollution-generating layer. The improved soil layer forcibly discharges the pollutants present in this layer along with the pore water. The remaining pollutants are permanently stored and will not be discharged unless a vacuum pressure exceeding the vacuum pressure of the history is applied. The improved soil layer has a significantly higher barrier function than a cover with permeable sand.

[0034] The thickness of the improved soil layer is determined by the internal height of the airtight loading box used in the rapid vacuum consolidation method. For example, the difference in construction costs between a 10cm and a 50cm improved soil layer is due to the cost of manufacturing the airtight loading box, as the construction time is almost the same. Therefore, the construction cost increases only slightly as the improved soil layer gets thicker. In contrast, with the sand capping method, the construction cost increases almost proportionally as the thickness increases. The construction cost for a 10cm and a 50cm thickness differs significantly. From the perspective of ensuring the long-term sustainability of the sand capping function, a thickness of around 50cm for the improved soil layer is recommended.

[0035] The solution to problem 4 of the present invention is to create a capping layer necessary for improving the environment for benthic plants and animals. In the bottom sediment improvement method, a capping layer is created on the upper surface of the improved soil layer, which has high barrier function, using a bottom-open capping installation box, with a capping material that improves the habitat for benthic plants and animals. By forming a composite capping layer of improved soil layer and capping material, the invention is characterized by the restoration of both a sustainable and good aquatic environment and a habitat for benthic plants and animals.

[0036] The second problem of the present invention is to dramatically shorten the consolidation time. The third problem is to prevent the bottom sediment (seabed ground) from rising when an airtight loading box installed on the seabed is pulled up. The rising of the bottom sediment is caused by the adhesion force between the bulkheads of the airtight loading box and the bottom sediment. Shortening the consolidation time and the rising of the bottom sediment are inversely related, depending on the size of the gap between the bulkheads. The solution to this lies in the airtight loading box used in the rapid vacuum consolidation method of the present invention. The gap between the bulkheads of the airtight loading box is set to the lower limit gap at which the bottom sediment does not rise when the bulkhead is pulled up, even when the adhesion force between the bulkheads and the bottom sediment is broken. As a result, the horizontal consolidation drainage distance of the airtight loading box is set to 1 / 2 of the lower limit gap between the bulkheads, which dramatically shortens the consolidation time.

[0037] Here is a supplementary explanation regarding the "minimum spacing at which the bottom sediment does not rise together with the bulkhead when the adhesion between the bulkhead and the bottom sediment (seabed ground) is severed." The solution to these conflicting problems is to sever the adhesion between the bulkhead and the bottom sediment (explained later). If the adhesion is severed, the uplift will be eliminated. Theoretically, the spacing between bulkheads can be shortened indefinitely, but there is a practical limit. This is referred to as the minimum spacing. Currently, a bulkhead spacing of about 10 cm is set as the minimum spacing. For reference, the vertical drain spacing in onshore vacuum consolidation methods is usually about 80 cm. Compared to this, the consolidation time is reduced to 1 / 64. This is described as a significant reduction in consolidation time. The internal height of the airtight loading box used for surface improvement of the bottom sediment is about 50 cm.

[0038] The solution to problem 3 of the present invention, which is the co-uplift of the sediment, is to break the adhesion between the partition wall and the sediment. The method for breaking the adhesion involves forming a water boundary film at the boundary between the partition wall, which has a drain function, and the sediment. The formation of this boundary film will be explained through the work process. The gas-water separation tank of the airtight loading container of the present invention is connected to the vacuum device and the compressed air device by their respective dedicated routes. Furthermore, the vacuum route and compressed air route within the airtight loading container are shared.

[0039] The main work steps of the rapid vacuum consolidation method for sediment improvement according to the present invention consist of a process of placing an airtight loading box, a vacuum consolidation process, a process of breaking the adhesion between the box's partition walls and the sediment, and a process of lifting the airtight loading box. In the process of breaking the adhesion, a compressed air device is operated to generate high water pressure that widens the gap between the ceiling surface of the box and the sediment surface, and a boundary membrane of water is formed at the boundary between the partition walls, which have a drain function, and the sediment, thereby breaking the adhesion between the partition walls and the sediment. In the lifting process, the airtight loading box is lifted up by pressing the sediment surface with high water pressure to prevent the sediment from rising together with the box. This rapid vacuum consolidation method is characterized by the fact that, due to the boundary membrane effect and the effect of the lower limit spacing of the partition walls, water pollutants are removed at a predetermined consolidation settlement, increasing density and strength, and significantly shortening the consolidation time for sediment improvement. The principle of breaking the adhesion is that the water remaining in the drain is pressurized and pushed out onto the partition wall surface. Since the sediment has finished consolidating, water cannot penetrate it and instead forms a water boundary film, breaking the adhesion.

[0040] The rapid vacuum consolidation method of the present invention is based on the premise of introducing the high-vacuum consolidation system of vacuum boiling consolidation described in Patent Document 2. The second problem of the present invention is to dramatically shorten the consolidation time, and the final touch to shortening the time is a further evolution of the method described in Patent Document 2. That is, measures to drastically reduce the amount of water vapor generated in the vacuum and measures to counter the heat of vaporization are incorporated into the high-vacuum consolidation system.

[0041] This is a description of a vacuum boiling consolidation specification airtight loading box used in the rapid vacuum consolidation method of the present invention. Several layers of drain sheets, which also serve as a screen and horizontal drains, are attached to the ceiling surface of the box. A gas-water separation tank, to which the horizontal drains are connected, is installed on the top surface of the box, and a submersible heater is installed in this gas-water separation tank. The structure of the gas-water separation tank is designed to have the minimum necessary horizontal cross-sectional area and to always contain residual water. As a result, the gas-water interface of the box (described later) is limited to the gas-water separation tank, and the generation of water vapor is significantly reduced. Thus, the airtight loading box of the present invention becomes a vacuum boiling consolidation specification airtight loading box in which a high vacuum for vacuum boiling consolidation is continuously maintained.

[0042] The present invention's rapid vacuum consolidation method first shortens the consolidation time by setting the spacing between the partitions of the airtight loading box to the lowest possible spacing that prevents the bottom sediment from rising. This ensures the effectiveness of the lower spacing. Next, to suppress the generation of water vapor, an underwater heater is installed in the gas-water separation tank, and the gas-water interface is limited to the gas-water separation tank, thereby ensuring a significant reduction in water vapor. This airtight loading box is a box designed for vacuum boiling consolidation. This box is a major factor in further advancing the high-vacuum consolidation system. The present invention's rapid vacuum consolidation method, which ensures these two significant effects, incorporates a high-vacuum consolidation system. As a result, the fine bubble effect is doubled in this consolidation system, improving the bottom sediment, dramatically increasing the barrier function of the improved soil layer, and the consolidation time is dramatically shortened due to the synergistic effect of the lower spacing of the box partitions and the enhanced fine bubble effect.

[0043] Here is some supplementary explanation regarding the gas-water interface. Water evaporates actively in a vacuum, expanding up to 1,700 times its original volume. Vapor pressure acts as a pressure increase, the exact opposite of a pressure decrease, significantly reducing the degree of vacuum. The high-vacuum consolidation system described in Patent Document 2 reduces the pressure by capturing the generated water vapor as ice (frost) in a cold trap, and then sends dry air to the vacuum pump, significantly reducing the load on the vacuum pump. In addition to this pressure reduction, the present invention limits the generated water vapor to a gas-water separation tank, drastically reducing the amount of water vapor generated itself. This is incorporated into the system. The vacuum consolidation method is the same as the principle of vacuum drying in terms of the principle of water evaporation. The rate at which water evaporates depends on the molecular structure of water. Other factors that affect evaporation include the gas-water interface area, temperature, and airflow. The larger the interface area, the faster the evaporation rate. The higher the temperature, the faster the evaporation rate. When water vapor moves across the interface, the increase in humidity is suppressed, so the evaporation rate is faster. Here, the element that can be re-examined is the gas-water interface area.

[0044] The structure of the airtight loading box in conventional vacuum consolidation dredging methods is a box-shaped structure with an open bottom. A gas-water separation tank is attached to the upper exterior surface of the box, and a thin vacuum tank communicating with the gas-water separation tank is provided on the interior ceiling surface. The gas-water separation tank has the minimum horizontal cross-sectional area necessary to install a submersible pump. The problem lies with the thin vacuum tank, which acts as a horizontal drain. Initially, it is full of water, but soon a two-layer flow of water and air is created. The interface between the two layers becomes the evaporation surface. Naturally, since water vapor is lighter than water, the water vapor is rapidly drawn in. The solution to this phenomenon in this invention is to replace the thin vacuum tank with several layers of drain sheets that act as a horizontal drain. The sheets are layered to enhance their function as a horizontal drain. Furthermore, the minimum necessary amount of water remains in the gas-water separation tank, so that the gas-water interface is located inside the gas-water separation tank, and this is set to the smallest possible horizontal area. In conventional vacuum consolidation systems, the area of ​​the gas-water interface is the horizontal cross-sectional area of ​​the airtight loading chamber, whereas in this invention, it is the horizontal cross-sectional area of ​​the gas-water separation tank. The difference in area is extreme. As a result, the cost-effectiveness of rapid depressurization using high-vacuum consolidation systems is dramatically improved. Furthermore, the purpose of the underwater heater is to counteract the heat of vaporization and suppress the drop in water temperature in the gas-water separation tank.

[0045] In the rapid vacuum consolidation method of the present invention, the advanced high-vacuum consolidation system, in addition to shortening the consolidation time due to the effect of the lower limit spacing of the partition walls of the present invention, further reduces the consolidation time by approximately half due to the enhanced fine bubble effect. For reference, the vertical drain spacing in conventional onshore vacuum consolidation methods is usually around 80 cm. Compared to this, the consolidation time is reduced to 1 / 64, and then further reduced by approximately half. This is described as a dramatic reduction.

[0046] Conventional airtight loading boxes are used in vacuum consolidation dredging methods intended for dredging, and their height is approximately 1 meter. Rapid vacuum consolidation methods intended for bottom sediment improvement while also increasing depth require airtight loading boxes to be approximately 10 meters high if the ground (bottom sediment) settlement is 1 meter. Airtight loading boxes for taller boxes have different structures due to manufacturing constraints. For example, a partition wall with a drain function is one to which a drain sheet is attached to the partition wall. If the drain spacing is 10 cm, the maximum height of the airtight loading box that can be manufactured is approximately 1 meter.

[0047] In the airtight loading box of a tall box used in the rapid vacuum consolidation method of the present invention, lightweight, tensile-strength vertical drains, such as plastic, are installed in parallel in the divided spaces inside the box. When the box is lifted after being driven into the seabed or riverbed, the spacing of the vertical drains in the box is set to the lower limit spacing that prevents the bottom sediment from rising together with the vertical drains while the adhesion force between the vertical drains and the bottom sediment is broken. The airtight loading box is characterized by significantly shortening the consolidation time by setting its horizontal consolidation drainage distance to half the lower limit spacing of the vertical drains. The high vacuum consolidation system is also introduced in the airtight loading box of a tall box.

[0048] In bottom sediment improvement methods for rivers and other bodies of water where water depth is insufficient, the water depth is increased not by dredging, but by the settlement of the bottom sediment (seabed ground) using a rapid vacuum consolidation method, and an impermeable improved soil layer is formed on the bottom sediment, with this improved soil layer containing the pollution-generating layer below. The bottom sediment improvement method that also increases water depth according to the present invention is characterized by not removing organic sludge from the bottom sediment and not bringing in any capping material.

[0049] The solution to problem 4 of the present invention is to create appropriate sand cover necessary for improving the environment for benthic plants and animals. In the case of composite sand cover where a good aquatic environment is ensured, the thickness of the sand cover only needs to be considered for benthic plants and animals. However, in the case of single sand cover, ensuring a good aquatic environment takes priority. There should be no gaps at the joints of the sand cover that occur during construction. However, small gaps are not a problem with composite sand cover. Whether it is single sand cover or composite sand cover, the challenge here is to develop a sand cover installation method that can reduce construction costs by accurately and quickly setting the density and thickness of the sand cover, and a sand cover installation box that makes this possible. Furthermore, the structure of the sand cover installation box is basically the same as that of an airtight loading box. The differences are that there is no drain function in the partition walls, and there is a sand cover material slip prevention mechanism at the bottom of the partition walls.

[0050] In the sand-covering enclosure of the present invention, the enclosure is a steel box with an open bottom. A horizontal drain, which also serves as a screen, is installed on the ceiling of the enclosure, and a gas-water separation tank, to which an underwater vibration device and the horizontal drain are connected, is installed on the top of the enclosure. The gas-water separation tank is connected to a vacuum device and a compressed air device via dedicated routes. Furthermore, the interior of the enclosure is divided into partitioned spaces, and a narrow anti-slip strip (described later) is placed around the bottom of the partition to prevent the sand-covering material from falling out. The spacing of the partitions is set to the upper limit to prevent the sand-covering material from falling out when it is loaded into the enclosure, and the loading and unloading of the sand-covering material is characterized by being pushed out with high water pressure. The internal height of the sand-covering enclosure, which is the thickness of the sand-covering material, is approximately 10 cm to 15 cm.

[0051] In a sand capping construction method using a sand capping installation box, the main work processes of the method consist of loading the sand capping material into the box, installing the box on the seabed, and loading and unloading the sand capping material. In the loading of the sand capping material process, in the stockyard of the work vessel, the sand capping installation box is vibrated and driven into the wet sand capping material while operating a vacuum device to remove excess pore water and compact the sand capping material, making the pore ratio of the sand capping material below the critical pore ratio, and then the sand capping material is loaded into the box. Next, in the installation process of the box, the sand capping installation box loaded with sand capping material is lifted and installed in a predetermined position on the seabed. In the loading and unloading process of the sand capping material, a compressed air device is operated to push out the sand capping material with high water pressure to create the sand cap. The sand capping construction method of the present invention is characterized by creating sand capping of a predetermined thickness and density below the critical pore ratio using a sand capping installation box with an open bottom.

[0052] The specified density is the density when the critical void ratio is below it. The critical void ratio is the void ratio at which the volume of the sand cover material does not increase or decrease when sheared. By loading the sand cover material to a density below the critical void ratio, volume expansion occurs during shearing. Here, the requirement that the sand cover material loaded into the box does not fall out under its own weight is usually that the frictional force on the circumferential surface of the partition wall be greater than the weight of the sand cover material. If relying on circumferential frictional force, the void ratio of the sand cover material must be significantly below the critical void ratio. If the void ratio of the sand cover material is only slightly lowered from the critical void ratio, the sand cover material will fall out. Managing this compaction is not easy. The requirements for the sand cover material of this invention not to fall off under its own weight are: the void ratio of the sand cover material is below the critical void ratio and volume expansion occurs during shear; there is a minute width of shear stopper at the bottom of the partition wall of the box-shaped structure to prevent the sand cover material from falling off; the shear stopper causes the sliding surface of the sand cover material to move from the circumferential surface of the partition wall into the bottom sediment by an amount corresponding to the width of the shear stopper; and the sliding resistance force (shear stress) generated on the sliding surface of the sand cover material that would otherwise fall off under its own weight is large. Note that, in the case of sliding resistance force where the void ratio of the sand cover material is below the critical void ratio, the relationship is shear resistance force > frictional resistance force. The minute width of the shear stopper is approximately the width of the grain size of the sand cover material. The width of the shear stopper should be such that the grain size of the sand cover material catches on it and prevents shearing. The width of the shear stopper is a few millimeters. Furthermore, the specification of the upper limit spacing of the partition walls is defined by the shear strength of the sand cover material at the critical void ratio. The weight of the sand covering material between the partitions is determined by the spacing between the partitions. If the spacing between the partitions is defined as the upper limit spacing at which the sand covering material does not fall out, this spacing is determined by the aforementioned shear strength of the sand covering material.

[0053] The construction of the sand cover can be carried out in stages, using appropriate sand cover material in the necessary areas depending on the situation. Since the method of this invention does not involve dropping the sand cover material into the sea, it does not spread pollution. Therefore, even if the sand cover material contains a small amount of silt, it is not a problem as long as the necessary shear strength is ensured. Sand without silt is expensive. This expands the effective use of inexpensive sandy soil. Furthermore, as a measure to prevent the erosion of the sand cover material, a sand cover material with adjusted particle size of gravel and sand is preferable. The use of steel slag, which is effective in promoting seaweed growth, can also be considered. Effects of the Invention

[0054] The present invention provides a new sediment improvement method that stabilizes the seabed by increasing the density and strength of the surface layer of the sediment, which is organic sludge, through consolidation settlement using a rapid vacuum consolidation method, thereby forming an improved soil layer. In parallel, substances that cause water pollution in the sediment are forcibly discharged by consolidation wastewater. This regenerates the organic sludge sediment into an improved soil layer that is impermeable and has a high barrier function, exceeding that of capping with permeable sand. Meanwhile, the wastewater containing dissolved pollutants is separated into water and pollutants and treated. The stable, impermeable improved soil layer contains the underlying pollution-generating layer, providing a novel sediment improvement method that regenerates a healthy water environment in a sustainable way without the need for capping materials or the generation of dredged soil.

[0055] In the airtight loading box of the sediment improvement method of the present invention, the consolidation time is shortened inversely proportional to the square of the drainage distance as the partition wall spacing decreases. On the other hand, the probability of sediment uplift increases sharply as the partition wall spacing decreases, as the proportion of the sediment's adhesive force increases. This conflicting problem is solved by forming a water boundary film at the boundary between the partition wall and the sediment to break the adhesive force, and the partition wall spacing is set to the lower limit of the minimum spacing at which sediment uplift does not occur. Furthermore, this method dramatically shortens the consolidation time through the synergistic effect of the enhanced fine bubble effect achieved by preparing a vacuum-consolidated gas-water separation tank and introducing a high-vacuum consolidation system, and the effect of the lower limit of the partition wall spacing of the airtight loading box, thereby reducing construction costs and greatly contributing to the realization of a new sediment improvement method that restores a sustainable and good water environment. [Brief explanation of the drawing]

[0056] [Figure 1] Vertical cross-sectional view of the self-submersible airtight loading box of the present invention (cross-sectional view along line CC in Figure 2) [Figure 2] Similarly, a plan view (section view along line AA in Figure 1) [Figure 3] Similarly, the cross-sectional view of line BB in Figure 1. [Figure 4] Similarly, a diagram showing the progress of the sediment improvement work. [Figure 5] Vertical cross-sectional view (FF line cross-section in Figure 6) of a workboat equipped with the airtight load box of the tall box according to the present invention. [Figure 6] Similarly, a plan view (cross-sectional view of line DD in Figure 5) [Figure 7] Similarly, the EE line cross-section in Figure 5. [Figure 8] Similarly, a diagram showing the work status of bottom sediment improvement, which also involves increasing the depth of the seabed. [Figure 9] Vertical cross-sectional view of a workboat equipped with the sand-covering box of the present invention (cross-sectional view along line HH in Figure 10) [Figure 10] Similarly, the plan view (cross-section of line GG in Figure 9) [Figure 11] Similarly, a diagram showing the work status of sand capping. [Modes for carrying out the invention]

[0057] The embodiments of the present invention will be described below with reference to Figures 1 to 11. Figures 1 to 4 describe a bottom sediment improvement method targeting the surface layer of the bottom sediment to a depth of approximately 50 cm, and the airtight loading box used therein. This airtight loading box is a self-submerging airtight loading box that can automatically dive, surface, and move horizontally in all directions on the seabed. This device and mechanism are introduced from the airtight loading box described in Patent Document 3. Figures 5 to 8 describe a bottom sediment improvement method that also increases depth, and the airtight loading box used therein. This airtight loading box is envisioned to be up to approximately 10 m in height and is mounted on a dedicated work vessel. The box tower is removable. This is related to the space under the girders of a bridge in the case of a river. Figures 9 to 11 describe a bottom sediment improvement method that creates a composite sand cover by creating a sand cover that improves the growth environment for benthic plants and animals on the upper layer of an improved soil layer with high sand cover function, and the sand cover installation box used therein. The sand-covering installation box is mounted on a specialized work vessel.

[0058] Figure 1 is a vertical cross-sectional view of a self-submersible airtight loading box (cross-sectional view along line CC in Figure 2), Figure 2 is a plan view of the same (cross-sectional view along line AA in Figure 1), Figure 3 is a cross-sectional view along line BB in Figure 1, and Figure 4 is a diagram showing the work situation of bottom sediment improvement in the consolidation process. In the figures, 1X is a self-submersible airtight loading box. 2X is an airtight loading box, and 3X is a steel box-shaped rigid floating body (ballast tank). The self-submersible airtight loading box 1X is formed by integrating the rigid floating body 3X onto the entire upper surface of the airtight loading box 2X. In the diagram, 1 is a bulkhead with drainage function, 2 is a box tower, 2a is a power supply unit, 2b is a vacuum unit, 2c is a compressed air unit, 2d is a wastewater storage tank, 3 is a gas-water separation tank, 4 is an underwater vibration device, 5 is a drain pipe with an automatically opening / closing lid for the airtight loading box, 6 is a ballast water inlet with an automatically opening / closing lid for the rigid floating body, and 7 is a submersible pump for the rigid floating body (ballast tank). A1 is the sea surface, B2 is the seabed surface, and C3 is the seabed (seabed ground). The upper floors of the box tower 2 are equipped with a power supply unit (generator) 2a, a vacuum unit 2b, and a compressed air unit 2c, while the lower floors are equipped with a wastewater storage tank 2d. The interior of the airtight loading box 2X is divided into partitioned spaces by bulkheads 1 with drainage function. The lower limit spacing of bulkheads 1 is assumed to be about 10 cm, and this can be in only one direction. For example, the horizontal cross-sectional area of ​​the partitioned space is 10 cm × 50 cm. For simplicity, the diagram omits the representation of pipelines such as vacuum paths and compressed air paths.

[0059] The work process for the seabed improvement method using a self-submerging airtight loading box 1X and rapid vacuum consolidation consists of the following steps: a diving step for the airtight loading box 2X, a placement step for the box 2X, a vacuum consolidation step for the box 2X and wastewater treatment step, a step to break the adhesion between the bulkhead 1 of the box 2X and the seabed sediment C3, and a lifting (floating) step for the box 2X. In the diving step for the box 2X, the automatic ballast water inlet 6 on the side of the rigid floating body 3X is opened to supply ballast water, allowing the box 2X to submerge and be set in place at the predetermined seabed position. In the placement step for the box 2X, the drainage pipe 5 of the airtight loading box 2X is opened, and the underwater vibration device 4 is operated to vibrate and place the box 2X until the seabed surface B2 of the seabed sediment is in contact with the ceiling surface of the box 2X. The vacuum consolidation and wastewater treatment process for the box 2X involves closing the drain pipe 5 and automatic ballast water inlet 6 of the airtight loaded box 2X, and operating the vacuum device 2b to perform the predetermined vacuum boiling consolidation. Meanwhile, the wastewater containing dissolved pollutants is sent from the gas-water separation tank 3 to the wastewater storage tank 2d for water treatment. In the process of breaking the adhesion, the vacuum device 2b is stopped, and the compressed air device 2c is operated. Using the stagnant water in the gas-water separation tank 3, high water pressure is generated to widen the gap between the ceiling surface of the box 2X and the seabed surface B2. At the same time, a boundary membrane of water is formed at the boundary between the bulkhead 1, which has a drain function, and the seabed C3, thereby breaking the adhesion between the bulkhead 1 and the seabed C3. The lifting (floating) process prevents the bottom sediment C3 from rising together with the seabed surface B2 by pressing it down with high water pressure, and the airtight loading box 2X is floated up by discharging the ballast water from the rigid floating body 3X with a submersible pump 7.

[0060] Figure 5 is a vertical cross-sectional view (cross-sectional view along the FF line in Figure 6) of a work vessel equipped with the airtight load box of the tall box type according to the present invention, Figure 6 is a plan view (cross-sectional view along the DD line in Figure 5), Figure 7 is a cross-sectional view along the EE line in Figure 5, and Figure 8 is a diagram showing the work situation of bottom sediment improvement in the consolidation process which also includes depth increase. In the figures, 1Y is a dedicated work vessel equipped with the airtight load box, 2Y is the airtight load box of the tall box type, 3Y is a barge, 1 is a bulkhead with a drain function, 2 is a box tower, 3 is a steam-water separation tank, 4 is an underwater vibration device, 5 is a drain pipe for the airtight load box with an automatically opening and closing lid, 8 is a three-dimensional support column for the box, 9 is a vertical drain, and 10 is a winch. A power generator 2a, a vacuum device 2b, a compressed air device 2c, and a wastewater storage tank 2d are installed on the floors of the box tower 2. It is structurally impossible to make the airtight load box 2Y of the tall box type self-submersible. Furthermore, the structure will differ due to manufacturing constraints.

[0061] In the fabrication of the airtight load-bearing box 2Y, if the spacing between the partition walls 1 is 10 cm, the maximum box height is approximately 1 m. In the airtight load-bearing box 2Y, lightweight, tensile-strength vertical drains 9 made of a material such as plastic are installed in parallel in the divided spaces separated by partition walls 1. For example, if the horizontal cross-sectional area of ​​the divided space is 1 m x 5 m, 49 vertical drains 9 with a width of 100 cm are installed in parallel at 10 cm intervals.

[0062] The work process for the seabed improvement method using a rapid vacuum consolidation method with a tall, airtight, loadable box 2Y consists of a diving process for box 2Y, a placement process for box 2Y, a vacuum consolidation and wastewater treatment process for box 2Y, a process to break the adhesion between the bulkhead 1 of box 2Y and the seabed sediment C3, and a lifting (floating) process for box 2Y. The difference from the method using a self-submerging, airtight, loadable box 1X is that the lowering and raising of the airtight, loadable box 2Y in the diving and lifting processes is performed by a winch 10. The other processes are the same.

[0063] The airtight load-carrying box 2Y of the tall box requires ingenuity in its manufacture. A sealed work entrance is provided at the bottom of the airtight load-carrying box 2Y and the bulkhead 1. In addition, a pair of drain connecting plates for joining vertical drains 9 are fixed in parallel at the upper and lower parts of the divided space, and the spacing between the drain connecting plates, i.e., the spacing between the vertical drains 9, is set to the lower limit spacing so that the bottom sediment C3 does not rise together when the airtight load-carrying box 2Y is lifted. The method of installing the vertical drains 9 is as follows: First, the sealed lid and work entrance are opened, the vertical drains 9 are suspended from the upper drain connecting plate at the innermost part to the lower drain connecting plate, the upper end of the vertical drains 9 is joined to the upper drain connecting plate, and then, while applying a predetermined tension, the lower end of the vertical drains 9 is joined to the lower drain connecting plate. This work is carried out by working backward from the innermost part to the front, thereby securing the lower working space and setting the spacing between the vertical drains 9 to the lower limit spacing so that the bottom sediment C3 does not rise together.

[0064] Figure 9 is a vertical cross-sectional view (cross-sectional view along line HH in Figure 10) of a workboat equipped with the sand-covering box of the present invention, Figure 10 is a plan view (cross-sectional view along line GG in Figure 9), and Figure 11 is a diagram showing the sand-covering box being placed at a predetermined seabed position and the sand-covering material being loaded and unloaded during sand-covering construction. The structure of the sand-covering box is basically the same as that of the airtight loading box 2X. The differences are that bulkhead 1 does not have a drain function, and there is a sand-covering material slip-prevention device at the bottom of bulkhead 1. In the figures, 1Z is a dedicated workboat equipped with the sand-covering box, 2Z is ​​the sand-covering box, 3Z is a barge, 4Z is a sand-covering material stockyard, 3 is a steam-water separation tank, 4 is an underwater vibrator, 11 is a trolley, 12 is a three-dimensional support column for the overhead crane, and 13 is a crane girder. The sand-covering thickness is equal to the internal height of the sand-covering box 2Z, and its height is approximately 10 cm to 15 cm. Furthermore, the trolley 11 is a device that, in combination with a chain block, moves the lifted sand-covering installation box 2Z on the crane girder 13.

[0065] The sand capping construction method using the sand capping box 2Z consists of the loading of sand capping material into box 2Z, the installation of box 2Z on the seabed, and the loading and unloading of sand capping material. Naturally, this method does not include a consolidation process. The sand capping box 2Z is ​​suspended from a trolley 11 that travels on a crane girder 13. The sand capping material is stored in a wet state in the sand capping material stockyard 4Z of the work vessel 1Z. The loading of sand capping material is carried out in the sand capping material stockyard 4Z, where the sand capping box is vibrated and driven into the wet sand capping material while operating a vacuum device, compacting the sand capping material while removing excess pore water, and loading the sand capping material into box 2Z after the pore ratio of the sand capping material is below the critical pore ratio. Next, the installation of box 2Z is ​​carried out by lifting the sand capping box 2Z loaded with sand capping material with the trolley 11 and installing it in the designated position on the seabed. Next, the loading and unloading process for the sand cover involves operating the compressed air device 2c to push out the sand cover material with high water pressure to create the sand cover. The sand cover creation method of the present invention creates sand cover with a predetermined thickness and density below the limit void ratio in a sand cover installation box 2Z with an open bottom. [Explanation of Symbols]

[0066] 1X Self-submarine type airtight loading case 2X Airtight Loading Box 3X Rigid Floating Body (Ballast Tank) 1Y A specialized work vessel equipped with a high-cab airtight loading box. 2Y High box airtight load box, 3Y barge 1Z Dedicated workboat equipped with sand-covering installation box 2Z Sand-covered installation box 3Z barge 4Z Sand Covering Material Stockyard 1. Partition wall of the box 2 Box-type tower 2a power supply 2b Vacuum device 2c Compression device 2d Drainage storage tank 3. Steam-water separation tank 4 Underwater vibration device 5. Drainage pipe with automatic opening and closing lid for airtight loading container 6. Ballast water inlet for rigid floating body 7. Submersible pump for rigid floating body (ballast tank) 8. Three-dimensional support columns for the box-shaped structure 9. Vertical drain 10 winches 11 Trolley 12. Three-dimensional support columns for overhead cranes 13 Crane girder A1 Sea surface B2 Bottom sediment seabed C3 Bottom sediment (seabed ground)

Claims

1. In the rapid vacuum consolidation method for improving sediment bottom, the structure of the airtight loading box used in this method is a box shape with an open bottom. A drain sheet that also serves as a screen and acts as a horizontal drain is attached to the ceiling of the box, and a gas-water separation tank to which the horizontal drain is connected is installed on the top of the box. The inside of the box is divided into partitioned spaces by partitions that have a drain function. By applying vacuum pressure and water pressure to this airtight loading box, the surface layer of the sediment is consolidated and settled. This sediment improvement method involves increasing the density and strength of the surface sediment to form an improved soil layer that stabilizes on the seabed. At the same time, substances that cause water pollution in the sediment are forcibly discharged through consolidated drainage, resulting in the regeneration of an impermeable, highly effective improved soil layer. Meanwhile, the drainage containing dissolved pollutants is separated into water and pollutants for water treatment. The impermeable improved soil layer seals off the underlying pollution-generating layer, eliminating the need for capping materials and generating dredged soil, thus enabling the sustainable regeneration of a healthy water environment.

2. The bottom sediment improvement method of claim 1 is characterized by creating a sand cover on the upper layer of the improved soil layer with a sand cover material that improves the habitat environment for benthic plants and animals, thereby forming a composite sand cover of the improved soil layer and the sand cover, and thereby restoring both a sustainable and good water environment and a habitat environment for benthic plants and animals.

3. An airtight loading box used in the rapid vacuum consolidation method of claim 1, wherein the spacing of the partition walls of the box is set at a lower limit spacing so that the bottom sediment does not rise together with the partition walls when the box is lifted, thereby reducing the horizontal consolidation drainage distance of the airtight loading box to half the lower limit spacing of the partition walls, thereby significantly shortening the consolidation time.

4. In the rapid vacuum consolidation method using the airtight loading box according to claim 3, the main work steps of the method consist of a process of placing the airtight loading box, a vacuum consolidation process, a process of breaking the adhesion between the box's partition walls and the bottom sediment, and a process of lifting the airtight loading box. In the process of breaking the adhesion, a compressed air device is operated to generate high water pressure that pushes open the space between the ceiling surface of the box and the bottom sediment surface, and a boundary membrane of water is formed at the boundary between the partition walls, which have a drain function, and the bottom sediment, thereby breaking the adhesion between the partition walls and the bottom sediment. In the lifting process, the bottom sediment surface is pressed down with high water pressure to prevent the bottom sediment from rising together and the airtight loading box is lifted. Due to the boundary membrane effect and the lower limit spacing effect of the partition walls, the consolidation time is significantly shortened, no sand covering material is required, and dredged soil is not generated, making this a sustainable bottom sediment improvement method characterized by the regeneration of a good water environment.

5. An airtight loading box for use in the rapid vacuum consolidation method of claim 1, wherein several layers of drain sheets that also serve as a screen and horizontal drains are attached to the ceiling surface of the box, a gas-water separation tank to which the horizontal drains are connected is installed on the upper surface of the box, an underwater heater is installed in this gas-water separation tank, and the structure of the gas-water separation tank is such that the horizontal cross-sectional area is the minimum necessary and there is always residual water, thereby limiting the gas-water interface of the box to the gas-water separation tank, and significantly reducing the generation of water vapor, thereby ensuring a high vacuum for vacuum boiling consolidation is continuously maintained.

6. The sediment improvement method of the rapid vacuum consolidation method of claim 1 is characterized in that the airtight loading box used in the method is a box having the features of both the airtight loading box of claim 3 and claim 5, thereby ensuring the effect of the lower limit spacing of the box's partitions in shortening the consolidation time and having the effect of significantly suppressing the generation of water vapor, and furthermore, a high vacuum consolidation system is introduced into the consolidation system of the rapid vacuum consolidation method using the box, thereby doubling the fine bubble effect of the consolidation system and improving the sediment, resulting in an improved soil layer with significantly higher barrier function, and the consolidation time is dramatically shortened due to the synergistic effect of the lower limit spacing of the box's partitions and the enhanced fine bubble effect.

7. An airtight loading box for use in the rapid vacuum consolidation method of claim 1, wherein lightweight, tensile-strength vertical drains are installed in parallel in the divided space inside the box, and the spacing between the vertical drains is set to the lower limit so that the bottom sediment does not rise together when the box is lifted, thereby significantly shortening the consolidation time as the horizontal consolidation drainage distance of the box is half the lower limit spacing of the vertical drains.

8. A bottom sediment improvement method for rivers, etc., where the water depth is insufficient, characterized in that the water depth is increased not by dredging but by ground subsidence of the bottom sediment using a rapid vacuum consolidation method, and an impermeable improved soil layer is formed in the bottom sediment, and this improved soil layer contains the pollution-generating layer in the lower layer, and no organic sludge bottom sediment is removed and no capping material is brought in, thus a bottom sediment improvement method that also increases the water depth of rivers, etc.

9. A sand-covering box used in a bottom sediment improvement method for creating a synthetic sand cover according to claim 2, wherein the box is box-shaped with an open bottom, a horizontal drain that also serves as a screen is attached to the ceiling of the box, a steam-water separation tank to which an underwater vibration device and a horizontal drain are connected is installed on the top of the box, the inside of the box is divided into partition spaces by partition walls, a minute width slip-prevention strip is placed around the bottom of the partition walls to prevent the sand cover material from falling out, and when loading the sand cover material into the box, the spacing between the partition walls is set to the upper limit spacing so that the sand cover material does not fall out, and the loading and unloading of the sand cover material is performed by pushing it out with high water pressure.

10. The sand capping construction method using a sand capping installation box according to claim 9, wherein the main work steps of the method consist of a step of loading sand capping material into the box, a step of installing the box on the seabed, and a step of loading and unloading sand capping material, wherein the step of loading sand capping material involves vibrating and driving the sand capping installation box into the stockyard of the sand capping material on the wet sand capping material while operating a vacuum device to remove excess pore water and compact the sand capping material, thereby loading the sand capping material into the box with the pore ratio of the sand capping material below the critical pore ratio, and then the step of installing the box involves lifting the sand capping installation box loaded with sand capping material and installing it at a predetermined position on the seabed, and the step of loading and unloading sand capping material involves operating a compressed air device to push out the sand capping material with high water pressure to create sand capping, and the sand capping construction method is characterized in that sand capping of a predetermined thickness and density below the critical pore ratio is created using a sand capping installation box with an open bottom.

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