Ground improvement methods

Abstract

To provide a ground improvement method capable of forming a lightweight ground with more stable quality by suppressing disappearance or connection of air bubbles contained in a lightweight ground material.SOLUTION: When forming a lightweight ground LG having a large number of air bubbles B by preparing a lightweight ground material LM by mixing slurry-like muddy water 3, a foamed foam 5 having a large number of air bubbles B, and a solidifying material 4, and pouring and solidifying the lightweight ground material LM, the lightweight ground material LM is mixed with cellulose nanofibers F.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a ground improvement method, and more particularly, to a ground improvement method capable of suppressing the disappearance of bubbles contained in a lightweight ground material and the connection between bubbles to form a lightweight ground with more stable quality.

Background Art

[0002] A ground improvement method (so-called SGM lightweight geotechnical method) is carried out in which a slurry-like muddy water prepared by adding water to raw soil, a foamed foam having a large number of bubbles, and a solidifying material are mixed to prepare a slurry-like lightweight ground material, and the lightweight ground material is placed and solidified to form a lightweight ground having a large number of bubbles (for example, see Patent Document 1). Conventionally, since the film of bubbles contained in the lightweight ground material is in a relatively unstable state, the bubbles contained in the lightweight ground material were relatively likely to disappear from when the lightweight ground material was prepared until it solidified. In addition, the bubbles contained in the lightweight ground material were relatively likely to connect to each other, and variations were likely to occur in the size of the bubbles contained in the lightweight ground. Therefore, there was a problem that variations were likely to occur in the quality of the lightweight ground.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a ground improvement method capable of suppressing the disappearance of bubbles contained in a lightweight ground material and the connection between bubbles to form a lightweight ground with more stable quality.

Means for Solving the Problems

[0005] To achieve the above objective, the present invention provides a ground improvement method that involves mixing a slurry-like mud, foamed foam having numerous air bubbles, and a solidifying agent to produce a lightweight ground material, and then casting and solidifying the lightweight ground material to form a lightweight ground having numerous air bubbles. The bubbles in the foamed foam are formed using a gas with a higher volume ratio of carbon dioxide than the air at the construction site. The lightweight ground material is characterized by being mixed with cellulose nanofibers. [Effects of the Invention]

[0006] According to the present invention, by mixing cellulose nanofibers with lightweight ground material, the stability of the coating on each air bubble contained in the lightweight ground material can be improved. This suppresses the disappearance of air bubbles contained in the lightweight ground material and the bonding of air bubbles with each other, making it possible to form a lightweight ground with more stable quality. [Brief explanation of the drawing]

[0007] [Figure 1] This is an explanatory diagram illustrating a schematic embodiment of the ground improvement method of the present invention. [Figure 2] This is a schematic diagram illustrating a lightweight ground material with added cellulose nanofibers. [Modes for carrying out the invention]

[0008] The ground improvement method of the present invention will be described below based on the embodiments shown in the figures.

[0009] The present invention is an improved ground improvement method (so-called SGM lightweight soil method) that involves mixing a slurry-like mud mixture prepared by adding water to raw soil, foamed foam having numerous air bubbles, and a solidifying agent such as cement to create a slurry-like lightweight ground material, and then pouring and solidifying this lightweight ground material to form a lightweight ground with numerous air bubbles.

[0010] In recent years, reducing carbon dioxide (CO2) emissions, a greenhouse gas, in civil engineering and construction projects has become a crucial issue in order to mitigate global warming. Conventional SGM lightweight earthworks use air to form air bubbles in the lightweight ground material, and the air is retained in these air bubbles (voids) in the lightweight ground. However, if the gas that forms these air bubbles in the lightweight ground material can be replaced with a gas that has a higher volume ratio of carbon dioxide than air, it will be possible to store a large amount of carbon dioxide in the lightweight ground, contributing to the reduction of carbon dioxide emissions in civil engineering and construction projects.

[0011] However, the solubility of carbon dioxide in water is significantly higher than that of air. Furthermore, carbon dioxide reacts with the alkali in the cement-based solidifying agent used in the SGM lightweight earthwork method (Ca(OH)2 + CO2 → CaCO3 + H2O). Therefore, in the SGM lightweight earthwork method, simply using a gas with a higher volume percentage of carbon dioxide than air as a substitute for air makes it difficult to prevent the disappearance of carbon dioxide-rich bubbles in the lightweight ground material, and it is not possible to form a lightweight ground with stable quality and a large number of bubbles.

[0012] The inventors investigated various methods to suppress the disappearance of bubbles formed with a gas that has a higher volume proportion of carbon dioxide than air in lightweight ground materials. As a result, they found that when cellulose nanofibers are mixed with the lightweight ground material, the cellulose nanofibers adhere to the coating (interface) of each bubble B, increasing the mechanical strength of the bubble interface. Furthermore, they found that by mixing cellulose nanofibers with the lightweight ground material to improve the stability of the coating of each bubble contained in the lightweight ground material, it becomes possible to form a lightweight ground with stable quality and a large number of bubbles, even when the bubbles in the lightweight ground material are formed with a gas that has a higher volume proportion of carbon dioxide than air. They also found that even when the bubbles in the lightweight ground material are formed with air, mixing cellulose nanofibers with the lightweight ground material can form a lightweight ground with even more stable quality.

[0013] Therefore, in the present invention, in a ground improvement method for forming lightweight ground (SGM lightweight earthwork method), lightweight ground having numerous air bubbles is formed by mixing cellulose nanofibers with the lightweight ground material. Cellulose nanofibers are materials obtained by subdividing cellulose from trees, plants, etc., to the nano level through mechanical treatment, chemical treatment, or a combination thereof.

[0014] Typical specifications for cellulose nanofibers include a fiber length of several nanometers and a fiber width of several nanometers to several tens of micrometers. There is some variation in fiber width; for example, low-defibration fibers produced during the manufacturing process of cellulose nanofibers from wood also fall under the category of cellulose nanofibers. Raw materials for cellulose nanofibers include, for example, wood pulp, bamboo pulp, and softwood pulp.

[0015] The ground improvement method of the present invention will be described in more detail below with reference to Figure 1. In the embodiment illustrated below, the case in which the air bubbles B of the lightweight ground material LM are formed with a gas CG in which the volume ratio of carbon dioxide C is higher than that of air is illustrated.

[0016] The volume percentage of carbon dioxide (C) in the air (atmosphere near the ground surface) varies slightly depending on altitude, but the volume percentage of carbon dioxide (C) in normal air is approximately 0.03-0.04%. When forming the bubbles B of the lightweight ground material LM with gaseous CG that has a higher volume percentage of carbon dioxide (C) than air, for example, the bubbles B of the lightweight ground material LM are formed with gaseous CG having a volume percentage of carbon dioxide (C) of 0.4% or more, more preferably 2% or more, and even more preferably 10% or more. Alternatively, the bubbles B of the lightweight ground material LM are formed with gaseous CG having a volume percentage of carbon dioxide (C) that is, for example, 10 times or more, more preferably 50 times or more, and even more preferably 250 times or more, than the volume percentage of carbon dioxide (C) contained in the air at the construction site. The upper limit of the volume percentage of carbon dioxide (C) in the gaseous CG that has a higher volume percentage of carbon dioxide (C) to form the bubbles B of the lightweight ground material LM is, for example, 97% or less.

[0017] As illustrated in Fig. 1, in the ground improvement system 1 of this ground improvement method, a slurry-like lightweight ground material LM is produced by mixing a slurry-like muddy water 3 prepared by adding water to raw soil 2 collected from underwater or on land, a foamed foam 5 having a large number of bubbles B, and a solidifying material 4 such as cement, and then placing it. The foamed foam 5 is obtained by coating the bubbles B with a film of a highly surface-active substance (surfactant).

[0018] In this ground improvement method, the slurry-like muddy water 3, the solidifying material 4 (for example, cement), and the foamed foam 5 are supplied to a lightweight soil kneader 6. In the line for supplying the muddy water 3 to the lightweight soil kneader 6, the raw soil 2 is collected from underwater or on land, and gravel and foreign matters contained in the raw soil 2 are removed. Next, the raw soil 2 is put into a vibrating screen 9 using a construction machine 8 or the like, and the raw soil 2 is classified by the vibrating screen 9 to remove particles with a relatively large particle size (for example, a particle size of 5 mm to 10 mm or more) from the raw soil 2. Then, the raw soil 2 classified by the vibrating screen 9 is deflocculated by a deflocculator 10, and the deflocculated raw soil 2 is put into a sludge storage tank 11. In the sludge storage tank 11, water is added to the raw soil 2 to produce a slurry-like muddy water 3.

[0019] When forming the bubbles B of the lightweight ground material LM with a gas CG having a higher volume ratio of carbon dioxide C than air as in this embodiment, it is advisable to increase the carbon dioxide concentration of the muddy water 3 in anticipation of the carbon dioxide C contained in the gas CG forming the bubbles B dissolving in the moisture of the lightweight ground material LM or reacting with the alkali of the solidifying material 4. Preferably, the carbon dioxide concentration of the muddy water 3 is made saturated.

[0020] In this embodiment, the carbon dioxide concentration of the muddy water 3 is increased by using a carbonic acid solution CW (carbonated water) in which carbon dioxide C is dissolved as the water added to the raw soil 2. For example, a high-pressure container 40 in which carbon dioxide C is previously stored is connected to the sludge storage tank 11, and carbon dioxide C is supplied into the sludge storage tank 11 by the high-pressure container 40, so that carbon dioxide C can be mixed and dissolved in the muddy water 3.

[0021] Next, the muddy water 3 stored in the sludge storage tank 11 is sent to the lightweight soil mixing machine 13 by the sludge pump 12. Then, the density and flow value of the muddy water 3 are adjusted by the lightweight soil mixing machine 13. When the raw material soil 2 is sandy soil, additives such as bentonite are added as necessary to adjust the muddy water 3 to a predetermined density and flow value. The muddy water 3 (adjusted soil) whose density and flow value have been adjusted by the lightweight soil mixing machine 13 is pumped to the lightweight soil kneading machine 6.

[0022] In the line for supplying the solidifying agent 4 to the lightweight soil kneading machine 6, the solidifying agent 4 is supplied from the storage facility 14 (silo) in which the solidifying agent 4 is stored to the lightweight soil kneading machine 6. When forming the bubbles B of the lightweight ground material LM with a gas CG having a higher volume ratio of carbon dioxide C than air as in this embodiment, anticipating that the carbon dioxide C contained in the gas CG forming the bubbles B reacts with the alkali of the solidifying agent 4, it is advisable to previously mix carbon dioxide C into the solidifying agent 4 to increase the carbon dioxide concentration of the solidifying agent 4. In this embodiment, a high-pressure container 40 (40A) in which carbon dioxide C is previously stored is connected to the storage facility 14, and the carbon dioxide C is mixed and dissolved in the solidifying agent 4 stored in the storage facility 14.

[0023] Also, when forming the bubbles B of the lightweight ground material LM with a gas CG having a higher volume ratio of carbon dioxide C than air, the pH of the lightweight ground material LM may decrease due to the reaction between the alkali of the solidifying agent 4 and carbon dioxide C. If the pH of the lightweight ground material LM becomes lower than the specified value, the long-term strength of the lightweight ground LG may decrease. Therefore, it is preferable to previously determine the mixing amount (cement addition amount) of the solidifying agent 4 that can suppress the pH of the lightweight ground material LM from becoming lower than the specified value through an indoor mix design test or the like.

[0024] In the line for supplying the foamed foam 5 to the lightweight soil kneading machine 6, a dilution liquid 17 obtained by diluting the foaming agent 15 with water 16 is supplied to the foaming machine 19 by the dilution liquid pump 18. In this embodiment, a liquid emulsifier containing the cellulose nanofiber F stored in the storage tank 22 is supplied to the dilution liquid pump 18, so that the dilution liquid 17 added with the cellulose nanofiber F is supplied to the foaming machine 19.

[0025] In this embodiment, when the bubbles B of the lightweight ground material LM are formed with a gas CG that has a higher volume ratio of carbon dioxide C than air, it is preferable to increase the carbon dioxide concentration of the diluent 17 in order to suppress the dissolution of carbon dioxide C contained in the gas CG that forms the bubbles B into the water contained in the diluent 17. Preferably, the carbon dioxide concentration of the diluent 17 is brought to a saturated state.

[0026] In this embodiment, a carbonated solution CW (carbonated water) in which carbon dioxide C is dissolved is used as the water 16 used for dilution. For example, a high-pressure container 40 containing pre-stored carbon dioxide C can be connected to a water tank 20 storing the water 16 used for dilution, and carbon dioxide C can be supplied into the water tank 20 by the high-pressure container 40, thereby mixing and dissolving carbon dioxide C in the water 16 used for dilution. Alternatively, for example, a high-pressure container 40 can be connected to a liquid tank 21 storing a foaming agent 15, and carbon dioxide C can be supplied into the liquid tank 21 by the high-pressure container 40, thereby mixing and dissolving carbon dioxide C in the foaming agent 15.

[0027] The foaming machine 19 is further supplied with gas CG, which forms the bubbles B of the foamed foam 5, by a compressor 30 (30A). In this embodiment, a high-pressure container 40 (40B) in which carbon dioxide C is pre-stored is connected to the compressor 30A, and carbon dioxide C is supplied from the high-pressure container 40B to the compressor 30A, thereby supplying gas CG from the compressor 30A to the foaming machine 19 with a higher volume ratio of carbon dioxide C than the air at the construction site. The compressor 30 to which the high-pressure container 40B can be connected can be easily manufactured, for example, by improving the air supply structure of a conventionally used air compressor.

[0028] In the foaming machine 19, a foamed foam 5 having numerous fine bubbles B is produced using the supplied diluent 17 and gas CG. The foamed foam 5 produced by the foaming machine 19 is sent to the lightweight soil mixer 6. In the lightweight soil mixer 6, the supplied mud 3 (adjusted mud), solidifying agent 4, and foamed foam 5 are mixed together by a mixer to produce a slurry-like lightweight ground material LM containing numerous bubbles B.

[0029] As in this embodiment, when the bubbles B of the lightweight ground material LM are formed with gas CG, which has a higher volume ratio of carbon dioxide C than air, some of the carbon dioxide C contained in the gas CG that forms the bubbles B of the foamed foam 5 may be consumed by dissolving in the water contained in the mud 3 or by reacting with the alkali of the solidifying agent 4 during the process of producing the lightweight ground material LM with the lightweight soil mixer 6. Therefore, when the bubbles B of the lightweight ground material LM are formed with gas CG, which has a higher volume ratio of carbon dioxide C than air, it is preferable to increase the mixing ratio of foamed foam 5 in the lightweight ground material LM compared to when bubbles B are formed with air. An appropriate mixing ratio of foamed foam 5 that can sufficiently secure bubbles B in the lightweight ground material LM can be determined in advance by conducting laboratory mix design tests.

[0030] As illustrated in the image in Figure 2, in the lightweight ground material LM mixed with added cellulose nanofibers F, the coating (interface) of each air bubble B is covered and protected by the surfactant contained in the foamed foam 5 and the cellulose nanofibers F. Therefore, compared to when cellulose nanofibers F are not added, the mechanical strength of the interface of each air bubble B contained in the lightweight ground material LM is increased due to the emulsification stability provided by the cellulose nanofibers F, making it more difficult for each air bubble B to disappear and for the air bubbles B to connect with each other. Furthermore, because the coating of each air bubble B is protected by cellulose nanofibers F, the carbon dioxide C contained in the gas CG that forms the air bubbles B is suppressed from dissolving to the outside of the coating and from reacting with alkali.

[0031] As illustrated in Figure 1, the unsolidified lightweight ground material LM prepared by the lightweight soil mixer 6 is pumped to the concrete casting machine 7, and the concrete casting machine 7 then casts the lightweight ground material LM onto the seabed or land where ground improvement is to be carried out. During the process of pumping the unsolidified lightweight ground material LM to the concrete casting machine 7, it is preferable to maintain a positive pressure state inside the pumping pipe 24 that pumps the lightweight ground material LM in order to suppress the escape of air bubbles B from the lightweight ground material LM.

[0032] In this embodiment, the lightweight ground material LM before solidification is pumped into the pumping pipe 24 by a pressure pump 23, and compressed gas CG, which has a higher volume ratio of carbon dioxide C than the air at the construction site, is supplied into the pumping pipe 24 by a compressor 30 (30B). This configuration maintains a positive pressure state inside the pumping pipe 24 that pumps the lightweight ground material LM, and the lightweight ground material LM before solidification is pumped to the concrete pouring machine 7. In this embodiment, a high-pressure container 40 (40C) containing carbon dioxide C is connected to the compressor 30B, and carbon dioxide C is supplied from the high-pressure container 40C to the compressor 30B. Furthermore, if the amount of compressed gas CG supplied by the compressor 30B exceeds the amount of gas CG with a high volume ratio of carbon dioxide C supplied from the high-pressure container 40C, the compressor 30B can be configured to supply air from the construction site along with gas CG with a high volume ratio of carbon dioxide C supplied from the high-pressure container 40C, thereby supplying compressed gas CG with a higher volume ratio of carbon dioxide C than the air from the construction site.

[0033] When the poured lightweight ground material LM solidifies, a lightweight ground LG is formed, containing numerous independent voids created by air bubbles B. Inside the lightweight ground LG, gas CG, with a higher volume ratio of carbon dioxide C than air, is contained within these numerous independent voids. The unit volume weight of the hardened lightweight ground material LM is 8-13 kN / m³. 3 Compared to conventional ground materials, lightweight ground material LM is relatively light and possesses sufficient strength against ground subsidence, earthquakes, and liquefaction. Therefore, by installing lightweight ground material LM using this ground improvement method, a stable lightweight ground LG can be formed.

[0034] Thus, according to the present invention, by mixing cellulose nanofibers F with the lightweight ground material LM, the stability of the coating of each air bubble B contained in the lightweight ground material LM can be improved by the emulsification stability provided by the cellulose nanofibers F. This suppresses the disappearance of air bubbles B contained in the lightweight ground material LM and the bonding of air bubbles B together, thereby forming a lightweight ground LG with more stable quality.

[0035] As in this embodiment, adding cellulose nanofibers F during the manufacturing process of the lightweight ground material LM before it is placed on the ground stabilizes the film of air bubbles B contained in the lightweight ground material LM by the cellulose nanofibers F before the lightweight ground material LM is placed on the ground by the grounding machine 7. Therefore, it is advantageous to reduce the rate at which air bubbles B contained in the lightweight ground material LM disappear.

[0036] In particular, as in this embodiment, when cellulose nanofibers F are added to the foamed foam 5 during the process of producing the foamed foam 5 using the foaming machine 19, the coating of air bubbles B contained in the foamed foam 5 becomes stable due to the cellulose nanofibers F at a stage prior to the process of mixing the mud 3, solidifying agent 4, and foamed foam 5 using the lightweight soil mixer 6. Therefore, it is even more advantageous to reduce the rate at which air bubbles B disappear during the mixing process by the lightweight soil mixer 6. Furthermore, when cellulose nanofibers F are added during the process of producing the foamed foam 5, the dispersibility of the cellulose nanofibers F causes the air bubbles B contained in the foamed foam 5 to become uniformly dispersed. Therefore, the mud 3, solidifying agent 4, and foamed foam 5 can be mixed uniformly in a short time during the mixing process by the lightweight soil mixer 6, which is advantageous in shortening the mixing time by the lightweight soil mixer 6. Shortening the mixing time by the lightweight soil mixer 6 is also advantageous in reducing the rate at which air bubbles B contained in the lightweight ground material LM disappear.

[0037] In this invention, it is sufficient to mix the cellulose nanofibers F into the lightweight ground material LM before it has completely solidified, and the cellulose nanofibers F can be added and mixed in a process other than the process of producing the foamed foam 5 as exemplified above. For example, cellulose nanofibers F can be added to the raw soil 2, the water added to the raw soil 2, or the adjusted mud 3 during the process of producing the slurry-like mud 3. Alternatively, cellulose nanofibers F can be added to the solidifying agent 4. Furthermore, cellulose nanofibers F can be added in the process of mixing the mud 3, foamed foam 5, and solidifying agent 4 using a lightweight soil mixer 6. In this case, since it is only necessary to put the cellulose nanofibers F into the lightweight soil mixer 6 along with the mud 3, solidifying agent 4, and foamed foam 5, the lightweight ground material LM with added cellulose nanofibers F can be produced very easily. Additionally, cellulose nanofibers F can be added and mixed in the process of pumping the produced lightweight ground material LM to the concrete placement machine 7. Furthermore, for example, after the lightweight ground material LM is placed by the concrete placement machine 7, cellulose nanofibers F can be added to and mixed with the lightweight ground material LM before it solidifies. In this embodiment, a liquid emulsifier containing cellulose nanofibers F was used, but for example, powdered cellulose nanofibers F can also be added.

[0038] As described above, in conventional methods that do not add cellulose nanofibers F, it was difficult to form a lightweight ground LG with stable quality by forming the bubbles B of the lightweight ground material LM with gas CG, which has a higher volume ratio of carbon dioxide C than air. In contrast, in the present invention, by mixing cellulose nanofibers F with the lightweight ground material LM, even when the volume ratio of carbon dioxide C contained in the bubbles B of the lightweight ground material LM is high, the emulsification stability of cellulose nanofibers F effectively suppresses the disappearance of bubbles B. Therefore, even when the bubbles B of the foamed foam 5 are formed with gas CG, which has a higher volume ratio of carbon dioxide C than the air at the construction site, it becomes possible to form a lightweight ground LG with a large number of bubbles B with stable quality.

[0039] In lightweight ground LG formed by mixing cellulose nanofibers F, the numerous independent air bubbles B (voids) in the lightweight ground LG are each covered with cellulose nanofibers F. As a result, carbon dioxide C contained in the gas CG forming the air bubbles B is less likely to leak out from the outside of the coating of air bubbles B. Therefore, by adopting the ground improvement method of the present invention, it becomes possible to stably store carbon dioxide C, a factor in global warming, in the improved soil (lightweight ground LG), and contribute to the reduction of carbon dioxide C in civil engineering and construction work.

[0040] Furthermore, in this ground improvement method, it is possible to incorporate a large amount of carbon dioxide (C) into the lightweight ground material LM by mixing gaseous CG, which has a higher volume ratio of carbon dioxide (C) than the air at the construction site, or a carbonated solution CW containing dissolved carbon dioxide (C), during the process of producing the lightweight ground material LM. Specifically, for example, a large amount of carbon dioxide (C) can be incorporated into the lightweight ground material LM by mixing carbon dioxide (C) into the water added to the raw soil 2, or the water 16 used to dilute the solidifying agent 4 and the foaming agent 15. Alternatively, a large amount of carbon dioxide (C) can be incorporated into the lightweight ground material LM by using a carbonated solution CW in the water added to the raw soil 2 or the water 16 used to dilute the foaming agent 15.

[0041] By mixing a large amount of carbon dioxide (C) into the lightweight ground material LM, the carbon dioxide (C) contained in the gas CG that forms the bubbles B in the lightweight ground material LM becomes less likely to dissolve due to the moisture in the lightweight ground material LM. Therefore, when forming the bubbles B in the lightweight ground material LM with gas CG that has a higher volume ratio of carbon dioxide (C) than air, it is advantageous to bring the carbon dioxide concentration of the lightweight ground material LM closer to saturation, thereby forming a lightweight ground LG with a stable quality and a large number of bubbles B.

[0042] As in this embodiment, by supplying compressed gas CG, which has a higher volume ratio of carbon dioxide C than the air at the construction site, into the pressure pipe 24 connected to the concrete-casting machine 7 that casts the lightweight ground material LM, and thereby pressurizing the lightweight ground material LM to the concrete-casting machine 7, carbon dioxide C can be effectively mixed into the lightweight ground material LM during the pressurizing process. Furthermore, by pressurizing the lightweight ground material LM under positive pressure using compressed gas CG with a higher volume ratio of carbon dioxide C, it is advantageous in suppressing the outflow of carbon dioxide C contained in the lightweight ground material LM to the outside before it is cast.

[0043] As in this embodiment, by mixing carbon dioxide (C) that has been pre-stored in a high-pressure container 40 or the like into the lightweight ground material LM, carbon dioxide (C) can be mixed into the lightweight ground material LM relatively easily. In particular, by using a high-pressure container 40, gaseous CG with a high volume ratio of carbon dioxide (C) can be easily supplied, so a large amount of carbon dioxide (C) can be efficiently mixed into the lightweight ground material LM. Therefore, it is advantageous for increasing the amount of carbon dioxide (C) stored in the lightweight ground LG. Furthermore, by using a high-pressure container 40, it becomes possible to store carbon dioxide (C) emitted outside the construction site in the soil.

[0044] The method of mixing carbon dioxide (C) into lightweight ground material LM (slurry 3, solidifying agent 4, foamed foam 5) is not limited to using a high-pressure container 40 or a carbonated solution CW; carbon dioxide (C) can be mixed into lightweight ground material LM in various other ways. For example, carbon dioxide (C) emitted by machinery used at the construction site can also be mixed into lightweight ground material LM.

[0045] Examples of machinery used at the construction site include construction machinery and diesel engines for ships (such as crane ships equipped with grab buckets) used for collecting soil from the seabed and land, construction machinery 8 for feeding raw soil 2 into the vibrating screen 9, and various machines used for producing the lightweight ground material LM (compressor 30A, diluent pump 18, foaming machine 19, etc.). Other examples include a compressor 30B for pumping the lightweight ground material LM to the concrete casting machine 7, a generator for supplying power to the various machines used in construction, and the concrete casting machine 7.

[0046] When using carbon dioxide (C) emitted by machinery at a construction site, for example, the exhaust port of the machine emitting the carbon dioxide (C) is connected by piping to the air intake port of the compressor 30 that supplies the carbon dioxide (C). Alternatively, for example, piping is installed to send the carbon dioxide (C) from the exhaust port of the machine emitting the carbon dioxide (C) to a facility where the lightweight ground material LM containing the carbon dioxide (C) is stored (e.g., a sludge storage tank 11, storage facility 14, water storage tank 20, liquid storage tank 21, or storage tank 22). At construction sites where it is unacceptable to contain pollutants in the exhaust gas emitted by the machinery in the soil, for example, filters are installed in the aforementioned piping to remove pollutants such as particulate matter (PM), nitrogen oxides (e.g., NO and NO2), and volatile organic compounds (VOCs) from the exhaust gas emitted by the machinery.

[0047] By mixing carbon dioxide (C) emitted from machinery at the construction site into the lightweight ground material LM, the amount of carbon dioxide (C) emitted into the atmosphere during the construction of this ground improvement method can be effectively reduced. In particular, since the amount of carbon dioxide (C) emitted by the concrete casting machine 7 and the compressor 30 is relatively large in this ground improvement method, recovering the carbon dioxide (C) emitted by the concrete casting machine 7 and the compressor 30 and using it as carbon dioxide (C) to be mixed into the lightweight ground material LM can effectively reduce the amount of carbon dioxide (C) emitted in this ground improvement method. Furthermore, by utilizing the exhaust gas emitted from machinery used at the construction site, the heat contained in the exhaust gas can be used to heat the lightweight ground material LM before it solidifies during the process of pumping it through the pressure pipe 24. Heating the lightweight ground material LM before it solidifies is advantageous in shortening the curing period until the placed lightweight ground material LM solidifies.

[0048] In the above-described embodiment, the example shown was that the bubbles B in the lightweight ground material LM are formed with a gas CG that has a higher volume ratio of carbon dioxide C than air. However, even when bubbles B are formed with air, mixing cellulose nanofibers F into the lightweight ground material LM can further improve the stability of the coating on each bubble B contained in the lightweight ground material LM. Therefore, compared to the conventional method without mixing cellulose nanofibers F, this method is advantageous in reducing the rate at which bubbles B contained in the lightweight ground material LM disappear, making it possible to form a lightweight ground LG with more stable quality. [Explanation of Symbols]

[0049] 1. Ground Improvement System 2 Raw material soil 3 muddy water 4. Solidifying agent 5. Foam 6. Lightweight soil mixer 7. Driving machine 8 Construction machinery 9 Vibrating sieve 10 Desilting machine 11 Sludge tank 12. Sludge pump 13 Lightweight soil preparation machine 14 Storage facilities 15 Foaming agent 16 water 17 Diluted solution 18 Diluent pump 19 Foaming machine 20 Water storage tanks 21 Liquid storage tank 22 Storage tanks 23 Pressure pump 24 Pressure pipe 30, 30A, 30B compressors 40, 40A~40C high-pressure container F Cellulose Nanofiber LM Lightweight Ground Materials B bubbles LG Lightweight Ground C Carbon dioxide CG (a gas with a higher volume proportion of carbon dioxide than air) CW Carbonated Solution

Claims

1. In a ground improvement method in which a lightweight ground material is prepared by mixing a slurry-like muddy water, foamed foam having numerous air bubbles, and a solidifying agent, and then the lightweight ground material is poured and solidified to form a lightweight ground with numerous air bubbles, A ground improvement method characterized by forming the bubbles in the foamed foam with a gas having a higher volume ratio of carbon dioxide than the air at the construction site, and mixing cellulose nanofibers with the lightweight ground material.

2. The ground improvement method according to Claim 1, wherein when preparing the lightweight ground material, the foamed foam with the cellulose nanofibers added, the muddy water, and the solidifying agent are mixed together.

3. The ground improvement method according to claim 1 or 2, wherein when preparing the lightweight ground material, the muddy water in which carbon dioxide has been dissolved to increase the carbon dioxide concentration, the foamed foam, and the solidifying agent are mixed.

4. The ground improvement method according to claim 1 or 2, wherein the lightweight ground material is mixed with a liquid emulsifier containing cellulose nanofibers.

5. The ground improvement method according to claim 1 or 2, wherein compressed gas having a higher volume ratio of carbon dioxide than the air at the construction site is supplied to a pipe connected to a concrete-casting machine that casts the lightweight ground material, and through which the lightweight ground material passes, thereby pressurizing the lightweight ground material to the concrete-casting machine.

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