Ground improvement method

Abstract

To realize solidification technology applicable under a ground water level for the ground through precipitation of calcium carbonate using microorganism.SOLUTION: A ground improvement method comprises the steps of: constructing an injection well 12 and a pumping-up well 14 in the ground 10; filling chemical including oxygen-contained organic substance and calcium salt in a supply tank 16; forming a circulation passage for the chemical by discharging the chemical from the injection well 12 to the ground and returning it to the supply tank 16 by sucking it from the pumping-up well 14; and forming a solidified body 32 comprising calcium carbonate in the ground by generating carbon dioxide through fermentation of the oxygen-contained substance under anaerobic surroundings and decomposition (anaerobic metabolism) by an oxidation-reduction reaction and reacting the carbon dioxide with calcium salt. At first, activation of microorganism is promoted using comparatively low concentrated chemical of calcium salt, and upon completion of an activation phase the precipitation efficiency is improved using highly concentrated chemical of the calcium salt.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] This invention relates to a ground improvement method, and particularly to a method for forming a solidified body composed of calcium carbonate in soil by using microbial fermentation and other anaerobic metabolisms.

Background Art

[0002] Conventionally, cement-based solidifying materials have been mainly used for improving soft or high water content geology and ground. However, due to the recent demand for reducing carbon dioxide emissions, the practical application of a ground improvement method with relatively low environmental impact using microorganisms is expected.

[0003] For example, in Patent Document 1, a technique is disclosed in which carbonate ions are generated by injecting cultured calcifying bacteria (urease-producing microorganisms) into the ground together with a nutrient source, urea, and a calcium source to solidify the ground. In addition, Patent Document 2 discloses a technique for increasing the liquefaction resistance by reducing the water saturation of the ground with nitrogen gas or carbon dioxide gas generated by microbial metabolism, or for increasing the liquefaction resistance by generating a highly viscous biofilm in the interstitial water of the ground using microorganisms and increasing its viscosity. Patent Document 3 discloses a ground strengthening technique in which oxygen-containing organic substances and calcium salts are added to soil, and calcium carbonate formed by using the anaerobic metabolism of microorganisms existing in the soil crosslinks between soil particles.

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the technology of Patent Document 1 is based on an aerobic reaction in which urease enzyme discharged by calcifying bacteria decomposes urea to generate carbonate ions, and there is a problem that it cannot be applied to the ground below the groundwater level. In addition, regarding the technology of Patent Document 2, nitrogen gas and carbon dioxide gas due to microbial metabolism do not always remain in the gaps of the soil, and the biofilm may disappear if the supply of nutrients is cut off. In any case, there are doubts about its permanence. In the case of the technology of Patent Document 3, it can be evaluated in that it can be applied to the ground below the groundwater level by utilizing anaerobic fermentation of microorganisms, and precipitation of calcium carbonate has also been confirmed. However, there is room for further improvement in order to enhance its practicality as a ground improvement method.

[0006] This invention has been devised in view of such a situation, and aims to establish a ground solidification technology using microorganisms to precipitate calcium carbonate that can be applied below the groundwater level.

Means for Solving the Problems

[0007] To achieve the above object, a first ground improvement method according to this invention comprises a step of constructing an injection well and a pumping well in the ground, a step of filling a chemical solution containing oxygenated organic matter and calcium salt in a supply tank, a step of discharging this chemical solution from the injection well into the ground and sucking it from the pumping well and returning it to the supply tank to form a circulation path of the chemical solution in the ground, and a step of generating carbon dioxide by fermenting the oxygenated organic matter by microorganisms in the ground under an anaerobic environment or other anaerobic metabolism (decomposition by oxidation-reduction reaction, etc.), and forming a solidified body composed of calcium carbonate in the ground by the reaction of this carbon dioxide and the calcium salt. In the initial stage of starting the method, a low-concentration chemical solution with a relatively low concentration of the calcium salt is used to promote the activation of microorganisms, and when a predetermined activation period ends, a high-concentration chemical solution with a higher concentration of the calcium salt than at the beginning is used to enhance the precipitation efficiency of calcium carbonate.

[0008] The second ground improvement method according to the present invention includes a first step of constructing an injection well and a pumping well in the ground surrounded by a water cutoff wall, a second step of filling a supply tank with a chemical solution containing oxygenated organic matter and calcium salt, a third step of discharging this chemical solution from the injection well and diffusing it into the ground, a fourth step of generating carbon dioxide by fermenting the oxygenated organic matter in an anaerobic environment by microorganisms in the ground or other anaerobic metabolism, and precipitating calcium carbonate in the ground by the reaction between this carbon dioxide and the calcium salt, and a fifth step of recovering the chemical solution in the soil through the pumping well and returning it to the supply tank after a predetermined curing period, and repeating the third to fifth steps until a solidified body of calcium carbonate is formed in the ground. In the initial stage of the method, a low-concentration chemical solution with a relatively low concentration of the calcium salt is used to promote the activation of microorganisms. When a predetermined activation period ends, a high-concentration chemical solution with a higher concentration of the calcium salt than at the beginning is used to increase the precipitation efficiency of calcium carbonate.

[0009] The third ground improvement method according to the present invention includes a step of constructing an injection well in the ground, a step of filling a culture tank with the soil of the ground and a low-concentration chemical solution containing oxygenated organic matter and a relatively low-concentration calcium salt, and activating microorganisms in the soil under anaerobic conditions for a predetermined period, a step of transferring the low-concentration chemical solution containing the activated microorganisms into a supply tank, a step of making the chemical solution into a high-concentration chemical solution containing oxygenated organic matter and a relatively high-concentration calcium salt by additionally charging calcium salt into the supply tank, a step of discharging this high-concentration chemical solution and the activated microorganisms from the injection well into the ground, and a step of forming a solidified body composed of calcium carbonate in the ground by generating carbon dioxide by fermenting the oxygenated organic matter in an anaerobic environment by the activated microorganisms or other anaerobic metabolism, and the reaction between this carbon dioxide and the calcium salt.

[0010] It is desirable to contain metal components such as iron, nickel, cobalt, and manganese (metal powders, metal compounds, metal oxides, metal hydroxides, aqueous solutions containing metal ions, etc.) in the above high-concentration chemical solution.

Advantages of the Invention

[0011] In the case of the ground improvement method according to this invention, at the beginning of the ground improvement method, a low-concentration chemical solution with a relatively low concentration of calcium salt is supplied to the microorganisms in the ground to promote the activation of the microorganisms. After the activation period has passed, by supplying a high-concentration chemical solution with a higher concentration of calcium salt than at the beginning to the microorganisms, the precipitation efficiency of calcium carbonate can be increased, and as a result, it becomes possible to form a solidified body with sufficient quality in the ground. When metal components such as iron are contained in the high-concentration chemical solution, the gaps between soil particles can be blocked by iron oxide, which contributes to the improvement of the strength and formation speed of the solidified body.

Brief Description of the Drawings

[0012]

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Best Mode for Carrying Out the Invention

[0013] FIG. 1 shows the components for realizing the first ground improvement method according to this invention, and a water injection wellhead 12 and a pumping wellhead 14 constructed in the ground 10 to be improved, and a supply tank 16 installed on the ground are used. The supply tank 16 is filled with a chemical solution 18 composed of an aqueous solution of an oxygen-containing organic substance and a calcium salt. There are no particular limitations on the number and arrangement of the water injection wellhead 12 and the pumping wellhead 14.

[0014] The supply tank 16 and the water injection wellhead 12 are connected via a water injection pipe 20, and a water injection pump 22 is installed in the middle thereof. Further, between the supply tank 16 and the pumping well 14, they are connected via a pumping pipe 24, and a pumping pump 26 is installed in the middle thereof. A well point 28 is arranged at the lower end of the pumping well 14 and is connected to the lower end of the pumping pipe 24.

[0015] Here, when the injection pump 22 is operated, the chemical solution in the supply tank 16 is supplied to the injection well 12 and penetrates into the ground 10. Also, when the pumping pump 26 is operated, the chemical solution in the ground 10 is sucked through the well point 28 and returned into the supply tank 16 via the pumping pipe 24. As a result, a circulation path of the chemical solution is formed inside the ground 10.

[0016] FIG. 2 illustrates the arrangement pattern of the injection well 12 and the pumping well 14, showing six injection wells 12 arranged in a rectangular shape with respect to the ground 10 and two pumping wells 14 arranged inside thereof. The dashed arrow in the figure indicates the flow (circulation path) of the chemical solution, and the rectangle indicated by the two-dot chain line indicates the improvement range (formation range of the solidified body) 30.

[0017] Hereinafter, according to the flowchart of FIG. 3, the working process of the first ground improvement method will be described. First, as a preparation stage, the supply tank 16 is filled with a low-concentration chemical solution 18 (S10). Here, the "low concentration" is based on comparison with the concentration of the high-concentration chemical solution used in later processes, and in the chemical solution 18 composed of oxygen-containing organic matter + calcium salt, it particularly means that the concentration of calcium salt is relatively low. Even when the calcium salt concentration is 0, it is included in the "low concentration" mentioned here.

[0018] Next, the injection pump 22 and the pumping pump 26 are operated to circulate and supply the low-concentration chemical solution into the ground 10 (S12). Since the chemical solution has a relatively low calcium salt concentration as described above and is suitable for the growth of microorganisms, it can promote the activation (growth) of the microorganisms originally present in the ground 10.

[0019] A part of the chemical solution that has penetrated into the soil is returned into the supply tank 16 through the pumping well 14 and the pumping pipe 24 together with the microorganisms and groundwater. For the chemical solution returned to this supply tank 16, after ensuring the required concentration and amount by additional input of the chemical agent (S14), it is re-injected into the ground 10 through the injection pipe 20 and the injection well 12 together with the microorganisms.

[0020] Then, when a predetermined "microorganism activation period" ends (S16 / Y), the chemical solution in the supply tank 16 is concentrated (S18). Specifically, calcium salt is additionally input into the supply tank 16 and adjusted to a concentration, for example, about twice that of the calcium salt concentration in the original chemical solution. However, there is no limitation to the concentration difference. Also, the "high concentration" mentioned here is not limited to one type of concentration, and different calcium salt concentrations can be applied step by step as long as they are higher than the original calcium salt concentration.

[0021] On the contrary, when the "microorganism activation period" has not ended (S16 / N), the microorganism activation process with the low-concentration chemical solution is continued (S12, S14). The "microorganism activation period" will be described later.

[0022] The concentrated chemical solution circulates between the ground 10 and the supply tank 16 by the operation of the injection pump 22 and the pumping pump 26 and is supplied to the microorganisms in the ground 10 (S20). As a result, the oxygen-containing organic matter is decomposed by aerobic fermentation and other anaerobic metabolisms by the activated microorganisms in the soil, and a large amount of carbon dioxide is generated. A large amount of calcium carbonate is generated by the reaction of this carbon dioxide with the high-concentration calcium salt, and a solidified body (improved body) 32 is formed in the ground 10. Even in the process of forming the solidified body 32, the chemical agent returned to the supply tank 16 is appropriately replenished with the chemical agent (S22). After ensuring the required concentration and required amount of the chemical solution, it is re-injected into the ground 10 together with the microorganisms through the injection pipe 20 and the injection well 12. At this time, fine particles (CaCO 3 (initial precipitate)) are also returned to the soil, so the rate of increase in the particle size of the precipitate is improved. In this supply tank 16, the growth of the activated microorganisms returned from the ground also proceeds due to the replenished chemical agent.

[0023] Inspections such as check boring are carried out on the ground 10 at an appropriate timing, and the degree of completion of the solidified body 32 is observed (S24). Then, when it is confirmed that the solidified body 32 with the required strength and volume has been formed (S26 / Y), the circulation supply of the chemical solution is stopped, and the improvement work is completed. If the degree of completion of the solidified body 32 does not reach the predetermined standard during the above observation (S26 / N), the formation process of the solidified body 32 by supplying a high-concentration chemical solution is continued (S20 to S24).

[0024] The above-mentioned "activation (growth) period of microorganisms" means the period required for the target microorganisms to grow sufficiently by the circulation supply of a low-concentration chemical solution and be activated so that a solidification reaction occurs. For example, the soil of the ground to be improved is sampled, and the period required for activation is preset through a culture experiment in the laboratory.

[0025] Alternatively, when it is confirmed that the growth of the microorganisms has reached a certain level or more by observing the chemical solution returned to the supply tank 16 with a microscope or measuring the copy number of the rRNA gene, it can be determined that the "activation (growth) period of microorganisms" has ended. Also, the amounts of calcium ions and nitrate ions in the chemical solution returned to the supply tank 16 can be measured, and the metabolic activity of the microorganisms can be estimated from the change in the decrease rate.

[0026] In addition, regarding the determination of the degree of completion of the solidified body in S24 and S26 above, instead of actually observing the degree of completion of the solidified body, based on the data of the improvement work previously carried out in the same construction area, the period required for the completion of the solidified body in the ground of the area is estimated, and it can also be recognized that the required solidified body 32 has been formed after this period has elapsed.

[0027] In the case of this ground improvement method, in the activation (growth) stage of microorganisms, since the concentration of calcium salt is relatively low and suppressed, the formation of calcium carbonate does not proceed so much. However, since an environment suitable for the growth of microorganisms can be formed in the soil, the activation (growth) thereof is promoted. And when sufficient growth of microorganisms is observed, by supplying a chemical solution containing a relatively high concentration of calcium salt into the ground 10, it becomes possible to efficiently generate calcium carbonate.

[0028] The nitrogen gas generated along with the precipitation of CaCO 3 by microbial metabolism is discharged to the outside through the pumping well 14 during pumping. If the nitrogen gas remains between the soil particles, the action of cross-linking between the soil particles by CaCO 3 will be inhibited. However, by removing the nitrogen gas through the pumping well 14 in this way, a decrease in the strength of the solidified body 32 can be effectively prevented.

[0029] Figure 4 is a graph showing the quality of the solidified body 32 formed by this ground improvement method. Figure 4(a) is a graph showing the measurement results of the uniaxial compressive strength, and Figure 4(b) is a graph showing the measurement results of the permeability coefficient. In both cases, the values of "soil only" which is the soil as it is sampled at the same site as the comparison target, and "simple chemical agent addition" which is the numerical values of the solidified body formed by simply adding one type of concentration of chemical agent at the same site are graphed.

[0030] As shown in the figure, it can be seen that the numerical values of the solidified body by the "present patent technology" for the uniaxial compressive strength are much higher than those of "soil only" and "simple chemical agent addition". Regarding the permeability coefficient as well, it has been shown that the numerical value of the solidified body by the "present patented technology" is much lower compared to that of "only soil" and "simple chemical agent addition". As a result, it was confirmed that the solidified body 32 by the present patented technology can deposit calcium carbonate more efficiently than the simple chemical agent addition, and can solidify the ground by filling the soil voids, contributing to countermeasures against liquefaction and reduction of groundwater flow in the soil.

[0031] In the case of the present invention, since the chemical solution is supplied to the ground 10 by the circulation method accompanying water injection and pumping, there is an advantage that the generation of waste liquid can be suppressed. Also, it is possible to adjust the strength of the solidified body by the amount of pumping / water injection, circulation time (number of exchanges), etc. That is, it has been confirmed that the strength of the solidified body increases and the permeability coefficient decreases as the number of exchanges of the chemical solution increases, and it is possible to lead the finish of the solidified body to the strength and permeability coefficient according to the requirements.

[0032] Also, since it utilizes the anaerobic reaction of microorganisms, it can be applied to solidification in the deep underground. When depending on the reaction of aerobic microorganisms, it is limited to solidification in the shallow layer within 1 m from the surface.

[0033] Also, no bad odors such as ammonia due to urea decomposition and hydrogen sulfide accompanying the sulfate reduction reaction occur. Since the chemical solution is neutral, there is no need for pH control, and it can be said that it is an environmentally friendly construction method. There is also an advantage that the composition of the liquid agent does not need to be changed even for soils with different pH values, and the applicable range is wide. Furthermore, since the chemical solution has no viscosity, there is no need to inject it into the ground at high pressure, and it can also be applied to the underground of structures.

[0034] The oxygen-containing organic substances contained in the above chemical solution may be compounds capable of generating carbon dioxide by fermentation by microorganisms, and examples thereof include organic acids, alcohols, saccharides, and proteins. The organic acid is preferably a carboxylic acid having 2 to 10 carbon atoms, more preferably succinic acid, propionic acid or pyruvic acid. In particular, succinic acid is a common acid produced from cellulose, which is the main component of terrestrial plants, and is also preferable because it may enhance nitrate reduction by soil microorganisms as an electron donor. The alcohol is, for example, ethanol. The saccharides include monosaccharides, disaccharides and polysaccharides, and examples thereof include glucose or sucrose.

[0035] In addition, the concentration (dissolved concentration) of the oxygen-containing organic substance in the above chemical solution can be appropriately changed according to the state of the soil and the like, but is preferably in the range of 1 to 100 mM, more preferably in the range of 5 to 50 mM.

[0036] As the calcium salt contained in the above chemical solution, one or more of calcium halide, calcium hydroxide, calcium acetate, calcium carbonate, etc. can be used. Preferably, calcium halide, more preferably calcium chloride (CaCl 2 ). The concentration of the calcium salt in the chemical solution can be appropriately changed according to the state of the soil and the like, but is preferably in the range of 100 to 5000 mM, more preferably in the range of 200 to 2000 mM. Within this range, the specific concentrations of the calcium salt in the activation stage of the microorganisms and the precipitation stage of calcium carbonate are set.

[0037] This invention ferments oxygen-containing organic substances by microorganisms in an anaerobic environment, thereby generating carbon dioxide. As the microorganisms to be used, those capable of producing carbon dioxide by anaerobic fermentation using oxygen-containing organic substances as fermentation raw materials can be widely used. When the oxygen-containing organic substance is an organic acid, for example, the microorganism performs fermentation represented by the following reaction formula to produce carbon dioxide. [Formula 1] C x H y O z→C x-1 H y O z-2 +CO 2

[0038] Here, anaerobic fermentation means that fermentation by microorganisms proceeds in an anaerobic environment or a hypoxic environment.

[0039] Specific examples of the microorganisms used in this invention include microorganisms having a gene (mmdA) encoding methylmalonyl-CoA decarboxylase. Such microorganisms are more preferably microorganisms belonging to the phylum Bacteroidetes, particularly microorganisms belonging to the order Bacteroidales. These microorganisms having the mmdA gene have the function of removing carbon dioxide from methylmalonyl-CoA, and are particularly preferable in that they efficiently generate carbon dioxide when using organic acids such as succinic acid as oxygen-containing organic substances. As the microorganisms, those originally present in the soil are used.

[0040] In the present invention, carbon dioxide is generated by the fermentation action of oxygen-containing organic substances by microorganisms and other anaerobic metabolisms under anaerobic conditions, and by reacting this with a calcium salt, calcium carbonate (CaCO 3 ) is precipitated. More specifically, as shown in the following reaction formula, first, the generated carbon dioxide dissolves in the water in the soil to generate carbonate ions (CO 3 2- ), and when the carbonate ions react with calcium ions (Ca 2+ ) derived from the calcium salt, calcium carbonate (CaCO 3 ) precipitates (calcifies). [Formula 2] CO 2 +H 2 O→H 2 CO 3 →CO 3 2- +2H + Ca 2+ + CO 3 2- → CaCO 3 ↓

[0041] By filling the calcium carbonate generated as described above into the spaces between particles or cracks in the ground, a geological improvement effect can be obtained, which increases the ground strength and decreases the porosity and permeability.

[0042] By adding a powder or aqueous solution of nitrate to the above chemical solution, a nitrate reduction reaction can also be advanced in the soil. Due to such a nitrate reduction reaction, denitrification (NO 3 → N 2 ) occurs, which is beneficial in that it increases the carbonate ions by raising the pH of the nearby soil and promotes the formation of calcium carbonate. Typically, the nitrate reduction reaction can be represented by the following equation. [Equation 3] CH 3 COO - + 2.6H + + 1.6NO 3 - → 2CO 2 + 0.8N 2 + 2.8H 2 O

[0043] As the nitrate added, nitrates of alkali metals or alkaline earth metals can be used, and preferably sodium nitrate (NaNO 3 ) is used because of its ease of handling. When adding the calcium salt in the form of an aqueous solution, the concentration of nitrate in the aqueous solution can be appropriately changed according to the state of the soil, etc., but preferably, it is in the range of 10 to 250 mM. However, in the activation stage of microorganisms, the concentration of nitrate is set relatively low, and when entering the formation stage of the solidified body, it can be adjusted so that the concentration becomes relatively high.

[0044] FIG. 5 shows an example of taking liquefaction countermeasures against the ground 10 located under the existing building 40 by using the first ground improvement method. In this case, the injection well 12 and the pumping well 14 are constructed at positions avoiding the existing building 40, and the supply tank 16 is installed. By operating the injection pump 22 and the pumping pump 26, a circulation path for the chemical solution is formed in the ground 10 located directly below the existing building 40.

[0045] FIG. 6 illustrates the arrangement pattern of the injection well 12 and the pumping well 14. Three injection wells 12 and three pumping wells 14 are arranged opposite to each other at a predetermined distance. The broken-line arrow in the figure indicates the flow (circulation path) of the chemical solution, and the rectangle indicated by the two-dot chain line shows the improvement range 30 directly below the building.

[0046] Also in this case, a relatively low-concentration chemical solution is supplied to the ground 10 during the activation stage of the microorganisms. When the microorganisms are activated beyond a predetermined level, a relatively high-concentration chemical solution is supplied to the ground 10. And the circulation supply of the high-concentration chemical solution is continued until the solidified body 32 of a predetermined quality is formed.

[0047] FIG. 7 shows an example of taking liquefaction countermeasures against the ground 10 surrounded by the water cutoff wall 42 by using the second ground improvement method. In this case, the injection well 12 and the pumping well 14 are constructed inside the water cutoff wall 42, and the supply tank 16 is installed outside the water cutoff wall 42.

[0048] FIG. 8 illustrates the arrangement pattern and the construction procedure of the injection well 12 and the pumping well 14. The injection well 12 and the pumping well 14 are respectively arranged on the diagonal line of the ground 10 surrounded in a rectangular shape by the water cutoff wall 42. In this case, the chemical solution discharged from the injection well 12 is blocked by the water cutoff wall 42, and it is suppressed from flowing out of the improvement range. Therefore, it is not necessary to constantly operate the pumping pump 26, and a predetermined curing period is provided, which is a characteristic point (FIG. 8(b)).

[0049] Next, according to the flowchart of FIG. 9, the details of the working process in the second ground improvement method will be described. First, the supply tank 16 is filled with the low-concentration chemical solution 18 (S30). Next, the water injection pump 22 is operated, and as shown in FIG. 8(a), the low-concentration chemical solution is diffused and infiltrated into the ground 10 surrounded by the water cutoff wall 42 (S32). Next, the operation of the water injection pump 22 is stopped, and as shown in FIG. 8(b), the ground 10 where the chemical solution has spread is cured (S34).

[0050] After a predetermined curing period has elapsed, as shown in FIG. 8(c), the pumping pump 26 is operated, and the chemical solution in the water cutoff wall 42 is recovered into the supply tank 16 through the pumping well 14 (S36). At this point, the supply tank 16 is replenished with the necessary chemical solution (S38).

[0051] The processes of S32 to S38 are repeated until the above-mentioned "activation (growth) period of microorganisms" ends (S40 / N). On the other hand, when the "activation (growth) period of microorganisms" ends (S40 / Y), after the chemical solution concentration in the supply tank 16 is increased (S44), the high-concentration chemical solution is diffused from the injection well 12 into the ground 10 by operating the water injection pump 22 (S46). When the high-concentration chemical solution has sufficiently spread to the ground 10 within the water cutoff wall 42, the operation of the water injection pump 22 is stopped, and a predetermined curing period is ensured (S48).

[0052] After this curing period has elapsed, the pumping pump 26 is operated, and the chemical solution and microorganisms in the water cutoff wall 42 are recovered into the supply tank 16 through the pumping well 14 (S50). At this point, the supply tank 16 is replenished with the necessary chemical solution (S52).

[0053] The ground 10 is inspected by check boring or the like at a predetermined timing, and the degree of completion of the solidified body 32 is observed (S54). When the formation of the solidified body 32 with the required strength and volume is confirmed (S56 / Y), the improvement work is completed. On the other hand, when the degree of completion of the solidified body 32 does not reach the predetermined standard (S56 / N), the formation process of the solidified body 32 by "injecting high-concentration chemical solution → ensuring the curing period → recovering high-concentration chemical solution and microorganisms → replenishing the chemical solution → observing the solidified body" is repeated (S46 to S54).

[0054] FIG. 10 shows the components for realizing the third ground improvement method according to the present invention. A water injection well 12 and an air vent well 50 constructed in the ground 10 to be improved, a supply tank 16 installed on the ground, and a culture tank 52 are used. The culture tank 52 is filled with a low-concentration chemical solution 54 composed of an aqueous solution of an oxygen-containing organic substance and a calcium salt, and soil 56 collected from the ground 10 to be improved.

[0055] The supply tank 16 and the water injection well 12 are connected via a water injection pipe 20, and a water injection pump 22 is installed in the middle thereof. In addition, an exhaust pipe 60 connected to a vacuum pump 58 is inserted into the air vent well 50. The culture tank 52 and the supply tank 16 are connected via a transfer pipe 62, and a transfer pump 64 is installed in the middle thereof.

[0056] Hereinafter, the working process in the third ground improvement method will be described according to the flowchart of FIG. 11. First, as a preparation stage, the in-situ soil 56 and the low-concentration chemical solution 54 are filled into the culture tank 52 (S60), and the microorganisms in the soil 56 are cultured under anaerobic conditions (S62).

[0057] Then, when the above-mentioned "activation (growth) period of microorganisms" ends (S64 / Y), the transfer pump 64 is operated, and the activated microorganisms in the culture tank 52 are transferred into the supply tank 16 together with the chemical solution via the transfer pipe 62 (S66). While the "activation (growth) period of microorganisms" has not ended (S64 / N), the cultivation of microorganisms with a low-concentration chemical solution continues (S62).

[0058] For the chemical solution transferred into the supply tank 16, calcium salts are additionally added and adjusted to become a high-concentration chemical solution (S68), and then supplied into the ground 10 from the injection well 12 by the operation of the injection pump 22 (S70).

[0059] As a result, the oxygen-containing organic matter is aerobically fermented by the activated microorganisms, generating a large amount of carbon dioxide. A large amount of calcium carbonate is generated by the reaction of this carbon dioxide with the high-concentration calcium salts, and a solidified body 32 is formed in the ground 10. The nitrogen gas generated during this period is sucked and discharged to the ground through the vent well 50 (S72). During the formation process of the solidified body, the supply tank 16 is replenished with a low-concentration chemical solution containing the activated microorganisms from the culture tank 52, and chemicals necessary to increase its concentration are added as needed (S74).

[0060] For the ground 10, inspections such as check boring are carried out at an appropriate timing, and the degree of completion of the solidified body 32 is observed (S76). And when the formation of the solidified body 32 with the required strength and volume is confirmed (S78 / Y), the supply of the chemical solution is stopped, and the improvement work is completed. On the contrary, when the degree of completion of the solidified body 32 does not reach the predetermined standard (S78 / N), the formation process of the solidified body 32 by supplying the high-concentration chemical solution and the activated microorganisms is continued (S70 to S74).

[0061] FIG. 12 illustrates the arrangement pattern of the injection wells 12 in this third ground improvement method, showing a plurality of injection wells 12 arranged in a row at a predetermined interval from each other. The dashed arrows in the figure indicate the flow of the chemical solution, and the circular shape indicated by the two-dot chain line shows the improvement range 30.

[0062] In the case of this third ground improvement method, since the activated microorganisms in the culture tank 52 are successively supplied to the ground 10 through the injection well 12, it is not necessary to install the pumping well 14 to form a circulation path of the low-concentration chemical solution in the ground 10 and activate the microorganisms living in the circulation path.

[0063] By mixing metal components such as iron into the high-concentration chemical solution of the first to third ground improvement methods, it is possible to improve the strength and formation speed of the solidified body 32. Specifically, at the timing when the activation period of the microorganisms ends and calcium salts are additionally added to the low-concentration chemical solution to make it a high-concentration chemical solution, iron powder, iron compounds, iron oxides and hydroxides, iron ion-containing aqueous solutions, etc. are added, and subsequent replenishment is carried out as necessary, so that the voids in the formed solidified body 32 are filled with iron oxide, and as a result, the strength and formation speed of the solidified body 32 can be improved.

[0064] Fig. 13(a) shows an SEM (scanning electron microscope) image of the solidified body 32 formed by appropriately adding iron powder to the high-concentration chemical solution in the first ground improvement method, and Fig. 13(b) is an enlarged view of the rectangular region therein. Fig. 14 is a diagram showing the analysis results of EDS (energy dispersive X-ray spectroscopy) at specific points (Pt. 1 to Pt. 5) in Fig. 13(b). Iron oxide (FeO) is distributed at Pt. 1 and Pt. 2, calcium carbonate (CaCO 3 ) is present at Pt. 3, and silicon (Si) is mainly distributed at Pt. 4 and Pt. 5.

[0065] Fig. 15 is a diagram that clearly shows the distribution status of silicon (Si) in the SEM image of Fig. 13(a) compared with other elements. Fig. 16 is a diagram that clearly shows the distribution status of iron oxide (FeO) in the same SEM image, and Fig. 17 is a diagram that clearly shows the distribution status of calcium carbonate (CaCO 3 ) Combining these figures, we can see that the spaces that would normally be gaps (voids) between soil particles (Si) are filled with iron oxide (FeO), and the boundaries are clogged with calcium carbonate (CaCO 3 ).

[0066] Figure 18 is a graph showing the quality of the solidified body 32 formed using a high-concentration chemical solution mixed with an appropriate amount of iron powder in the first ground improvement method. Figure 18(a) is a graph showing the measurement results for the uniaxial compressive strength, and figure 18(b) is a graph showing the measurement results for the hydraulic conductivity. For comparison, both graphs show the values ​​for "soil only," which is soil taken from the same site, the values ​​for a solidified body formed at the same site by simply adding one type of chemical at a certain concentration, "simple chemical addition," and the values ​​for a solidified body formed by the first ground improvement method, "this patented technology," which are plotted alongside the values ​​for a solidified body formed by adding iron powder to the first ground improvement method, "this patented technology + Fe addition."

[0067] As shown in the figure, the values ​​for the solidified body formed by "this patented technology + Fe addition" are far higher in uniaxial compressive strength than "soil only," "simple chemical addition," and "this patented technology." In addition, the permeability coefficient of the solidified body by "this patented technology + Fe addition" is shown to be much lower than "soil only", "simple chemical addition", and "this patented technology".

[0068] The same effect can be achieved by mixing metals other than iron, such as nickel, cobalt, manganese, etc., into the high-concentration chemical solution. It is sufficient that these metal components are at least present in the high-concentration chemical solution, and the timing of their mixing is not limited to "when converting a low-concentration chemical solution into a high-concentration chemical solution". In other words, it is possible to mix metal components into the low-concentration chemical solution and add calcium salt to it to make it a "high-concentration chemical solution containing metal components".

Explanation of symbols

[0069] 10 Ground 12 injection wellheads 14 pumping wellheads 16 supply tanks 18 chemical solutions 20 injection pipes 22 injection pumps 24 pumping pipes 26 pumping pumps 28 well points 30 improvement range 32 solidified bodies 40 existing buildings 42 water cutoff walls 50 air vent wells 52 culture tanks 54 chemical solutions 56 soil 58 vacuum pumps 60 exhaust pipes 62 transfer pipes 64 transfer pumps

Claims

1. constructing a water injection well and a water pumping well in the ground; A step of filling a supply tank with a chemical solution containing an oxygen-containing organic substance and a calcium salt; A step of discharging the chemical solution from the water injection well into the ground and sucking it from the pumping well and returning it to the supply tank to form a chemical solution circulation path in the ground; a step of generating carbon dioxide by fermenting the oxygen-containing organic matter in an anaerobic environment or by other anaerobic metabolism using microorganisms in the ground, and forming a solidified body made of calcium carbonate in the ground by reacting the carbon dioxide with the calcium salt; A ground improvement method comprising: At the beginning, the calcium salt concentration was kept relatively low to promote the activation of microorganisms. This ground improvement method is characterized in that, at the end of a predetermined activation period, the efficiency of calcium carbonate precipitation is increased by using a high-concentration chemical solution in which the concentration of the calcium salt is higher than that at the beginning.

2. A first step of constructing a water injection well and a pumping well in ground surrounded by a water impermeable wall; A second step of filling a chemical solution containing an oxygen-containing organic substance and a calcium salt into a supply tank; A third step of discharging the chemical solution from the water injection well and dispersing it into the ground; a fourth step of generating carbon dioxide by fermenting the oxygen-containing organic matter in an anaerobic environment or by other anaerobic metabolism using microorganisms in the ground, and precipitating calcium carbonate in the ground by reacting the carbon dioxide with the calcium salt; and a fifth step of recovering the chemical solution in the soil through the pumping well and returning it to the supply tank after a predetermined curing period has elapsed. A ground improvement method in which the third to fifth steps are repeated until a solidified body of calcium carbonate is formed in the ground, At the beginning, the calcium salt concentration was kept relatively low to promote the activation of microorganisms. This ground improvement method is characterized in that, at the end of a predetermined activation period, the efficiency of calcium carbonate precipitation is increased by using a high-concentration chemical solution in which the concentration of the calcium salt is higher than that at the beginning.

3. 3. The ground improvement method according to claim 1, wherein the supply tank is appropriately replenished with the chemical solution to ensure a required concentration and amount of the chemical solution.

4. 3. The ground improvement method according to claim 1, wherein nitrogen gas produced by anaerobic metabolism by microorganisms is discharged to the ground through the pumping well.

5. constructing a water injection well in the ground; A step of filling a culture tank with the soil of the ground, oxygen-containing organic matter, and a low-concentration chemical solution containing a relatively low concentration of calcium salt, and activating microorganisms in the soil under anaerobic conditions for a predetermined period of time; A step of transferring a low-concentration chemical solution containing activated microorganisms into a supply tank; adding calcium salt to the supply tank to convert the chemical solution into a high-concentration chemical solution containing oxygen-containing organic matter and a relatively high concentration of calcium salt; Releasing the high-concentration chemical solution and activated microorganisms from the water injection well into the ground; a step of generating carbon dioxide by fermenting the oxygen-containing organic matter in an anaerobic environment or by other anaerobic metabolism using the activated microorganisms, and forming a solidified body made of calcium carbonate in the ground by reacting the carbon dioxide with the calcium salt; This is a ground improvement method consisting of the following.

6. An air vent well has been constructed in the above ground.

6. The ground improvement method according to claim 5, wherein nitrogen gas produced by anaerobic metabolism by microorganisms is discharged to the ground through the air vent well.

7. The ground improvement method according to claim 5 or 6, characterized in that the supply tank is appropriately replenished with a low-concentration chemical solution containing activated microorganisms from the culture tank, and calcium salt is additionally added to ensure the required chemical solution concentration and amount.

8. 6. The ground improvement method according to claim 1, 2 or 5, characterized in that the high-concentration chemical solution contains a metal component.

9. 9. The ground improvement method according to claim 8, wherein the metal component is iron.

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