JP7579741B2 - Ground improvement methods - Google Patents

Description

The present invention relates to a ground improvement method.

Patent Document 1 discloses a ground improvement method for suppressing liquefaction of ground due to earthquakes. In this ground improvement method, a first solution in which urease is dissolved in water and a second solution in which urea and a calcium salt are dissolved in water are injected into the ground. The first and second solutions are injected into the ground separately or in a mixed state.

In the ground into which the first and second solutions have been injected, urea is hydrolyzed by urease to produce carbon dioxide and ammonia. This carbon dioxide reacts with calcium ions produced from the calcium salt, causing calcium carbonate to precipitate in the ground. This allows the ground to be modified so that it is less susceptible to liquefaction.

Patent No. 5599032

In the above-mentioned ground improvement method, it is possible to form the first solution using plants such as beans that are easily available. In this case, the first solution is formed by extracting the components contained in the plants, such as urease, into water.

When the first and second solutions formed as described above are mixed together, the plant-derived components contained in the first solution are precipitated as precipitates containing proteins and the like due to the action of the calcium salt in the second solution. These precipitates prevent the first and second solutions from penetrating the ground, making it difficult for the first and second solutions to penetrate a wide area of the ground.

As a result, it becomes difficult to precipitate calcium carbonate over a wide area of the ground using the first and second solutions. Therefore, the calcium carbonate cannot effectively suppress liquefaction of the ground.

The means for solving the above problems and their effects will be described below.
In a ground improvement method for solving the above problem, a first solution is prepared by mixing a calcium compound with an enzyme aqueous solution obtained by extracting urease contained in plants. Since the first solution in which the enzyme aqueous solution and calcium compound are mixed causes precipitates such as proteins to be precipitated, the precipitates are removed from the first solution. The first solution from which the precipitates have been removed and a second solution in which urea is dissolved in water are then injected into the ground.

According to the above method, the enzyme aqueous solution obtained by extracting urease contained in a plant contains plant-derived components. Therefore, when a calcium compound is mixed into the enzyme aqueous solution to form the first solution, the plant-derived components are precipitated as precipitates containing proteins and the like by the action of calcium chloride. Then, the first solution and the second solution are injected into the ground with the precipitates such as proteins removed from the first solution. Therefore, the precipitates do not prevent the first and second solutions from penetrating the ground. Therefore, the first and second solutions can be penetrated into a wide area of the ground, and calcium carbonate can be precipitated by the first and second solutions in a wide area of the ground. As a result, the calcium carbonate can effectively suppress liquefaction of the ground.

In the above-mentioned ground improvement method, it is considered that the first solution and the second solution are injected into the ground in a mixed state.
The calcium carbonate is more likely to precipitate as the first solution and the second solution are mixed together. According to the above method, the first solution and the second solution are injected into the ground in a mixed state, so that calcium carbonate can be efficiently precipitated in the ground.

In the above-mentioned ground improvement method, it is considered that the precipitation of the precipitate from the first solution is carried out by stirring the first solution in a state in which a defoaming agent has been added.
The precipitation of proteins and other precipitates from the first solution is carried out by mixing the enzyme aqueous solution with a calcium compound and stirring the mixture, which tends to cause foaming of the first solution consisting of the enzyme aqueous solution and the calcium compound. However, according to the above method, since a defoamer is added to the first solution, foaming of the first solution during stirring can be suppressed.

In the above ground improvement method, when calcium carbonate precipitates in the ground by injecting the first and second solutions into the ground, it is possible to preliminarily adjust the molar concentration of urea in the second solution so that the calcium carbonate precipitation ratio relative to the sand mass of the ground is 1 to 4%.

According to the above method, by adjusting the molar concentration of urea in the second solution as described above, it is possible to efficiently precipitate calcium carbonate in the ground while keeping the amount of materials such as urea to a minimum.

In the above-mentioned ground improvement method, when calcium carbonate precipitates in the ground by injecting the first and second solutions into the ground, it is possible to preliminarily adjust the amount of urease dissolved in the first solution so that the particle size and amount of precipitated calcium carbonate are predetermined values.

According to the above method, by adjusting the amount of urease dissolved in the first solution as described above, the particle size and amount of calcium carbonate precipitated in the ground can be adjusted to values appropriate for suppressing liquefaction of the ground.

1 is a schematic diagram showing a solution injection plant used in the ground improvement method of this embodiment. 1 is a graph showing the change in the precipitation ratio of calcium carbonate with respect to the change in the molar concentration of urea in a second solution. 1 is a graph showing the relationship between the amount of sword beans used to form the first solution and the change in calcium carbonate precipitation ratio over time. 1 is a schematic diagram showing another example of a solution injection plant.

Hereinafter, one embodiment of the ground improvement method will be described with reference to Figs. 1 to 3.
Fig. 1 shows a schematic diagram of a solution injection plant for injecting a solution for suppressing liquefaction into the ground. As shown in Fig. 1, the solution injection plant includes water tanks 1 and 2, mixers 3 and 4, grout pumps 5 and 6, a flow rate pressure control device 7, a branch pressure control device 8, and a mixing unit 9. As the mixing unit 9, a T-shaped pipe, a pressure injector having a mixing head, a line mixer, or the like may be used.

The water tank 1 is for storing an enzyme aqueous solution obtained by extracting urease contained in plants. The water tank 1 is equipped with a submersible pump 1a. The submersible pump 1a is for pumping up the enzyme aqueous solution in the water tank 1 and supplying it to the mixer 3. The mixer 3 stirs and mixes the enzyme aqueous solution with a calcium compound such as calcium chloride. As a result, a first solution is formed in the mixer 3, in which the enzyme aqueous solution is mixed with the calcium compound. The grout pump 5 sucks in the first solution in the mixer 3 and discharges the first solution to the diversion pressure management device 8. This first solution is injected into the ground via the diversion pressure management device 8 and the mixing unit 9.

The water tank 2 is for storing water and is equipped with a submersible pump 2a. The submersible pump 2a is for pumping up the water in the water tank 2 and supplying it to the mixer 4. The mixer 4 stirs and mixes the water and urea. As a result, a second solution in which urea is dissolved in water is formed in the mixer 4. The grout pump 6 sucks in the second solution in the mixer 4 and discharges the second solution to the flow rate pressure control device 7. This second solution is supplied to the mixing unit 9 via the flow rate pressure control device 7 and the branch pressure control device 8, and is mixed with the first solution in the mixing unit 9 and injected into the ground.

Next, a ground improvement method using the above-mentioned solution injection plant will be described. In this ground improvement method, the following first, second, and third steps are carried out in this order.
[First step]
This step is for producing the first solution described above. In this step, a predetermined amount of crushed sword bean seeds is placed in a water-permeable bag, and the bag is immersed in water stored in a water tank 1 for about 1 to 4 hours. As a result, components such as urease contained in the sword bean seeds are extracted, and the water in the water tank 1 becomes an aqueous enzyme solution. It is preferable that the water tank 1 has a stirring device for stirring the aqueous enzyme solution so as to prevent precipitation of the components extracted from the sword bean seeds.

The enzyme aqueous solution in the water tank 1 is sent to the mixer 3 by the submersible pump 1a. In the mixer 3, a predetermined amount of calcium chloride is added while the enzyme aqueous solution is being stirred, and an antifoaming agent is also added. The stirring time by the mixer 3 at this time is set to, for example, one minute or more. As a result, a first solution is formed in the mixer 3 by mixing calcium chloride with the enzyme aqueous solution.

[Second step]
This step is for removing precipitates such as proteins that precipitate from the first solution from the first solution. In this step, the first solution is stirred in the mixer 3. When the aqueous enzyme solution and calcium chloride in the first solution are mixed, the plant-derived components contained in the aqueous enzyme solution are precipitated as precipitates containing proteins by the action of calcium chloride. The precipitates containing proteins that have precipitated in this way are allowed to settle, and the precipitates are removed from the first solution in the mixer 3.

While the above-mentioned first and second steps are being performed, the second solution is formed using the water tank 2 and mixer 4. In detail, water stored in the water tank 2 is sent to the mixer 4 by the submersible pump 2a. In the mixer 4, a predetermined amount of urea is introduced while stirring the water. The stirring time by the mixer 4 at this time is set to, for example, one minute or more. As a result, a second solution in which urea is dissolved in water is formed in the mixer 4.

[Third step]
This process is for injecting the first solution and the second solution into the ground. In this process, the first solution from which precipitates have been removed is sent from the mixer 3 to the diversion pressure control device 8 by the grout pump 5. Also, the second solution is sent from the mixer 4 to the diversion pressure control device 8 by the grout pump 6. The first solution and the second solution are sent to the mixing unit 9 via the diversion pressure control device 8, mixed in the mixing unit 9, and then injected into the ground.

Next, the molar concentration of urea in the second solution used in the above-mentioned ground improvement method will be described.
In determining the molar concentration of urea, the following experiment was carried out. This experiment was conducted to investigate the change in the precipitation ratio of calcium carbonate in the ground with respect to the change in the molar concentration of urea. The precipitation ratio of calcium carbonate represents the percentage of the mass of calcium carbonate contained per unit mass of dry sand in the ground.

In the above experiment, the amount of sword beans used to form the first solution is constant, and the molar ratio of calcium chloride in the first solution to urea in the second solution is fixed at a predetermined value (e.g., "1"). In this experiment, under the above conditions, the first solution and the second solution are injected into dry sand while the molar concentration of urea in the second solution is changed stepwise, and the calcium carbonate precipitation ratio in the sand is measured.

Figure 2 shows the results of this experiment. As can be seen from Figure 2, in the region where the molar concentration of urea in the second solution is low, the calcium carbonate precipitation ratio is low. If the calcium carbonate precipitation ratio is less than 1%, there is a risk that liquefaction of the ground cannot be suppressed. On the other hand, in the region where the molar concentration of urea in the second solution is high, the calcium carbonate precipitation ratio increases as the molar concentration increases. However, if the molar concentration of urea becomes too high, the increase in the calcium carbonate precipitation ratio relative to the increase in the molar concentration of urea tends to become slower. And, if the calcium carbonate precipitation ratio becomes higher than 4%, the above-mentioned tendency becomes stronger.

These experimental results show that adjusting the molar concentration of urea in the second solution in the above ground improvement method so that the calcium carbonate precipitation ratio is 1-4% is preferable for efficiently precipitating calcium carbonate relative to the amount of urea used. Therefore, in the above ground improvement method, the molar concentration of urea in the second solution is adjusted so that the calcium carbonate precipitation ratio is 1-4%.

Next, the amount of urease dissolved in the first solution used in the above-mentioned ground improvement method will be described.
In determining the amount of urease dissolved, the following experiment was carried out to examine the change in the time required for calcium carbonate precipitation in the ground relative to the change in the amount of urease dissolved.

In the above experiment, the molar ratio of calcium chloride in the first solution to urea in the second solution is fixed to a predetermined value (e.g., "1"), and the molar concentration of urea in the second solution is constant. In this experiment, multiple first solutions with different amounts of urease dissolved are prepared, i.e., a first solution using 30 g of sword beans and a first solution using 90 g of sword beans. Then, under the above conditions, the first and second solutions are poured into dry sand, and the calcium carbonate precipitation ratio in the sand is measured at predetermined time intervals.

Figure 3 shows the results of this experiment. As can be seen from Figure 3, the first solution using 30 g of sword beans has a slower rate of calcium carbonate precipitation than the first solution using 90 g of sword beans. In other words, the first solution with a smaller amount of dissolved urease has a slower rate of calcium carbonate precipitation than the first solution with a larger amount of dissolved urease. This rate of calcium carbonate precipitation affects the particle size of the precipitated calcium carbonate. In other words, the slower the rate of calcium carbonate precipitation, the larger the particle size of the precipitated calcium carbonate.

Therefore, the less urease dissolved in the first solution, the slower the rate of calcium carbonate precipitation and the larger the particle size of the precipitated calcium carbonate. For this reason, if the amount of urease dissolved in the first solution is too much, the rate of calcium carbonate precipitation will be faster and the particle size of the calcium carbonate will be smaller, making it difficult to suppress liquefaction of the ground with the calcium carbonate. On the other hand, if the amount of urease dissolved in the first solution is too little, the particle size of the precipitated calcium carbonate will be larger, but the amount of calcium carbonate precipitated will be smaller, making it difficult to suppress liquefaction of the ground with the calcium carbonate.

As can be seen from these experimental results, it is preferable to adjust the amount of urease dissolved in the first solution so that the particle size and amount of precipitated calcium carbonate are values that can effectively suppress liquefaction of the ground. Therefore, in the above-mentioned ground improvement method, the amount of urease dissolved in the first solution is adjusted so that the particle size and amount of precipitated calcium carbonate are values that can effectively suppress liquefaction of the ground. When using sword beans to dissolve urease in the first solution, it is effective to use, for example, about 5 g to 90 g of sword beans per liter of the total amount of the first solution and the second solution.

Next, the effects of the ground improvement method of this embodiment will be described.
(1) The enzyme aqueous solution of the first solution is formed by soaking crushed jack bean seeds in water. Therefore, the enzyme aqueous solution contains plant-derived components. The first solution is formed by mixing calcium chloride with the enzyme aqueous solution. When calcium chloride is mixed with the enzyme aqueous solution to form the first solution, the plant-derived components contained in the enzyme aqueous solution are precipitated as precipitates containing proteins and the like by the action of calcium chloride. In the second step, the precipitates such as proteins thus precipitated are removed from the first solution. Then, in the third step, the first solution and the second solution from which the precipitates have been removed are injected into the ground. Therefore, the precipitates do not hinder the penetration of the first solution and the second solution into the ground. Therefore, the first solution and the second solution can be permeated into a wide range of the ground, and calcium carbonate can be precipitated by the first solution and the second solution in a wide range of the ground. As a result, the calcium carbonate can effectively suppress liquefaction of the ground.

(2) In the third step, the first solution and the second solution are mixed and then injected into the ground. The more the first solution and the second solution are mixed, the easier it is for calcium carbonate to precipitate. Therefore, by injecting the first solution and the second solution in a mixed state as described above into the ground, calcium carbonate can be efficiently precipitated in the ground.

(3) When forming the first solution in the first step, not only is calcium chloride added to the enzyme aqueous solution, but an antifoaming agent is also introduced and the solution is stirred. In the subsequent second step, precipitates such as proteins that precipitate from the first solution are removed, and in the third step, the first solution is sucked in by the grout pump 5 and discharged to the branch pressure control device 8 and the mixing unit 9. In the third step, if the first solution foams due to the stirring in the first step, it becomes difficult for the grout pump 5 to suck in and discharge the first solution.

However, since an antifoaming agent is added in the first step as described above, when the grout pump 5 pressure-feeds the first solution to the branch pressure control device 8 and the mixing unit 9, the first solution is prevented from becoming foamy, and the above-mentioned pressure-feeding is prevented from becoming difficult. Furthermore, since the first solution pressure-feeds by the grout pump 5 is not foamy, the first solution and the second solution are easily mixed in the mixing unit 9.

(4) When calcium carbonate is precipitated in the ground by injecting the first and second solutions into the ground, the molar concentration of urea in the second solution is adjusted so that the ratio of calcium carbonate precipitated to the mass of sand in the ground is 1 to 4%. This makes it possible to efficiently precipitate calcium carbonate in the ground while keeping the amount of materials such as urea to a minimum.

(5) When calcium carbonate precipitates in the ground by injecting the first and second solutions into the ground, the amount of urease dissolved in the first solution is adjusted so that the particle size and amount of precipitated calcium carbonate are set to predetermined values. This allows the particle size and amount of precipitated calcium carbonate precipitated in the ground to be appropriate values for suppressing liquefaction of the ground.

The above embodiment can be modified, for example, as follows: The above embodiment and the following modified examples can be combined with each other to the extent that no technical contradiction occurs.
The configuration of the solution injection plant may be changed as shown in Fig. 4. In this solution injection plant, one grout pump 10 sucks the first solution from the mixer 3 and the second solution from the mixer 4, and the grout pump 10 discharges the first and second solutions toward the branch flow pressure control device 8. The first and second solutions are supplied to a mixing unit 9 via the branch flow pressure control device 8, and are mixed with the first solution in the mixing unit 9 and injected into the ground.

The first solution and the second solution may be injected into the ground separately. In this case, the first solution and the second solution are mixed in the ground to precipitate calcium carbonate.
The molar concentration of urea in the second solution was adjusted so that the calcium carbonate precipitation ratio was 1 to 4%, but such an adjustment is not necessarily required.

In the first step, it is not always necessary to add a defoaming agent to the first solution.
The plant for forming the enzyme aqueous solution may be any plant that contains a large amount of urease. For example, beans other than sword beans, such as soybeans, adzuki beans, and peas, may be used. Also, it is not necessary to use beans, and plants of the Cucurbitaceae family, such as watermelon, white melon, Japanese cucumber, and sweet melon, may be used.

- A calcium compound other than calcium chloride may be used as the calcium compound for forming the first solution.

Reference Signs List 1... Water tank 1a... Submersible pump 2... Water tank 2a... Submersible pump 3, 4... Mixer 5, 6... Grout pump 7... Flow rate pressure control device 8... Branch flow pressure control device 9... Mixing unit 10... Grout pump

Claims (3)

A ground improvement method in which a calcium compound is mixed with an enzyme aqueous solution extracted from plants to produce a first solution, precipitates are removed from the first solution, and the first solution from which the precipitates have been removed and a second solution in which urea has been dissolved in water are injected into the ground. The ground improvement method according to claim 1, wherein the first solution and the second solution are mixed and injected into the ground. The ground improvement method according to claim 1 or 2, wherein the precipitation of the precipitate from the first solution is carried out by stirring the first solution in a state in which a defoaming agent has been added.