Unsaturated chassis improvement methods

Abstract

This invention provides a method for improving unsaturated ground that can be improved using a ground freezing method in unsaturated ground above the groundwater level where there is little water between soil particles. [Solution] The unsaturated ground improvement method according to the present invention is characterized by comprising: a first step of injecting a chemical solution into the unsaturated ground to increase the degree of saturation of the ground; and a second step of freezing the ground whose degree of saturation has been increased in the first step. Furthermore, if the unsaturated ground is cohesive soil, it is preferable to increase the degree of saturation of the ground to 60% or more in the first step.

Description

[Technical Field]

[0001] The present invention relates to a method for improving unsaturated ground using a ground freezing method. [Background technology]

[0002] One known ground improvement method is the ground freezing method, which stabilizes the ground by freezing soil containing groundwater (see Patent Document 1). [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-143539 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, the aforementioned ground freezing method is a method that freezes the water between soil particles, and is known as a method for improving saturated ground below the groundwater level. If this method is used on unsaturated ground above the groundwater level where there is little water between soil particles, the ground will not freeze sufficiently, making it difficult to improve the strength of the ground and resulting in a low ground improvement effect. For this reason, conventionally, the ground freezing method has not been adopted as a method for improving unsaturated ground above the groundwater level where there is little water between soil particles. In view of the above-mentioned problems, the present invention provides an unsaturated ground improvement method that enables the improvement of unsaturated ground above the groundwater level where there is little water between soil particles using a ground freezing method. [Means for solving the problem]

[0005] The unsaturated ground improvement method according to the present invention is characterized by comprising: a first step of injecting a chemical solution into unsaturated ground to increase the degree of saturation of the ground; and a second step of freezing the ground whose degree of saturation has been increased in the first step. Furthermore, if the unsaturated ground is cohesive soil, the first step is characterized by increasing the saturation level of the ground to 60% or more. Furthermore, it is characterized by the use of a suspension-type drug solution. According to the unsaturated ground improvement method of the present invention, the strength of the unsaturated ground can be improved by injecting a chemical solution into the unsaturated ground to increase the degree of saturation of the ground, and then freezing the ground with increased saturation. This makes it possible to improve the ground using the ground freezing method for unsaturated ground. [Brief explanation of the drawing]

[0006] [Figure 1] A diagram showing the results of the indoor strength test. [Modes for carrying out the invention]

[0007] Embodiment 1 The unsaturated ground improvement method according to Embodiment 1 comprises a first step of injecting a chemical solution into the unsaturated ground to increase the saturation level of the ground, and a second step of freezing the ground whose saturation level has been increased in the first step.

[0008] For the chemical solution used in the first step, for example, a suspension-type (cement-based) chemical solution or a solution-type (e.g., water glass-based) chemical solution may be used, as described later.

[0009] Furthermore, in the second step, a ground freezing method can be used in which a freezing pipe is installed in the ground with increased saturation, and then a refrigerant is supplied to the freezing pipe to freeze the ground around the freezing pipe. For the ground freezing method in question, it is sufficient to implement a well-known ground freezing method.

[0010] According to the unsaturated ground improvement method of Embodiment 1, by injecting a chemical solution into unsaturated ground above the groundwater level where there is little water between soil particles, moisture is replenished between soil particles and the degree of soil saturation increases. Then, by using a ground freezing method to freeze the soil whose saturation level has been increased, the water between soil particles freezes, causing the ground to freeze sufficiently, and this sufficient freezing increases the strength of the ground. Furthermore, since the chemical solution contains components that solidify the ground, the strength of the ground increases as a result of using this solution. The components in the chemical solution that solidify the ground include, for example, cement and hardening accelerators (such as water glass, sodium sulfate, and calcium carbonate). In other words, after the chemical injection but before the ground is frozen, the ground's strength increases due to the ground-solidifying action of the soil-solidifying components contained in the chemical. Then, by increasing the saturation of the ground through chemical injection, the ground is frozen using a ground freezing method, resulting in sufficient freezing of the ground. In other words, the synergistic effect of the chemical solution and the freezing effect increases the strength of the ground. Furthermore, even if the ground thaws, the ground's strength is maintained due to the ground-solidifying effect of the chemical solution.

[0011] Thus, according to the unsaturated ground improvement method of Embodiment 1, it is now possible to improve unsaturated ground using the ground freezing method. In particular, the unsaturated ground improvement method according to Embodiment 1 can significantly increase the strength of the ground before freezing, after freezing, and after thawing following chemical injection, and it is possible to realize a method that has an excellent ground improvement effect on unsaturated ground.

[0012] Embodiment 2 In implementing the unsaturated ground improvement method according to Embodiment 1 described above, the inventors confirmed through laboratory strength tests that the conditions under which the strength of the ground (improved ground) after chemical injection, before freezing, after freezing, and after thawing could be set to a desired strength or higher. In the experiment, soil from the unsaturated ground at the site where the unsaturated ground improvement method is actually planned to be implemented (hereinafter simply referred to as the "site") was collected (hereinafter referred to as "locally collected soil"), and a suspension-type (cement-based) chemical solution was injected into the locally collected soil to prepare three types of specimens with saturation degrees of 100%, 80%, and 60%, and the uniaxial compressive strength of the three types of specimens was measured. In addition, the design reference strength σa set by the Ground Freezing Method Association is that when the ground is cohesive soil, the uniaxial compressive strength = 3.0 N / mm 2 and when the ground is sandy soil, the uniaxial compressive strength = 4.5 N / mm 2 is the case. The "locally collected soil" used in the test was collected from the unsaturated ground of cohesive soil, so the design reference strength σa of the ground was set to the uniaxial compressive strength = 3.0 N / mm 2 In order to ensure that the uniaxial compressive strength of the ground before freezing, after freezing, and after thawing = 3.0 N / mm 2 after injecting the chemical solution, it was confirmed in the laboratory experiment how much the saturation degree of the ground should be frozen. That is, three types of specimens described above (that is, specimens with saturation degrees of 100%, 80%, and 60%) were prepared using the "locally collected soil", and the uniaxial compressive strength before freezing, after freezing, and after thawing was measured for the three types of specimens.

[0013] The results of the above-mentioned laboratory strength test are shown in Fig. 1. In addition, as shown in Fig. 1, the saturation degree of the "locally collected soil" was 35.2 - 45.8%. And the uniaxial compressive strength of the frozen soil obtained by freezing the "locally collected soil" as it was was 1.24 N / mm 2 (see the plot 〇 in Fig. 1). That is, it was found that the uniaxial compressive strength = 3.0 N / mm 2 which is the design reference strength σa, cannot be ensured by simply freezing the "locally collected soil". Then, as described above, in the three types of specimens with saturation levels of 100%, 80%, and 60%, injection solidified bodies (solidified bodies before freezing after injecting suspension-type chemicals (solidified bodies before freezing after chemical solution injection)), injection solidified body frozen soils (solidified bodies after freezing after injecting suspension-type chemicals (solidified bodies after freezing)), and injection solidified body thawings (solidified bodies after the injection solidified body frozen soils have thawed (solidified bodies after thawing)) were prepared respectively, and the uniaxial compressive strength of each injection solidified body was measured.

[0014] In addition, in Fig. 1, the data of the injection solidified body are plotted as △, the data of the injection solidified body frozen soil are plotted as □, and the data of the injection solidified body thawing are plotted as ◇. Also, in Fig. 1, the actual combat and dotted lines indicated by "a" are the predicted lines of the uniaxial compressive strength with respect to the saturation of the injection solidified body, the actual combat and dotted lines indicated by "b" are the predicted lines of the uniaxial compressive strength with respect to the saturation of the injection solidified body frozen soil, and the actual combat and dotted lines indicated by "c" are the predicted lines of the uniaxial compressive strength with respect to the saturation of the injection solidified body thawing.

[0015] As is clear from the test results in Fig. 1, in the specimen with a saturation of 60%, all of the injection solidified body, the injection solidified body frozen soil, and the injection solidified body thawing can ensure the uniaxial compressive strength = 3.0 N / mm which is the desired design standard strength σa. 2 It was confirmed that this could be ensured.

[0016] Therefore, in the unsaturated ground improvement method according to Embodiment 2, in the above-mentioned first step of increasing the saturation of the in-situ unsaturated ground, based on the results of the above-mentioned laboratory experiments, a suspension-type (cement-based) chemical solution is injected into the in-situ unsaturated ground so as to increase the saturation of the ground to 60% or more.

[0017] In addition, the saturation of the ground is calculated based on the following calculation formula (1) or the following calculation formula (2).

[0018] Sr = 100×(Vw / Vk)…(1) Vw is the volume of water contained in the soil, and Vk is the volume of the voids in the soil.

[0019] Sr = (wρs / eρw) ... (2) w is the water content ratio, ρs is the dry density of the soil, e is the void ratio, and ρw is the density of water.

[0020] In the first step, for example, soil samples are taken from the unsaturated ground at the site, the degree of saturation of a unit volume of the soil is calculated, and the amount of chemical solution required to increase the degree of saturation to 60% or more is calculated. Then, the amount of chemical solution needed to increase the saturation level of the target unsaturated ground to 60% or more is calculated, injected into the unsaturated ground at the site, and soil is collected from the injected ground to confirm whether the saturation level is 60% or more.

[0021] In the second step, after confirming that the soil saturation level at the site is 60% or higher, a freezing pipe is installed in the soil, and freezing liquid is supplied to the freezing pipe to freeze the soil around the pipe.

[0022] According to the unsaturated ground improvement method of Embodiment 2, when the soil of the unsaturated ground is cohesive soil, in the first step, the degree of saturation of the ground is increased to 60% or more, and then the ground is frozen using a ground freezing method, thereby increasing the strength of the ground before freezing, after freezing, and after thawing to a desired design standard strength σa, which is a uniaxial compressive strength of 3.0 N / mm². 2 This has enabled us to secure the necessary resources and improve unsaturated ground into stable, good ground.

[0023] Furthermore, according to the unsaturated ground improvement method of Embodiment 2, by using a suspension-type chemical solution (cement-based), the solidified state of the ground can be maintained even after thawing due to the cement component of the chemical solution, thereby reducing the decrease in ground strength.

[0024] The unsaturated ground improvement method according to Embodiment 2 is suitable for ground improvement (ground stabilization) in unsaturated ground sites such as the following. For example, in an unsaturated ground where the soil at the site is cohesive soil, there is an existing building on the unsaturated ground, and there is a removal target in the unsaturated ground. When removing the removal target from the unsaturated ground, in order not to affect the existing building above the unsaturated ground, it is necessary to ensure the strength of the ground under the existing building before proceeding. Therefore, in this case, first, construct a pit from the ground outside the existing building to reach the vicinity of the removal target in the unsaturated ground. Through this pit, inject a suspension-type (cement-based) chemical solution into the unsaturated ground around the removal target to increase the saturation of the ground to 60% or more. After the saturation of the ground reaches 60% or more, arrange a plurality of freezing pipes to freeze the ground around the removal target. Then, supply a refrigerant to the plurality of freezing pipes to freeze the ground around the removal target. The strength of the ground around the removal target can be ensured to be uniaxial compressive strength = 3.0 N / mm 2 or more, and the removal target can be removed under a stable ground condition. After removing the removal target, fill the part where the removal target was removed with a filler such as mortar. That is, the strength of the ground around the removal target in the unsaturated ground and the strength of the ground after removing the removal target can be ensured to be uniaxial compressive strength = 3.0 N / mm 2 or more, so that the removal work and the ground after removal will not affect the existing building above the ground. Incidentally, as the removal target, for example, steel materials such as H-shaped steel of a soil retaining wall by the parent pile cross-sheet pile method left in the ground are assumed.

[0025] As described above, according to the unsaturated ground improvement method according to Embodiment 2, when there is a removal target to be removed in the unsaturated ground under an existing building, the strength of the ground around the removal target can be improved, and the removal target can be removed under a stable ground condition so as not to affect the existing building.

[0026] Furthermore, if the ground at the site where the unsaturated ground improvement method according to the present invention is to be implemented is sandy soil, the design standard strength σa set by the Japan Ground Freezing Method Association is uniaxial compressive strength = 4.5 N / mm². 2 Therefore, the uniaxial compressive strength of the ground after chemical injection, before freezing, after freezing, and after thawing is 4.5 N / mm². 2 To ensure this can be achieved, the appropriate degree of soil saturation for freezing should be determined through laboratory experiments using soil samples taken from the field.

Claims

1. The first step involves injecting a chemical solution into unsaturated ground to increase its saturation level, The second step involves freezing the ground whose saturation level was increased in the first step, A method for improving unsaturated ground, characterized by having the following features.

2. The unsaturated ground improvement method according to claim 1, characterized in that, when the unsaturated ground is cohesive soil, the degree of saturation of the ground is increased to 60% or more in the first step.

3. The unsaturated ground improvement method according to claim 1 or 2, characterized in that a suspension-type chemical solution is used as the chemical solution.

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