Soil mix design method

Abstract

This invention provides a method for setting the strength of improved soil at long-term age in the field to a desired level or higher, based on short-term laboratory test results using hydraulic cement and in-situ soil. [Solution] The process includes a target N-value setting step S1, a target strength setting step S2, an indoor mix design test step S3, and a mix design selection step S4. In the target N-value setting step S1, the target N-value of the improved soil 10 is set. In the target strength setting step S2, the target strength q0 is set from the main relation equation of the short-term indoor strength ql(S): target strength q0 = main coefficient α × target N-value...(A). In the indoor mix design test step S3, the mixing ratio of hydraulic cement is changed and the short-term indoor strength ql(S) for each is measured. In the mix design selection step S4, a mixing ratio that exceeds the target strength q0 is selected from the multiple short-term indoor strengths ql(S).

Description

[Technical Field]

[0001] The present invention relates to a method for designing the mix of improved soil to fill voids after the removal of existing piles. [Background technology]

[0002] Traditionally, when rebuilding apartment buildings or other structures, the piles supporting the existing structure (hereinafter referred to as "existing piles") are extracted after the demolition and removal of the existing structure. Hereafter, this construction method will be referred to as the "existing pile extraction method." The existing pile extraction method is disclosed, for example, in Patent Document 1.

[0003] In the existing pile extraction method, the void left after the removal of the existing piles is filled with improved soil. The improved soil is a mixture of a self-hardening stabilizing liquid and the in-situ soil surrounding the piles. Furthermore, the self-hardening stabilizing liquid is, for example, a mixture of cement, bentonite, and water (hereinafter referred to as "cement-bentonite liquid"), and has the self-hardening property of hardening after filling, and its composition is set so that the improved soil mixed with the site soil exhibits a predetermined strength after a predetermined period of time.

[0004] A method for designing the formulation of the injection liquid (i.e., the self-hardening stabilizing liquid) used when forming improved soil by mixing on-site soil with a self-hardening stabilizing liquid is disclosed, for example, in Patent Document 2. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Patent No. 7152899 [Patent Document 2] Japanese Patent Publication No. 2008-31769 [Overview of the project] [Problems that the invention aims to solve]

[0006] The mix design method described in Patent Document 2 involves determining the relationship between the water / cement ratio and strength by conducting compression experiments on multiple test specimens formed by mixing soil from the ground to be constructed with injection liquids (self-hardening stabilizing liquids) with different water / cement ratios. Then, based on the determined water / cement ratio and the previously measured ground conditions, the water / cement ratio in the injection liquid is calculated.

[0007] However, this method had the following problems. (1) The improved soil (soil cement) filled into the void after the removal of existing piles has self-hardening properties and hardens after filling. Therefore, the strength (e.g., unconfined compressive strength) of the improved soil differs significantly between short-term age (e.g., 7 days) and long-term age (e.g., 56 days) after filling. Therefore, in the formulation design method of Patent Document 2, compression experiments on multiple samples are conducted over a long period of time.

[0008] (2) The soil generated at sites where the existing pile extraction method is applied (hereinafter referred to as "site soil") is usually different. Therefore, when measuring the uniaxial compressive strength ("site uniaxial compressive strength") at sites where the existing pile extraction method is applied, the test environment (temperature, humidity, vibration, etc.) varies greatly from site to site, so it takes even more time and effort to measure accurate data with less variability.

[0009] (3) It is relatively easy to measure the uniaxial compressive strength of improved soil using on-site soil in a laboratory where a stable test environment can be obtained (hereinafter referred to as "in-laboratory uniaxial compressive strength"). However, the indoor unconfined compressive strength at a short age (e.g., 7 days) and the indoor unconfined compressive strength at a long age (e.g., 56 days) usually differ significantly, and the indoor unconfined compressive strength at a long age and the field unconfined compressive strength at a long age also usually differ significantly.

[0010] Therefore, there was a need for a soil mix design method that could accurately predict the field unconfined compressive strength at a long age (e.g., 56 days) from the laboratory unconfined compressive strength at a short age (e.g., 7 days), and set the improved soil at a long age to a strength above the desired level.

[0011] On the other hand, hydraulic cement used in existing pile extraction methods can be broadly classified into ordinary cement and blast furnace cement. Ordinary cement is primarily made from limestone, clay, iron ore, and bauvesite, and is characterized by its low initial heat of hydration and slow hardening. Ordinary cement is also called "ordinary Portland cement." Blast furnace cement uses blast furnace slag, a by-product of blast furnaces, as part of its raw materials and is characterized by its high initial heat of hydration and rapid hardening. Blast furnace cement is classified into types A, B, and C depending on the amount of blast furnace slag powder it contains.

[0012] Because the production of ordinary cement generates a large amount of carbon dioxide, the carbon dioxide emissions resulting from the use of ordinary cement are enormous. Therefore, there is a strong demand to reduce carbon dioxide emissions in order to prevent global warming (for example, under the Kyoto Protocol).

[0013] In the existing pile extraction method described above, ordinary cement has conventionally been used as the hydraulic cement that makes up the cement-bentonite liquid. On the other hand, blast furnace cement produces less carbon dioxide during its manufacture, so by changing the ordinary cement used in the existing pile extraction method to blast furnace cement, a significant reduction in carbon dioxide emissions can be achieved.

[0014] This invention was devised to satisfy the above-mentioned requirements. Specifically, the first objective of this invention is to provide a soil mix design method that allows the improved soil at long-term age in the field to be set to a desired strength or higher based on short-term laboratory test results using hydraulic cement and in-situ soil. The second objective is to provide a soil mix design method that can significantly reduce carbon dioxide emissions using blast furnace cement. [Means for solving the problem]

[0015] According to the present invention, a method for designing the mix of improved soil using hydraulic cement, The improved soil is a long-term cured product of a mixture of in-situ soil and cement-bentonite slurry, which is a mixed liquid of hydraulic cement, bentonite, and water, a target N value setting step of setting a target N value for the improved soil; a target strength setting step of setting the target strength q0 from the main relational expression of target strength q0 for short-term indoor strength ql(S) at short-term material age = main coefficient α × target N value ···(A); an indoor mixing test step of measuring each said short-term indoor strength ql(S) by changing the mixing ratio of hydraulic cement in said mixture to a plurality of values; a mixing selection step of selecting the mixing ratio of hydraulic cement that exceeds said target strength from the plurality of measured said short-term indoor strengths ql(S); there is provided a mixing design method for improved soil comprising the above steps. Effects of the Invention

[0016] According to the method of the present invention, in the target strength setting step, the target strength q0 of the short-term indoor strength ql(S) that achieves the target N value of the improved soil can be easily set from the main relational expression (A).

[0017] Furthermore, in the indoor mixing test step, since the mixing ratio of hydraulic cement in the mixture is changed to a plurality of values and each corresponding short-term indoor strength ql(S) is measured, the relationship between the mixing ratio of hydraulic cement and the short-term indoor strength ql(S) can be obtained within a short period (for example, 7 days).

[0018] Furthermore, in the mixing selection step, the mixing ratio of hydraulic cement that exceeds the target strength q0 is selected from the plurality of measured short-term indoor strengths ql(S). Accordingly, using hydraulic cement and in-situ soil, the improved soil with long-term material age at the construction site can be easily adjusted to have a strength not less than a desired strength based on short-term test results in a laboratory. Brief Description of the Drawings

[0019] [Figure 1] It is an explanatory diagram of an existing pile pulling-out construction method to which the present invention is applied. [Figure 2]This is an overall flowchart of the soil improvement mix design method according to the present invention. [Figure 3] This is a test result showing the relationship (second relationship) between the long-term field strength qf(L) of improved soil and the deformation modulus E. [Figure 4] This is a test result showing the relationship (third relationship) between the long-term laboratory strength ql(L) and the long-term field strength qf(L) of improved soil. [Figure 5] This is a test result showing the relationship (fourth relationship) between the short-term laboratory strength ql(S) and the long-term laboratory strength ql(L) of improved soil. [Figure 6] This diagram illustrates the void volume V1 after the removal of existing piles and the soil volume V2 around the existing piles. [Figure 7] This figure shows an example of the test results from an indoor uniaxial compression test. [Modes for carrying out the invention]

[0020] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0021] (definition) The "N-value" is a dimensionless value indicating the strength of the ground obtained from the Standard Penetration Test (SPT). A higher N-value indicates harder and stronger ground, while a lower N-value indicates softer and weaker ground. The N-value is usually given as a value between 0 and 60, and in existing pile extraction methods, values ​​between 20 and 50 are used. In some cases, even lower N-values ​​may be used. The "deformation modulus" of the ground is similar to the elastic modulus of steel or reinforced concrete, and refers to the slope of the stress-strain diagram in the ground.

[0022] "Material age" refers to the number of days that have passed since the improved soil was filled into the container. In the examples described later, "short age" refers to the number of days elapsed since filling, which is 7 days. In the embodiments described later, "long-term age" refers to the number of days elapsed since filling, which is 28 days and 56 days. In this invention, the 28-day long-term aging period is applied to ordinary cement, while the 56-day long-term aging period is applied to blast furnace cement.

[0023] In the following examples, "ordinary cement" means "ordinary Portland cement," and "blast furnace cement" means "blast furnace cement type B." As will be described later, the present invention is not limited to ordinary Portland cement or blast furnace cement type B, but can be applied to other hydraulic cements.

[0024] In this invention, unconfined compressive strength is used as the strength of the improved soil. Unconfined compressive strength is measured in accordance with the "Unconfined Compression Test Method for Soil (JIS A 1216)".

[0025] Hereinafter, the unconfined compressive strength q of the improved soil measured at a field site where the existing pile extraction method was applied will be referred to as the "field unconfined compressive strength". The in-situ uniaxial compressive strength at a short age (Short) is abbreviated as "short-term in-situ strength" and is represented by the symbol qf(S). The long-term (Long) field uniaxial compressive strength is abbreviated as "long-term field strength" and represented by the symbol qf(L).

[0026] On the other hand, the unconfined compressive strength q of improved soil using field soil 4 in a laboratory where a stable test environment can be obtained is called the "in-laboratory unconfined compressive strength". The intra-laboratory uniaxial compressive strength at a short age is abbreviated as "short-term intra-laboratory strength" and is represented by the symbol ql(S). The long-term uniaxial compressive strength at the intra-calibration level is abbreviated as "long-term intra-calibration strength" and is represented by the symbol ql(L).

[0027] Figure 1 is an explanatory diagram of an existing pile extraction method to which the present invention is applied. In this diagram, the existing pile extraction method is carried out in the following order: (1) exposing the top of the existing pile 1, (2) setting the pile core, (3) drilling (friction cutting), (4) lifting the casing 2, (5) setting the wire, (6) lifting, and (7) injecting and mixing cement bentonite liquid 3. In drilling (friction cutting), it is preferable to rotate the casing 2 while spraying cement bentonite liquid 3 (hereinafter referred to as "CB liquid 3") from the discharge port of the casing head 2a, and insert it into the tip of the pile 1. Furthermore, when injecting and stirring the cement bentonite liquid 3, it is preferable to thoroughly mix the surrounding soil (site soil 4), which has turned into a muddy state during friction cutting, with the CB liquid 3.

[0028] Cement bentonite liquid (CB liquid 3) is a mixture of hydraulic cement, bentonite, and water. "Hydraulic cement" refers to cement that hardens after solidifying through a chemical reaction with water. Hydraulic cement has the characteristic of setting and hardening even underwater. The ordinary cement and blast furnace cement mentioned above are both types of hydraulic cement. Bentonite is a weakly alkaline clay with viscosity, and when a liquid made by dissolving bentonite in water is mixed with soil, it imparts viscosity to the soil.

[0029] Figure 2 is an overall flow chart of the soil mix design method for improved soil according to the present invention. In this invention, "improved soil" refers to a long-term hardened mixture of CB liquid 3 and on-site soil 4. "Site soil" refers to soil excavated from around existing pile 1 during drilling (friction cutting) at a site where the existing pile extraction method is applied, or an equivalent material. In this invention, "long-term" in "long-term hardened material" refers to the age at which the improved soil 10 containing hydraulic cement reaches a desired strength or higher. In the examples described later, the "long-term age" is 28 days (4 weeks) or 56 days (8 weeks). Furthermore, in the example described later, "short age" refers to 7 days (1 week).

[0030] In Figure 2, the soil mix design method of the present invention (hereinafter referred to as the "mix design method") comprises the following steps: a target N-value setting step S1, a target strength setting step S2, an indoor mix test step S3, and a mix selection step S4.

[0031] In the target N-value setting process S1, the target N-value for the improved soil 10 is set. In the target N-value setting process S1, the target N-value of the improved soil 10 to be used as backfill material for the area where existing piles are removed is set based on the boring data in the geological survey report. The N-value of the ground corresponding to the length of the existing piles (depth of the ground) is obtained from the boring data. For example, the target N-value of the improved soil 10 to be used as backfill material is set to correspond to the obtained N-value of the ground.

[0032] In the target strength setting process S2, the target strength q0 is set from the main relational equation q0 = α × target N value (A) of the indoor uniaxial compressive strength ql(S) at a short age.

[0033] In the indoor mix design test process S3, the mixing ratio of hydraulic cement in the mixture is varied in multiple ways, and the short-term indoor strength ql(S) for each is measured.

[0034] In the mix design selection process S4, the proportion of hydraulic cement exceeding the target strength q0 is selected from multiple measured indoor uniaxial compressive strengths ql(S).

[0035] The principal coefficient α mentioned above is determined from the relationship between the N value of improved soil 10, the deformation coefficient E of improved soil 10, the short-term field strength qf(S) of short-term age, the long-term field strength qf(L) of long-term age, the short-term laboratory strength ql(S) of short-term age, and the long-term laboratory strength ql(L) of long-term age. The method for determining the principal coefficient α in the principal relation (A) is explained below.

[0036] (Deformation coefficient E) It is known that the deformation coefficient E of improved soil 10 at a long age can be calculated using the following first relational equation. Deformation coefficient E = k1 × N = 700 × N (kN / m) 2 )···(1) Here, N is the N-value (dimensionless) which indicates the strength of the ground. The unit of the deformation modulus E is kN / m 2 That is the case. This first relation can be applied to improved soil 10 using either ordinary cement or blast furnace cement.

[0037] (Example 1) (Relationship between the long-term field strength qf(L) and deformation modulus E of improved soil 10) Figure 3 shows the test results illustrating the relationship (second relationship) between the long-term field strength qf(L) and deformation modulus E of improved soil 10. The long-term field strength qf(L) was experimentally obtained at multiple sites where the field soil 4 differed. These multiple sites consisted of more than 10 actual sites with different locations, times, and soil types. In this figure, (A) represents ordinary cement and (B) represents blast furnace cement. In each figure, the horizontal axis represents the long-term field strength qf(L), and the vertical axis represents the deformation modulus E.

[0038] Note that the correlation coefficient R in Figures 3(A) and 3(B) is R 2 The value was >0.74, indicating a strong positive correlation.

[0039] From the test results in Figures 3(A) and 3(B), the deformation modulus E and the long-term field strength qf(L) can be expressed by the following second relationship. E = k² × qf(L) ···(2)

[0040] In the case of ordinary cement, the coefficient k2 is in the range of 50 to 350, and preferably about 200. In the case of blast furnace cement, the coefficient k2 is preferably about 160.

[0041] In the target N-value setting process S1 shown in Figure 2, when a target N-value for the improved soil 10 is set, the target deformation coefficient E for the improved soil 10 is determined from the first relational equation (1). Furthermore, the long-term field strength qf(L) requires obtaining the target deformation coefficient E of the improved soil 10 based on the first relational equation (1). Therefore, from the first relation (1) and the second relation (2), the long-term field strength qf(L) can be set as the target strength for the long-term field.

[0042] (Relationship between long-term indoor strength ql(L) and long-term field strength qf(L)) Figure 4 shows the test results illustrating the relationship (third relationship) between the long-term laboratory strength ql(L) and the long-term field strength qf(L) of the improved soil 10. The long-term laboratory strength ql(L) was experimentally obtained in the laboratory using multiple (two locations in this example) field soils 4.

[0043] In this figure, (A) represents ordinary cement, and (B) represents blast furnace cement. In each figure, the horizontal axis represents the long-term laboratory strength ql (L), and the vertical axis represents the long-term field strength qf (L).

[0044] From the test results in Figures 4(A) and 4(B), the third relationship between the long-term field strength qf(L) and the long-term indoor strength ql(L) can be expressed by the following third relationship equation. qf(L) = k³ × ql(L) ···(3)

[0045] In the case of ordinary cement, the coefficient k3 is k3 = 0.23 to 0.62, preferably about 0.23, and in the case of blast furnace cement, the coefficient k3 is k3 = 0.11 to 0.69, preferably about 0.11. It is preferable to use a small value for the coefficient k3 as a safety measure.

[0046] From the third relation, it can be seen that the strength at long age (uniaxial compressive strength) is generally lower in field tests than in laboratory tests, and this needs to be taken into consideration when designing the mix for improved soil 10.

[0047] (Relationship between short-term indoor intensity ql(S) and long-term indoor intensity ql(L)) Figure 5 shows the test results illustrating the relationship (fourth relationship) between the short-term laboratory strength ql(S) and the long-term laboratory strength ql(L) of the improved soil. The short-term laboratory strength ql(S) was experimentally obtained in the laboratory using multiple field soil samples 4.

[0048] In this figure, (A) represents ordinary cement, and (B) represents blast furnace cement. In each figure, the horizontal axis represents the short-term indoor strength ql(S), and the vertical axis represents the long-term indoor strength ql(L).

[0049] Note that the correlation coefficient R in Figure 5 is R 2 The correlation was >0.90, indicating a strong positive correlation.

[0050] From the test results in Figures 5(A) and 5(B), the relationship between the short-term indoor strength ql(S) and the long-term indoor strength ql(L) can be expressed by the following fourth relation. ql(L) = k⁴ × ql(S)···(4)

[0051] The coefficient k4 is preferably about 1.7 for ordinary cement and preferably about 5.6 for blast furnace cement.

[0052] From the fourth relation, it can be seen that, even in the laboratory, the strength of improved soil 10 at a long age (28 days or 56 days) is generally greater than that of improved soil 10 at a short age (7 days) due to the hydraulic properties of the hydraulic cement, and this needs to be taken into consideration.

[0053] Using the first relation (1) to the fourth relation (4) described above, the principal coefficient α of the principal relation can be found. In other words, the principal coefficient α of the principal relation is found by α = q0 / N = k1 / k2 / k3 / k4 ... (5). From the values ​​of the coefficients mentioned above, the principal coefficient α is preferably about 9.0 for ordinary cement and preferably about 7.0 for blast furnace cement.

[0054] The first relational equation (1) described above is a general equation that can be applied to all types of ground to which the existing pile extraction method is applied, and can be applied to improved soil 10 using either ordinary cement or blast furnace cement. Furthermore, the second relation (2) described above was obtained experimentally at multiple sites where the site soil 4 differed, and can be similarly applied to new site soil 4 using ordinary cement or blast furnace cement. Furthermore, the aforementioned third relational expression (3) and fourth relational expression (4) are obtained experimentally in a laboratory where a stable test environment can be achieved, and can be similarly applied to new in-situ soil 4 using ordinary Portland cement or blast furnace cement. Note that the third relational expression (3) is a relational expression between on-site strength and laboratory strength, and on-site samples are collected by on-site core boring or the like. Therefore, for new in-situ soil 4 using ordinary Portland cement or blast furnace cement, the target strength q0 for the short-term laboratory strength ql(S) can be easily set from the main relational expression of target strength q0=α×target N value ···(A).

[0055] (Example 2) In the laboratory mixing test step S3 shown in FIG. 2, first, the void volume V1 after removing the existing pile and the soil volume V2 around the existing pile are calculated. FIG. 6 is an explanatory diagram of the void volume V1 after removing the existing pile and the soil volume V2 around the existing pile. In this figure, when L is the length (m) of the existing pile, D is the diameter (m) of the casing, and d is the diameter (m) of the existing pile, the void volume V1 after removing the existing pile and the soil volume V2 around the existing pile can be obtained by the following formulas (6) and (7). Void volume V1=(π·d 2 ) / 4×L···(6) Soil volume V2=(π·D 2 ) / 4×L-V1=π·(D 2 -d 2 ) / 4×L···(7)

[0056] Next, a plurality of mixing formulations are set by changing the mixing ratio of hydraulic cement in the mixture to a plurality of values. Table 1 shows a specific example of a mixing test table in which four types of mixing formulations are set.

[0057]

Table 1

[0058] In this example, the cement content is set to four types within the range of 200 to 500 kg / m 3 In addition, 1.0 m of CB liquid 3In contrast, the amount of bentonite is 50 kg. Also, the total amount of CB liquid 3 (mixed volume) is 1.0 m³. 3 The water volume is set to achieve this result. In other words, in this example, the volume ratio of the soil volume 4 to the unit volume of CB liquid 3 is set to V2 / V1, the weight of bentonite contained in the unit volume of CB liquid 3 is kept constant, the weight of hydraulic cement is varied, and the remainder is water. Note that the formulation test sheet is not limited to this example and can be set up as desired.

[0059] (Example 3) In the formulation selection process S4 shown in Figure 2, the intra-indoor uniaxial compressive strength (short-term intra-indoor strength ql(S)) is measured for each of the four formulations listed in Table 1 above. This measurement should be carried out using the following method. (1) Collect the necessary amount of samples from the soil layer targeted by the existing pile extraction method. (2) Select the type of hydraulic cement (e.g., blast furnace cement type B). (3) Prepare the sample and prepare the test specimen. The preparation of the sample and the preparation of the test specimen should be carried out in accordance with the Japanese Geotechnical Society standard "JGS0821-2000". (4) Curing. The sample filled into the mold should be cured in a moist state to prevent water from entering or leaving, and at a constant temperature of 20°C ± 3°C. (5) Indoor uniaxial compression test Uniaxial compression tests should be conducted in accordance with the "Uniaxial Compression Test Method for Soil (JIS A 1216)".

[0060] Figure 7 shows an example of the test results from an indoor uniaxial compression test. In this figure, the horizontal axis represents the amount of hydraulic cement added (kg / m³). 3 The vertical axis represents the short-term indoor strength ql(S) (indoor uniaxial compressive strength). From this diagram, the amount of cement in CB liquid 3 that will achieve the target indoor uniaxial compressive strength is determined. In other words, in the mix design selection step S4, a ratio of hydraulic cement exceeding the target strength is selected based on the relationship between the weight of hydraulic cement and the short-term indoor strength ql(S) based on multiple measured short-term indoor strengths ql(S).

[0061] According to the embodiment of the present invention described above, in the target strength setting step S2, the target strength q0 of the short-term indoor strength ql(S) from which the target N value of the improved soil 10 can be easily set from the main relation (A).

[0062] Furthermore, in the indoor mix design test process S3, the mixing ratio of hydraulic cement in the mixture is varied to multiple values, and the short-term indoor strength ql(S) for each is measured. This allows the relationship between the mixing ratio of hydraulic cement and the short-term indoor strength ql(S) to be determined in a short period of time (e.g., 7 days).

[0063] Furthermore, in the mix selection step S4, a proportion of hydraulic cement exceeding the target strength q0 is selected from multiple measured short-term laboratory strengths ql(S). This makes it easy to set the strength of the improved soil 10 at the site over a long period of time to the desired level, based on short-term test results in the laboratory using hydraulic cement and site soil 4.

[0064] Furthermore, as described above, the method of the present invention can be applied equally to both ordinary cement and blast furnace cement. Therefore, by using blast furnace cement, which has not been conventionally used as a hydraulic cement, carbon dioxide emissions can be significantly reduced.

[0065] In the above-described examples, the ordinary cement is "ordinary Portland cement" and the blast furnace cement is "blast furnace cement type B," but the present invention is not limited to this example and can be applied to other hydraulic cements.

[0066] Furthermore, the present invention is not limited to the "existing pile extraction method" described above, but can also be applied to other construction methods.

[0067] The scope of the present invention is not limited to the embodiments described above, but is indicated by the claims, and includes all modifications within the meaning and scope of equivalence to the claims. [Explanation of Symbols]

[0068] α: Main coefficient, k1, k2, k3, k4: Coefficients, D: Casing diameter, d: Existing pile diameter, L: Existing pile length, qf(S): Short-term field strength, qf(L): Long-term field strength, ql(S): Short-term laboratory strength, ql(L): Long-term laboratory strength, q0: Target strength, V1: Void volume, V2: Soil volume, 1: Existing pile, 2: Casing, 2a: Casing head, 3: Cement bentonite liquid (CB liquid), 4: Field soil, 10: Improved soil

Claims

1. A method for designing the mix design of improved soil using hydraulic cement, The improved soil is a long-term hardened product of a mixture of cement bentonite liquid, which is a mixture of hydraulic cement, bentonite, and water, and the site soil. A target N-value setting step for setting a target N-value for the improved soil, A target strength setting step in which the target strength q0 is set from the main relation equation of the short-term intra-industrial strength ql(S) of the short-term age material, q0 = main coefficient α × target N value ... (A), A laboratory mix design test step in which the mixing ratio of hydraulic cement in the mixture is changed to multiple values ​​and the short-term laboratory strength ql(S) of each is measured, A method for designing the mix of improved soil, comprising a mix selection step of selecting the mixing ratio of hydraulic cement that exceeds the target strength from a plurality of measured short-term indoor strengths ql(S).

2. The method for designing the mix of improved soil according to claim 1, wherein the principal coefficient α is determined from the relationship between the N value of the improved soil, the deformation coefficient E of the improved soil, the short-term field strength qf(S) at a short age, the long-term field strength qf(L) at a long age, the short-term laboratory strength ql(S), and the long-term laboratory strength ql(L) at a long age.

3. The principal coefficient α is, The first relational expression between the deformation coefficient E and N value of the improved soil at the long age, The second relationship between the deformation coefficient E and the long-term field strength qf(L), obtained at multiple sites where the field soil differs, A method for designing the mix of improved soil according to claim 2, obtained from a third relational expression between the long-term field strength qf(L) and the long-term laboratory strength ql(L), and a fourth relational expression between the short-term laboratory strength ql(S) and the long-term laboratory strength ql(L), obtained in a laboratory using a plurality of the aforementioned field soils.

4. The first relation is E = k1 × N ... (1), In multiple sites where the soil conditions differ, the second relation, E = k² × qf ... (2), was determined experimentally. In the laboratory, the third relation, qf = k3 × ql ... (3), and the fourth relation, ql = k4 × q0 ... (4), were experimentally determined. Using the first to fourth relational equations, The method for designing the mix of improved soil according to claim 3, wherein the principal coefficient α is determined by the formula α = q0 / N = k1 / k2 / k3 / k4...(5).

5. In the case of ordinary cement, the coefficient k2 in the second relational equation is k2 = 50 to 350, preferably about 200. In the case of blast furnace cement, the coefficient k2 of the second relation is preferably about 160, the method for designing the mix of improved soil according to claim 4.

6. In the case of ordinary cement, the coefficient k3 in the third relation described above is k3 = 0.23 to 0.62, preferably about 0.

23. In the case of blast furnace cement, the coefficient k3 of the third relation is k3 = 0.11 to 0.69, preferably about 0.11, the method for designing the mix of improved soil according to claim 4.

7. In the case of ordinary cement, the coefficient k4 in the fourth relation is approximately 1.

7. In the case of blast furnace cement, the coefficient k4 of the fourth relation is approximately 5.6, the method for designing the mix of improved soil according to claim 4.

8. In the case of ordinary cement, the principal coefficient α is preferably about 9.

0. In the case of blast furnace cement, the main coefficient α is preferably about 7.0, the method for designing the mix of improved soil according to claim 4.

9. In the aforementioned indoor mix design test process, the void volume V1 after the removal of the existing piles and the volume of soil around the existing piles V2 are calculated. The volume ratio of the soil volume at the site to the unit volume of the cement bentonite liquid is set to V2 / V1. A method for designing the mix of improved soil according to claim 1, wherein the weight of bentonite contained in a unit volume of the cement bentonite liquid is kept constant, the weight of the hydraulic cement is varied to multiple values, the remainder is water, and the short-term indoor strength ql(S) of each is measured.

10. The method for designing the mix of improved soil according to claim 1, wherein in the mix selection step, the mixing ratio of the hydraulic cement that exceeds the target strength is selected based on the relationship between the weight of the hydraulic cement and the short-term indoor strength ql(S).

11. The method for designing the mix of improved soil according to claim 1, wherein in the step of setting the target N value, the target N value of the improved soil is set as the backfill material for the portion where existing piles are removed, based on the boring data in the geological survey report.

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