JP2020203998A - Soil improvement method - Google Patents

Abstract

To provide a method for determining an oxidation reduction potential (ORP) of a chemical admixture and a suitable addition amount thereof for suppressing an elution amount of hexavalent chromium to the environmental standard value, the elution amount varying according to construction conditions when preparing stabilized soil.SOLUTION: When preparing stabilized soil by mixing and stirring a cement slurry obtained by mixing water and cement and original soil, a chemical admixture slurry is produced beforehand by mixing a cement slurry and a chemical admixture having an oxidation-reduction potential (ORP) of 50 mV or less and, thereafter, the chemical admixture slurry is discharged into the original soil and mixed by stirring. At that time, the oxidation-reduction potential (ORP) and the addition amount of the chemical admixture, which suppress an elution amount of hexavalent chromium to the environmental standard value (0.05 mg/L or less) or less than a minimum determination value (0.02 mg/L), are determined by a specific mixing test.SELECTED DRAWING: Figure 9

Description

The present invention relates to a ground improvement method for creating a stable-treated soil by stirring and mixing raw soil and cement slurry, and particularly, an elution amount of hexavalent chromium contained in the prepared stable-treated soil (hereinafter, "hexavalent chromium elution amount"). ) Is related to the ground improvement method focusing on the suppression.

The amount of hexavalent chromium eluted from the stable-treated soil created by stirring and mixing raw soil and cement slurry is "0 per 1 L of test solution" according to the environmental standard of the Ministry of the Environment (environmental standard for soil pollution). It is stipulated that it should be 0.05 mg or less (hereinafter referred to as "environmental standard value").

However, there are cases where the environmental standard value is exceeded when the ability of cement to fix substances such as heavy metals is hindered by organic matter or clay minerals in the soil. In particular, when the raw soil is loam soil containing a large amount of clay minerals or humus soil containing a large amount of humic acid, which significantly inhibits the solidification of the stable-treated soil, the strength of the stable-treated soil develops slowly and is contained in cement. It is known that the hexavalent loam of humic acid is not sufficiently immobilized (insolubilized) and the amount of elution increases. As a countermeasure, it is necessary to use a more expensive solidifying material for hexavalent chromium (solidifying material for special soil), which poses an economic burden.

On the other hand, Patent Document 1 and Patent Document 2 have introduced a technique that is effective in suppressing the elution amount of hexavalent chromium contained in cement.

Japanese Unexamined Patent Publication No. 2018-14507 Japanese Unexamined Patent Publication No. 2016-169317

In both Patent Document 1 and Patent Document 2, an admixture (cement additive or ground stabilizing admixture) having an oxidation-reduction potential (hereinafter referred to as "ORP") having a low ORP of -400 mV to -450 mV or less and cement or It is a technology that has the effect of reducing hexavalent chromium contained in cement by adding it to cement milk (cement slurry) or the like. However, although the ratio of the main component for purifying the admixture and the ORP and pH of the admixture are shown, the addition amount thereof is 0.001 to 1 part by mass or 0.01 to 10 parts by mass with respect to 100 parts by mass of cement. It is only mentioned as a department.

The redox potential (ORP) is an Oxidation-Reduction Potential, also called a Redox potential, and is an oxide (M ^{Z +} ) and a reduce (M ^{(ZN) +} ) coexisting in a solution. It is supposed to mean an energy level (potential) determined by the equilibrium state between them.

On the other hand, the amount of hexavalent chromium that elutes from the stable-treated soil varies widely depending on the construction conditions, let alone the ORP of the admixture to surely suppress the amount of hexavalent chromium that elutes to the environmental standard value. It is necessary to appropriately select the combination with the amount of admixture added. However, neither Patent Document 1 nor Patent Document 2 describes a method for determining the combination of the above-mentioned ORP of the admixture and the amount of the admixture added.

The present invention has been made in view of such problems, and the construction conditions for creating stable treated soil (type and amount of cement added, soil properties of raw soil, flow value immediately after stirring and mixing, target strength, etc.) ) To ensure that the amount of hexavalent chromium eluted (hereinafter referred to as "hexavalent chromium elution amount") is the environmental standard value, the amount of admixture ORP and admixture added (hereinafter referred to as "admixture addition"). It proposes a method for determining an appropriate combination of "quantity".

In addition, in construction in agricultural production areas and in the vicinity of natural parks, it is required to reduce the elution amount of hexavalent chromium, which is a carcinogen, to zero (non-detection). It is a suggestion.

The present invention is a ground improvement method for suppressing the amount of hexavalent chromium eluted from a stable-treated soil formed by stirring and mixing a cement slurry in which water and cement are kneaded and raw soil.

Then, as a typical embodiment, an admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of 50 mV or less is produced in advance, and then the admixture slurry is placed in the raw soil. The oxidation-reduction potential (oxidation-reduction potential) of the admixture suppresses the amount of hexavalent chromium eluted to less than the environmental standard value (0.05 mg / L or less) or the lower limit of quantification (0.02 mg / L). It is characterized in that the amount of the ORP) and the admixture added is determined by the combination test of (a) or (b) shown below.

(A) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, the amount of hexavalent chromium eluted from the stable-treated soil is less than the environmental standard value or the lower limit of quantification. The oxidation-reduction potential (ORP) of the admixture that seems to be suppressed and the amount of admixture added are estimated.
After using the estimated amount of the admixture added as the provisional amount, an admixture compounding test was conducted in which the admixture was added to confirm the effect of suppressing the amount of hexavalent chromium eluted.
After confirming that the amount of hexavalent chromium eluted was suppressed to less than the environmental standard value or the lower limit of quantification, the redox potential (ORP) and provisional addition amount of the admixture used in the admixture compounding test were improved. It is determined as the redox potential (ORP) of the admixture used in the above and the amount of the admixture added.

(B) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, the amount of hexavalent chromium eluted from the stable-treated soil is less than the environmental standard value or the lower limit of quantification. Estimate the oxidation-reduction potential (ORP) of the admixture and the amount of admixture added, which are thought to suppress up to
After using the estimated admixture addition amount as the provisional addition amount, an admixture compounding test was conducted using a plurality of admixture addition amounts including the provisional addition amount.
Based on the correlation between the amount of admixture added and the amount of hexavalent chromium eluted after addition of the admixture based on the results of the admixture compounding test, the amount of hexavalent chromium eluted is less than the environmental standard value or the lower limit of quantification. Ask for quantity,
The redox potential (ORP) of the admixture used in the admixture compounding test and the admixture addition amount are determined as the redox potential (ORP) of the admixture used for ground improvement and the admixture addition amount.

According to the present invention, even if the solidifying material typified by cement is not over-administered or the more expensive solidifying material is not used, the elution amount of hexavalent chromium from the prepared stable-treated soil is suppressed and the elution amount is reduced. It can be less than the environmental standard value or the lower limit of quantification (not detected). Therefore, not only is it economically efficient by reducing the amount of solidifying material and additives used, but it is also environmentally friendly by suppressing the amount of hexavalent chromium that elutes, and it is possible to improve the ground with high reliability. ..

It is a figure which shows the embodiment of the ground improvement construction method which concerns on this invention, and is the graph which shows the relationship between the uniaxial compression strength and hexavalent chromium elution amount of the stable-treated soil. A graph showing the relationship between the table flow value and the uniaxial compressive strength. The graph which shows the relationship between the table flow value and the amount of hexavalent chromium elution. The graph which showed the relationship between the uniaxial compression strength and the amount of hexavalent chromium elution shown in FIG. 1 as a single approximate curve. The graph which shows the relationship between the redox potential (ORP) and the amount of hexavalent chromium elution at the time of high flow value and low intensity. The graph which shows the relationship between the redox potential (ORP) and the amount of hexavalent chromium elution in the case of low flow value and high intensity. The graph which shows the relationship between the addition amount of the admixture which makes the hexavalent chromium elution amount an environmental standard value, and the redox potential (ORP). The graph which shows the relationship between the addition amount of the admixture which the hexavalent chromium elution amount becomes the lower limit of quantification and the redox potential (ORP). The graph which shows the relationship between the addition amount of an admixture and the elution amount of hexavalent chromium after the addition of an admixture. Explanatory drawing which shows the procedure from manufacturing of admixture slurry to construction by underground discharge.

Next, an embodiment of the ground improvement method according to the present invention will be described based on more specific examples.

In the test described below, loam soil, which contains a large amount of clay minerals that significantly inhibit solidification and is difficult to fix (insolubilize) hexavalent chromium, is used as the sample soil (raw soil). The loam soil used in this example is collected from Yotsukaido City, Chiba Prefecture, and has a wet density of 1.369 g / cm ³ and a water content of 121%.

1. 1. Mixing test of stable treated soil As mentioned above, since the elution status of hexavalent chromium differs depending on the construction conditions for creating stable treated soil, the construction conditions (stirring and mixing) in the ground improvement work generally performed as a countermeasure against soft ground Considering the combination of the flow value immediately after and the target strength of the stable treated soil), the compounding test (uniaxial compression test and six) under two construction conditions, one is when the amount of hexavalent chromium is the largest and the other is when the amount of hexavalent chromium is relatively small. Hexavalent chromium dissolution test) is performed.

The combinations of construction conditions are shown in (a) to (d) below.
(A) When the flow value immediately after stirring and mixing is applied at a low flow value to create a stable treated soil with low strength.
(B) When the flow value immediately after stirring and mixing is applied at a low flow value to create high-strength stable treated soil.
(C) When the flow value immediately after stirring and mixing is applied at a high flow value to create a stable treated soil with low strength.
(D) When the flow value immediately after stirring and mixing is applied at a high flow value to create a highly strong stable treated soil.

The flow value immediately after stirring and mixing is the stable treated soil within 5 to 20 minutes after stirring and mixing the admixture slurry (as will be described later, the cement slurry and the admixture are kneaded) and the raw soil. Here, it refers to the flow value measured in the flow test (JIS R5201). The flow value based on the flow test is a table flow value, and is hereinafter referred to as a "TF value".

The above-mentioned low flow value refers to construction with a TF value of approximately 110 mm to 115 mm, and a high flow value refers to construction with a TF value of approximately 150 mm or more. The above-mentioned low strength means the strength of the stable treated soil having a uniaxial compressive strength of about 100 kN / m ² created as a countermeasure against liquefaction, and the high strength means 3000 kN / m ² created as a structural foundation. It refers to the strength of stable treated soil, which exceeds the uniaxial compressive strength.

1) Preparation of specimen and various tests In order to obtain the compounding conditions (water-cement ratio of cement slurry and cement addition amount) that satisfy the above construction conditions (a) to (d), a compounding test was conducted with the cement addition amount shown below. Do.

The amount of cement added shall be four levels of 200 kg, 300 kg, 400 kg, and 500 kg per 1 m ^{3 of} raw soil. In the compounding test, the cement slurry and the sample based on the water-cement ratio that resulted in the above low flow value (about 110 mm to 115 mm in TF value) or high flow value (about 150 mm or more in TF value) regardless of the addition amount. A specimen shall be prepared from stable-treated soil in which soil (raw soil) is stirred and mixed, and various tests shall be conducted on the specimen.

In the above various tests, the uniaxial compressive strength of the specimen after 7 days of standard curing (uniaxial compressive test JIS A 1216) and the amount of hexavalent chromium eluted (hexavalent chromium elution test, Notification No. 46 and No. 77 of the Environment Agency). (Method of raising) is requested. Note that the description of test methods and procedures for various tests is omitted here.

In addition, the cement used in the compounding test shows good strength development in loam soil, humus soil, etc., but is said to be a solidifying material for general soft soil (cement-based solidifying material) in which hexavalent chromium is relatively eluted. To do.

2) Summary of compounding test results Table 1 shows the TF value, uniaxial compressive strength, and hexavalent chromium elution amount as the compounding test results using the above specimens. Then, based on Table 1, FIGS. 1, 2, 3, and 4 are created.

FIG. 1 shows the relationship between the uniaxial compressive strength and the amount of hexavalent chromium eluted, and FIG. 2 shows the relationship between the table flow value and the uniaxial compressive strength. Furthermore, FIG. 3 shows the relationship between the table flow value and the amount of hexavalent chromium eluted. FIG. 4 shows the relationship between the uniaxial compressive strength of FIG. 1 and the amount of hexavalent chromium eluted by a single and common approximate curve.

3) Features obtained from the compounding test results (1) Relationship between uniaxial compression strength and hexavalent chromium elution amount (Figs. 1 and 4)
FIG. 1 is a diagram showing the relationship between the uniaxial compressive strength of the stabilized soil and the amount of hexavalent chromium eluted, and the correlation is based on the amount of cement added (200 kg / m ³ , 300 kg / m ³ , 400 kg / m ³ , 500 kg / m ³⁾ . It is a graph shown for each m ³ ). Further, FIG. 4 is a graph showing the above FIG. 1 with a single and common approximate curve, and is a graph showing the relationship between the uniaxial compression strength of the stable-treated soil and the amount of hexavalent chromium eluted. Its features are shown below.
-The uniaxial compressive strength of stable treated soil and the amount of hexavalent chromium eluted are inversely proportional. That is, in any cement addition amount, the hexavalent chromium elution amount decreases as the uniaxial compressive strength increases.
-As is clear from FIGS. 1 and 4, the amount of hexavalent chromium eluted decreases as the amount of cement added increases. This indicates that the uniaxial compressive strength of the stable treated soil increased with the increase in the amount of cement added, and the amount of hexavalent chromium eluted was suppressed by promoting the immobilization (insolubilization) of hexavalent chromium.
-As is also clear from FIGS. 1 and 4, since the uniaxial compressive strength of the stable-treated soil exceeds 4000 kN / m ² , the amount of hexavalent chromium eluted is almost flat even if the amount of cement added increases.

(2) Relationship between TF value and uniaxial compressive strength (Fig. 2)
FIG. 2 is a graph showing the relationship between the TF value immediately after stirring and mixing the stabilized soil and the uniaxial compressive strength, and the correlation is based on the cement addition amounts (200 kg / m ³ , 300 kg / m ³ , 400 kg / m ³ , etc.). It is a graph showing every 500 kg / m ³ ). Its features are shown below.

-The TF value immediately after stirring and mixing the stabilized soil is inversely proportional to the uniaxial compressive strength. That is, when the TF value increases with the same amount of addition, the uniaxial compressive strength decreases.

-The uniaxial compressive strength under the same TF value is proportional to the amount of cement added. That is, regardless of whether the TF value is 110 mm or 150 mm, the larger the amount of cement added, the larger the uniaxial compression strength.

(3) Relationship between TF value and hexavalent chromium elution amount (Fig. 3)
FIG. 3 is a diagram showing the relationship between the TF value immediately after stirring and mixing the stabilized soil and the amount of hexavalent chromium eluted, and the correlation is based on the amount of cement added (200 kg / m ³ , 300 kg / m ³ , 400 kg / m ^3). , 500 kg / m ³ ). Its features are shown below.

-The TF value immediately after stirring and mixing the stabilized soil is proportional to the amount of hexavalent chromium eluted. That is, regardless of the amount of cement added, the amount of hexavalent chromium eluted increases as the TF value increases.
-The amount of cement added and the amount of hexavalent chromium eluted under the same TF value are inversely proportional. That is, under the same TF value, the amount of hexavalent chromium eluted decreases as the amount of cement added increases.

(4) Summary of characteristics obtained from the results of the compounding test In the compounding test of the stable-treated soil, the properties of the sample soil (raw soil), the amount of cement added, the strength of the stable-treated soil (uniaxial compressive strength), and the strength at the time of construction The amount of hexavalent chromium eluted from the stable-treated soil fluctuated greatly due to differences in construction conditions such as the flow value (TF value) immediately after stirring and mixing.

More specifically, the solidifying material for general soft soil is a cement (solidifying material) having high solidifying performance but a remarkable amount of hexavalent chromium elution as described above, and has a reducing effect of an admixture described later (a solidifying material). It can be said that it is the most suitable cement for confirming the effect of suppressing the amount of hexavalent chromium eluted). In other words, it is a cement that exhibits a characteristic that the amount of hexavalent chromium eluted increases when the admixture is not added, and an appropriate ORP of the admixture and its admixture are added under the condition that the amount of hexavalent chromium eluted increases. Since the amount is determined, it can be said that it is the most suitable cement for confirming the effect of suppressing the amount of hexavalent chromium eluted.

Although it is a small amount of test data, the uniaxial compressive strength of the stable-treated soil increases as the amount of cement added increases, and hexavalent chromium in the stable-treated soil is more likely to be immobilized (insolubilized), making it difficult for hexavalent chromium to elute. A tendency was also confirmed. This shows the same phenomenon as the elution of hexavalent chromium from well-cured solid concrete.

(5) Estimation of hexavalent chromium elution amount based on construction conditions From the above compounding test results, the estimated value of hexavalent chromium elution amount under the construction conditions (a) to (d) is obtained and shown below.

(A) When the flow value immediately after stirring and mixing is a low flow value and low strength: 0.401 mg / L
The compounding conditions are a water-cement ratio of 27% and a cement addition amount of 196 kg / m ³ .

(A) When the flow value immediately after stirring and mixing is a low flow value and high strength: 0.147 mg / L
The compounding conditions are a water-cement ratio of 21% and a cement addition amount of 353 kg / m ³ .

(C) When the flow value immediately after stirring and mixing is a high flow value and low strength: 0.416 mg / L
The compounding conditions are a water-cement ratio of 129% and a cement addition amount of 210 kg / m ³ .

(D) When the flow value immediately after stirring and mixing is a high flow value and high strength: 0.196 mg / L
Blending conditions: Water-cement ratio 74%, cement addition amount 407 kg / m ³ .

The procedure for obtaining each of the above estimated values will be omitted.

The elution amount is estimated to be 0.147 mg / L even under the construction condition (a) where the hexavalent chromium elution amount is the smallest, and the hexavalent chromium elution amount is the environmental standard value (0.05 mg / L) under all construction conditions. The result exceeded (L or less).

2. 2. Mixing test to add admixture In the above-mentioned compounding test, it was confirmed that hexavalent chromium exceeding the environmental standard value was eluted from the stable treated soil prepared under all the construction conditions (a) to (d). The ground improvement method in which the amount of hexavalent chromium eluted is less than the environmental standard value or the lower limit of quantification (hereinafter, both are collectively referred to as "control standard value") will be described below. The environmental standard value for the amount of hexavalent chromium eluted from the stable treated soil is defined as "0.05 mg or less per 1 L of test solution".
In addition, in this hexavalent chromium dissolution test, a test solution is prepared (dissolution method) by the method listed in Notification No. 77 of the Environment Agency, and the quantification method (measurement method) is the diphenylcarbazide absorptiometry (JIS K0102). It was done in 65.2.1) of. The lower limit of quantification (measurement limit) by the quantification method is 0.02 mg / L, and a value less than the lower limit of quantification is treated as non-detection (ND).
The quantification method is not limited to the diphenylcarbazide absorptiometry, and may be any of the frame atomic absorption method, the electrically heated atomic absorption method, the ICP issuance analysis method, the ICP mass spectrometry method, and the flow analysis method. good.

In the present invention, as described above, unstable hexavalent chromium in cement is reduced to stable chromium other than hexavalent by using an admixture with a low ORP (generally, it means about 50 mV or less). It is a technology to do. That is, the reducing effect (effect of suppressing the amount of hexavalent chromium elution) differs depending on the ORP of the admixture and the amount of the admixture added, and a method for obtaining an appropriate ORP as an admixture and the amount of the admixture added and the method thereof. It proposes a construction method.

In the compounding test to add the admixture described below, the admixture ORP and admixture required to suppress the amount of hexavalent chromium eluted from the stable treated soil to the control standard value (environmental standard value or less than the lower limit of quantification). The method of determining the addition amount will be described.

In addition, for the preparation of stable-treated soil to be carried out to obtain the water-cement ratio of cement slurry and the amount of cement added to satisfy the above-mentioned construction conditions (flow value immediately after stirring and mixing and target strength of stable-treated soil). In order to distinguish between the compounding test conducted in advance and the compounding test in which an admixture is added to confirm the effect of suppressing the amount of hexavalent chromium eluted from the stabilized soil described below, the stabilized soil is used. The compounding test performed prior to the formation is called a "cement compounding test", and the compounding test for adding the admixture is called an "admixture compounding test".

1) Admixture compounding test In the above-mentioned cement compounding test, the cement addition condition was 150% of water-cement ratio and cement addition than the estimated elution amount of hexavalent chromium (0.416 mg / L) under the construction condition (c). The amount of hexavalent chromium eluted (0.425 mg / L in Table 1) is larger when the amount is 200 kg / m ³ , and it is larger than the estimated amount of hexavalent chromium (0.147 mg / L) under the construction condition (a). However, the hexavalent chromium elution amount (0.110 mg / L in Table 1) was smaller when the cement addition conditions were 30% water-cement ratio and the cement addition amount was 400 kg / m ³ . Therefore, the admixture compounding test shall be conducted under the compounding conditions shown below.

The conditions of the admixture compounding test below are a compounding test for the purpose of confirming the effect of the admixture on suppressing the amount of hexavalent chromium eluted, and the amount of hexavalent chromium eluted from the stable treated soil shown below is the largest. It shall be carried out under "blending condition 1" and "blending condition 2" in which the elution amount is relatively small.

-Mixing condition 1 [When the amount of hexavalent chromium eluted (0.425 mg / L) is the largest]
The water-cement ratio is 150% and the cement addition amount is 200 kg / m ³ .

-Mixing condition 2 [When the amount of hexavalent chromium eluted (0.110 mg / L) is relatively small]
The water-cement ratio is 30% and the cement addition amount is 400 kg / m ³ .

The admixture used in the following admixture compounding test is a mixture of an admixture having an ORP of -533 mV, -230 mV, and -127 mV, and an admixture having an ORP of -127 mV and pure water. The ORP is performed with an admixture adjusted to approximately -100 mV to 100 mV. As the admixture, one produced by using calcium polysulfide and a thiosulfate compound as main materials was used.

For the admixture slurry in which the admixture is added to the cement slurry, the water-cement ratio of the cement slurry is set to 150% (in the case of "blending condition 1") or 30% (in the case of "blending condition 2"), and then. 0.1%, 0.3%, 0.5%, 1.0%, 5.0%, 10.0% equivalent amount per weight of the cement (200 kg / m ³ or 400 kg / m ³ ) , Addition / kneading admixture slurry.

A specimen was prepared from stable-treated soil in which the admixture slurry and sample soil were mixed by stirring, and the uniaxial compression test and hexavalent chromium elution test after standard curing of the specimen were carried out in this admixture combination test. It is a test item to be carried out.

The following describes an example of weighing up to the preparation of the specimen (when the cement addition amount is 200 kg / m ³ , the water-cement ratio is 150%, and the admixture addition amount is 1.0%), and the procedure thereof.

(1) Specifications of admixture combination test ・ Wet density of sample soil: 1.369 g / cm ³
・ Cement addition amount: 200kg / m ³
・ Water-cement ratio: 150%
・ Admixture added: 1.0% of cement added
-The ORP of the admixture: -533 mV, -230 mV, -127 mV, -77 mV, -44 mV, 0 mV, 15 mV, 87 mV.

(2) Measurement of sample soil ・ Sample soil: 800 g
800 ÷ 1.369 = 584.4 cm ³ = 5.844 ^-4 m ³
This means that 5.844 ^-4 m ³ of sample soil was weighed by weighing 800 g.

-Cement: 5.844 ^-4 x (200 x 1000) = 116.88 g
This means that 200 kg of cement was added per 1 m ^{3 of} sample soil.

-Admixture: 116.88 x (1/100) ≒ 1.169 g
This means that 1.0% of the admixture was added to the cement per 200 kg / ?.

-Total solution amount: 116.88 x (150/100) = 175.32 g
The total amount of the aqueous solution to which the kneading water and the admixture are added is referred to as the total solution amount, and here, the ratio (150%) of the cement to the total solution amount is defined as the water-cement ratio. The amount of kneading water shall be the amount of water shown below.

-Kneading water: Total solution amount-Admixture amount = 175.32-1.169 = 174.15 g
The water-cement ratio is, for example, 150%.

(3) Preparation of specimen The preparation procedure is shown below.

Step 1: The above cement (116.88 g) and kneaded water (174.15) are kneaded to prepare a cement slurry.

Step 2: An admixture (1.169 g) is added to and kneaded with the cement slurry to prepare an admixture slurry.

Step 3: The admixture slurry is added to the sample soil (800 g) and mixed by stirring to prepare a stable treated soil to which the admixture is added.

Step 4: A specimen is prepared from the above-mentioned stabilized soil.

Step 5: After standard curing of the above specimen for 7 days, uniaxial compression test (JIS A 1216) and hexavalent chromium elution test (preparation of test solution by the method listed in Notification No. 77 of the Environmental Agency and 65.2.1 of JIS K 0102) Quantitatively).

2) Summary of admixture compounding test The results of the uniaxial compression test and hexavalent chromium elution test are shown in Table 2 (blending condition 1) and Table 3 (blending condition 2). 5 and 6 are created based on Tables 2 and 3.

FIG. 5 is a diagram showing the relationship between the ORP of the admixture and the amount of hexavalent chromium eluted after the admixture is added by an approximate curve for each admixture addition amount under the compounding condition 1 (when the hexavalent chromium elution amount is the largest). ing. FIG. 6 is a diagram showing the relationship between the ORP of the admixture and the amount of hexavalent chromium eluted after the admixture is added by an approximate curve for each admixture addition amount under the compounding condition 2 (when the hexavalent chromium elution amount is relatively small). It has become.

From FIGS. 5 and 6, the ORP when the amount of hexavalent chromium eluted after the addition of the admixture becomes 0.05 mg / L and 0.02 mg / L is determined for each admixture addition amount (0.1% to 10%). Ask for. In FIGS. 5 and 6, the hexavalent chromium elution amount of 0.05 mg / L is shown by the solid horizontal line, and the hexavalent chromium elution amount of 0.02 mg / L is shown by the broken horizontal line. There is. The reading results are shown in Table 4.

Here, in Table 4, the columns of the admixture addition amount of 0.1% and 0.3% under the compounding condition 1 are blank because the hexavalent chromium elution amount in both addition amounts reaches the lower limit of quantification. Because it is not.

7 and 8 are created based on Table 4. In addition, each of FIGS. 7 and 8 is a diagram in which each plot point (reading value) is displayed by an approximate curve.

FIG. 7 shows the case where the amount of hexavalent chromium eluted is 8.5 times the environmental standard value (hexavalent chromium elution amount 0.425 mg / L), which is the largest (blending condition 1), and the elution amount is 2. When the amount is relatively small (hexavalent chromium elution amount 0.110 mg / L) (blending condition 2), the amount of admixture ORP and admixture added that suppresses the amount of hexavalent chromium elution to 0.05 mg / L. It is a graph showing the relationship with. Further, FIG. 8 is a graph showing the relationship between the amount of the admixture added that suppresses the above-mentioned amount of hexavalent chromium elution to 0.02 mg / L and the ORP of the admixture.

3) Features seen from the admixture compounding test (1) Relationship between the redox potential of the admixture and the amount of hexavalent chromium eluted after the admixture is added (Figs. 5 and 6)
The features seen from the approximate curves of FIG. 5 (blending condition 1) and FIG. 6 (blending condition 2) are shown below.

-The ORP of the admixture and the amount of hexavalent chromium eluted after the admixture is added are proportional. That is, as the ORP of the admixture increases, the amount of hexavalent chromium eluted also increases. This means that the effect of suppressing the amount of hexavalent chromium eluted decreases as the ORP of the admixture increases.

-The amount of admixture added and the amount of hexavalent chromium eluted after the admixture is added are inversely proportional. That is, when the ORP of the admixture is at the same level (for example, when the ORP of the admixture is -127 mV), the amount of hexavalent chromium eluted decreases as the amount of admixture added increases, and hexavalent chromium elutes. The effect of suppressing the amount is increasing.

-That is, when trying to suppress the amount of hexavalent chromium eluted from stable soil to the same level of control standard value (for example, environmental standard value), if the admixture has a low ORP, the control standard value should be reduced with a small amount of admixture added. However, it means that a large amount of admixture must be added when trying to meet the control standard value with an admixture having a high ORP.

(2) Relationship between the amount of admixture added and the ORP of the admixture (FIGS. 7 and 8)
FIG. 7 shows the relationship between the amount of admixture added that suppresses the amount of hexavalent chromium eluted from the stable treated soil to 0.05 mg / L and the ORP of the admixture by an approximate curve, and FIG. 8 shows the amount of hexavalent chromium eluted. The figure shows the relationship between the amount of the admixture added and the ORP of the admixture, which suppresses the amount to 0.02 mg / L, by an approximate curve.

From FIG. 7, the amount of admixture added and the ORP of the admixture when the stable treated soil (blending condition 1) having a hexavalent chromium elution amount of 0.425 mg / L is suppressed to the environmental standard value (0.05 mg / L or less). The relationship with is described. The values in parentheses indicate the case of stable treated soil (blending condition 2) in which the amount of hexavalent chromium eluted is 0.110 mg / L.

-When the amount of admixture added is 0.1%: The ORP of the admixture is -280 mV or less (-70 mV or less).
-When the amount of admixture added is 0.3%: The ORP of the admixture is -175 mV or less (-25 mV or less).
-When the amount of admixture added is 0.5%: The ORP of the admixture is -120 mV or less (5 mV or less).
-When the amount of admixture added is 1.0%: The ORP of the admixture is -65 mV or less (25 mV or less).
-When the amount of admixture added is 3.0%: The ORP of the admixture is 10 mV or less (45 mV or less).
-When the amount of admixture added is 5.0%: The ORP of the admixture is 25 mV or less (50 mV or less).
-When the amount of admixture added is 10.0%: The ORP of the admixture is 30 mV or less (50 mV or less).
FIG. 8 describes the relationship between the amount of the admixture added and the ORP of the admixture when the environmental standard value is suppressed to less than the lower limit of quantification (not detected).

-When the amount of admixture added is 0.1%: The OR of the admixture is less than P-545 mV (less than -160 mV).
-When the amount of admixture added is 0.3%: The ORP of the admixture is less than -210 mV (less than -75 mV).
-When the amount of admixture added is 0.5%: The ORP of the admixture is less than -130 mV (less than -40 mV).
-When the amount of admixture added is 1.0%: The ORP of the admixture is less than -120 mV (less than -5 mV).
-When the amount of admixture added is 3.0%: The ORP of the admixture is less than -15 mV (less than 20 mV).
-When the amount of admixture added is 5.0%: The ORP of the admixture is less than 10 mV (less than 25 mV).
-When the amount of admixture added is 10.0%: The ORP of the admixture is less than 15 mV (less than 25 mV).
That is, in order to suppress the stable treated soil having a hexavalent chromium elution amount of 0.425 mg / L to the environmental standard value, if the ORP of the admixture is 280 mV or less, the admixture addition amount is cleared at 0.1%. It indicates that the amount of the admixture added when the ORP of the admixture is raised to 10 mV or less must be increased to 3.0%.

Further, in order to suppress the elution amount of hexavalent chromium to less than the lower limit of quantification, if the ORP of the admixture is -545 mV or less, the admixture addition amount is cleared at 0.1%, and the ORP of the admixture is adjusted. It indicates that the amount of admixture added when the amount is increased to less than -15 mV must be increased to 3.0%.

(3) Characteristics seen from the relationship between the ORP of the admixture and the amount of the admixture added From the above, the characteristics seen from the relationship between the ORP of the admixture and the amount of the admixture added will be described.

-Upper limit of the amount of admixture added In both FIGS. 7 and 8, the relationship between the ORP of the admixture and the amount of admixture added is proportional to the amount of admixture added up to 5.0%. When the amount exceeds 5.0%, the proportional relationship disappears. In other words, until the admixture addition amount is up to 5.0%, even if the ORP of the admixture rises, the control standard value is cleared by increasing the admixture addition amount, but when the admixture addition amount exceeds 5%, the control standard is cleared. There is no change in the ORP of the admixture that clears the value. For example, in the case of compounding condition 2, the ORP of the admixture that clears the control standard value is the same value when the admixture addition amount is 5.0% and 10.0%.

From the above, it can be said that the amount equivalent to 5.0% per cement weight is the upper limit of the amount of admixture added that has the effect of suppressing the amount of hexavalent chromium eluted.

-Upper limit of ORP of admixture The upper limit of ORP of admixture means the upper limit of reducing power (effect of suppressing hexavalent chromium elution amount) as an admixture, in other words, of the above-mentioned admixture addition amount. It can be said that the upper limit value (5.0%) is the upper limit of the ORP of the admixture. However, the upper limit is the amount of hexavalent chromium eluted from the stable-treated soil, the case where the amount of hexavalent chromium eluted is suppressed to the environmental standard value (0.05 mg / L or less), and the lower limit of quantification (0.02 mg / L). The ORP differs depending on the case of suppressing to less than L).

The upper limit of the ORP of the admixture under each condition is shown below.
・ When suppressing the amount of hexavalent chromium eluted after adding the admixture to the environmental standard value ・ When the amount of hexavalent chromium eluted from the stable treated soil is large, the ORP is 25 mV.
・ ORP is 50 mV when the amount of hexavalent chromium eluted from stable soil is relatively small.
・ When the amount of hexavalent chromium eluted after the addition of the admixture is suppressed to less than the lower limit of quantification ・ When the amount of hexavalent chromium eluted from the stable treated soil is large, the ORP is 10 mV.
・ ORP is 25 mV when the amount of hexavalent chromium eluted from stable soil is relatively small.
-Effect of suppressing hexavalent chromium elution amount by admixture ORP and admixture addition amount As mentioned above, the requirement to suppress the amount of hexavalent chromium elution from stable treated soil to the control standard value is required by the admixture ORP. The amount of admixture required to meet is different. That is, it can be said that a large amount of an admixture having a high ORP may be added, or a small amount of an admixture having a low ORP may be added. The method for determining the ORP of the admixture and the amount of the admixture added will be described below.

4) Method for determining the ORP of the admixture and the amount of admixture added For the admixture required to suppress the amount of hexavalent chromium eluted from the stable-treated soil to less than the environmental standard value or the lower limit of quantification (both are the control standard values). The addition conditions (hereinafter referred to as "appropriate addition conditions of the admixture") differ depending on the combination of the ORP of the admixture and the addition amount of the admixture, and there are various choices.

In other words, the ORP of the admixture and the amount of the admixture added, which are the appropriate addition conditions of the admixture, are the admixtures necessary to suppress the amount of hexavalent chromium eluted from the stable-treated soil until it reaches the control standard value. The ORP and the amount of the admixture added are combined, and the method for determining the ORP of the admixture and the amount of the admixture added will be described below.

In the following examples, the amount of hexavalent chromium eluted from stable-treated soil (0.425 mg / L) under compounding condition 1 (cement addition condition is 150% water-cement ratio, cement addition amount 200 kg / m ³ ) is an environmental standard. An example of suppressing the value (0.05 mg / L or less) will be described.

The ORP of the admixture used in the admixture compounding test should be 50 mV or less as a guide, but considering that the amount of hexavalent chromium eluted from the stable treated soil is 0.425 mg / L, past results It is described as assuming that an admixture having an ORP of -127 mV is used.

(1) Method of determining the amount of admixture added in the admixture compounding test using only one point This method is based on past actual data and empirical values, and one point (pinpoint) is that an admixture with an ORP of -127 mV is used. ) Only the admixture combination test (hexavalent chromium dissolution test) will be described to determine the amount of admixture added.

The procedure is shown below.

Step 1: Estimate the amount of admixture added that is considered necessary to suppress to the environmental standard value The amount of hexavalent chromium eluted from the stabilized soil (specimen) is 0.425 mg / L, which is the environmental standard value of 8. Since it is 5 times (0.425 / 0.05), the amount of admixture added that seems to suppress the amount of hexavalent chromium eluted to the environmental standard value (0.05 mg / L) is at least 0. It is estimated that 5% or more, and in some cases up to about 0.7%, is necessary.

Step 2: Conducting an admixture compounding test based on the provisional addition amount After setting the maximum addition amount (0.7%) estimated in step 1 as the provisional addition amount, the admixture compounding test based on the provisional addition amount (only one point) To determine the amount of hexavalent chromium eluted after adding the admixture.

Step 3: Determining the ORP of the admixture and the amount of admixture added After confirming that the amount of hexavalent chromium eluted after adding the admixture has cleared the environmental standard value, the provisional addition amount (0.7%) Is the amount of the admixture added to the admixture slurry.

The ORP (-127 mV) of the admixture used in the admixture compounding test and the provisional addition amount (0.7%) are determined as the ORP of the admixture used for ground improvement and the admixture addition amount.

The amount of admixture added determined by this method is shown below.

Admixture addition amount = 200 x (0.7 ÷ 100) = 1.4 kg / m ³
The amount of admixture added determined by this method is 0.7% per cement addition amount (per cement weight), and 1.4 kg of admixture is added per 1 m ^{3 of} raw soil.

In this method, since the admixture addition amount is determined by the admixture compounding test of only one point as described above, the admixture addition amount must be inevitably set on the safe side (large amount addition), and as a result, the admixture addition amount must be set. It will force an excessive amount of admixture added.

(2) Method of determining the amount of admixture added in a multi-point admixture combination test This method is a procedure for estimating the amount of admixture added that is considered to be an environmental standard value from past actual data and empirical values. (Procedure 1) is the same as the above method, but an admixture compounding test is performed at a plurality of points including the estimated addition amount (provisional addition amount), and an appropriate admixture addition amount is determined in the admixture compounding test. There is a feature in what you want.

The procedure is shown below.

Step 1: Estimate the amount of admixture added to suppress the environmental standard value In the same way as above, consider the amount of hexavalent chromium eluted from the stable treated soil and use an admixture with an ORP of -127 mV. It is estimated that the amount of admixture added to suppress the amount of hexavalent chromium eluted from the stabilized soil to the environmental standard value will be 0.5% to 0.7%.

Step 2: Implementation of admixture compounding test at multiple points Addition of admixture at multiple points of 0.3%, 0.5%, 0.7% including the estimated addition amount (0.5% to 0.7%) The admixture compounding test is carried out based on the amount, and the amount of hexavalent chromium eluted after the admixture is added at the plurality of points is determined.

Step 3: Creating a correlation between the amount of admixture added and the amount of hexavalent chromium eluted after adding the admixture From the amount of hexavalent chromium eluted after adding the admixture (shown below) obtained in step 2, the amount of admixture added FIG. 9 is created as a correlation diagram between this and the amount of hexavalent chromium eluted after the addition of the admixture.
-Hexavalent chromium elution amount when the admixture addition amount is 0.3%: 0.077 mg / L
-Hexavalent chromium elution amount when the admixture addition amount is 0.5%: 0.036 mg / L
-Hexavalent chromium elution amount when the admixture addition amount is 0.7%: 0.027 mg / L (estimated elution amount)
The hexavalent chromium elution amount is a numerical value quoting the test results in Table 2, but the hexavalent chromium elution test with an admixture addition amount of 0.7% has not been carried out in the above admixture combination test. Therefore, the hexavalent chromium elution amount (0.027 mg / L) when the admixture addition amount was 0.7% was estimated from the test results of the admixture addition amount of 0.5% and 1.0%.

Step 4: Reading the admixture addition amount From the correlation diagram of FIG. 9, the admixture addition amount (0.43%) at the point where the hexavalent chromium elution amount is 0.05 mg / L is read and obtained. The amount of the admixture added here is not limited to the correlation diagram, and may be obtained by a correlation equation.

Step 5: Determination of admixture addition amount The admixture addition amount of 0.43% obtained in step 4 is determined as the admixture addition amount of the admixture slurry. The amount of the admixture added (0.43%) is rounded up to 0.5% in order to suppress the variation in the field and the amount of hexavalent chromium eluted from the stable-treated soil as much as possible. The amount of hexavalent chromium eluted from is an environmental standard value (0.05 mg / L or less).

The ORP (-127 mV) of the admixture used in the admixture compounding test and the admixture addition amount (0.5%) obtained in the admixture compounding test using the plurality of points are used as the ORP of the admixture used for ground improvement. Determined as the amount of admixture added.

The amount of admixture added by this method is shown below.
Admixture addition amount = 200 x (0.5 ÷ 100) = 1.0 kg / m ³
1.0 kg of admixture will be added per 1 m ^{3 of} raw soil.

In this method, the amount of the admixture added (0.5%) is more economical than the amount of the admixture added (0.7%) obtained in the above-mentioned only one admixture compounding test. However, it is required to perform an admixture combination test with a plurality of points (2 points or more preferably 3 points) each time.

(3) Method for determining the ORP of the admixture The ORP of the admixture and the amount of the admixture added required to suppress the amount of hexavalent chromium eluted from the stable-treated soil to the control standard value are large amounts of the admixture having a high ORP. Although it is possible to add it to, it can be said that it is economically desirable to add a small amount of an admixture having a low ORP. However, considering the variation in construction, it is desirable that the amount of admixture added is at least 0.1% or more per weight of cement (the lower limit of the amount of admixture added when the variation in construction is taken into consideration).

On the other hand, the upper limit of the amount of admixture added was set to 5.0% per weight of cement in the above example, but when the amount of hexavalent chromium eluted from the stable-treated soil is large (blending condition 1), the admixture It has been confirmed in the previous admixture combination test that the addition amount is only about 5 mV in the ORP of the admixture at 5.0% and 3.0%. From this result, even if the ORP of the admixture is almost the same, considering that the admixture addition amount is reduced to 3%, the admixture addition amount should be 3.0% in consideration of economical construction. It is desirable to set the upper limit (the upper limit of the amount of admixture added when economic efficiency is taken into consideration). Further, by slightly lowering the ORP of the admixture, the control standard value can be cleared even if the upper limit of the admixture addition amount is 1.0%, so it is economical to limit the admixture addition amount to 1.0%. It can be said that it is a target and more desirable.

The relationship between the ORP of the admixture and the amount of admixture added required to suppress the amount of hexavalent chromium eluted from the stable-treated soil to the control standard value is closely related, and the lower and upper limits of the above-mentioned admixture addition amount are set. Based on this, the method for determining the ORP of the admixture will be described below.

In the above-mentioned admixture compounding test, the admixture compounding test was conducted after estimating the ORP of the admixture used in the admixture compounding test in advance from the amount of hexavalent chromium eluted from the stable-treated soil. Depending on the ORP of, the amount of admixture added may be excessive or too small. In order to effectively implement this technology, it is important to select an appropriate admixture ORP, and the guideline for obtaining an appropriate admixture ORP will be described below.

-When lowering the ORP of the admixture in the admixture combination test In the admixture combination test, the admixture addition amount exceeds the upper limit of 3.0% (more preferably 1.0%) and the admixture addition amount is excessive. If such a result is obtained, the ORP of the admixture used in the admixture combination test is again subjected to the admixture combination test with an admixture having a lower ORP, and the admixture addition amount is recalculated. For example, in an admixture compounding test using an admixture of -127 mV, when the admixture addition amount exceeds 3.0% (more preferably 1.0%), the ORP is lower than -127 mV. The mixture is changed to an admixture (for example, an admixture of -230 mV or -533 mV), and the admixture compounding test is performed again to determine the amount of the admixture added.

This method is a method in which the upper limit (for example, 3%) of the admixture addition amount is determined in advance in consideration of the soil properties of the raw soil, and when the admixture addition amount exceeds the upper limit. Is a method in which the ORP of the admixture is lowered and then the admixture addition amount is recalculated in the admixture compounding test. By the way, as a guideline for the upper limit, for example, in the case of cohesive soil where small clay lumps (lumps) are likely to occur during stirring and mixing, the upper limit is set to 3.0%, and sandy soil and the like where good stirring and mixing can be easily obtained. In the case of targeting, the upper limit of the amount of admixture added shall be 1.0% to 2.0%, and it shall be decided in consideration of workability.

・ When raising the ORP of the admixture during the admixture compounding test The lower limit of the admixture addition amount is 0.1% of the admixture addition amount (considering the construction variation), considering the influence of quality variation in the construction work. (Lower limit of the amount of admixture added) is a guide. Therefore, if the amount of the admixture added is less than 0.1% in the admixture combination test, the admixture is mixed again with an admixture showing an ORP higher than that of the admixture used in the admixture combination test. Perform a drug combination test and recalculate the amount of admixture added. For example, in an admixture compounding test using an admixture having an ORP of -127 mV, when the admixture addition amount is less than 0.1%, an admixture having an ORP higher than -127 mV (for example, ORP is -77 mV). The admixture will be changed to (1), and the admixture combination test will be conducted again to recalculate the admixture addition amount.

It is desirable to change the lower limit of the admixture addition amount according to the soil properties of the raw soil as described above. For example, in the case of cohesive soil, 0.2% to 0.3% of the admixture addition amount should be changed. The lower limit should be 0.1% as the lower limit of the amount of admixture added when sandy soil is used.

The ORP of the admixture can be selected from not only the -533 mV, -230 mV, and -127 mV used in this admixture compounding test but also the admixture showing the ORP in the range of approximately -550 mV to 50 mV. Various choices can be made for the combination of the ORP and the amount of the admixture added. However, it is desirable to select the ORP of the admixture so that the amount of the admixture added is in the range of 0.1% to 3.0% per weight of the cement added to the stable-treated soil. In the ground improvement method for suppressing the amount of hexavalent chromium elution while paying attention to economic efficiency and workability, an appropriate combination of the admixture ORP and the admixture addition amount is important and indispensable.

In the example of the admixture compounding test, the amount of hexavalent chromium eluted from the stable-treated soil is considered to be necessary to suppress the amount of hexavalent chromium eluted to the environmental standard value (control standard value). Although the ORP of the agent and the amount of admixture added were estimated, even if only the soil properties of the raw soil (soil classification, water content ratio, wet density, etc.) are known, it will be eluted from the stable-treated soil based on many past achievements. It is possible to estimate the amount of hexavalent chromium eluted and the approximate amount of admixture ORP and admixture added to suppress the amount of hexavalent chromium eluted to the control standard value.

(4) Timing of admixture compounding test for adding admixture The admixture compounding test for determining the appropriate addition conditions for the admixture is a cement compounding test conducted prior to the preparation of stable-treated soil, and the cement compounding test is used. A case where the hexavalent chromium elution amount exceeding the control standard value is detected from the sample (stable treated soil) (timing 1) and a case where the cement compounding test and the admixture compounding test are performed in parallel (timing 2). ) Is roughly divided into two timings.

[Timing 1]
The case of timing 1 refers to a case where the admixture compounding test is performed after hexavalent chromium exceeding the control standard value is detected from the specimen at the time of the cement compounding test.

In the previous example, hexavalent chromium exceeding the control standard value was detected from the specimen under the compounding conditions (water-cement ratio 129%, cement addition amount 210 kg / m ³ ) determined in the construction condition (c), for example. After confirming that (detection amount 0.416 mg / L), the above-mentioned admixture combination test will be performed.

More specifically, the admixture is added to a cement slurry having a water-cement ratio of 120% at 0.7% per cement weight (for example, 210 kg / m ³ ) (when performing an admixture compounding test using only one point) or 0.3. Hexavalent chromium in a specimen (stable treated soil) in which the admixture slurry to which%, 0.5%, and 0.7% (when performing an admixture compounding test using multiple points) and the sample soil are stirred and mixed. A dissolution test will be conducted.

[Timing 2]
The case of timing 2 refers to a case where the cement compounding test and the admixture compounding test are performed in parallel at the time of the cement compounding test described above. This case can be said to be an effective means when there is not enough time to start the construction (creation of stable soil) and it is difficult in terms of process to carry out at the above-mentioned [Timing 1].

Specifically, based on the soil properties of the raw soil, the type of cement, the amount of addition, etc. at the time of the cement compounding test, the amount of hexavalent chromium eluted from the stable treated soil and its hexavalent The ORP of the admixture and the amount of the admixture added, which are thought to suppress the amount of chromium elution to the control standard value, are estimated, and the cement compounding test and the admixture compounding test are performed in parallel.

Therefore, in the admixture compounding test, the admixture addition amount is determined by setting the admixture addition amount to only one point (pinpoint) based on the admixture ORP and the admixture addition amount that are considered to be more reliably suppressed to the control standard value. There are many.

In the previous embodiment, 0.7% per cement weight (per cement addition amount) under all 16 addition conditions (4 cement addition amounts x 4 water-cement ratios) shown in Table 1. The admixture compounding test in which a considerable amount of admixture is added and the cement compounding test will be conducted in parallel. Inevitably, the ORP of the admixture used in this admixture compounding test tends to use an admixture having an ORP lower than -127 mV used in the above-mentioned admixture compounding test (for example, -230 mV).

In the case of [Timing 1], even if a plurality of points of the admixture combination test are performed, the admixture combination test is 2 to 3 points, but in the case of [Timing 2], 16 points or at least 9 points ( This is an admixture compounding test (3 types of cement addition x 3 types of water-cement ratio). Therefore, it can be said that the admixture combination test at a plurality of points according to the above [timing 1] is rational and desirable.

5) Uniaxial compressive strength of stable treated soil applicable to this technology and TF value immediately after stirring and mixing In the admixture compounding test, the case of compounding condition 1 with the largest amount of hexavalent chromium elution (hexavalent chromium elution amount 0.425 mg) / L) and the case of compounding condition 2 in which the elution amount thereof was relatively small (hexavalent chromium elution amount 0.110 mg / L) were carried out in two cases.

Under compounding condition 1, a test piece was prepared from a stable-treated soil having a TF value of 156 mm immediately after stirring and mixing. As a result, the uniaxial compressive strength of the test piece was 31 kN / m ² , and hexavalent chromium was eluted from the stable-treated soil. The result was that the amount was 0.425 mg / L. Further, under compounding condition 2, a test piece was prepared from a stable-treated soil having a TF value of 115 mm immediately after stirring and mixing, and as a result, the uniaxial compressive strength of the test piece was 3850 kN / m ² , and hexavalent from the stable-treated soil. The result was that the amount of chromium eluted was 0.110 mg / L.

From these results, the quality control range of the uniaxial compressive strength of the stable treated soil applicable to this technology and the TF value (flow value) immediately after stirring and mixing is shown below.

(1) Applicable range of uniaxial compressive strength of stable-treated soil Under compounding condition 1, the suppressive effect of admixture was confirmed under the condition that the uniaxial compressive strength of the specimen was 31 kN / m ² for stable-treated soil. There is. It is clear that the uniaxial compressive strength of the stable-treated soil is inversely proportional to the amount of hexavalent chromium eluted, and if the stable-treated soil is 50 kN / m ² or more, which is larger than the uniaxial compressive strength, the stable-treated soil of 31 kN / m ² described above is used. It can be inferred that the amount of hexavalent chromium eluted is smaller than that. Therefore, it can be said that this technique can be applied to stable treated soil that requires a uniaxial compressive strength of 50 kN / m ² or more.

(2) Scope of application of the flow value immediately after stirring and mixing As described above, the TF value (flow value) immediately after stirring and mixing is proportional to the amount of hexavalent chromium eluted from the stable-treated soil. In the previous test, the effect of suppressing the amount of hexavalent chromium eluted from the stable-treated soil with a TF value of 156 mm was confirmed under compounding condition 1. Therefore, it can be said that this technology can be applied to stable treated soil whose TF value (flow value) is controlled within 155 mm.

The means for controlling the flow value immediately after stirring and mixing is not limited to the table flow test described in this embodiment, and for example, a numerical value using a slump cone, a flow cone, a viscometer, or the like may be used as an index.

In the admixture compounding test so far, admixtures produced using calcium polysulfide (lime sulfur mixture) or thiosulfate compound as the main material were used, but if the ORP of the admixture is approximately 50 mV or less, the main raw material is used. It is not limited.

Hexavalent chromium is produced by baking trivalent chromium at a high temperature, but it is said that it becomes trivalent chromium when it is reduced by a reducing agent. The technique for suppressing the amount of hexavalent chromium eluted is a technique for reducing hexavalent chromium contained in cement with a reducing agent having a low ORP. Therefore, it goes without saying that the admixture described in the present technology has a stronger reducing power (suppressing power) as the ORP of the admixture is lower, and becomes a more desirable admixture (reducing agent). Further, the admixture may be a liquid or a solid (powder).

The main raw materials as an admixture are not only the above-mentioned calcium polysulfide or thiosulfate compound, but also potassium sulfide, iron (II) sulfate, tin (II) chloride, oxalic acid, calcium thioglycolate, etc. As long as the ORP as is 50 mV or less, any of them may be used.

Further, in this embodiment, cement as a solidifying material for general soft soil was used, but the type of cement is not limited, and ordinary Portland cement, blast furnace cement, cement-based solidifying material for special soil, early-strength cement, and fly are used. It is possible to suppress the elution amount of hexavalent chromium in any cement such as ash cement and lime-based solidifying material containing cement.

3. 3. Construction Examples The following describes construction examples when the water-cement ratio is 150%, the cement addition amount is 200 kg / m ³ , and the admixture addition amount is 1%. In this construction example, the cement and the kneading water are kneaded by a batch type mixer, and 800 kg of cement is weighed and kneaded per batch to produce a cement slurry.

1) Weighing weight per batch Cement: 800kg
Admixture: 800 x (1/100) = 8 kg
Kneading water: Total solution amount-volume of admixture = {800 x (150/100)}-(8 / 1.25) ≒ 1194 kg
In order to prevent the addition of the admixture from affecting the expression strength (uniaxial compressive strength) of the stable treated soil, it is desirable to include the admixture in the kneading water (total solution amount) to determine the water-cement ratio. .. At that time, the specific gravity of the admixture is 1.25 g / cm ³ , and the volume of the admixture is the value obtained by dividing the admixture weight by the specific gravity.

2) Construction procedure using the admixture slurry The construction procedure using the admixture slurry is shown in the flowchart of FIG.

Step 1: Weigh 1194 kg of kneaded water and put it into the mixer section.

Step 2: Weigh 800 kg of cement and put it into the mixer section. In the mixer section, the kneading blades (stirring blades) are constantly rotating, and the kneading water and cement are kneaded to form a cement slurry as the cement is added.

Step 3: Weigh 8 kg of the admixture and put it into the mixer section. The total amount of the solution after adding the admixture is 1200 liters, and the water-cement ratio at this time is 150%. The calculation formula is shown below.

Total solution amount: 1194+ (8 / 1.25) ≒ 1200 liters Water-cement ratio: (total solution amount / per weight of cement) × 100 = (1200/800) × 100 = 150%
Step 4: Kneading water, cement and an admixture are kneaded in a mixer section for a predetermined time to produce an admixture slurry, which is stored in an agitator tank.
Step 5: An admixture slurry is produced by repeating steps 1 to 4.
Step 6: The admixture slurry is pumped to the ground improvement machine by a grout pump.
Step 6: The admixture slurry and the raw soil (in-situ soil) are agitated and mixed by the above-mentioned ground improvement machine to prepare stable treated soil.

The admixture in step 3 may be weighed and added before the kneading water in step 1 is weighed and added. Further, the admixture slurry may be weighed and injected into the cement slurry in the middle of pumping in step 6, and the admixture and the cement slurry are mixed before the cement slurry is pumped and injected into the original ground (original soil). It should be done.

Further, the ground improvement machine used here may be any type such as a trencher type, a horizontal stirring type, a high pressure injection stirring type, and the like, and the ground improvement machine is not limited. For example, as a typical trencher type ground improvement machine, the one described in Japanese Patent Application Laid-Open No. 2005-307675 according to the present applicant can be mentioned.

Claims (9)

It is a ground improvement method that suppresses the amount of hexavalent chromium that elutes from the stable-treated soil that is created by stirring and mixing the cement slurry, which is a mixture of water and cement, and the raw soil.
An admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of 50 mV or less is produced in advance, and then the admixture slurry is discharged into the raw soil and stirred and mixed.
The redox potential (ORP) of the admixture and the admixture addition amount that suppress the hexavalent chromium elution amount to less than the environmental standard value (0.05 mg / L or less) or the lower limit of quantification (0.02 mg / L). A ground improvement method, which is determined by the compounding test of (a) or (b) shown below.
(A) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, the amount of hexavalent chromium eluted from the stable-treated soil is less than the environmental standard value or the lower limit of quantification. The oxidation-reduction potential (ORP) of the admixture that seems to be suppressed and the amount of admixture added are estimated.
After using the estimated amount of the admixture added as the provisional amount, an admixture compounding test was conducted in which the admixture was added to confirm the effect of suppressing the amount of hexavalent chromium eluted.
After confirming that the amount of hexavalent chromium eluted was suppressed to less than the environmental standard value or the lower limit of quantification, the redox potential (ORP) and provisional addition amount of the admixture used in the admixture compounding test were determined. It is determined as the redox potential (ORP) of the admixture used for ground improvement and the amount of admixture added.
(B) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, the amount of hexavalent chromium eluted from the stable-treated soil is less than the environmental standard value or the lower limit of quantification. The oxidation-reduction potential (ORP) of the admixture that seems to be suppressed and the amount of admixture added are estimated.
After using the estimated admixture addition amount as the provisional addition amount, an admixture compounding test was conducted using a plurality of admixture addition amounts including the provisional addition amount.
Based on the correlation between the amount of admixture added and the amount of hexavalent chromium eluted after addition of the admixture based on the results of the admixture compounding test, the amount of hexavalent chromium eluted is less than the environmental standard value or the lower limit of quantification. Ask for quantity,
The redox potential (ORP) of the admixture used in the admixture compounding test and the admixture addition amount are determined as the redox potential (ORP) of the admixture used for ground improvement and the admixture addition amount. The ground improvement method according to claim 1, wherein the admixture combination test is carried out under the conditions (c) or (d) shown below.
(C) After the hexavalent chromium elution amount exceeding the environmental standard value or the quantification lower limit value or more is detected from the specimen of the cement compounding test in the cement compounding test performed prior to the preparation of the stable treated soil. At the timing, the admixture combination test is performed.
(D) The admixture compounding test is also performed in parallel with the timing of performing the cement compounding test prior to the preparation of the stable treated soil. The admixture slurry has an oxidation-reduction potential (ORP) of the admixture of 10 mV or less in the range of 0.1% to 3.0% per weight of cement to which the admixture is added to the raw soil. The ground improvement method according to claim 1 or 2, wherein the amount of hexavalent chromium eluted from the stable-treated soil is suppressed to the environmental standard value by adding the soil. The admixture slurry has an oxidation-reduction potential (ORP) of the admixture of -15 mV or less and is in the range of 0.1% to 3.0% per weight of cement to which the admixture is added to the raw soil. The ground improvement method according to claim 1 or 2, wherein the amount of hexavalent chromium eluted from the stable-treated soil is suppressed to less than the lower limit of quantification. The stable-treated soil formed by stirring and mixing the admixture slurry and the raw soil is characterized in that the soil is required to have a uniaxial compressive strength of 50 kN / m ² or more. Item 3. The ground improvement method according to Item 3. The stable-treated soil formed by stirring and mixing the admixture slurry and the raw soil is characterized in that the quality control is performed so that the flow value immediately after the stirring and mixing is within 155 mm in terms of the table flow value. The ground improvement method according to claim 5. The ground according to any one of claims 1 to 6, wherein the admixture slurry is produced by weighing each of the water, cement and admixture and kneading them with a batch mixer. How to improve. Ground improvement that suppresses the amount of hexavalent chromium that elutes from the stable-treated soil that is created by stirring and mixing cement slurry, which is a mixture of water and cement, and the raw soil, to the environmental standard value (0.05 mg / L or less). It's a method
An admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of 10 mV or less is produced in advance, and then the admixture slurry is discharged into the raw soil and stirred and mixed.
In carrying out an admixture compounding test for determining the redox potential (ORP) of the admixture and the admixture addition amount that suppresses the hexavalent chromium elution amount to the environmental standard value.
The admixture compounding test is the timing after the hexavalent chromium elution amount exceeding the environmental standard value is detected in the specimen of the cement compounding test in the cement compounding test performed prior to the preparation of the stable-treated soil, or Performed in parallel with the timing of performing the cement compounding test.
In the admixture compounding test of (e) or (f) shown below, the amount of the admixture added was determined to be in the range of 0.1% to 3.0% per weight of the cement added to the raw soil. Ground improvement method characterized by doing.
(E) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, the mixture that seems to suppress the amount of hexavalent chromium eluted from the stable-treated soil to the environmental standard value. Estimate the oxidation-reduction potential (ORP) of the agent and the amount of admixture added,
After using the estimated amount of the admixture added as the provisional amount, an admixture compounding test was conducted in which the admixture was added to confirm the effect of suppressing the amount of hexavalent chromium eluted.
After confirming that the amount of hexavalent chromium eluted is suppressed to the environmental standard value, the redox potential (ORP) of the admixture used in the admixture combination test and the provisional addition amount are used for ground improvement. It is determined as the redox potential (ORP) of the agent and the amount of the admixture added.
(F) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, the mixture that seems to suppress the amount of hexavalent chromium eluted from the stable-treated soil to the environmental standard value. Estimate the oxidation-reduction potential (ORP) of the agent and the amount of admixture added,
After using the estimated admixture addition amount as the provisional addition amount, the admixture combination test was performed with a plurality of admixture addition amounts surrounding the provisional addition amount.
From the correlation between the amount of admixture added and the amount of hexavalent chromium eluted after the admixture was added based on the results of the admixture compounding test, the amount of admixture added at which the hexavalent chromium elution amount is an environmental standard value was obtained.
The redox potential (ORP) of the admixture used in the admixture compounding test and the admixture addition amount are determined as the redox potential (ORP) of the admixture used for ground improvement and the admixture addition amount. A ground improvement method that suppresses the amount of hexavalent chromium eluted from the stable-treated soil created by stirring and mixing the cement slurry, which is a mixture of water and cement, and the raw soil to less than the lower limit of quantification (0.02 mg). There,
An admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of -15 mV or less is produced in advance, and then the admixture slurry is discharged into the raw soil and stirred and mixed.
In carrying out an admixture compounding test for determining the redox potential (ORP) of an admixture that suppresses the elution amount of hexavalent chromium to less than the lower limit of quantification and the amount of admixture added.
The admixture compounding test is the timing after the hexavalent chromium elution amount equal to or higher than the lower limit of quantification is detected in the specimen of the cement compounding test in the cement compounding test performed prior to the preparation of the stable-treated soil. Perform in parallel with the timing of the cement compounding test.
In the admixture compounding test of (g) or (h) shown below, the amount of the admixture added was determined to be in the range of 0.1% to 3.0% per weight of the cement added to the raw soil. Ground improvement method characterized by doing.
(G) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, it is considered that the amount of hexavalent chromium eluted from the stable-treated soil is suppressed to less than the lower limit of quantification. Estimate the redox potential (ORP) of the agent and the amount of admixture added,
After using the estimated amount of the admixture added by the admixture as the provisional addition amount, an admixture compounding test was conducted in which the admixture was added to confirm the effect of suppressing the amount of hexavalent chromium eluted.
After confirming that the amount of hexavalent chromium eluted is suppressed to less than the lower limit of quantification, the redox potential (ORP) of the admixture used in the admixture compounding test and the provisional addition amount are used for ground improvement. It is determined as the redox potential (ORP) of the admixture to be added and the amount of the admixture added.
(H) Based on the soil properties of the raw soil or the result of the cement compounding test conducted prior to the preparation of the stable-treated soil, it is considered that the amount of hexavalent chromium eluted from the stable-treated soil is suppressed to less than the lower limit of quantification. Estimate the redox potential (ORP) of the agent and the amount of admixture added,
After using the estimated amount of the admixture added by the admixture as the provisional addition amount, the admixture combination test was performed with a plurality of admixture addition amounts including the provisional addition amount.
From the correlation between the amount of admixture added and the amount of hexavalent chromium eluted after addition of the admixture based on the results of the admixture compounding test, the amount of admixture added at which the amount of hexavalent chromium eluted is less than the lower limit of quantification was determined.
The redox potential (ORP) of the admixture used in the admixture compounding test and the admixture addition amount are determined as the redox potential (ORP) of the admixture used for ground improvement and the admixture addition amount.

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