JP6639723B1 - Ground improvement method - Google Patents

Abstract

An object of the present invention is to provide a method for determining an oxidation-reduction potential (ORP) of an admixture and an appropriate addition amount thereof for suppressing the elution amount of hexavalent chromium, which varies depending on construction conditions when constructing a stabilized soil, to an environmental standard value. I do. SOLUTION: In producing a stabilized soil by stirring and mixing a cement slurry obtained by kneading water and cement with a raw soil, the cement slurry is mixed with an admixture having an oxidation-reduction potential (ORP) of 50 mV or less. After the kneaded admixture slurry is manufactured in advance, the admixture slurry is discharged into the original soil and stirred and mixed. At this time, the redox potential (ORP) of the admixture and the amount of the admixture added to suppress the elution amount of hexavalent chromium below the environmental standard value (0.05 mg / L or less) or the lower limit of quantification (0.02 mg / L) Is determined by a specific formulation test. [Selection diagram] FIG.

Description

  The present invention relates to a soil improvement method for preparing a stabilized soil by stirring and mixing a raw soil and a cement slurry, and particularly relates to an elution amount of hexavalent chromium (hereinafter referred to as a “hexavalent chromium elution amount”) contained in the stabilized soil prepared. The present invention relates to a ground improvement method that focuses on suppressing the above.

  The amount of hexavalent chromium eluted from the stabilized soil created by stirring and mixing the original soil and the cement slurry was determined by the Ministry of the Environment's environmental standards (separate table of environmental standards for soil contamination) as "0 per liter of test solution. 0.05 mg or less "(hereinafter referred to as" environmental reference value ").

  However, in some cases, when the ability of cement to fix substances such as heavy metals, etc. possessed by cement is impaired by organic matter or clay minerals in the soil, the value exceeds the environmental standard value. In particular, when the raw soil is a loam soil containing a large amount of clay minerals or a humus soil containing a lot of humic acid, which significantly inhibits the solidification of the stabilized soil, the strength of the stabilized soil is slow to develop, and the cement soil is hardly developed. It is known that hexavalent chromium is not sufficiently immobilized (insolubilized) and the amount of elution increases. As a countermeasure, a more expensive solidification material for hexavalent chromium (solidification material for special soil) must be used, and an economic burden is a problem.

  On the other hand, Patent Literature 1 and Patent Literature 2 disclose techniques that are effective in suppressing the elution amount of hexavalent chromium contained in cement.

JP 2018-145077 A JP-A-2006-169317

  In both Patent Literature 1 and Patent Literature 2, the redox potential (hereinafter, referred to as “ORP”) is −400 mV to −450 mV or less, and a low ORP admixture (cement additive or ground stabilization admixture) and cement or This 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, the ORP and the pH of the admixture are indicated, the addition amount is 0.001 to 1 part by mass or 0.01 to 10 parts by mass with respect to 100 parts by mass of the cement. It is only mentioned as a department.

Note that the oxidation-reduction potential (ORP) is an oxidation-reduction potential, which is also referred to as a redox (redox) potential, and is defined as an oxidant (M ^{Z +} ) and a reductant (M ^{(ZN) +} ) which coexist in a solution. It means an energy level (potential) determined by an equilibrium state therebetween.

  On the other hand, the amount of hexavalent chromium eluted from the stabilized soil varies widely depending on the construction conditions, and even more so, the amount of hexavalent chromium eluted must be reduced to the environmental standard value. It is necessary to appropriately select a combination with the amount of the admixture added. However, neither Patent Literature 1 nor Patent Literature 2 describes a method for determining a combination of the above-described ORP of the admixture and the amount of the admixture added.

  The present invention has been made in view of such problems, and has been applied to the conditions for constructing a stabilized soil (type and amount of cement, soil properties of raw soil, flow value immediately after stirring and mixing, target strength, etc.) ), The amount of hexavalent chromium eluted (hereinafter referred to as “hexavalent chromium elution amount”) must be set as an environmental standard value in order to ensure that the ORP of the admixture and the amount of admixture added (hereinafter “additive admixture”). Quantities ") are proposed.

  In construction near agricultural production areas and natural parks, the elution of hexavalent chromium, which is a carcinogenic substance, is required to be as low as possible (no detection). It is a suggestion.

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

  As a typical embodiment, an admixture slurry in which the cement slurry is kneaded with an admixture having an oxidation-reduction potential (ORP) of 50 mV or less is manufactured in advance, and the admixture slurry is mixed in the raw soil. The redox potential of the admixture which suppresses the elution amount of hexavalent chromium to less than the environmental standard value (0.05 mg / L or less) or the lower limit of quantification (0.02 mg / L) upon stirring and mixing. ORP) and the amount of the admixture are determined by a compounding test (a) or (b) shown below.

(A) The amount of hexavalent chromium eluted from the stabilized soil is less than the environmental standard value or the lower limit of quantification, based on the soil properties of the original soil or the results of a cementation test performed prior to the formation of the stabilized soil. Estimating the oxidation-reduction potential (ORP) of the admixture and the amount of the admixture which are considered to be suppressed,
After the estimated admixture addition amount was set to the provisional addition amount, the admixture was added to the admixture to perform an admixture blending test to confirm the effect of suppressing the elution amount of hexavalent chromium at the provisional addition amount,
After confirming that the hexavalent chromium elution amount was suppressed to less than the environmental standard value or the lower limit of quantification, the oxidation-reduction potential (ORP) and the provisional addition amount of the admixture used in the admixture compounding test were improved. Is determined as the oxidation-reduction potential (ORP) of the admixture and the amount of the admixture added.

(B) The amount of the hexavalent chromium eluted from the stabilized soil is less than the environmental standard value or the lower limit of quantification, based on the soil properties of the original soil or the results of a cement mixing test performed prior to the formation of the stabilized soil. The oxidation-reduction potential (ORP) of the admixture and the amount of the admixture,
With the estimated admixture addition amount as a provisional addition amount, an admixture blending test was performed with a plurality of admixture addition amounts including the provisional addition amount,
From the correlation between the admixture additive amount based on the admixture test results and the hexavalent chromium elution amount after the admixture addition, the hexavalent chromium elution amount is less than the environmental standard value or the lower limit of quantification. Find the quantity,
The oxidation-reduction potential (ORP) of the admixture used in the admixture compounding test and the amount of the admixture added are determined as the oxidation-reduction potential (ORP) of the admixture used for ground improvement and the amount of the admixture added.

  According to the present invention, even if the solidifying material represented by cement is not excessively administered or a more expensive solidifying material is used, the amount of hexavalent chromium eluted from the stabilized soil that has been formed is suppressed, and the amount of the eluting 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 economical to reduce the amount of solidifying materials and additives used, but also the environment is excellent due to the suppression of the amount of hexavalent chromium elution, and reliable ground improvement can be performed. .

FIG. 4 is a view showing an embodiment of the ground improvement method according to the present invention, and is a graph showing the relationship between the unconfined compressive strength of the stabilized soil and the elution amount of hexavalent chromium. 4 is a graph showing a relationship between a table flow value and a uniaxial compression strength. 4 is a graph showing a relationship between a table flow value and a hexavalent chromium elution amount. 2 is a graph showing the relationship between the uniaxial compressive strength and the amount of hexavalent chromium eluted shown in FIG. 1 as a single approximate curve. The graph which shows the relationship between the oxidation-reduction potential (ORP) and the hexavalent chromium elution amount in the case of high flow value and low intensity | strength. The graph which shows the relationship between the oxidation-reduction potential (ORP) and the hexavalent chromium elution amount in the case of low flow value and high intensity | strength. The graph which shows the relationship between the addition amount of the admixture whose hexavalent chromium elution amount becomes the environmental standard value, and the oxidation-reduction potential (ORP). The graph which shows the relationship between the addition amount of the admixture whose hexavalent chromium elution amount becomes the lower limit of quantification and the oxidation-reduction potential (ORP). 5 is a graph showing the relationship between the amount of admixture added and the amount of hexavalent chromium eluted after addition of the admixture. Explanatory drawing which shows the procedure from manufacture of an admixture slurry to construction by underground discharge.

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

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

1. As described above, the elution status of hexavalent chromium differs depending on the construction conditions for creating stabilized soil, as described above. Taking into account the combination of the flow value immediately after and the target strength of the stabilized soil, the mixing test (uniaxial compression test and 6 (Chromium (VI) dissolution test).

The combinations of the construction conditions are shown in (A) to (D) below.
(A) When the flow value immediately after stirring and mixing is set at a low flow value to create a low-strength stabilized soil.
(A) When the flow value immediately after stirring and mixing is set at a low flow value to create a high-strength stabilized soil.
(C) When the flow value immediately after stirring and mixing is applied at a high flow value to create a low-strength stabilized soil.
(D) When the flow value immediately after stirring and mixing is applied at a high flow value to create a high-strength stabilized soil.

  The fluidity value immediately after the stirring and mixing is defined as the stabilizing soil within 5 to 20 minutes after the mixing and mixing of the admixture slurry (as will be described later, the kneaded cement slurry and the admixture) with the original soil. , And here, the flow value measured by a flow test (JIS R5201). The flow value based on the flow test is a table flow value, and is hereinafter referred to as “TF value”.

The above-mentioned low fluidity value refers to construction with a TF value of about 110 mm to about 115 mm, and the high fluidity value refers to construction with a TF value of about 150 mm or more. Further, the above low strength refers to the strength of a stabilized soil having a uniaxial compressive strength of about 100 kN / m ² created as a measure against liquefaction, and the high strength refers to 3000 kN / m ² created as a structural foundation. The strength of the stabilized soil that exceeds the uniaxial compressive strength.

1) Preparation of test specimens and various tests To determine the blending conditions (water-cement ratio of cement slurry and the amount of cement added) satisfying the above-mentioned construction conditions (A) to (D), a blending test was performed using the following cement added amounts. Do.

The amount of cement to be added is four levels of 200 kg, 300 kg, 400 kg, and 500 kg per 1 m ³ of the original soil. In addition, in the compounding test, the cement slurry and the sample with the water-cement ratio which became the above-mentioned low flow value (approximately 110 mm to 115 mm in TF value) or high flow value (approximately 150 mm or more in TF value) were obtained at any amount of addition. Specimens shall be prepared from the stabilized soil obtained by mixing and mixing the soil (original soil), and various tests using the specimen shall be performed.

  In the various tests described above, the uniaxial compression strength (uniaxial compression test JIS A 1216) and the amount of hexavalent chromium elution (hexavalent chromium elution test, notified by the Environment Agency Nos. 46 and 77) under the standard curing for 7 days were determined. Method). Here, the description of the test methods and procedures of various tests is omitted.

  In addition, the cement used in the compounding test shows good strength in loam soil and humus soil, etc., but hardening material for general soft soil (cement-based hardening material) that hexavalent chromium is relatively eluted. I do.

2) Summary of Mixing Test Results Table 1 shows the TF value, unconfined compressive strength, and hexavalent chromium elution amount as the mixing test results using the above test 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. FIG. 3 shows the relationship between the table flow value and the elution amount of hexavalent chromium. FIG. 4 shows the relationship between the uniaxial compressive strength and the amount of hexavalent chromium eluted in FIG. 1 by a single and common approximate curve.

3) Characteristics obtained from the results of the blending test (1) Relationship between uniaxial compressive strength and elution amount of hexavalent chromium (FIGS. 1 and 4)
FIG. 1 is a graph showing the relationship between the unconfined compressive strength of the stabilized soil and the amount of hexavalent chromium eluted. The correlation is shown by the amounts of cement added (200 kg / m ³ , 300 kg / m ³ , 400 kg / m ³ , 500 kg / m ³ ). FIG. 4 is a graph showing FIG. 1 as a single and common approximate curve, and is a graph showing the relationship between the uniaxial compressive strength of the stabilized soil and the elution amount of hexavalent chromium. The features are shown below.
・ The uniaxial compressive strength of the stabilized soil and the amount of hexavalent chromium eluted are inversely proportional. That is, the amount of hexavalent chromium eluted decreases as the unconfined compressive strength increases, regardless of the amount of cement added.
-As is clear from Figs. 1 and 4, the elution amount of hexavalent chromium decreases as the amount of cement added increases. This indicates that the uniaxial compressive strength of the stabilized soil increased as the amount of cement added increased, and the amount of hexavalent chromium eluted was suppressed by promoting the fixation (insolubilization) of hexavalent chromium.
· As also apparent from FIGS. 1, 4, from the treated soil unconfined compressive strength of approximately 4000kN / m ² exceeds the hexavalent chromium eluted amount amount added cement is increased indicates a nearly flat state.

(2) Relationship between TF value and uniaxial compression strength (Fig. 2)
FIG. 2 is a diagram showing the relationship between the TF value and the unconfined compressive strength of the stabilized soil immediately after stirring and mixing, and the correlation is shown by the cement addition amounts (200 kg / m ³ , 300 kg / m ³ , 400 kg / m ³ , 500 kg / m ³ ). The features are shown below.

  -The TF value of the stabilized soil immediately after stirring and mixing is in inverse proportion to the uniaxial compressive strength. That is, when the TF value increases at the same addition amount, the uniaxial compressive strength decreases.

  -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 uniaxial compressive strength increases as the amount of added cement increases.

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

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

(4) Summary of the characteristics obtained from the results of the compounding test In the compounding test of the stabilized soil, the properties of the sample soil (original soil), the amount of cement added, the strength of the stabilized soil (uniaxial compressive strength), The elution amount of hexavalent chromium eluted from the stabilized soil was greatly changed due to the difference in the working conditions such as the flow value (TF value) immediately after the stirring and mixing.

  More specifically, the solidifying material for general soft soil is a cement (solidifying material) which has a high solidifying performance but a remarkable elution amount of hexavalent chromium as described above, and has a reducing effect of an admixture (described later). It can be said that this cement is most suitable for confirming the hexavalent chromium elution amount suppression effect). In other words, this cement is characterized by the fact that the amount of hexavalent chromium eluted increases when no admixture is added, and the appropriate ORP of the admixture and the addition of the admixture under the conditions where the amount of hexavalent chromium elutes increase. Therefore, it can be said that this cement is the most suitable cement for confirming the effect of suppressing the elution of hexavalent chromium.

  Although the test data is small, the uniaxial compressive strength of the stabilized soil increases as the amount of cement added increases, and hexavalent chromium in the stabilized soil becomes more difficult to be eluted due to the progress of immobilization (insolubilization) of hexavalent chromium in the stabilized soil. A trend was also confirmed. This shows the same phenomenon that hexavalent chromium is not eluted from sufficiently cured hard concrete.

(5) Estimation of Hexavalent Chromium Leaching Amount by Execution Conditions From the results of the above blending test, an estimated value of the hexavalent chromium elution amount under the above-mentioned execution conditions (A) to (D) was obtained and is shown below.

(A) When the flow value immediately after stirring and mixing is a low flow value and low in 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
Mixing conditions: water cement ratio 74%, cement addition amount 407 kg / m ³ .

  Note that the procedure for obtaining the above estimated values will be omitted.

  Even under the working condition (a) where the hexavalent chromium elution amount was the smallest, the elution amount was estimated to be 0.147 mg / L, and the hexavalent chromium elution amount was the environmental standard value (0.05 mg / L or less).

2. Mixing test in which an admixture is added In the above-described mixing test, it was confirmed that hexavalent chromium exceeding the environmental standard value eluted from the stabilized soil prepared under all the construction conditions (A) to (D). The following describes a soil improvement method in which the amount of hexavalent chromium eluted is less than the environmental standard value or the lower limit of quantification (hereinafter, referred to collectively as “management standard value”). In addition, the environmental standard value of the amount of hexavalent chromium eluted from the stabilized soil is set to be "0.05 mg or less per 1 L of test solution".
In addition, in this hexavalent chromium elution test, a test solution was prepared (elution method) according to the method listed in the Notification of the Environment Agency No. 77, and the quantification method (measurement method) was diphenylcarbazide absorption spectrophotometry (JIS K0102). 65.2.1). 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 diphenylcarbazide absorption spectrophotometry, and may be any of flame atomic absorption method, electric heating atomic absorption method, ICP issuance analysis method, ICP mass analysis method, and flow analysis method. good.

  As described above, the present invention reduces unstable hexavalent chromium in cement to stable chromium other than hexavalent with a low ORP (generally, about 50 mV or less) admixture. Technology. That is, the reduction effect (the effect of suppressing the amount of hexavalent chromium elution) differs depending on the ORP of the admixture and the amount of the admixture, and a method for determining an appropriate ORP as the admixture and the amount of the admixture and a method for obtaining the appropriate amount of the admixture. It proposes a construction method.

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

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

1) Admixture blending test In the cement blending test described above, the cement addition condition was 150% water-cement ratio and cement added more than the estimated elution amount of hexavalent chromium (0.416 mg / L) under construction conditions (c). The amount of hexavalent chromium elution (0.425 mg / L in Table 1) was larger when the amount was 200 kg / m ³ , and also from the estimated elution amount of hexavalent chromium (0.147 mg / L) under the construction conditions (a). Also, when the cement addition condition was 30% of the water cement ratio and the cement addition amount was 400 kg / m ^{3, the} hexavalent chromium elution amount (0.110 mg / L in Table 1) became smaller. Therefore, the admixture compounding test is performed under the following compounding conditions.

  The conditions of the following admixture blending test are blending tests aimed at confirming the effect of suppressing the amount of hexavalent chromium eluted by the admixture, and the amount of hexavalent chromium eluted from the stabilized soil shown below is the largest "Compounding condition 1" and "Compounding condition 2" in which the elution amount is relatively small.

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

・ Compounding condition 2 [When hexavalent chromium elution amount (0.110 mg / L) is relatively small]
The water cement ratio is 30%, and the amount of cement added is 400 kg / m ³ .

  The admixture used in the following admixture compounding test was prepared by mixing an admixture having an ORP of -533 mV, -230 mV, and -127 mV with 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 using calcium polysulfide and a thiosulfate compound as main materials was used.

The admixture slurry in which the admixture is added to the cement slurry has a water-cement ratio of the cement slurry of 150% (in the case of "formulation condition 1") or 30% (in the case of "formulation condition 2"). 0.1%, 0.3%, 0.5%, 1.0%, 5.0%, 10.0% equivalent to the weight of the cement (200 kg / m ³ or 400 kg / m ³ ) And the added and kneaded admixture slurry.

  A specimen was prepared from the stabilized soil obtained by mixing and mixing the admixture slurry and the sample soil, and a uniaxial compression test and a hexavalent chromium elution test after the specimen was subjected to standard curing were performed. Test items to be performed.

Hereinafter, an example of the measurement up to the preparation of the test piece (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 will be described.

(1) wet density of specifications and the sample soil admixture formulations tested: 1.369g / cm ³
-Addition amount of cement: 200 kg / m ³
・ Water cement ratio: 150%
-Additive amount: 1.0% of cement amount
-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 a sample soil of 5.844 ^-4 m ^{3 was} measured by weighing 800 g.

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

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

-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 called the total solution amount, and here, the ratio of the cement to the total solution amount (150%) is the water-cement ratio. The amount of kneading water is 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 test specimen The preparation procedure is shown below.

  Procedure 1: The cement (116.88 g) and the kneading water (174.15) are kneaded to prepare a cement slurry.

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

  Step 3: Add the admixture slurry to the sample soil (800 g), stir and mix to prepare a stabilized soil with the admixture added.

  Procedure 4: A specimen is prepared from the stabilized soil.

  Procedure 5: After standard curing of the above specimen for 7 days, a uniaxial compression test (JIS A 1216) and a hexavalent chromium elution test (preparation of a test solution by the methods listed in the notification of the Environment Agency No. 77 and preparation of JIS K0102 65.2.1) Quantification).

2) Summary of admixture blending test The results of the uniaxial compression test and the 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 graph showing the relationship between the ORP of the admixture and the amount of hexavalent chromium eluted after addition of the admixture under the blending condition 1 (when the elution amount of hexavalent chromium is the largest) by an approximate curve for each addition amount of the admixture. ing. FIG. 6 is a graph showing the relationship between the ORP of the admixture and the amount of hexavalent chromium eluted after the addition of the admixture in the blending condition 2 (when the elution amount of hexavalent chromium is relatively small) with an approximate curve for each admixture addition amount. Has become.

  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 was determined for each of the admixture addition amounts (0.1% to 10%). Ask for. In FIGS. 5 and 6, the hexavalent chromium elution amount is shown by a solid horizontal line, and the hexavalent chromium elution amount is shown by a broken horizontal line. I have. Table 4 shows the reading results.

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

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

  FIG. 7 shows that the amount of hexavalent chromium eluted was 8.5 times the environmental standard value (hexavalent chromium elution amount: 0.425 mg / L), which was the largest (mixing condition 1), and the amount of hexavalent chromium eluted was 2. In the case of a relatively small amount of 2 times (hexavalent chromium elution amount: 0.110 mg / L) (formulation condition 2), the ORP of the admixture and the amount of the admixture to suppress the hexavalent chromium elution amount to 0.05 mg / L Is a graph showing the relationship between FIG. 8 is a graph showing the relationship between the additive amount of the admixture and the ORP of the admixture for suppressing the elution amount of hexavalent chromium to 0.02 mg / L.

3) Characteristics observed from the admixture blending test (1) Relationship between the oxidation-reduction potential of the admixture and the amount of hexavalent chromium eluted after addition of the admixture (FIGS. 5 and 6)
The characteristics seen from the approximate curves in FIG. 5 (mixing condition 1) and FIG. 6 (mixing condition 2) are shown below.

  The ORP of the admixture and the amount of hexavalent chromium eluted after the addition of the admixture 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 elution amount of hexavalent chromium decreases with an increase in the ORP of the admixture.

  -The amount of the admixture added and the amount of hexavalent chromium eluted after the addition of the admixture 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 elution amount of hexavalent chromium decreases as the amount of the admixture increases, and the elution of hexavalent chromium decreases. The effect of controlling the amount is increasing.

  In other words, when the amount of hexavalent chromium eluted from the stabilized soil is to be suppressed to the same level of the control standard value (for example, the environmental standard value), if the admixture with a low ORP is used, the control standard value with a small amount of the admixture is used. This means that a large amount of the admixture must be added if the control standard value is to be satisfied with the admixture having a high ORP.

(2) Relationship between admixture additive amount and ORP of admixture (FIGS. 7 and 8)
FIG. 7 shows an approximate curve showing the relationship between the amount of the admixture added to suppress the amount of hexavalent chromium eluted from the stabilized soil to 0.05 mg / L and the ORP of the admixture, and FIG. 8 shows the amount of the hexavalent chromium eluted. Is a graph showing, as an approximate curve, the relationship between the additive amount of the admixture and ORP of the admixture, which suppresses ORP to 0.02 mg / L.

  FIG. 7 shows that the amount of the admixture added and the ORP of the admixture when the stabilized soil (mixing condition 1) in which the hexavalent chromium elution amount was 0.425 mg / L was suppressed to the environmental standard value (0.05 mg / L or less). The relationship with is described. In addition, the numerical value in a parenthesis shows the case of the stabilization treatment soil (mixing condition 2) whose hexavalent chromium elution amount is 0.110 mg / L.

-When the amount of the admixture is 0.1%: ORP of the admixture is -280 mV or less (-70 mV or less).
-When the amount of the admixture is 0.3%: the ORP of the admixture is -175 mV or less (-25 mV or less)
-When the amount of the admixture is 0.5%: ORP of the admixture is -120 mV or less (5 mV or less).
-When the amount of the admixture is 1.0%: ORP of the admixture is -65 mV or less (25 mV or less).
-When the additive amount of the admixture is 3.0%: The ORP of the admixture is 10 mV or less (45 mV or less).
-When the amount of the admixture is 5.0%: The ORP of the admixture is 25 mV or less (50 mV or less).
-When the amount of the admixture is 10.0%: The ORP of the admixture is 30 mV or less (50 mV or less).
Referring to FIG. 8, the relationship between the additive amount of the admixture and the ORP of the admixture when the environmental reference value is suppressed below the lower limit of quantification (non-detection) will be described.

-When the amount of the admixture is 0.1%: OR of the admixture is less than P-545 mV (less than -160 mV).
When the amount of the admixture is 0.3%: the ORP of the admixture is less than -210 mV (less than -75 mV)
When the amount of the admixture is 0.5%: the ORP of the admixture is less than -130 mV (less than -40 mV)
When the amount of the admixture is 1.0%: the ORP of the admixture is less than -120 mV (less than -5 mV)
When the amount of the admixture is 3.0%: the ORP of the admixture is less than -15 mV (less than 20 mV)
When the amount of the admixture is 5.0%: the ORP of the admixture is less than 10 mV (less than 25 mV)
When the amount of the admixture is 10.0%: ORP of the admixture is less than 15 mV (less than 25 mV)
That is, in order to suppress the stabilized 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 addition amount of the admixture is cleared by 0.1%, This indicates that when the ORP of the admixture is increased to 10 mV or less, the amount of the admixture added must be increased to 3.0%.

  In order to suppress the elution amount of hexavalent chromium below the lower limit of quantification, if the ORP of the admixture is -545 mV or less, the addition amount of the admixture is cleared by 0.1%, and the ORP of the admixture is reduced. This indicates that the amount of the admixture added when the voltage is increased to less than -15 mV must be increased to 3.0%.

(3) Characteristics observed from the relationship between the ORP of the admixture and the added amount of the admixture The characteristics observed from the relationship between the ORP of the admixture and the added amount of the admixture are described above.

-Upper limit of the amount of admixture In both of Figs. 7 and 8, there is a proportional relationship between the ORP of the admixture and the amount of admixture until the amount of admixture is 5.0%. When the amount exceeds 5.0%, the proportional relationship is not observed. That is, the control standard value is cleared by increasing the admixture addition amount even if the ORP of the admixture is increased up to 5.0% of the admixture addition amount, but the control standard value is exceeded when the admixture addition amount exceeds 5%. The ORP of the admixture that clears the value no longer changes. For example, in the case of the blending condition 2, the ORP of the admixture that clears the management reference value when the admixture addition amount is 5.0% and 10.0% has the same value.

  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 the admixture which shows the effect of suppressing the elution amount of hexavalent chromium.

The upper limit of the ORP of the admixture The upper limit of the ORP of the admixture means the upper limit of the reducing power (the effect of suppressing the elution amount of hexavalent chromium) as the admixture. The upper limit (5.0%) can be said to be the upper limit of the ORP of the admixture. However, the upper limit is the amount of hexavalent chromium eluted from the stabilized soil, the case where the amount of hexavalent chromium is eluted to an environmental standard value (0.05 mg / L or less), and the lower limit of quantification (0.02 mg / L). The ORP is different 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 the amount of hexavalent chromium eluted after adding the admixture is suppressed to the environmental standard value-When the amount of hexavalent chromium eluted from the stabilized soil is large, the ORP is 25 mV
ORP is 50 mV when the amount of hexavalent chromium eluted from the stabilized soil is relatively small
-When the amount of hexavalent chromium eluted after adding the admixture is suppressed to below the lower limit of quantification.-When the amount of hexavalent chromium eluted from the stabilized soil is large, the ORP is 10 mV.
-ORP is 25mV when the amount of hexavalent chromium eluted from the stabilized soil is relatively small.
・ Suppressive effect of ORP of admixture and the amount of hexavalent chromium eluted by the amount of admixture added The requirement that the amount of hexavalent chromium eluted from the stabilized soil be controlled to the control standard value is, as mentioned above, a requirement by the ORP of the admixture. The amount of admixture required to fill varies. That is, it can be said that a large amount of the admixture with a high ORP may be added, or a small amount of the admixture with a low ORP may be added. Hereinafter, a method of determining the ORP of the admixture and the amount of the admixture added will be described.

4) ORP of admixture and method for determining the amount of admixture added The amount of admixture required to suppress the amount of hexavalent chromium eluted from the stabilized soil to less than the environmental standard value or the lower limit of quantification (both 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 amount of the admixture, and there are various choices.

  In other words, the ORP of the admixture and the amount of the admixture, which are suitable conditions for the addition of the admixture, are the admixture necessary to suppress the amount of hexavalent chromium eluted from the stabilized soil until the elution amount reaches the control standard value. The method for determining the ORP of the admixture and the amount of the admixture is described below.

In the following examples, the amount of hexavalent chromium eluted (0.425 mg / L) from the stabilized soil under the blending condition 1 (the cement addition condition was 150% water cement ratio and the cement addition amount was 200 kg / m ³ ) was defined as an environmental standard. An example will be described in which the value is suppressed to a value (0.05 mg / L or less).

  In addition, the ORP of the admixture used in the admixture compounding test is 50 mV or less as a guide. However, considering that the elution amount of hexavalent chromium from the stabilized soil is 0.425 mg / L, It is described as using an admixture having an ORP of -127 mV.

(1) Method of determining the amount of admixture to be added by an admixture blending test using only one point This method is based on the use of an admixture with an ORP of -127 mV based on past data and experience. )), A method of determining the amount of the admixture to be added in the admixture blending test (hexavalent chromium elution test).

  The procedure is shown below.

Procedure 1: Estimation of the amount of admixture that is considered to be necessary to control to the environmental standard value The hexavalent chromium elution amount of the stabilized soil (specimen) is 0.425 mg / L, which is the environmental standard value of 8. Since the amount of hexavalent chromium eluted to 5 times (0.425 / 0.05) is considered to be suppressed to the environmental standard value (0.05 mg / L), the amount of the additive added is considered to be at least 0.1%. It is estimated that 5% or more, and in some cases up to about 0.7%, is necessary.

Procedure 2: Implementation of admixture compounding test based on provisional addition amount The maximum addition amount (0.7%) estimated in Step 1 was used as the provisional addition amount, and the admixture compounding test based on the provisional addition amount (one point only) To determine the amount of hexavalent chromium eluted after the addition of the admixture.

Step 3: Determination of ORP of admixture and addition amount of admixture It is confirmed that the elution amount of hexavalent chromium after addition of the admixture meets the environmental standard value, and 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 mixing test and the provisional addition amount (0.7%) are determined as the ORP of the admixture used for ground improvement and the addition amount of the admixture.

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

Additive amount = 200 × (0.7 ÷ 100) = 1.4 kg / m ³
Admixture amount determined by this method, the cement addition amount per a (weight per cement) 0.7%, and adding the admixture of 1.4kg to the original soil 1 m ³ per.

  In this method, since the amount of the admixture to be added is determined by only one point of the admixture test as described above, the amount of the admixture to be added must be on the safe side (large amount). An excessive amount of the admixture will be imposed.

(2) Method for determining the amount of admixture to be added in a plurality of admixture compounding tests This method is based on the results of past data and experience to estimate the amount of admixture that is considered to be an environmental standard value. (Procedure 1) is the same as the above-described method, except that an admixture blending test is performed at a plurality of points including the estimated addition amount (temporary addition amount), and an appropriate admixture addition amount is determined in the admixture blending test. There is a characteristic in what you want.

  The procedure is shown below.

Step 1: Estimation of the amount of admixture that seems to be necessary to control to the environmental standard value In the same manner as above, considering the amount of hexavalent chromium eluted from the stabilized soil, use an admixture with an ORP of -127 mV. It is estimated that the amount of the admixture considered to be necessary 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: Performing a multipoint admixture blending test Addition of a 0.3%, 0.5%, 0.7% multipoint admixture including the estimated addition amount (0.5% to 0.7%) The admixture blending test is performed based on the amount, and the elution amount of hexavalent chromium after addition of the admixture at the above-mentioned plurality of points is determined.

Step 3: Creating a correlation between the amount of admixture added and the amount of hexavalent chromium eluted after addition of the admixture From the amount of hexavalent chromium eluted after addition of admixture determined in step 2 (shown below), the amount of admixture added FIG. 9 is created as a correlation diagram between the amount of hexavalent chromium and the amount of hexavalent chromium eluted after the admixture is added.
-Elution amount of hexavalent chromium when the additive amount is 0.3%: 0.077 mg / L
-Elution amount of hexavalent chromium when the amount of admixture added is 0.5%: 0.036 mg / L
-Elution amount of hexavalent chromium when the admixture addition amount is 0.7%: 0.027 mg / L (estimated elution amount)
The hexavalent chromium elution amount is a numerical value obtained by citing the test results in Table 2, but the hexavalent chromium elution test with the admixture addition amount of 0.7% was not performed in the above admixture compounding test. Therefore, the amount of hexavalent chromium elution (0.027 mg / L) when the amount of the admixture was 0.7% was estimated from the test results of the amounts of the admixture of 0.5% and 1.0%.

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

Step 5: Determination of the amount of admixture added The amount of admixture 0.43% determined in step 4 is determined as the amount of admixture added to the admixture slurry. In addition, in order to minimize variations in the field and the amount of hexavalent chromium eluted from the stabilized soil as much as possible, the amount of the admixture (0.43%) is rounded up to 0.5% to thereby increase the stability of the stabilized soil. Of the hexavalent chromium from the product becomes the environmental standard value (0.05 mg / L or less).

  The ORP (-127 mV) of the admixture used in the admixture blending test and the admixture additive amount (0.5%) determined in the admixture blending test at the plurality of points were used as the ORP of the admixture used for ground improvement. Determined as the amount of admixture added.

The amount of the admixture added by this method is shown below.
Additive amount = 200 × (0.5 ÷ 100) = 1.0 kg / m ³
Hara Sat 1 m ³ per the adding admixtures of 1.0 kg.

  According to this method, the admixture addition amount (0.5%) can be made more economical than the admixture addition amount (0.7%) obtained in the above-mentioned single-point admixture blending test. However, each time, it is required to carry out an admixture compounding test at a plurality of points (two or more points, preferably three points).

(3) Method for determining the ORP of the admixture The amount of the ORP of the admixture and the amount of the admixture required to suppress the amount of hexavalent chromium eluted from the stabilized soil to the control standard value are determined by using a large amount of the admixture with a high ORP. Can be added, but it is economically preferable to add a small amount of an admixture having a low ORP. However, in consideration of the variation in construction, it is desirable that the additive amount of the admixture be at least 0.1% or more per weight of cement (the lower limit of the additive amount in consideration of the dispersion of construction).

  On the other hand, the upper limit of the amount of the admixture was 5.0% equivalent to the weight of cement in the above-described example, but when the amount of hexavalent chromium eluted from the stabilized soil was large (formulation condition 1), the admixture was added. It has been confirmed in the previous admixture compounding test that the ORP of the admixture is only about 5 mV difference when the added amount is 5.0% and 3.0%. From this result, even when the ORP of the admixture is almost the same, considering that the addition amount of the admixture is reduced to 3%, the addition amount of the admixture is 3.0% in consideration of economical construction. It is desirable to set the upper limit (the upper limit of the amount of the admixture added in consideration of economy). Also, 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%. And more desirable.

  The relationship between the ORP of the admixture and the amount of the admixture required to control the amount of hexavalent chromium eluted from the stabilized soil to the control reference value is closely related. Based on this, the method for determining the ORP of the admixture will be described below.

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

-When lowering the ORP of the admixture in the admixture test, the admixture addition amount exceeds the upper limit of 3.0% (more desirably 1.0%) in the admixture mixture test, and the admixture addition amount is excessive. In the case where the result becomes as follows, the ORP of the admixture used in the admixture blending test is again subjected to the admixture blending test with a lower ORP admixture, and the amount of the admixture added is determined again. For example, when the admixture addition amount exceeds 3.0% (more desirably 1.0%) in the admixture compounding test using the admixture of -127 mV, the ORP is lower than -127 mV. After changing to an admixture (for example, an admixture of -230 mV or -533 mV), the admixture blending test is performed again, and the amount of the admixture added is determined again.

  This method is a method in which the upper limit (for example, 3%) of the amount of the admixture is determined in advance in consideration of the soil properties of the original soil. Is a method of lowering the ORP of the admixture and again calculating the amount of the admixture added in the admixture blending test. By the way, the standard of the upper limit is, for example, in the case of a viscous soil or the like in which a small clay lump (dama) or the like is liable to be generated during stirring and mixing, the upper limit is set to 3.0%, and sandy soil or the like in which good stirring and mixing is easily obtained. In the case of (1), the upper limit of the additive amount of the admixture is set to 1.0% to 2.0%, and is determined in consideration of workability.

・ In the case of increasing 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 dispersion of construction) in consideration of the influence of quality variation and the like in the execution work. The lower limit of the amount of the admixture added in this case) is a guide. Therefore, if the amount of the admixture added is less than 0.1% in the admixture blending test, the admixture having an ORP higher than the ORP of the admixture used in the admixture blending test is mixed again. Perform a formulation 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 amount of the admixture added is less than 0.1%, the admixture having an ORP higher than -127 mV (for example, an ORP of -77 mV). And the admixture blending test is performed again to determine the amount of the admixture added again.

  It is desirable that the lower limit of the amount of the admixture be changed in the same manner as described above, depending on the soil properties of the original soil. For example, when cohesive soil is targeted, 0.2% to 0.3% of the amount of the admixture is added. The lower limit is preferably set to 0.1% when the target is sandy soil.

  The ORP of the admixture is selected not only from −533 mV, −230 mV, and −127 mV used in the present admixture blending test, but also from an admixture exhibiting an ORP in a range of approximately −550 mV to 50 mV. Various selections are possible for the combination of the ORP and the additive amount. However, it is desirable to select the ORP of the admixture so that the amount of the admixture is in the range of 0.1% to 3.0% corresponding to the weight of cement added to the stabilized soil. In the ground improvement method for suppressing the amount of hexavalent chromium elution in consideration of economy and workability, an appropriate combination of the ORP of the admixture and the addition amount of the admixture is important and indispensable.

  In the examples of the admixture compounding test, the amount of hexavalent chromium eluted from the stabilized soil was adjusted so that the amount of hexavalent chromium eluted was considered to be necessary to control the elution amount to the environmental standard value (control standard value). Although the ORP of the agent and the additive amount of the admixture were estimated, even if only the soil properties of the original soil (soil classification, water content ratio, wet density, etc.) are known, it is eluted from the stabilized soil by many past results. It is possible to estimate the amount of hexavalent chromium eluted and the approximate amount of ORP of the admixture and the amount of admixture that are necessary to suppress the amount of hexavalent chromium eluted to the control reference value.

(4) Timing of the admixture compounding test for adding the admixture The admixture compounding test for determining the appropriate addition conditions of the admixture is performed in the cement compounding test conducted prior to the formation of the stabilized soil. The case where the elution amount of hexavalent chromium exceeding the control standard value is detected from the specimen (stabilized soil) (Timing 1), and the case where the cement mixing test and the admixture mixing test are performed in parallel (Timing 2) ) Are roughly divided into two timings.

[Timing 1]
The case of timing 1 refers to a case in which an admixture blending test is performed after hexavalent chromium exceeding a control reference value is detected from a specimen during the cement blending test.

In the previous embodiment, for example, hexavalent chromium exceeding the control standard value was detected from the specimen under the blending conditions (129% water cement, 210 kg / m ^{3 of} cement added) determined under the application conditions (c). (Confirmed amount: 0.416 mg / L), and then the above-mentioned admixture compounding test is performed.

More specifically, an admixture is added to a cement slurry having a water-cement ratio of 120% at a cement weight of 0.7% per cement weight (for example, 210 kg / m ³ ) (for a single-point admixture compounding test) or 0.3%. %, 0.5%, 0.7% (when conducting the admixture blending test at multiple points) hexavalent chromium by a specimen (stabilized soil) obtained by stirring and mixing the admixture slurry with the admixture and the sample soil A dissolution test will be performed.

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

  Specifically, the amount of hexavalent chromium eluted from the stabilized soil and its hexavalent value based on the past performance and experience, etc., based on the soil properties of the original soil, the type of cement, The ORP of the admixture which is considered to suppress the chromium elution amount to the control reference value and the admixture addition amount are estimated, and the cement mixing test and the admixture mixing test are performed in parallel.

  Therefore, in the admixture compounding test, the amount of the admixture to be added is determined with only one point (pinpoint) of the ORP of the admixture and the amount of the admixture that are considered to be more reliably suppressed to the control reference value. There are many.

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

  In the case of the above [Timing 1], even if a plurality of admixture compounding tests are performed, two to three points of the admixture compounding test are performed. The admixture compounding test is performed with the amount of cement added 3 times the ratio of water cement 3). Therefore, it can be said that a plurality of admixture compounding tests according to [Timing 1] are rational and desirable.

5) Uniaxial compressive strength of stabilized soil applicable to this technology and TF value immediately after mixing by mixing In the admixture compounding test, the case of compounding condition 1 where the amount of hexavalent chromium eluted was the highest (0.425 mg of hexavalent chromium eluted) / L) and the case of blending condition 2 in which the elution amount is relatively small (hexavalent chromium elution amount: 0.110 mg / L).

Under the blending condition 1, a specimen was prepared using a stabilized soil having a TF value of 156 mm immediately after stirring and mixing. As a result, the specimen had a uniaxial compressive strength of 31 kN / m ² and a hexavalent chromium elution from the stabilized soil. The result was that the amount was 0.425 mg / L. In addition, under the blending condition 2, a specimen was prepared from a stabilized soil having a TF value of 115 mm immediately after stirring and mixing. As a result, the specimen had a uniaxial compressive strength of 3850 kN / m ² and a hexavalent strength from the stabilized soil. The result that the elution amount of chromium was 0.110 mg / L was obtained.

  From these results, the quality control range of the unconfined compressive strength of the stabilized soil applicable to the present technology and the TF value (flow value) immediately after stirring and mixing are shown below.

(1) Applicable range of unconfined compressive strength of stabilized soil Under mixing condition 1, the inhibitory effect of the admixture was confirmed under the condition that the unconfined compressive strength of the specimen was 31 kN / m ^2. I have. It is clear that the unconfined compressive strength of the stabilized soil and the amount of hexavalent chromium eluted are inversely proportional. If the stabilized soil is 50 kN / m ² or more, which is larger than the unconfined compressive strength, the stabilized treated soil of 31 kN / m ² is used. It can be inferred that the hexavalent chromium elution amount is smaller than that. Therefore, it can be said that the present technology can be applied to a stabilized soil that requires a uniaxial compressive strength of 50 kN / m ² or more.

(2) Applicable range of 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 stabilized soil. In the previous test, the effect of suppressing the amount of hexavalent chromium eluted from the stabilized soil having a TF value of 156 mm under the blending condition 1 was confirmed. Therefore, it can be said that the present technology can be applied to a stabilized soil in which the TF value (flow value) is controlled within 155 mm.

  The means for managing the flow value immediately after stirring and mixing is not limited to the table flow test described in the present embodiment. For example, a numerical value obtained by 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, an admixture produced using calcium polysulfide (lime-sulfur mixture) or a thiosulfate compound as a main material was used. However, 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 is said to become trivalent chromium when reduced by a reducing agent. The technique for suppressing the elution amount of hexavalent chromium is a technique of reducing hexavalent chromium contained in cement with a reducing agent having a low ORP. Therefore, in the admixture described in the present technology, it goes without saying that the lower the ORP of the admixture, the stronger the reducing power (suppression power), and the more desirable the admixture (reducing agent). The admixture may be a liquid or a solid (powder).

  The main raw materials as admixtures are not only the above-mentioned calcium polysulfide or thiosulfate compounds, but also potassium iodide, iron (II) sulfate, tin (II) chloride, oxalic acid, calcium thioglycolate, etc. Any value may be used as long as the ORP is 50 mV or less.

  Further, in the present 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, fly cement, etc. It is possible to suppress the elution amount of hexavalent chromium in any cement such as ash cement and lime-based solidifying material including cement.

3. Example of execution Hereinafter, an example of execution in a case where the water cement ratio is 150%, the cement addition amount is 200 kg / m ³ , and the admixture addition amount is 1% will be described. In this construction example, the kneading of the cement and the kneading water is performed by a batch mixer, and 800 kg of cement per batch is measured and kneaded to produce a cement slurry.

1) Weighing weight per batch Cement: 800kg
Admixture: 800 x (1/100) = 8 kg
Kneading water: total amount of solution−volume of admixture = {800 × (150/100)} − (8 / 1.25) ≒ 1194 kg
In order to prevent the addition of the admixture from affecting the development strength (uniaxial compressive strength) of the stabilized soil, it is desirable to include the admixture in the kneading water (total solution volume) and 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 a value obtained by dividing the weight of the admixture 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.

  Procedure 1: 1194 kg of kneading water is weighed and put into a mixer section.

  Procedure 2: 800 kg of cement is weighed and put into the mixer section. In the mixer section, the kneading blades (stirring blades) are constantly rotating, and the kneading water and the cement are kneaded with the introduction of the cement to form a cement slurry.

  Procedure 3: 8 kg of the admixture is weighed and charged into the mixer section. The total solution volume after the addition of the admixture is 1200 liters, and the water-cement ratio at this time is 150%. The calculation formula is shown below.

Total solution volume: 1194+ (8 / 1.25) ≒ 1200 liters Water cement ratio: (total solution volume / per weight of cement) × 100 = (1200/800) × 100 = 150%
Step 4: The kneading water, cement and admixture are kneaded in a mixer 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 a ground improvement machine by a grout pump.
Step 6: The admixture slurry and the original soil (in situ soil) are stirred and mixed by the above-mentioned ground improvement machine to form a stabilized soil.

  The measuring and charging of the admixture in the procedure 3 may be before the measuring and charging of the kneading water in the procedure 1. Also, the admixture and the cement slurry may be mixed and mixed before the cement slurry is pumped and injected into the original ground (original soil). It is good.

  Further, the ground improvement machine used here may be of any type such as a trencher type, a horizontal stirring type, a high pressure injection stirring type, etc., and is not limited to the ground improvement machine. For example, a representative example of a trencher-type ground improvement machine may be the one described in JP-A-2005-307675 of the present applicant.

Claims (9)

A soil improvement method for suppressing the amount of hexavalent chromium eluting from a stabilized slurry that is created by stirring and mixing a cement slurry obtained by kneading water and cement with the original soil,
After previously producing an admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of 50 mV or less, discharging the admixture slurry into the raw soil and stirring and mixing,
The oxidation-reduction potential (ORP) of the admixture and the additive amount of the admixture, which suppress the elution amount of hexavalent chromium to less than an environmental standard value (0.05 mg / L or less) or a lower limit of quantification (0.02 mg / L), A ground improvement method characterized by being determined by a compounding test of (a) or (b) shown below.
(A) The amount of hexavalent chromium eluted from the stabilized soil is less than the environmental standard value or the lower limit of quantification based on the soil properties of the original soil or the results of a cementation test performed prior to the formation of the stabilized soil. Estimating the oxidation-reduction potential (ORP) of the admixture and the amount of the admixture which are considered to be suppressed,
After the estimated admixture addition amount was set to the provisional addition amount, the admixture was added to the admixture to perform an admixture blending test to confirm the effect of suppressing the elution amount of hexavalent chromium at the provisional addition amount,
After confirming that the hexavalent chromium elution amount was suppressed to less than the environmental standard value or the lower limit of quantification, the oxidation-reduction potential (ORP) and the provisional addition amount of the admixture used in the admixture compounding test were determined. The redox potential (ORP) of the admixture used for ground improvement and the amount of the admixture added are determined.
(B) The amount of hexavalent chromium eluted from the stabilized soil is less than the environmental standard value or the lower limit of quantification, based on the soil properties of the original soil or the results of a cement mixing test performed prior to the formation of the stabilized soil. Estimating the oxidation-reduction potential (ORP) of the admixture and the amount of the admixture which are considered to be suppressed,
With the estimated admixture addition amount as a provisional addition amount, an admixture blending test was performed with a plurality of admixture addition amounts including the provisional addition amount,
From the correlation between the admixture additive amount based on the admixture test results and the hexavalent chromium elution amount after the admixture addition, the hexavalent chromium elution amount is less than the environmental standard value or the lower limit of quantification. Find the quantity,
The oxidation-reduction potential (ORP) of the admixture used in the admixture compounding test and the amount of the admixture added are determined as the oxidation-reduction potential (ORP) of the admixture used for ground improvement and the amount of the admixture added. The ground improvement method according to claim 1, wherein the admixture compounding test is performed under the following condition (c) or (d).
(C) In a cement blending test performed prior to the formation of the stabilized soil, after a hexavalent chromium elution amount exceeding the environmental standard value or exceeding the lower limit of quantification is detected from a specimen of the cement blending test. At the timing, the admixture compounding test is performed.
(D) The admixture compounding test is also performed in parallel with the timing of the cement compounding test performed prior to the formation of the stabilized soil.   In the admixture slurry, the admixture has an oxidation-reduction potential (ORP) of 10 mV or less, and the admixture is added in a range of 0.1% to 3.0% by weight of cement added to the raw soil. The soil improvement method according to claim 1 or 2, wherein the amount of hexavalent chromium eluted from the stabilized 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 the admixture is added in a range of 0.1% to 3.0% by weight of cement added to the raw soil. The soil improvement method according to claim 1 or 2, wherein the amount of hexavalent chromium eluted from the stabilized soil is suppressed to less than the lower limit of quantification by adding the chromium to the stabilized soil. The stabilized treated soil formed by stirring and mixing the admixture slurry and the raw soil is intended for a material requiring a uniaxial compressive strength of 50 kN / m ² or more. Item 6. The ground improvement method according to item 3 or 4.   The stabilized treated soil formed by stirring and mixing the admixture slurry and the raw soil is quality-controlled so that the flow value immediately after the stirring and mixing is within 155 mm as a table flow value. The ground improvement method according to claim 5, wherein   The ground according to any one of claims 1 to 6, wherein the admixture slurry is manufactured by weighing each of the water, cement, and the admixture and kneading the mixture with a batch mixer. Improvement method. Ground improvement to reduce the amount of hexavalent chromium eluted from the stabilized soil created by mixing and mixing the cement slurry with water and cement and the original soil to the environmental standard value (0.05 mg / L or less) The method
After preliminarily producing an admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of 10 mV or less, discharging the admixture slurry into the original soil and stirring and mixing,
In conducting an admixture compounding test to determine the redox potential (ORP) of the admixture and the amount of the admixture added, which suppress the elution amount of the hexavalent chromium to the environmental standard value,
The admixture compounding test is a timing after the hexavalent chromium elution amount exceeding the environmental standard value is detected from the specimen of the cement mixing test in a cement mixing test performed prior to the formation of the stabilized soil, or Performed in parallel with the timing of performing the cement mixing test,
In the admixture blending test of (e) or (f) shown below, the amount of the admixture was determined to be in the range of 0.1% to 3.0% by weight of the cement added to the raw soil. Ground improvement method characterized by performing.
(E) From the soil properties of the original soil or the results of a cement mixing test conducted prior to the formation of the stabilized soil, the mixing which seems to suppress the amount of hexavalent chromium eluted from the stabilized soil to the environmental standard value. The oxidation-reduction potential (ORP) of the agent and the amount of the admixture added,
After the estimated admixture addition amount was set to the provisional addition amount, the admixture was added to the admixture to perform an admixture blending test to confirm the effect of suppressing the elution amount of hexavalent chromium at the provisional addition amount,
After confirming that the hexavalent chromium elution amount was suppressed to the environmental standard value, the redox potential (ORP) of the admixture used in the admixture compounding test and the interim addition amount were used to improve the ground. The redox potential (ORP) of the agent and the amount of the admixture added are determined.
(F) From the soil properties of the original soil or the results of a cement blending test conducted prior to the formation of the stabilized soil, the mixing which seems to suppress the amount of hexavalent chromium eluted from the stabilized soil to the environmental standard value. The oxidation-reduction potential (ORP) of the agent and the amount of the admixture added,
With the estimated admixture addition amount as a provisional addition amount, the admixture blending test was performed at a plurality of admixture addition amounts surrounding the provisional addition amount,
From the correlation between the amount of the admixture added and the amount of hexavalent chromium eluted after the addition of the admixture based on the results of the admixture compounding test, the amount of the hexavalent chromium eluted was determined as an environmental standard value to determine the amount of the admixture added,
The oxidation-reduction potential (ORP) of the admixture used in the admixture compounding test and the amount of the admixture added are determined as the oxidation-reduction potential (ORP) of the admixture used for ground improvement and the amount of the admixture added. A soil improvement method that controls the amount of hexavalent chromium eluted from a stabilized soil created by mixing and mixing a cement slurry in which water and cement are kneaded with the original soil to below the lower limit of quantification (0.02 mg). So,
After previously preparing an admixture slurry obtained by kneading the cement slurry and an admixture having an oxidation-reduction potential (ORP) of −15 mV or less, discharging the admixture slurry into the raw soil and stirring and mixing,
In conducting an admixture compounding test to determine the oxidation-reduction potential (ORP) of the admixture and the amount of the admixture added, which suppress the elution amount of the hexavalent chromium below the lower limit of quantification,
The admixture compounding test is a timing after the hexavalent chromium elution amount equal to or more than the lower limit of quantification is detected from the specimen of the cement mixing test in a cement mixing test performed prior to the formation of the stabilized soil, or Perform in parallel with the timing of performing the cement blending test,
In the admixture compounding test of (g) or (h) shown below, the amount of the admixture was determined to be in the range of 0.1% to 3.0% by weight of the cement added to the raw soil. Ground improvement method characterized by performing.
(G) From the soil properties of the original soil or the results of a cement mixing test conducted prior to the formation of the stabilized soil, a mixing that seems to suppress the amount of hexavalent chromium eluted from the stabilized soil to less than the lower limit of quantification. The oxidation-reduction potential (ORP) of the agent and the amount of the admixture added,
After the estimated admixture addition amount by the admixture was a provisional addition amount, an admixture addition test was performed at the provisional addition amount to confirm the effect of suppressing the elution amount of hexavalent chromium by adding the admixture,
After confirming that the hexavalent chromium elution amount was 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 were used for ground improvement. Is determined as the redox potential (ORP) of the admixture to be added and the amount of the admixture added.
(H) From the soil properties of the original soil or the results of a cement mixing test performed prior to the formation of the stabilized soil, a mixing that seems to suppress the amount of hexavalent chromium eluted from the stabilized soil to less than the lower limit of quantification. The oxidation-reduction potential (ORP) of the agent and the amount of the admixture added,
The admixture added amount by the estimated admixture was taken as a provisional addition amount, and the admixture blending test was performed at a plurality of admixture addition amounts including the provisional addition amount,
From the correlation between the amount of the admixture added and the amount of hexavalent chromium eluted after the addition of the admixture based on the results of the admixture compounding test, determine the amount of the admixture in which the amount of hexavalent chromium eluted is less than the lower limit of quantification,
The oxidation-reduction potential (ORP) of the admixture used in the admixture compounding test and the amount of the admixture added are determined as the oxidation-reduction potential (ORP) of the admixture used for ground improvement and the amount of the admixture added.

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