Modifier for expansive soil, modified expansive soil, and method for producing modified expansive soil

Abstract

The present invention is a modifier for expansive soil that contains at least one selected from salts, cements, and limes in expansive soil.　In addition, the present invention is a modified expansive soil that contains at least expansive soil and a modifier for expansive soil.　In addition, the present invention is a method for producing a modified expansive soil in which the modifier for expansive soil is added to and mixed with expansive soil and a modified expansive soil is produced, preferably a method for producing a modified expansive soil in which 0-10 mass parts of salts, 0-30 mass parts of cements, and 0-20 mass parts of limes are added and mixed per 100 mass parts of expansive soil as a modifier for expansive soil, and a modified expansive soil is produced (provided that the sum of the amounts of salts, cements, and limes added is not 0 mass parts).

Description

Expansive soil modifier, modified expansive soil, and method for manufacturing modified expansive soil

[0001] The present invention relates to an expansive soil modifier that increases the compressive strength of expansive soil and suppresses the swelling and shrinkage of expansive soil, a modified expansive soil modified with the expansive soil modifier, and a method for producing the modified expansive soil.In this specification, expansive soil refers to soil or sand that swells when it absorbs moisture and water and shrinks when it dries.

[0002] Known examples of expansive soil include bentonite and black cotton soil (hereinafter referred to as "BCS"). Of these, BCS is found in large quantities in tropical and subtropical regions and is a soil containing swelling clay minerals such as montmorillonite. This BCS swells and turns muddy when it absorbs moisture, and shrinks when it dries. Therefore, if a road is laid on top of BCS left in the ground, the BCS will repeatedly shrink and swell due to the changes in wetness and dryness between the dry and rainy seasons, causing cracks in the road and resulting in its collapse (see Non-Patent Document 1).

[0003] Currently, the main countermeasure for road collapse caused by BCS is to replace the surface layer of the ground containing BCS with high-quality soil. For example, Non-Patent Document 1 proposes a method of placing high-quality replacement soil to a thickness of 1 m directly below the planned road and to a depth of 3 m on both sides of the road to suppress changes in the moisture content (i.e., changes in volume) of the remaining BCS (see Figure 4, BCS countermeasure work adopted in the project).

[0004] Furthermore, Non-Patent Document 2 proposes the following methods a) to g) as means for preventing cracks in structures built on BCS in India: a) Removing the expansive soil and installing non-expansive soil at the foundation of the structure; b) Minimizing differential settlement by using a reinforced, highly rigid foundation; c) Increasing the flexibility of the structure to allow for large differential settlement; d) Pre-wetting the expansive soil to minimize subsequent changes in water content; e) Applying large stresses to the foundation to balance the expansion pressure; f) Inserting an appropriate non-cohesive layer between the expansive soil and the foundation; and g) Foundation and ground improvement methods using piers, piles, and beams. Furthermore, under-reamed piles and multi-under-reamed piles for larger structures are widely used in India (see Figure 11, Construction Sequence for Multi-Under-Reamed Pile).

[0005] However, the above methods are thought to have the following problems: i) The method of replacing with high-quality soil and sand requires the delivery of replacement soil and the removal of the replaced BCS, and furthermore, in road construction work in urban areas, it is necessary to secure a storage location for the removed BCS. ii) Because the under-reamed pile method is uneconomical for small-scale structures, an easier and cheaper method of constructing foundations is required (see "8. Conclusion" on page 51 of Non-Patent Document 2).

[0006] Okita et al., "A Study on Countermeasures for Expansive Soil in Ethiopia," 69th Annual Academic Conference of the Japan Society of Civil Engineers (September 2014), pp. 1331-1332; D. Mohan et al., "India's Black Cotton Soil," Soil and Foundations, 26-11 (248), November, 1978, pp. 47-51

[0007] Therefore, an object of the present invention is to provide a method for modifying expansive soil to suppress swelling and shrinkage of the soil and sand, and to produce a modified expansive soil that is strong and resistant to collapse.

[0008] Therefore, the inventors conducted extensive research to achieve the above-mentioned objectives and found that the expansive soil modifier, the modified expansive soil, and the method for manufacturing the modified expansive soil having the following configurations can achieve the above-mentioned objectives, and completed the present invention described below.

[0009] [1] An expansive soil modifier containing at least one or more selected from salts, cements, and limes. [2] A modified product of expansive soil containing at least expansive soil and the expansive soil modifier described in [1] above. [3] A method for producing a modified product of expansive soil, which comprises adding at least the expansive soil modifier described in [1] above to expansive soil and mixing the mixture to produce a modified product of expansive soil. [4] A method for producing a modified product of expansive soil, which comprises adding at least 0 to 10 parts by mass of salts, 0 to 30 parts by mass of cements, and 0 to 20 parts by mass of limes to 100 parts by mass of expansive soil and mixing the mixture (excluding cases where the total amount of salts, cements, and limes added is 0 part by mass). [5] A method for producing a modified expansive soil as described in [3] above, which produces a modified expansive soil by adding a modifier in portions, through at least the following steps (A) to (D): (A) a step for producing a primary mixture, in which the modifier is added to the expansive soil and mixed to produce a primary mixture; (B) a step for producing a crushed product of the primary mixture, in which the primary mixture is cured and then crushed to produce a crushed product; (C) a step for producing a secondary mixture, in which the modifier is added to the crushed product for a second time and mixed to produce a secondary mixture; and (D) a step for compacting the secondary mixture. [6] A method for producing a modified expansive soil as described in [5] above, which further comprises a compaction step for compacting the primary mixture produced in the step (A) for producing the primary mixture. [7] A method for producing a modified expansive soil as set forth in [5] above, wherein the amount of modifier added in installments is 0.01 to 20 parts by mass in the first addition and 1 to 20 parts by mass in the second addition, relative to 100 parts by mass of the expansive soil. [8] A method for producing a modified expansive soil as set forth in [5] above, wherein the curing period of the primary mixture is 7 days to 1 year. [9] A method for producing a modified expansive soil as set forth in [5] above, wherein the particle size of the crushed material is 200 mm or less.

[10] A method for producing a modified expansive soil as set forth in [5] above, wherein water is added during the primary addition of the modifier in the case of (a) below, and water is added during the secondary addition of the modifier in the case of (b) below. (a) When stirring and mixing the modifier is difficult or impossible due to the high viscosity of the expansive soil. (b) When the compressive strength of the secondary mixture after curing is 0.5 N / mm or less at the time of crushing. ２

[11] The method for producing a modified expansive soil according to any one of [3] to

[10] above, wherein the expansive soil is black cotton soil.

[0010] In the present invention, the method for producing a modified expansive soil using the above-mentioned modifying material modifies the expansive soil, suppresses swelling and shrinkage of the soil, and can produce a modified soil with high strength.

[0011] As described above, the present invention relates to an expansive soil conditioner, an improved product of expansive soil, and a method for producing the improved product of expansive soil. The present invention will be described in detail below.

[0012] 1. Expansive soil conditioner The expansive soil conditioner contains at least one or more selected from salts, cements, and limes. The expansive soil is soil and sand containing mainly montmorillonite, which expands when absorbing water and shrinks when drying. In the present invention, an example of the expansive soil is black cotton soil. For example, black cotton soil has a liquid limit (W Ｌ ) is 46-97%, and the plasticity index (I Ｐ ) is 21 to 63%, which means that the liquid limit and plastic limit are relatively high. The salts are at least one selected from alkali metal salts and alkaline earth metal salts, specifically at least one selected from sodium chloride, table salt, potassium chloride, calcium chloride, magnesium chloride, sodium carbonate, potassium carbonate, etc. Furthermore, in the present invention, seawater and wood ash containing these salts can also be used. For example, if road construction is along the coast, seawater can be used, and if it is in a grassland, forest area, or grain-producing area, wood ash made by burning vegetation can be used.

[0013] The salts are preferably added in an amount of 0 to 10 parts by mass per 100 parts by mass of expansive soil. If the salts are added in an amount within the above range, excluding 0 parts by mass, the liquid limit of the modified expansive soil will decrease and the dry density will increase, thereby suppressing shrinkage and swelling. Furthermore, if there is no need to modify the liquid limit and / or dry density of the expansive soil, there is no need to add salts (i.e., 0 parts by mass). The salts are more preferably added in an amount of 1 to 8 parts by mass per 100 parts by mass of expansive soil.

[0014] The cement may be one or more selected from blast furnace cement type A, blast furnace cement type B, blast furnace cement type C, Portland cement, silica cement, fly ash cement, ecocement, and cement-based solidification materials. The cement-based solidification material is a composite material with cement as the base material, and the other solidification components and their blend ratios are determined based on the degree of difficulty of solidification, such as for general soft soil, special soil (general-purpose type), or high organic soil, as well as the conditions at the solidification site. For example, commercially available cement-based solidification materials include Geoset (registered trademark, manufactured by Taiheiyo Cement Corporation) 200 for general-purpose use and Geoset (registered trademark, manufactured by Taiheiyo Cement Corporation) 225 for high organic soil. Among the above cements, blast furnace cement type B, Portland cement, and cement-based solidification materials are suitable as modifiers for expansive soil due to their low cost and high solidification performance.

[0015] The cements are preferably added in an amount of 0 to 30 parts by mass per 100 parts by mass of the expansive soil. If the cements are added in an amount within the above range, excluding 0 parts by mass, the strength development is high and the swelling and shrinkage rates are low. The cements are more preferably added in an amount of 10 to 20 parts by mass per 100 parts by mass of the expansive soil.

[0016] The lime is one or more selected from slaked lime, quicklime, calcium hydroxide, calcium oxide, gypsum, and calcium sulfate. Of these limes, slaked lime or calcium hydroxide is preferred because they have high strength development and are easy to handle. The lime is preferably added in an amount of 0 to 20 parts by mass per 100 parts by mass of expansive soil. If the lime is added in the above range excluding 0 part by mass, the strength development is high and the swelling rate and shrinkage rate are low. The lime is more preferably added in an amount of 5 to 15 parts by mass per 100 parts by mass of expansive soil.

[0017] 2. Manufacturing Method of Modified Expansive Soil The manufacturing method of the modified expansive soil of the present invention includes a method of adding the modifier all at once to produce a modified product (lump-addition method), and a method of adding the modifier in two parts to produce a modified product (divided addition method). Generally, if the amount of modifier added is the same, the modified product produced by the divided addition method has higher strength and lower swelling and shrinkage than the lump-addition method. The bulk addition method is a commonly used addition method and requires no further explanation. Therefore, only the divided addition method will be explained.

[0018] The method for adding the modifier in portions includes at least (A) a step of preparing a primary mixture, (B) a step of preparing a crushed product of the primary mixture, (C) a step of preparing a secondary mixture, and (D) a step of compacting the secondary mixture. Each of these steps will be described below.

[0019] (A) Primary Mixture Preparation Process This process involves the primary addition of the modifier to the expansive soil and mixing to prepare a primary mixture. The primary addition ratio of the modifier is preferably 0.01 to 20 parts by mass per 100 parts by mass of the expansive soil. If the primary addition ratio of the modifier is within the above range, the liquid limit of the modified expansive soil is lowered and the dry density is increased, as well as the strength of the modified expansive soil and suppression of swelling and shrinkage of the modified soil. The primary addition ratio of the modifier is more preferably 5 to 15 parts by mass per 100 parts by mass of the expansive soil. The device used to mix the expansive soil with the primary or secondary modifier is not particularly limited, and generally, any mixer used for mixing concrete or mortar may be used, including tilting mixers, forced mixers, drum mixers, gravity mixers, and hand mixers. Furthermore, examples of mixing equipment used on-site include backhoes, bucket mixers, and batch or continuous mixers dedicated to soil cement. Furthermore, the process of preparing the primary mixture may include a compaction step as an optional step. The compaction method used in the present invention is not particularly limited, and examples include rolling compaction using a vibrating roller or a vibration damper. Furthermore, if the strength of the modifier is insufficient, it is advisable to add more modifier to the crushed material and compact it in this step.

[0020] (B) Step of preparing crushed material from primary mixture This step is a step of curing the primary mixture and then crushing the primary mixture to prepare a crushed material. Here, the curing method is not particularly limited, but air-dry curing is preferred because it is easy to cure a large area of ​​road. The curing period depends on the type and amount of modifier added, but is preferably 7 days to 1 year. If the curing period is within this range, the primary mixture after curing has a strength that allows it to be crushed. Furthermore, the compressive strength of the primary mixture after curing is preferably 0.2 to 4.0 N / mm ２ The compressive strength is 0.2 N / mm ２ If the pressure is less than 1.2 N / mm, the strength of the secondary mixture may not be strong enough to be crushed (it may become muddy when pressurized). ２If the compressive strength exceeds 0.4 to 1.0 N / mm, it may be difficult to crush the material or to apply a rolling pressure after crushing. ２ The particle size of the crushed material is preferably 200 mm or less. If the particle size exceeds 200 mm, rolling compaction becomes difficult. The particle size is preferably 150 mm or less. The crushing method is not particularly limited, but examples include crushing the primary mixture after curing with the bucket or caterpillar of a backhoe, or crushing using a special crusher equipped with a sieve.

[0021] (C) Secondary Mixture Preparation Step This step involves secondarily adding the modifier to the crushed material and mixing it to prepare a secondary mixture. The secondary addition ratio of the modifier is 1 to 20 parts by mass per 100 parts by mass of the expansive soil. If the secondary addition ratio of the modifier is within the above range, as in the case of the primary addition, the dry density of the modified expansive soil is increased, the strength of the modified product is improved, and swelling and shrinkage are suppressed. The secondary addition ratio of the modifier is preferably 5 to 15 parts by mass per 100 parts by mass of the expansive soil.

[0022] (D) Compaction of the secondary mixture This step is a step of compacting the secondary mixture. In this step, it is preferable to cure the mixture after compaction to prevent freezing in winter and drying in summer. Examples of the curing include the use of a curing sheet or a curing mat, a combination of these curing methods with water spraying or endothermic curing, and air-drying curing.

[0023] In the present invention, when it is necessary to adjust the viscosity of the expansive soil and the strength of the modified product, water may be added at the time of the first addition of the modifier and / or at the time of the second addition of the modifier. The conditions for adding water are as follows: (a) When the viscosity of the expansive soil is high, it is difficult or impossible to stir and mix the modifier; (b) When the compressive strength of the secondary mixture after curing is 0.5 N / mm when crushed. ２The above cases: Here, "cases where the high viscosity of expansive soil makes it difficult or impossible to mix the modifier" refers to cases where mixing is impossible or where the mixture is not uniform, although this cannot be stated unambiguously as mixing depends on the performance of the mixing equipment. The water used can be tap water, river water, lake water, seawater, treated sewage water, etc. The amount of water to be added should be determined by actually preparing a mixture using the water to be used. When using water other than tap water, it is preferable to confirm that it does not adversely affect the solidification performance.

[0024] The present invention will be described in more detail below with reference to examples, but is not limited to these examples. 1. Materials Used The materials used in these examples are shown in Table 1. The properties of black cotton soil a, bentonite, and black cotton soil b used in these examples are shown in Table 2.

[0025]

[0026]

[0027] 2. Measurement of Swelling Ratio of Expansive Soil Due to Salts Eighty milliliters of aqueous solutions containing various concentrations of salt a (shown in Table 3 as the amount of salt a added) were placed in 100-ml measuring cylinders. Next, 40 grams of black cotton soil a (expansive soil) with a natural water content (the soil's water content in its natural state) was dropped into the solution. The graduated cylinder was immediately read at the water level, and the increased volume was calculated as the volume of black cotton soil a before water absorption. Next, the mixture of the aqueous solution and black cotton soil a was stirred, and the mixture was left to stand for 1 day and 7 days. The graduated cylinder was read at the boundary between the swollen black cotton soil a and the aqueous solution, and this was determined as the volume of black cotton soil a after water absorption. The swelling ratio of black cotton soil a was then calculated using the following formula (1). The results are shown in Table 3. Swelling rate of black cotton soil a (%) = 100 × (volume of black cotton soil a after water absorption − volume of black cotton soil a before water absorption) / volume of black cotton soil a before water absorption (1) As shown in Table 3, the swelling rate of black cotton soil a decreases as the amount of added salt a increases, so salt a can suppress the swelling of black cotton soil a.

[0028]

[0029] 2. Measurement of Compressive Strength, Swelling Ratio, and Shrinkage Ratio of Modified Materials Produced by Bulk Addition of Modifier (1) Production of Modified Materials by Bulk Addition of Modifier Bentonite (expansive soil) was kneaded using an electric mixer (manufactured by Hitachi Koki) while adding mixing water containing or not containing salt b to the bentonite until the bentonite reached its liquid limit (the water content at the boundary between the plastic and liquid states). Bentonite slurries were obtained. The liquid limits were 169% for Examples 4 to 6, 90% for Examples 7 to 9, 85% for Examples 10 to 12, and 169% for Comparative Example 2. The liquid limits in this specification were measured in accordance with JGS T 142-1997, "Liquid Limit Test Method for Soil Using Fall Cone." Next, cement a (cement) and slaked lime a (lime) were added to the above slurry in the amounts shown in Table 4, and the mixture was kneaded in a Hobart mixer to produce a modified product.

[0030] (2) Measurement of compressive strength of modified product The modified product was placed in a mold with an inner diameter of 50 mm and a height of 100 mm to form a molded specimen, which was then sealed and cured. The compressive strength of the specimen at an age of 7 days and 28 days was measured in accordance with JIS A 1216 "Unconfined Compression Test Method for Soil." The results are shown in Table 4. As shown in Table 4, the compressive strength could be measured in Examples 4 to 12, but not in Comparative Example 2.

[0031] (3) Measurement of Swelling Ratio and Shrinkage Ratio of Modified Product The residues of the modified product at ages of 7 days and 28 days, which were generated in the compressive strength measurement, were crushed through a sieve with a nominal mesh size of 9.5 mm to obtain crushed material. Next, in the same manner as in the measurement of the swelling ratio of expansive soil due to salts, the crushed material was dropped into water in a measuring cylinder, and the increased volume was calculated as the volume of the expansive soil before water absorption. Furthermore, after stirring the mixture of water and crushed material, the mixture was left to stand for 3 hours and 7 days, the volume of the crushed material after water absorption was measured, and the swelling ratio of the crushed material (modified product) was calculated using the following formula (2). These swelling ratios are shown in Table 4. As shown in Table 4, the swelling ratios of Examples 4 to 12 were all lower than those of Comparative Example 2. Swelling ratio of crushed material (%) = 100 × (volume of crushed material after water absorption - volume of crushed material before water absorption) / volume of crushed material before water absorption ... (2)

[0032] Next, the crushed material was packed into a boat-shaped container made by splitting a cylinder with an inner diameter of 50 mm and a height of 100 mm lengthwise, and dried by air drying indoors for 1 day, 7 days, and 21 days. After that, the packed material was removed, and its length and width were measured, and the shrinkage rate of the packed material (modified material) was calculated using the following formula (3). These shrinkage rates are shown in Table 4. Packed material shrinkage rate (%) = 100 × (volume of packed material before drying - volume of packed material after drying) / volume of packed material before drying (3) As shown in Table 4, the shrinkage rates of Examples 4 to 12 were equal to or lower than those of Comparative Example 2. Note that the starting point for the material age in measuring the swelling rate and shrinkage rate of the modified material was the time when the compressive strength was measured (i.e., when the above residue was generated).

[0033]

[0034] 3. Preparation of Modified Products by Adding Modifier in Portions and Measurement of Compressive Strength, Swelling Ratio, and Shrinkage Ratio of the Modified Products (1) Preparation of Primary Mixtures and Crushed Products Thereof In a manner similar to that used for preparing modified products by adding the modifier all at once, bentonite was prepared by adding mixing water containing or not containing salt b to the bentonite and kneading the mixture using the electric mixer until the bentonite reached its liquid limit. The liquid limit values ​​were 169% for Examples 13-17, 90% for Examples 18-22, and 85% for Examples 23-26. Next, the modifier was added to the bentonite slurry in a primary mixture according to the formulations in Table 5 and mixed to prepare primary mixtures. The primary mixtures were then placed in polyethylene bags and sealed and cured for 28 days. The primary mixtures were then crushed through a sieve with a nominal mesh size of 9.5 mm to prepare crushed primary mixtures.

[0035]

[0036] (2) Preparation of Modified Product and Measurement of Compressive Strength, Swelling Ratio, and Shrinkage Ratio Next, according to the formulation in Table 5, the modifier was added to the crushed product of the primary mixture and kneaded in a Hobart mixer to prepare a modified product (secondary mixture). Furthermore, the secondary mixture was placed in a mold with an inner diameter of 50 mm and a height of 100 mm to form a molded specimen, which was then sealed and cured. The compressive strength of the modified product (secondary mixture) at ages of 7 days and 28 days was measured in the same manner as above. The residue of the modified product at ages 7 days and 28 days obtained from the compressive strength measurement was crushed through a sieve with a nominal mesh size of 9.5 mm, and the swelling ratio and shrinkage ratio of the crushed product were measured in the same manner as above. These results are shown in Table 5. As shown in Table 5, all examples had high strength and low swelling ratios and shrinkage ratios.

[0037] (3) Measurement of compressive strength, swelling ratio, and shrinkage ratio of modified material using black cotton soil a Using black cotton soil a, salt a, and cement b, the compressive strength, swelling ratio, and shrinkage ratio of the modified material were measured in the same manner as above, according to the formulations in Table 6. These results are shown in Table 6. As shown in Table 6, compared to Comparative Example 3, Examples 27 to 34 had higher compressive strength and lower swelling ratios and shrinkage ratios. Incidentally, the test water content was 33.3% for Examples 27 to 30, 52.6% for Examples 31 to 34, and 22.5% for Comparative Example 3.

[0038]

[0039] (4) Measurement of compressive strength, swelling ratio, and shrinkage ratio of modified material using black cotton soil b The compressive strength, swelling ratio, and shrinkage ratio of the modified material were measured in the same manner as above using black cotton soil b, seawater, and slaked lime b. The results are shown in Table 7. As shown in Table 7, the longer the air-drying period of the modified material, the higher the compressive strength and the smaller the swelling and shrinkage.

[0040]

[0041] (5) Measurement of compressive strength, swelling ratio, and shrinkage ratio of modified product using black cotton soil b 2.6 parts by mass of slaked lime was added to 100 parts by mass of black cotton soil b and mixed to prepare a primary mixture. Next, the primary mixture was air-cured for the period shown in Table 8, and water and cement b in the amounts shown in Table 8 were added to the primary mixture to prepare a modified product (secondary mixture). Furthermore, the compressive strength of the modified product was measured in the same manner as above. These results are shown in Table 8. As shown in Table 8, the longer the air-curing period of the primary mixture, the higher the compressive strength.

[0042]

Claims

1. An expansive soil conditioner containing at least one selected from salts, cements, and limes.

2. An expansive soil modifier comprising expansive soil and at least the expansive soil modifier according to claim 1.

3. A method for producing a modified expansive soil, comprising adding at least the expansive soil modifier according to claim 1 to and mixing the expansive soil to produce a modified expansive soil.

4. The method for producing a modified expansive soil according to claim 3, comprising adding and mixing at least 0 to 10 parts by mass of salts, 0 to 30 parts by mass of cements, and 0 to 20 parts by mass of lime as expansive soil modifiers to 100 parts by mass of expansive soil to produce a modified expansive soil (however, this does not include cases where the total amount of salts, cements, and limes added is 0 parts by mass).

5. A method for producing a modified expansive soil as claimed in claim 3, which produces a modified expansive soil by adding a modifier in portions through at least the following steps (A) to (D): (A) a step for producing a primary mixture by adding the modifier to the expansive soil for the first time and mixing to produce a primary mixture, (B) a step for producing a crushed product of the primary mixture by curing the primary mixture and then crushing the primary mixture to produce a crushed product, (C) a step for producing a secondary mixture by adding the modifier to the crushed product for the second time and mixing to produce a secondary mixture, and (D) a step for compacting the secondary mixture.

6. A method for producing an improved expansive soil according to claim 5, further comprising a compaction step of compacting the primary mixture prepared in the step (A) of preparing the primary mixture.

7. A method for producing a modified expansive soil as described in claim 5, wherein the amount of the modifier added in portions is 0.01 to 20 parts by mass in the first addition and 1 to 20 parts by mass in the second addition, per 100 parts by mass of the expansive soil.

8. The method for producing an improved expansive soil according to claim 5, wherein the curing period of the primary mixture is from 7 days to 1 year.

9. A method for producing an improved expansive soil according to claim 5, wherein the particle size of the crushed material is 200 mm or less.

10. The method for producing a modified expansive soil according to claim 5, wherein water is added when the modifier is added for the first time in the case of (a) below, and water is added when the modifier is added for the second time in the case of (b) below: (a) When the viscosity of the expansive soil is high, stirring and mixing the modifier is difficult or impossible; (b) When the compressive strength of the secondary mixture after curing is less than 0.5 N / mm at the time of crushing; ２ In the above case 11. A method for producing an improved expansive soil according to any one of claims 3 to 10, wherein the expansive soil is black cotton soil.

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