Method for improving sandy soft ground

Abstract

To provide a method for improving sandy soft ground capable of improving the strength of soft ground, mainly sandy soil, at low cost without using cement.SOLUTION: By mixing a soil improvement material consisting of a substance that has a basicity of 2 or more and contains 10 mass% or more of alumina and a steel slag ballast with a sandy soft ground, the strength of the soil is improved. As the substance that has a basicity of 2 or more and contains 10 mass% or more of alumina, an electric arc furnace reducing slag containing mayenite (12CaO 7Al2O3) can be used. It is preferable that the mixing ratio of the electric arc furnace reducing slag to the steel slag ballast be 10 mass% or less.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present invention relates to a method for improving soft sandy ground by using crushed steelmaking slag stone. [Background technology]

[0002] Looking at the current social situation, the disposal of construction by-products such as generated soil and waste from construction projects has become a major social issue. Until now, generated soil from construction projects has often been disposed of as surplus soil or inferior soil. The active and effective use of such generated soil is a social requirement not only from the standpoint of economy and workability, but also from an environmental perspective.

[0003] For example, Japan is home to a large amount of soft clayey soil, including volcanic ash clayey soil, and many construction projects in such areas involve handling this soft clayey soil. In order to proactively improve this soft soil and make effective use of construction waste soil, an effective and economical ground improvement method was required.

[0004] Therefore, the inventors have been researching a method for improving the soil quality of soft ground by effectively utilizing steelmaking slag, which is available at low cost, and using carefully selected crushed steelmaking slag as an improvement material for soft mud for many years. They have established a soil improvement method using a specific steelmaking slag as the main material, which can provide sufficient improvement effects, and filed a patent application under Japanese Patent Application No. 2022-4780. This prior invention is based on the knowledge that crushed steelmaking slag with a porous mineral structure and a water absorption rate of 3% or more, or a basicity (CaO / SiO2) of 3 or more is appropriate, and by mixing crushed steelmaking slag with the existing soil in a volume ratio of 2:1, they have succeeded in significantly increasing the CBR of about 2%, which is a clayey soil that cannot stand on its own, to a CBR of 40 to 100% after mixing and curing. The CBR will be described later.

[0005] According to this prior invention, by mixing inexpensive steel slag with soft clayey soil and curing it, the CBR value is significantly improved and the soil strength is also improved, so it has come to be effectively used as a roadbed or road body at construction sites. However, it was found that the soil improvement effect of this prior invention was effective in the case of clayey soil, but was not effective in the case of soft soil mainly composed of sand.

[0006] Patent Document 1 describes a method for improving soft ground by mixing crushed stone, cement, and steel slag with dredged soil. This method of using cement-based solidification materials can also produce strength in sandy soil. However, there are problems with the deterioration of the working environment, such as the generation of cement dust, and the use of cement increases the construction costs, so there are also cost issues, and many issues remain for practical use. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2018-184823 A Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, the object of the present invention is to provide a method for improving soft sandy ground that can solve the above-mentioned problems of the conventional method and increase the strength of soft ground mainly composed of sandy soil at low cost without using cement. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors have investigated the reason why the soil improvement method of the prior invention is effective for clayey soil but not for sandy soil. As a result, the gravel of sandy soil has collapsed into hard or glassy particles, and has less fine particles than clayey soil. As a result, the interfacial area for ion reaction is small, and the pozzolanic reaction of ion components (reaction between CaO in steelmaking slag and soluble silica in soil), which is the key point of the prior invention, does not occur sufficiently in sandy soil. Then, the inventors have found that it is possible to improve soft ground mainly composed of sandy soil by using the hydration reaction of meyanite (12CaO 7Al2O3) instead of the pozzolanic reaction of the prior invention.

[0010] The method for improving soft sandy ground of the present invention was developed based on the above findings, and is characterized by improving the soil strength by mixing a soil improvement material consisting of a substance with a basicity of 2 or more and containing 10 mass% or more of alumina, and crushed steelmaking slag stone, into soft sandy ground.

[0011] In addition, electric furnace reduced slag containing meyanite (12CaO 7Al2O3) can be used as a substance having a basicity of 2 or more and containing 10 mass% or more of alumina. In this case, it is preferable that the mixing ratio of electric furnace reduced slag to crushed steel slag is 10 mass% or less. Effect of the Invention

[0012] In the method for improving soft sandy ground of the present invention, a material with a basicity of 2 or more and containing 10% by mass or more of alumina and crushed steel slag are used as soil improvement materials. The mechanism of improvement of soft sandy ground using this soil improvement material is that the moisture in the soft sandy soil reacts with the meyanite present in the soil improvement material to reduce the moisture in the soil as a whole, and the clayey soil is hydrophobized, thereby developing strength. In other words, the main reaction is hydrophobization by the hydration reaction of meyanite, rather than dehydration by the slake reaction of quicklime (free lime) contained in crushed steel slag, as in the conventional method, or physical absorption of moisture by porosity.

[0013] In addition, electric furnace reduced slag containing meyanite (12CaO 7Al2O3) can be used as a material with a basicity of 2 or more and containing 10 mass% or more of alumina. Like steelmaking slag, electric furnace reduced slag is a by-product of the steelmaking process and can be obtained at a very low cost. This makes it possible to increase the strength of soft ground, mainly consisting of sandy soil, at low cost without using expensive cement as in the past. In addition, the method for improving sandy soft ground of the present invention has the advantage of not generating dust as in the case of using cement, and not worsening the working environment. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a diagram showing particle size classifications and names of soil particles. [Diagram 2] FIG. 1 is a pseudo-ternary phase diagram of electric furnace reducing slag. [Diagram 3] 1 is a graph showing CBR of a comparative example and an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The embodiment of the present invention will be described below. The sandy soil targeted by the present invention has a larger soil particle size than the clayey soil targeted by the prior invention. As shown in FIG. 1, in engineering terms, soil particles with a particle size of less than 75 μm are called fine particles, those with a particle size of up to 2 mm are called sand, and those with a particle size of up to 75 mm are called gravel. Clayey soil is mainly composed of fine particles, while sandy soil is mainly composed of sand and gravel. As the particle size of the soil particles of clayey soil and sandy soil differs, the interfacial area is also significantly different, and the total interfacial area of ​​the particles of sandy soil with a large particle size is much smaller than that of clayey soil with a small particle size. As a result, the pozzolanic reaction of the ionic components that worked effectively in the prior invention does not work effectively in sandy soil. In actual soil, fine particles, sand, and gravel are mixed, so the boundary between clayey soil and sandy soil is not necessarily clear, but clayey soil and sandy soil are distinguished by the particle size of the main soil particles.

[0016] In the present invention, a material with a basicity of 2 or more and containing 10% or more by mass of alumina and crushed steel slag are used as soil improvement materials. They may be mixed with the existing soil at the site, or they may be mixed in advance to prepare a soil improvement material, which is then mixed with the existing soil. Steel slag is mainly composed of CaO, SiO2, and Al2O3, and further contains MnO, FeO, MgO, etc. In particular, it contains about 30 to 60% by mass of CaO, and some of the CaO is not completely melted in the slag, and exists in the steel slag as unmelted free lime (free lime) by several percent. Steel slag is mainly crushed and sieved, and the grain size is adjusted to about 0 to 25 mm (including 0 to 30 mm and 0 to 40 mm), to produce crushed steel slag, which is used as civil engineering materials such as roadbed materials.

[0017] Even if only this crushed steel slag is mixed into sandy soil, the CBR, which indicates the soil strength, will increase to a certain extent. Here, CBR is a value that indicates the roadbed bearing capacity ratio specified in JIS A 1211 "CBR test method". Specifically, a piston with a diameter of 5 cm is penetrated into the target soil at a speed of 1 mm / min, and the load at penetrations of 2.5 mm and 5.0 mm is read, and the ratio to the standard load is calculated. Normally, soil is added with a target CBR of 20%, but if the existing soil is sandy and its CBR is about 2%, where a person cannot stand, it is not possible to reach that level by simply mixing crushed steel slag into the existing soil. As mentioned above, in the case of sandy soil, the dehydration effect due to the digestion reaction of free lime alone cannot sufficiently improve the soil strength.

[0018] Therefore, in the present invention, a substance having a basicity of 2 or more and containing 10 mass% or more of Al2O3 is mixed with the crushed steel slag. Such a substance is not particularly limited, but from an economical point of view, electric furnace reduced slag containing meyanite (12CaO 7Al2O3) can be used. A representative example of the composition of electric furnace reduced slag is shown in Table 1. For reference, the composition of the steel slag used in the prior invention is also shown. As shown in Table 1, electric furnace reduced slag contains a large amount of Al2O3. Moreover, since the basicity is 2 or more, it contains meyanite. Since meyanite is a mineral phase that exists in an area with low SiO2, meyanite is not formed if the basicity is less than 2.

[0019] [Table 1]

[0020] Table 2 shows a representative compositional representation of a pseudo-ternary system that focuses only on CaO, SiO2, and Al2O3 in the electric furnace reducing slag mentioned above. Figure 2 shows the pseudo-ternary phase diagram.

[0021] [Table 2]

[0022] In the present invention, as described above, a material having a basicity of 2 or more and containing 10% by mass or more of alumina and crushed steel slag are used as soil improvement materials. However, since electric furnace reduced slag may violate environmental standards due to fluorine, when using electric furnace reduced slag, it is preferable to mix it at a ratio of 10% by mass or less with non-fluorine-contaminated steel slag. The preferred mixing ratio is about 3 to 8%.

[0023] This soil improvement material is mixed into the soft sandy ground. The mixing ratio of the soil improvement material to the existing soil is preferably 20% to 33%. In actual construction sites, the existing soil and the soil improvement material are temporarily placed in separate piles and mixed by the number of times of the bucket of a power shovel. If the number of times is 4:1, the mixing ratio is 20%, and if it is 2:1, the mixing ratio is 33%. In addition, when improving shallow ground in a wide area such as a parking lot of a mass retailer or a container yard of a port facility, a construction method can be used in which the soil improvement material is spread evenly on the top of the soft sandy ground, and then the two are mixed at once using a surface soil mixer / mixer called a stabilizer. EXAMPLES

[0024] Comparative examples and examples of the present invention are given below. Comparative Example 1 First, the effect of improving the strength of soft soil was investigated using only electric furnace reduced slag containing meyanite (12CaO 7Al2O3) with CBR. The existing soil before improvement was a soft clayey soil with a water content of 30% and sand, with a CBR of ≦1%. When only electric furnace reduced slag was mixed with this soft soil at 20% and 33%, the CBR was 50% and 100%, respectively, demonstrating a significant improvement effect.

[0025] However, it was found that the F content of this electric furnace reduced slag was significantly higher than the environmental standard (F content) set by the Soil Contamination Countermeasures Act, and that it could not be sold on its own to the market. In other words, while the F content in the Soil Contamination Countermeasures Act is 4000mg / kg or less, the F content of the electric furnace reduced slag used was 24000mg / kg. Furthermore, no strength development was observed after curing.

[0026] In response to this situation, the inventors investigated the mixing ratio of the conventional converter slag-based improver without F contamination and the electric furnace reduced slag described above at various ratios. As a result, they found that there is a nearly linear relationship between the mixing ratio and the F content. In other words, they confirmed that in order to use electric furnace reduced slag with an F content of 24,000 mg / kg in a state that satisfies the environmental standards, it is essential to limit the mixing ratio to 10% or less.

[0027] Comparative Example 2 The porous steel slag crushed stone (water absorption rate 4.9%) used in the prior invention was mixed with a sandy soft ground with a CBR of 2% in a volume ratio of soil to steel slag crushed stone = 2:1. As a result, the CBR immediately after mixing was 6%, and even after one week of curing, it only improved to 10%. Although the target CBR for subgrade soil is CBR ≥ 20%, it did not reach that level. In addition, the high basicity steel slag (basicity 4.6) used in the prior invention was mixed with a sandy soft ground with a CBR of 2% in a volume ratio of 2:1, but the CBR immediately after mixing was 10%, and even after one week of curing, it only improved to 11%. Thus, the soil improvement effect of the prior invention was insufficient for sandy soft ground. The composition of the steel slag crushed stone used here is as shown in Table 1 above.

[0028] Example 1 Thus, even if only crushed steel slag was mixed into the sandy soft ground with a CBR of 2%, a CBR of 20% could not be achieved. Therefore, a soil improvement material made by mixing 8% electric furnace reduced slag with crushed steel slag, as shown in Table 1 above, was mixed in a volume ratio of current soil:soil improvement material = 2:1. As a result, the CBR was 40% immediately after mixing, and after allowing one week of curing, it increased to 58%.

[0029] Example 2 For a sandy soft ground with gravel and a water content of 25% and a CBR of 1.2%, 33% of the high basicity crushed steel slag used in the prior invention was mixed, but the CBR only improved to 12%. When the electric furnace reduced slag shown in Table 1 was then added at a ratio of 10% to the crushed steel slag and 3% to the existing soil, the CBR immediately after mixing was 50%, and after a one-week curing period, it improved significantly to 70%. In addition, the F content of the soil improvement material in which 10% of the electric furnace reduced slag was mixed with the crushed steel slag was 2500 mg / kg, which satisfied the environmental standard of 4000 mg / kg or less.

[0030] The CBR data of the above-mentioned Comparative Example 2, Example 1, and Example 2 are shown in the graph of Figure 3. As shown in this graph, according to Examples 1 and 2 of the present invention, the CBR can be made to be 20% or more, which is the target value for the roadbed, and can be further improved by curing for one week.

[0031] As described above, according to the present invention, soft sandy ground can be improved by mixing crushed steel slag and reduced slag from an electric furnace, so that the soil strength of the ground can be increased to a specified level, and the efficiency and cost of ground improvement can be improved. In addition, the cost of removing the existing soft soil from the system and the cost of purchasing and replacing good quality soil are no longer necessary, so that ground improvement work can be carried out more quickly and at a lower cost. Furthermore, steel slag generated as a by-product in the steelmaking process can be effectively used when working on soft ground. As a result, steel slag can be effectively used as a civil engineering material other than its conventional use, the sales channels for steel slag can be expanded, and the need to transport steel slag to a final disposal site can be significantly reduced.

Claims

1. A method for improving soft sandy ground, characterized in that the soil strength is improved by mixing a soil improvement material consisting of a substance with a basicity of 2 or more and containing 10 mass% or more of alumina and crushed steelmaking slag stone with the soft sandy ground.

2. Meyanite (12CaO 7Al) is a material with a basicity of 2 or more and containing 10 mass% or more of alumina. 2 O 3 2. The method for improving soft sandy ground according to claim 1, characterized in that electric furnace reduced slag containing slag oxide is used.

3. 3. The method for improving soft sandy ground according to claim 2, characterized in that the mixing ratio of electric furnace reduced slag to crushed steelmaking slag stone is 10 mass% or less.

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