Recycled sand, ground improvement method, and ground structure

Abstract

To provide recycled sand capable of stably securing quality (hydraulic filling CBR value, water permeability, particle size distribution) required for sand for forming a sand mat in a sand mat method, and to provide a ground improvement method and a ground structure utilizing the recycled sand.SOLUTION: Recycled sand is obtained by carrying out melting treatment of general waste and industrial waste at 1200°C or higher in a waste melting furnace in which coke and lime stone are added as secondary materials, rapidly crushing the generated melt with water, eliminating metal iron by magnetic separation, and carrying out mechanical pulverization treatment or continuous grinding treatment, and has quality of a hydraulic filling CBR value of 10% or more, and a water permeability coefficient in 10-4 m / s. The recycled sand is laid on a weak ground 1 so that a site CBR value becomes 7 to 10%, to form a sand mat 2.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to recycled sand for forming a sand mat in the sand mat method, and a ground improvement method and a ground structure using the recycled sand.

Background Art

[0002] The sand mat method is a method applied when constructing an earth structure such as an embankment on soft ground. By laying sand on the soft ground to form a sand mat, it is a method for promoting upper drainage for consolidation of the soft layer and ensuring the trafficability of construction machinery. As sand for forming a sand mat in such a sand mat method, natural sand such as sea sand and river sand has generally been used in the past (for example, Patent Document 1), but from the viewpoint of reducing environmental impact, the use of recycled sand is desirable.

[0003] Known recycled sands include blast furnace granulated slag, recycled sand derived from concrete blocks, recycled sand derived from construction waste soil, recycled sand derived from bricks, etc., crushed sand (sand obtained by artificially crushing natural rock), etc., but each has the following problems. <Blast furnace granulated slag> The pH value is 11 or more and it is highly alkaline, which has an adverse effect on plant growth. In addition, since there are many unreacted CaO components, solidification occurs after laying construction and the water permeability decreases. <Recycled sand derived from concrete blocks> Similar to steel slag / blast furnace granulated slag, the pH value is 11 or more and it is highly alkaline, which has an adverse effect on plant growth. In addition, since there are many unreacted cement components, solidification occurs after laying construction and the water permeability decreases. <Recycled sand derived from construction waste soil> The quality (compacted CBR value, water permeability, particle size distribution) varies greatly depending on the construction waste soil, and it is difficult to ensure stable quality. <Recycled sand derived from bricks, etc.> The quality (compacted CBR value, water permeability, particle size distribution) varies greatly depending on the raw material type, and it is difficult to ensure stable quality. <Crushed sand> (sand obtained by artificially crushing natural rock) It has good water permeability, but other qualities (compacted CBR value, particle size distribution) vary depending on the rock source.

[0004] In the case of the recycled sand known in the prior art as described above, it is impossible to stably ensure the qualities (compacted CBR value, water permeability, particle size distribution) required for the sand to form sand mat in the sand mat method.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] The problem to be solved by the present invention is to provide recycled sand capable of stably ensuring the qualities (compacted CBR value, water permeability, particle size distribution) required for the sand to form sand mat in the sand mat method, as well as a ground improvement method and a ground structure using the recycled sand.

Means for Solving the Problems

[0007] According to one aspect of the present invention, the following recycled sand is provided Recycled sand for forming sand mat in the sand mat method, which is obtained by rapidly pulverizing with water a melt generated by melting general waste and industrial waste at a high temperature of 1200°C or higher in a waste melting furnace to which coke and limestone are added as auxiliary materials, removing metallic iron content by magnetic separation, and performing mechanical pulverization treatment or grinding treatment, having a particle size distribution with an average particle size of 0.6 to 0.8 mm, and 100% by mass being 5 mm or less, 98 to 99% by mass being 2.5 mm or less, 1 to 2% by mass being more than 2.5 mm and 5 mm or less, and 2 to 4.5% by mass being 0.075 mm or less, having a compacted CBR value of 10% or more and a hydraulic conductivity of 10 -4 m / s or more (1×10 -4 m / s or more and 1×10 -3 m / s or less) and having such qualities.

[0008] According to another aspect of the present invention, there is provided a ground improvement method including a step of laying the recycled sand of the present invention on soft ground so that the in-situ CBR value becomes 7 to 10% to form a sand mat.

[0009] According to still another aspect of the present invention, there is provided a ground structure in which a sand mat made of the recycled sand of the present invention is formed on soft ground and the in-situ CBR value is 7 to 10%.

[0010] Here, the "soft ground" in the present invention qualitatively refers to a ground composed of soft clay or loose sand, having weak soil strength, high durability and compressibility, and quantitatively refers to a ground with an N value of less than 5. The N value is a numerical value serving as a standard for determining the degree of compaction and strength of soil and is obtained by a standard penetration test. The standard penetration test is a test in which a hammer attached to the tip of a measuring iron rod instrument is freely dropped from a predetermined height, and the number of blows until the instrument is introduced 30 cm into the soil is the N value. That is, the higher the N value, the more compact the soil and the more capable it is of withstanding heavy buildings. It is generally said that in many cases, general housing construction is possible when the N value is 5 or more.

Advantages of the Invention

[0011] According to the present invention, it is possible to provide recycled sand capable of stably ensuring the quality (water-compacted CBR value, water permeability, particle size distribution) required for sand for forming a sand mat in the sand mat method, and a ground improvement method and a ground structure using the recycled sand.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0013] First, the recycled sand of the present invention will be described. The recycled sand of the present invention is obtained by rapidly pulverizing with water a melt generated by melting general waste and industrial waste at a high temperature of 1200°C or higher in a waste melting furnace (hereinafter referred to as a "coke bed type waste melting furnace") that adds coke and limestone as auxiliary materials, removing metallic iron by magnetic separation, and performing mechanical pulverization or grinding treatment.

[0014] Here, the "general waste" in the present invention refers to household waste and business-related general waste (excluding industrial waste generated in business activities), and the "industrial waste" refers to ordinance industrial waste (also called "designated industrial waste") defined by local governments in ordinances (for example, dehydrated sludge from sewage treatment facilities). These general waste and industrial waste can be treated at general waste treatment facilities, and the above-mentioned coke bed type waste melting furnace is also included in general waste treatment facilities. In the following description, general waste and industrial waste are collectively referred to as "waste".

[0015] In the coke bed type waste melting furnace, when coke and limestone are added as auxiliary materials at the time of waste input, the following effects can be obtained by the coke and limestone added as auxiliary materials. That is, the following effects can be obtained by adding coke as an auxiliary material. (1) The ambient temperature at the lower part of the furnace becomes a high temperature state of 1700 to 1800°C, and the waste is completely melted in this high temperature state, so that the ash content in the waste is completely melted (the unmolten part disappears). (2) The furnace atmosphere becomes a high temperature reducing atmosphere (a high temperature atmosphere without oxygen), and by its action, harmful substances such as heavy metals in the waste migrate to the gas phase, and the content of harmful substances in the melt is kept low. Therefore, safety comparable to that of natural sand can be obtained. In addition, the following effects can be obtained by adding limestone as an auxiliary material. (3) By adjusting the addition ratio of limestone according to the type of waste, the concentrations of the main components (silica content / SiO2, lime content / CaO) of the melt are stabilized throughout the year. (4) Stabilization of the main component concentration of the melt and high-temperature melting promote the separation of slag and metallic iron during rapid pulverization with water, improving the separability in the subsequent magnetic separation process. As a result, the content of metallic iron in the obtained recycled sand becomes very low. (5) By stabilizing the main component concentration of the melt throughout the year, the quality (water-compacted CBR value, water permeability, particle size distribution) of the obtained recycled sand also remains stable throughout the year. In addition, the compaction performance improves due to the stable particle size distribution.

[0016] Table 1 shows the results of measuring the main components, pH value, particle size distribution, designed CBR value, water-compacted CBR value, and permeability coefficient of the recycled sand actually obtained from the coke bed type waste melting furnace. Here, the main components, pH value, and particle size distribution are the annual measurement results (average values) of the recycled sand obtained from three coke bed type waste melting furnaces. Also, the designed CBR value, water-compacted CBR value, and permeability coefficient are the summarized measurement results for the recycled sand obtained from multiple coke bed type waste melting furnaces.

[0017]

Table 1

[0018] In Table 1, the dimensions shown in "Particle Size Distribution" indicate the sieve mesh sizes defined by the nominal aperture of the sieves specified in JIS Z 8801-1, and the mass% indicates the mass ratio passing through the sieve. Also, "Average" (average particle diameter) refers to the particle diameter at which the cumulative mass becomes 50% in the particle size curve drawn as a graph with the sieve mesh size on the horizontal axis and the mass percentage of the aggregate mass passing through the sieve with respect to the aggregate mass applied to the sieve on the vertical axis. Regarding the particle size (grain size), the particle size (grain size) of the molten slag is defined in "5.4.2 Particle Size and Coarse Particle Ratio of Molten Slag Fine Aggregate" of JIS A 5031:2016 (Melting Slag Aggregate for Concrete Obtained by Melting and Solidifying General Waste, Sewage Sludge, or Their Incineration Ash).

[0019] On the one hand, for the design CBR value, the test method is specified in JIS A 1211 (CBR test method). An infiltration test is carried out using a CBR tester consisting of a loading device, a load cell, a penetration piston, and an infiltration measurement device. While penetrating a piston with a diameter of 5 cm at 1 mm / min, the loads at penetration amounts of 2.5 mm and 5.0 mm are read to obtain the CBR value (design CBR value). Also, for the water-compacted CBR value, JIS A 1211 (CBR test method) is applied mutatis mutandis. In the water-compacted CBR test, instead of compacting with a rammer, the sample is immersed in water and dropped into the mold together with the sample. After digging, a perforated weight plate of 5 kg is placed on it, and after standing for 15 minutes, it is penetrated to obtain the CBR value. On the other hand, for the permeability coefficient, the test method is specified in JIS A 1218 (Soil permeability test method). The test specimen is set in a cylinder, immersed in water, and vacuum-deaerated to a saturated state. The constant head method (constant water level difference, measuring the water volume passing through the soil sample) is adopted to determine the permeability.

[0020] As shown in Table 1, the variation in the main component is about ±2 mass% of the annual average value, the variation in the pH value is about ±0.5 of the annual average value, and the variation in the particle size distribution is about ±1 mass% of the annual average value. It can be seen that all are stable throughout the year.

[0021] Here, the quality standards required for the sand for forming the sand mat in the sand mat method are generally as follows. · Water-compacted CBR value: 2% or more · Permeability coefficient: 10 -4 m / s or more · Particle size distribution: sandy (5 mm or less) Among these, as shown in Table 1, the water-compacted CBR value is 10% or more, far exceeding the quality standard (2% or more). Therefore, conventionally, when natural sand was laid to form a sand mat so that the in-situ CBR value would be 7 - 10%, the sand mat thickness was about 100 cm as a guideline. However, when using the recycled sand of the present invention, the sand mat thickness is about 40 cm, or about 50 cm allowing for a margin, enabling shortening of the sand mat construction process and leading to reduction of construction costs. Note that the in-situ CBR value of 7 - 10% is the condition where the surface layer is passed by people and light vehicles. Also, the in-situ CBR value is measured based on JIS A 1222 (In-situ CBR test method).

[0022] As described above, the recycled sand of the present invention has a quality with a water-compacted CBR value of 10% or more. The upper limit of the water-compacted CBR value is not particularly limited, but as past performance values, about 18% is the maximum value.

[0023] On the other hand, as shown in Table 1, for the permeability coefficient, it stably satisfies the range of 10 -4 m / s or more.

[0024] Also, regarding the particle size distribution, it stably satisfies the sandy state (5 mm or less) of the quality standard. Furthermore, the recycled sand of the present invention is obtained by performing mechanical crushing treatment or grinding treatment. As a result of performing mechanical crushing treatment or grinding treatment to adjust the particle size, as shown in Table 1, the variation in the particle size distribution is small and stable. Therefore, the compaction performance is improved. Note that the mechanical crushing treatment can be performed by crushing an object between a rotating tooth and a fixed tooth with a device having a rotating shaft. Also, the mechanical grinding treatment can be performed by putting the object into a rotating drum and grinding the object.

[0025] Next, a ground improvement method and ground structure using the recycled sand of the present invention will be described. Figure 1 conceptually shows in cross-section the ground structure according to an embodiment of the present invention. In the ground structure shown in this figure, a sand mat 2 made of the recycled sand of the present invention described above is formed on the soft ground 1, and its in-situ CBR value is 7 - 10%. From the perspective of the ground improvement method, the ground improvement method of the present invention includes a step of laying the recycled sand of the present invention described above on the soft ground 1 so that the in-situ CBR value becomes 7 - 10% to form the sand mat 2. As described above, since the recycled sand of the present invention has a high water-compacted CBR value of 10% or more, the thickness of the sand mat 2 is about 40 cm when using the recycled sand of the present invention, compared to about 100 cm when using natural sand conventionally. When forming the sand mat 2 on the soft ground 1, the surface layer portion of the soft ground 1 may be scarified by a thickness corresponding to the thickness of the sand mat 2, and the sand mat 2 may be formed on the soft ground 1, or the sand mat 2 may be directly formed on the soft ground 1 without scarifying the surface layer portion of the soft ground 1. In any case, the recycled sand of the present invention described above is laid on the soft ground 1 so that the in-situ CBR value becomes 7 - 10% to form the sand mat 2.

[0026] Figure 2 conceptually shows in cross-section the ground structure according to another embodiment of the present invention. In the ground structure shown in this figure, a mixed soil 3 in which 20 - 40% by volume of the recycled sand of the present invention described above is mixed with the soil of the soft ground 1 (hereinafter referred to as "existing soil") is laid on the sand mat 2 shown in Figure 1. From the perspective of the ground improvement method, the ground improvement method of the present invention includes a step of laying the mixed soil 3 in which 20 - 40% by volume of the recycled sand of the present invention described above is mixed with the existing soil on the sand mat 2 shown in Figure 1.

[0027] In this way, by laying the mixed soil 3, which is a mixture of the existing soil and the recycled sand of the present invention, on the sand mat 2, the soft ground 1 can be improved into arable farming soil, and the amount of residual soil (existing soil) generated by hoeing the surface layer of the soft ground 1 can be reduced. The mixing ratio of the recycled sand of the present invention to the existing soil is 20 to 40% by volume. When the mixing ratio of the recycled sand is less than 20% by volume, depending on the quality of the soft ground 1 (existing soil), the improvement effect such as the permeability coefficient may be small. When the mixing ratio of the recycled sand exceeds 40% by volume, there is a concern that the water retention of the mixed soil 3 may decrease. Note that the thickness of the mixed soil 3 may be appropriately set according to the application. For example, when it is used as arable farming soil, it can be about 20 to 40 cm.

Example

[0028] For soft ground with an N value of less than 5, ground improvement construction was carried out under the following conditions. Note that before forming the sand mat, about 40 cm of the surface layer of the soft ground was hoed. · Target area: about 1500 m 2 · Sand mat thickness: about 40 cm on average · Amount of recycled sand used: about 900 t (about 600 m 3 ) · Quality of the recycled sand used Water-compacted CBR value: 11.5% Design CBR value: 23% Permeability coefficient: 2.1×10 -4 m / s Average particle size: 0.61 mm Percentage of material passing through a 5 mm sieve: 100% Percentage of material passing through a 2.5 mm sieve: 99% Percentage of material passing through a 0.075 mm sieve: 4% · Thickness of the mixed soil: about 30 cm on average · Mixed soil used: 30% by volume of recycled sand mixed with the existing soil

[0029] Although the N-value of the soft ground was less than 5 as described above, by laying the recycled sand on the soft ground to form a sand mat with an average thickness of about 40 cm, the in-situ CBR value could be made 7 - 10%. The ground with an N-value less than 5 is a soft ground where steel bars are inserted into the ground surface and can be easily penetrated by hand, and neither people nor light vehicles can pass through it. On the contrary, when the in-situ CBR value reaches 7 - 10%, people and light vehicles can pass through as described above.

[0030] In addition, in this embodiment, a mixed soil in which 30% by volume of recycled sand was mixed with the existing soil was laid on the sand mat with an average thickness of about 30 cm. As a result, it became available as arable farming soil.

Description of Signs

[0031] 1 Soft ground 2 Sand mat 3 Mixed soil

Claims

1. Recycled sand for forming a sand mat in the sand mat method, which is obtained by rapidly pulverizing with water a melt generated by melting general waste and industrial waste at a high temperature of 1200 °C or higher in a waste melting furnace to which coke and limestone are added as auxiliary materials, removing metallic iron content by magnetic separation, and performing mechanical pulverization treatment or grinding treatment, having an average particle size of 0.6 to 0.8 mm, and a particle size distribution of 100% by mass or less of 5 mm or less, 98 to 99% by mass of 2.5 mm or less, 1 to 2% by mass of more than 2.5 mm and 5 mm or less, and 2 to 4.5% by mass of 0.075 mm or less, the recycled sand having a quality with a water-compacted CBR value of 10% or more and a permeability coefficient of 1×10−4 m / s or more and less than 1×10−3 m / s.

2. A ground improvement method including a step of laying the recycled sand according to Claim 1 on soft ground so that the in-situ CBR value becomes 7 to 10% to form a sand mat.

3. The ground improvement method according to Claim 2, wherein in the step of forming the sand mat, the sand mat thickness is made to be an average of 40 cm or more and 50 cm or less.

4. The ground improvement method according to Claim 2 or 3, including a step of laying a mixed soil in which 20 to 40% by volume of the recycled sand according to Claim 1 is mixed with the soil of the soft ground on the sand mat.

5. The ground improvement method according to Claim 4, which is for improving arable cultivated soil.

6. A ground structure in which a sand mat made of the recycled sand according to Claim 1 is formed on soft ground and the in-situ CBR value is 7 to 10%.

7. The ground structure according to Claim 6, wherein the thickness of the sand mat is an average of 40 cm or more and 50 cm or less.

8. The ground structure according to Claim 6 or 7, wherein a mixed soil in which 20 to 40% by volume of the recycled sand according to Claim 1 is mixed with the soil of the soft ground is laid on the sand mat.

9. The ground structure according to Claim 8, which is for arable use.

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