Construction method of low water permeable layer and low water permeable layer structure

Abstract

To provide a construction method of a low water permeable layer and a low water permeable layer structure which can shorten a construction time.SOLUTION: A method for constructing a low water permeable layer includes: a step S1 of repeating spreading of a low water permeable soil material and rolling the spread soil material, and thereby previously manufacturing a block-shaped precast soil material composed of laminated soil layers; and steps S2 and S3 of densely arranging the plurality of manufactured precast soil materials, then providing a sealing material in a gap between the precast soil materials and sealing the gap with the sealing material, and thereby constructing the low water permeable layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a construction method and structure of a low-permeability layer that is provided around a facility such as a concrete pit in a low-level radioactive waste disposal site. [Background technology]

[0002] Conventionally, in facilities that must be constructed to minimize groundwater infiltration, such as concrete pits in low-level radioactive waste disposal sites, the concrete pit is surrounded by bentonite, which has low permeability, and the surrounding area is covered with soil using a conventional method (see, for example, Patent Document 1 and Non-Patent Document 1). The covering soil layer is usually compacted using a compaction machine such as a roller or striking hammer (see, for example, Patent Document 2 and Non-Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2023-48432 [Patent Document 2] Japanese Patent Application Publication No. 2023-49301 [Non-patent literature]

[0004] [Non-Patent Document 1] "Permeability characteristics and evaluation method of hydraulic conductivity of gravel-sand-bentonite mixture soil", Toshiyuki Tanaka et al., Journal of the Japan Society of Civil Engineers, Vol. 64, No. 1, pp. 101-110, 2008 [Non-patent document 2] "Estimation of spatial distribution of bentonite-based low-permeability layers using geostatistics," Fumihiko Ono et al., Proceedings of the 2006 Autumn Meeting of the Atomic Energy Society of Japan, p.105, 2006 Summary of the Invention [Problem to be solved by the invention]

[0005] By the way, if the cover soil layer is strongly compacted using a compaction machine such as a roller or hammer, the permeability of the cover soil layer will be reduced, but since the compaction machine will have to be moved back and forth along the surface of the cover soil layer many times, there is a risk that the construction time will increase.

[0006] In order to solve these problems, the inventors have conducted extensive research into construction methods that can shorten construction time, and as a result have arrived at the present invention, which utilizes the anisotropic permeability of precast soil materials produced by compaction.

[0007] The present invention has been made in view of the above, and aims to provide a construction method for a low-permeability layer and a low-permeability layer structure that can shorten construction time. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems and achieve the object, the construction method of a low-permeability layer according to the present invention is a method of constructing a low-permeability layer, characterized by comprising the steps of: producing a block-shaped precast soil material consisting of stacked soil layers in advance by repeating the rolling out of low-permeability soil material and the compaction of the rolled out soil material, layer by layer; and constructing a low-permeability layer by arranging a plurality of the produced precast soil materials closely together and then providing a sealing material in the gaps between the precast soil materials to seal them.

[0009] Furthermore, another construction method of a low-permeability layer according to the present invention is characterized in that, in the above-mentioned invention, the placement orientation of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material.

[0010] Furthermore, another method for constructing a low-permeability layer according to the present invention is characterized in that, in the above-mentioned invention, the orientation of the precast soil material is set so that the direction of water flow expected around the area where the low-permeability layer is constructed is parallel to the stacking direction of the soil layers in the precast soil material.

[0011] The low-permeability layer structure according to the present invention is characterized in that it comprises a plurality of block-shaped precast soil materials consisting of soil layers in which low-permeability soil materials are stacked and compacted closely together, and sealing materials are provided in the gaps between the precast soil materials.

[0012] Furthermore, another low-permeability layer structure according to the present invention is characterized in that, in the above-mentioned invention, the orientation of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material.

[0013] Another low-permeability layer structure according to the present invention is characterized in that, in the above-mentioned invention, the direction of water flow expected around the low-permeability layer is set parallel to the stacking direction of the soil layers in the precast soil material. [Effects of the Invention]

[0014] The method for constructing a low-permeability layer according to the present invention is a method for constructing a low-permeability layer, which comprises the steps of: previously manufacturing a block-shaped precast soil material consisting of stacked soil layers by repeatedly rolling out low-permeability soil material and compacting the rolled-out soil material, layer by layer; and constructing a low-permeability layer by arranging a plurality of the manufactured precast soil materials closely together and then providing a sealing material in the gaps between the precast soil materials to seal them, thereby achieving the effect of shortening construction time compared to conventional construction methods in which strong rolling is carried out by repeatedly going back and forth with a roller or the like at the construction site.

[0015] Furthermore, according to another construction method of a low-permeability layer of the present invention, the placement direction of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material, thereby achieving the effect of being able to set the desired water-blocking performance in a specified direction.

[0016] Furthermore, according to another method for constructing a low-permeability layer of the present invention, the precast soil material is oriented so that the expected direction of water flow around the area where the low-permeability layer is constructed is parallel to the stacking direction of the soil layers in the precast soil material, thereby achieving the effect of improving performance as a water-blocking layer.

[0017] Furthermore, the low-permeability layer structure of the present invention is a low-permeability layer structure that includes a plurality of block-shaped precast soil materials consisting of soil layers made of densely packed, layer-by-layer laminated low-permeability soil materials, and sealing material provided in the gaps between the precast soil materials. This has the effect of enabling construction to be carried out in a shorter time than conventional structures that require strong compaction by repeatedly going back and forth with a roller or the like at the construction site.

[0018] Furthermore, according to another low-permeability layer structure of the present invention, the orientation of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material, thereby achieving the effect of being able to set the desired water-blocking performance in a specified direction.

[0019] Furthermore, according to another low-permeability layer structure of the present invention, the direction of the expected water flow around the low-permeability layer and the stacking direction of the soil layers in the precast soil material are set to be parallel, thereby achieving the effect of improving performance as a water-blocking layer. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a schematic flow diagram showing an embodiment of a low-permeability layer construction method and a low-permeability layer structure according to the present invention. [Figure 2] FIG. 2 is a schematic vertical cross-sectional view showing an embodiment of a low-permeability layer structure according to the present invention. [Figure 3] FIG. 3 is a schematic vertical cross-sectional view showing an example of the arrangement of precast soil materials according to this embodiment, where (1) shows a case where the size of the precast soil materials is large, and (2) shows a case where the size is small. [Figure 4] FIG. 4 shows an example of the vertical depth distribution of the dry density of a bentonite layer constructed using the in-situ compaction method (source: Non-Patent Document 2). [Figure 5] FIG. 5 is a diagram showing an example of the vertical distribution of hydraulic conductivity. [Figure 6] Figure 6 is a schematic diagram of the ground below the groundwater level WL, where (1) is ground where the soil layer composition direction is horizontal to the groundwater flow, and (2) is ground where the composition direction is vertical. [Figure 7] FIG. 7 is a diagram showing an example of the relationship between the length of one side of a block and the porosity. DETAILED DESCRIPTION OF THE INVENTION

[0021] Hereinafter, embodiments of the low-permeability layer construction method and low-permeability layer structure according to the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to these embodiments.

[0022] As shown in Figure 1, the method for constructing a low-permeability layer according to an embodiment of the present invention is a method for constructing a low-permeability layer, and includes step S1 of manufacturing block-shaped precast soil materials in advance, step S2 of densely arranging the manufactured precast soil materials in the construction area, and then step S3 of sealing the gaps between the precast soil materials. In this way, a low-permeability layer (cover soil) can be constructed.

[0023] FIG. 2 shows an application example of a low-permeability layer structure 10 constructed by the above-described method. As shown in this figure, the low-permeability layer structure 10 according to the embodiment of the present invention comprises a plurality of block-shaped precast soil materials 12 and a sealing material 16 provided in the gaps 14 between the precast soil materials 12. The low-permeability layer structure 10 is provided around a location where infiltration of groundwater or the like is undesirable, such as a concrete pit 1 in a low-level radioactive waste disposal site. In the example of FIG. 2, the low-permeability layer structure 10 is provided as cover soil in the areas to the sides and above the concrete pit 1 provided in the ground G.

[0024] Precast soil material 12 is a block-shaped precast material consisting of layers of soil stacked by repeatedly extruding low-permeability soil material and compacting the extruding soil material layer by layer. In the example shown in Figure 2, precast soil material 12A, with soil layers 18 stacked vertically, is installed in the area above concrete pit 1, and precast soil material 12B, with soil layers 18 stacked horizontally, is installed in the area to the side of concrete pit 1. Because groundwater tends to flow horizontally above concrete pit 1 and vertically below concrete pit 1, precast soil materials 12A and 12B are installed above and to the side of concrete pit 1 with the soil layers 18 stacked in different directions X. This arrangement improves the performance of the water barrier in each area, as described below.

[0025] (Method of manufacturing precast soil materials) In the above step S1, the precast soil material 12 can be manufactured, for example, by the following procedure. In addition to this procedure, the precast soil material 12 may also be manufactured using the method described in Patent Document 2. The precast soil material 12 may be manufactured in a factory or the like away from the construction site, or may be manufactured at or near the construction site.

[0026] First, the soil material is rolled out onto a horizontal surface within the formwork, and the top surface of the soil material is compacted using a compaction machine such as a sand rammer. The soil material is a low-permeability material, and may be composed of, for example, bentonite alone, or a mixture of bentonite and locally generated soil. The thickness of the soil material rolled out is kept constant. Next, soil material of the same composition is rolled out on top of the compacted soil material, and the top surface is compacted using a compaction machine. This rolling and compaction of the soil material is repeated until the stack of soil material reaches a certain height. Once it reaches a certain height, part of the formwork is removed, and the top of the stack is shaped using a large cutter. In this way, a layered block of precast soil material 12 can be produced.

[0027] (Precast soil material placement method) In the above step S2, when multiple precast soil materials 12 are closely arranged, as will be described later, it is preferable to arrange the precast soil materials 12 in a direction that increases permeability, since permeability varies depending on the direction of compaction. In this way, a structure that makes it difficult for water to penetrate into the low-permeability layer to be constructed can be constructed. For example, as shown in Figure 3, when the precast soil materials 12 are arranged so that the layering direction X of the soil layer 18 in each precast soil material 12 is horizontal, the permeability is low in the horizontal direction (high water impermeability) and high in the vertical direction (low water impermeability).

[0028] On the other hand, when multiple precast soil materials 12 are piled up for construction, gaps 14 are formed between the precast soil materials 12, which become passageways for water. Increasing the size of the precast soil materials 12 reduces the number of precast soil materials 12 required to cover the same volume of soil, thereby reducing the gaps 14. By sealing these gaps 14 with sealing material 16, construction with higher watertightness is possible.

[0029] (Directional dependence of permeability of precast soil materials) Next, we will explain the directional dependence of permeability of precast soil materials. Figure 4 shows the vertical depth distribution of the dry density of a bentonite layer that was compacted by rolling horizontally every 10 cm thick layer using a vibrating roller and a pneumatic impact hammer (see Non-Patent Document 2 mentioned above). This bentonite layer is made of a material in which simple bentonite has been adjusted to a predetermined moisture content. According to Non-Patent Document 2, the dry density ρ d (Mg / m 3 The relationship between the water content and the permeability coefficient k (m / s) is given by the following equation (1):

[0030]

number

[0031] From the relationship in Figure 4 and the relationship in equation (1), the relationship between the permeability coefficient and depth is as shown in Figure 5.

[0032] Generally, in ground composed of several soil layers with different hydraulic conductivity, the average hydraulic conductivity of the entire ground varies depending on the flow of water (groundwater) and the orientation of the soil layers (layer direction). The average hydraulic conductivity when the soil layer structure is parallel (horizontal) to the water flow is k h , the average hydraulic conductivity in the vertical case is k v In this case, the respective values ​​can be calculated using the following equations (2) and (3). n is the hydraulic conductivity of each soil layer, H1,…,H n is the thickness of each soil layer, and H is the total soil thickness.

[0033] When the direction of water flow and the direction of soil layer composition are parallel (see Figure 6 (1))

[0034]

number

[0035] When the direction of water flow and the direction of soil layer formation are perpendicular (see Figure 6 (2))

[0036]

number

[0037] For example, if the soil layer is the bentonite layer mentioned above, the average horizontal hydraulic conductivity is k h =1.86×10 -13 m / s, and the average hydraulic conductivity in the vertical direction is k v =1.73×10 -13m / s. In this way, the permeability of a bentonite layer constructed by compacting each layer depends on the layering direction X of the soil layer. In this case, by forming the soil layer perpendicular to the groundwater flow (the direction of the flow is parallel to the layering direction X of the soil layer), it is possible to construct a low-permeability layer with a lower permeability coefficient than when the soil layer is installed horizontally. Also, by changing the orientation of the soil layer relative to the groundwater flow, construction can be carried out that makes it difficult for water to penetrate into the interior covered by the soil layer.

[0038] In the above example, the difference in hydraulic conductivity between the horizontal and vertical directions is small because the bentonite layer is made of bentonite alone, which has extremely low permeability. However, if other soil materials are used, the difference in hydraulic conductivity between the horizontal and vertical directions may become larger. For example, if the precast soil material 12 is made of a soil material containing a mixture of gravel and bentonite, the difference in the anisotropy of the hydraulic conductivity is thought to be larger than in the case of a bentonite layer made of bentonite alone (see, for example, the above-mentioned Non-Patent Document 1). In this case, the difference in the values ​​of the hydraulic conductivity calculated from the above equations (2) and (3) becomes more clear, and the layering direction X of the soil layer 18 in the precast soil material 12 significantly contributes to the water-blocking performance.

[0039] (Gaps between precast soil materials) Next, the gaps between the precast soil materials formed in step S3 above will be described. As shown in Figure 3, if the length of one side of a cubic precast soil material 12 (hereinafter referred to as a block) is L and the length of the gap 14 is d, the number N of blocks required to cover an area of ​​volume V is approximately expressed as in equation (4).

[0040]

number

[0041] Furthermore, when N blocks are placed in an area of ​​volume V, the proportion of voids (porosity P (%)) is roughly expressed as in equation (5).

[0042]

number

[0043] When the length of the gap 14 is 1 mm, the relationship between the length of one side of the block and the porosity is as shown in Figure 7. As shown in this figure, the porosity can be reduced as the block size increases. There is no standard for porosity, but to reduce the porosity, it is preferable to use large blocks.

[0044] (Sealing gaps) The gaps 14 between the blocks when they are arranged can be closed, for example, by the following method. Note that the following method will be explained using an example in which the blocks are stacked vertically, but it can also be done in the same way when the blocks are stacked horizontally.

[0045] First, the first layer of blocks is arranged in the front-to-back and left-to-right directions on a substantially horizontal construction surface. Next, a sealing material 16 is poured into the gaps 14 between the arranged blocks in the front-to-back and left-to-right directions. Next, the sealing material 16 is laid on the top surfaces of the arranged blocks, and the second layer of blocks is arranged on top of it in the front-to-back and left-to-right directions. Subsequently, the process of pouring the sealing material 16 into the gaps 14 between the arranged blocks in the front-to-back and left-to-right directions, and the process of laying the sealing material 16 on the top surfaces of the blocks and arranging the blocks on top of it in the front-to-back and left-to-right directions are repeated, stacking the blocks in multiple layers from bottom to top. This method allows the sealing material 16 to seal the small gaps 14 between the blocks. For example, a cement-based material such as cement mixed with water is preferable as the sealing material 16 used to seal the gaps 14 between the blocks, but a water-stopping material such as a bentonite-based material can also be used.

[0046] According to the construction method of this embodiment, because precast materials are used, construction time can be shortened compared to conventional construction methods in which strong compaction is performed by repeatedly going back and forth with a roller or the like at the construction site. Furthermore, the construction method of this embodiment makes it possible to construct a low-permeability covering soil (waterproof layer). Furthermore, the low-permeability layer structure 10 of this embodiment makes it possible to reduce groundwater infiltration into the concrete pit 1.

[0047] In addition, by changing the orientation of the blocks, it is possible to set the desired water-blocking performance in a specific direction. For example, by changing the orientation of the blocks depending on the location of a structure such as a concrete pit 1, it is possible to impart water-blocking performance according to the location.

[0048] Furthermore, although the gaps 14 are a factor that increases permeability (decreases water impermeability), the use of large-sized blocks can make the gaps 14 relatively small. In other words, by increasing the size of the blocks, the gaps 14 become relatively small, thereby suppressing the factor that increases permeability (decreases water impermeability).

[0049] As explained above, the low-permeability layer construction method of the present invention is a method of constructing a low-permeability layer, which includes the steps of: previously manufacturing block-shaped precast soil material made up of stacked soil layers by repeatedly rolling out low-permeability soil material and compacting the rolled-out soil material, layer by layer; and constructing a low-permeability layer by arranging a plurality of the manufactured precast soil materials closely together and then providing a sealing material in the gaps between the precast soil materials to seal them. Therefore, construction time can be shortened compared to conventional construction methods that involve strong rolling by repeatedly going back and forth with a roller or the like at the construction site.

[0050] Furthermore, according to another construction method of a low-permeability layer of the present invention, the placement orientation of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material, so that the desired water-blocking performance can be set in a specified direction.

[0051] Furthermore, according to another method for constructing a low-permeability layer of the present invention, the precast soil material is oriented so that the expected direction of water flow around the area where the low-permeability layer is constructed is parallel to the stacking direction of the soil layers in the precast soil material, thereby improving its performance as a water-blocking layer.

[0052] Furthermore, the low-permeability layer structure of the present invention is a low-permeability layer structure that includes a plurality of block-shaped precast soil materials consisting of soil layers made of densely packed, layer-by-layer laminated low-permeability soil materials, and sealing material provided in the gaps between the precast soil materials. This allows for construction in a shorter time than conventional structures that require strong compaction by repeatedly going back and forth with a roller or the like at the construction site.

[0053] Furthermore, according to another low-permeability layer structure of the present invention, the orientation of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material, so that the desired water-blocking performance can be set in a specified direction.

[0054] Furthermore, according to another low-permeability layer structure of the present invention, the direction of the expected water flow around the low-permeability layer is set to be parallel to the stacking direction of the soil layers in the precast soil material, thereby improving the performance as a water-blocking layer.

[0055] The Sustainable Development Goals (SDGs) are 17 international goals that were adopted at the United Nations Summit in September 2015. The low-permeability layer construction method and low-permeability layer structure according to this embodiment can contribute to achieving one of the 17 SDGs, for example, goal 9, "Build resilient infrastructure, promote inclusive and sustainable industrialization, and foster innovation." [Industrial Applicability]

[0056] As described above, the low-permeability layer construction method and low-permeability layer structure of the present invention are useful for low-permeability layers installed around facilities such as concrete pits in low-level radioactive waste disposal sites, and are particularly suitable for shortening construction time. [Explanation of symbols]

[0057] 10 Low permeability layer structure 12 Precast soil materials 14 Gap 16 Sealing material 18 Soil layer X stacking direction

Claims

1. A method for constructing a low permeability layer, comprising: a step of pre-manufacturing a block-shaped precast soil material consisting of stacked soil layers by repeating the process of extruding low-permeability soil material and rolling the extruding soil material layer by layer; a step of constructing a low-permeability layer by densely arranging the manufactured plurality of precast soil materials and then providing a sealing material in the gaps between the precast soil materials to seal them; A method for constructing a low-permeability layer, comprising:

2. 2. The method for constructing a low-permeability layer according to claim 1, wherein the orientation of the precast soil material is determined based on the directional dependency of the permeability coefficient of the precast soil material.

3. 3. The method for constructing a low-permeability layer according to claim 2, characterized in that the precast soil material is arranged so that the direction of water flow expected around the area where the low-permeability layer is constructed is parallel to the stacking direction of the soil layers in the precast soil material.

4. A structure of a low permeability layer, A low-permeability layer structure comprising a plurality of block-shaped precast soil materials consisting of soil layers made of densely packed, layer-by-layer compacted low-permeability soil materials, and sealing material provided in the gaps between the precast soil materials.

5. 5. The low-permeability layer structure according to claim 4, wherein the orientation of the precast soil material is set based on the directional dependency of the permeability coefficient of the precast soil material.

6. The low-permeability layer structure described in claim 5, characterized in that the direction of water flow expected around the low-permeability layer is set to be parallel to the stacking direction of the soil layers in the precast soil material.

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