Soil improvement method and soil improvement structure for landfill utilization

Abstract

To provide a ground improvement method and a ground improvement structure capable of maintaining an impervious function of a cohesive soil layer even if a foundation pile is driven while suppressing differential settlement of a reclaimed land when the reclaimed land is provided in the ground having the cohesive soil layer and used as a land resource.SOLUTION: A soil improvement method for using a reclaimed land according to the present invention is a soil improvement method for using, as a land resource, a reclaimed land formed by constructing a revetment on a water bottom ground containing cohesive soil, in which a strength modified soil layer 25 is formed by stirring and mixing, in situ, a modifier 41 for hardening the cohesive soil into the cohesive soil at a position deeper than a surface layer portion of a water bottom ground 3, and the modifier 41 is not stirred and mixed into the cohesive soil of the surface layer portion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a ground improvement method and a ground improvement structure for constructing a reclaimed land on a waterbed ground mainly containing clay soil and utilizing the reclaimed land as a land resource. [Background technology]

[0002] Conventionally, a final disposal site for industrial waste and general waste has been an offshore waste disposal site 5 as shown in Figure 6. The offshore waste disposal site 5 closes off the sea surface area with a seawall 13, and waste is dumped inside the seawall for landfill disposal. Such an offshore waste disposal site 5 needs to have a high waterproofing function so that the wastewater generated from the waste does not contaminate the surrounding seawater or the bottom ground 3.

[0003] For this reason, it is common to install impermeable walls 19 made of steel sheet piles or steel pipe sheet piles with improved watertightness at the joints on the revetment 13. In some cases, as shown in Figure 6, two impermeable walls 19 are installed, with a waterproof material 21 placed between them to enhance the waterproofing function of the impermeable walls 19.

[0004] Furthermore, in order to prevent wastewater from flowing into the ground, a waterproof sheet may be laid on the bottom of the area where the waste is to be buried, or a waterproof layer may be formed on the surface of the bottom ground 3 using soil-based waterproof material, thereby enhancing the waterproof function of the bottom.

[0005] As shown in Figure 6, there are also cases where the offshore waste disposal site 5 is constructed on the water bottom ground 3 made of soft clay soil, and the water-impermeable properties of the clay soil are utilized. In this case, the ground itself has a water-impermeable function, so the above-mentioned water-impermeable treatment can be omitted or simplified. When constructing the offshore waste disposal site 5 by utilizing the water-impermeable properties of the water bottom ground 3, the water bottom ground 3 must have a permeability coefficient of 1 x 10 -6 It is required to have a clayey soil layer 9 of less than cm / s.

[0006] In addition, as a measure to enhance the waterproofing function of the offshore waste disposal site 5, Patent Document 1 proposes a method of pouring an alkaline solidification material inside the revetment, solidifying the alkaline solidification material to form a waterproof layer with a certain thickness, and then filling the inside of the waterproof layer with waste or fill material. In addition, Patent Document 2 proposes a waterproof structure that combines a waterproof wall made by continuously pouring waterproof sheet piles with a waterproof backfill material formed by pre-mixing soil-based waterproof material and riprap. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2022-41360 [Patent Document 2] Patent No. 5054388 [Non-patent literature]

[0008] [Non-Patent Document 1] Committee on Advanced Technology for the Use of Managed Offshore Disposal Sites: Guidelines for the Advanced Use of Managed Offshore Disposal Sites in Ports - Construction of Piles Penetrating the Bottom Impermeable Layer -, March 2019<URL:https: / / www.mlit.go.jp / kowan / content / 001321270.pdf> Summary of the Invention [Problem to be solved by the invention]

[0009] The above-mentioned offshore waste disposal site 5 receives waste inside the seawall 13, and then the waste layer 15 is covered with soil 17 to become a landfill, which is then utilized as a land resource. In order to utilize this reclaimed land flexibly, it is desirable to be able to construct both structures on direct foundations and structures on pile foundations.

[0010] However, if a foundation structure 7 is constructed directly on the offshore waste disposal site 5, there is a risk that the soft clayey soil layer 9 will be unevenly consolidated, causing differential settlement, as shown in Figure 7. In this way, a ground having a soft clay layer 9 is preferable from the viewpoint of ensuring watertightness at the bottom of the offshore waste disposal site 5, but from the viewpoint of utilizing the reclaimed land, there is a problem in that uneven settlement is likely to occur due to the construction of structures 7.

[0011] On the other hand, when constructing a pile foundation structure 27 (bridge, etc.) at an offshore waste disposal site 5, as shown in Figure 8, the foundation piles 29 are driven down to the sandy soil layer 11 (bearing layer) formed below the clayey soil layer 9, so that differential settlement as shown in Figure 7 is unlikely to occur. However, because the foundation piles 29 penetrate the waste layer 15 and the clayey soil layer 9, when the foundation piles 29 are driven, the tip of the piles may drag waste and wastewater into the clayey soil layer 9. If waste is dragged in, the clayey soil in the clayey soil layer 9 will be disturbed, and there is a risk that the water-blocking function of the clayey soil layer 9 will be reduced.

[0012] Therefore, Non-Patent Document 1 recommends the double pipe method as a pile driving method for preventing waste from being carried in (see Figure 3-1 in Non-Patent Document 1). The double pipe method is a method in which waste is excavated and removed using the all-casing method, and the piles are then driven into a waste-free space. According to this method, the foundation piles 29 pass through the waste-free casing and penetrate the waste layer 15, so the foundation piles 29 can be driven into the clayey soil layer 9 without bringing waste into the clayey soil layer 9. If no waste is carried in during pile driving, the clayey soil of the clayey soil layer 9 will adhere to the periphery of the foundation pile 29 after pile driving, so the water-blocking function of the clayey soil layer 9 can be maintained even after pile driving.

[0013] On the other hand, in order to suppress the uneven settlement as shown in FIG. 7, it is effective to solidify and improve the soft clay layer 9 in advance. However, when the clayey soil layer 9 is solidified and improved, the clayey soil loses its inherent self-compacting properties, and as a result, when the foundation pile 29 is driven in as shown in Figure 8, the clayey soil no longer adheres to the periphery of the foundation pile 29, which causes the problem that waste and sewage are more likely to flow out from that area.

[0014] As described above, when a reclaimed land such as an offshore waste disposal site 5 is established on the submerged ground 3 having a soft clayey soil layer 9 and the reclaimed land is utilized as a land resource, it is desirable not to solidify and improve the clayey soil layer 9 from the viewpoint of maintaining the water-blocking function of the submerged ground 3, but it is desirable to solidify and improve the clayey soil layer 9 from the viewpoint of preventing differential settlement.

[0015] Therefore, Patent Document 1 describes an example of forming a bottom water-blocking layer on the bottom surface of the waste layer (see Figure 7 of Patent Document 1), but because this water-blocking layer is made by solidifying a solidification material, there is a risk that it will not be able to maintain its water-blocking function when foundation piles are driven in.

[0016] Furthermore, Patent Document 2 describes an example in which a water-blocking layer formed using a soil-based water-blocking material is provided at the bottom of the waste layer (see Figure 1 of Patent Document 2), but this method is based on the premise of a permeable sandy soil base and does not anticipate soft ground, so this method does not suppress differential settlement as shown in Figure 7. As described above, the conventional method has the problem that after a landfill is established on a clayey soil ground, restrictions are imposed on how the land can be used as a land resource, making it difficult to use the landfill flexibly.

[0017] The present invention has been made to solve the above-mentioned problems, and aims to provide a ground improvement method and a ground improvement structure that, when a reclaimed land is created on ground having a clayey soil layer and used as a land resource, can suppress uneven settlement of the reclaimed land and maintain the water-blocking function of the clayey soil layer even when foundation piles are driven into the reclaimed land. [Means for solving the problem]

[0018] (1) The ground improvement method for utilizing reclaimed land according to the present invention is a ground improvement method for utilizing reclaimed land formed by constructing a revetment on a water bottom ground containing clayey soil as a land resource, A modifier that hardens the clayey soil is stirred and mixed in situ into the clayey soil at a position deeper than the surface layer of the waterbed ground to form a strength-modified soil layer; The modifier is not mixed into the clayey soil of the surface layer.

[0019] (2) In addition, in the above-mentioned (1), a soil-based water-blocking material that improves the water-blocking properties of the clayey soil in the surface layer is stirred and mixed in situ, and a water-blocking improved soil layer is formed on the strength-improved soil layer.

[0020] (3) The ground improvement structure for utilizing reclaimed land according to the present invention is a ground improvement structure for utilizing a reclaimed land formed by constructing a revetment on a water bottom ground containing clayey soil as a land resource, An unmodified soil layer formed on the surface of the bottom ground; a strength-improved soil layer formed at a position deeper than the unimproved soil layer; The strength-modified soil layer is made of strength-modified soil in which a modifier that hardens the clayey soil is mixed into the clayey soil to modify its strength, The unmodified soil layer is made of the clayey soil to which the modifier has not been mixed.

[0021] (4) The ground improvement structure for utilizing reclaimed land according to the present invention is a ground improvement structure for utilizing a reclaimed land formed by constructing a revetment on a water bottom ground containing clayey soil as a land resource, A water-impermeable improved soil layer formed on the surface layer of the bottom ground; a strength-improved soil layer formed at a position deeper than the water-impermeable improved soil layer; The water-impermeable improved soil layer is made of water-impermeable improved soil whose water impermeability has been improved by mixing the clayey soil with a soil-based water-impermeable material that improves the water impermeability of the clayey soil, The strength-modified soil layer is made of strength-modified soil, the strength of which has been modified by mixing the clayey soil with a modifier that hardens the clayey soil.

[0022] (5) In the above (3) or (4), the modifying agent is a material that reacts with the clayey soil and hardens, The strength-modified soil layer is obtained by mixing the modifier with the clayey soil.

[0023] (6) In addition, in the above (5), the material is made from converter-type iron and steel slag.

[0024] (7) In the above (4), the soil-based water-shielding material is a clay mineral, The water-impermeable improved soil layer exhibits water-impermeable properties by mixing the soil-based water-impermeable material with the clayey soil. [Effects of the Invention]

[0025] In the present invention, a modifier is mixed in situ into the clayey soil at a depth deeper than the surface of the submerged ground to form a strength-modified soil layer, and by not mixing the modifier into the clayey soil at the surface, differential settlement can be suppressed and the water-blocking function of the ground can be maintained even when foundation piles are driven in. Therefore, it is possible to construct both structures with direct foundations and structures with pile foundations on reclaimed land built on a submerged ground containing clayey soil, allowing for flexible use of the reclaimed land. [Brief explanation of the drawings]

[0026] [Figure 1] FIG. 1 is an explanatory diagram of a ground improvement structure according to a first embodiment. [Figure 2] FIG. 2 is a diagram illustrating a suitable layer thickness of the unimproved soil layer in the ground improvement structure of FIG. 1. [Figure 3] FIG. 4 is an explanatory diagram of another aspect of the ground improvement structure according to the first embodiment. [Figure 4] FIG. 10 is an explanatory diagram of a ground improvement structure according to a second embodiment. [Figure 5] 5 is a diagram illustrating a ground improvement method for forming the ground improvement structure of FIG. 4. FIG. [Figure 6] FIG. 1 is a diagram illustrating a conventional marine waste disposal site. [Figure 7] This is a diagram explaining the issues involved in constructing a structure with a foundation directly on the offshore waste disposal site shown in Figure 6. [Figure 8] FIG. 7 is a diagram illustrating the issues involved in constructing a structure with a pile foundation at the offshore waste disposal site shown in FIG. 6. DETAILED DESCRIPTION OF THE INVENTION

[0027] [Embodiment 1] A ground improvement structure 1 according to a first embodiment of the present invention will be described below with reference to FIG. Figure 1 shows a submarine ground 3 to which the ground improvement structure 1 of this embodiment is applied, a marine waste disposal site 5 which is a reclaimed land built on the submarine ground 3, and a structure 7 with a direct foundation structure built on the marine waste disposal site 5.

[0028] The waterbed 3 has a clayey soil layer 9 formed on the seabed and a sandy soil layer 11 formed below the clayey soil layer 9. The clayey soil layer 9 is an impermeable stratum composed mainly of soft clay, and is a weak layer in which consolidation settlement may occur due to the load of the structure 7. The clayey soil composition ratio in the clayey soil layer 9 is, for example, 50% or more. Here, a weak layer refers to a layer whose N value obtained in a standard penetration test (Japanese Industrial Standards JIS A1219 "Standard penetration test method for soil") is 4 or less. In addition, clayey soil refers to soil that falls into the fine-grained soil category (containing more than 50% clay or silt with a particle size of less than 0.075 mm) in the geotechnical engineering standard "Engineering Classification Method for Geotechnical Materials" (JGS0051-2000). The sandy soil layer 11 is a permeable stratum made of soil containing a large amount of sand, and has the strength to support the load of the offshore waste disposal site 5 and the structure 7 .

[0029] The marine waste disposal site 5 has a seawall 13 that divides the sea surface area into a waste landfill area, a waste layer 15 formed by waste landfilled inside the seawall 13, and a cover soil 17 that is provided to cover the waste layer 15.

[0030] The revetment 13 has a double impermeable wall 19 and a waterproof material 21 filled between the impermeable walls 19. The impermeable wall 19 is formed from steel sheet piles (or steel pipe sheet piles) connected continuously in the horizontal direction, and its lower ends are driven down to the sandy soil layer 11. The joints connecting adjacent steel sheet piles (or steel pipe sheet piles) in the impermeable wall 19 are ensured to have sufficient water-stopping properties, so that wastewater will not flow into the seawater.

[0031] The structure 7 is constructed using a spread foundation structure, and is built on a spread foundation that is laid directly on the cover soil 17.

[0032] The ground improvement structure 1 of this embodiment is an improvement of the submerged ground 3 in the area surrounded by the revetment 13, and as shown in Figure 1, has an unmodified soil layer 23 formed on the surface of the submerged ground 3 and a strength-improved soil layer 25 formed at a position deeper than the unmodified soil layer 23. The surface layer of the water bottom ground 3 refers to a portion of the water bottom ground 3 from the ground surface to a depth of about 1 to 2 m.

[0033] The unmodified soil layer 23 is a layer formed from conventional clayey soil to which no modifier, which will be described later, has been mixed, and has the same properties as the clayey soil layer 9. To prevent wastewater leakage from the waste layer 15, the permeability coefficient of the unimproved soil layer 23 is set to 1 × 10 -6 It is desirable that the speed be less than cm / s.

[0034] The strength-modified soil layer 25 is a layer made of strength-modified soil obtained by mixing conventional clayey soil (clayey soil of the clayey soil layer 9) with a modifier that hardens the clayey soil to strengthen it.

[0035] The modifier may be made of any material that can modify the soft clay to a predetermined strength, but examples of materials that can be used include steel slag, cement, cement-based solidification materials, quicklime, and lime-based solidification materials.

[0036] A specific example of the modifier is a material that develops strength through a hydration reaction with clay over time, such as a calcia modifier. Calcia modifier is a material that uses converter steelmaking slag, a type of steel slag, as a raw material, with its components controlled and particle size adjusted. By using calcia modifier as a modifier, steelmaking slag can be effectively utilized.

[0037] By mixing the above-mentioned modifier with the clayey soil of the clayey soil layer 9, the weak clayey soil is modified to form strength-modified soil with a predetermined strength, and this strength-modified soil forms the strength-modified soil layer 25.

[0038] In the ground improvement structure 1 of this embodiment configured as described above, a highly water-impermeable unimproved soil layer 23 is formed on the surface of the waterbed ground 3 in contact with the waste layer 15, thereby preventing wastewater from the waste layer 15 from flowing into the waterbed ground 3. In other words, by forming the unmodified soil layer 23 on the surface of the submerged ground 3, the water-blocking function of the bottom of the offshore waste disposal site 5 is ensured.

[0039] Furthermore, since a high-strength strength-modified soil layer 25 is formed below the unmodified soil layer 23, even if a local vertical load is applied to the waterbed ground 3 due to the construction of the structure 7, the clayey soil layer 9 below the strength-modified soil layer 25 can be uniformly consolidated, thereby suppressing uneven settlement.

[0040] Furthermore, according to the ground improvement structure 1 of this embodiment, an unmodified soil layer 23 is formed on the surface of the submerged ground 3, so that even if a pile foundation structure 27 (see Figure 8) is installed on the reclaimed land that is the offshore waste disposal site 5, the waterproofing function of the bottom of the offshore waste disposal site 5 can be maintained.

[0041] When constructing a pile foundation structure 27 at the offshore waste disposal site 5 in Figure 1, the foundation piles 29 are driven through the cover soil 17, waste layer 15, unmodified soil layer 23, strength-modified soil layer 25, and clayey soil layer 9 into the sandy soil layer 11. At this time, the clayey soil of the unmodified soil layer 23 has the inherent self-compacting properties of the ground, so it adheres closely to the periphery of the foundation piles 29 after the foundation piles 29 are driven in. Therefore, even if a pile-foundation structure 27 is constructed on the offshore waste disposal site 5 in Figure 1, the waterproofing function of the bottom of the offshore waste disposal site 5 is maintained.

[0042] As mentioned above, when driving foundation piles 29 into the offshore waste disposal site 5, it is desirable to use the double-pipe pile method described in Non-Patent Document 1 to prevent waste from being dragged into the bottom ground 3. In this double-pipe pile method, the depth of the hole drilled into the clayey soil layer for the outer pipe (casing) is set to 1 to 2 m, so it is desirable to set the thickness of the unimproved soil layer 23 to a thickness that is not less than the depth of the hole drilled for the casing (see Figure 2). In Figure 2, 31 is the casing, and 32 is the fill material injected between the casing 31 and the foundation pile 29.

[0043] As described above, the reclaimed land (offshore waste disposal site 5) built on the ground improvement structure 1 of this embodiment is unlikely to experience differential settlement even if a direct foundation structure 7 is installed, and the water-blocking function of the bottom ground 3 can be maintained even if a pile foundation structure 27 is installed. Therefore, it becomes possible to flexibly use the reclaimed land as a land resource.

[0044] The ground improvement structure 1 in Figure 1 is an example in which the clayey soil layer 9 remains below the strength-improved soil layer 25, but as shown in Figure 3, it is also possible to form the strength-improved soil layer 25 up to the top end of the sandy soil layer 11 without leaving the clayey soil layer 9 below the strength-improved soil layer 25. By doing so, consolidation settlement of the clayey soil layer 9 does not occur, and it is possible to prevent the subsidence phenomenon of the reclaimed land itself. Also in this case, since an unmodified soil layer 23 is formed on the surface of the submerged ground 3, the waterproofing function of the bottom of the marine waste disposal site 5 can be maintained even if a pile foundation structure 27 is constructed.

[0045] [Embodiment 2] In the ground improvement structure 1 described in the first embodiment, an unmodified soil layer 23 is formed on the surface of the waterbed ground 3. However, if the water-blocking property of the conventional clayey soil is low or if the construction plan for the pile foundation structure 27 is known in advance, it is possible to improve the water-blocking property of the waterbed ground 3 by modifying the clayey soil on the surface. Such an example is shown in Figure 4.

[0046] The ground improvement structure 33 of this embodiment is an improvement of the submerged ground 3 in the area surrounded by the seawall 13 of the offshore waste disposal site 5, and as shown in Figure 4, has a water-impermeable improved soil layer 35 formed on the surface of the submerged ground 3, and a strength-improved soil layer 25 formed at a position deeper than the water-impermeable improved soil layer 35. 4 are the same as those in FIGS. 1 and 3, the same reference numerals are used and the explanation is omitted, and only the impermeable improved soil layer 35 will be explained below.

[0047] The water-impermeable improved soil layer 35 is a layer formed from water-impermeable improved soil obtained by improving the water impermeability of conventional clayey soil (clayey soil of the clayey soil layer 9) by improving its properties. The water-impermeable improved soil is formed by mixing the clayey soil of the clayey soil layer 9 with a soil-based water-impermeable material that improves the water-impermeability of the clayey soil.

[0048] Soil-based waterproofing materials have high water-blocking properties and deformation-following ability, and there are no particular restrictions on the constituent materials as long as they can improve the water-blocking properties of the clayey soil without impairing its self-compacting properties. For example, clay minerals, coal ash, fibrous materials, etc. can be used. Furthermore, an example of a soil-based waterproofing material using clay minerals is bentonite.

[0049] By mixing the above-mentioned soil-based waterproofing material with the clayey soil of the clayey soil layer 9, the clayey soil is modified to form waterproof improved soil with the specified waterproofing properties, and this waterproof improved soil forms the waterproof improved soil layer 35.

[0050] In addition, the thickness of the water-impermeable improved soil layer 35 is preferably about 1 to 2 m so as not to interfere with the installation of foundation piles using the double pipe pile method, as with the unimproved soil layer 23 in embodiment 1 (see Figure 2).

[0051] In the ground improvement structure 33 of this embodiment configured as described above, a water-impermeable modified soil layer 35 is formed in the surface layer of the submerged ground 3 that contacts the waste layer 15, thereby improving the water-impermeable function of the bottom of the offshore waste disposal site 5 compared to the ground improvement structure 1 of Figure 1. Therefore, this embodiment is preferable because it can ensure the waterproofing function of the bottom of the offshore waste disposal site 5 even when the clayey soil layer 9 has low waterproofing properties.

[0052] Furthermore, the impermeable improved soil that forms the impermeable improved soil layer 35 has improved deformation followability and self-compacting properties compared to conventional clayey soil, due to the inclusion of an earth-based waterproofing material. Therefore, even if a structure 27 with a pile foundation structure is constructed as shown in FIG. 4, the water-impermeable improved soil is securely adhered to the periphery of the foundation piles 29 of the structure 27, making it easier to maintain the water-impermeable function. Therefore, this embodiment is suitable for constructing a pile foundation structure 27 on a reclaimed land, since it can reliably maintain the waterproofing function of the bottom of the offshore waste disposal site 5.

[0053] Also in this embodiment, a high-strength strength-modified soil layer 25 is formed below the water-impermeable modified soil layer 35, so even if a local vertical load is applied to the submerged ground 3 due to the construction of the structure 7, the clayey soil layer 9 below the strength-modified soil layer 25 can be uniformly consolidated, thereby suppressing uneven settlement.

[0054] Next, a ground improvement method for forming the ground improvement structure 33 shown in FIG. 4 will be described. The ground improvement method of this embodiment involves stirring and mixing an improvement material in situ into the clayey soil at a position deeper than the surface of the submerged ground 3 to form a strength-improved soil layer 25, and then stirring and mixing an earth-based waterproofing material in situ into the clayey soil at the surface to form a water-impermeable improved soil layer 35.

[0055] There are no particular limitations on the method for mixing the modified material or soil-based waterproofing material in situ with the existing clayey soil of the submerged ground 3, but it is desirable to use, for example, the method described in Japanese Patent No. 6936456. An example of the construction procedure based on the above document is shown in Figure 5.

[0056] In the method described below, as shown in Figure 5, a stirring and mixing device 47 is used, which has a casing 37 that can take in clay soil inside, a stirring and mixing means 39 that can move up and down inside the casing 37, and a material supply path 45 for supplying a modifier 41 or a soil-based waterproofing material 43 toward the stirring and mixing means 39. The stirring and mixing device 47 is provided on a work boat (not shown) that can move on the sea, and can be raised and lowered along a leader erected on the work boat.

[0057] In Fig. 5, the portion marked as the surface layer of the clayey soil layer 9 corresponds to the layer where the water-impermeable improved soil layer 35 in Fig. 4 is formed. Also, the portion marked as the middle layer corresponds to the layer where the strength-improved soil layer 25 in Fig. 4 is formed.

[0058] First, the casing 37 is injected into the clayey soil layer 9 of the waterbed ground 3. In the process of injecting the casing 37, the clayey soil is taken into the casing 37. When the injection of the casing 37 reaches the target depth (the depth at which the bottom end of the middle layer is reached), the injection of the casing 37 is stopped (FIGS. 5(a) and 5(b)). Here, when the strength-improved soil layer 25 is formed up to the upper end of the sandy soil layer 11 as shown in FIG. 3, the casing 37 may be press-fitted up to the lower end of the deep layer (upper end of the sandy soil layer 11).

[0059] Next, the stirring and mixing means 39 and the material supply passage 45 are lowered to the lower end position within the casing 37. The stirring and mixing means 39 is, for example, a rotating stirring and mixing blade, and in addition to the role of stirring and mixing the modifier 41 and the soil-based water-blocking material 43 into the clayey soil, it also plays a role of loosening the clayey soil inside the casing 37. By rotating the stirring and mixing means 39 and lowering it inside the casing 37, the clayey soil inside the casing 37 is loosened from top to bottom (Fig. 5(c)).

[0060] Next, the modifier 41 is introduced into the casing 37 through the material supply path 45. The introduced modifier 41 is discharged from the outlet located directly above the stirring and mixing means 39 and supplied to the stirring and mixing means 39. The amount of modifier 41 introduced here is the amount required to modify the middle part of the clayey soil layer 9 (Fig. 5(d)).

[0061] After the modifier 41 is added, the stirring and mixing means 39 is rotationally driven to stir and mix the clayey soil and modifier 41 in the casing 37. At this time, while rotating the stirring and mixing means 39, it is simultaneously raised and lowered in short strokes, moving the stirring and mixing position of the stirring and mixing means 39 from bottom to top. By stirring and mixing the modifier 41 into the clayey soil inside the casing 37 in the above manner, strength-modified soil 49 is formed by mixing the clayey soil and the modifier 41 (FIG. 5(e)).

[0062] When the position of the stirring and mixing means 39 in the casing 37 reaches the upper end of the middle part of the clayey soil layer 9, the soil-based waterproof material 43 is introduced into the casing 37 through the material supply path 45. The amount of the soil-based waterproof material 43 introduced here is, for example, the amount required to improve the surface part of the clayey soil layer 9. In this embodiment, the material supply path 45 is configured to follow the up and down movement of the stirring and mixing means 39, and the relative distance between the stirring and mixing means 39 and the outlet of the material supply path 45 is always kept at a constant close distance (Figure 5(f)).

[0063] Then, similarly to FIG. 5(e), the stirring and mixing means 39 is rotationally driven to stir and mix the clayey soil and the soil-based waterproof material 43 in the casing 37. The clayey soil in the casing 37 is stirred and mixed with the soil-based water-blocking material 43 to form water-blocking improved soil 51, which is a mixture of the clayey soil and the soil-based water-blocking material 43 (FIG. 5(g)).

[0064] When the position of the stirring and mixing means 39 in the casing 37 reaches the upper end of the clay soil layer 9, the stirring and mixing means 39 is stopped (FIG. 5(h)). Thereafter, the casing 37 is pulled out from the waterbed ground 3, and the top of the water-impermeable modified soil 51 is leveled (FIG. 5(i)). Through the above steps, the formation of the strength-improved soil 49 and the formation of the water-impermeable improved soil 51 at one point is completed.

[0065] Thereafter, by changing the position as necessary and repeating the same process as above, strength-improved soil 49 and water-impermeable improved soil 51 are formed over the entire target area, thereby forming strength-improved soil layer 25 and water-impermeable improved soil layer 35.

[0066] As described above, by supplying the modifier 41 directly into the submerged ground 3 and stirring and mixing it into the clayey soil in situ, it is possible to control whether or not the modifier 41 is mixed for each depth, and therefore it is possible to form a strength-modified soil layer 25 at a position deeper than the surface of the submerged ground 3. Furthermore, as shown in Figure 5, by changing the material supplied depending on the depth, the formation of the strength-improved soil 49 and the water-impermeable improved soil 51 can be completed in one construction run, so the creation of the strength-improved soil layer 25 and the water-impermeable improved soil layer 35 can be carried out simultaneously, which is efficient.

[0067] The method illustrated in FIG. 5 can also be used to form the ground improvement structure 1 of the first embodiment (see FIG. 1). When forming the ground improvement structure 1, it is advisable to complete construction at one point by pulling out the casing 37 without supplying the soil-based waterproofing material 43 in the step of Figure 5(f). In this way, it is possible to improve the clayey soil deeper than the surface layer while leaving the clayey soil in the surface layer unimproved, so that an unimproved soil layer 23 can be formed in the surface layer of the submerged ground 3.

[0068] Furthermore, in the above-described first and second embodiments, the offshore waste disposal site 5 is given as an example of a landfill site, but the landfill sites covered by the present invention are not limited to this and also include, for example, offshore landfill treatment facilities for contaminated soil. In either case, the present invention is suitable when the structure is installed in ground having a soft clay layer, requires a water-blocking function to prevent the water contained inside from leaking into the ground, and the reclaimed land is to be used as a land resource. [Explanation of symbols]

[0069] 1 Ground improvement structure (embodiment 1) 3 Underwater ground 5. Marine waste disposal site 7 Structures (direct foundation structure) 9 Cohesive soil layer 11 Sandy soil layer 13 Seawall 15 Waste layer 17 Soil cover 19 Impermeable wall 21 Water-shielding material 23 Unmodified soil layer 25 Strength-improved soil layer 27 Structures (Pile foundation structure) 29 Foundation piles 31 Casing 32 Filling material 33 Ground Improvement Structure (Embodiment 2) 35 Water-impermeable improved soil layer 37 Casing 39 Stirring and mixing means 41 Modifier 43 Soil-based waterproofing material 45 Material supply route 47 Stirring and mixing equipment 49 Strength-improved soil 51 Water-impermeable soil

Claims

1. A ground improvement method for utilizing a reclaimed land formed by constructing a revetment on a water bottom ground containing clayey soil as a land resource, comprising: A modifier that hardens the clayey soil is stirred and mixed in situ into the clayey soil at a position deeper than the surface layer of the waterbed ground to form a strength-modified soil layer; A ground improvement method for landfill utilization in which the modifying material is not mixed into the clayey soil in the surface layer.

2. 2. A ground improvement method for landfill utilization as described in claim 1, wherein a soil-based water-blocking material that improves the water-blocking properties of the clayey soil in the surface layer is stirred and mixed in situ to form a water-blocking improved soil layer on top of the strength-improved soil layer.

3. A ground improvement structure for utilizing reclaimed land formed by constructing a revetment on a water bottom ground containing clayey soil as a land resource, An unmodified soil layer formed on the surface of the bottom ground; a strength-improved soil layer formed at a position deeper than the unimproved soil layer; The strength-modified soil layer is made of strength-modified soil in which a modifier that hardens the clayey soil is mixed into the clayey soil to modify its strength, The unmodified soil layer is a ground improvement structure for use on a landfill site, which is made of clayey soil to which the modifier has not been mixed.

4. A ground improvement structure for utilizing reclaimed land formed by constructing a revetment on a water bottom ground containing clayey soil as a land resource, A water-impermeable improved soil layer formed on the surface layer of the bottom ground; a strength-improved soil layer formed at a position deeper than the water-impermeable improved soil layer; The water-impermeable improved soil layer is made of water-impermeable improved soil whose water impermeability has been improved by mixing the clayey soil with a soil-based water-impermeable material that improves the water impermeability of the clayey soil, The strength-modified soil layer is a ground improvement structure for use on a reclaimed land, and is made of strength-modified soil whose strength has been modified by mixing the clayey soil with a modifier that hardens the clayey soil.

5. The modifying material is a material that reacts with the clayey soil and hardens, 5. The ground improvement structure for use on a reclaimed land according to claim 3, wherein the strength of the strength-improved soil layer is imparted by mixing the modifier with the clayey soil.

6. 6. The ground improvement structure for use on a landfill site according to claim 5, wherein the material is made from converter-based steel slag.

7. The soil-based water-shielding material is a clay mineral, 5. The ground improvement structure for use on reclaimed land according to claim 4, wherein the water-impermeable improved soil layer exhibits water-impermeable properties by mixing the soil-based water-impermeable material with the clayey soil.

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