Method for constructing deformation prevention structures and method for melting frozen soil

Abstract

To provide a method for constructing a deformation suppression structure that suppresses deformation of surrounding ground when frozen soil thaws, and a method for thawing frozen soil.SOLUTION: A method for constructing a deformation suppression structure 17 that suppresses deformation of surrounding ground when a frozen soil wall 11 formed around a frozen pipe 5 in ground 1 by a freezing method thaws comprises: a boring step of forming a boring hole 13 passing near the frozen pipe 5 in a state where the frozen soil wall 11 is frozen; and an underground structure forming step of filling the boring hole 13 with a cement-based material in a state where the frozen soil wall 11 is frozen to form a cement-based underground structure 15 made of the cement-based material.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for constructing a deformation suppression structure and a method for melting frozen soil. [Background technology]

[0002] Conventionally, a freezing method as disclosed in the following Patent Document 1 has been known as a method for temporarily improving ground involved in construction work. In the freezing method, freezing pipes through which a refrigerant flows are buried in the ground, and the ground containing groundwater is frozen to create frozen soil. The ground that has solidified as frozen soil has strength and water-stopping properties, and can prevent ground collapse and groundwater inflow that occur during ground excavation during construction work. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 07-268856 Summary of the Invention [Problem to be solved by the invention]

[0004] In the freezing method, the ground that is frozen expands in volume by several percent due to the freezing expansion of groundwater. When the frozen soil is thawed after the completion of the work, the volume of the ground shrinks, and the ground does not necessarily return to its pre-freezing state. As a result, there is a risk that deformation (e.g., land subsidence) will occur in the surrounding ground when thawing. As a method for suppressing deformation of the surrounding ground, it is possible to carry out ground improvement by injecting a chemical solution, but it is difficult to inject a chemical solution into frozen ground. Even if chemical solution is injected into the ground after thawing, it is difficult to determine the timing of the injection, and there are cases where the deformation of the ground that occurs when thawing can no longer be suppressed. In view of such problems, the present invention aims to provide a method for constructing a deformation suppression structure that suppresses deformation of the surrounding ground when the frozen soil thaws, and a frozen soil thawing method. [Means for solving the problem]

[0005] The method for constructing a deformation suppression structure of the present invention is a method for constructing a deformation suppression structure that suppresses deformation of the surrounding ground when frozen soil formed around a freezing pipe in the ground by a freezing method thaws, and includes a boring process for forming a borehole that passes near the freezing pipe while the frozen soil is frozen, and an underground structure formation process for filling the borehole with a predetermined filling material while the frozen soil is frozen, to form an underground structure made of the filling material.

[0006] In the underground structure forming step, a cement-based material may be introduced into the borehole as a filler material and hardened to form the underground structure. In addition, in the underground structure forming step, a water-absorbing and swelling soil-based material may be introduced into the borehole as a filler material to form the underground structure.

[0007] In the method for constructing a deformation suppression structure of the present invention, a plurality of freezing pipes may extend in parallel, and the underground structure may have rod-shaped portions extending parallel to the freezing pipes and / or rod-shaped portions extending in a direction perpendicular to the freezing pipes. The frozen soil may include a portion in which a part of the low-permeability layer is frozen, and a borehole passing through the low-permeability layer may be formed in the boring step. The boring step may also include a borehole passing through the frozen soil.

[0008] The frozen soil thawing method of the present invention is a thawing method for melting frozen soil formed around a freezing pipe in the ground by a freezing method, and after an underground structure is formed by any of the above-mentioned methods for constructing a deformation suppression structure, the cooling of the frozen soil is stopped and the frozen soil is thawed. Effect of the Invention

[0009] According to the present invention, a method for constructing a deformation suppression structure that suppresses deformation of the surrounding ground when frozen soil thaws and a frozen soil thawing method can be provided. [Brief description of the drawings]

[0010] [Figure 1]1A is a perspective view showing a schematic diagram of the state of the ground when the method for constructing a deformation suppression structure and the frozen soil melting method are applied, and FIG. 1B is a horizontal cross-sectional view of the main part. [Diagram 2] FIG. 1(a) is a vertical cross-sectional view showing a schematic diagram of the ground in the first and fourth embodiments, and FIG. [Diagram 3] FIG. 1(a) is a vertical cross-sectional view showing a schematic diagram of the ground in the second and fifth embodiments, and FIG. 1(b) is a horizontal cross-sectional view of a main part thereof. [Figure 4] FIG. 13(a) is a vertical cross-sectional view showing a schematic diagram of the ground in the third and sixth embodiments, and FIG. 13(b) is a horizontal cross-sectional view of a main part thereof. [Diagram 5] 1(a) and 1(b) are cross-sectional views sequentially showing a method of filling a borehole with bentonite pellets using an insertion tool. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, an embodiment of the method for constructing a deformation suppression structure and the frozen soil thawing method according to the present invention will be described in detail with reference to the drawings. Fig. 1(a) is a perspective view showing a schematic state of ground 1 to which the method for constructing a deformation suppression structure and the frozen soil thawing method are applied, and Fig. 1(b) is a horizontal cross-sectional view of the main part. In Fig. 1(a), the ground 1 is removed to avoid cluttering the drawing. In the freezing method of this embodiment, a vertical wall of frozen soil (frozen soil wall 11 described later) is formed in the ground.

[0012] A plurality of freezing pipes 5 are buried in the ground 1, extending vertically downward from the horizontal ground surface in a rod shape. The plurality of freezing pipes 5 are arranged at equal intervals in a roughly straight line in a plan view. For example, the arrangement pitch of the freezing pipes 5 is about 4 m, and the length of the freezing pipes 5 is about 30 m. A refrigerant pipe 7 for circulating a refrigerant is connected to the upper end of each freezing pipe 5 on the ground, and the refrigerant is introduced into and discharged from each freezing pipe 5 through the refrigerant pipe 7. The ground around each freezing pipe 5 is cooled by circulating a very low temperature refrigerant through each freezing pipe 5. For example, antifreeze or liquid nitrogen is used as the refrigerant.

[0013] The ground around each freezing pipe 5 is cooled and frozen, forming a cylindrical frozen soil 9 with each freezing pipe 5 at its center. For example, if the arrangement pitch of the freezing pipes 5 is about 4 m as described above, and each freezing pipe 5 has the ability to freeze the ground within a radius of about 2.5 m, adjacent cylindrical frozen soil 9 are connected to each other in the arrangement direction of the freezing pipes 5, forming a frozen soil wall 11 extending in the arrangement direction of the freezing pipes 5 in the ground. The formed frozen soil wall 11 has strength and water-stopping properties, and can prevent ground collapse and groundwater inflow due to ground excavation for surrounding construction work. After that, for example, when the frozen soil wall 11 is no longer needed after the construction work is completed, the flow of the refrigerant to the freezing pipes 5 is stopped, and the cooling of the frozen soil wall 11 is stopped, and the frozen soil wall 11 is melted.

[0014] Here, when the frozen soil wall 11 is formed, the volume of the frozen ground expands by several percent due to the freezing expansion of groundwater. Conversely, when the frozen soil wall 11 is melted, the volume of the ground shrinks, but it does not necessarily return completely to the state before freezing. As a result, there is a risk that deformation (e.g., ground subsidence, etc.) will occur in the surrounding ground when melting. Therefore, in the frozen soil melting method of this embodiment, a deformation suppression structure for suppressing deformation of the surrounding ground is constructed in the ground before the cooling of the frozen soil wall 11 is stopped. Hereinafter, first to sixth embodiments of the construction method of such a deformation suppression structure will be described respectively. The construction method of the deformation suppression structure in the first to sixth embodiments includes a boring process and an underground structure formation process, which will be described respectively. The boring process and the underground structure formation process are performed in a state in which the refrigerant continues to flow through the freezing pipe 5 and the frozen soil wall 11 is frozen.

[0015] [First embodiment] A first embodiment of a method for constructing a deformation suppression structure will be described below. Fig. 2(a) is a vertical cross-sectional view showing a schematic diagram of a ground 1 in this embodiment, and Fig. 2(b) is a horizontal cross-sectional view of a main part thereof. (Boring process) In the boring process of this embodiment, a plurality of vertical boreholes 13 are formed from the ground so as to pass through the frozen soil wall 11. As a specific example, as shown in FIG. 2, the intermediate positions between the freezing pipes 5 are bored from the ground parallel to the freezing pipes 5 to form a plurality of vertical boreholes 13. In the example of FIG. 2, the boreholes 13 are formed at the same pitch as the freezing pipes 5, but the boreholes 13 may be thinned out as appropriate. The boreholes 13 reach a depth slightly below the lower end of the frozen soil wall 11, and the diameter of the boreholes 13 is, for example, about 20 cm. In this boring process, a known vertical boring method is used, the frozen ground contained in the frozen soil wall 11 is drilled, and the boreholes 13 pass through the frozen soil wall 11.

[0016] (Underground structure formation process) In the underground structure forming process of this embodiment, a cement-based material is injected into the borehole 13, and the borehole 13 is filled with the cement-based material. As the cement-based material, for example, concrete, mortar, cement paste, cement milk, etc. can be used. The cement-based material may also contain additives as appropriate. When the injected cement-based material hardens, a plurality of cement-based underground structures 15 made of the cement-based material are formed. The cement-based underground structures 15 are vertically present in the frozen soil wall 11 in the form of rods. In this way, a deformation suppression structure 17 made of a plurality of cement-based underground structures 15 is constructed in the frozen soil wall 11. Note that it is not essential that the deformation suppression structure 17 is made of a plurality of cement-based underground structures 15 as described above, and the deformation suppression structure 17 may be made of one cement-based underground structure 15.

[0017] After the above-mentioned deformation suppression structure 17 is constructed, the flow of the refrigerant to the freezing pipe 5 is stopped, and the cooling of the frozen soil wall 11 is stopped, causing the frozen soil wall 11 to melt. At this time, the volume of the ground around the freezing pipe 5 is reduced and its strength is also reduced due to the melting of the frozen soil wall 11, but the rigidity of the deformation suppression structure 17 supports the surrounding ground, suppressing deformation of the ground (e.g., subsidence, etc.). In addition, since the surface of the cement-based underground structure 15 is roughened following the roughness of the inner wall surface of the borehole 13, this surface roughness is expected to provide adhesion between the cement-based underground structure 15 and the surrounding ground, and deformation of the ground is efficiently suppressed.

[0018] In order to suppress deformation of the surrounding ground due to the melting of the frozen soil wall 11, it is possible to implement deformation suppression measures for a wide area of ​​the ground surrounding the frozen soil wall 11, but this is not realistic in terms of labor, cost, amount of waste generated, etc. In contrast, the method of constructing a deformation suppression structure in this embodiment is a realistic measure in which a part of the frozen soil wall 11 is replaced with a cement-based underground structure 15, and it is possible to suppress the labor, cost, and amount of waste generated for deformation suppression measures.

[0019] Second Embodiment A second embodiment of the method for constructing a deformation suppression structure will now be described. Fig. 3(a) is a vertical cross-sectional view showing a schematic diagram of the ground 1 in this embodiment, and Fig. 3(b) is a horizontal cross-sectional view of a main part thereof. (Boring process) In the boring process of this embodiment, a plurality of horizontal boreholes 23 are formed so as to pass through the frozen soil wall 11 at positions near each freezing pipe 5. The boreholes 23 extend in the arrangement direction of the freezing pipes 5 to a length that reaches the full horizontal width of the frozen soil wall 11, and a plurality of the boreholes 23 are formed so as to be lined up in the vertical direction. The diameter of the boreholes 23 is, for example, about 20 cm. Such boreholes 23 are formed, for example, by using a known curved boring method.

[0020] (Underground structure formation process) In the underground structure formation step of this embodiment, a cement-based material is injected into the borehole 23 in the same manner as in the first embodiment, and the borehole 23 is filled with the cement-based material. When the injected cement-based material hardens, a plurality of cement-based underground structures 25 made of the cement-based material are formed. The cement-based underground structures 25 are horizontally arranged in rod form within the frozen soil wall 11. In this manner, a deformation suppression structure 27 made of a plurality of cement-based underground structures 25 is constructed within the frozen soil wall 11. Note that it is not essential that the deformation suppression structure 27 is made of a plurality of cement-based underground structures 25 as described above, and the deformation suppression structure 27 may be made of a single cement-based underground structure 25.

[0021] This type of deformation suppression structure 27 also provides the same effect as the first embodiment, suppressing deformation of the ground when the frozen soil wall 11 melts. Note that both the deformation suppression structure 17 of the first embodiment and the deformation suppression structure 27 of this embodiment may be constructed. In this case, a column-beam structure made of a cement-based material is present in the ground 1, further suppressing deformation of the ground.

[0022] Third Embodiment A third embodiment of the method for constructing a deformation suppression structure will be described. FIG. 4(a) is a vertical cross-sectional view showing a schematic diagram of the ground 1 in this embodiment, and FIG. 4(b) is a horizontal cross-sectional view of a main part thereof. As shown in FIG. 4(a), in this embodiment, a horizontal low-permeability layer 2 (e.g., a layer of clay) exists in the ground 1. Each freezing pipe 5 extends to a depth that reaches the low-permeability layer 2, for example, and a part of the low-permeability layer 2 exists within the frozen range of the ground 1 by the freezing pipe 5. Each freezing pipe 5 may extend to a position deeper than the low-permeability layer 2 by vertically penetrating the low-permeability layer 2. The frozen soil wall 11 includes a part of the low-permeability layer 2, that is, in the area where the frozen soil wall 11 and the low-permeability layer 2 overlap, the low-permeability layer 2 is in a frozen state. In this way, the part of the frozen soil wall 11 where the low-permeability layer 2 is frozen is called the "low-permeability frozen part 12". Here, the low-permeability layer 2 refers to a layer with a small permeability coefficient. According to technical books (e.g., soil engineering handbooks), the permeability coefficient of the ground is 1.0×10 -5 ~1.0×10 -7In accordance with this, the low-permeability layer 2 in this embodiment is a layer having a permeability coefficient of 1.0×10 -5 ~1.0×10 -7 It may be defined as a layer having a thickness of less than or equal to 1 cm / sec.

[0023] (Boring process) In the boring process of this embodiment, a plurality of horizontal boreholes 33 are formed so as to pass through the low permeability frozen portion 12 of the frozen soil wall 11 at positions near each freezing pipe 5. The method of forming the boreholes 33 is the same as that of the boreholes 23 of the second embodiment.

[0024] (Underground structure formation process) In the underground structure formation step of this embodiment, a cement-based material is injected into the borehole 33, filled and hardened in the same manner as in the second embodiment, to form a plurality of cement-based underground structures 35 made of the cement-based material. The cement-based underground structures 35 are horizontally arranged in rod-like form in the low-permeability frozen portion 12 of the frozen soil wall 11. In this manner, a deformation suppression structure 37 made of a plurality of cement-based underground structures 35 is constructed in the low-permeability frozen portion 12 of the frozen soil wall 11. Note that it is not essential that the deformation suppression structure 37 is made of a plurality of cement-based underground structures 35 as described above, and the deformation suppression structure 37 may be made of a single cement-based underground structure 35.

[0025] With this type of deformation suppression structure 37, the same effect as in the first embodiment can be obtained, and deformation of the ground when the frozen soil wall 11 melts can be suppressed.

[0026] In addition, generally, the low-permeability layer has a property that it is difficult to discharge the moisture that it holds. Therefore, when the low-permeability layer freezes / thaws, the held moisture expands / contracts without being discharged from the layer, and as a result, the low-permeability layer itself expands / contracts significantly. Based on this knowledge, the low-permeability frozen portion 12 of the frozen soil wall 11 in particular contracts significantly when thawed, and is likely to be a cause of deformation of the ground. Therefore, in this embodiment, by constructing the deformation suppression structure 37 in the low-permeability frozen portion 12, deformation of the ground when the frozen soil wall 11 melts is efficiently suppressed. In addition, in this embodiment, the borehole 33 is formed horizontally without crossing the low-permeability layer 2 in the vertical direction, so that it is possible to prevent the borehole 33 from becoming a water path and deteriorating the low-permeability function of the low-permeability layer 2.

[0027] [Fourth embodiment] A fourth embodiment of the method for constructing a deformation suppression structure will be described. The method for constructing this embodiment differs from the method for constructing this embodiment in that a water-absorbing and swelling soil material (e.g., bentonite) is filled into the borehole 13 instead of a cement-based material. Since the appearance of the deformation suppression structure 47 constructed in this embodiment is similar to the deformation suppression structure 17 in the first embodiment (FIG. 2), the method for constructing this embodiment will be described mainly with reference to FIG. 2.

[0028] (Boring process) The boring process of this embodiment is carried out in the same manner as the boring process of the first embodiment, and a plurality of boreholes 13 are formed in the frozen soil wall 11 as shown in Figs. 2(a) and 2(b).

[0029] (Underground structure formation process) In the underground structure formation step of this embodiment, a water-absorbent swelling soil material is introduced and filled into the borehole 13. The water-absorbent swelling soil material has the property of swelling and increasing in volume when it absorbs water. In this embodiment, compacted bentonite is used as the water-absorbent swelling soil material. Here, for example, bentonite pellets 41 with a particle size of about 5 mm are continuously inserted into the borehole 13 to fill the borehole 13. The insertion and placement of the bentonite pellets 41 into the borehole 13 is performed using an insertion tool 81 shown in FIG. 5.

[0030] As shown in FIG. 5(a), the inserting tool 81 has a cylindrical storage section 83 having a diameter slightly smaller than that of the borehole 13. The storage section 83 can store the bentonite pellets 41 and can be inserted into the borehole 13. A lid 83a is attached to the tip of the storage section 83. Furthermore, the inserting tool 81 has a pipe material 85, and the storage section 83 is provided at the tip of the pipe material 85. By pushing the pipe material 85 into the borehole 13 from the ground, the storage section 83 moves toward the back of the borehole 13. In the hollow part of the pipe material 85, an extrusion rod 87 for extruding the bentonite pellets 41 from the storage section 83 is inserted. The extrusion rod 87 can reciprocate in the axial direction within the hollow part of the pipe material 85. When the extrusion rod 87 is pushed into the pipe material 85 by an operation from the ground, an extrusion plate 89 provided at the tip of the extrusion rod 87 protrudes into the storage section 83.

[0031] The bentonite pellets 41 are stored in the storage section 83 of the inserting tool 81 as described above, and the lid 83a is attached. The storage section 83 is inserted and pushed into the borehole 13 from the ground, and as shown in FIG. 5(a), the storage section 83 reaches just before the bentonite pellets 41' that have been previously filled. When the pipe material 85 is pulled back from this state while holding down the extrusion rod 87 by an operation on the ground, the lid 83a falls off the storage section 83, and the bentonite pellets 41 are left on the bentonite pellets 41', as shown in FIG. 5(b). After that, the pipe material 85 and the extrusion rod 87 are pulled back by an operation on the ground, and the inserting tool 81 is retrieved. In the example of FIG. 5, the lid 83a is left in the borehole 13 together with the bentonite pellets 41, but the structure may be such that the lid 83a is retrieved together with the storage section 83. In this case, for example, the cover 83a may be connected to the storage section 83 via a hinge so as to be able to open and close.

[0032] The insertion and installation of the bentonite pellets 41 is repeated using the insertion tool 81, so that the borehole 13 is filled with the bentonite pellets 41. As a result, as shown in FIG. 2, a plurality of bentonite-based underground structures 45 consisting of an aggregate of bentonite pellets 41 are formed. The bentonite-based underground structures 45 exist vertically in rod form within the frozen soil wall 11. In this way, a deformation inhibition structure 47 consisting of a plurality of bentonite-based underground structures 45 is constructed within the frozen soil wall 11. Note that it is not essential that the deformation inhibition structure 47 is composed of a plurality of bentonite-based underground structures 45 as described above, and the deformation inhibition structure 47 may be composed of a single bentonite-based underground structure 45.

[0033] After the above-mentioned deformation suppression structure 47 is constructed, the flow of the refrigerant to the freezing pipe 5 is stopped, and the cooling of the frozen soil wall 11 is stopped, so that the frozen soil wall 11 melts. Then, the groundwater contained in the frozen soil wall 11 returns to liquid, and the bentonite pellets 41 in the borehole 13 come into contact with the groundwater and swell. At this time, the volume of the ground around the frozen soil wall 5 is reduced and the strength is also reduced due to the melting of the frozen soil wall 11, but the rigidity of the deformation suppression structure 47 supports the surrounding ground, and deformation of the ground (for example, subsidence, etc.) is suppressed. In addition, the volume of the ground around the frozen soil wall 5 is reduced due to the melting of the frozen soil wall 11, but at least a part of the reduction in the volume of the surrounding ground is compensated for by the expansion of the bentonite-based underground structure 45, so that deformation of the ground is further suppressed.

[0034] Fifth embodiment A fifth embodiment of the method for constructing a deformation suppression structure will be described. The method for constructing this embodiment differs from the method for constructing this embodiment in that a water-absorbing and swelling soil material (e.g., bentonite) is filled into the borehole 23 instead of a cement-based material. Since the appearance of the deformation suppression structure 57 constructed in this embodiment is similar to the deformation suppression structure 27 in the second embodiment (FIG. 3), the method for constructing this embodiment will be described mainly with reference to FIG. 3.

[0035] (Boring process) The boring process of this embodiment is carried out in the same manner as the boring process of the second embodiment, and a plurality of boreholes 23 are formed in the frozen soil wall 11 as shown in Figs. 3(a) and 3(b).

[0036] (Underground structure formation process) In the underground structure formation step of this embodiment, an insertion tool 81 (FIG. 5) is used as in the fourth embodiment, and the borehole 23 is filled with bentonite pellets 41. As a result, a plurality of bentonite-based underground structures 55 consisting of an aggregate of bentonite pellets 41 are formed as shown in FIG. 3. The bentonite-based underground structures 55 exist horizontally in rod form within the frozen soil wall 11. In this way, a deformation inhibition structure 57 consisting of a plurality of bentonite-based underground structures 55 is constructed within the frozen soil wall 11. Note that it is not essential that the deformation inhibition structure 57 is composed of a plurality of bentonite-based underground structures 55 as described above, and the deformation inhibition structure 57 may be composed of a single bentonite-based underground structure 55.

[0037] Such a deformation suppression structure 57 also provides the same effect as the fourth embodiment, suppressing deformation of the ground when the frozen soil wall 11 melts. Note that both the deformation suppression structure 47 of the fourth embodiment and the deformation suppression structure 57 of this embodiment may be constructed. In this case, a column-beam structure made of a bentonite-based material is present in the ground 1, further suppressing deformation of the ground.

[0038] Sixth Embodiment A sixth embodiment of the method for constructing a deformation suppression structure will be described. The method for constructing this embodiment differs from the method for constructing this embodiment in that a water-absorbing and swelling soil material (e.g., bentonite) is filled into the borehole 33 instead of a cement-based material. Since the appearance of the deformation suppression structure 67 constructed in this embodiment is similar to the deformation suppression structure 37 in the third embodiment (FIG. 4), the method for constructing this embodiment will be described mainly with reference to FIG. 4.

[0039] (Boring process) The boring process of this embodiment is carried out in the same manner as the boring process of the third embodiment, and multiple boreholes 33 are formed in the low-permeability frozen portion 12 of the frozen soil wall 11, as shown in Figures 4(a) and 4(b).

[0040] (Underground structure formation process) In the underground structure forming step of this embodiment, an insertion tool 81 (FIG. 5) is used as in the fourth embodiment, and the borehole 33 is filled with bentonite pellets 41. As a result, as shown in FIG. 4, a plurality of bentonite-based underground structures 65 consisting of an aggregate of bentonite pellets 41 are formed. The bentonite-based underground structures 65 are horizontally present in the low-permeability frozen portion 12 of the frozen soil wall 11 in the form of a rod. In this way, a deformation suppression structure 67 consisting of a plurality of bentonite-based underground structures 65 is constructed in the low-permeability frozen portion 12 of the frozen soil wall 11. Note that it is not essential that the deformation suppression structure 67 is composed of a plurality of bentonite-based underground structures 65 as described above, and the deformation suppression structure 67 may be composed of one bentonite-based underground structure 65.

[0041] The deformation suppression structure 67 has the same effect as the fourth embodiment, and the deformation of the ground when the frozen soil wall 11 melts is suppressed. As in the third embodiment, the deformation suppression structure 67 is constructed in the low-permeability frozen portion 12, and the deformation of the ground when the frozen soil wall 11 melts is efficiently suppressed. In this embodiment, the borehole 33 is formed horizontally without vertically crossing the low-permeability layer 2, so that the borehole 33 can be prevented from becoming a water path and deteriorating the low-permeability function of the low-permeability layer 2. In addition, the borehole 33 formed in the low-permeability layer 2 is filled with the bentonite-based underground structure 65 having low permeability, so that the deterioration of the low-permeability function of the low-permeability layer 2 is further suppressed.

[0042] Comparing the first to third embodiments with the fourth to sixth embodiments, in the first to third embodiments, the cement-based underground structures 15, 25, 35 remain in the ground 1 after the frozen soil wall 11 has completely thawed. In contrast, in the fourth to sixth embodiments, what remains in the ground 1 after the frozen soil wall 11 has completely thawed is the bentonite-based underground structures 45, 55, 65, which are soil-based materials and are closer to the original ground 1. Therefore, the fourth to sixth embodiments can reduce the load on the environment of the ground 1 compared to the first to third embodiments.

[0043] The present invention can be embodied in various forms, including the above-described embodiment, with various modifications and improvements based on the knowledge of those skilled in the art. In addition, the following modified examples can be constructed by utilizing the technical matters described in the above-described embodiment. The configurations of each embodiment may be used in appropriate combination.

[0044] For example, the deformation suppression structure 17 (FIG. 2) in the first embodiment, the deformation suppression structure 27 (FIG. 3) in the second embodiment, and the deformation suppression structure 37 (FIG. 4) in the third embodiment may be constructed alone, or any two of the three may be constructed together, or all three may be constructed together. Similarly, the deformation suppression structure 47 (FIG. 2) in the fourth embodiment, the deformation suppression structure 57 (FIG. 3) in the fifth embodiment, and the deformation suppression structure 67 (FIG. 4) in the sixth embodiment may be constructed alone, or any two of the three may be constructed together, or all three may be constructed together. Furthermore, any two or more of the deformation suppression structures 17, 27, 37, 47, 57, and 67 may be constructed together.

[0045] In the fourth to sixth embodiments, the bentonite pellets 41 are used as the bentonite to be inserted into the boreholes 13, 23, and 33. Instead of the bentonite pellets 41, compacted bentonite lumps may be used. For example, the bentonite lumps are cylindrical with a diameter slightly smaller than the diameter of the boreholes 13, 23, and 33 (for example, a diameter of 20 cm) and a length of about 50 cm. Such bentonite lumps may be filled in the boreholes 13, 23, and 33 so as to be continuously aligned in the axial direction of the cylinders. In addition, in order to facilitate alignment of the bentonite lumps in the axial direction of the cylinders, engaging portions that engage with each other may be formed on the lower and upper surfaces of the cylinders of the bentonite lumps. For example, as the engaging portion, a conical portion may be formed on the lower surface of the cylinder so as to protrude coaxially, and a conical hole into which the conical portion fits may be formed on the upper surface of the cylinder.

[0046] The bentonite lump as described above may be inserted into the borehole 13, 23, 33 using the insertion tool 81 (FIG. 5) described above. In this case, the bentonite lump has a diameter slightly smaller than the inner diameter of the storage section 83 and is stored in the storage section 83. Also, a mixture of cylindrical bentonite lump and bentonite pellets 41 may be used. In this case, the bentonite lump and the bentonite pellets 41 may be stored together in the storage section 83 of the insertion tool 81.

[0047] Also, it is not essential to use the insertion tool 81. For example, when cylindrical bentonite blocks as described above are used, the bentonite blocks may be inserted into the boreholes 13, 23, 33 one by one from above ground to fill the boreholes 13, 23, 33. When bentonite pellets 41 are used, the bentonite pellets 41 may be poured into the boreholes 13, 23, 33 to fill them. As the water-absorbing and swelling soil material forming the underground structure, bentonite-mixed soil or the like may be used instead of bentonite.

[0048] In the first to sixth embodiments, after the boring process, optical fiber sensors may be installed in the boreholes 13, 23, 33, and the optical fiber sensors embedded in the deformation suppression structure may be used to measure ground strain, thereby monitoring the behavior of the surrounding ground when the frozen soil wall 11 melts.

[0049] In the first to sixth embodiments, the boreholes 13, 23, 33 pass through the frozen soil wall 11, but this configuration is not essential. The boreholes 13, 23, 33 may pass through an area outside the frozen soil wall 11 in the vicinity of the freezing pipe 5. Even if the deformation suppression structures 17, 27, 37, 47, 57, 67 are constructed in the area outside the frozen soil wall 11, deformation of the ground due to melting of the frozen soil wall 11 can be suppressed so long as the area is relatively close to the frozen soil wall 11.

[0050] In this case, for example, the area of ​​the ground surrounding the freezing pipe 5 where the ground temperature is below 0°C can be determined as inside the frozen soil wall 11, and the area where the ground temperature is 0°C or higher can be determined as outside the frozen soil wall 11. In this type of freezing method, temperature sensors for monitoring the ground temperature are buried at various locations in the ground surrounding the freezing pipe 5, and the areas inside the frozen soil wall 11 and the areas outside the frozen soil wall 11 as described above can be distinguished based on the temperatures measured by these temperature sensors.

[0051] When the boreholes 13, 23, 33 pass outside the frozen soil wall 11, in order to fully obtain the effect of suppressing deformation of the ground, it is preferable that the positions at which the deformation suppression structures 17, 27, 37, 47, 57, 67 are constructed are close to the frozen soil wall 11. From this viewpoint, when the boreholes 13, 23, 33 pass outside the frozen soil wall 11, it is preferable that the boreholes 13, 23, 33 pass through a position where the ground temperature is less than +3°C, and it is further preferable that the boreholes 13, 23, 33 pass through a position where the ground temperature is less than +1°C.

[0052] Moreover, it is preferable that the boreholes 13, 23, 33 pass through the frozen soil wall 11 (that is, a position where the ground temperature is below 0°C), and more preferably pass through a position where the ground temperature is below -2°C.

[0053] When the borehole 13, 23, 33 passes outside the frozen soil wall 11 as described above, there is a possibility that liquid groundwater exists in the borehole 13, 23, 33 after the formation. In this case, in the underground structure formation process in the fourth to sixth embodiments, the bentonite pellets 41 in the storage section 83 of the insertion tool 81 (FIG. 5) may come into contact with groundwater immediately after the lid 83a of the storage section 83 is opened. In this case, the bentonite pellets 41 may swell before being completely discharged from the storage section 83, and the outlet of the storage section 83 may be blocked, which may prevent the bentonite pellets 41 from being smoothly filled in the borehole 13, 23, 33. In order to suppress such blocking of the outlet of the storage section 83, the fourth to sixth embodiments may use a method as described below.

[0054] As a method for suppressing the clogging of the outlet of the storage section 83 of the insertion tool 81, in the underground structure formation process, after the bentonite pellets 41 are stored in the storage section 83, ethanol is poured into the storage section 83 before the insertion into the borehole 13, 23, 33. According to this method, when the cover 83a of the storage section 83 is opened in the groundwater in the borehole 13, 23, 33, the swelling of the bentonite pellets 41 is delayed only while the ethanol in the storage section 83 is sufficiently replaced by groundwater. In this way, the bentonite pellets 41 are smoothly discharged outside the storage section 83 before swelling, and the possibility of smoothly filling the borehole 13 is increased. In addition, since ethanol has a property of being easily mixed with water, it is sufficiently diluted by the surrounding groundwater, and there is almost no adverse effect on the surroundings due to the ethanol remaining in the borehole 13, 23, 33.

[0055] As another method for suppressing the outlet blockage of the storage section 83 of the insertion tool 81, an ice film is formed in advance on the grain surface of the bentonite pellets 41. Then, in the underground structure formation step, the bentonite pellets 41 covered with the ice film are stored in the storage section 83, and the borehole 13, 23, 33 is filled by the insertion tool 81. According to this method, when the cover 83a of the storage section 83 is opened in the groundwater in the borehole 13, 23, 33, the swelling of the bentonite pellets 41 is delayed by the time it takes for the ice film to melt. Then, the bentonite pellets 41 are smoothly discharged outside the storage section 83 before swelling, and the possibility of smoothly filling the borehole 13, 23, 33 is increased. In addition, in order to form an ice film on the grain surface of the bentonite pellets 41, for example, a method may be used in which the bentonite pellets 41 are immersed in liquid nitrogen to cool them, and then immersed in water to freeze the water attached to the grain surface of the bentonite pellets 41 on the grain surface.

[0056] As yet another method for suppressing clogging of the outlet of the storage section 83 of the insertion tool 81, a salt of an alkali metal ion identical to the interlayer cation of the bentonite is mixed with the bentonite to form the bentonite pellets 41. As a specific example, at least one of sodium chloride, sodium carbonate, and sodium hydrogen carbonate is mixed with sodium-type bentonite to form the bentonite pellets 41. As another example, calcium chloride is mixed with calcium-type bentonite to form the bentonite pellets 41. Then, in the underground structure formation process, the bentonite pellets 41 are stored in the storage section 83, and smoothly filled into the boreholes 13, 23, and 33 by the insertion tool 81.

[0057] Normally, when bentonite comes into contact with water, water molecules hydrate with the interlayer cations, and the distance between the unit layers increases, causing swelling. In contrast, the bentonite pellets 41 described above contain alkali metal ions that are the same as the interlayer cations of bentonite, so the hydration of water molecules with the interlayer cations is suppressed, and as a result, the swelling of the bentonite pellets 41 that come into contact with groundwater is delayed. This increases the possibility that the bentonite pellets 41 will be smoothly discharged outside the storage section 83 before they swell, and will be smoothly filled into the boreholes 13, 23, and 33. [Explanation of symbols]

[0058] 1...ground, 2...low-permeability layer, 5...frozen pipe, 9...frozen soil, 11...frozen soil wall, 12...low-permeability frozen section, 13, 23, 33...borehole, 15, 25, 35...cement-based underground structure, 45, 55, 65...bentonite-based underground structure, 17, 27, 37, 47, 57, 67...deformation prevention structure.

Claims

1. A method for constructing a deformation suppression structure that suppresses deformation of the surrounding ground when frozen soil formed around a freezing pipe in the ground by a freezing method melts, a boring step of forming a borehole passing through the frozen soil while the refrigerant continues to flow through the freezing pipe and the frozen soil is frozen; A method for constructing a deformation suppression structure, comprising: an underground structure formation process for filling the borehole with a predetermined filling material to form an underground structure made of the filling material while the flow of refrigerant through the freezing pipes continues and the frozen soil is frozen.

2. The frozen soil forms a vertical frozen soil wall formed around the plurality of parallel extending freezing pipes and extending in the arrangement direction of the freezing pipes, In the boring step, A method for constructing a deformation suppression structure as described in claim 1, wherein a plurality of the boreholes are formed within the frozen soil wall extending in the extension direction of the freezing pipes and aligned in the arrangement direction of the freezing pipes, and / or a plurality of the boreholes are formed within the frozen soil wall extending in the arrangement direction of the freezing pipes and aligned in the extension direction of the freezing pipes.

3. In the underground structure formation process, 3. A method for constructing a deformation suppression structure as described in claim 1 or 2, wherein a cement-based material is introduced into the borehole as the filling material and hardens to form the underground structure.

4. In the underground structure formation process, 3. A method for constructing a deformation suppression structure as described in claim 1 or 2, wherein a water-absorbing and swelling soil-based material is introduced into the borehole as the filling material to form the underground structure.

5. A plurality of the freezing tubes extend in parallel, The method for constructing a deformation suppression structure described in any one of claims 1 to 4, wherein the underground structure has a rod-shaped portion extending parallel to the freezing pipe and / or a rod-shaped portion extending in a direction perpendicular to the freezing pipe.

6. The frozen soil includes a part of a low-permeability layer that is partially frozen, The method for constructing a deformation suppression structure according to any one of claims 1 to 5, wherein the boring step forms a borehole passing through the low permeability layer.

7. A method for melting frozen soil formed around a freezing pipe in the ground by a freezing method, comprising: A method for melting frozen soil, comprising forming an underground structure by the method for constructing a deformation suppression structure according to any one of claims 1 to 6, and then stopping cooling of the frozen soil to melt the frozen soil.

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