Liquefaction countermeasure structure and liquefaction countermeasure method

Abstract

To provide a liquefaction countermeasure structure and a liquefaction countermeasure method that can more reliably suppress liquefaction of the ground directly below and around buildings.SOLUTION: A liquefaction countermeasure structure according to one embodiment is a liquefaction countermeasure structure 1 that prevents liquefaction of the ground G directly below a building S. The liquefaction countermeasure structure 1 includes a peripheral improvement body 12 that is buried in the ground G and has a frame shape in plan view. At least a part of the peripheral improvement body 12 is inclined vertically downward toward the center of the peripheral improvement body 12 in plan view. The structure further includes a liquefaction countermeasure section that is a portion of the ground G inside the peripheral improvement body 12 in plan view where liquefaction countermeasures have been applied.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a liquefaction countermeasure structure and a liquefaction countermeasure method.

Background Art

[0002] Patent Document 1 describes a liquefaction countermeasure structure formed in the ground below an existing structure. The liquefaction countermeasure structure includes a pair of vertical underground walls and a pair of inclined underground walls located between the pair of vertical underground walls. The pair of inclined underground walls face each other so as to form an inverted V shape. The pair of vertical underground walls are formed so as to sandwich the foundation ground located directly below the existing structure from the left and right in the transverse direction of the existing structure. The vertical underground walls are columnar ground improvement bodies formed by a ground mixing improvement method in which a cement-based solidifying material and the original ground are stirred and mixed. The lower end of the vertical underground wall reaches (is rooted in) the non-liquefied layer. Also, the upper end of the vertical underground wall is located at a position higher than the groundwater level.

[0003] Each of the pair of inclined underground walls is formed from the side of the existing structure downward toward the lower part of the existing structure. By forming the pair of inclined underground walls, the ground below the existing structure is separated into a first region located above the inclined underground walls and a second region located below the inclined underground walls. The inclined underground walls are not joined to other structures and are structurally independent. The upper end of the inclined underground wall is located between the existing structure and the vertical underground wall, and the lower end of the inclined underground wall is located at the lower central part of the existing structure. The upper end of the inclined underground wall faces the ground surface, and the lower end of the inclined underground wall abuts or is slightly inserted into the non-liquefied layer.

[0004] In this liquefaction countermeasure structure, the first region located above the inclined underground walls may liquefy. When the first region liquefies and ground settlement occurs in the existing structure, the muddy water pressure in the first region acts on the inclined underground wall. When the muddy water pressure in the first region acts, the inclined underground wall rotates with the lower end as a fulcrum and presses the second region. As a result, the force (constraint force) for suppressing the second region surrounded by the vertical underground wall and the inclined underground wall increases, and it becomes difficult for the second region to liquefy.

[0005] Patent Document 2 describes a ground improvement body including a wall-shaped ground improvement portion and an inclined wall-shaped ground improvement portion. The ground improvement body is a grid-shaped ground improvement body constructed continuously in the X direction and the Y direction. In the wall-shaped ground improvement portion, four vertical walls are arranged in a rectangular shape so as to surround the structure in plan view. The lower end portion of the wall-shaped ground improvement portion is embedded in the non-liquefiable layer.

[0006] The inclined wall-shaped ground improvement portion is constructed in a quadrangular pyramid shape. The inclined wall-shaped ground improvement portion is composed of four inclined walls that are inclined obliquely inward from the entire circumference of the upper end portion of the wall-shaped ground improvement portion. The lower end portions of the inclined wall-shaped ground improvement portion are joined and closed. The load of the structure during an earthquake is received by the inclined wall-shaped ground improvement portion and transmitted near the ground surface.

[0007] Patent Document 3 describes a ground improvement structure for improving the soft ground located directly below a structure. The ground improvement structure includes a planar grid-shaped wall-shaped ground improvement body and a plurality of compacted piles extending vertically inside the wall-shaped ground improvement body in plan view. The compacted piles are columnar or block-shaped ones constructed while applying vibration or pressure to the ground for the purpose of compacting the ground. The compacted piles are columnar or block-shaped piles constructed by methods such as the sand compaction method or the non-excavation pile method while compacting the ground to increase the density.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0009] In the above-described structure, an inclined improvement body that inclines inward in plan view as it goes downward is constructed inside the vertical ground improvement body that has a rectangular shape in plan view and extends in the vertical direction. However, in such a structure, although liquefaction of the ground located around the building in plan view can be suppressed, liquefaction of the ground located directly below the building may not be reliably suppressed. Therefore, it is required to more reliably suppress liquefaction of the ground located directly below and around the building.

[0010] An object of the present disclosure is to provide a liquefaction countermeasure structure and a liquefaction countermeasure method that can more reliably suppress liquefaction of the ground located directly below and around a building.

Means for Solving the Problems

[0011] (1) The liquefaction countermeasure structure according to the present disclosure is a liquefaction countermeasure structure for preventing liquefaction of the ground located directly below a building. The liquefaction countermeasure structure includes an outer peripheral improvement body that is buried in the ground and formed in a frame shape in plan view. At least a part of the outer peripheral improvement body inclines toward the center side of the outer peripheral improvement body in plan view as it goes vertically downward. The liquefaction countermeasure structure further includes a liquefaction countermeasure portion that is a portion where liquefaction countermeasures are applied to the ground inside the outer peripheral improvement body in plan view.

[0012] This liquefaction countermeasure structure includes an outer peripheral part improvement body formed in a frame shape in plan view. At least a part of the outer peripheral part improvement body is inclined so as to face the central side of the outer peripheral part improvement body in plan view as it goes vertically downward. In this way, by making at least a part of the outer peripheral part improvement body oblique to the vertical direction, the rigidity of the ground improvement body against horizontal forces can be improved. This liquefaction countermeasure structure has a liquefaction countermeasure part which is a part where liquefaction countermeasures are taken inside the outer peripheral part improvement body in plan view. By providing the liquefaction countermeasure part directly below the building, liquefaction in the ground directly below the building can be suppressed. And since the outer peripheral part improvement body is provided outside the liquefaction countermeasure part in plan view, liquefaction of the ground located around the building can be suppressed. Therefore, liquefaction of the ground located directly below and around the building can be more reliably suppressed.

[0013] (2) In the above (1), the outer peripheral part improvement body may have a polygonal shape in plan view. At least a part of one side of the outer peripheral part improvement body in plan view may be inclined so as to face the central side of the outer peripheral part improvement body in plan view as it goes vertically downward. In this case, in at least a part of one side of the outer peripheral part improvement body in plan view, the rigidity of the ground improvement body against horizontal forces can be improved.

[0014] (3) In the above (1) or (2), the liquefaction countermeasure part may be a plurality of central improvement bodies extending in the vertical direction. In this case, since the distance from the outer peripheral part improvement body to the central improvement body becomes shorter as it goes vertically downward, the restraint effect by the improvement body can be enhanced as it goes vertically downward. Therefore, the amount of the liquefaction countermeasure part can be reduced, such as by reducing the number of central improvement bodies, so that the cost for the improvement body can be reduced and the construction period can be shortened.

[0015] (4) In any of the above (1) to (3), the ground may have a liquefied layer and a non-liquefied layer located below the liquefied layer, and the lower end of the peripheral improvement body may reach the non-liquefied layer. In this case, since the lower end of the peripheral improvement body reaches the non-liquefied layer below the liquefied layer, the liquefaction of the ground located around the building can be more reliably suppressed.

[0016] (5) The liquefaction countermeasure method according to the present disclosure is a liquefaction countermeasure method for preventing the liquefaction of the ground located directly below the building. The liquefaction countermeasure method includes a step of forming a liquefaction countermeasure portion in the ground by performing a liquefaction countermeasure on the ground located directly below the building, and a step of forming a peripheral improvement body formed in a frame shape in plan view so as to surround the liquefaction countermeasure portion in plan view. At least a part of the peripheral improvement body is inclined so as to face the central side of the peripheral improvement body in plan view as it goes vertically downward.

[0017] This liquefaction countermeasure method performs a liquefaction countermeasure on the ground located directly below the building to form a liquefaction countermeasure portion. Therefore, the liquefaction of the ground directly below the building can be suppressed. Since the peripheral improvement body is provided outside the liquefaction countermeasure portion in plan view, the liquefaction of the ground located around the building can be suppressed. At least a part of the peripheral improvement body is inclined so as to face the central side of the peripheral improvement body in plan view as it goes vertically downward. Therefore, by making at least a part of the peripheral improvement body oblique with respect to the vertical direction, the rigidity of the ground improvement body against the horizontal force can be improved. Thus, the liquefaction of the ground located directly below and around the building can be more reliably suppressed.

Advantages of the Invention

[0018] According to the present disclosure, the liquefaction of the ground located directly below and around the building can be more reliably suppressed.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

[0020] Hereinafter, embodiments of the liquefaction countermeasure structure and the liquefaction countermeasure method according to the present disclosure will be described with reference to the drawings. In the description of the drawings, the same or corresponding elements are denoted by the same reference numerals, and redundant descriptions are omitted as appropriate. The drawings may be drawn with some parts simplified or exaggerated for ease of understanding, and the dimensional ratios and the like are not limited to those described in the drawings.

[0021] FIG. 1 is a plan view showing a liquefaction countermeasure structure 1 according to an embodiment. FIG. 2 is a cross-sectional view taken along line A-A of FIG. 1. As shown in FIGS. 1 and 2, the liquefaction countermeasure structure 1 is located directly below the building S. In FIG. 1, in order to clarify the structure of the liquefaction countermeasure structure 1, the liquefaction countermeasure structure 1 is shown by a solid line and the building S is shown by a dashed line. For example, the area of the liquefaction countermeasure structure 1 in plan view is larger than the area of the building S in plan view, and the building S is accommodated inside the liquefaction countermeasure structure 1 in plan view. The liquefaction countermeasure structure 1 has a rectangular (square) shape in plan view.

[0022] The liquefaction countermeasure structure 1 is constructed to prevent liquefaction of the ground G located directly below the building S. The ground G has a liquefied layer G1 and a non-liquefied layer G2 located below the liquefied layer G1. For example, the ground G has been previously judged to be liquefiable, and as a result of the liquefaction judgment, the liquefied layer G1 and the non-liquefied layer G2 have been specified.

[0023] The liquefaction countermeasure structure 1 has a plurality of ground improvement bodies 10 embedded in the ground G. The ground improvement bodies 10 have a wall shape. The ground improvement bodies 10 are formed, for example, by hardening the mixed and agitated original ground (ground G) and cement-based solidifying materials. The plurality of ground improvement bodies 10 include a plurality of central improvement bodies 11 located directly below the building S and outer peripheral improvement bodies 12 located outside the central improvement bodies 11 in plan view.

[0024] The outer peripheral improvement bodies 12 have a frame shape in plan view. For example, the upper ends of the outer peripheral improvement bodies 12 have a plurality of connecting portions 12b connected to the upper ends of the central improvement bodies 11. The outer peripheral improvement bodies 12 have, for example, a polygonal shape in plan view. As an example, the shape of the outer peripheral improvement bodies 12 in plan view is rectangular. The plurality of connecting portions 12b are arranged along the sides of the outer peripheral improvement bodies 12 in plan view.

[0025] The central improvement body 11 is a liquefaction countermeasure part, which is a part of the ground G where liquefaction countermeasures are taken inside the outer peripheral improvement body 12 in plan view. For example, in plan view, the central improvement bodies 11 are arranged in a grid pattern. In this case, since the liquefaction countermeasure part is the central improvement body 11 formed in a grid pattern in plan view, liquefaction in the ground G directly under the building S can be more reliably suppressed. The plurality of central improvement bodies 11 includes a plurality of central improvement bodies 13 extending along a first direction D1 which is a horizontal direction, and a plurality of central improvement bodies 14 extending along a second direction D2 which is a horizontal direction and intersects (orthogonal as an example) the first direction D1.

[0026] The central improvement body 11 extends in a third direction D3 that intersects (orthogonal as an example) both the first direction D1 and the second direction D2. As an example, the third direction D3 is a vertical direction. For example, the liquefaction countermeasure part is a plurality of central improvement bodies 11 extending in the vertical direction. The length of the central improvement body 13 in the third direction D3 becomes shorter as it approaches the end of the liquefaction countermeasure structure 1 in the first direction D1 (the upper end of the outer peripheral improvement body 12).

[0027] The plurality of central improvement bodies 13 includes, for example, a first central improvement body 13b having a portion reaching the non-liquefied layer G2, and a second central improvement body 13c not having a portion reaching the non-liquefied layer G2 (remaining in the liquefied layer G1). The maximum value of the length of the second central improvement body 13c in the third direction D3 is smaller than the maximum value of the length of the first central improvement body 13b in the third direction D3.

[0028] For example, the first central improvement body 13b is located closer to the center in the second direction D2 of the liquefaction countermeasure structure 1, and the second central improvement body 13c is located on both end sides in the second direction D2 of the liquefaction countermeasure structure 1. That is, in the second direction D2, a pair of second central improvement bodies 13c are arranged at positions sandwiching the first central improvement body 13b.

[0029] For example, the maximum values of the lengths (the lengths in the third direction D3) of the plurality of first central improvement bodies 13b are the same as each other. In contrast, the maximum values of the lengths of the plurality of second central improvement bodies 13c are different from each other. The maximum value of the length of the second central improvement body 13c located on the end side in the second direction D2 of the liquefaction countermeasure structure 1 is smaller than the maximum value of the length of the second central improvement body 13c located on the central side in the second direction D2 of the liquefaction countermeasure structure 1.

[0030] That is, the maximum value of the length of the second central improvement body 13c becomes smaller as it approaches the end in the second direction D2 of the liquefaction countermeasure structure 1. An outer peripheral improvement body 12 is provided below the second central improvement body 13c in the ground G. An unimproved region G3, which is a region of the ground G that is not improved, is formed between the lower end of the second central improvement body 13c and the outer peripheral improvement body 12 in the ground G.

[0031] The plurality of central improvement bodies 14 include, for example, a first central improvement body 14b having a portion reaching the non-liquefied layer G2 and a second central improvement body 14c not having a portion reaching the non-liquefied layer G2. The maximum value of the length of the second central improvement body 14c is smaller than the maximum value of the length of the first central improvement body 14b. For example, the first central improvement body 14b is located closer to the center in the first direction D1 of the liquefaction countermeasure structure 1, and the second central improvement body 14c is located on the end side in the first direction D1 of the liquefaction countermeasure structure 1.

[0032] That is, in the first direction D1, a pair of second central improvement bodies 14c are arranged at positions sandwiching the first central improvement body 14b. For example, the maximum values of the lengths of the plurality of first central improvement bodies 14b are the same as each other. The maximum values of the lengths of the plurality of second central improvement bodies 14c are different from each other. The maximum value of the length of the second central improvement body 14c located on the end side in the first direction D1 of the liquefaction countermeasure structure 1 is smaller than the maximum value of the length of the second central improvement body 14c located on the central side in the first direction D1 of the liquefaction countermeasure structure 1.

[0033] The length of the second central improvement body 14c becomes shorter as it approaches the end of the liquefaction countermeasure structure 1 in the first direction D1. Below the second central improvement body 14c in the ground G, an outer peripheral improvement body 12 is provided. An unimproved region G3 described above is formed between the lower end of the second central improvement body 14c and the outer peripheral improvement body 12 in the ground G.

[0034] For example, the lower end of the outer peripheral improvement body 12 reaches the non-liquefiable layer G2. For example, in plan view, the outer peripheral improvement body 12 is provided so as to surround the building S. At least a part of the outer peripheral improvement body 12 is inclined so as to face the central side of the outer peripheral improvement body 12 in plan view as it goes vertically downward. For example, at least a part of one side of the outer peripheral improvement body 12 in plan view is inclined so as to face the central side of the outer peripheral improvement body 12 in plan view as it goes vertically downward. As an example, the four sides of the outer peripheral improvement body 12 in plan view are inclined so as to face the central side of the outer peripheral improvement body 12 in plan view as it goes vertically downward.

[0035] In the cross-section when the outer peripheral improvement body 12 is cut by a plane extending in the third direction D3, a pair of outer peripheral improvement bodies 12 approach each other as they go downward. That is, in the cross-section, the pair of outer peripheral improvement bodies 12 are formed in an inverted V shape. The cross-sectional area of the cross-section when the outer peripheral improvement body 12 is cut by a plane extending in the horizontal direction becomes smaller as it goes vertically downward. As an example, the outer peripheral improvement body 12 has a frustum of a pyramid shape.

[0036] Next, an example of the steps of the liquefaction countermeasure method according to the present embodiment will be described. The liquefaction countermeasure method according to the present embodiment prevents the liquefaction of the ground G located directly below the building S. Below, an example of the steps of executing the liquefaction countermeasure method by constructing the liquefaction countermeasure structure 1 described above will be described.

[0037] First, a liquefaction countermeasure part is formed in the ground G by taking liquefaction countermeasures for the ground G located directly below the building S (step of forming a liquefaction countermeasure part). At this time, a plurality of central improvement bodies 11 extending in the third direction D3 are formed in the ground G located directly below the building S.

[0038] Next, an outer peripheral improvement body 12 formed in a frame shape in plan view is formed so as to surround the central improvement body 11 (liquefaction countermeasure part) in plan view (step of forming an outer peripheral improvement body). As shown in FIG. 3, for example, a hole for inserting the rod M1 of the boring machine M is formed so that the hole is inclined with respect to the third direction D3, and the rod M1 is inserted into the hole.

[0039] Then, while rotating the rod M1, ultra-high-pressure water is injected onto the inner wall of the hole, and a ground hardening agent is poured into the hole while excavating the hole. At this time, the poured ground hardening agent is stirred by, for example, the stirring blades of the rod M1. When the rod M1 moves obliquely upward while pouring and stirring the ground hardening agent, the region where the ground hardening agent is poured expands, and a columnar ground improvement body 12c extending obliquely with respect to the vertical direction is formed. Note that the central improvement body 11 can also be formed by the same method as described above.

[0040] The rod M1 is inserted at a position adjacent to the location where the ground improvement body 12c is formed in plan view, and a plurality of ground improvement bodies 12c are formed continuously in the horizontal direction by the same method as described above. Then, by forming a plurality of ground improvement bodies 12c having a frustum-shaped cylindrical shape so as to surround the plurality of central improvement bodies 11, the outer peripheral improvement body 12 is completed.

[0041] Next, the effects obtained from the liquefaction countermeasure structure 1 and the liquefaction countermeasure method according to the present embodiment will be described. As shown in FIGS. 1 and 2, the liquefaction countermeasure structure 1 and the liquefaction countermeasure method according to the present embodiment include an outer peripheral part improvement body 12 formed in a frame shape in plan view. At least a part of the outer peripheral part improvement body 12 is inclined so as to face the central side of the outer peripheral part improvement body 12 in plan view as it goes downward in the vertical direction. In this way, by making at least a part of the outer peripheral part improvement body 12 oblique to the vertical direction, the rigidity of the ground improvement body 10 against the horizontal force can be improved.

[0042] FIG. 4 shows the inertial force (lateral force from the unimproved ground), bending moment diagram (M diagram), and axial force diagram (N diagram) of the liquefaction countermeasure structure according to the comparative example having the improvement body 100 arranged along the vertical direction. FIG. 5 shows the inertial force, bending moment diagram, and axial force diagram of the liquefaction countermeasure structure 1 according to the present embodiment having the outer peripheral part improvement body 12.

[0043] As shown in FIGS. 4 and 5, in the case of the liquefaction countermeasure structure 1 having the outer peripheral part improvement body 12 arranged obliquely to the vertical direction, the lateral force of the seismic force can be borne and transmitted as the axial force of the outer peripheral part improvement body 12 (like a folded plate structure). And in the outer peripheral part improvement body 12, since the rigidity can be made higher than that of the improvement body 100 arranged along the vertical direction, the deformation of the liquefaction countermeasure structure 1 during an earthquake or the like can be suppressed.

[0044] Also, as shown in FIGS. 1 and 2, the outer peripheral part improvement body 12 is arranged obliquely with respect to the vertical direction so as to approach the central improvement body 11 as it goes downward. Therefore, since the interval between the ground improvement bodies 10 at the lower part of the liquefaction layer G1 is narrowed, the restraining effect of the ground improvement bodies 10 on the inner ground is enhanced, and it becomes possible to prevent the ground improvement bodies 10 at the central part of the liquefaction countermeasure structure 1 in plan view from reaching the non-liquefaction layer G2. Further, since the outer peripheral part improvement body 12 is arranged obliquely with respect to the vertical direction so as to approach the central improvement body 11 as it goes downward, the length of a part of the improvement bodies of the central improvement body 11 can be shortened as in the case of the second central improvement bodies 13c and 14c described above.

[0045] Furthermore, the liquefaction countermeasure structure 1 has a liquefaction countermeasure part (central improvement body 11) which is a part where liquefaction countermeasures are taken inside the outer peripheral part improvement body 12 in plan view. By providing the liquefaction countermeasure part directly below the building S, liquefaction in the ground G directly below the building S can be suppressed. And since the outer peripheral part improvement body 12 is provided outside the liquefaction countermeasure part in plan view, liquefaction of the ground G located around the building S can be suppressed. Therefore, liquefaction of the ground G located directly below and around the building S can be more reliably suppressed.

[0046] In the present embodiment, the outer peripheral part improvement body 12 may have a polygonal shape in plan view. At least one side portion of the outer peripheral part improvement body 12 in plan view may be inclined so as to go toward the central side of the outer peripheral part improvement body 12 in plan view as it goes vertically downward. In this case, the rigidity of the ground improvement body 10 against the horizontal force can be improved at at least one side portion of the outer peripheral part improvement body 12 in plan view.

[0047] In this embodiment, the liquefaction countermeasure part is a plurality of central improvement bodies 11 extending in the vertical direction. In this case, since the distance from the outer peripheral improvement body 12 to the central improvement body 11 becomes shorter as it goes vertically downward, the restraint effect by the ground improvement body 10 can be enhanced as it goes vertically downward. Therefore, the amount of the liquefaction countermeasure part can be reduced by reducing the number of the central improvement bodies 11 or the like, so that the cost for the ground improvement body 10 can be reduced and the construction period can be shortened.

[0048] In this embodiment, the ground G has a liquefied layer G1 and a non-liquefied layer G2 located below the liquefied layer G1, and the lower end of the outer peripheral improvement body 12 reaches the non-liquefied layer G2. In this case, since the lower end of the outer peripheral improvement body 12 reaches the non-liquefied layer G2 below the liquefied layer G1, the liquefaction of the ground G located around the building S can be more reliably suppressed.

[0049] In the liquefaction countermeasure method according to this embodiment, for example, after forming the central improvement body 11, the outer peripheral improvement body 12 is formed. When the central improvement body 11 is formed after forming the outer peripheral improvement body 12, there is a concern that the rod M1 of the boring machine M or the like may contact the outer peripheral improvement body 12 when forming the central improvement body 11. On the other hand, when the outer peripheral improvement body 12 is formed after forming the central improvement body 11, since the outer peripheral improvement body 12 is formed below the lower end of the central improvement body 11, the possibility of the rod M1 or the like contacting the central improvement body 11 can be reduced. However, the central improvement body 11 may be formed after forming the outer peripheral improvement body 12.

[0050] Next, a modification example of the liquefaction countermeasure structure according to the present disclosure will be described. Some configurations of the liquefaction countermeasure structure according to each of the modification examples described later are the same as some configurations of the liquefaction countermeasure structure 1 described above. Therefore, hereinafter, the descriptions overlapping with the description of the liquefaction countermeasure structure 1 will be appropriately omitted by attaching the same reference numerals.

[0051] FIG. 6 is a cross-sectional view showing the liquefaction countermeasure structure 21 according to the first modification. As shown in FIG. 6, the liquefaction countermeasure structure 21 is different from the above-described liquefaction countermeasure structure 1 in that the end portion of the central improvement body 23 in the first direction D1 is separated from the outer peripheral improvement body 12. An unimproved region G4 is formed between the end portion of the central improvement body 23 (central improvement body 14) in the first direction D1 and the outer peripheral improvement body 12. In this case, since the central improvement body 23 is smaller than the above-described central improvement body 13, the amount of the liquefaction countermeasure portion can be further reduced, so that the cost of the ground improvement body can be further reduced. Note that the end portion of the central improvement body 14 in the second direction D2 may be separated from the outer peripheral improvement body 12, and an unimproved region G4 may be formed between the end portion of the central improvement body 14 in the second direction D2 and the outer peripheral improvement body 12.

[0052] FIG. 7 is a cross-sectional view showing the liquefaction countermeasure structure 31 according to the second modification. As shown in FIG. 7, the liquefaction countermeasure structure 31 does not have an unimproved region G3 between the lower end of the central improvement body 33 in the ground G and the outer peripheral improvement body 12. That is, the central improvement body 33 is provided over the entire region between the pair of outer peripheral improvement bodies 12 in the first direction D1. In this case, the rigidity of the liquefaction countermeasure structure 31 can be further improved. Further, the central improvement body 14 may be provided over the entire region between the pair of outer peripheral improvement bodies 12 in the second direction D2.

[0053] FIG. 8(a) is a plan view showing the liquefaction countermeasure structure 41 according to the third modification. FIG. 8(b) is a cross-sectional view taken along line B-B of FIG. 8(a). As shown in FIGS. 8(a) and 8(b), the liquefaction countermeasure structure 41 includes an outer peripheral improvement body 42 having a rectangular shape in plan view, a central improvement body 43 extending in the first direction D1 and the third direction D3 inside the outer peripheral improvement body 42 in plan view, and a central improvement body 44 extending in the second direction D2 and the third direction D3 inside the outer peripheral improvement body 42 in plan view. The central improvement body 43 is formed in a wall shape having a thickness in the second direction D2, and the central improvement body 44 is formed in a wall shape having a thickness in the first direction D1.

[0054] In the ground G, an underground structure 45 is buried. The underground structure 45 is, for example, an oil tank. The underground structure 45 is arranged at an adjacent position to the liquefaction countermeasure structure 41 in plan view. The outer peripheral improvement body 42 has a rectangular shape in plan view, similar to the outer peripheral improvement body 12 described above.

[0055] The underground structure 45 is arranged at an adjacent position to one side of the outer peripheral improvement body 42 in plan view. The portion of the said one side of the outer peripheral improvement body 42 in plan view is inclined so as to go toward the center side of the outer peripheral improvement body 42 in plan view as it goes vertically downward. In the portion of the said one side, the outer peripheral improvement body 42 is inclined with respect to the third direction D3 (vertical direction) so as to move away from the underground structure 45 as it goes downward. Thereby, the influence on the underground structure 45 due to the construction of the liquefaction countermeasure structure 41 can be reduced. The minimum value X of the separation distance from the underground structure 45 to the outer peripheral improvement body 42 in the first direction D1 (horizontal direction) is, for example, 50 cm or more.

[0056] FIG. 9(a) is a plan view showing a liquefaction countermeasure structure 51 according to a fourth modification. FIG. 9(b) is a cross-sectional view taken along the line C-C of FIG. 9(a). As shown in FIGS. 9(a) and 9(b), the liquefaction countermeasure structure 51 has an outer peripheral improvement body 52 presenting a frame shape in plan view and a liquefaction countermeasure portion 55 located inside the outer peripheral improvement body 52 in plan view.

[0057] The outer peripheral improvement body 52 has, for example, a structure similar to the outer peripheral improvement body 12 described above. That is, the cross-section of the outer peripheral improvement body 52 when cut by a plane extending in the vertical direction is such that the portions of the pair of outer peripheral improvement bodies 52 approach each other as they go vertically downward. The liquefaction countermeasure portion 55 is provided above the outer peripheral improvement body 52 in the ground G.

[0058] For example, the liquefaction countermeasure section 55 is formed by a compaction method. In this case, the liquefaction countermeasure section 55 has a plurality of compaction piles 53 located inside the ground G. The compaction piles 53 are, for example, sand piles constructed in the ground G and are portions where the density of the soil is increased by being subjected to vibration or impact.

[0059] For example, in plan view, the plurality of compaction piles 53 are formed in a staggered pattern. However, in plan view, the plurality of compaction piles 53 may be formed in a grid pattern, and the arrangement and number of the compaction piles 53 are not particularly limited. For example, the lower end of the peripheral improvement body 52 reaches the non-liquefiable layer G2, and the lower end of the compaction pile 53 does not reach the non-liquefiable layer G2. The length of the compaction pile 53 located on the outer edge side of the liquefaction countermeasure structure 51 in plan view is shorter than the length of the compaction pile 53 located on the central side of the liquefaction countermeasure structure 51 in plan view. Thus, in the liquefaction countermeasure structure 51, the length of the compaction pile 53 can be shortened.

[0060] Note that the liquefaction countermeasure section 55 may be formed by a pore water pressure dissipation method instead of the compaction method. In this case, for example, in the liquefaction countermeasure section 55, a drain material made of an artificial material is driven into the ground G so as to extend in the vertical direction. This drain material is, as an example, a spiral drain. Then, by allowing the excess pore water generated during an earthquake to flow into and be drained by the above drain material, an increase in the excess pore water pressure is suppressed. Thus, the mode of the liquefaction countermeasure section 55 can be changed as appropriate.

[0061] The embodiments and various modifications of the liquefaction countermeasure structure and the liquefaction countermeasure method according to the present disclosure have been described above. However, the liquefaction countermeasure structure and the liquefaction countermeasure method according to the present disclosure are not limited to the above-described embodiments or modifications, and may be further modified within the scope of the gist described in the claims. That is, the structure, shape, size, material, number, and arrangement mode of each part of the liquefaction countermeasure structure, and the content and order of the steps of the liquefaction countermeasure method can be appropriately changed within the scope of the above gist. For example, in the above-described embodiment, an example of adopting the jet mixing method in the method for constructing the ground improvement body has been described. However, in the method for constructing the ground improvement body, it is not necessary to adopt the jet mixing method, and for example, a mechanical mixing method may be adopted. Thus, in the method for constructing the ground improvement body, various ground improvement methods can be adopted.

[0062] For example, in the above-described embodiment, the outer peripheral improvement body 12 having a rectangular shape in plan view has been described. However, the shape of the outer peripheral improvement body in plan view is not limited to a rectangular shape, and may be a polygonal shape such as a triangular shape, a quadrangular shape, or a hexagonal shape, or a circular shape or an oval shape, and is not particularly limited.

[0063] In the above-described embodiment, an example in which the four sides of the outer peripheral improvement body 12 in plan view are inclined so as to face the central side of the outer peripheral improvement body 12 in plan view as they go vertically downward has been described. However, one side, two sides, three sides, or five or more sides of the outer peripheral improvement body in plan view may be inclined so as to face the central side of the outer peripheral improvement body in plan view as they go vertically downward.

Description of Reference Numerals

[0064] 1…Liquefaction countermeasure structure, 10…Ground improvement body, 11…Central improvement body, 12…Peripheral improvement body, 12b…Connection part, 12c…Ground improvement body, 13…Central improvement body, 13b…First central improvement body, 13c…Second central improvement body, 14…Central improvement body, 14b…First central improvement body, 14c…Second central improvement body, 21…Liquefaction countermeasure structure, 23…Central improvement body, 31…Liquefaction countermeasure structure, 33…Central improvement body, 41…Liquefaction countermeasure structure, 42…Peripheral improvement body, 43…Central improvement body, 44…Central improvement body, 45…Underground structure, 51…Liquefaction countermeasure structure, 52…Peripheral improvement body, 53…Compaction pile, 55…Liquefaction countermeasure part, G…Ground, G1…Liquefied layer, G2…Non-liquefied layer, G3, G4…Ungraded area, M…Boring machine, M1…Rod, S…Building.

Claims

1. A liquefaction countermeasure structure for preventing liquefaction of the ground located directly below a building, comprising: an outer peripheral improvement body that is buried in the ground and formed in a frame shape in plan view; at least a part of the outer peripheral improvement body is inclined so as to face the central side of the outer peripheral improvement body in plan view as it goes vertically downward; further comprising a liquefaction countermeasure part which is a part of the ground where liquefaction countermeasures are taken inside the outer peripheral improvement body in plan view; a liquefaction countermeasure structure.

2. The outer peripheral improvement body has a polygonal shape in plan view; at least a part of one side of the outer peripheral improvement body in plan view is inclined so as to face the central side of the outer peripheral improvement body in plan view as it goes vertically downward. The liquefaction countermeasure structure according to Claim 1.

3. The liquefaction countermeasure part is a plurality of central improvement bodies extending in the vertical direction. The liquefaction countermeasure structure according to Claim 1 or Claim 2.

4. The ground has a liquefied layer and a non-liquefied layer located below the liquefied layer; the lower end of the outer peripheral improvement body reaches the non-liquefied layer. The liquefaction countermeasure structure according to Claim 1 or Claim 2.

5. A liquefaction countermeasure method for preventing liquefaction of the ground located directly below a building, comprising: forming a liquefaction countermeasure part in the ground by taking liquefaction countermeasures on the ground located directly below the building; forming an outer peripheral improvement body formed in a frame shape in plan view so as to surround the liquefaction countermeasure part in plan view; and at least a part of the outer peripheral improvement body is inclined so as to face the central side of the outer peripheral improvement body in plan view as it goes vertically downward. A liquefaction countermeasure method.

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