Construction method for liquefaction countermeasures using retaining walls

Abstract

To provide a construction method for a liquefaction countermeasure structure using an earth retention wall, which carries out efficient construction of a connection part between an outer periphery retention wall 2 where a reinforcement steel member 3 as a core member is installed and a wall-shaped ground improvement body 4 constructed inside thereof, while enhancing the strength continuity and integration of the connection part.SOLUTION: A construction method for a liquefaction countermeasure structure comprising an outer periphery retention wall 2 as a continuous underground wall where a reinforcement steel member 3 as a core member is installed in a soil-cement column row wall and a wall-shaped ground improvement body 4 partitioning the inside of the outer periphery retention wall 2 to a plurality of partitions constructs an auxiliary column row wall part 6 made of soil cement so as to overlap with a column row of the outer periphery retention wall 2 at a connection part between an inner peripheral surface of the outer periphery retention wall 2 and an end part of the wall-shaped ground improvement body 4. The end part of the wall-shaped ground improvement body 4 is constructed so as to overlap with the auxiliary column row wall part 6.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present invention relates to a construction method for a liquefaction countermeasure work using a retaining wall for a building constructed on liquefiable ground that is at risk of liquefaction.

Background Art

[0002] It is known that grid-shaped ground improvement constructed with appropriate mutual spacing and depth is effective as a liquefaction countermeasure for building sites, and by continuously constructing uniaxial columnar ground improvement, it has been implemented as a liquefaction countermeasure in numerous buildings to date.

[0003] On the other hand, as a retaining work method, the soil-cement column row continuous wall method, in which columnar ground improvement is performed by a multi-axis ground improvement machine, is implemented as a high-rigidity retaining wall that requires water stopping performance by inserting H-shaped steel into the improved body.

[0004] Grid-shaped ground improvement for liquefaction countermeasures reduces the decrease in ground reaction coefficient of the ground caused by liquefaction and reduces the input of seismic motion to the pile head in the liquefiable ground, so it is possible to reduce the decrease in pile bearing capacity in the liquefiable ground.

[0005] In addition, liquefiable ground is sandy soil ground with high water level in excavated ground, and the above-mentioned soil-cement column row continuous wall method is often adopted as a retaining work method. However, H-shaped steel is inserted inside the retaining wall, and grid-shaped ground improvement requires appropriate improved body strength, so connection at the joint with the uniaxial columnar ground improved body is accompanied by difficulty.

[0006] Regarding the construction of this joint, for example, there is a construction method in which ground improvement by high-pressure injection is applied to connect the retaining wall and the grid-shaped ground improvement part.

[0007] In addition, Patent Document 1 discloses a foundation structure that can suppress liquefaction of the ground in an inner region surrounded by a wall-shaped ground improvement body, comprising a grid-shaped ground improvement body formed in a grid pattern in the liquefiable layer, a wall-shaped ground improvement body formed on the outer periphery of the grid-shaped ground improvement body and surrounding an underground building constructed above the grid-shaped ground improvement body, and an out-of-plane stiffness-granting member embedded in the wall-shaped ground improvement body to impart out-of-plane stiffness to the wall-shaped ground improvement body.

[0008] The foundation structure described in Patent Document 1 involves surrounding a grid-shaped ground improvement body with a wall-shaped ground improvement body, thereby suppressing groundwater infiltration into the inner region enclosed by the wall-shaped ground improvement body. Furthermore, by imparting out-of-plane rigidity to the wall-shaped ground improvement body with an out-of-plane rigidity-granting member embedded in the wall-shaped ground improvement body, the resistance of the wall-shaped ground improvement body to earth pressure is increased. As a result, even if the ground in the outer region of the wall-shaped ground improvement body liquefies during an earthquake, the failure of the wall-shaped ground improvement body is suppressed, thus ensuring watertightness.

[0009] Furthermore, as a construction procedure in a specific embodiment, first, a wall-like ground improvement body is formed in a frame shape in plan view, spanning the liquefiable layer and the supporting layer using the soil-cement column method. At that time, connecting column-shaped improvement bodies are connected to the inner wall surface of the wall-like ground improvement body. Next, core materials are driven into and embedded in each column-shaped improvement body that makes up the wall-like ground improvement body. Then, the ground in the inner area surrounded by the wall-like ground improvement body is excavated to form an underground space for an underground building. At that time, the upper part of the wall-like ground improvement body functions as a retaining wall to suppress the collapse of the ground around the underground space.

[0010] Next, in the ground at the bottom of the underground space, a grid-like ground improvement body is formed in a grid pattern in plan view, spanning the liquefiable layer and the supporting layer using the soil-cement columnar construction method. At this time, connecting columnar improvement bodies are connected to the columnar improvement bodies in a wall-like manner, thereby connecting the ends of the grid-like ground improvement body to the inner wall surface of the wall-like ground improvement body. In this construction method, when constructing the wall-like ground improvement body, the connecting columnar improvement bodies that constitute the ends of the grid-like ground improvement body are connected to the inner wall surface of the wall-like ground improvement body in advance. This allows the connecting columnar improvement bodies to be connected to the columnar improvement bodies at a position away from the wall-like ground improvement body. Therefore, the excavation auger etc. used to construct the columnar improvement bodies does not interfere with the core material embedded in the wall-like ground improvement body, improving workability and suppressing damage to the core material.

[0011] Furthermore, Patent Document 2 discloses a ground improvement structure for suppressing soil liquefaction by creating a grid-like ground improvement body beneath an underground space or underground structure, wherein the outer perimeter wall portion of the grid-like ground improvement body extends upward and is arranged around the underground space or underground structure, and a reinforcing ground improvement body is created on the outer surface of the outer perimeter wall portion of the grid-like ground improvement body, and a steel core material is embedded in the reinforcing ground improvement body, thereby enabling the outer perimeter wall portion of the grid-like ground improvement body to be used as a retaining wall. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2013-002078 [Patent Document 2] Japanese Patent Publication No. 2020-176381 [Overview of the project] [Problems that the invention aims to solve]

[0013] As mentioned above, there is a challenge in construction regarding the joint between the retaining wall constructed using a soil-cement column-type continuous wall method with reinforcing steel materials and the grid-like ground improvement body constructed inside it.

[0014] Furthermore, even in a construction method, such as the invention described in Reference Document 1, in which connecting columnar improved bodies that constitute the ends of a grid-like ground improved body are pre-connected to the inner wall surface of the wall-like ground improved body when constructing a wall-like ground improved body as a retaining wall, it is necessary to switch construction machinery between the construction of the retaining wall portion, which is usually constructed using a multi-axis auger that has high construction efficiency, and the construction of the connecting columnar improved bodies. This presents problems such as poor construction efficiency at the connection point, as well as concerns regarding the structural integrity of the connection point between the retaining wall, the connecting columnar improved bodies, and the grid-like ground improved body, which have different rigidities.

[0015] The present invention aims to solve the problems of the prior art described above, and provides a method for constructing liquefaction countermeasures using a retaining wall that can efficiently construct the connection between the outer retaining wall, into which reinforcing steel is erected as a core material, and the wall-like ground improvement body made of soil cement that divides the interior of the retaining wall into multiple sections, and can also enhance the structural continuity and integrity of the connection. [Means for solving the problem]

[0016] The present invention relates to a method for constructing liquefaction countermeasures for a building constructed on liquefiable ground susceptible to liquefaction, wherein the liquefaction countermeasures consist of an outer perimeter retaining wall as a continuous underground wall in which reinforcing steel is erected as a core material in a soil-cement columnar wall, and a wall-shaped ground improvement body made of soil-cement that divides the interior of the outer perimeter retaining wall into a plurality of sections, wherein an auxiliary columnar wall section made of soil-cement is constructed parallel to the outer perimeter retaining wall and overlaps with the columnar wall of the outer perimeter retaining wall at the portion corresponding to the connection between the inner surface of the outer perimeter retaining wall and the end of the wall-shaped ground improvement body, and the construction is carried out such that the end of the wall-shaped ground improvement body overlaps with the auxiliary columnar wall section.

[0017] Typically, the outer perimeter retaining wall, which serves as an earth retaining wall or watertight wall, must be constructed before the wall-like ground improvement bodies that are built inside it in a grid pattern or in multiple parallel rows. Furthermore, considering the overall structural stability of the outer perimeter retaining wall and the wall-like ground improvement bodies, it is desirable to connect the ends of the inner wall-like ground improvement bodies to the outer perimeter retaining wall.

[0018] However, when reinforcing steel is installed as a core material in the outer retaining wall, construction to connect to the outer retaining wall becomes difficult. In addition to being a difficult area to construct from the start, the ends of the outer retaining wall and the wall-shaped ground improvement body, which are usually of equal width, are typically met at a right angle or nearly a right angle, which may create a weak point in terms of stress transmission.

[0019] In contrast, the construction method of the present invention solves the above-mentioned construction problems and stress transmission problems by interposing an auxiliary column wall section made of soil cement at the connection point between the outer retaining wall and the end of the wall-shaped ground improvement body, so as to overlap with the column row of the outer retaining wall.

[0020] In other words, by interposing an auxiliary column wall section made of soil cement, the stress concentration at the corners of the connection between the outer retaining wall and the end of the wall-shaped ground improvement body, which had occurred in the past, is alleviated. Furthermore, since the construction of the auxiliary column wall section can be carried out before the erection of the reinforcing steel members of the outer retaining wall, interference between the construction machinery and the reinforcing steel members during excavation can be eliminated.

[0021] Regarding the outer perimeter retaining wall, typically, similar to the construction of soil-cement column walls, excavation and injection of cement-based suspension are carried out using a multi-axis ground improvement machine with a multi-axis auger, while the injected cement-based suspension and soil are mixed together, and reinforcing steel members are erected afterward. However, if the auxiliary column wall section is also constructed using the soil-cement column method with a multi-axis auger, construction can be carried out using the same multi-axis auger at the construction site, without having to use different construction machines for each construction location.

[0022] Furthermore, in that case, in the portion where the column rows of the outer retaining wall and the column rows of the auxiliary column wall overlap, interference between the multi-axis auger, which is used as construction machinery, and the reinforcing steel can be eliminated by constructing the column rows of the auxiliary column wall prior to the installation of reinforcing steel members into the outer retaining wall, as described above.

[0023] Furthermore, if the wall-shaped ground improvement body made of soil cement located inside the outer retaining wall is also constructed by the soil cement column row method using a multi-axis auger, for example, one type of multi-axis auger with a minimum of about 3 to 5 rows can be commonly used for constructing the outer retaining wall, the auxiliary column row wall portion, and the wall-shaped ground improvement body.

[0024] It should be noted that it is not absolutely necessary to carry out the construction with all the same type of construction machinery; for example, the wall-shaped ground improvement body portion can also be constructed as a soil cement continuous wall constructed by an equal-thickness continuous wall ground improvement machine.

[0025] The present invention is to construct a liquefaction countermeasure work for a building constructed on liquefied ground that has a risk of liquefaction. When a plurality of bearing piles that receive the vertical load of the building are required, each bearing pile can be dispersed and driven into a plurality of sections partitioned by the above-mentioned wall-shaped ground improvement body (see Fig. 1).

[0026] In addition, regarding the relationship between the outer retaining wall and the building constructed on the liquefied ground, two cases are conceivable: one is that the outer retaining wall is constructed outside the outer periphery of the building with a horizontal gap from the outer periphery of the building (see Fig. 1 and Fig. 2), and the other is that the outer periphery of the building is integrated inside the outer retaining wall (not shown).

Effects of the Invention

[0027] In the construction method of the present invention, by interposing an auxiliary column row wall made of soil cement at the joint between the outer retaining wall and the end of the wall-shaped ground improvement body so as to overlap with the column row of the outer retaining wall, the stress concentration at the corner of the joint between the outer retaining wall and the end of the wall-shaped ground improvement body, which occurs in conventional construction methods, is alleviated. In addition, since the construction of the auxiliary column row wall portion can be performed before the reinforcing steel members of the outer retaining wall are installed, interference between the construction machinery and the reinforcing steel members during excavation can be eliminated.

[0028] Furthermore, if the auxiliary column wall section is also constructed using the soil-cement column method with a multi-axis auger, in line with the construction of the outer perimeter retaining wall which is usually constructed using a multi-axis auger, then construction can be carried out using the same construction machinery at the construction site, without having to use different construction machinery for each section. [Brief explanation of the drawing]

[0029] [Figure 1] This is a conceptual vertical cross-sectional view showing one form of liquefaction countermeasures using a retaining wall to which the present invention is applied. [Figure 2] Figure 1 is a horizontal cross-sectional view conceptually illustrating the relationship between the building, the outer retaining wall, and the wall-like ground improvement structure. [Figure 3] This is a horizontal cross-sectional view of a key part illustrating the positional relationship between the outer retaining wall, the auxiliary column row wall section, and the wall-shaped ground improvement body, as well as the construction procedure, in one embodiment of the construction method for liquefaction countermeasures using a retaining wall of the present invention. [Figure 4] This is a horizontal cross-sectional view of a key part illustrating the positional relationship between the outer retaining wall, the auxiliary column row wall section, and the wall-shaped ground improvement body in another embodiment of the construction method for liquefaction countermeasures using retaining walls of the present invention. [Figure 5] This is a perspective view showing the area inside the outer retaining wall after excavation. [Figure 6] This shows an example of construction using a triple-row multi-axis auger. (a) is a horizontal cross-sectional view when the auxiliary column wall is single-layered, and (b) is a horizontal cross-sectional view when the auxiliary column wall is double-layered. [Figure 7] This shows an example of construction using a 5-row multi-axis auger. (a) is a horizontal cross-sectional view when the auxiliary column wall is single-layered, and (b) is a horizontal cross-sectional view when the auxiliary column wall is double-layered. [Modes for carrying out the invention]

[0030] Specific embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to the embodiments described below.

[0031] Figures 1 and 2 conceptually illustrate one form of liquefaction countermeasures using retaining walls to which the present invention is applied. In this example, as a liquefaction countermeasure when constructing a building 1 on liquefiable ground L (liquefiable layer) where liquefaction is a risk, an outer retaining wall 2 is constructed around the perimeter of the building 1, penetrating the liquefiable ground L to reach non-liquefiable ground (non-liquefiable layer) NL. Furthermore, the inside of the outer retaining wall 2 is divided into multiple sections by wall-shaped ground improvement bodies 4.

[0032] In this example, multiple support piles 5 that reach the supporting ground S and directly support the vertical load of building 1 are placed within each section separated by the wall-like ground improvement body 4.

[0033] The outer perimeter retaining wall 2 is a continuous underground wall constructed by installing reinforcing steel materials 3, such as H-shaped steel, as core material into a soil-cement column wall. Multi-axis auger type construction equipment is often used for excavation and mixing with injected cement grout, etc.

[0034] Conventionally, construction machinery for building soil-cement column walls or a continuous wall ground improvement machine of equal thickness has been used for constructing the wall-shaped ground improvement body 4. In the illustrated example, the wall-shaped ground improvement body 4 extends in the direction of the beams of the building 1 and partitions the outer retaining wall 2 in one direction. However, depending on the cross-sectional shape and size of the building 1, the wall-shaped ground improvement bodies can also be arranged in a grid pattern inside the outer retaining wall 2.

[0035] Regarding the construction of the liquefaction countermeasures described above, there are issues with the construction of the retaining wall with reinforcing steel members installed and the grid-like ground improvement body constructed inside it, where the reinforcing steel members become an obstacle to construction, and with the integrity of the connection between the retaining wall and the grid-like ground improvement body. The embodiments described below solve these problems.

[0036] Figure 3 shows the positional relationship between the outer retaining wall 2, the auxiliary column row wall section 6, and the wall-shaped ground improvement body 4, as well as the construction procedure, in one embodiment of the construction method for liquefaction countermeasures using the retaining wall of the present invention.

[0037] In the construction method of the present invention, an auxiliary column wall section 6 made of soil cement is interposed at the connection point between the outer perimeter retaining wall 2 and the end of the wall-shaped ground improvement body 4, so as to overlap with the column row of the outer perimeter retaining wall 2. In this case, the construction of the auxiliary column wall section 6 can be carried out before the erection of the reinforcing steel members 3 of the outer retaining wall 2, thus eliminating interference between construction machinery such as a multi-axis auger and the reinforcing steel members 3 during the construction of the auxiliary column wall section 6.

[0038] Furthermore, by interposing the auxiliary column wall section 6 made of soil cement, the cross-section of the connection between the outer retaining wall 2 and the end of the wall-shaped ground improvement body 6 is enlarged, and stress concentration in this section is alleviated.

[0039] In the construction of the inner grid-like or unidirectional wall-like ground improvement body 4 surrounded by the outer retaining wall 2, the ends of the wall-like ground improvement body 4 can be overlapped with the auxiliary column wall section 6, and similarly there is no interference with the reinforcing steel members 3 of the outer retaining wall 2.

[0040] Regarding the outer perimeter retaining wall 2, normally, similar to the construction of soil-cement column walls, a multi-axis ground improvement machine using a multi-axis auger is used to excavate and inject a cement-based suspension, while the injected cement-based suspension and soil are mixed together, and then the reinforcing steel members 3 are erected. However, in the present invention, the auxiliary column wall section 6 can also be constructed using the soil-cement column method with a multi-axis auger, so the same multi-axis auger can be used for the construction of both the outer perimeter retaining wall 2 and the auxiliary column wall section 6, thus avoiding the complicated task of changing construction machinery at the construction site.

[0041] Furthermore, in that case, in the portion where the column rows of the outer retaining wall 2 and the column rows of the auxiliary column wall section 6 overlap, as described above, interference between the multi-axis auger, which is used as construction machinery, and the reinforcing steel can be eliminated by constructing the column rows of the auxiliary column wall section 6 prior to the installation of the reinforcing steel members 3 on the outer retaining wall 2.

[0042] The numbers (1) to (6) in parentheses in Figure 3 indicate an example of the construction procedure when constructing the overlapping portion of the outer retaining wall 2 and the auxiliary column wall section 6 using a construction device equipped with a triple-axis auger. However, it is not necessary to follow the order of (1) to (6). As long as there is no interference with the reinforcing steel members 3 erected in the outer retaining wall 2 during the construction of the auxiliary column wall section 6, any construction procedure can be determined considering on-site workability, stability of the construction area, etc.

[0043] As an example of dimensions, a design could be created where the drilling diameter (the diameter of each individual drilling hole constituting the multi-unit structure) for the outer retaining wall 2, the auxiliary column wall section 6, and the wall-shaped ground improvement body 4 is 900 mm, the drilling pitch (the distance between the centers of each drilling hole) is 600 mm, H-shaped steel of H-700 x 300 x 13 x 24 is used as the reinforcing steel material 3 to be inserted, and the overlapping portion (maximum portion) of the auxiliary column wall section 6 and the wall-shaped ground improvement body 4 is 400 mm. However, these dimensions are merely an example and are not limiting.

[0044] Figure 4 shows the positional relationship between the outer retaining wall 2, the auxiliary column row wall section 6, and the wall-shaped ground improvement body 7 in another embodiment of the construction method for liquefaction countermeasures using the retaining wall of the present invention.

[0045] The connection between the outer retaining wall 2 and the end of the wall-like ground improvement body is made by interposing an auxiliary column wall section 6 made of soil cement so as to overlap with the column row of the outer retaining wall 2, as in the embodiment shown in Figure 3 above. However, the embodiment shown in Figure 4 shows a case in which the construction of the wall-like ground improvement body section 7 is carried out using a uniform-thickness continuous wall ground improvement machine instead of the soil cement column row method.

[0046] Thus, based on on-site construction conditions, design conditions, and the arrangement of construction machinery, the wall-shaped ground improvement body 7 can be constructed using a continuous wall ground improvement machine of equal thickness.

[0047] In this case as well, the construction of the outer retaining wall 2 and the auxiliary column wall section 6 can be streamlined at the construction site by using a common multi-axis ground improvement machine.

[0048] Furthermore, the construction of the outer perimeter retaining wall 2 and the auxiliary column row wall section 6 is the same as in the embodiment shown in Figure 3.

[0049] As an example of dimensions, the design includes a drilling diameter of 850 mm for the outer retaining wall 2 and the auxiliary column wall section 6, a drilling pitch of 600 mm, the use of H-600 x 200 x 11 x 17 H-shaped steel as the reinforcing steel material 3 to be inserted, a width of 850 mm for the equal-thickness section of the wall-shaped ground improvement body 7, and a 500 mm overlap between the auxiliary column wall section 6 and the wall-shaped ground improvement body 7. However, these dimensions are merely an example and are not limiting.

[0050] Figure 5 is a perspective view showing the state after excavation of the inside of the outer retaining wall 2 in the present invention. Note that the reinforcing steel members installed within the outer retaining wall 2 are not shown.

[0051] Figure 6 shows an example of construction using a triple-type multi-axis auger in the present invention. Figure 6(a) shows the case where the auxiliary column wall is single-layered, which corresponds to the embodiment shown in Figure 3 above.

[0052] Figure 6(b) shows an example where the auxiliary column wall section is doubled. By doubling the auxiliary column wall section 6, the distance between the reinforcing steel members 3 of the outer retaining wall 2 and the end of the wall-shaped ground improvement body 4 can be increased, and stress concentration at the connection point between the outer retaining wall 2 and the end of the wall-shaped ground improvement body 4 can be further reduced.

[0053] Figure 7 shows an example of construction using a five-segment multi-axis auger, where (a) is an example where the auxiliary column wall section is single-layered, and (b) is an example where the auxiliary column wall section is double-layered. Regarding the construction of the outer perimeter retaining wall 2 and the auxiliary column row wall section 6, construction efficiency can be increased by using construction machinery such as the 4-series or 5-series multi-axis auger, in addition to the 3-series multi-axis auger described above, and the auxiliary column row wall section 6 that overlaps with the outer perimeter retaining wall 2 can be constructed over a wide area in a single construction phase. [Explanation of symbols]

[0054] L... Liquefiable ground NL... Non-liquefiable ground S...Supporting ground, 1… Building 2… Perimeter retaining wall 3…Reinforcement steel 4…Wall-shaped ground improvement body (column type) 5…Support pile 6…Auxiliary column wall section 7…Wall-shaped ground improvement body (equal thickness type)

Claims

1. A method for implementing liquefaction countermeasures for buildings constructed on liquefiable ground that is at risk of liquefaction, The aforementioned liquefaction countermeasure consists of an outer perimeter retaining wall, which is a continuous underground wall in which reinforcing steel is installed as a core material in a row of soil-cement columns, and a wall-like ground improvement body made of soil-cement that divides the interior of the outer perimeter retaining wall into multiple sections. At the portion of the inner surface of the outer perimeter retaining wall that connects with the end of the wall-shaped ground improvement body, an auxiliary column wall section made of soil cement is constructed parallel to the outer perimeter retaining wall and overlaps with the column row of the outer perimeter retaining wall. A method for constructing liquefaction countermeasures using a retaining wall, wherein the end of the wall-shaped ground improvement body is constructed so as to overlap with the auxiliary column wall section, The auxiliary column wall section is constructed such that it overlaps only with the portion that connects to the end of the wall-shaped ground improvement body, which is a part of the inner circumference of the column row of the outer perimeter retaining wall, and is longer than the width of the wall-shaped ground improvement body in a direction parallel to the inner surface of the outer perimeter retaining wall. A method for constructing liquefaction countermeasures using a retaining wall, characterized in that the outer perimeter retaining wall and the auxiliary column wall section are constructed using a soil-cement column construction method with a multi-axis auger, and in the portion where the column rows of the outer perimeter retaining wall and the column rows of the auxiliary column wall section overlap, the construction of the column rows of the auxiliary column wall section is carried out prior to the installation of the reinforcing steel materials into the outer perimeter retaining wall.

2. A method for constructing liquefaction countermeasures using a retaining wall as described in claim 1, characterized in that the wall-shaped ground improvement body consists of a soil-cement column wall constructed by a soil-cement column construction method using a multi-axis auger.

3. A method for constructing liquefaction countermeasures using a retaining wall as described in claim 1, characterized in that the wall-shaped ground improvement body consists of a soil-cement continuous wall constructed by a uniform-thickness continuous wall ground improvement machine.

4. A method for constructing liquefaction countermeasures using a retaining wall according to any one of claims 1 to 3, characterized in that a plurality of support piles supporting the building are driven in by distributing them among the plurality of sections separated by the wall-like ground improvement body.

5. A method for constructing liquefaction countermeasures using a retaining wall according to any one of claims 1 to 3, characterized in that the outer perimeter retaining wall is constructed outside the outer perimeter of the building, with a horizontal gap between it and the outer perimeter of the building.

Images