JP2011007031A - Structure and construction method for countermeasure against liquefaction - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce damage to an existing structure by liquefaction.SOLUTION: This structure for a countermeasure against liquefaction includes a plurality of buttresses (underground walls) 3 built on the ground G under the outer peripheral part 2a of an existing tank 2 and the ground G therearound and an inner ground-improved body 4 formed by completely improving the ground G under the center 2b of the existing tank 2. The buttresses 3 are plate-like ground-improved bodies formed by a cement-based deep layer mixing method, and disposed radially around the center axis of the tank 2 at predetermined intervals between adjacent buttresses 3. The inner ground-improved body 4 is improved by the cement-based deep layer mixing method. The buttresses 3 are built starting at the inside of the tank 2 by removing a foundation slab 11 in the tank 2. While a solidification material is supplied, a stirrer for stirring the solidification material with the soil at the original position is inserted into the ground, and the stirrer is moved vertically and horizontally in a level attitude for building the buttresses.

Description

  The present invention relates to a ground liquefaction countermeasure structure and a liquefaction countermeasure construction method for supporting an existing structure.

In recent years, refurbishment has been carried out to adapt existing tanks that have been built with past earthquake resistance standards to the new standards.
For example, as a liquefaction countermeasure method for an existing tank, a permeation solidification method such as a chemical solution injection method for injecting a chemical solution into the ground directly below the foundation of the existing tank is performed. In this construction method, a boring survey is performed after injecting a drug into the ground, and it is confirmed whether a predetermined ground strength is secured.
In addition, a cement-based deep mixing treatment method is being performed in which a solidified material such as cement-based slurry or powder is sprayed while mechanically stirring the ground. The deep mixing treatment method has an advantage that the ground within a predetermined range can be improved reliably. However, there is a drawback that it is difficult to improve the ground directly under the tank.
Therefore, according to Patent Document 1, in a configuration in which the stirring mixer that is inserted into the ground and stirs the original soil of the ground while supplying the solidified material is held in a horizontal posture so as to be movable in the vertical and horizontal directions, There have been proposed a ground improvement device for inserting a stirring mixer around the tank and entering the lower ground of the tank, and a liquefaction prevention method using this ground improvement device.

JP 2007-162337 A

However, the conventional liquefaction countermeasure method has the following problems.
In the liquefaction countermeasure method using the infiltration solidification method such as the chemical solution injection method, there is a problem that it is difficult to uniformly infiltrate the chemical solution into the ground, and it is difficult to confirm how much the whole solution has penetrated.
In addition, in the liquefaction countermeasure method using the deep mixing method in which the in-situ soil of the ground and the solidified material are mixed by stirring, the ground under the tank is completely improved, which increases the cost of the improvement and takes time.

  The present invention has been made in view of the above-described problems, and a liquefaction countermeasure structure and a liquid countermeasure that partially improve the ground on which an existing structure is constructed to reduce damage to the structure due to liquefaction. The purpose is to provide a method of construction.

In order to achieve the above object, the liquefaction countermeasure structure according to the present invention is a liquefaction countermeasure structure that reduces the damage to the existing structure due to liquefaction, on the ground below and around the outer periphery of the existing structure, The ground wall in the ground is formed by stirring the soil and the solidified material, and the underground wall in the direction perpendicular to or close to the outer periphery of the existing structure has a predetermined direction in the direction parallel to the outer periphery of the existing structure. It is characterized in that a plurality are arranged at intervals.
Moreover, in the liquefaction countermeasure structure according to the present invention, the existing structure may be a circular structure in plan view, and the underground wall may be arranged radially with respect to the existing structure.

In the present invention, it is formed by stirring the in-situ soil and solidified material in the lower part of the outer periphery of the existing structure and in the surrounding ground, and the direction orthogonal to or near the orthogonal to the outer periphery of the existing structure. The multiple underground walls are arranged at predetermined intervals in a direction parallel to the outer periphery of the existing structure, so that the underground walls constrain the ground between the underground walls and shear deformation during an earthquake Therefore, damage to existing structures due to liquefaction can be reduced.
And by arranging a plurality of underground walls at predetermined intervals, the amount of ground improvement can be reduced and labor and cost can be reduced compared to the liquefaction countermeasure structure that improves the entire ground. The construction period can be shortened.

The liquefaction countermeasure structure according to the present invention is characterized in that the lower ground in the center of the existing structure is improved.
In the liquefaction countermeasure structure according to the present invention, a ground improvement body formed by stirring the original soil of the ground and the solidified material is partially disposed on the ground below the center of the existing structure. It may be provided.
Moreover, in the liquefaction prevention structure according to the present invention, the lower ground in the center of the existing structure may be improved by injecting and solidifying the chemical.
In this invention, the intensity | strength of the ground which supports the existing structure can be made high by improving the lower ground of the center part of the existing structure.

Further, in the liquefaction countermeasure structure according to the present invention, the underground wall has a lower end part embedded in the non-liquefied layer of the ground, and the improvement of the ground below the central part of the existing structure is non-liquid. It is good also as a structure which has not reached the formation layer.
In the present invention, since the bottom of the underground wall is embedded in the non-liquefaction layer of the ground, the displacement of the underground wall during an earthquake is suppressed, and the ground between adjacent underground walls is shear-deformed. It is possible to suppress this, and the uneven settlement of existing structures can be prevented.
In addition, since the improvement of the ground below the center of the existing structure has not reached the non-liquefied layer, there is no liquefaction between the non-liquefied layer and the ground improvement body below the center of the existing structure. Because the layer is interposed and part of the vibration transmitted from the non-liquefied layer to the ground improvement body below the center of the existing structure during the earthquake is absorbed by this liquefied layer, the vibration transmitted to the existing structure is absorbed. Can be reduced.
Moreover, compared with the case where the ground below the center part of the existing structure is improved so as to reach the non-liquefiable layer, the amount of ground improvement is small, and the cost and construction period can be shortened.

In the liquefaction countermeasure structure according to the present invention, the solidifying material is preferably cement or a cement-based solidifying material.
In the present invention, the strength of the underground wall can be increased and the strength of the ground supporting the existing structure can be increased by using the cement or cement-based solidified material as the solidified material.

Further, in the liquefaction countermeasure structure according to the present invention, the liquefaction strength of the partially improved ground where the underground wall is disposed is calculated by calculating the shear stress (τ _d ) generated in the ground before the improvement at the time of the earthquake. _d ) Calculate the shear strain (γ _i ) of the ground before the improvement based on _d ), calculate the equivalent shear stiffness (G _eq ) of the ground after the improvement, The generated shear strain (γ _i ) is obtained, the equivalent shear stiffness (G _eq ) corresponding to the shear strain (γ _i ) is re-determined, and the re-determined equivalent shear stiffness (G _eq ) is used to determine the shear strain (γ _i ). To calculate the shear strain (γ _i ) until it converges to a certain value, and the excess pore water pressure ratio (Δu / σ _ν ′) generated in the unmodified ground based on the determined shear strain (γ _i ) determined, based on the excess pore water pressure ratio _(Δu / σ ν ') Characterized in that it is evaluated.
In the present invention, by evaluating in advance the liquefaction strength of the partially improved ground, it is possible to appropriately plan the shape of the underground wall, so that damage to existing structures due to liquefaction can be prevented, and excess It is possible to prevent the ground improvement.

The liquefaction countermeasure method according to the present invention is a liquefaction countermeasure method that reduces damage to existing structures due to liquefaction, while supplying solidification material to the lower ground and the surrounding ground of the outer periphery of the existing structure. A stirrer / mixer that stirs and mixes with the in-situ soil is inserted, and the stirrer / mixer is moved in the vertical and horizontal directions in a horizontal posture to form an underground wall.
In the present invention, a stirrer / mixer that stirs and mixes with the in-situ soil while supplying the solidified material to the lower ground and the surrounding ground of the outer periphery of the existing structure is inserted, and the stirrer / mixer in the ground in the vertical direction and By moving in each horizontal direction, the underground wall can be easily formed in the lower ground and the peripheral ground of the outer peripheral portion of the existing structure.

Moreover, in the liquefaction countermeasure method according to the present invention, the step of forming the underground wall is preferably performed from the inside of the existing structure.
In the present invention, by forming the underground wall from the inside of the existing structure, it is not necessary to dodge the pipes embedded in the ground around the existing structure, and the underground wall can be formed efficiently. . Moreover, when improving the lower ground of the center part of the existing structure, it is easy to perform the ground improvement work.
In addition, you may perform the process of forming an underground wall from the exterior of the existing structure.

  According to the present invention, the underground wall is disposed on the lower ground and the surrounding ground of the outer peripheral portion of the existing structure, thereby reducing the damage of the existing structure due to liquefaction and the existing structure due to the earthquake. As well as suppressing the vibration of the existing structure, the amount of ground improvement can be reduced compared with the case of improving the entire lower ground of the existing structure, labor and cost can be reduced, and the construction period can be shortened. .

(A) shows an example of the liquefaction countermeasure structure by 1st embodiment of this invention, and shows the AA sectional view taken on the line of (b), (b) is the BB sectional drawing of (a). FIG. 2 is a diagram showing a unit periodic structure of the liquefaction countermeasure structure shown in FIG. It is a figure which shows the example of the simple chart used for calculation of the deformation amount of the partial improvement ground used when determining the shape of the buttress of this invention. The embodiment of the simple evaluation method of the liquefaction strength of the partially improved ground of the present invention is shown and is a flowchart showing the entire procedure. It is a figure which shows the relationship between a shear strain and a shear stress. It is a figure which shows the relationship between a shear rigidity fall rate and a shear strain. It is a figure for calculating | requiring an excess pore water pressure ratio. It is a schematic diagram of a liquefaction intensity curve. It is a figure which shows the process of forming a buttress in the lower ground of a tank with a ground improvement apparatus, and shows the state which constructed the prior improvement part. It is a figure which shows the state which inserted the stirring mixer in the prior improvement part in FIG. It is a figure which shows the state which made the stirring mixer reach the lower ground of a tank. It is a figure which shows the state which dropped the stirring mixer to the bottom part of the prior improvement part. (A) shows an example of the liquefaction countermeasure structure by 2nd embodiment of this invention, and shows the CC sectional view taken on the line (b), (b) is the DD sectional view taken on the line (a). (A) shows an example of the liquefaction countermeasure structure by 3rd embodiment of this invention, and shows the EE sectional view taken on the line (b), (b) is the FF sectional view taken on the line (a). (A) shows an example of the liquefaction countermeasure structure according to the fourth embodiment of the present invention, and is a sectional view taken along the line GG of (b), and (b) is a sectional view taken along the line HH of (a). (A) is a figure which shows the outline | summary of the experiment which verifies the relationship between the improvement depth of the ground under a structure, and the amount of settlement of a structure, (b) is a figure which shows the detail of the experiment case of (a). It is a figure which shows the amount of settlement of experiment cases C1-C4 by making the settlement amount of experiment case C0 into 1 as a result of the experiment shown in FIG. (A) shows an example of the liquefaction countermeasure structure by the modification of embodiment of this invention, and shows the II sectional view taken on the line of (b), (b) is the JJ sectional view taken on the line of (a).

Hereinafter, a liquefaction prevention structure according to an embodiment of the present invention will be described with reference to FIGS.
As shown in FIGS. 1 (a) and 1 (b), the liquefaction countermeasure structure 1 according to the present embodiment is a structure of ground improvement performed as a liquefaction countermeasure on the supporting ground of an existing tank 2 that has not been liquefied. A plurality of buttresses (underground walls) 3 formed on the ground G below the outer peripheral portion 2a of the tank 2 and the surrounding ground G, and the ground G below the center portion 2b of the existing tank 2 It is comprised from the internal ground improvement body 4 improved entirely. In the present invention, the outer peripheral portion 2a of the tank 2 indicates a portion in a predetermined range on the outer peripheral side of the tank 2, and the inner side of the outer peripheral portion 2a is defined as a central portion 2b.

The tank 2 is formed in a columnar shape and includes a basic slab 11 on the bottom surface.
The buttress 3 is a plate-like ground improvement body formed by a cement-based deep mixing method, and a plurality of buttresses 3 are arranged radially around the central axis of the tank 2 as shown in FIG. A predetermined interval is provided between the buttresses 3.
The internal ground improvement body 4 has been improved by a cement-based deep mixing process.

In such a liquefaction countermeasure structure 1, in order to evaluate the liquefaction strength of the partially improved ground G1 in which the buttress 3 is disposed, first, the partially improved ground G1 is made of a unit periodic structure G2 as shown in FIG. It is constructed as an assembly, and the rigidity of the unit periodic structure G2 is obtained based on the mathematical homogenization theory, and this is set as the equivalent rigidity of the entire partially improved ground G1, and this equivalent rigidity and the partially improved ground G1 after construction are affected. The amount of deformation of the partially improved ground G1 is predicted and calculated in advance from the external force to be applied.
And the shear stress which arises in the ground G before improvement at the time of an earthquake is calculated, and the shear strain of the ground G before improvement is calculated based on this shear stress. The equivalent shear stiffness of the partially improved ground G1 after the improvement is calculated, the shear strain generated in the unmodified ground between the buttresses 3 in the partially improved ground G1 after the improvement is obtained, and the equivalent shear stiffness corresponding to the shear strain is determined again. .
Then, using the re-determined equivalent shear stiffness, calculation is performed until the shear strain converges to a constant value, the shear strain is determined, and the excess pore water pressure ratio generated in the unmodified ground is determined based on the determined shear strain.
Then, the liquefaction strength of the partially improved ground G1 is evaluated based on this excess pore water pressure ratio.
A simple method for evaluating the liquefaction strength of this partially improved ground is described below.

First, the improvement rate R of the unit periodic structure G2 is obtained.
As shown in FIG. 2, the partially improved ground G <b> 1 is formed in a ring shape with the ground G below and around the outer peripheral portion of the tank 2 in which the buttress 3 is disposed. Unit periodic structure G2 is obtained by dividing the partial ground improved G1 evenly arcuate, having a thickness of l buttress 3 of _R and original ground G3 Metropolitan the surrounding.
Here, 1 unit periodic structure G2 shape length l _x in the x direction as shown in FIG. _2, the inside of the rectangle in the y direction length l _y, buttress 3 having a thickness of l _R is the y-direction Think of it as being arranged.
At this time, the improvement rate R indicating the ratio of the horizontal area of the buttress 3 to the horizontal area of the unit periodic structure G2 is expressed by the following equation.

The value of the improvement rate R varies depending on the situation and can be set arbitrarily, but 20 to 60% is a guide.
Further, the improvement rate R is corrected if the deformation amount of the partially improved ground G1 described later is calculated and the design condition is not satisfied.

Next, the equivalent rigidity of the partially improved ground G1 is obtained.
The unit periodic structure G2 is composed of two elastic bodies having different rigidity, that is, an unmodified and low-rigidity ground G3 (having its rigidity as ES), and a buttress 3 that has been improved to be high in rigidity (having its rigidity as ER). And the composite can be regarded as a single homogeneous body having a rigidity that can be regarded as equivalent to the rigidity of the entire composite (hereinafter referred to as equivalent rigidity E ^H ). The equivalent stiffness E ^H of the homogeneous body can be obtained as follows using the characteristics and shape of the unit periodic structure G2 as parameters based on the mathematical homogenization theory.

That is, according to the mathematical homogenization theory, the elastic coefficient C ^H of one homogeneous body equivalent to a composite of two elastic bodies is expressed by the following equation when expressed in a matrix.

In the above formula, C is an elastic matrix of the unit periodic structure G2 as a micro periodic structure.
X is a response displacement when a unit macro strain I is given to the micro periodic structure, and in three dimensions, it consists of six components as shown in the following equation.

Further, the inverse matrix (compliance matrix) of the elastic modulus C ^H of the homogeneous body is expressed by the following equation, and the equivalent stiffness in each direction can be obtained from this equation.

In the above equation, Ex ^H is the axial stiffness in the x direction of the homogeneous body, Ey ^H is the axial stiffness in the y direction, Ez ^H is the axial stiffness in the z direction, Gxy ^H is the shear stiffness in the xy plane, and Gyz ^H is y−. The shear stiffness in the z plane, Gzx ^H, is the shear stiffness in the zx plane (xz plane), and each stiffness of the homogeneous body obtained by the above equation is the unit periodic structure G2 and its aggregate This represents the equivalent rigidity of the entire partially improved ground G1.
In this calculation method, the relationship between the equivalent stiffness in each direction obtained by the above equation and the improvement rate R is preliminarily made into a simple chart by using the pattern of the unit periodic structure G2 as a parameter. By using this, the equivalent rigidity (axial rigidity and shear rigidity) in each direction of the partially improved ground G1 is easily obtained.

As shown in a specific example in FIG. 3, the simplified chart has an improvement rate R on the horizontal axis and an equivalent stiffness Ex ^H , Ey ^H , Ez ^H , Gxy ^H , Gyz ^H , Gzx ^H (buttress 3 ES / ER or GS / GR (ratio of the stiffness of the base ground G3 to the stiffness of the buttress 3) and the aspect ratio of the unit periodic structure G2 This is created using lx / ly as a parameter.
The simple charts shown in FIGS. 3A to 3F are for the case where lx / ly = 1, 2, and 3, and the parameters ES / ER or GS / GR are set to 0, 0.2, 0,. 4, 0.6, and 0.8.

  By preparing such a simple chart in advance, the equivalent rigidity in each direction of the partially improved ground G1 can be immediately obtained from only the improvement rate R, the rigidity ER or GR of the buttress 3 and the rigidity ES or GS of the original ground G3. Can be sought.

Next, the deformation amount of the partially improved ground G1 is predicted and calculated.
Assuming that the entire liquefied layer around the partially improved ground G1 is liquefied, the external force applied to the partially improved ground G1 after liquefaction is calculated. At this time, since the entire internal ground improvement body 4 inside the partial improvement ground G1 has been improved, the inside of the partial improvement ground G1 is not liquefied, and external force is applied from the internal ground improvement body 4 to the partial improvement ground G1. It shall not take.
Then, the deformation amount of the partially improved ground G1 is calculated from the equivalent rigidity and the external force acting on the partially improved ground G1. The calculation can be performed by using the two-dimensional elastic finite element method or by modeling the entire partially improved ground G1 as a shear bar as a simpler method, and almost the same result is obtained in any case. .

  If the deformation amount calculated above satisfies the design conditions, the improvement rate R is determined as a final decision. If the calculation result of the deformation amount does not satisfy the design condition, the improvement rate R is changed and the above procedure is repeated. That is, if the deformation amount is excessive, the improvement rate is insufficient, so that the improvement rate R is increased, and if the deformation amount is excessively small, the improvement rate R is excessive, so that the improvement rate R is decreased. The above procedure may be repeated until the desired result is obtained. Of course, if necessary at that time, that is, if the condition cannot be satisfied only by correcting the improvement rate R, the rigidity of the buttress 3 may be reviewed together.

Next, the liquefaction strength of the partially improved ground G1 is evaluated.
The evaluation method of liquefaction strength is composed of a series of steps (i) to (vii) in the flowchart shown in FIG. 4, and the details thereof will be described below.

(I) The shear stress τ _d generated in the ground G during the earthquake is calculated. The shear stress τ _d is calculated from the seismic response analysis or is calculated using the formula (1) used in the simple liquefaction calculation method.

In the equation (1), M: magnitude, α _max : ground surface maximum acceleration, z: depth, σ _v ': vertical effective stress, σ _v : vertical total stress, g: gravity acceleration.

(Ii) The shear strain γ _i of the ground G before improvement is calculated using the shear stress τ _d obtained by the above equation (1) and FIG. FIG. 5 is created based on the equation (2).

In the equation (2), h _max is a maximum attenuation constant and is 0.2 to 0.25 in the case of sand. Although the initial shear stiffness G _so and the reference shear strain γ _rf vary depending on the type of sand and the restraint pressure, typical examples are G _so = 50 MPa, γ _rf = 0.1%, and h _max = 0.2.

(Iii) According to the method described in the calculation method described above, the improvement rate of the partially improved ground G1, the shape of the buttress 3, the improved pattern, and the shear rigidity G _so and G _I of the ground G and the buttress 3 using FIG. The equivalent shear rigidity G _eq is calculated.

(Iv) Using the shear stress τ _d obtained in (i) above and the equivalent shear stiffness G _eq provisionally calculated in (iii) above, the shear strain γ _i generated in the ground between the buttresses 3 by the equation (3) Ask.

(V) Using the shear strain γ _i obtained in (iv) above and FIG. 6, the equivalent shear stiffness G _eq corresponding to the shear strain γ _i is determined again.

(Vi) the equivalent shear modulus G _eq re determined, using tau _d obtained in (1), (3) by a shear strain gamma _i calculated again, the above calculation of the shear strain gamma _i is constant Until it converges to the value of.

(Vii) Using the determined shear strain γ _i , the excess pore water pressure ratio Δu / σ _ν ′ is determined according to FIG.
FIG. 7 is obtained using the equation (2) and the following equations (4) to (6).

Here, R ₁ is defined as a repeated shear stress ratio, and R ₂₀ is a liquefaction strength, which is defined as a shear stress ratio reaching liquefaction 20 times. N _l is the number of repetitions that led to liquefaction. k is an experimental constant and takes a value of about -0.25.
FIG. 8 shows a schematic diagram of a liquefaction strength curve.
Since the equation (4) can be regarded as approaching liquefaction by 1 / N _l after one iteration of R _l, the cumulative damage degree R _n when 20 repeated shears occur can be expressed as the equation (5). The excess pore water pressure ratio generated in this state can be expressed by equation (6), which is a proposed equation of De Alba. α _rf is an experimental constant, but takes a value of about 0.7 for loose sand.

In step (vii), the excess pore water pressure ratio Δu / σ _ν ′ is obtained using FIG. 7, and if the excess pore water pressure ratio Δu / σ _ν ′ is less than 1.0, the ground G is liquefied. However, the excess pore water pressure will rise to some extent, and the liquefaction strength of the partially improved ground G1 can be evaluated. Therefore, if it is estimated that the excess pore water pressure ratio Δu / σ _v ′ is 1.0 or more and the ground G is liquefied, the improvement rate R and other conditions are reset, and the excess pore water pressure ratio Δu The above procedure is repeated until / _σν ′ becomes less than 1.0, and the shape of buttress 3 is determined.

  Thus, since the liquefaction strength of the partially improved ground G1 is evaluated in advance and the shape of the buttress 3 is determined, damage to the tank 2 due to liquefaction can be prevented and excessive ground improvement can be performed. Can be prevented.

Next, the construction method (liquefaction countermeasure construction method) of the liquefaction countermeasure structure mentioned above is demonstrated using drawing.
First, the foundation slab 11 in the tank 2 shown in FIG. 1 is removed, and the surface of the ground G is exposed inside the tank 2.
And as shown in FIG. 9, the ground improvement apparatus 21 is installed in the tank 2, and the buttress 3 is created from the tank 2 inside.

  As shown in FIG. 9, the ground improvement device 21 of this embodiment has a back mixer or the like as a base machine 22 and a swing mixer 23 mounted with a stirring mixer (trencher) 24. In the improvement device, the agitating mixer 24 is directly mounted on the tip of the swing arm 23 in a vertical posture (that is, in a state where the tip is directed downward), whereas in the ground improvement device 21 of the present embodiment, the improvement device 21 is. A vertical arm 25 is connected to the tip of the swing arm 23 by a swing jack 26 so as to be swingable, and the stirring mixer 24 is always in a substantially horizontal posture (that is, the tip is connected to the tip (lower end)) of the vertical arm 25. The rear end portion is fixed so as to be in a state of facing forward.

The stirring mixer 24 basically has a well-known structure, and an endless chain 31 fitted with stirring blades (not shown) is wound between a pair of sprockets 32 and driven to circulate. Various solidified materials can be supplied into the ground G in an ejected state by the abbreviated solidified material supplying means, and the solidified material and the in-situ soil are driven by circulating the chain 31 while supplying the solidified material into the ground G. Can be efficiently stirred and mixed.
Further, the agitating mixer 24 is equipped with a forward jack 34 provided with a reaction force plate 33 at the rear thereof, and the agitating mixer is configured to take the reaction force from the rear ground by extending the forward jack 34 rearward. The whole 24 can be pushed forward.
The total length of the agitating mixer 24 and the stroke of the forward jack 34 are determined in consideration of the width dimension d _{1 of the} buttress 3 to be formed (see FIG. 1) and the width dimension d ₃ of the advance improvement portion 41 described later. What is necessary is just to set so that the buttress 3 can be formed efficiently, and when the required stroke of the jack 34 for advance is especially large, what is necessary is just to employ | adopt a multistage expansion | extension type jack as needed.

A construction procedure when the buttress 3 is formed by the ground improvement device 21 having the above-described configuration will be described with reference to FIGS. First, as a preparation step, as shown in FIG. 9, a pre-improved portion 41 for inserting the stirring mixer 24 into the ground G is formed on the ground G below the outer peripheral portion 2 a of the tank 2.
The pre-improvement portion 41 may be formed as appropriate, but is stirred and mixed with the in-situ soil while supplying the solidified material (in this embodiment, cement or cement-based solidified material) to the original ground using a conventional ground improvement device. In this way, the viscosity of the pre-improving portion 41 is set to a state that is sufficiently flexible and semi-fluid enough to allow the stirring mixer 24 to be inserted.
Incidentally, pre-improvement section 41 is to be a buttress 3 integral with eventually formed by stirring and mixing machine 24, and the extent of the whole of the width d ₃ is at least buttress 3 can be inserted while the horizontal posture The depth is at least the depth that reaches the bottom of the buttress 3 to be formed.

After the advance improvement part 41 is formed, the agitation mixer 24 is inserted into the advance improvement part 41 by operating the base machine 22 as shown in FIG. The jack 34 is extended rearward and the reaction force plate 33 is pressed against the rear wall of the pre-improvement unit 41, whereby the entire stirring mixer 24 is advanced and its tip is pressed against the front wall of the pre-improvement unit 41.
The chain 31 of the agitating mixer 24 is circulated and driven, and the forward jack 34 is further extended while jetting cement (cement paste or cement milk) or cement-based solidifying material as a solidifying material, and the stirring mixer 24 The tip portion is allowed to enter the ground G around the tank 2, whereby the solidified material is stirred and mixed with the in-situ soil. At that time, the driving direction of the chain 31 may be set to a direction (a counterclockwise direction in the drawing) in which the tip end portion of the stirring mixer 24 easily bites into the ground G.

As shown in FIG. 11, when the advancing jack 34 is fully extended and almost the entire length of the agitating mixer 24 enters the ground G around the tank 2, the agitating and mixing is continued by operating the base machine 22 while continuing the agitating and mixing. The machine 24 is gradually lowered.
Then, as shown in FIG. 12, when the stirring mixer 24 reaches the bottom of the advance improvement section 41 (that is, the bottom of the buttress 3 to be formed), the operation at this stage is finished.
Therefore, the forward jack 34 is degenerated, the entire agitating mixer 24 is pulled up to the upper part of the advance improvement section 41, the position is shifted slightly to the side, and the above operation is repeated. Repeating over the circumference, the buttress 3 is finally arranged radially around the tank 2, and the buttress 3 is completed when the solidified material is cured after a predetermined period of time.

Next, the internal ground improvement body 4 shown in FIGS. 1A and 1B is formed.
The internal ground improvement body 4 is formed after or after the buttress 3 is formed. The internal ground improvement body 4 uses the ground improvement device 21 according to the present embodiment shown in FIG. 9 or the conventional ground improvement device to make the ground G below the central portion 2b of the tank 2 entirely solidified and in situ. The ground is improved by stirring and mixing, and is completed when the solidified material is cured after a predetermined period of time.
Then, the foundation slab 11 is restored, and the liquefaction countermeasure structure 1 is applied to the support ground of the tank 2.

Next, the operation of the liquefaction countermeasure structure according to the first embodiment will be described with reference to the drawings.
According to the liquefaction countermeasure structure 1 according to the first embodiment, a plurality of buttresses 3 are radially arranged on the ground G below the outer peripheral portion 2a of the tank 2 and the surrounding ground G. 3 can restrain the ground G between the buttresses 3, and the strength of the ground G below and around the outer peripheral portion of the tank 2 can be increased. Further, since the buttress 3 is formed by stirring and mixing the cement-based solidified material and the in-situ soil, the rigidity of the entire ground G supporting the tank 2 can be increased.

In the liquefaction countermeasure structure 1 according to the first embodiment described above, a plurality of buttresses 3 are disposed radially on the ground G below the outer peripheral portion 2a of the tank 2 and the surrounding ground G, thereby It is possible to reduce the damage of the tank 2 due to the conversion and to suppress the vibration of the tank 2 due to the earthquake.
Moreover, compared with the conventional liquefaction countermeasure structure that improves the entire ground G below the tank 2, the amount of ground improvement can be reduced, labor and cost can be reduced, and the construction period can be shortened.

In addition, according to the liquefaction countermeasure method according to the first embodiment, the buttress 3 is moved by the ground improvement device 21 in which the agitating mixer 24 that agitates and mixes the solidified material and the in-situ soil moves in a horizontal posture in the vertical and horizontal directions. By forming, reliable ground improvement is possible. Moreover, since the ground G around the tank 2 can be improved from the tank 2, the buttress 3 can be efficiently created, the labor for forming the buttress 3 can be reduced, and the construction period can be shortened. it can.
In addition, since the buttress 3 is formed from the inside of the tank 2, it is not necessary to perform work while passing through the piping of the tank 2 disposed outside the tank 2, so that the buttress 3 can be easily formed.

Next, other embodiments will be described with reference to the accompanying drawings. However, the same or similar members and parts as those of the above-described first embodiment are denoted by the same reference numerals, and description thereof is omitted. A configuration different from the embodiment will be described.
As shown in FIG. 13, in the liquefaction countermeasure structure 51 according to the second embodiment, the internal ground improvement body 54 is used as a partial improvement ground, and the ground is improved in a lattice shape.
At this time, the conditions such as the improvement rate of the internal ground improvement body 54 and the rigidity of the ground improvement part are the same as the buttress 3 according to the first embodiment, with the partial ground improvement body as a set of unit periodic structures G2. The liquefaction strength of the ground improvement body may be evaluated and determined based on this.
Further, when determining the conditions of the buttress 3, the liquefaction strength is calculated in consideration of the external force received from the internal ground improvement body 54 in the case of liquefaction, and is determined based on the liquefaction strength.

  In the liquefaction countermeasure structure 51 according to the second embodiment, a plurality of buttresses 3 are radially arranged on the ground G below the outer peripheral portion 2a of the tank 2 and the surrounding ground G, so that the first The same effects as the liquefaction countermeasure structure 1 according to the embodiment are obtained.

As shown in FIG. 14, in the liquefaction countermeasure structure 61 according to the third embodiment, the internal ground improvement body 64 is improved by an osmotic solidification method for infiltrating a chemical solution.
At this time, since the strength against liquefaction of the ground G is increased by the chemical solution, the buttress 3 is planned in consideration of the influence of the chemical solution injection.
In the liquefaction countermeasure structure 61 according to the third embodiment, a plurality of buttresses 3 are radially disposed on the ground G below the outer peripheral portion 2a of the tank 2 and the surrounding ground G, thereby The same effects as the liquefaction countermeasure structure 1 according to the embodiment are obtained.
Further, since the internal ground improvement body 64 is improved by a permeation solidification method for infiltrating the chemical liquid, the internal ground improvement body can be injected by injecting the chemical liquid from the side even when the basic slab 11 of the tank 2 is not removed. 64 can be formed.

As shown in FIG. 15, in the liquefaction countermeasure structure 81 according to the fourth embodiment, the lower end 3a of the buttress 3 is rooted in the non-liquefaction layer G5 of the ground G, and the center 2b of the tank 2 is The lower end 84a of the internal ground improvement body 84 formed on the lower ground G does not reach the non-liquefied layer G5, and the liquefied layer G6 is interposed between the non-liquefied layer G5 and the internal ground improvement body 84. Intervene.
The depth h1 of the internal ground improvement body 84 is preferably about 50 to 80% of the depth h2 of the liquefied layer G6. The internal ground improvement body 84 may be formed by a cement-based deep mixing method, or may be formed by an osmotic solidification method in which a chemical solution is infiltrated. Moreover, even if the internal ground improvement body 84 is formed by this other method.
The internal ground improvement body 84 may be an improvement of the entire region in which the internal ground improvement body 84 is formed, or may be improved in a lattice shape as in the second embodiment.

According to the liquefaction countermeasure structure 81 according to the fourth embodiment, the same effect as the liquefaction countermeasure structure 1 according to the first embodiment is obtained, and the buttress 3 is embedded in the non-liquefaction layer G5. Thus, since the displacement of the buttress 3 at the time of the earthquake is suppressed, it is possible to suppress the ground G between the adjacent buttresses 3 from being sheared and to prevent the tank 2 from being settling down.
Further, since the liquefied layer G6 is interposed between the non-liquefied layer G5 and the internal ground improvement body 84, a part of vibration transmitted from the non-liquefied layer G5 to the internal ground improvement body 84 at the time of an earthquake is generated. Since it is absorbed by the liquefied layer G6, vibration transmitted to the tank 2 can be reduced.
Moreover, since this Embodiment can reduce the construction amount of the internal ground improvement body 84 compared with the case where the lower end part 84a of the internal ground improvement body 84 is formed so that it may reach the non-liquefaction layer G5, Cost reduction and construction period can be shortened.

Here, an experiment was conducted to verify the relationship between the improvement depth of the ground below the structure and the amount of settlement of the structure.
As shown in FIG. 16 (a), the ground is improved by compacting the ground G corresponding to the non-liquefied layer, and the steel block 91 is disposed thereon, and a predetermined centrifugal vibration experiment (q = 100 kPa) is performed. The amount of settlement of the steel block 91 was measured. In FIG. 16A, the ground improved region is a ground improved region 92, and the ground improved region 92 is formed in a columnar shape having a cross section that is larger than the steel block 91 in plan view.
In this experiment, five cases of experiment cases C0 to C4 were performed. In these cases C0 to C5, the depth h1 of the ground improvement region 92 is different, and FIG. 16B shows the improvement ratio (h1 / h2) with respect to the depth h1 and the depth h2 of the ground G, respectively.

  As can be seen from the figure showing the amount of settlement of the steel block in FIG. 17, the amount of settlement is almost the same in case C2 (improvement ratio 0.53) and case C4 (improvement ratio 1.00). As a result, the amount of subsidence of the steel block 91 is equivalent to that when the ground is improved to a total depth h2 by improving the ground at a depth of about 53% or more with respect to the depth h2 of the ground G. It can be seen that it can be suppressed.

As mentioned above, although embodiment of the liquefaction countermeasure structure by this invention was described, this invention is not limited to said embodiment, In the range which does not deviate from the meaning, it can change suitably.
For example, in the embodiment described above, the buttress 3 is disposed below the outer peripheral portion 2a of the tank 2 and the surrounding ground G, and the ground G below the central portion 2b of the tank 2 is improved. The buttress 3 may be disposed below the outer peripheral portion 2a of the tank 2 and the surrounding ground G without improving the ground G below the central portion 2b.
In the above embodiment, the buttress 3 is constructed from the inside of the tank 2 after the foundation slab 11 is removed, but the ground improvement device 21 according to the present embodiment is used from the outside of the tank 2. Buttress 3 may be created.
In the above embodiment, the shape of the buttress 3 is determined on the basis of the liquefaction strength of the partially improved ground G1, but the buttress 3 is also examined using a finite element method (FEM) or the like. Also good.
In the second embodiment, the internal ground improvement body 54 is a partial improvement ground improved in a grid shape, but a partial improvement in which columnar ground improvement bodies arranged at predetermined intervals are provided. It may be the ground.
In the present embodiment, the liquefaction countermeasure structure is applied to the ground G that supports the cylindrical tank 2, but as shown in FIG. The liquefaction countermeasure structure according to this embodiment may be applied.

1, 51, 61 Liquefaction countermeasure structure 2 Tank (existing structure)
3 Buttress (underground wall)
4, 54, 64 Internal ground improvement body 11 Basic slab 21 Ground improvement device 22 Base machine 24 Stir mixer G Ground G1, G4 Partially improved ground G5 Non-liquefied layer G6 Liquefied layer τ _d Shear stress γ _i Shear strain G _eq Equivalent shear stiffness Δu / σ _ν 'Excess pore water pressure ratio

Claims (10)

A liquefaction countermeasure structure that reduces damage to existing structures due to liquefaction,
It is formed by stirring the in-situ soil and solidification material of the ground below and around the outer periphery of the existing structure, and is oriented in a direction perpendicular to or substantially perpendicular to the outer periphery of the existing structure. 2. A liquefaction countermeasure structure, wherein a plurality of underground walls are arranged at predetermined intervals in a direction parallel to the outer periphery of the existing structure.   The liquefaction countermeasure structure according to claim 1, wherein the ground below the center of the existing structure is improved.   The lower end of the underground wall is embedded in the non-liquefied layer of the ground, and the improvement of the ground below the central portion of the existing structure does not reach the non-liquefied layer. Item 3. The liquefaction countermeasure structure according to Item 2.     2. The ground improvement body formed by stirring the original soil and solidification material of the ground is partially disposed on the ground below the center of the existing structure. The liquefaction countermeasure structure according to 2 or 3.   The liquefaction countermeasure structure according to claim 2 or 3, wherein the lower ground in the central portion of the existing structure is improved by injecting a chemical solution and solidifying it by penetration.   The liquefaction countermeasure structure according to any one of claims 1 to 5, wherein the solidifying material is cement or a cement-based solidifying material. The liquefaction strength of the partially improved ground where the underground wall is disposed is
Calculate the shear stress generated during the earthquake on the ground before improvement,
Calculate the shear strain of the ground before improvement based on the shear stress,
Calculate the equivalent shear stiffness of the ground after improvement,
Obtain the shear strain generated in the unimproved ground between the underground walls in the improved ground,
Re-determining the equivalent shear stiffness according to the shear strain;
Using the re-determined equivalent shear stiffness, calculate the shear strain until the shear strain converges to a certain value, determine the shear strain,
Based on the determined shear strain, find the excess pore water pressure ratio that occurs in the unmodified ground,
The liquefaction countermeasure structure according to any one of claims 1 to 6, wherein the structure is evaluated based on the excess pore water pressure ratio.   The liquefaction countermeasure structure according to claim 1, wherein the existing structure is a circular structure in plan view, and the underground wall is arranged radially with respect to the existing structure. A liquefaction countermeasure method that reduces damage to existing structures due to liquefaction,
Insert a stirring mixer that stirs and mixes with the in-situ soil while supplying solidification material to the lower ground and surrounding ground of the outer periphery of the existing structure, and moves the stirring mixer in the vertical and horizontal directions in a horizontal position. A liquefaction countermeasure method characterized by forming underground walls. The liquefaction countermeasure method according to claim 9, wherein the step of forming the underground wall is performed from the inside of the existing structure.

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