JP7355680B2 - Liquefaction evaluation method for pile foundation improved ground and pile foundation ground improvement method - Google Patents

Description

The present invention relates to a liquefaction evaluation method for pile foundation improved ground and a pile foundation ground improvement construction method.

Conventionally, as a measure to prevent ground liquefaction, a method of construction in which a lattice-like wall is created in plan view using cement-based improvement in the ground, restraining the original ground within the lattice and reducing the shear strain of the ground during an earthquake. It has been known.

As a method for evaluating the liquefaction of the improved ground that has undergone such liquefaction prevention, for example, as shown in Patent Document 1, there is a method of simply determining the rigidity of the ground improvement wall and the original ground combined.
Patent Document 1 describes an equivalent shear wave velocity that shows the correlation between the equivalent shear wave velocity of improved ground in which ground improvement bodies are periodically arranged in a plan view in the original ground and the ground improvement rate of the improved ground. A method has been proposed that uses graphs to evaluate the seismic response of buildings built on the improved ground.

Japanese Patent Application Publication No. 2007-170904

By the way, conventionally, piles driven into the ground have not been used as a countermeasure against liquefaction. Therefore, there is a need for a specific method for evaluating liquefaction in ground improvement that takes such piles into consideration, and there is room for improvement in this respect.

The present invention has been made in view of the above-mentioned problems, and provides a method for evaluating liquefaction of improved ground on pile foundations and a method for improving ground on pile foundations, which can efficiently evaluate liquefaction of improved ground in consideration of pile foundations. With the goal.

In order to achieve the above object, the liquefaction evaluation method of pile foundation improved ground according to the present invention is a pile foundation for evaluating the possibility of liquefaction in pile foundation improved ground where the pile foundation is restrained by a grid-like ground improvement body. A method for evaluating liquefaction of improved ground, comprising: creating a first calculation chart obtained from the equivalent shear rate of a pile-like ground improvement body whose improved shape is improved to a pile-like shape; A step of creating a second calculation chart obtained from the shear rate, and converting the material values of the pile-like ground improvement body into material values of the pile foundation using the first calculation chart, and converting the material values of the pile foundation and the original a step of determining the relationship between the shear stress τ and the strain γ with respect to the rigidity of the ground, a step of determining the relationship between the shear stress τ and the strain γ of the grid-like ground improvement body using the second calculation chart, and the pile foundation. Based on the relationship between shear stress τ and strain γ with the stiffness of the original ground, and the relationship between the shear stress τ and strain γ of the grid-like ground improvement body, liquefaction safety is determined by the ratio of shear stress generated in the ground before and after improvement. The present invention is characterized by comprising the steps of determining the shear stress ratio, and determining the presence or absence of liquefaction based on the magnitude of the shear stress ratio.

Further, the pile foundation ground improvement method according to the present invention is a pile foundation ground improvement method for constructing the pile foundation improved ground using the above-mentioned liquefaction evaluation method for the pile foundation improved ground, wherein the pile foundation is added to the original ground. and a step of constructing a lattice-like ground improvement body so as to surround the pile foundation in a lattice-like manner.

In the present invention, liquefaction can be evaluated using the first calculation chart based on the pile-like ground improvement body, which is corrected by replacing it with the material values of the pile foundation. That is, when calculating the liquefaction safety factor, the apparent strength of the original ground can be increased in consideration of the rigidity of the pile foundation.
As a result, in the present invention, the rigidity of a pile foundation improved ground that combines a pile foundation and a lattice-like ground improvement body can be determined by a simple method, and even if the pile foundation improved ground takes into account the effects of the pile foundation. Liquefaction can be evaluated. In addition, the liquefaction safety factor can be obtained by a simple method such as using the first calculation chart of the conventionally used pile-type ground improvement body and converting it to the material value of the pile foundation, so that liquefaction can be evaluated efficiently. can do.

Moreover, the liquefaction evaluation method for pile foundation improved ground according to the present invention may be characterized in that the pile foundation is an existing pile.

In this case, when constructing a pile foundation improved ground that combines existing piles and grid-like ground improvement bodies, liquefaction can be evaluated using the liquefaction evaluation method described above. In this case, construction can be carried out without driving a new pile foundation, so it is possible to construct an efficient and highly rigid improved ground.

According to the method for evaluating liquefaction of improved ground on pile foundations and the method for improving ground on pile foundations of the present invention, liquefaction of improved ground in consideration of pile foundations can be efficiently evaluated.

1 is a perspective view schematically showing a pile foundation improved ground to which a liquefaction evaluation method according to an embodiment of the present invention is applied. FIG. 2 is a plan view of the pile foundation improved ground shown in FIG. 1. It is a figure which shows a pile-like ground improvement body typically, Comprising: (a) is a perspective view, (b) is a top view. It is a figure which shows typically a grid-like ground improvement body, Comprising: (a) is a perspective view, (b) is a top view. 4 is an equivalent shear wave velocity graph of the pile-shaped ground improvement body shown in FIG. 3. 5 is an equivalent shear wave velocity graph of the grid-like ground improvement body shown in FIG. 4. FIG. It is a figure showing an example of a liquefaction examination flow of improved ground. It is a figure showing the liquefaction safety factor (FL value) of the improved ground before and after improvement.

DESCRIPTION OF EMBODIMENTS Hereinafter, a liquefaction evaluation method for pile foundation improved ground and a pile foundation ground improvement construction method according to an embodiment of the present invention will be described based on the drawings.

As shown in FIGS. 1 and 2, the liquefaction evaluation method for pile foundation improved ground 1 according to the present embodiment is applied to the original ground on which a plurality of existing piles 2 (pile foundations) such as steel pipe piles and concrete piles have been constructed. In G, liquefaction is evaluated for the pile foundation improved ground 1 that has been improved to form a grid-like ground improvement body 3 that is grid-like in plan view. In the lattice-like ground improvement body 3, a plurality of lattices surrounding the existing piles 2 are formed. The existing pile 2 is formed into a columnar shape.
In addition, in this embodiment, although the existing pile 2 is targeted as a pile foundation, a newly installed pile may be applied to the pile foundation by the liquefaction evaluation method of this embodiment.

First, an equivalent shear wave velocity graph of the pile foundation improved ground 1 is created. In this embodiment, the shape of the existing pile 2 and the improved shape of the grid-like ground improvement body 3 are selected. Specifically, as shown in FIGS. 3(a) and 3(b), the first calculation chart (see FIG. 5) obtained from the equivalent shear rate of the pile-like ground improvement body 4 whose improved shape has been improved to a pile-like shape is At the same time, as shown in FIGS. 4(a) and 4(b), a second calculation chart (see FIG. 6) obtained from the equivalent shear rate of the grid-like ground improvement body 3 is created.
These calculation charts are used to determine the equivalent rigidity of the original ground G and the ground improvement bodies 3 and 4, depending on the shape of the improvement body.

The first calculation chart shown in FIG. 5 calculates the equivalent shear wave velocity V _{s_eq} using a periodic structure model expressed on a micro scale in the pile ground improvement body 4 shown in FIGS. 3(a) and 3(b). This is a calculation chart. In the case of the pile-shaped ground improvement body 4, the radius r of the pile-shaped ground improvement body 4 is changed for calculation. The curve group shown in the first calculation chart shows characteristics depending on the pile shape, and the rate of increase in the equivalent shear wave velocity V _{S_eq} with respect to the increase in the ground improvement rate R (%) is small. For example, when R = 60%, the It only corresponds to the equivalent shear wave velocity V _{S_eq, which} is about twice as high. The ground improvement rate R is the area of the ground improvement body divided by the total area of the improved ground.

The second calculation chart shown in FIG. 6 is a calculation chart obtained by determining the equivalent shear wave velocity V _{S_eq} in the grid-like ground improvement body 3 using a periodic structure model expressed on a micro scale. In the case of the lattice-shaped ground improvement body 3, the thickness t of the lattice-shaped ground improvement body 3 in the lattice width direction is changed for calculation. The curve group shown in the second calculation chart shows characteristics depending on the lattice shape, and a remarkable improved shape effect on the overall stiffness has occurred.

Set a and the ground improvement rate R in each calculation chart, mechanically or analytically determine the elastic matrix C and response displacement vector Calculate C ^H. Note that a in the figure is (shear wave velocity V _SI of the ground improvement body/shear wave velocity V _SG of the original ground).
Next, the equivalent shear wave velocity V ^H of the ground improvement body is calculated from equation (2) using the equivalent shear elastic modulus G ^H of the ground improvement body obtained as a component of the equivalent elasticity matrix CH.
Here, in equations (1) and (2), C is the elastic matrix of the unit periodic structure as above, I is the unit matrix, and X is the time when unit macro strain is applied to the unit periodic structure as above. , where y is the microscale, Y is the area of the unit periodic structure, and ρ is the average density of the improved ground. The unit macro strain means that the average strain of the improved ground is 1.

Next, the equivalent shear wave velocity V ^H of the improved ground is calculated by changing a and the ground improvement rate R sequentially, and using a as a parameter, the equivalent shear wave velocity V ^H of the improved ground and the ground improvement rate of the improved ground are calculated. By finding a group of curves showing the correlation of , an equivalent shear wave velocity graph corresponding to the selected improved shape can be obtained.

When using the equivalent shear wave velocity graphs shown in Figures 5 and 6, first set (shear wave velocity V _sI of the ground improvement body / shear wave velocity V _sG of the original ground), and then use the equivalent shear wave velocity graph. Therefore, the optimal combination of a and soil improvement rate R is selected from a design perspective. For example, in the case of the pile-shaped ground improvement body 4, if the equivalent shear wave velocity V ^H of the improved ground is set to approximately 1.4 times that of the original ground, the ground improvement rate will be 35% in the range of a = 4 to 20 as shown in Figure 5. You can see that it is sufficient to do this.

Here, the liquefaction evaluation method of the pile foundation improved ground 1 according to the present embodiment can use a liquefaction study flow in the improved ground as shown in FIG. 7, as an example. The liquefaction study flow shown in Figure 7 is based on “Simple liquefaction evaluation method for sandy ground restrained by grid-like improvement bodies”, Journal of the Japan Society of Civil Engineers C (Geosphere Engineering), vol. 68, No. 2, 297- 304, 2012). The liquefaction study flow shown in FIG. 7 may be used, or other liquefaction study flow may be adopted. Note that the liquefaction study flow shown in FIG. 7 is described in the above-mentioned literature, so a detailed explanation will be omitted here.

Next, the material value of the pile-like ground improvement body 4 is converted to the material value of the existing pile 2 using the first calculation chart shown in FIG. 5, and as shown in FIG. The relationship between rigid shear stress τ and strain γ (curve S2 shown in FIG. 8) is determined. Specifically, regarding the shear wave velocity V _SI of the first calculation chart of the pile-shaped ground improvement body 4, the material of the existing pile 2 is used instead of the material of the pile-shaped ground improvement body 4. For example, if the shear wave velocity V _SI of the existing pile 2 is 2000 m/s and the pile-like ground improvement body 4 is 500 m/s, the ratio a is 4 for the existing pile 2 and 4 for the pile-like ground improvement body. Double.

Next, the equivalent repeated shear stress ratio (τ _d /σ′ _v ) is calculated. The shear stress τ _d generated in the ground during an earthquake is calculated from earthquake response analysis. Then, the shear strain γ _I of the restrained ground is calculated. Then, the initial shear strain γ _I for the shear stress τ _d determined above is determined using equation (3). Here, G _eq is the equivalent shear rigidity of the grid-like ground improvement body 3.

In accordance with the value of the initial shear strain γ _I obtained from equation (3), the equivalent shear rigidity G _eq of the lattice-shaped ground improvement body 3 is determined using the first calculation chart shown in FIG. Then, this cycle is repeated several times until γ _I converges, and γ _I = γ _{I_eq} is determined. Here, γ _{I_eq} is the equivalent initial shear strain occurring in the improved ground.

Next, as shown in FIG. 8, the liquefaction safety factor (FL value) before and after the improvement is determined. That is, the relationship between the rigid shear stress τ and strain γ of the existing pile 2 and the original ground G (curve S2 in FIG. 8), and the shear stress τ and strain of the grid-like ground improvement body 3 obtained from the second calculation chart. Based on the relationship of γ (curve S3 in FIG. 8), a liquefaction safety factor consisting of the shear stress ratio (τ/σ′ _v ) generated in the ground before and after improvement is determined.

Specifically, the corresponding shear stress ratio (τ/σ′ _v ) is determined from the equivalent initial shear strain γ _{I_eq} of the improved ground determined above and the initial shear strain γ _{I_Iq} at which liquefaction occurs.

As shown in FIG. 8, the liquefaction intensity is corrected so that the strain γ _{I_Iq} at which the original ground G reaches liquefaction does not change. The graph in FIG. 8 shows deformation (strain γ _I ) on the horizontal axis and shear stress ratio (τ/σ′ _v ) on the vertical axis. Here, it is assumed that the shear strain γ _{I_Iq} at which liquefaction occurs in the original ground G does not change regardless of the presence or absence of the existing pile 2, and it is assumed that the stress generated in the ground during liquefaction increases. In other words, when the shear strain γ _{I_Iq} at which liquefaction occurs in the original ground G, the stress generated in the ground during liquefaction is not the graph S1 of the τ - γ relationship of the original ground G, but the stiffness of the original ground G and the existing pile 2. It is determined based on the numerical value from the graph S2 of the τ-γ relationship of the ground with .

The assumption of the homogenization method is that the pile deforms in shear together with the surrounding ground, but in reality, the pile does not undergo the same shear deformation as the ground, but deforms as a bending material, so the homogenization method The shear stiffness of the composite ground is relatively overestimated.

Next, based on FIG. 8, the presence or absence of liquefaction is determined based on the magnitude of the shear stress ratio. Here, the FL value is the ratio (τ/σ′ _v ) of the shear stress of the ground before and after improvement to the shear stress τ _Iq at which liquefaction occurs.

Before the improvement, the external force (equivalent cyclic shear stress ratio: τ _d /σ′ _v ) was larger than the liquefaction strength (shear stress ratio generated in the ground during liquefaction: τ _Iq /σ′ _v ), so the liquefaction Even if it is determined that there is a risk (FL value < 1), after appropriate improvement, the shear strain caused by external force will be reduced due to the improvement effect, and the shear stress ratio (τ _eq /σ' _v ) becomes smaller than the liquefaction strength, and it is determined that there is no risk of liquefaction (FL value>1).

Next, the effects of the liquefaction evaluation method of the pile foundation improved ground and the pile foundation ground improvement method described above will be explained in detail based on the drawings.
In this embodiment, as shown in FIG. 8, liquefaction can be evaluated using a first calculation chart based on the pile-like ground improvement body 4 that is corrected by replacing it with the material values of the existing piles 2. That is, when calculating the liquefaction safety factor, the apparent strength of the original ground G can be increased in consideration of the rigidity of the existing pile 2 in calculation.

As a result, in this embodiment, the rigidity of the pile foundation improved ground 1 that combines the existing piles 2 and the lattice-like ground improvement body 3 can be determined by a simple method, and the pile foundation improvement considering the effect of the existing piles 2 is possible. Liquefaction can be evaluated even in ground 1. Moreover, the liquefaction safety factor can be determined by a simple method such as using the first calculation chart of the conventionally used pile-shaped ground improvement body 4 shown in FIG. 5 and converting it to the material value of the existing pile 2. Liquefaction can be evaluated efficiently.

In addition, in this embodiment, since the existing piles 2 are adopted as the pile foundation, when constructing the pile foundation improved ground 1 that combines the existing piles 2 and the lattice-like ground improvement body 3, the liquid of this embodiment Liquefaction can be evaluated using a liquefaction evaluation method. In this case, construction can be carried out without driving a new pile foundation, so it is possible to construct an efficient and highly rigid improved ground.

As described above, in the pile foundation improved ground liquefaction evaluation method and pile foundation ground improvement construction method according to the present embodiment, liquefaction of improved ground considering the pile foundation can be efficiently evaluated.

The embodiments of the liquefaction evaluation method for pile foundation improved ground and the pile foundation ground improvement method according to the present invention have been described above, but the present invention is not limited to the above-described embodiments, and may be provided within the scope of the spirit thereof. It can be changed as appropriate.

For example, the pile foundation 2 targeted in this embodiment can be either an existing pile or a newly installed foundation pile.
Moreover, the pile foundation is not limited to a columnar shape, and may be a rectangular pile in cross-section.

Furthermore, in the embodiment described above, one foundation pile (pile foundation) is arranged in one lattice of the lattice-like ground improvement body 3, but a plurality of pile foundations are arranged in one lattice. It is also applicable when As a liquefaction evaluation method in this case, for example, the first calculation chart based on the pile-shaped ground improvement body 4 (see Figure 5) and the second calculation chart based on the grid-like ground improvement body 3 (see Figure 6) are independently used. Liquefaction evaluation can be carried out by calculating. In other words, if the pile types of multiple pile foundations are the same, approximately, the improvement rate is determined based on the area of the multiple piles relative to the area of the ground within the grid, and the first calculation chart based on the pile-shaped soil improvement body 4 is used. It can be calculated by In addition, if the pile types of multiple piles are different (for example, the first existing pile A, the second existing pile B), approximately the pile rigidity can be simply calculated as (the first existing pile A + the second existing pile B). /2, and the rigidity can be determined by the area ratio with the ground in the grid and calculated in the same way. Note that the calculation method using the first calculation chart when a plurality of pile foundations are arranged between grids is not limited to this, and any appropriate calculation method can be adopted.

In addition, it is possible to appropriately replace the components in the above-described embodiments with well-known components without departing from the spirit of the present invention.

1 Pile foundation improved ground 2 Existing piles (pile foundation)
3 Grid-shaped ground improvement body 4 Pile-shaped ground improvement body G Original ground

Claims (3)

A liquefaction evaluation method for pile foundation improved ground for evaluating the possibility of liquefaction in pile foundation improved ground where the pile foundation is restrained by a grid-like ground improvement body, comprising:
a step of creating a first calculation chart obtained from the equivalent shear rate of a pile-like ground improvement body whose improved shape is improved to a pile-like shape;
creating a second calculation chart obtained from the equivalent shear rate of the grid-like ground improvement body;
A step of converting the material values of the pile-like ground improvement body into material values of the pile foundation using the first calculation chart, and determining the relationship between the shear stress τ and strain γ with the stiffness of the pile foundation and the original ground. and,
determining the relationship between shear stress τ and strain γ of the grid-like ground improvement body using the second calculation chart;
It consists of the shear stress ratio generated in the ground before and after improvement based on the relationship between the shear stress τ and strain γ with the rigidity of the pile foundation and the original ground, and the relationship between the shear stress τ and strain γ of the lattice-like ground improvement body. A process of determining the liquefaction safety factor,
determining the presence or absence of liquefaction based on the magnitude of the shear stress ratio;
A liquefaction evaluation method for pile foundation improved ground, characterized by having the following. The liquefaction evaluation method for pile foundation improved ground according to claim 1, wherein the pile foundation is an existing pile. A pile foundation ground improvement method for constructing the pile foundation improved ground using the liquefaction evaluation method for pile foundation improved ground according to claim 1 or 2,
a step of driving the pile foundation into the original ground;
constructing a lattice-like ground improvement body so as to surround the pile foundation in a lattice-like manner;
A pile foundation ground improvement method characterized by having.

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