JP6769223B2 - How to design press-fit piles in liquefied ground - Google Patents

Description

The present invention relates to a method for designing a press-fit pile in liquefied ground.

In liquefaction, when a horizontal force during an earthquake acts on the ground, the shear deformation of the ground increases the interstitial water pressure between the sand particles, which reduces the effective stress and makes it difficult to transfer stress between the sand particles. This is a phenomenon in which the fluidity increases and eventually the vertical bearing capacity is lost, which tends to occur in loosely saturated sandy ground (hereinafter, the ground where liquefaction is likely to occur is referred to as liquefaction ground).

As measures to prevent liquefaction, in addition to the so-called sand compaction pile (SCP) method, in which sand piles are created in the ground to compact the ground, the pore water pressure is reduced by draining, and mortar and chemicals are used in the soil particle gap. There are various construction methods such as filling and injecting into the sand to solidify and lowering the groundwater level, but the SCP construction method has been widely adopted from the viewpoint of cost and reliability.

Japanese Unexamined Patent Publication No. 2007-309091

In the SCP method, the sand thrown into the ground through the casing is statically or dynamically press-fitted after the casing is removed to expand the diameter, thereby creating a sand pile in the ground and forming the sand pile. The area that extends between the piles is compacted, but when implementing this method, the volume ratio of the sand piles is evaluated so that the liquefaction of the original ground is appropriately prevented or suppressed by the press-fitted sand piles. There is a need to.

In performing the above evaluation, the evaluation method called the C method is mainly used these days, and in the evaluation method, in the area extending between the piles of the sand pile when compacting by forming the sand pile. The target N value is set to N _1, and it is assumed that the gap ratio of the inter-pile region becomes smaller by the body integral of the press-fitted sand pile, and the sand pile is in the form of improvement rate or press-fitting rate while considering the fine grain content. The volume ratio of is determined.

Here, since sand piles are created by press-fitting and expanding the diameter of sand thrown into the original ground, their N value tends to be larger than the N value of the inter-pile area, but during an earthquake, N As the value is larger, the sand pile bears more horizontal force during an earthquake, while the load on the inter-pile area is smaller. Therefore, in the conventional C method with the target N value as N ₁ , in the inter-pile area. There has been a problem that the degree of compaction becomes larger than necessary and the cost for preventing liquefaction becomes excessive.

In particular, from the viewpoint of effective use of resources or consideration for the environment, steel slag may be used instead of natural sand (Patent Document 1), but in such a construction method, N of press-fit piles made of steel slag is used. The above-mentioned tendency becomes more remarkable because the value becomes much larger than the N value of the sand pile and the difference from the N value in the inter-pile region also widens.

The present invention has been made in consideration of the above-mentioned circumstances, and provides a method for designing a press-fit pile in liquefied ground, which can reasonably design a press-fit pile made of granules such as sand and slag in the SCP method. With the goal.

In order to achieve the above object, as described in claim 1, the method for designing a press-fitting pile in the liquefied ground according to the present invention is to press-fit granules such as sand and slag into the original ground in a columnar shape in the original ground. This is a method of designing a press-fitting pile in a liquefied ground in which a region extending between the piles of the press-fitting ground is compacted by constructing a press-fitting pile in the ground, and the N value of the ground is N ₀ . The following equation, where the target N value in the region when compacting the original ground is N ₁ , and the reduction rate with respect to the increase amount of the N value according to the fine grain content of the original ground is β,
N ₁ ′ = (N ₁ −N ₀ ) / β + N ₀ (1)
From, the calculated N value N ₁ ′ when there is no fine particle content is obtained, then the gap ratio e ₁ after press-fitting in the region is obtained using the N ₁ ′, and then the press-fitting in the original ground is performed. With the previous gap ratio as e ₀ ,
a _s = (e ₀ −e ₁ ) / (1 + e ₀ ) (2)
In the design method of the press pile in liquefaction ground for calculating an improved rate a _s of the press-pile from,
Evaluate the N value N _p of the press-fit pile and
Using the N _p and the improvement rate a _s , the N value N ₂ of the composite ground composed of the press-fitted pile and the region is calculated by the following equation.
_{N 2 = a s · N p} + (1-a s) · N 1 (3)
Then, from the N ₂ , the target N value reduction coefficient R is obtained from the following equation.
R = N ₁ / N ₂ (4)
From calculated, the the R · N ₁ formed by multiplying the N ₁ and R, by calculating the a new target N value in place of N ₁ (1) and equation (2), the improvement ratio a _s and calculates a correction improvement ratio a _'s in place of.

Further, in the method for designing a press-fit pile in liquefied ground according to the present invention, at least a part of the granules is composed of steel slag.

The method for designing a press-fit pile in a liquefied ground according to the present invention is to press-fit granules into a columnar shape into the original ground to create a press-fit pile in the original ground, so that between the piles of the press-fit pile in the original ground. It is applied to the method of compacting the expanding area, and the method includes the sand compaction pile method in the narrow sense where the press-fit pile is a sand pile, the slag compaction pile method where the press-fit pile is a pile made of slag, and others. Construction methods using press-fit piles made of granular material are included.

The term sand compaction pile (SCP) method is used in the present specification to mean a compaction method using a press-fit pile made of granular material, including a slag compaction pile method.

In the method for designing a press-fitted pile in liquefied ground according to the present invention, first, the N value of the original ground is N ₀ , the target N value in the above-mentioned inter-pile region when compacting the original ground is N ₁ , and the original ground Let β be the reduction rate for the increase in N value according to the fine grain content, and the following equation,
N ₁ ′ = (N ₁ −N ₀ ) / β + N ₀ (1)
From this, the calculated N value N ₁ ′ when there is no fine particle size is obtained (step 101).

The N value N ₀ of the original ground may be obtained by using the liquefied ground to be compacted as the original ground and conducting a ground survey on the original ground, and the target N value N ₁ is the pile described above. It may be appropriately defined as a design giving condition for preventing or suppressing liquefaction in the inter-region. The reduction rate β is calculated by the following equation, where F _c (%) is the fine particle content of the original ground.
β = 1.05-0.51 · logF _c
It can be calculated from.

Next, N ₁ ′ is used to obtain the gap ratio e ₁ after press-fitting in the inter-pile region (step 102).

For the gap ratio e ₁ , the effective loading pressure is σ _v ′ (kN / m ² ), and the maximum gap ratio and the minimum gap ratio before press-fitting in the original ground are e _max and e _min , respectively.
D _r1 = 21 · √ (N ₁ ′ / (0.7 + σ _v ′ / 98))
e ₁ = e _max − (D _r1 / 100) ・ (e _max −e _min )
The maximum gap ratio e _max and the minimum gap ratio e _min can be obtained from the following equations, respectively.
e _max = 0.02 · F _c +1.0
e _min = 0.008 ・ F _c +0.6
You can find it from.

Next, let e _{0 be} the gap ratio before press-fitting in the original ground, and the following equation,
a _s = (e ₀ −e ₁ ) / (1 + e ₀ ) (2)
From calculates the improvement ratio a _s of pressed piles (step 103).

The gap ratio e ₀ is calculated by the following equation.
D _r0 = 21 · √ (N ₀ / (0.7 ₊ σ _v ′ / 98))
e ₀ = e _max − (D _r0 / 100) ・ (e _max −e _min )
You can find it from.

Step 101 to 103 described above is a procedure according to the conventional method C, pressed pile in a conventional sand compaction pile method is the radial and pitch using a modification ratio a _s obtained in such a sequence of steps It is decided.

However, in the above procedure, since the contribution of the press-fitting pile itself having the compaction strength to the prevention of liquefaction is not taken into consideration, the larger the N value of the press-fitting pile, the safer the design. Therefore, in the present invention, the above-mentioned improvement rate is modified by the following procedure.

First, the N value N _p of the press-fit pile is evaluated (step 104).

The N value N _p of the press-fit pile was subjected to a laboratory test in advance with the same granular material and the same physical constant (for example, density and compaction) as the press-fit pile to be constructed on the original ground to be prevented from liquefaction. Above, the strength constants (internal friction angle φ and adhesive force C) obtained in the laboratory test are used to determine from the relationship between the conventionally known internal friction angle φ and the N value, or the relationship between the adhesive force C and the N value. be able to.

On the other hand, N value N _p of the press piles to original ground to become liquefaction prevention object, prior to the construction, the same granules as those used in the main construction is press-fitted to the original ground in the same press-fitting method Preliminary work to create a press-fit pile can be performed, and a standard penetration test can be performed on the press-fit pile to obtain it. If the diameter of the press-fit pile is set to a certain size and only the pitch is determined by design, 1 N _p may be obtained by performing a standard penetration test on the main press-fitting pile, and after the pitch is determined by the design results described later, the second and subsequent piles may be constructed according to the pitch.

Then, by using the N _p and improvement ratio a _s, the following equation,
_{+ N T = a s · N} p (1-a s) · N 1 (3)
The N value N _T of the composite ground is calculated from (step 105).

Here, the composite ground is composed of a press-fitted pile and an inter-pile area, and the N value includes not only the N value after press-fitting in the inter-pile area (target N value N ₁ ) but also the N value. The N value N _{p of the} press-fit pile is reflected.

Next, from _NT , the target N value reduction coefficient R is calculated by the following equation.
R = N ₁ / _NT (4)
Calculate from (step 106).

According to the conventional C method, the inter-pile area becomes the target N value N ₁ by compaction with the press-fit pile, but when assuming the actual behavior during an earthquake, the press-fit pile with a large N value becomes the N value. A larger amount of horizontal force is borne during an earthquake, and the horizontal seismic force acting on the inter-pile area is reduced accordingly. Therefore, when the N value of the press-fitted pile is large, the inter-pile area is tightened to the target N value N _1. Hardening is an over-design.

Therefore, in the present invention, the concept of the target N value reduction coefficient is newly introduced, and the ratio of the target N value N ₁ to the N value N _T of the composite ground is set as the target N value reduction coefficient R (Equation (4)). By multiplying this by the target N value N ₁ , the target N value N ₁ is reduced (step 107).

By the way, assuming the target N value reduction coefficient R by the equation (4) has sufficient engineering validity in consideration of the sharing of the horizontal seismic force.

Next, Eq. (1), where R · N ₁ obtained by multiplying R by N ₁ is set as a new target N value to replace N ₁ .
N ₂ ′ = (RN ₁ −N ₀ ) / β + N ₀
Substitute in to obtain N ₂ ′ (step 108).

Next, using N ₂ ′, the following equation,
D _r2 = 21 · √ (N ₂ ′ / (0.7 + σ _v ′ / 98))
e ₂ = e _max − (D _r2 / 100) ・ (e _max −e _min )
The gap ratio e ₂ after press-fitting in the inter-pile region is newly calculated from (step 109).

Next,
a _s2 = (e ₀ −e ₂ ) / (1 + e ₀ ) (2)
From this, the correction improvement rate a _s2 of the press-fitted pile is calculated (step 110).

According to the series of steps described above, by considering the compaction strength of the press-fit pile itself, it is possible to reduce the target N value in the inter-pile region, and thus the design of the press-fit pile can be rationalized.

As long as it is possible to create the granules that make up the press-fitting pile by press-fitting and expanding the diameter of the original ground according to the procedure adopted in the conventional SCP method, for example, what kind of material should be used? It is optional and can be composed mainly of granules (φ material) that exhibit shear strength due to the internal friction angle φ, typically sand.

On the other hand, when at least a part of the above-mentioned granular material is composed of a so-called c-φ material that exhibits shear strength due to the internal friction angle φ and the adhesive force c, particularly steel slag, the press-fitting pile itself is tightened. As the compaction strength increases, the above rationalization in the design of press-fit piles becomes even more pronounced.

In the above-described configuration, the granules may be entirely composed of steel slag, or only a part thereof may be composed of steel slag and the rest may be composed of sand.

As the steel slag, blast furnace slag generated during pig iron production and steelmaking slag produced during the steelmaking process can be used.

The flowchart which showed the implementation procedure of the design method of the press-fitting pile in the liquefied ground which concerns on this embodiment. It is a figure which showed the press-fitting pile 1 to be designed together with the original ground 2 as the liquefied ground into which they are press-fitted, (a) is a vertical sectional view, and (b) is an arrow seen from the direction of AA. Vision.

Hereinafter, embodiments of a method for designing a press-fit pile in liquefied ground according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a flowchart showing a method of designing press-fit piles in the liquefied ground according to the present embodiment, and FIG. 2 shows the press-fit piles 1 to be designed together with the original ground 2 as the liquefied ground into which they are press-fitted. It is a vertical cross-sectional view shown, and the diameter and pitch of the press-fit pile 1 shown in the figure are calculated by the design method of the press-fit pile according to the present embodiment.

In order to calculate the diameter and pitch of the press-fit pile 1 using the press-fit pile design method according to the present embodiment, first, as shown in FIG. 1, the N value of the original ground 2 is set to N ₀ and the original ground 2 is compacted. The following equation, where the target N value in the inter-pile area 3 is N ₁ , and the reduction rate for the increase in N value according to the fine particle content of the original ground 2 is β.
N ₁ ′ = (N ₁ −N ₀ ) / β + N ₀ (1)
From this, the calculated N value N ₁ ′ when there is no fine particle size is obtained (step 101).

The N value N ₀ of the original ground 2 is acquired in advance by a ground survey, and the target N value N ₁ is appropriately set as a design condition for preventing or suppressing liquefaction in the inter-pile region 3.

The reduction rate β is calculated by the following equation, where F _c (%) is the fine particle content of the original ground 2.
β = 1.05-0.51 · logF _c
It may be calculated from.

Next, using the calculated N value N ₁ ′ obtained by Eq. (1), the gap ratio e ₁ after press fitting in the inter-pile region 3 is obtained (step 102).

The gap ratio e ₁ is based on the following equation, where the effective loading pressure is σ _v ′ (kN / m ² ), the maximum gap ratio before press-fitting in the original ground 2 is e _max , and the minimum gap ratio is e _min .
D _r1 = 21 · √ (N ₁ ′ / (0.7 + σ _v ′ / 98))
e ₁ = e _max − (D _r1 / 100) ・ (e _max −e _min )
The maximum gap ratio e _max and the minimum gap ratio e _min can be obtained from the following equations, respectively.
e _max = 0.02 · F _c +1.0
e _min = 0.008 ・ F _c +0.6
You can find it from.

Next, let e _{0 be} the gap ratio before press-fitting in the original ground 2, and the following equation
a _s = (e ₀ −e ₁ ) / (1 + e ₀ ) (2)
From calculates the improvement ratio a _s of pressed pile 1 (step 103).

The gap ratio e ₀ is calculated by the following equation.
D _r0 = 21 · √ (N ₀ / (0.7 ₊ σ _v ′ / 98))
e ₀ = e _max − (D _r0 / 100) ・ (e _max −e _min )
You can find it from.

Next, (2) the temporarily obtained improved rates a _s in formula, to correct the following procedure.

First, the N value N _p of the press-fit pile 1 is evaluated (step 104).

The N value N _p of the press-fit pile 1 is the same granular material as the press-fit pile to be constructed on the original ground 2 and has the same physical constants (for example, density and compaction) after being subjected to an indoor test in advance. Using the strength constants (internal friction angle φ and adhesive force C) obtained in the test, it may be appropriately determined from the conventionally known relationship between the internal friction angle φ and the N value or the relationship between the adhesive force C and the N value. ..

The particles constituting the press-fitting pile 1 are made of steel slag, particularly a high-strength slag material obtained by physically or chemically treating steelmaking slag, or a part thereof replaced with sand as a base material, and cement thereof. It is desirable to add and mix a system solidifying material and silica fume, and adjust the particle size according to the standards set by the government.

Then, by using the N _p and improvement ratio a _s, the following equation,
_{+ N T = a s · N} p (1-a s) · N 1 (3)
The N value N _T of the composite ground is calculated from (step 105).

Here, the composite ground is composed of the press-fitted pile 1 and the inter-pile area 3, and the N value thereof is only the N value after press-fitting in the inter-pile area 3 (target N value N ₁ ). Instead, the N value N _p of the press-fit pile 1 is reflected.

Next, from _NT , the target N value reduction coefficient R is calculated by the following equation.
R = N ₁ / _NT (4)
Calculate from (step 106).

Next, the target N value N ₁ is reduced by multiplying the calculated target N value reduction coefficient R by the target N value N ₁ (step 107).

Next, the R · N _1, as a new target N value in place of the target N value N ₁ (1) formula,
N ₂ ′ = (RN ₁ −N ₀ ) / β + N ₀
Substitute in to obtain N ₂ ′ (step 108).

Next, using N ₂ ′, the following equation,
D _r2 = 21 · √ (N ₂ ′ / (0.7 + σ _v ′ / 98))
e ₂ = e _max − (D _r2 / 100) ・ (e _max −e _min )
The gap ratio e ₂ after press fitting in the pile-to-pile region 3 is newly calculated from (step 109).

Next,
a _s2 = (e ₀ −e ₂ ) / (1 + e ₀ ) (2)
From this, the correction improvement rate a _s2 of the press-fitting pile 1 is calculated (step 110).

Once the correction improvement rate a _s2 is calculated, it is possible to obtain the diameter and pitch of the press-fit pile 1 by using it, for example, if the press-fit pile 1 is arranged in a square and its cross-sectional area is a fixed value. For example,
Pitch of pressed pile 1 = √ (cross-sectional area / a _s2 of the press pile 1)
You can find it from.

As described above, according to the method of designing the press-fit pile in the liquefied ground according to the present embodiment, the concept of the target N value reduction coefficient is newly introduced, and the target N value N with respect to the N value N _T of the composite ground is newly introduced. The ratio of ₁ is set as the target N value reduction coefficient R, and by multiplying this by the target N value N ₁ , the initial target N value N ₁ is reduced, so liquefaction prevention by compaction of the press-fit pile 1 itself is prevented. Contribution to the target N value reduction coefficient will be reflected, and thus the design of the press-fitting pile 1 can be rationalized.

Further, according to the method of designing the press-fit pile in the liquefied ground according to the present embodiment, since the press-fit pile 1 is composed of the steelmaking slag as the base material, the compaction strength of the press-fit pile 1 itself becomes higher. , The design of the press-fitting pile 1 can be performed more rationally.

In the present embodiment, the N value N _p of the press-fitting pile 1 is determined by a laboratory test, but instead of this, the same granules used in the main construction are subjected to the same press-fitting method prior to the main construction. Preliminary construction to create a press-fit pile by press-fitting into the original ground 2 may be performed, and a standard penetration test may be performed on the press-fit pile to obtain the result, or the diameter of the press-fit pile may be a fixed size (fixed value). ), If only the pitch is determined by the design, the preliminary construction is omitted, and N _p is obtained by performing a standard penetration test on the first press-fitting pile in the main construction, and calculated in the above-described embodiment below. The second and subsequent press-fitting piles 1 may be constructed according to the pitch.

1 Press-fitting pile 2 Original ground (liquefied ground)
3 Pile area

Claims (2)

Liquefied ground that compacts the area of the original ground that extends between the piles of the press-fitting pile by press-fitting granular materials such as sand and slag into the original ground to create a press-fitting pile in the original ground. The N value of the original ground was N ₀ , the target N value in the region when compacting the original ground was N ₁ , and the fine grain content of the original ground was determined. Let β be the reduction rate for the increase in N value,
N ₁ ′ = (N ₁ −N ₀ ) / β + N ₀ (1)
From, the calculated N value N ₁ ′ when there is no fine particle content is obtained, then the gap ratio e ₁ after press-fitting in the region is obtained using the N ₁ ′, and then the press-fitting in the original ground is performed. With the previous gap ratio as e ₀ ,
a _s = (e ₀ −e ₁ ) / (1 + e ₀ ) (2)
In the design method of the press pile in liquefaction ground for calculating an improved rate a _s of the press-pile from,
Evaluate the N value N _p of the press-fit pile and
Using the N _p and the improvement rate a _s , the N value N ₂ of the composite ground composed of the press-fitted pile and the region is calculated by the following equation.
_{N 2 = a s · N p} + (1-a s) · N 1 (3)
Then, from the N ₂ , the target N value reduction coefficient R is obtained from the following equation.
R = N ₁ / N ₂ (4)
From calculated, the R · N ₁ made by multiplying the R to the N _1, by calculating the target N value N as a new target N value in place of ₁ (1) and (2), wherein the improvement design method for press-pile in liquefaction soil and calculates a correction improvement ratio a _'s in place of the rate a _s. The method for designing a press-fit pile in liquefied ground according to claim 1, wherein at least a part of the granules is composed of steel slag.

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