JP7447652B2 - Evaluation method of pull-out resistance - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for evaluating a pull-out resistance force for evaluating the pull-out resistance force of a knot portion that is borne when a knotted pile is pulled out.

BACKGROUND ART Conventionally, knotted piles have often been used as foundation piles to accommodate larger and higher-rise buildings, as they can ensure high vertical bearing capacity while suppressing increases in pile length. For example, as shown in Patent Document 1, a knotted pile has an enlarged bottom at the lower end of the shaft and one or more knots at the middle of the shaft in order to increase the vertical supporting force. It is being

Patent No. 4856903

The knots provided in the knotted pile can bear not only the vertical support force but also the pull-out resistance force, but the pull-out resistance force that can be borne differs depending on the embedment length. Specifically, when a predetermined pulling force is applied to a knotted pile, if the knot has sufficient penetration length, or if the knot is entirely buried in a hard intermediate layer or support layer, etc. If the joint is in the ground, it continues to resist the pull-out force in the ground and bears the pull-out resistance force for a long period of time.

On the other hand, when the penetration length of the joint is shallow, although the joint resists the pull-out force in the ground, a fracture surface reaching the ground surface from the joint is generated in the ground, and the pull-out resistance is lost. In this way, knots with a shallow penetration length are often not reflected in the design, even though they have pull-out resistance until a fracture surface occurs in the ground.

For this reason, when knotted piles are required to have pull-out resistance, regardless of their size, the knots should be designed at a depth that ensures sufficient penetration length, in order to improve workability and economy. This resulted in an unfavorable design plan.

The present invention has been made in view of such problems, and its main purpose is to improve the pull-out resistance of the joints in cast-in-place concrete knotted piles. provide an evaluation method.

In order to achieve such an object, the method for evaluating the pullout resistance of the present invention includes evaluating the pullout resistance of the knots during pulling out of a knotted pile that includes a shaft portion and a knot portion provided on the shaft portion. A method for evaluating pull-out resistance for the purpose of A verification step of comparing the range of influence of stress and the penetration length of the joint to verify whether the range of influence of underground stress reaches the ground surface; In the case where the joint reaches the ground surface, the pull-out resistance force of the joint is determined by the weight of the soil clod between the fracture surface that occurs from the joint to the ground surface and the shaft of the knotted pile, the weight of the soil mass, and the mass of the fracture surface. An evaluation step of calculating based on shear resistance.

Further, in the evaluation method of the pullout resistance force of the present invention, in the evaluation step, a pullout resistance force calculated assuming that the fracture surface has a cylindrical shape and a pullout resistance force calculated assuming a cone shape are compared, It is characterized in that the smaller numerical value is adopted as the pull-out resistance force of the knot .

According to the pull-out resistance evaluation method of the present invention, it is verified whether or not the range of influence of underground stress reaches the ground surface based on the penetration length of the knot and the range of influence of underground stress. The pull-out resistance of the knot can be appropriately evaluated based on the following. This makes it possible to reflect not only the vertical support force but also the pull-out resistance force in the design of joints with a shallow penetration length, where pull-out resistance force has not been considered in the design in the past.

In addition, when knots are provided for the purpose of resisting the pull-out force that acts on a knotted pile, the penetration length of the knot can be set according to the required pull-out resistance force, which improves safety. This makes it possible to design rational knotted piles that are both economical and economical.

According to the present invention, it is possible to appropriately evaluate the pull-out resistance of a joint provided in a cast-in-place concrete joint pile.

It is a figure showing an outline of a knotted pile in an embodiment of the present invention. It is a figure which shows the detail of the joint part provided in the knotted pile in embodiment of this invention. It is a figure showing the knotted pile in which the upward truncated conical part of the knot is embedded in the surface layer according to an embodiment of the present invention. FIG. 3 is a diagram showing the penetration length of a joint and the influence range of underground stress in an embodiment of the present invention. It is a figure showing the flow of the evaluation method of the pull-out resistance force in a joint part in an embodiment of the present invention. It is a figure which shows the state of the pulling-out experiment of the model pile in embodiment of this invention. It is a graph which shows the relationship between the pile head displacement and the pile head load obtained from the pull-out experiment of the model pile in embodiment of this invention. It is a figure which shows the pull-out resistance mechanism and destruction form of the knotted pile in embodiment of this invention. It is a graph which shows the relationship between (length Ls affected by underground stress/protrusion width Dn) and diameter expansion ratio Dc in an embodiment of the present invention. It is a figure showing the clod of soil formed right above a node when a fracture surface occurs in the ground in an embodiment of the present invention.

The present invention evaluates the pull-out resistance force exerted by the knots when the knotted pile is pulled out, with respect to cast-in-place concrete knotted piles. The details of the method for evaluating the pull-out resistance of knotted piles will be explained below with reference to FIGS. 1 to 10.

As shown in FIGS. 1(a) and 1(b), the knotted pile 1 that supports the building has a pile length that reaches the support layer G3, and has a shaft portion 2 and an expanded bottom portion provided at the lower end of the shaft portion 2. 3, and a joint portion 4 provided at the intermediate portion of the shaft portion 2.

As shown in FIG. 2, the joint portion 4 includes a cylindrical portion 41 having a larger diameter than the shaft portion 2, an upward truncated conical portion 42 located above the cylindrical portion 41, and a downward conical portion located below the cylindrical portion 41. It has a shape that combines the base portion 43.

As shown in FIGS. 1(a) and 1(b), the knot portion 4 having such a shape has at least the downward truncated conical portion 43 rooted in the supporting layer G3 or an intermediate layer G2 such as a gravel layer, and is a knot. When a pushing force is applied to the attached pile 1, the expanded bottom portion 3 and the knot portion 4 resist this force. On the other hand, when a pullout force is applied to the knotted pile 1, the pullout resistance mechanism differs depending on how the upward truncated conical portion 42 of the knotted portion 4 is buried in the ground.

For example, in a state where the upward truncated conical portion 42 of the knot 4 is provided on the surface layer G1 as shown in FIG. 3, if the penetration length H of the knot 4 is sufficiently secured, the The joint portion 4 always resists the pulling force in the ground without causing any damage to the ground.

When the penetration length H of the joint 4 is sufficiently secured, as shown in FIG. , refers to the case where the influence range A of underground stress falls within the ground. Note that the range A of influence of underground stress refers to the range in which the bearing effect is exerted when the upward truncated conical portion 42 presses the ground.

On the other hand, if the penetration length H of the knot 4 is not sufficient, that is, if the influence range A of the underground stress does not fit within the ground as shown in Fig. 4(b), the knot 4 will be pulled out in the ground. Although it resists the force, as shown in FIGS. 10(a) and 10(b), a fracture surface Fs reaching the ground surface is generated in the ground, and the resisting force is lost. In other words, even if the joint portion 4 does not have a sufficient penetration length H, it bears the pull-out resistance force until the fracture surface Fs occurs in the ground.

<Extraction experiment using model pile>
The pull-out resistance mechanism of the knots 4 when a pull-out force is applied to the above-mentioned knotted pile 1 (whether or not the range of influence of underground stress reaches the ground surface) and the knots when the penetration length H is shallow. In order to confirm the fracture form (shape of fracture surface Fs) occurring in the surrounding ground of section 4, the following experiment was conducted.

As shown in FIGS. 6(a) to 6(c), a centrifugal force model experiment was conducted using a half-split model pile 1' to confirm the resistance mechanism of the joint 4' during extraction. Dry Toyoura sand was used as the ground material, and a model ground G' with a predetermined relative density was created in a container.

On the other hand, with reference to FIG. 2, the dimensions of the model pile 1' were as follows: shaft diameter D0 = 48 mm, node diameter D = 68 mm, node protrusion width Dn = 10 mm, and upper inclination angle θn = 20°. This model pile 1' was penetrated into the model ground G' at three penetration lengths H (Case 1 to Case 3) as shown in FIGS. 6(a) to 6(c).

Case 1 corresponds to a case where the penetration length H is set to 20Dn, and the penetration length H of the joint portion 4 is sufficient (comparative example). On the other hand, Case 2 and Case 3 correspond to the case where the penetration length H of the joint portion 4 is shallow; in Case 2, the penetration length H is set to 12Dn, and in Case 3, the penetration length H is set to 6Dn. In addition, a displacement meter DG is installed at the top of the model pile 1'.

After burying such a model pile 1' in the model ground G', a pulling force was applied to the model pile 1', and from the measurement results of the displacement meter DG, the displacement of the pile head and the pile as shown in Fig. 7 were obtained. The relationship of head load was obtained. In addition, FIGS. 8(a) to 8(c) are contours representing displacements in the ground around the model pile 1' for these Cases 1 to 3 through image analysis.

In Case 1 in which the knot 4 in FIG. 8(a) is fully embedded, no fracture surface that reaches the ground surface is observed in the model ground G' near the knot 4. Looking at FIG. 7, it can be seen that as the pile head displacement increases, the pile head load also increases, and the joint portion 4 constantly resists the pulling force.

In both Cases 2 and 3 in which the penetration length H in FIGS. 8(b) and 8(c) is insufficient, a fracture surface Fs reaching the ground surface is formed in the model ground G' near the joint 4. Looking at Figure 7, in Case 2 where the penetration length H is shallower than in Case 1, the pile cap load gradually decreases as the pile cap displacement increases, and the joint 4 gradually loses its ability to resist the pulling force. I can see how it goes.

Furthermore, in Case 3 where the penetration length H is the shallowest, the pile cap load decreases when the pile cap displacement is small, and then levels off. In other words, it can be seen that the knot 4 loses its ability to resist the pulling force at an early point after the pulling force is applied to the model pile 1'.

Furthermore, looking at the failure form when the penetration length H is shallow, in Case 2, a cylindrical failure surface Fs is generated in the model ground G' near the joint 4. In Case 3, it can be seen that a cone-shaped fracture surface Fs is generated in the model ground G' near the node 4'.

Based on the above experimental results, we verified the pull-out resistance mechanism of the knot A when the knotted pile 1 is pulled out, and calculated the pull-out resistance of the knot 4 using the optimal evaluation formula selected based on the verification results. I decided to evaluate it.

≪≪Evaluation method of pull-out resistance force≫≫
Below, the procedure for evaluating the pull-out resistance force due to the knots 4 provided on the knotted pile 1 will be explained with reference to the flowchart of FIG. 5.

≪Check dimensions of knotted pile: STEP 1≫
First, check the dimensions of the knotted pile 1. Specifically, as shown in FIG. 2, check the shaft diameter D0, knot diameter D, protrusion width Dn of the knot 4 with respect to the shaft 2, knot upper angle θn, and penetration length H of the knot 4. do.

Here, the protrusion width Dn is 1/2 of the difference between the knot diameter D and the shaft diameter D0, and the knot upper angle θn is the upward truncated conical part when the knotted pile 1 is vertically halved. 42 and the outer circumferential surface of the shaft portion 2. Further, it is preferable to calculate the diameter expansion ratio Dc, which is calculated by dividing the node diameter D by the shaft diameter D0.

≪Estimating and verifying the range of influence of underground stress: STEP 2≫
Next, in order to estimate the pull-out resistance mechanism of the knot 4 when a pull-out force is applied to the knotted pile 1, we will examine the influence range A of underground stress in the surrounding ground of the knot 4 and the embedment length H of the knot 4. , and verify whether the influence range A of underground stress is within the ground.

<Method for estimating the length Ls affected by underground stress>
In this embodiment, the length Ls affected by underground stress is used for comparison. The underground stress influence length Ls corresponds to the longest part in the vertical direction in the underground stress influence range A, as shown in FIGS. 4(a) and 4(b). Regarding the length Ls affected by underground stress, the inventor calculated the difference between (length Ls affected by underground stress/protrusion width Dn) and the diameter expansion ratio Dc based on the following analysis, as shown in FIG. 9(a). We found that there is a relationship as shown in

The analysis assumes that four types of knotted piles 1 with different diameter expansion ratios D0 are buried in consolidated silt, as shown in Fig. 9(b), and the behavior of the knotted piles 1 when pulled out is analyzed. We conducted a simulation analysis using FEM. It is assumed that the ultimate degree of frictional force of the inclined surface of the upward truncated conical portion 42 is infinite.

First, a pulling force was applied to the knotted pile 1, and the vertical stress distribution of the ground generated in the ground around the knot 4 when the displacement of the knotted pile 1 reached 10% of the knot diameter D was determined. Next, based on this distribution, the distance to the position where the vertical stress decreases to 10% of the bearing pressure strength of the node 4 was determined, and this distance was defined as the underground stress influence length Ls.

As a result, it was found that the larger the diameter expansion ratio Dc, the larger the loading area of the joint 4 on the ground, and the wider the influence range A of underground stress. Therefore, when we plotted the relationship between the length Ls affected by underground stress divided by the protrusion width Dn and the diameter expansion ratio Dc, we found that the length Ls affected by underground stress It has been found that the value obtained by dividing by the protrusion width Dn decreases as the diameter expansion ratio Dc increases, and converges to a certain constant value.

Based on this analysis result, regression analysis is performed to create an approximate curve, and by substituting the protrusion width Dn and diameter expansion ratio Dc of the knotted pile 1 confirmed in STEP 1 to this neighborhood curve, each knotted pile 1 is It was decided to estimate the length Ls affected by the corresponding underground stress.

The influence length Ls of the underground stress estimated in this way is compared with the penetration length H of the joint 4 to verify whether the influence range A of the underground stress falls within the ground.

≪Evaluation process of pull-out resistance: STEP 3-1≫
As shown in FIG. 4(a), when the penetration length H is longer than the underground stress influence length Ls, the influence range A of underground stress falls within the ground. In other words, when the knotted pile 1 is pulled out and the knotted portion 4 resists this, it can be estimated that no damage will occur in the ground near the knotted portion 4 that reaches the ground surface from the knotted portion 4.

Therefore, the pull-out resistance force F (Fa) of the knot 4 may be calculated by appropriately employing an evaluation formula that has conventionally been used when the penetration length H of the knot 4 is sufficiently secured. Note that the pull-out resistance force F (Fa) can be calculated, for example, using the following equation (1).

Equation (1) given as an example assumes that a cylindrical sheared surface is formed vertically upward from the outermost edge of the cylindrical portion 41 in the joint portion 4. This method evaluates the pull-out resistance force F (Fa) borne by the knot 4 based on the ultimate shear resistance of the ground near the shear plane. The details of equation (1) are given in Japanese Patent No. 4856903.

Fa=f _si ×A _NP・・・・・・・・・・・・・・・・・・・・・(1)
A _NP : Area of cylindrical surface receiving shear force (m ² )
f _si : Ultimate shear resistance of the ground near the shear plane (kN/m ² )

≪Evaluation process of pull-out resistance: STEP 3-2≫
On the other hand, when the underground stress influence length Ls is longer than the penetration length H, the underground stress influence range A does not fall within the ground and reaches the ground surface. That is, when the knotted pile 1 is pulled out, when the knotted portion 4 resists this, it can be estimated that a fracture surface Fs that reaches the ground surface from the knotted portion 4 is generated in the ground near the knotted portion 4.

Therefore, the pull-out resistance force F of the joint 4 is the sum of the weight of the resistance soil clod formed on the joint 4 due to the formation of the fracture surface Fs, and the shear resistance force of the resistance soil mass and the fracture surface Fr. . By the way, in the pull-out experiment using the model pile described above, when the fracture surface Fs that reaches the ground surface from the node 4 occurs, the fracture form is cylindrical or conical, as shown in FIGS. 6(b) and 6(c). There are two ways.

Therefore, when the influence range A of the underground stress reaches the ground surface, the pull-out resistance force F is as shown in FIG. 10(a). Calculations are performed for each of the cases in which a broken soil mass C2 is formed and a cone-shaped broken soil mass C2 as shown in FIG. 10(a) is formed. Note that the cone-shaped fractured soil mass C2 is a soil mass surrounded by the fracture surface Fr, the shaft portion 2, and the upward truncated conical portion 42 of the joint portion 4.

The pulling resistance force F (Fr) when assuming a cylindrical broken earth mass C1 based on the node diameter D can be calculated using the following equation (2). Moreover, the pull-out resistance force F (Fc) when assuming a cone-shaped broken soil mass C2 can be calculated using the following equation (3).

Fr=Wg+Rf・・・・・・・・・・・・・・・・・・・・・(2)
Wg: Weight of cylindrical soil just above the joint (kN)
Rf: Shear resistance force of cylindrical surface during cylindrical fracture (kN)

Fc=Wg+Fv・・・・・・・・・・・・・・・・・・・・・(3)
Wg: Weight of cone-shaped soil (kN)
Fv: Vertical component of shear resistance force of cone-shaped fracture surface (kN)

In this way, the pull-out resistance force F (Fr) calculated by the above formula (2) is compared with the pull-out resistance force F (Fc) calculated by the formula (3), and the smaller value is determined as the pull-out resistance force of the knotted pile 1. Adopted as F.

As mentioned above, based on the penetration length H of the joint 4 and the influence range A of the underground stress, it is verified whether or not the influence range A of the underground stress reaches the ground surface, and based on the verified result, the joint The pull-out resistance force F of No. 4 can be appropriately evaluated. This makes it possible to reflect not only the vertical support force but also the pull-out resistance force in the design of the shallow joint portion 4 with the penetration length H, for which the pull-out resistance force F has not been considered in the design in the past.

In addition, when the knot 4 is provided for the purpose of resisting the pull-out force acting on the knotted pile 1, the penetration length H of the knot 4 can be adjusted depending on the magnitude of the required pull-out resistance force F. It becomes possible to design a rational knotted pile 1 that is both safe and economical.

It goes without saying that the method for evaluating the pull-out resistance of the present invention is not limited to the above-described embodiments, and that various changes can be made without departing from the spirit of the present invention.

For example, in the present embodiment, the knot portion 4 has a shape that is a combination of a cylindrical portion 41, an upward truncated conical portion 42, and a downward truncated conical portion 43. However, the shape is not limited to this, and any shape of the joint portion 4 can be employed.

In addition, in this embodiment, when estimating the length Ls affected by underground stress, an experiment is conducted using the case where the ground is solidified silt, and an approximate curve related to the relationship with the diameter expansion ratio Dc is calculated. Indicated. These approximate curves may be created by suitably conducting similar experiments in accordance with the type of ground on which the joint portion 4 is to be provided. In this way, even when the ground is not solidified silt, it is possible to estimate the length Ls affected by underground stress.

1 Knotted pile 2 Shaft part 3 Expanded bottom part 4 Knot part 41 Cylindrical part 42 Upward truncated conical part 43 Downward truncated conical part G1 Surface layer G2 Intermediate layer G3 Support layer C1 Destruction clod (cylindrical shape)
C2 Destruction clod (cone shape)
1' Model pile 2' Shaft 4' Joint G' Model ground

Claims (2)

A method for evaluating the pulling resistance of a knotted pile including a shaft and a joint provided on the shaft, the method comprising:
an estimation step of estimating the range of influence of underground stress in the ground surrounding the joint when a pull-out force is applied to the joint;
a verification step of comparing the range of influence of the underground stress and the penetration length of the joint, and verifying whether the range of influence of the underground stress reaches the ground surface;
When the range of influence of the underground stress reaches the ground surface, the pull-out resistance of the knot is determined by the weight of the soil mass between the fracture surface that occurs from the knot to the ground surface and the shaft of the knotted pile. and an evaluation step of calculating based on the weight of the soil clod and the shear resistance of the fracture surface,
A method for evaluating pull-out resistance, comprising: In the method for evaluating pullout resistance according to claim 1,
In the evaluation step, the pull-out resistance force calculated assuming that the fracture surface has a cylindrical shape is compared with the pull-out resistance force calculated assuming a cone shape, and the smaller value is determined as the pull-out resistance force of the knot. A method for evaluating pull-out resistance, which is characterized in that it is adopted .

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