Evaluation method of pile enlarged part

Abstract

To provide an evaluation method of a pile enlarged part applicable to prediction of pull-out resistance force of a cast-in-place concrete pile and displacement generated to a structure associated with pull-out force for the cast-in-place concrete pile having an enlarged part.SOLUTION: An evaluation method of a pile enlarged part evaluates pull-out resistance-displacement character of an enlarged part 3 of a cast-in-place concrete pile 1 on which pull-out force acts through a prediction curve by hyperbolic curve function or a predication curve by exponential function. The prediction curve by the hyperbolic curve function is applied when a tilt angle θ of an upper surface of the enlarged part 3 to a shaft 2 of the cast-in-place pile 1 is equal to or less than 20°, and the prediction curve by the exponential function is applied when the tilt angle θ is more than 20, for a case where the enlarged part 3 expands its diameter as going downward.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating an enlarged portion of a cast-in-place pile on which a pull-out force acts. [Background technology]

[0002] When column spans are increased to build taller buildings or to create more interior space, the axial force per column can increase. As the axial force of columns increases, the load acting on piles also increases. Therefore, the lower end of a cast-in-place concrete pile can be enlarged to increase the bearing area against the supporting ground and thereby increase bearing capacity. For example, Patent Document 1 discloses an expanded diameter pile in which enlarged sections with a larger diameter than the shaft of the pile are formed at multiple locations along the pile's length.

[0003] In structures with pile foundations, there is a risk of pull-out forces acting on the piles due to shaking during earthquakes, buoyancy, etc. In particular, if an enlarged section is provided on the pile, the pull-out resistance force increases dramatically. Therefore, in structures where pull-out forces act on the piles, it is necessary to design the structure to ensure safety by predicting the pull-out resistance force of the pile itself and the deformation that occurs in the structure due to the pull-out force.

[0004] Here, for piles without enlarged sections, a method for predicting the relationship between pull-out resistance and pull-out amount (displacement amount) has been proposed in the Architectural Institute's "Architectural Foundation Design Guidelines 2019." On the other hand, for piles with enlarged sections, there is no proposal for a method for predicting the relationship between pull-out resistance and pull-out amount. Additionally, one method that can be considered is to predict the relationship between the pull-out resistance force of the expanded section and the amount of pull-out using numerical analysis such as FEM, but numerical analysis that can take into account the effects of the construction process of cast-in-place piles is difficult, and no practical method has been established. Furthermore, for example, Patent Document 2 discloses a method for calculating the pull-out resistance of a cast-in-place pile with an enlarged portion, but does not predict the deformation that occurs in the structure due to the pull-out force. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-138561 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-174251 Summary of the Invention [Problem to be solved by the invention]

[0006] The objective of the present invention is to propose an evaluation method for the enlarged portion of a cast-in-place concrete pile that can be applied to predict the pull-out resistance of the cast-in-place concrete pile itself and the amount of displacement that occurs in the structure due to the pull-out force. [Means for solving the problem]

[0007] To solve the above problems, the present invention provides a method for evaluating the enlarged portion of a pile, which is subject to a pull-out force, by using a prediction curve based on a hyperbolic function or a prediction curve based on an exponential function to evaluate the pull-out resistance force-displacement characteristics of the enlarged portion of a cast-in-place concrete pile. For an enlarged portion having an inclined portion at its upper end that increases in diameter as it goes downward, the prediction curve based on the hyperbolic function is preferably used when the inclination angle θ of the upper surface of the enlarged portion relative to the pile axis is 20° or less, and the prediction curve based on the exponential function is preferably used when the inclination angle θ is greater than 20°. Furthermore, the prediction curve based on the hyperbolic function is preferably Equation 1, and the prediction curve based on the exponential function is preferably Equation 2. The coefficients C1 and C2 in Equation 1 are linear functions of the inclination angle θ. The coefficient α in Equation 2 is a constant, and the order n is an exponential function of the inclination angle θ.

[0008]

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[0009] This method for evaluating enlarged pile sections enables simple and accurate design of structures that use cast-in-place concrete piles with enlarged sections as pull-out resistance members. Specifically, predicting the pull-out resistance force (load)-displacement characteristics and using this to calculate buoyancy countermeasures and the stress and displacement acting on the structure during an earthquake makes it possible to design structures that ensure safety. In addition, the pull-out amount or ground spring value according to the load level (long-term, during an earthquake) can be easily calculated (for example, by hand). [Effects of the Invention]

[0010] According to the pile enlargement evaluation method of the present invention, it is possible to predict the pull-out resistance force of a cast-in-place concrete pile having an enlargement, as well as the amount of displacement that occurs in the structure due to the pull-out force. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a side view showing an enlarged portion of a site entrance concrete pile. [Figure 2] FIG. 1 is a diagram showing the relationship between the ground conditions and the test piles in the comparative example and examples 1 and 2. [Figure 3] FIG. 10 is a diagram showing the relationship between the ground conditions and the test piles in Examples 3 and 4. [Figure 4] 1 is a graph showing the relationship between the pull-out load measured in a pull-out test and the displacement of the pile head. [Figure 5] 1 is a graph showing the distribution of pull-out resistance force of a test pile having an enlarged portion, where (a) is Example 1, (b) is Example 2, (c) is Example 3, and (d) is Example 4. [Figure 6] 1 is a graph showing the relationship between the normalized pull-out resistance force and the normalized displacement for Examples 1 to 4 and the comparative example. [Figure 7] FIG. 10 is an explanatory diagram showing the dimensions of the enlarged portion. [Figure 8] 1 shows plots showing the relationship between the orthogonal pull-out load and the orthogonal displacement for Examples 1 to 4, and an approximation curve using a hyperbolic function. [Figure 9]FIG. 10(a) is a diagram showing the relationship between the coefficient C1 and the tilt angle, and FIG. 10(b) is a diagram showing the relationship between the coefficient C2 and the tilt angle. [Figure 10] 10 is a graph showing a prediction curve calculated using a hyperbolic function. [Figure 11] 1 shows plots showing the relationship between the orthogonal pull-out load and the orthogonal displacement for Examples 1 to 4, and an approximation curve using an exponential function. [Figure 12] 10A is a diagram showing the relationship between the coefficient α and the tilt angle θ, and FIG. 10B is a diagram showing the relationship between the order n and the tilt angle θ. [Figure 13] 1 is a graph showing a predicted curve calculated using an exponential function. DETAILED DESCRIPTION OF THE INVENTION

[0012] FIG. 1 shows a cast-in-place concrete pile 1. In this embodiment, a method for evaluating an enlarged portion 3 when a pull-out force acts on the cast-in-place concrete pile 1 will be described. The cast-in-place concrete pile 1 has an enlarged portion 3 (expanded bottom portion) at the lower end of the shaft portion 2. The upper end of the enlarged portion 3 has an outer diameter D equal to the outer diameter D of the shaft portion 2. The enlarged portion 3 has an outer diameter D that decreases from the upper end downward. e In this embodiment, the expanded portion 3 is located in sandy soil.

[0013] The evaluation of the enlarged portion 3 is carried out based on the pull-out resistance force-displacement characteristics of the enlarged portion 3 of the cast-in-place concrete pile 1 to which the pull-out force acts. The pull-out resistance force-displacement characteristics of the enlarged portion 3 are evaluated using a prediction curve based on a hyperbolic function or a prediction curve based on an exponential function. When the inclination angle θ of the top surface of the enlarged portion 3 relative to the pile axis is small (for example, 20° or less), the prediction curve based on a hyperbolic function (Equation 1) is used, and when the inclination angle θ is large (for example, more than 20°), the prediction curve based on an exponential function (Equation 2) is used.

[0014]

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[0015] Note that coefficients C1 and C2 in Equation 1 are linear functions of the tilt angle θ, and are expressed by Equations 3 and 4 in this embodiment. Coefficient α in Equation 2 is a constant and has a value expressed by Equation 5. Furthermore, degree n is an exponential function of the tilt angle θ, and is expressed by Equation 6 in this embodiment. C1=0.0008θ-0.057...Equation 3 C2=-0.0083θ+1.056...Equation 4 α=0.143...Equation 5 n=16800·θ 2.5 ...Formula 6

[0016] The pile enlargement evaluation method of this embodiment enables simple and accurate design of structures that use cast-in-place concrete piles 1 with enlarged sections 3 as pull-out resistance members. Specifically, predicting the pull-out resistance force (load)-displacement characteristics and using this to calculate buoyancy countermeasures and the stress and displacement acting on the structure during an earthquake makes it possible to design structures that ensure safety. In addition, the pull-out amount or ground spring value according to the load level (long-term, during an earthquake) can be easily calculated, for example, by hand. Moreover, since the prediction formula is based on the diameter and scale of a general enlarged section 3, it is highly versatile.

[0017] Below are the results of an experiment conducted to confirm the relationship between the pull-out resistance of the enlarged portion 3 of a cast-in-place concrete pile 1 in sandy soil and the inclination angle θ. In this experiment, an in-situ pull-out load test was conducted on several enlarged portions 3 with different upper inclination angles θ. As a result, it was confirmed that the pull-out resistance changes depending on the inclination angle θ, that the relationship between the bearing capacity coefficient for the pull-out resistance and the inclination angle θ can be expressed as an exponential function, and that the relationship between the orthogonalized pull-out resistance and the orthogonalized displacement can be approximated by a hyperbolic function when the inclination angle θ is small (θ≦20°) and by an exponential function when the inclination angle θ is large (θ>20°).

[0018] Table 1 shows the test piles on which the tests were conducted. As shown in Table 1, pull-out load tests were conducted on test piles with enlarged sections with inclination angles θ of 12°, 17°, 21°, and 30°. As a comparative example, pull-out load tests were also conducted on test piles without enlarged sections. The pull-out load tests were conducted using a step-loading method in accordance with JGS1813-2002 ("Methods and Commentary for Vertical Load Tests on Piles," Geotechnical Society of Japan, 2002).

[0019] [Table 1]

[0020] Figures 2 and 3 show the measurement locations of the test piles. The ground at the test site was mainly composed of alluvial deposits, and the enlarged section was installed in a stratum 10 to 20 m below the ground surface. The pull-out resistance of the sandy soil was measured. The measurement items were the pile head load, the displacement of the pile head and enlarged section, and the strain of the main reinforcement. Measurements were performed using multiple strain gauges installed in the shaft and enlarged section of the test pile, and an internal displacement meter installed in the enlarged section, as shown in Figures 2 and 3. As shown in Figures 2 and 3, in Examples 1 to 3, the pull-out resistance of the enlarged section (enlarged base section) formed at the tip of the test pile was measured. The test pile in Example 4 was a multi-stage enlarged pile with an enlarged section (enlarged base section) formed at the tip and an enlarged base section (intermediate enlarged diameter section) formed in the middle. In Example 4, the pull-out resistance of the intermediate enlarged diameter section was measured.

[0021] The pull-out force P is calculated from the strain measurement value inside the pile body, and was estimated using Equation 7, referring to "Technology and Current Status of High-Strength Concrete" (Architectural Institute of Japan, 2009). The tensile strength of concrete σ t is evaluated using Equations 8 to 10 depending on the magnitude of the strain. P=σ t A c +E s A s ε Equation 7 where σ t : Tensile strength of concrete ε: Measured strain A c:Concrete cross-sectional area E s : Young's modulus of main reinforcement A s : Cross-sectional area of ​​main reinforcement

[0022]

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[0023] Figure 4 shows the relationship between the pull-out load P0 and the pile head displacement δ0 measured in the pull-out test. As shown in Figure 4, the relationship between the pull-out load and displacement for the test piles (Examples 1 and 2) with a small inclination angle θ of the enlarged section tends to decrease slightly after reaching the maximum tensile resistance. Similarly, the pull-out load for the comparative example without an enlarged section also tended to decrease after reaching the maximum tensile resistance. On the other hand, for the test piles (Examples 3 and 4) with a large inclination angle θ, the pull-out load continued to increase.

[0024] Figures 5(a) to (d) show the pull-out resistance distribution of the test piles in Examples 1 to 4. The pull-out resistance was estimated from the strain measurement results using Equations 7 to 10. As shown in Figures 5(a) to (d), it was confirmed that the pull-out resistance changed significantly in the enlarged section.

[0025] FIG. 6 shows the relationship between the normalized pull-out resistance force and the normalized displacement amount in Examples 1 to 4 and the comparative example. b is the pull-out resistance of the expansion section calculated from Figure 4, A t is the lateral area of ​​the inclined surface at the top of the enlarged section shown in Figure 7, δ b is the vertical displacement of the expansion, D e is the diameter of the enlarged portion. Note that the curve for Example 2 in FIG. b / D e The pull-out resistance of the comparative example was calculated from the results of measuring the friction of the shaft at the same depth as the enlarged portion of Examples 1 and 2.

[0026] As shown in Figure 6, the curves of the relationship between pull-out resistance and displacement in Examples 1 to 3 show that the pull-out resistance value remains constant as the displacement increases. On the other hand, in Example 4, where the inclination angle θ is large, the pull-out resistance continues to increase even as the displacement increases. These results confirm that the pull-out resistance increases as the inclination angle θ increases, that is, the inclination angle θ affects the pull-out behavior.

[0027] Figure 8 shows the normalized load and normalized displacement δ b / D e The relationship between A and B is shown. w is the projected area of ​​the inclined portion of the upper part of the enlarged section onto the horizontal plane, as shown in Figure 7. (P b / A w ) u is 0.1δ b / D e P in b / A w As shown in Figure 8, the initial stiffness of the test pile gradually decreased as the inclination angle θ increased. The solid line in Figure 8 is a curve approximating the test results of Examples 1 to 4 using the hyperbolic function of Equation 1. As shown in Figure 8, Examples 1 and 2 almost matched the approximate curve. On the other hand, Examples 3 and 4 deviated from the approximate curve. Note that coefficients C1 and C2 are coefficients obtained by curve fitting of the results of the in-situ pull-out load test. The relationship between coefficients C1 and C2 and the inclination angle θ generally changes linearly, as shown in Figure 9, and can be expressed by Equations 3 and 4.

[0028]

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[0029] Figure 10 shows the prediction curves based on equations 1, 3, and 4. The four prediction curves shown in Figure 10 are, from top to bottom, the results of equations 3 and 4 with the inclination angle θ set to 10°, 12°, 15°, and 20°. The prediction curves shown in Figure 9 are similar to the approximation curves created from the test results in Figure 8. Therefore, it was confirmed that when the inclination angle θ is between 10° and 20°, it is possible to evaluate the enlarged portion by using the predicted curve based on the hyperbolic function of Equation 1 and calculating the coefficients C1 and C2 using Equations 3 and 4.

[0030] Fig. 11 shows an approximation curve based on the exponential function shown in Equation 2, and plots of the test results for Examples 1 to 4. When the test results were approximated using Equation 2, the results for Examples 3 and 4 were nearly identical, as shown in Fig. 11. For sand, a prediction curve was estimated using Equation 2 with a coefficient α of 0.23 and an order n of 2.70 (dashed line in Fig. 11). The prediction curve nearly matched the test results (approximation curve) for Example 4.

[0031]

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[0032] Figure 12(a) shows the relationship between the coefficient α and the tilt angle θ, and Figure 12(b) shows the relationship between the order n and the tilt angle θ. In this test, a clear correlation between the coefficient α and the tilt angle θ could not be confirmed, so the average value was used for the coefficient α (Equation 5). As shown in Figure 12(b), the value of the order n decreases rapidly and nonlinearly as the tilt angle θ increases. For the order n, an exponential function was used, which provides a relatively good approximation, as shown in Equation 6. α=0.143...Equation 5 n=16800·θ 2.5 ...Formula 6

[0033] The solid lines in Figure 13 show the prediction curves obtained by substituting Equations 5 and 6 into Equation 2. The five prediction curves (solid lines) shown in Figure 13 are, from top to bottom, the results of Equation 6 with inclination angles θ of 10°, 15°, 20°, 25°, and 30°. The prediction curve with an inclination angle θ of 30° was similar to the prediction curve (dashed line) estimated using Equation 2 for sand with coefficient α = 0.23 and order n = 2.70. Therefore, when the tilt angle θ is large, it was confirmed that it is possible to evaluate the enlarged portion by using the predicted curve based on the exponential function of Equation 2 and calculating the coefficient α and order n using Equations 5 and 6.

[0034] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and each of the above-described components can be appropriately modified within the scope of the invention. The magnitude of the inclination angle θ of the expanded portion 3 is not limited, and may be, for example, 30° or more. In the above embodiment, a prediction curve based on a hyperbolic function is used when the inclination angle θ of the upper surface of the enlarged portion relative to the axis of the pile is 20° or less, and a prediction curve based on an exponential function is used when the inclination angle θ is more than 20°. However, the boundary value of the inclination angle θ when switching between the prediction curve based on the hyperbolic function and the prediction curve based on the exponential function is not limited to 20°. [Explanation of symbols]

[0035] 1. Cast-in-place concrete piles 2 Shaft 3 Enlarged section

Claims

1. A method for evaluating an enlarged portion of a pile, characterized in that the pull-out resistance force-displacement characteristics of the enlarged portion of a cast-in-place concrete pile, to which a pull-out force is applied, are evaluated using a prediction curve based on a hyperbolic function or a prediction curve based on an exponential function.

2. The enlarged portion has an inclined portion whose diameter increases downward, 2. The pile enlargement evaluation method according to claim 1, characterized in that when the inclination angle θ of the upper surface of the enlargement with respect to the pile axis is 20° or less, a prediction curve based on the hyperbolic function is used, and when the inclination angle θ is more than 20°, a prediction curve based on the exponential function is used.

3. The pile enlargement evaluation method according to claim 2, wherein the predicted curve by the hyperbolic function is expressed by Equation 1. [Equation 1]

4. The coefficient C １ and coefficient C ２ The pile enlargement part evaluation method according to claim 3, characterized in that: is a linear function of the inclination angle θ.

5. The pile enlargement part evaluation method according to claim 2, wherein the prediction curve by the exponential function is expressed by Equation 2. [Equation 2]

6. The coefficient α is a constant, The pile enlargement portion evaluation method according to claim 5, wherein the order n is an exponential function of the inclination angle θ.

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