Method for evaluating ultimate tip bearing capacity of single pile

Abstract

To propose a method for evaluating ultimate tip bearing capacity of a single pile in order to evaluate with high accuracy the tip bearing capacity of a cast-in-place pile and an embedded pile in ground conditions where a clayey soil layer exists below a thin sandy soil layer supporting the tip of the pile.SOLUTION: This is a method for evaluating ultimate tip bearing capacity of a single pile, applicable to pile 1, consisting of a cast-in-place pile or an embedded pile. The ultimate tip bearing capacity of the pile, qp0.1, is evaluated using Equation 1. A load distribution angle θ in the bearing layer below pile 1 is calculated using Equation 2, which is a linear function of a ratio of bearing layer thickness H below pile 1 to a pile end diameter D. qp0.1={1+2(H / D)tanθ}2*qc...Equation 1 tanθ=f(H / D)...Equation 2 Where, H / D: effective bearing layer thickness ratio qc: ultimate bearing capacity of the lower layer (=6cu) cu: undrained shear strength or cohesion of the lower layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating the ultimate tip bearing capacity of a single pile. [Background technology]

[0002] In the case of buried piles, which are prefabricated piles inserted into boreholes formed in the ground, a base-hardening fluid is sometimes injected into the tip of the borehole to form a base-hardening part at the tip of the pile (see, for example, Patent Document 1). In this case, the tip of the borehole is allowed to reach the supporting layer, and the base-hardening part is formed while inserted into the supporting layer.

[0003] Such bored piles must have sufficient end bearing capacity to prevent excessive settlement due to the load acting on the pile. Therefore, the end bearing capacity of bored piles must be evaluated in advance. One method for evaluating the end bearing capacity of cast-in-place piles and bored piles is to calculate the end bearing capacity Rp by multiplying the pile end area Ap by the average end N-value Np and the end bearing capacity coefficient α (see Equation 1). Rp=α·Np·Ap Equation 1

[0004] In ground conditions where the bearing layer supporting the tip of a pile is a thin sandy layer with a clayey soil layer underneath, as the ratio of the thickness of the sandy soil layer below the pile, H, to the pile tip diameter, D (effective bearing layer thickness ratio), H / D, decreases, the possibility of a truncated cone-shaped mass of sandy soil forming below the pile penetrating into the clayey soil layer increases. If such a failure occurs, the bearing capacity of the pile will decrease. Therefore, when the bearing layer is thin, it is necessary to properly evaluate the tip bearing capacity and design according to the site.

[0005] Non-Patent Document 1 presents Equation 2 as a method for evaluating the tip bearing capacity of thin-layer bearing piles. q p0.1 ={1+2(H / D)tanθ} 2 ·q c ...Formula 2 where q p0.1 : Ultimate end bearing capacity (when the end settlement ratio Sp / D reaches 0.1) H / D: Effective bearing layer thickness ratio θ: Load distribution angle in the supporting layer (sandy soil) (vertical 1: horizontal 0.3) tanθ=0.3 q c : Ultimate bearing capacity of the lower layer (clay soil) q c =6c u c u :Undrained shear strength or cohesion of the lower layer Sp: Tip subsidence [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-16867 [Non-patent literature]

[0007] [Non-Patent Document 1] Architectural Institute of Japan, "Guidelines for Architectural Foundation Design," September 15, 2021, pp. 217-219 Summary of the Invention [Problem to be solved by the invention]

[0008] The load distribution angle θ (tanθ) is constant regardless of the effective bearing layer thickness ratio H / D. Therefore, as the effective bearing layer thickness ratio H / D increases, the ultimate end bearing capacity q p0.1 Therefore, when using Equation 2, it is common to set an upper limit value that corresponds to the bearing capacity when the bearing layer is thick (when it is determined by the bearing layer).

[0009] The present invention aims to propose a method for evaluating the ultimate tip bearing capacity of a single pile in order to evaluate with high accuracy the tip bearing capacity of cast-in-place piles and buried piles in ground conditions where a clayey soil layer exists below a thin sandy soil layer supporting the tip of the pile. [Means for solving the problem]

[0010] The present invention, which solves the above-mentioned problems, is a method for evaluating the ultimate tip bearing capacity of a single pile, which is applied to a pile consisting of a cast-in-place pile or a bored pile. The pile is supported on a bearing layer (upper layer) mainly made of sandy soil layered on a lower layer mainly made of clay soil. The ultimate tip bearing capacity q of the pile is p0.1 is evaluated by Equation 3, and the load distribution angle θ in the upper layer below the pile is calculated from Equation 4, which is a linear function of the ratio of the upper layer thickness H below the pile to the tip diameter D of the pile. q p0.1 ={1+2(H / D)tanθ} 2 ·q c ...Formula 3 where q p0.1 : Ultimate end bearing capacity (= bearing capacity when the end settlement ratio Sp / D reaches 0.1) Sp: Tip subsidence H / D: Effective bearing layer thickness ratio θ: Load distribution angle in the support layer tanθ=f(H / D) Equation 4 q c : Ultimate bearing capacity of the lower layer c u :Undrained shear strength or cohesion of the lower layer q c =6c u

[0011] This method for evaluating the ultimate end bearing capacity of a single pile allows for accurate evaluation of the ultimate end bearing capacity even in a thin layer of supporting ground, which in turn enables economical design of pile foundations and, in turn, rationalized construction, such as cost reduction and resource conservation.

[0012] It is desirable that tan θ is in the range of 0.2 to 0.4. If the formula 4 is expressed as the formula 5, the ultimate tip bearing force can be evaluated with higher accuracy. tanθ=a(H / D)+b Equation 5 [Effects of the Invention]

[0013] According to the method for evaluating the ultimate tip bearing capacity of a single pile of the present invention, it is possible to evaluate the tip bearing capacity of cast-in-place piles and buried piles with high accuracy in ground conditions where a clayey soil layer exists below a thin sandy soil layer supporting the tip of the pile. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 2 is a conceptual diagram showing the lower end portion of the pile according to the present embodiment. [Figure 2] 10 is a graph showing the comparison results between the centrifugal model experiment results and the evaluation method of the ultimate tip bearing capacity of a single pile according to the present embodiment, and showing the relationship between tan θ and H / D. FIG. [Figure 3] 10 is a graph showing the comparison results between the centrifugal model experiment results and the calculated values ​​by the method for evaluating the ultimate tip bearing capacity of a single pile of this embodiment, and is a graph showing the relationship between qp0.1 and H / D. [Figure 4] 10 is a graph showing the relationship between qp0.1 and H / D, with the centrifugal model experiment results and the calculated values ​​when tanθ is kept constant at 0.3. DETAILED DESCRIPTION OF THE INVENTION

[0015] In this embodiment, we will assume a two-layer ground consisting of a clayey soil layer (lower layer L2) stacked beneath a thin sandy soil layer (upper layer L1), and explain a pile 1 (cast-in-place pile or buried pile) supported by this two-layer ground. The pile 1 is designed to ensure sufficient tip resistance against the expected surcharge load. Figure 1 shows the tip (lower end) of the pile 1. As shown in Figure 1, the tip of the pile 1 is inserted into the upper layer L1, which is the supporting layer.

[0016] Ultimate end bearing capacity of pile 1 q p0.1 is evaluated using Equation 6. q p0.1 ={1+2(H / D)tanθ} 2 ·q c ...Formula 6 where q p0.1 : Ultimate end bearing capacity (= bearing capacity when the end settlement ratio Sp / D reaches 0.1) Sp: Tip subsidence H / D: Effective bearing layer thickness ratio H: Upper layer thickness below the pile D: Pile tip diameter θ: Load distribution angle in the support layer (upper layer) q c : Ultimate bearing capacity of the lower layer (=6c u ) c u :Undrained shear strength or cohesion of the lower layer

[0017] The load distribution angle θ in the lower layer L2 of the pile 1 is calculated from Equation 7, which is a linear function of the ratio (=H / D) of the layer thickness H of the supporting layer below the pile (upper layer L1) to the tip diameter D of the pile 1. tanθ=f(H / D)=a(H / D)+b...Equation 7

[0018] As described above, according to the method for evaluating the ultimate tip bearing capacity of a single pile of this embodiment, even if the supporting ground is thin, the ultimate tip bearing capacity q p0.1 As a result, economical design of pile foundations becomes possible, which in turn leads to cost reduction, resource saving, and other streamlined construction.

[0019] A centrifugal loading experiment was conducted using a model simulating a thin-layer bearing pile to confirm the evaluation method of this embodiment. Four cases (No. 1 to No. 4 in Table 1) were carried out in this experiment. As shown in Table 1, the effective bearing layer thickness ratios H / D for No. 1 to No. 4 were 4, 2, 2, and 1, the effective overburden pressure σv' at the pile bottom depth was 155, 155, 336, and 346 kPa, and the undrained shear strength or cohesion strength c of the lower layer was 155, 155, 336, and 346 kPa. u The values ​​are 75.3, 70.1, 124, and 124 kPa. The experimental results are shown in Table 1. As shown in Table 1, the ultimate end bearing capacity q p0.1 The obtained values ​​were 2.31 to 4.13 MPa.

[0020] [Table 1]

[0021] Test results: Ultimate end bearing capacity q p0.1 , the effective bearing layer thickness ratio H / D, and the undrained shear strength c of the lower layeru Figure 2 shows the relationship between tanθ, calculated back from Equation 6, and the effective support layer thickness ratio H / D. Figure 2 also plots the experimental results. As shown in Figure 2, tanθ decreases as the effective support layer thickness ratio H / D increases. Figure 2 also confirms that tanθ can be approximated by the linear function of Equation 8. The dashed line in Figure 2 shows the case where tanθ is kept constant (=0.3). tanθ=0.43-0.045(H / D)...Equation 8

[0022] Figure 3 shows the ultimate end bearing capacity q p0.1 8 is a graph showing the relationship between the effective bearing layer thickness ratio H / D and the ultimate end bearing capacity q calculated by substituting Equation 8 into Equation 6. p0.1 The values ​​of undrained shear strength c in Eq. u The case where tanθ is set to 70.1 kPa is shown by a dashed line, the case where tanθ is set to 75.3 kPa is shown by a broken line, and the case where tanθ is set to 124 kPa is shown by a solid line. p0.1 The results of the evaluation and experimental results are shown in Figure 4. u The calculations were carried out for pressures of 70.1, 75.3 and 124 kPa.

[0023] As shown in Figure 3, the experimental results (plots) and the evaluation results (curves) from Equation 6 were almost identical. On the other hand, in Figure 4, where tan θ was set to a constant value (=0.3), the plots and calculation results did not necessarily match. Therefore, it was confirmed that the evaluation method of the ultimate tip bearing capacity of a single pile of this embodiment (the method of obtaining tan θ from Equation 7) improves evaluation accuracy compared to the conventional method where tan θ is set to a constant value.

[0024] 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 present invention. For example, upper and lower limits may be set for tan θ calculated using Equation 7 and Equation 8. The lower limit for tan θ may be, for example, 0.2, and the upper limit may be, for example, 0.4. In other words, tan θ calculated using Equation 7 and Equation 8 may be set within the range of 0.2 to 0.4. [Explanation of symbols]

[0025] 1 stake L1 upper layer L2 lower layer

Claims

1. A method for evaluating the ultimate tip bearing capacity of a single pile, which is applied to piles consisting of cast-in-place piles or bored piles, It is supported by a supporting layer made up mainly of sandy soil layered on a lower layer made up mainly of clayey soil. The ultimate end bearing capacity q of the pile ｐ０．１ is evaluated in Eq. A method for evaluating the ultimate tip bearing capacity of a single pile, characterized in that the load distribution angle in the supporting layer below the pile is calculated from Equation 2, which is a linear function of the ratio of the thickness H of the supporting layer below the pile to the tip diameter D of the pile. q ｐ０．１ = {1 + 2(H / D)tanθ} ２ ·q ｃ ··· Equation 1 Here, q ｐ０．１ : Ultimate end bearing capacity (= bearing capacity when the end settlement ratio Sp / D reaches 0.1) Sp: Tip subsidence H / D: Effective support layer thickness ratio θ: Load distribution angle in the support layer tanθ=f(H / D)...Formula 2 q ｃ : Ultimate bearing capacity of the lower layer c ｕ : Undrained shear strength or cohesion of the lower layer q ｃ =6c ｕ

2. 2. The method for evaluating the ultimate tip bearing capacity of a single pile according to claim 1, wherein tan θ is within a range of 0.2 to 0.

4.

3. 2. The method for evaluating the ultimate tip bearing capacity of a single pile according to claim 1, wherein said formula 2 is expressed by formula 3. tanθ=a(H / D)+b...Formula 3

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