Evaluation method of load-settlement relation of intermediate enlarged diameter part and cast-in-place concrete pile

Abstract

To propose: an evaluation method of a load-settlement relation of an intermediate enlarged diameter part which enables formulation of a ground spring, and which enables improvement in settlement prediction accuracy of a multistage enlarged pile having the intermediate enlarged diameter part, regarding the multistage enlarged pile; and a cast-in-place concrete pile which is designed utilizing the evaluation method of a load-settlement relation of the intermediate enlarged diameter part.SOLUTION: In an evaluation method of a load-settlement relation of an intermediate enlarged diameter part, load-settlement characteristics of the intermediate enlarged diameter part of a cast-in-place concrete pile is evaluated by a formula 1. A cast-in-place concrete pile is designed based on the settlement prediction result by setting a ground spring of the intermediate enlarged diameter part by this evaluation method of a load-settlement relation of the intermediate enlarged diameter part, and by performing settlement prediction of a multistage enlarged pile by a hybrid method using the ground spring.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for evaluating the "load-subsidence relationship of an intermediate expansion portion" in a pile having an intermediate expansion portion, and to a cast-in-place concrete pile designed using said evaluation method. [Background technology]

[0002] Increasing the span of columns to build taller buildings or to create more interior space can result in a larger axial force per column. As the axial force of columns increases, the load acting on the piles also increases. For this reason, the bottom end of cast-in-place concrete piles can be enlarged to increase the bearing area against the supporting ground and thus increase the bearing capacity. In addition, in high-rise buildings and towers, the aspect ratio is large, and the overturning moment is prominent, resulting in excessive pull-out force. If an enlarged diameter section is formed not only at the bottom end of the pile but also at the middle part (between the tip and the top end), the bearing capacity of the pile can be further increased and the pull-out resistance of the pile can be increased. In recent years, cast-in-place concrete piles having multiple enlarged diameter sections have been developed and applied. For example, Patent Document 1 discloses a multi-stage enlarged pile in which enlarged diameter sections are formed at multiple locations along the length of the pile. Patent document 1 discloses a method for evaluating multi-stage enlarged piles in which a vertical cylindrical sliding surface is assumed for each enlarged portion, with the diameter being the diameter of the enlarged portion and the effective height being the range over which the bearing effect of the enlarged portion extends, and the ultimate vertical bearing capacity is calculated by subtracting the pile's own weight from the sum of the ultimate circumferential friction force generated on the vertical cylindrical sliding surface of each enlarged portion, the ultimate resistance force of the ground at the pile tip, and the ultimate circumferential friction force of the shaft, and the multi-stage enlarged pile is evaluated based on the magnitude of the calculated ultimate vertical bearing capacity.

[0003] For pile foundations, in addition to evaluating the bearing capacity of each pile, it is also important to consider the settlement of the entire building. Methods for detailed consideration of the settlement of buildings include the finite element method and hybrid method, but in practical design, the hybrid method is often used because it is relatively easy to parametrically consider pile specifications. In the hybrid method, piles are replaced by beam elements and then divided into multiple elements, and a ground spring (load-subsidence relationship) that simulates the resistance characteristics of the ground near the pile corresponding to the depth is assigned to the nodes between the elements. The accuracy of the subsidence prediction depends on the setting of the ground spring. For buildings with enlarged base piles, it has been confirmed through comparison with actual measurement results during the construction process that the settlement of buildings can be predicted with high accuracy by setting a nonlinear spring that simulates the resistance characteristics of the pile tip and periphery, which has been proposed based on the results of numerous load tests.

[0004] On the other hand, for multi-stage enlarged piles that also have an enlarged section in the middle (middle enlarged section), the method for setting the ground spring (method for evaluating the load-subsidence relationship) in the middle enlarged section has not been established, and there are few examples of verification through experiments, analyses, or actual measurements in actual buildings, so the current situation is that the prediction accuracy has not yet been verified. Therefore, in order to improve the settlement prediction accuracy of multi-stage enlarged piles, it is necessary to understand the settlement characteristics of the middle enlarged section. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2003-138561 A Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to propose a method for evaluating the load-settlement relationship of a multi-stage enlarged pile with an intermediate enlarged portion, which enables the formulation of ground springs and thereby improves the accuracy of predicting the subsidence of multi-stage enlarged piles, and further aims to propose a cast-in-place concrete pile designed using this method for evaluating the load-settlement relationship of an intermediate enlarged portion. [Means for solving the problem]

[0007] The present invention, which aims to solve the above problems, is a method for evaluating the load-settlement relationship of an intermediate expansion part of a cast-in-place concrete pile, which evaluates the load-settlement characteristics of the intermediate expansion part by Equation 1. The cast-in-place concrete pile of the present invention is a cast-in-place concrete pile in which a ground spring of the intermediate expansion part is set by this method for evaluating the load-settlement relationship of the intermediate expansion part, subsidence of the multi-stage enlarged pile is predicted by a hybrid method using the ground spring, and the pile is designed based on the result of the predicted subsidence. The coefficient α' for the initial tangent slope of the curve is calculated by multiplying the coefficient α (proposed value for the tip of the pile) by (D2-D1) / D2 (D1: diameter of the shaft of the pile). The coefficient α' relating to the initial tangent gradient of the curve is preferably set to 0.08 for sandy soil and 0.11 for gravelly soil. Moreover, the order n', which determines the curve shape, is preferably set within the range of 2.0 to 7.0, and more preferably set to 4.3 for sandy soil and 4.5 for gravelly soil.

[0008]

number

[0009] According to the method for evaluating the load-subsidence relationship of the intermediate expansion part, the average load-subsidence relationship of the intermediate expansion part can be obtained for each soil type. Therefore, it is possible to set the ground spring of the intermediate expansion part when predicting the subsidence of the multi-stage expansion pile by the hybrid method, and thus to predict the subsidence of the multi-stage expansion pile with high accuracy. Effect of the Invention

[0010] According to the present invention's method for evaluating the load-subsidence relationship of an intermediate expansion portion, it is possible to formulate the ground spring for a multi-stage expansion pile having an intermediate expansion portion, and ultimately to improve the accuracy of predicting the subsidence of the multi-stage expansion pile. [Brief description of the drawings]

[0011] [Figure 1] This is a front view showing an example of a multi-stage enlarged pile and the ground conditions where the application of the multi-stage enlarged pile is expected. [Diagram 2] FIG. 1 is a diagram showing an analytical model. [Diagram 3] Graphs showing the skeleton curves of ground springs, where (a) is the circumferential resistance spring and (b) is the pile tip spring. [Figure 4] 1A and 1B are diagrams showing assumed models of the resistance mechanism in the intermediate expansion section, where (a) is a cylindrical surface shear type and (b) is a projected surface support type. [Diagram 5] 1 is a graph showing the relationship between the load on the pile head and the amount of settlement, where (a) is when the maximum value on the horizontal axis is 200 mm, and (b) is when the maximum value on the horizontal axis is 30 mm. [Figure 6] 1 is a graph showing the relationship between the load and the sinking amount of the intermediate expansion section, where (a) is the case where the maximum value on the horizontal axis is 200 mm, and (b) is the case where the maximum value on the horizontal axis is 30 mm. [Figure 7] 13 is a graph showing the curve shape of the ground spring of the intermediate expansion section. [Figure 8] FIG. 1 is a diagram showing an overview of the test piles and ground for the in-situ loading test. [Figure 9] FIG. 2 is a diagram showing a loading device, where (a) is a plan view and (b) is an elevation view. [Figure 10] Graphs showing the results of an in-situ loading test, where (a) is the axial force distribution and (b) is the circumferential resistance force distribution. [Figure 11] 1 is a graph showing the load-subsidence relationship of the pile head, intermediate expansion section, and pile tip. [Figure 12] 1 is a graph showing the relationship between peripheral resistance force τ and sectional displacement δ. [Figure 13] 11 is a graph comparing the relationship between the axial force difference Pm and the displacement Sm of the intermediate expansion portion based on an embodiment and past data. [Figure 14] The graph shows the standardized Pm-Sm relationships for each soil type to the {(Pm / Am) / (Pm / Am)u}-Sm / D2 relationship and compared with the formula proposed in the Foundation Guidelines. (a) is for sandy soil, and (b) is for gravelly soil. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] In this embodiment, a design method for a cast-in-place concrete pile (multi-stage enlarged pile 1) having multiple enlarged sections will be described. Figure 1 shows a multi-stage enlarged pile. As shown in Figure 1, the multi-stage enlarged pile 1 has an enlarged section (enlarged bottom section 3) at its lower end, and also has an enlarged section (intermediate enlarged bottom section 2) above (in the middle section) the enlarged bottom section 3 at a predetermined interval. The upper end of the intermediate expanded diameter section 2 has an outer diameter D1 equivalent to the outer diameter D1 of the shaft section 4 (a portion other than the expanded diameter section). The upper part of the intermediate expanded diameter section 2 expands in diameter from the upper end downward at an inclination angle θ1 until it reaches a maximum outer diameter D2. The lower end of the intermediate expanded diameter section 2 has an outer diameter D1 equivalent to the outer diameter D1 of the shaft section 4. The lower part of the intermediate expanded diameter section 2 expands in diameter from the lower end upward at an inclination angle θ2 until it reaches a maximum outer diameter D2. The location where the intermediate expanded diameter section 2 is formed is not limited, but in this embodiment, it is formed at a position corresponding to the support layer as shown in FIG. 1. The upper end of the expanded bottom portion 3 has an outer diameter D1 equal to the outer diameter D1 of the shaft portion 4. The expanded bottom portion 3 has a shape that expands in diameter from the upper end downward at an inclination angle θ3 to an outer diameter D3.

[0013] The design of the multi-stage enlarged pile 1 is carried out based on the results of a prediction of the settlement of the multi-stage enlarged pile 1 using a hybrid method using a ground spring. The ground spring of the intermediate enlarged section 2 used in the hybrid method is set after understanding the settlement characteristics of the intermediate enlarged section 2. The hybrid method is a settlement analysis method that combines elastic theory and the finite element method and incorporates local nonlinear springs of the ground near the piles. The hybrid method makes it possible to evaluate the complex settlement behavior caused by interactions between piles and between the piles and the ground in group pile foundations such as pile raft foundations. In an analysis using the hybrid method, first, the piles and foundation beams are replaced with beam elements (finite elements) and modeled in 3D. Next, the ground is treated as an elastic body (springs based on elastic theory), and the deformation coefficient is set taking into account nonlinearity. In addition, soil springs that show local nonlinearity between the piles and the ground are placed on the periphery and tip of the piles. The interactions between each foundation member are then evaluated using Steinbrenner's approximate solution and Mindlin's first solution.

[0014] The settlement characteristics (load-settling characteristics) of the intermediate expansion section 2 are evaluated by Equation 1. In this embodiment, the coefficient α' relating to the initial tangent gradient of the curve is set to 0.08 for sandy soil and 0.11 for gravelly soil. The order n' determining the curve shape is preferably within the range of 2.0 to 7.0, and more preferably 4.3 for sandy soil and 4.5 for gravelly soil.

[0015]

number

[0016] According to the method for evaluating the load-subsidence relationship of the intermediate expansion part of this embodiment, the average load-subsidence relationship of the intermediate expansion part can be obtained for each soil type. Therefore, it is possible to set the ground spring of the intermediate expansion part 2 when predicting the subsidence of the multi-stage expansion pile 1 by the hybrid method, and therefore it is possible to predict the subsidence of the multi-stage expansion pile 1 with high accuracy.

[0017] Below, we will explain the results of conducting an in-situ loading test to verify the design method for the cast-in-place concrete pile of this embodiment.

[0018] (1) Study on the effect of ground spring settings in the hybrid method First, a comparative study was conducted on the ground spring settings to confirm the effect that the ground spring settings of the intermediate expansion section 2 in the hybrid method have on the accuracy of settlement prediction. The ground spring setting conditions are shown in Table 1. As shown in Table 1, four types (four cases) of ground springs were set for the intermediate expansion section 2, and the differences in the analysis results for each case were compared for a single pile.

[0019] [Table 1]

[0020] The analysis model is shown in Figure 2. As shown in Figure 2, the test pile 10 has one intermediate expansion part 2, and the other part is a pile consisting of a shaft part 4 with a constant outer diameter. The dimensions of the test pile 10 are shaft diameter D1 = 1.0 m, intermediate expansion part diameter D2 = 2.0 m, and pile length 19.5 m. In the analysis model, the pile is replaced with a beam element, divided into 10 parts shallower than the intermediate expansion part and 4 parts deeper than the intermediate expansion part, and a ground spring having a skeleton curve shown in Figure 3 is given to each node. The range from the pile head to GL-11.0 m is the friction cut section. Note that Figure 3 is a graph showing the skeleton curve of the ground spring. A hyperbolic circumferential resistance spring is given to the node of the shaft part 4, and an exponential pile tip spring (see Equation 2) shown in the Building Foundation Structure Design Guidelines (Architectural Institute of Japan, 2019: hereinafter referred to as the "Foundation Guidelines") is given to the node of the pile tip to take nonlinearity into consideration. In the analysis, all springs are replaced with multi-linear type. Table 2 shows the ground resistance and spring type at the nodes of the intermediate expansion section 2 assumed for each case.

[0021]

number

[0022] [Table 2]

[0023] In case 4, the coefficient α multiplied by (D2-D1) / D2 is the instantaneous sinking (displacement) S at the center of the ring-shaped intermediate expansion section 2 due to a uniformly distributed circular load in a uniform semi-infinite elastic body, relative to the horizontal projection plane of the ring-shaped intermediate expansion section 2 (see Figure 4). m Assuming (Equation 3), the immediate settlement S at the tip of a pile of the same diameter is also assumed. p This takes into consideration the ratio of the sinking rigidity (Equation 5) to (Equation 4). Figure 4 is a hypothetical model of the resistance mechanism in the intermediate expansion section 2.

[0024]

number

[0025] Initial stiffness K0 and ultimate resistance τ of hyperbolic spring max The exponential spring coefficient α, order n, and ultimate bearing capacity were set as shown in Table 1. The peripheral resistance spring from the pile head to GL-11.0m was set to the ultimate resistance capacity τ max =0.2kN / m 2 The elastic modulus of the beam element is set to 3.85×10 based on the actual strength of the concrete pile and the amount of reinforcing steel. 7 kN / m 2 The ground model around the piles was based on the ground conditions from the load tests. The ground conditions are shown in Table 3. The deformation coefficient was calculated from the PS logging results from the test site. The load was set to the sum of the ultimate values ​​of the ground springs in cases 3 and 4 (15,391 kN; equivalent to ultimate bearing capacity). Note that the analysis takes into account the effects of interactions between each node via the ground using Mindlin's solution.

[0026] [Table 3]

[0027] Figure 5 shows the pile head load P0-pile head settlement S0 relationship for each case, with two maximum values ​​on the horizontal axis (200mm or 30mm). As shown in Figure 5(a), in cases 3 and 4, where the ground spring in the intermediate expansion section 2 is exponential, the degree of increase in P0 during large deformation is greater than in cases 1 and 2, where the ground spring is hyperbolic. Also, as shown in Figure 5(b), at P0=5,130kN, which is equivalent to the long-term allowable bearing capacity (1 / 3 of the ultimate bearing capacity) in cases 3 and 4, the pile head settlement S0 in case 4 is reduced by approximately 2.7mm (approximately 19%) compared to the pile head settlement S0 in case 3.

[0028] FIG. 6 shows the load P m -Subsidence amount S m The relationship is shown by changing the maximum value on the horizontal axis to two values ​​(200 mm or 30 mm). Here, P m was organized as the difference in axial force between the upper and lower parts of the intermediate enlarged diameter part 2 of the test pile 10. m -Sm The relationship results are also shown for reference. As shown in Figure 6(a), P m -S m The curve shape of the relationship is the axial force difference P m Although there are some differences, cases 3 and 4 correspond to the load test results, and it can be seen that they can be roughly evaluated using an exponential skeleton curve. Also, as shown in Figure 6(b), during the initial settlement, case 4 has a larger displacement (settlement) S than case 3. m This is because the coefficient α related to the initial tangent gradient in Equation 2 is set to (D2-D1) / D2 = 0.5 times, which increases the initial stiffness of the curve and reduces the amount of initial settlement.

[0029] Figure 7 shows the curved shapes of the ground springs in the intermediate expansion section 2 set in cases 3 and 4. As shown in Fig. 7, the initial stiffness in case 4 is greater than the initial stiffness in case 3, which has the same shape as the proposed equation for pile tip springs for sandy soil (α = 0.23, n = 2.7), and in the initial range, the shape is closer to the proposed equation for pile tip springs for gravelly soil (α = 0.12, n = 3.3). It is estimated that this effect is reflected in the approximately 19% reduction in pile head settlement S0 seen in Fig. 5(b). From the above, it was confirmed that the ground spring settings (skeleton curve and its initial rigidity) of the intermediate expansion section 2 affect the accuracy of predicting the settlement of the entire pile.

[0030] (2) In-situ loading test Next, the results of the in-situ vertical compression load test are shown. Figure 8 shows an overview of the test pile and the ground. The test site is on a diluvial plateau, with the ground conditions being alternating deposits of sandy soil and clayey soil. The groundwater level is at a depth of 3.3 m, and a fine sand layer with an N value of about 30-40 and a shear wave velocity Vs of 260-300 m / s is distributed at a depth of 12-23 m. The test pile 10 has a shaft diameter D1 of 1.0 m, a pile tip depth of 19.5 m, and a diameter D2 of the intermediate enlarged section of 2.0 m (projection length D p2 The intermediate expansion part 2 is installed at a depth of 15.5 m in the fine sand layer, and the average N value around it is N ave=33. The area from the pile head to a depth of 11.0 m is wrapped with a 1.0 m diameter, 9 mm thick steel pipe to reduce friction with the ground, and consideration was given to efficiently transmit the load to the intermediate expansion section. The construction of the test pile 10 was carried out in accordance with the prescribed procedures for the multi-stage expansion pile method, which is classified as an earth drill method. The nominal strength of the concrete is 45 N / mm 2 The average core strength at 246 days was 67N / mm 2 It was.

[0031] The loading test was conducted in accordance with the Geotechnical Society of Japan standard. Figure 9 shows an overview of the loading device used in the loading test. As shown in Figure 9, six reaction piles 20 were installed around the test pile 10, and a static pushing force was applied to the head of the test pile 10 by a hydraulic jack 40 while a reaction force was applied through a loading girder 30 connected to the piles. The loading method was a multi-cycle step loading method in which the load was applied stepwise. The loading steps were in 1.5 MN increments, and the load was maintained for 30 minutes at the new step. The main measurement items were the pile head load (pressure gauge), the displacement of the pile head, intermediate expansion part, and pile tip, and the strain of the pile body (gauge attached to the main reinforcement). The installation locations (measurement points) of the gauges for measuring displacement and strain are shown in Table 4.

[0032] [Table 4]

[0033] The loading test was carried out by applying a step load up to a maximum of 19.5MN, and then the loading was terminated when it was confirmed that the pile head displacement had reached 245mm, exceeding 0.1D2. Figure 10 shows the pile axial force obtained from the strain measurement results and the depth distribution of the peripheral resistance strength τ standardized by the peripheral area equivalent to the shaft, including the intermediate expansion section. Here, the Young's modulus E of the concrete used in the axial force evaluation is cwas calculated based on the relationship between the axial stress (pile head load / concrete cross-sectional area) and the measured strain value ε at measurement point G3, according to the ε history, and applied to each cross section. It has been confirmed that the axial force at measurement point G1 obtained from the measured strain value ε (calculated assuming that the concrete, main reinforcement, and steel pipes remain flat) is almost identical to the pile head load (with an error of roughly 3% or less). As shown in Figure 10, the axial force difference P above and below the intermediate expansion section 2 (between measurement points G4 and G5) is m is large, and the τ of the relevant section is 1,300 kN / m 2 Therefore, it can be seen that the intermediate expansion section 2 contributes to an increase in the vertical bearing capacity.

[0034] Figure 11 shows the load-subsidence relationship of the pile head, the intermediate enlarged diameter part 2, and the pile tip. Here, the axial force difference P m The displacement S of the intermediate enlarged diameter portion 2 is included in the peripheral resistance force of the shaft portion 4 and the rising portion (vertical portion of the intermediate enlarged diameter portion) in the relevant section. m The difference in axial force P when it becomes 0.1D2 (=200mm) m (Second limit resistance; ultimate design bearing capacity) is 11,000kN, and its horizontal projection area A m The bearing capacity is approximately 4,700 kN / m 2 (≒140N). This is the ultimate end bearing capacity q pAIJ (=120N). In Fig. 11, the axial force difference P m Based on the second limit resistance of P m -S m The curves of pile tip springs for gravel and sandy soils converted into a relationship are shown in Fig. 11. As shown in Fig. 11, the P m -S m The relationship shows a curve shape with a greater initial stiffness than the curve for sandy soil (almost identical to the curve for gravelly soil).

[0035] The relationship between the peripheral resistance force τ and the sectional displacement δ is shown in Figure 12. As shown in Figure 12, the peripheral resistance force τ of the shaft portion remains almost constant as the sectional displacement δ increases, whereas the peripheral resistance force τ of the intermediate expansion portion maintains an increasing trend, confirming that there is a difference in the support mechanism between the shaft portion 4 and the intermediate expansion portion 2.

[0036] (3) Comparison of load-settlement relationships between the pile tip and the intermediate enlarged section The load-settlement relationship of the intermediate expansion part is compared with the proposed formula (Formula 2) for the load-settlement relationship of the pile tip shown in the foundation guidelines. In this study, in addition to this embodiment, past in-situ loading test results were also compared as comparative examples (Comparative Examples 1 to 5). Table 5 shows the pile specifications and ground types of the six loading tests that were examined. As shown in Table 5, the shaft diameter D1 of the test pile 10 was 1.0 to 1.2 m, the diameter D2 of the intermediate expansion part was in the range of 1.5 to 2.0 m, and the lower inclination angle θ2 of the intermediate expansion part was 45° in all cases. The type of ground around the intermediate expansion part was sandy soil or gravel soil, and the average N value was 33 to 95.

[0037] [Table 5]

[0038] Figure 13 shows the P of the intermediate expansion part in each test. m -S m The relationship is shown below. Axial force difference P m are calculated at the strain gauge positions installed above and below the intermediate expansion section, and include the peripheral resistance forces of the shaft and rising parts in the relevant sections. As shown in Figure 13, the axial force difference P m The maximum value of is 3,700 to 14,100 kN, and the ratio of the maximum displacement to the diameter of the intermediate expansion section S m / D2 is 0.03 to 0.12, with some variation, but in all curves, the axial force difference P m In Fig. 13, the approximate curves fitted to each curve by Equation 2 are also shown. When fitting, the pile tip settlement (immediate settlement) S p is the displacement of the intermediate expansion section S m, pile tip diameter d p is the diameter of the intermediate enlarged diameter part D2, and the pile tip bearing capacity R p / A p is the bearing capacity of the intermediate expansion part P m / A m , pile tip ultimate bearing capacity (R p / A p ) u is the ultimate bearing capacity of the intermediate expansion section (P m / A m ) u (See formula 1) The ultimate bearing capacity of the intermediate expansion section (P m / A m ) u is the axial force difference P m The second limit resistance of A m The coefficient α and the order n are calculated by dividing by . Furthermore, the coefficient α and the order n are respectively set as the coefficient α' and the order n' as shown in Table 6. It was confirmed that the coefficient α' could be roughly fitted by multiplying the coefficient α by (D2-D1) / D2, except for the comparative example 3. Furthermore, the order n' could be fitted by giving a value larger than the order n (higher order).

[0039]

number

[0040] [Table 6]

[0041] As described above, when evaluating the load-settlement relationship of the intermediate expansion section 2 using Equation 1, it was found that not only the setting of the coefficient α relating to the initial stiffness, but also the setting of the order n relating to the curve shape are important influencing factors. In other words, it was confirmed that the load-settlement relationship of the intermediate expansion section 2 can be properly evaluated by appropriately setting the coefficient α' and order n'. In addition, the load-settlement relationship of the intermediate expansion section 2 obtained from the results of the full-scale load test was fitted with the proposed equation for pile tips in the Foundation Guidelines (Equation 2), and the average curves were obtained for each type of ground and compared with the proposed equation for pile tips. The comparison results are shown in Figure 14. Figure 14(a) shows sandy soil, and (b) shows gravel. soilAs shown in Figures 14(a) and (b), the average curves for all ground types showed shapes that exceeded those of the proposed formula. In other words, it was confirmed that the intermediate expansion section is able to suppress settlement for the same load better than the tip of a pile of the same diameter. Furthermore, from the average coefficient α' and order n' obtained by fitting for each ground type, the following statistical values ​​were obtained: α' = 0.08, n' = 4.3 for sandy soil, and α' = 0.11, n' = 4.5 for gravelly soil. This is considered to give the average load-settlement characteristics of the intermediate expansion section for each ground type.

[0042] Although an embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and each of the above-described components can be appropriately modified without departing from the spirit of the present invention. In the above embodiment, a multi-stage expansion pile 1 having an intermediate expansion portion 2 and an expansion bottom portion 3 was described, but the presence or absence of the expansion bottom portion 3 and the number and arrangement of the intermediate expansion portions 2 are not limited. [Explanation of symbols]

[0043] 1. Multi-stage enlarged piles (cast-in-place concrete piles) 2 Intermediate expansion section 3. Enlarged base 4 Shaft section 10 Test pile 20 Reaction piles 30 Loading girder 40 Hydraulic Jack

Claims

1. A method for evaluating the load-subsidence relationship of an intermediate expansion part of a cast-in-place concrete pile, comprising evaluating the load-subsidence characteristics of the intermediate expansion part using Equation 1. [0010]

2. 2. The method for evaluating the load-sinking relationship of an intermediate expansion portion according to claim 1, wherein the coefficient α' relating to the initial tangent gradient of the curve is a value calculated by Equation 2. [0025]

3. The method for evaluating the load-subsidence relationship of an intermediate expansion section according to claim 1, characterized in that the coefficient α' relating to the initial tangent gradient of the curve is set to 0.08 for sandy soil and 0.11 for gravelly soil.

4. 2. The method for evaluating the load-sinkage relationship of an intermediate expansion portion according to claim 1, wherein the degree n' that determines the curve shape is set within a range of 2.0 to 7.

0.

5. The method for evaluating the load-subsidence relationship of an intermediate expansion section according to claim 1, characterized in that the order n' determining the curve shape is set to 4.3 for sandy soil and 4.5 for gravelly soil.

6. A ground spring of the intermediate expansion portion is set by the evaluation method of the load-subsidence relationship of the intermediate expansion portion according to any one of claims 1 to 5, The settlement of the multi-stage expansion pile is predicted by the hybrid method using the ground spring. A cast-in-place concrete pile designed based on the results of said settlement prediction.

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