JP7535481B2 - Method for evaluating the vertical bearing capacity of wall piles - Google Patents

Description

The present invention relates to a method for evaluating the vertical bearing capacity of wall piles in order to evaluate the performance of wall piles.

As a method for evaluating the performance of a pile foundation, evaluation may be performed based on the vertical bearing capacity of the pile tip (see Patent Documents 1 and 2). On the other hand, as a foundation structure for a building, not only pile foundations but also wall piles formed continuously along the periphery of the building may be adopted. The vertical bearing capacity of such wall piles has been evaluated by replacing them with circular piles (cylindrical piles) having an equivalent cross-sectional area. Specifically, after replacing the wall piles with circular piles arranged directly under the columns, the bearing capacity formula was applied to determine the bearing capacity at the time of 0.1D settlement of the equivalent diameter D as the ultimate bearing capacity. For example, Patent Document 1 discloses a method for evaluating soil cement column-row wall piles, in which each soil cement column with a buried core material is regarded as a single pile and the ultimate bearing capacity is calculated.
Patent Document 2 discloses a method for evaluating a multi-stage expanded cast-in-place concrete pile having multiple expanded sections in the longitudinal direction of the pile, in which the ultimate vertical bearing capacity is calculated by subtracting the weight of the pile from the sum of the ultimate resistance force of the ground at the tip of the pile, the ultimate circumferential friction force generated on a vertical cylindrical sliding surface, the diameter of which is the diameter and the range over which the bearing effect of the expanded sections extends is the effective height, and the ultimate circumferential friction force of the shaft.
Non-Patent Document 1 discloses the measured values of the relationship between vertical load and settlement obtained from vertical load tests using partial test specimens of cast-in-place concrete circular piles and underground wall piles without enlarged bases.
According to the measurements in Non-Patent Document 1, in the case of wall piles, as the column spacing of a building increases, the equivalent diameter also increases, and the ultimate bearing capacity increases, but the estimated settlement tends to increase. In addition, if the ultimate bearing capacity of wall piles having a continuous wall-like body is evaluated by replacing them with independent equivalent circular piles, the rigidity in the direction in which the wall piles are continuous is not taken into account, and the amount of uneven settlement becomes excessive. On the other hand, when the ultimate bearing capacity is evaluated using the three-dimensional finite element method, it is necessary to model the continuous wall piles and the ground, which requires a huge amount of calculation time and a large amount of investigation costs.

JP 2017-119968 A JP 2006-152799 A

Japan Construction Federation, "BCS Foundation Pile Evaluation Study Group, Final Report," August 1999

The objective of the present invention is to propose a method for evaluating the vertical bearing capacity of wall piles that is easy to model, enables reduction in design time and costs, and allows for rational evaluation of the performance of wall piles.

The method for evaluating the vertical bearing capacity of a wall pile according to the first invention is a method for evaluating the vertical bearing capacity of a wall pile, and is characterized by comprising the steps of: creating a graph showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile based on actual measurement results showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile; or modeling the wall pile as a rectangular cross-section pile for multi-mass point analysis at the position where the pile load acts, performing an analysis using the modeled pile, and creating a graph showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile, in which the friction resistance acting on the pile peripheral surface is added to the tip pushing load; and calculating a value Ba obtained by multiplying the short side width B of the wall pile by a coefficient a within the range of 0.05 to 0.2 as the tip settlement at the time of the ultimate vertical bearing capacity of the wall pile, and then extracting the pushing load Ru of the wall pile corresponding to the tip settlement of the wall pile from the graph as the ultimate vertical bearing capacity of the wall pile.
According to this invention, first, the value Ba obtained by multiplying the short side width B of the wall pile by a coefficient a smaller than 1 is calculated as the tip settlement amount at the ultimate vertical bearing capacity of the wall pile, and then the pushing load Ru of the wall pile corresponding to the tip settlement amount is calculated as the ultimate vertical bearing capacity of the wall pile. Therefore, according to the evaluation method of the vertical bearing capacity of the wall pile of the present invention, the ultimate vertical bearing capacity of the wall pile is estimated based on data on the wall pile, so that the tip settlement amount and the ultimate vertical bearing capacity at the ultimate vertical bearing capacity can be evaluated more rationally than the conventional calculation method for circular piles. In addition, since the tip settlement amount and the ultimate vertical bearing capacity at the ultimate vertical bearing capacity are calculated based on the actual measurement results or analysis results of the wall pile, it is possible to reduce the design period and design costs while ensuring a rational performance evaluation of the wall pile. In addition, when comparing a pile with a rectangular cross section with a circular pile with the same cross-sectional area, the circumferential area of the circular pile is smaller. Therefore, a pile with a rectangular cross section has a larger circumferential area than a circular cross section, and can have a larger peripheral frictional resistance even on the same ground.In addition, in pile design, the vertical bearing capacity of the pile head is calculated by adding the ultimate bearing capacity of the pile tip to the frictional resistance around the pile.
In addition , by setting the coefficient a multiplied by the short side width B of the wall pile within the range of 0.05 to 0.2, the tip settlement Ba corresponding to the ultimate vertical bearing capacity of the wall pile can be accurately calculated from the actual measurement results or analysis results showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile.
The second invention is a method for evaluating the vertical bearing capacity of a wall pile, wherein the short side width B of the wall pile is the wall thickness if the wall thickness of the wall pile is uniform, and in the case of a wall pile having an enlarged base, is the equivalent short side width obtained by dividing the projected area onto a plane including the protruding portion of the enlarged base by the long side width of the wall pile.
According to this invention, in a wall pile with a uniform wall thickness and a wall pile having an expanded base, the wall width in the short side direction of the wall pile that resists ground reaction force can be set to the short side width of the wall pile, thereby establishing a vertical bearing mechanism for the wall pile.

The method for evaluating the vertical bearing capacity of wall piles of the present invention allows for easy modeling, reduces design time and costs, and enables rational performance evaluation of wall piles.

FIG. 1A is a perspective view showing an example of a wall pile according to this embodiment, and FIG. 1B is a multi-mass point analysis model used in a method for evaluating the vertical bearing capacity of the wall pile. FIG. 1A is a perspective view showing an example of a wall pile, and FIG. 1B is a perspective view of an analytical model in which the wall pile is replaced with a circular pile. 1 is a flowchart showing a procedure for designing a wall pile. 1 is a schematic diagram showing the relationship between the pushing load of a wall pile and the amount of settlement of the tip of the wall pile. FIG. This is a three-dimensional finite element analysis model used to calculate the amount of settlement. 1 is a schematic diagram showing the relationship between the pushing load of a pile and the amount of settlement at the tip. FIG. 11 is a perspective view showing an example of a wall pile according to another embodiment. This is a graph comparing the measured values of the relationship between the tip pushing load and the tip settlement of wall piles and circular piles. This is a graph comparing the actual measured values and analytical values for the relationship between the tip pushing load and the tip settlement of wall piles and circular piles.

The present invention is a method for evaluating the vertical bearing capacity of a wall pile, which calculates Ba, the short side width B of the wall pile multiplied by a coefficient a, as the tip settlement amount at the time of the ultimate vertical bearing capacity of the wall pile, based on an actual measurement value or an analytical value showing the relationship between the pushing load (vertical load intensity) of the wall pile and the tip settlement amount, and then calculates the pushing load Ru of the wall pile corresponding to the tip settlement amount of the wall pile as the ultimate vertical bearing capacity of the wall pile. The tip settlement amount of the wall pile is calculated from any one of the actual measurement value based on the load test result of the wall pile, the analytical value based on the three-dimensional finite element method analysis, and the analytical value based on the multi-mass point analysis result.
In this embodiment, a method for evaluating the vertical bearing capacity of wall piles when designing them in the case where wall piles are used as the foundation of a building will be described.
FIG. 1 shows an analytical model used in the method for evaluating the vertical bearing capacity of the wall pile of this embodiment. In this embodiment, the continuous wall pile 1 shown in FIG. 1(a) is modeled for multi-mass point analysis as a pile 11 with a rectangular cross section at the position where the pile load P _P acts, as shown in FIG. 1(b), and the behavior of the continuous wall pile 1 is simulated without increasing the number of elements 12, 12, ... on the analytical model by considering the constraint conditions between the piles 11 (such as giving the same displacement or a deformation distribution considering the shear rigidity and bending rigidity of a continuous wall). In addition, in the design of the pile, the vertical bearing capacity of the pile head is the ultimate bearing capacity of the pile tip plus the friction resistance around the pile. At this time, in the multi-mass point model of FIG. 1(b), the rectangular shape is taken into account in the evaluation of the peripheral friction resistance.
Conventionally, the amount of settlement of the pile tip to determine the ultimate vertical bearing capacity of a wall pile 1 was calculated by replacing the rectangular cross-sectional area at the tip of the wall pile with an equivalent circular pile 13 (see Figure 2), whereas the inventors of the present application have devised a method of evaluating the vertical bearing capacity of a wall pile in which the value Ba obtained by multiplying the short side width B of the wall pile 1 by a coefficient a that is smaller than 1 is calculated as the amount of tip settlement of the wall pile 1 at its ultimate vertical bearing capacity, and then the relationship between the pushing load of the wall pile and the amount of tip settlement of the wall pile is estimated, and the pushing load Ru of the wall pile corresponding to the amount of tip settlement is calculated as the ultimate vertical bearing capacity of the wall pile.
Next, as shown in Fig. 4, the value obtained by multiplying the ultimate vertical bearing capacity by a safety factor is calculated as the vertical bearing capacity at each required performance level (damage limit, service limit), and then the settlement at each required performance level is calculated from the pushing load of the wall pile and the tip settlement curve of the wall pile. The relationship between the pushing load of the wall pile tip and the tip settlement is estimated by Equation 1 based on the actual measured values of the vertical loading experiment of the wall pile described in Non-Patent Document 1. Details of Equation 1 will be described later.

The design procedure for the wall piles is explained below. Figure 3 shows the design procedure for the wall piles.
As shown in FIG. 3, in designing the wall pile 1, first, the ground characteristics, the shape of the wall pile, and the pile length are set.
Next, the ultimate vertical bearing capacity of the wall pile 1 is calculated.
In the calculation of the ultimate vertical bearing capacity of the wall pile 1, first, a graph showing the relationship between the pushing load of the wall pile 1 and the tip settlement of the wall pile 1 is created based on the actual measured values of the vertical loading experiment of the wall pile described in Non-Patent Document 1 or the analysis value of the wall pile. FIG. 4 shows a schematic diagram showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile. In this embodiment, the wall pile 1 is constructed by a column-type continuous wall, and the tip pushing load and tip settlement curve when the wall pile 1 is loaded with a vertical load are used as known measured values (see Non-Patent Document 1). Next, as shown in FIG. 1(b), the wall pile 1 is modeled for multi-mass point analysis as a pile 11 with a rectangular cross section at the position where the pile load P _P acts. By performing an analysis using this model, the frictional resistance acting on the peripheral surface of the pile is added to the tip pushing load, and a graph (FIG. 4) showing the relationship between the pushing load of the wall pile 1 and the tip settlement of the wall pile is obtained. The vertical load (indentation load) when the value Ba obtained by multiplying the short side width B of the wall pile 1 (pile 11) by a coefficient a smaller than 1 (0.1 in this embodiment) is taken as the tip settlement of the wall pile 1 is extracted from the graph in Figure 4, and this value is taken as the ultimate vertical load.
After calculating the ultimate vertical load, the allowable vertical load of the wall pile 1 is calculated as shown in Figure 3. Specifically, the allowable vertical bearing capacity in the service limit state and the allowable vertical load in the damage limit state are calculated using the ultimate vertical load. The allowable vertical load in the service limit state is the ultimate bearing load multiplied by a safety factor (1/3). The allowable vertical load in the damage limit state is the ultimate bearing load multiplied by a safety factor (2/3).
Next, as shown in FIG. 3, the settlement amount of the wall pile 1 for each limit state is calculated.
Then, as shown in Figure 3, the calculated ultimate vertical load, allowable vertical load, tip settlement, etc. are compared with the design limit values (service limit, damage limit), and if they are below the design limit values, the design of the foundation members is carried out; if they are above the design limit values, the pile shape and pile length of the wall piles are reset, and then the ultimate vertical load, allowable vertical load, tip settlement, etc. are calculated and a design is carried out so that each response value is below the design limit value.

As described above, according to the method for evaluating the vertical bearing capacity of the wall pile of this embodiment, the ultimate vertical bearing capacity (ultimate vertical load) of the wall pile 1 is estimated based on a known measured value (see Non-Patent Document 1) corresponding to the wall pile 1 or an analytical value, so that the tip settlement of the wall pile can be evaluated more rationally than the conventional calculation method in which the wall pile 1 is replaced with a circular pile 13. In addition, the method for estimating the relationship between the pushing load of the wall pile and the tip settlement amount using a multi-mass point analysis model is easier to model than the case of estimating it using a three-dimensional finite element method analysis, and it is possible to reduce the design period and design costs of the wall pile. The vertical load intensity of the ground (the value obtained by dividing the vertical load by the tip area) is originally affected by the tip shape. If the tip load intensity of the wall pile 1 is the same, the settlement amount at the ultimate bearing capacity is reduced by using the width of the wall pile to evaluate the vertical bearing capacity, making it a rational evaluation method.
In addition, the vertical load-tip settlement curve of a wall pile, which generally does not have a clear point of change, can be used to estimate the ultimate vertical load that corresponds to the actual situation.
Furthermore, by incorporating the shape of the wall pile 1 into the analysis model, the behavior of the wall pile 1 (load-subsidence relationship) can be estimated with high accuracy.
Furthermore, when the continuous wall piles 1 are modeled as independent piles 11 with rectangular cross sections, the behavior of the continuous wall piles can be simulated without increasing the number of elements in the analysis model by considering the constraint conditions between the piles (giving a deformation distribution that takes into account the same displacement and the shear stiffness and bending stiffness of a continuous wall). Also, when comparing the pile 11 with a rectangular cross section with a circular pile with the same cross section, the circumferential area of the circular pile is smaller. In other words, the rectangular cross section has a larger circumferential area than the circular pile, and the circumferential friction resistance can be increased even with the same ground.

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 case has been described in which a vertical load test is carried out as known data. However, if there is past test data or the like, this may be used as known data.
In the above embodiment, the coefficient multiplied by the short side width when calculating the ultimate vertical bearing capacity of the wall pile is set to 0.1 (10%), but the coefficient is not limited to a value smaller than 1. If the coefficient is set within the range of 0.05 to 0.2, the ultimate vertical load reflecting the actual measured value or the analytical value can be estimated in the vertical load intensity-tip settlement amount curve of the wall pile that generally does not have a clear change point. The reason for setting the coefficient a to 0.1 is based on the description in "6.2, Vertical Bearing Capacity of Pile" in the Foundation Guidelines (Architectural Institute of Japan: Architectural Foundation Structural Design Guidelines, 3rd Edition, November 2019), "Second limit resistance: Load when the indentation load is at its maximum. However, the pile tip settlement amount is within a range of 10% or less of the tip diameter." In addition, the lower limit value 0.05 of the coefficient a was set to a value below 0.1, taking into account the degree of variation in the wall pile's body dimensions, etc. Furthermore, the upper limit of 0.2 for coefficient a was set based on the results of a development in which, in an experiment using a model non-displacement pile by Takano and Kishida (Kishida Hideaki and Takano Akinobu: Distribution of contact pressure at the tip of a buried pile in sand (Part 2: Relationship between distribution of contact pressure and tip bearing capacity of a buried pile), Journal of the Architectural Institute of Japan, No. 261, pp. 25-40, November 1977), the amount of settlement at which the ground at the tip of the pile becomes completely plastic is 20% of the pile diameter (tip diameter).
In the above embodiment, the vertical load (compression load) vs. tip settlement curve of the wall pile 1 is created based on analytical values obtained by multi-mass point analysis, but the vertical load (compression load) vs. tip settlement curve of the wall pile may also be calculated based on actual measured values obtained from load test results of the wall pile or analytical values obtained from the results of three-dimensional finite element method analysis, etc.
The known data used when calculating the ultimate vertical bearing capacity of the wall pile 1 is not limited to that shown in the above embodiment, and may be collected, for example, by a vertical load test.
As shown in Figure 7, in the case where the wall pile 1 is a continuous underground wall pile having an expanded bottom portion 14, the width B of the wall pile 1 can be determined as the equivalent short side width obtained by dividing the projected area of the plane including the expanded bottom portion by the long side length of the wall pile.

The following shows the results of comparing the method of evaluating the vertical bearing capacity of wall piles of the present invention with conventional evaluation methods. Figure 8 shows the relationship between the load and settlement of a wall pile based on actual measurements, and the relationship between the load and settlement of a circular pile. Table 1 shows the cross-sectional specifications of the wall piles and circular piles. As shown in Table 1, the wall piles and circular piles were set to have the same cross-sectional dimensions.

As shown in Figure 8, it was confirmed that the use of wall pile 1 reduced the amount of settlement at the pile tip, allowing for a rational evaluation of the amount of settlement due to constant load.
Next, the measured data shown in Fig. 9 was compared with the data calculated using Equation 1. Fig. 10 shows the relationship between the load and the amount of settlement based on the measured values and the results of calculation using Equation 1.

As shown in Figure 9, the actual measurements and analytical values showed similar nonlinearity regardless of whether the pile was a wall pile or a circular pile.

1 Wall pile 11 Rectangular cross-section pile 12 Element 13 Circular pile a Coefficient B Short side width P _P pile load

Claims (2)

A method for evaluating the vertical bearing capacity of a wall pile, comprising:
A process of creating a graph showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile based on actual measurement results showing the relationship between the pushing load of the wall pile and the tip settlement of the wall pile, or modeling the wall pile as a rectangular cross-section pile for multi-mass point analysis at the position where the pile load acts, performing analysis using the modeled pile, and creating a graph showing the relationship between the pushing load of the wall pile, in which the frictional resistance acting on the pile peripheral surface is added to the tip pushing load, and the tip settlement of the wall pile;
A method for evaluating the vertical bearing capacity of a wall pile, comprising the steps of: calculating a value Ba obtained by multiplying the short side width B of the wall pile by a coefficient a in the range of 0.05 to 0.2 as the tip settlement amount of the wall pile at its ultimate vertical bearing capacity ; and then extracting from the graph the pushing load Ru of the wall pile corresponding to the tip settlement amount of the wall pile as the ultimate vertical bearing capacity of the wall pile. A method for evaluating the vertical bearing capacity of a wall pile as described in claim 1, characterized in that the short side width B of the wall pile is the wall thickness when the wall thickness of the wall pile is uniform, and in the case of a wall pile having an expanded base, is the equivalent short side width obtained by dividing the projected area onto a plane including the protruding portion of the expanded base by the long side width of the wall pile.

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