JP2021009071A - Pile evaluation method - Google Patents

Abstract

To provide a pile evaluation method capable of evaluating support performance of a pile simply and accurately.SOLUTION: A pile evaluation method comprises: acquiring an elastic wave propagation velocity based on the rise time difference of an actual impact elastic wave test from when hitting the head of a pile to generate a striking wave until the striking wave propagates as an elastic wave through the pile and a reflected wave reflected at the lower end of the pile propagates as an elastic wave to the head; determining whether or not the pile is longer than a predetermined reference value; when it is determined that the pile is longer than the reference value, reducing a measured value of the elastic wave propagation velocity of the pile to calculate a pile length; and when it is determined that the value is shorter than the reference value, calculating the pile length using the measured value of the elastic wave propagation velocity of the pile.SELECTED DRAWING: Figure 7

Description

The present invention relates to a pile evaluation method for evaluating pile support performance at the time of new construction or rebuilding of a building.

When constructing or rebuilding a building, there is an increasing demand for reusing the foundation piles of existing buildings in order to reduce construction costs and construction periods and to consider the global environment. When reusing an existing pile, it is necessary to confirm the pile length and the soundness of the material in order to evaluate its support performance. However, there are cases where design documents and construction records of existing buildings do not remain, so some kind of investigation will be conducted to evaluate the actual pile length.

As a pile evaluation method, the applicant measures the elastic wave velocity of the retreat at the pile head in the pile length evaluation based on the impact elastic wave test, and further considers the decrease in the propagation velocity due to the influence of the ground by numerical analysis. A method for improving the evaluation system for pile length was disclosed (see Patent Document 1).

Japanese Unexamined Patent Publication No. 2019-032303

An object of the present invention is to provide a pile evaluation method capable of easily and accurately evaluating the support performance of a pile.

ここで、ρ_pは杭の密度、A_pは杭の断面積、E_pは杭のヤング率、ρ_gは杭周地盤２ａの密度、A_gは杭周地盤２ａの断面積、E_gは杭周地盤２ａのヤング率である。 The pile evaluation method according to the present invention is
Actually, from hitting the head of a pile to generate a striking wave, the striking wave propagates as an elastic wave to the pile, and the reflected wave reflected at the lower end of the pile propagates to the head as an elastic wave. Steps to obtain the rise time difference of the impact elastic wave test of
The step of acquiring the elastic wave propagation velocity of the pile and
A step of determining whether the pile is longer than a predetermined reference value, and
When it is determined that the pile is longer than the reference value, the step of reducing the measured value of the elastic wave propagation velocity of the pile to calculate the pile length and
When it is determined that the pile is shorter than the reference value, the step of calculating the pile length using the measured value of the elastic wave propagation velocity of the pile, and
Have,
The calculation of the pile length for reducing the measured value of the elastic wave propagation velocity of the pile is characterized by using the following formula of the reduction rate or a chart derived from the formula.

Here, [rho _p is the density of the pile, A _p is the cross-sectional area of the pile, E _p is the Young's modulus of the pile, [rho _g is the density of the pile peripheral ground 2a, A _g is the cross-sectional area of the pile peripheral ground 2a, E _g is It is the young rate of the pile surrounding ground 2a.

According to the pile evaluation method according to the present invention, it is possible to easily and accurately evaluate the support performance of the pile.

The range of piles and pile surrounding ground to be considered in the pile evaluation method of the present embodiment is shown. The outline of the ground and piles subjected to the impact elastic wave test in the pile evaluation method of the present embodiment is shown. An analysis model of the three-dimensional finite element method performed to obtain the elastic wave propagation velocity reduction rate of the pile in the pile evaluation method of the present embodiment is shown. Relationship between elastic wave velocity of pile surrounding ground and reduction rate of elastic wave propagation velocity of pile in the result of three-dimensional finite element method analysis performed to obtain elastic wave propagation velocity reduction rate of pile in the pile evaluation method of this embodiment. Is shown. The elastic wave velocity of the pile circumference derived from the result of the three-dimensional finite element method analysis performed to obtain the elastic wave propagation velocity reduction rate of the pile in the pile evaluation method of the present embodiment and the pile circumference additional ground thickness to be considered. Shows the relationship. The relationship between the elastic wave velocity of the pile surrounding ground and the reduction rate of the elastic wave propagation velocity of the pile in the pile evaluation method of the present embodiment is shown. The flowchart of the pile evaluation method of this embodiment is shown.

Hereinafter, the pile evaluation method of one embodiment according to the present invention will be described with reference to the drawings.

FIG. 1 shows the range of piles and pile surrounding ground considered in the pile evaluation method of the present embodiment. FIG. 2 shows an outline of the ground and piles subjected to the impact elastic wave test in the pile evaluation method of the present embodiment.

The pile 1 to be evaluated by the pile evaluation method of the present embodiment is a rod-shaped member having a diameter of 2d, a circular cross section, and a length (H + D). The upper part of the pile 1 is buried in the surface layer ground 2, and the lower end is rooted in the supporting ground 3.

First, consider the wave equation for longitudinal waves propagating in a pile in the air. Considering the balance of the forces acting on a pile with a thickness of dx by applying the element method in the pile length direction, if the vertical downward direction is positive, the surface force expressed by the following equation acts.

ここで、ρ_pは杭の密度、A_pは杭の断面積、E_pは杭のヤング率である。 In addition, various physical property values used thereafter are expressed by the following equations using the defined variables.

Here, [rho _p is the density of the pile, the A _p cross-sectional area of the pile, the E _p is the Young's modulus of the pile.

The equations of motion of the piles that receive the acting force of equations (1-1) and (1-2) are expressed by the following equations.

Equation (1-6f) is a wave equation and represents how waves propagate at elastic wave velocities Vp and p.

Next, the longitudinal wave propagating in the pile 1 in the ground will be described.

It is assumed that the longitudinal wave propagating in the pile 1 in the ground vibrates integrally with the pile 1 in the pile peripheral ground 2a of the portion having the thickness r of the pile circumference. Then, the element method is applied in the pile length direction. Considering the balance of the forces acting on the pile element of thickness dx, if the vertical downward direction is positive, the surface force expressed by the following equation acts.

ここで、ρ_pは杭の密度、A_pは杭の断面積、σ_pは杭の軸応力、E_pは杭のヤング率、m_gは杭周地盤２ａの質量、ρ_gは杭周地盤２ａの密度、A_gは杭周地盤２ａの断面積、σ_gは杭周地盤２ａの軸応力、E_gは杭周地盤２ａのヤング率である。 In addition, various physical property values used thereafter are expressed by the following equations using the defined variables.

Here, [rho _p is the density of the pile, A _p is the cross-sectional area of the pile, sigma _p is axial stress of piles, E _p is the Young's modulus of the pile, m _g is the mass of the pile peripheral ground 2a, [rho _g is pile peripheral ground The density of 2a, _Ag is the cross-sectional area of the pile surrounding ground 2a, σ _g is the axial stress of the pile surrounding ground 2a, and E _g is the Young ratio of the pile surrounding ground 2a.

The equations of motion of the pile element that receives the acting force of equations (2-1) and (2-2) are expressed by the following equations. At this time, it is considered that the movement of the pile element is accompanied by the movement of the surrounding ground, and it is assumed that the longitudinal wave propagates in the range of d + r to which the pile peripheral ground is added with respect to the pile radius d.

Comparing the formula (1-6f) and the formula (2-10g) in consideration of the formula (2-6), it can be seen that the formula (2-11) represents the reduction rate of the elastic wave propagation velocity.

From the above examination, if the thickness r of the pile peripheral ground that vibrates integrally with the pile 1 is determined, the reduction rate of the elastic wave propagation velocity of the pile 1 in the ground can be obtained.

Next, a method of determining the thickness r of the pile surrounding ground will be described. In order to determine the thickness r of the pile peripheral ground 2a that vibrates integrally with the pile 1, a parametric study simulating the impact elastic wave test of the pile 1 is carried out.

FIG. 3 shows an analysis model of the three-dimensional finite element method performed to obtain the elastic wave propagation velocity reduction rate of the pile in the pile evaluation method of the present embodiment.

The analysis conditions are set as follows.
・ Conditions for pile 1 (diameter) 2d = 2m
(Length) H + D = 31m
(Unit volume weight) γ ＝ 9.8ρ _g ＝ 23kN / m ³
(Poisson's ratio) ν = 0.2
(Vertical wave propagation velocity) Vp (3D) = 4000m / s
(Young's modulus) E = 3.31 × 107 kN / m ²
・ Conditions for surface ground 2 (width) W = 10m
(Depth) H = 30m
(Unit volume weight) γ = 9.8ρ _g = 14 or 16 or 18 kN / m ³ (see Table 1)

(Poisson's ratio) ν = 0.49
・ Conditions for supporting ground 3 (width) W = 10m
(Depth) H = 11m
(Unit volume weight) γ ＝ 9.8ρ _g ＝ 18kN / m ³
(Poisson's ratio) ν = 0.49

ここで、γは単位体積重量、V_s,g は地盤の弾性波速度、G＝ρV_s,g ²は地盤のせん断弾性係数を示す。 Further, the surface layer ground 2 is set as shown in Table 1 below.

Here, γ is the unit volume weight, V _{s, g} is the elastic wave velocity of the ground, and G = ρV _{s, g} ² is the shear modulus of the ground.

FIG. 4 shows the elastic wave velocity of the pile surrounding ground and the elastic wave propagation velocity of the pile in the result of the three-dimensional finite element method analysis performed to obtain the elastic wave propagation velocity reduction rate of the pile in the pile evaluation method of the present embodiment. The relationship of reduction rate is shown.

As shown in FIG. 4, in the surface layer ground 2 of clay, silt or sand, as the elastic wave velocity V _{s, g} of the ground increases, the reduction rate of the elastic wave propagation velocity V _{p, p} of the pile 1 also increases.

FIG. 5 shows the elastic wave velocity of the pile surrounding ground derived from the result of the three-dimensional finite element method analysis performed to obtain the elastic wave propagation velocity reduction rate of the pile in the pile evaluation method of the present embodiment and the pile to be considered. The relationship between the circumference additional ground thickness is shown.

The thickness r of the pile peripheral ground 2a is obtained by substituting the equations (2-8) and (2-9) into the equation (2-11). Since the result shown in FIG. 5 shows that the radius of the pile 1 is 1 m, the value of r may be regarded as the ratio of the thickness of the pile peripheral ground 2a to the radius of the pile 1. As shown in FIG. 5, it can be seen that the larger the elastic wave velocity V _{s, g} of the surface layer ground 2, the thinner the thickness r of the pile peripheral ground 2a.

FIG. 6 shows the relationship between the elastic wave velocity of the pile peripheral ground and the reduction rate of the elastic wave propagation velocity of the pile in the pile evaluation method of the present embodiment.

In FIG. 6, the gradient of the regression line and the intercept value are extrapolated with respect to FIG. 4, and an approximate straight line having a mass γ = 20 kN / m ³ per unit volume corresponding to gravel is set. By using this chart, it is possible to obtain the reduction rate of the elastic wave propagation velocity V _{p, p} of the pile 1 with respect to the elastic wave velocity V _{s, g} of the surface layer ground 2 for each soil type.

FIG. 7 shows a flowchart of the pile evaluation method of the present embodiment. Here, the contents explained so far are made into a flowchart, and it is easily confirmed in each order.

First, in step 1, an elastic wave propagation test is performed (ST1). The elastic wave propagation test is performed as shown in Patent Document 1. First, the tip of the head 11 of the pile 1 is hit with a hammer or the like to generate a striking wave. The striking wave propagates through the pile 1 as an elastic wave. The propagated elastic wave is reflected at the lower end of the pile 1. The reflected elastic wave propagates through the pile 1 as a reflected wave and returns to the tip of the head 11. Then, the rise time difference from the generation of the striking wave at the head of the pile 1 to the propagation of the reflected wave to the head is acquired.

Next, in step 2, the elastic wave propagation velocity of the pile is acquired (ST2). The elastic wave propagation velocity of the pile 1 is obtained by V = l / Δt using the measurement interval l in the aerial part of the pile 1 and the measured time Δt.

Next, in step 3, it is determined whether or not the pile is longer than the reference value (ST3). When the pile 1 has a certain length or more, the change in the elastic wave propagation velocity due to the influence of the ground becomes remarkable. Therefore, a reference value at which the influence of the ground becomes strong is determined in advance, and it is determined whether or not it is longer than the reference value.

If it is determined in step 3 that the pile is longer than the reference value, the measured value of the elastic wave propagation velocity of the pile 1 is reduced in step 4 to calculate the pile length (ST4). In the calculation, a chart derived from the formula (2-11) and the formula (2-11) such as the graph of the reduction rate shown in FIG. 6 may be used.

If it is determined in step 3 that the pile is shorter than the reference value, the pile length is calculated using the measured value of the elastic wave propagation velocity of the pile 1 in step 5 (ST5).

As described above, according to the pile evaluation method of the present embodiment, it is possible to evaluate the pile length easily and accurately.

It should be noted that the pile length may be calculated by eliminating step 3 and subsequent steps and always reducing the measured value of the elastic wave propagation velocity of the pile 1.

ここで、ρ_pは杭の密度、A_pは杭の断面積、E_pは杭のヤング率、ρ_gは杭周地盤２ａの密度、A_gは杭周地盤２ａの断面積、E_gは杭周地盤２ａのヤング率である。 As described above, in the pile evaluation method of the present embodiment, after hitting the head of the pile 1 to generate a striking wave, the striking wave propagates as an elastic wave in the pile 1 and the reflected wave reflected at the lower end of the pile 1 is elastic. The step of acquiring the rise time difference of the actual impact elastic wave test until it propagates to the head as a wave, the step of acquiring the elastic wave propagation velocity of the pile 1, and whether the pile 1 is longer than the predetermined reference value. The step of determining whether or not, and the step of calculating the pile length by reducing the measured value of the elastic wave propagation velocity of the pile 1 when it is determined that the pile 1 is longer than the reference value, and the step of calculating the pile length, the pile 1 is shorter than the reference value. If it is determined that, the pile length is calculated by using the measured value of the elastic wave propagation velocity of the pile, and the calculation of the pile length for reducing the measured value of the elastic wave propagation velocity of the pile 1 is as follows. Use the reduction rate equation or a chart derived from the equation. Therefore, it is possible to easily evaluate the support performance of the pile.

Here, [rho _p is the density of the pile, A _p is the cross-sectional area of the pile, E _p is the Young's modulus of the pile, [rho _g is the density of the pile peripheral ground 2a, A _g is the cross-sectional area of the pile peripheral ground 2a, E _g is It is the young rate of the pile surrounding ground 2a.

The present invention is not limited to this embodiment. That is, in the description of the embodiment, many specific detailed contents are included for illustration purposes, but those skilled in the art may make various variations and changes to these detailed contents.

1 ... Pile 11 ... Head 12 ... Lower part 2 ... Ground surface 3 ... Ground

Claims (1)

Actually, from hitting the head of a pile to generate a striking wave, the striking wave propagates as an elastic wave to the pile, and the reflected wave reflected at the lower end of the pile propagates to the head as an elastic wave. Steps to obtain the rise time difference of the impact elastic wave test of
The step of acquiring the elastic wave propagation velocity of the pile and
A step of determining whether the pile is longer than a predetermined reference value, and
When it is determined that the pile is longer than the reference value, the step of reducing the measured value of the elastic wave propagation velocity of the pile to calculate the pile length and
When it is determined that the pile is shorter than the reference value, the step of calculating the pile length using the measured value of the elastic wave propagation velocity of the pile, and
Have,
The pile evaluation method for calculating the pile length for reducing the measured value of the elastic wave propagation velocity of the pile is characterized by using the following formula of the reduction rate or a chart derived from the formula.

Here, [rho _p is the density of the pile, A _p is the cross-sectional area of the pile, E _p is the Young's modulus of the pile, [rho _g is the density of the pile peripheral ground 2a, A _g is the cross-sectional area of the pile peripheral ground 2a, E _g is It is the young rate of the pile surrounding ground 2a.

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