JP7257897B2 - Pile evaluation method - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pile evaluation method for evaluating the supporting performance of piles when constructing or rebuilding a building.

BACKGROUND ART When constructing a new building or rebuilding it, there is an increasing demand for reusing the foundation piles of an existing building in order to reduce the construction cost and construction period and to consider the global environment. When reusing existing piles, it is necessary to confirm the pile length and soundness of materials in order to evaluate their bearing performance. However, in some cases, design documents and construction records for existing buildings do not exist, so some sort of survey is conducted to evaluate the actual pile length.

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

JP 2019-032303 A

An object of the present invention is to provide a pile evaluation method that enables simple and accurate evaluation of the support performance of piles.

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
After striking the head of a pile to generate an impact wave, the impact wave propagates through the pile as an elastic wave, and the reflected wave reflected at the lower end of the pile propagates to the head as an elastic wave. obtaining the rise time difference of the shock acoustic wave test of
obtaining the elastic wave propagation velocity of the pile;
determining whether the pile is longer than a predetermined reference value;
if the pile is determined to be longer than a reference value, reducing the measured value of the elastic wave propagation velocity of the pile to calculate the pile length;
if the pile is determined to be shorter than a reference value, calculating the pile length using the measured elastic wave velocity of the pile;
has
The calculation of the pile length that reduces the measured value of the elastic wave propagation velocity of the pile is characterized by using the following reduction rate formula or a diagram derived from the above formula.

Here, ρ _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, ρ _g is the density of the pile surrounding ground 2a, A _g is the cross-sectional area of the pile surrounding ground 2a, and E _g is It is the Young's modulus of the pile surrounding ground 2a.

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

The range of the pile and the surrounding ground of the pile considered in the pile evaluation method of this embodiment is shown. The outline of the ground and pile which performed the impact elastic wave test in the pile evaluation method of this embodiment is shown. 3 shows an analysis model of a three-dimensional finite element method performed to obtain the elastic wave propagation velocity reduction rate of piles in the pile evaluation method of the present embodiment. The relationship between the elastic wave velocity of the ground around the pile and the reduction rate of 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 this embodiment indicates The elastic wave velocity of the ground around the pile derived from the results of the three-dimensional finite element method analysis performed to determine the reduction rate of the elastic wave propagation velocity of the pile in the pile evaluation method of this embodiment and the additional ground thickness around the pile to be considered shows the relationship between 2 shows the relationship between the elastic wave velocity of the ground around the pile and the reduction rate of the elastic wave propagation velocity of the pile in the pile evaluation method of the present embodiment. 4 shows a flowchart of a pile evaluation method according to the present embodiment.

A pile evaluation method according to an embodiment of the present invention will be described below with reference to the drawings.

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

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

First, consider the wave equation for longitudinal waves propagating through piles in air. Applying the element method in the pile length direction and considering the balance of forces acting on a pile 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、E_pは杭のヤング率である。 Various physical property values used hereinafter are represented by the following equations using the defined variables.

where ρ _p is the pile density, A _p is the pile cross-sectional area, and E _p is the pile Young's modulus.

The equation of motion of the pile that receives the acting forces of Equations (1-1) and (1-2) is expressed by the following equations.

Equation (1-6f) is a wave equation, and expresses how a wave propagates at elastic wave velocity Vp,p.

Next, longitudinal waves propagating through the underground pile 1 will be described.

It is assumed that longitudinal waves propagating through the pile 1 in the ground vibrate together with the pile 1 in the ground 2a around the pile at a thickness r. Then, the element method is applied in the pile length direction. Considering the balance of forces acting on a 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。 Various physical property values used hereinafter are represented by the following equations using the defined variables.

where ρ _p is the density of the pile, A _p is the cross-sectional area of the pile, σ _p is the axial stress of the pile, E _p is the Young's modulus of the pile, m _g is the mass of the pile surrounding ground 2a, and ρ _g is the pile surrounding ground. The density of 2a, A _g 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's modulus of the pile surrounding ground 2a.

The equation of motion of the pile element that receives the acting forces of Equations (2-1) and (2-2) is expressed by the following equations. At this time, it is assumed that the motion of the pile element is accompanied by the motion of the surrounding ground, and the longitudinal wave propagates in the range of d + r, which is the pile radius d plus the pile surrounding ground.

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

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

Next, a method for determining the thickness r of the ground surrounding the pile will be described. In order to determine the thickness r of the pile surrounding ground 2a that vibrates together with the pile 1, a parametric study simulating an 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 this 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
(Longitudinal wave propagation velocity) Vp(3D) = 4000m/s
(Young's modulus) E=3.31×107kN/ ^m2
・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 of 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。 The surface ground 2 is set as shown in Table 1 below.

where γ is the unit weight, V _s,g is the elastic wave velocity of the ground, and G = ρV _s,g ² is the shear elastic modulus of the ground.

FIG. 4 shows the elastic wave velocity of the ground around the pile and the elastic wave propagation velocity of the pile in the results 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 this embodiment. Fig. 3 shows the relation of reduction rate.

As shown in FIG. 4, in the surface layer 2 of clay, silt, or sand, the faster the elastic wave velocity V _s,g of the ground, the larger the reduction rate of the elastic wave propagation velocity V _p,p of the pile 1 .

Fig. 5 shows the elastic wave velocity of the ground around the pile 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 this embodiment and the pile to be considered. Figure 1 shows the relationship of additional ground thickness around the perimeter.

The thickness r of the pile surrounding ground 2a is obtained by substituting the equations (2-8) and (2-9) into the equation (2-11). Since the results shown in FIG. 5 are for the radius of the pile 1 of 1 m, the value of r may be regarded as the ratio of the thickness of the pile surrounding ground 2a to the radius of the pile 1. As shown in FIG. 5, it can be seen that the thickness r of the pile surrounding ground 2a decreases as the elastic wave velocity V _s,g of the surface ground 2 increases.

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

In FIG. 6, by extrapolating the values of the slope and intercept of the regression line with respect to FIG. 4, an approximation straight line of mass per unit volume equivalent to pebbles γ=20 kN/m ³ 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 subsurface ground 2 for each soil type.

FIG. 7 shows a flowchart of the pile evaluation method of this embodiment. Here, the content explained so far is made into a flow chart, and it is easily confirmed for each order.

First, in step 1, an elastic wave propagation test is performed (ST1). Elastic wave propagation tests are performed as shown in US Pat. First, the tip of the head portion 11 of the pile 1 is hit with a hammer or the like to generate an impact wave. The impact wave propagates through the pile 1 as an elastic wave. The propagated elastic wave is reflected at the lower end of pile 1 . The reflected elastic wave propagates through the stake 1 as a reflected wave and returns to the tip of the head 11 . Then, the rise time difference between the generation of the impact wave at the head of the pile 1 and 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 determined by V=l/.DELTA.t, using the measured distance l in the air part of the pile 1 and the measured time .DELTA.t.

Next, in step 3, it is determined whether or not the pile is longer than the reference value (ST3). When the pile 1 exceeds a certain length, the change in elastic wave propagation speed due to the influence of the ground becomes significant. Therefore, a reference value at which the influence of the ground becomes stronger 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, in step 4, the measured value of the elastic wave propagation velocity of pile 1 is reduced to calculate the pile length (ST4). For the calculation, the graph derived from the equation (2-11) such as the graph of the reduction rate shown in the equation (2-11) and FIG. 6 may be used.

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

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

It should be noted that the pile length may be calculated by eliminating the 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, the head of the pile 1 is hit to generate an impact wave, and the impact wave propagates through the pile 1 as an elastic wave, and the reflected wave reflected at the lower end of the pile 1 is elastic. Obtaining the rise time difference of the actual impact elastic wave test until it propagates to the head as a wave, obtaining the elastic wave propagation speed of the pile 1, and whether the pile 1 is longer than a predetermined reference value a step of determining whether the pile 1 is longer than the reference value, a step of calculating the pile length by reducing the measured value of the elastic wave propagation velocity of the pile 1, and a step of calculating the pile length when the pile 1 is shorter than the reference value and calculating the pile length using the measured value of the elastic wave propagation velocity of the pile, and the calculation of the pile length that reduces the measured value of the elastic wave propagation velocity of the pile 1 is as follows. Use the rate of reduction formula or a chart derived from the above formula. Therefore, it is possible to easily evaluate the support performance of the pile.

Here, ρ _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, ρ _g is the density of the pile surrounding ground 2a, A _g is the cross-sectional area of the pile surrounding ground 2a, and E _g is It is the Young's modulus of the pile surrounding ground 2a.

It should be noted that the present invention is not limited by this embodiment. That is, in describing the embodiments, many specific details are included for the purposes of illustration, but those skilled in the art may make many variations and modifications to these details.

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

Claims (1)

After striking the head of a pile to generate an impact wave, the impact wave propagates through the pile as an elastic wave, and the reflected wave reflected at the lower end of the pile propagates to the head as an elastic wave. obtaining the rise time difference of the shock acoustic wave test of
obtaining the elastic wave propagation velocity of the pile;
determining whether the pile is longer than a predetermined reference value;
if the pile is determined to be longer than a reference value, reducing the measured value of the elastic wave propagation velocity of the pile to calculate the pile length;
if the pile is determined to be shorter than a reference value, calculating the pile length using the measured elastic wave velocity of the pile;
has
A pile evaluation method, wherein the calculation of the pile length that reduces the measured value of the elastic wave propagation velocity of the pile uses the following reduction rate formula or a diagram derived from the above formula.

Here, ρ _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, ρ _g is the density of the pile surrounding ground 2a, A _g is the cross-sectional area of the pile surrounding ground 2a, and E _g is It is the Young's modulus of the pile surrounding ground 2a.

Images