JP7236945B2 - Ground anchor tension evaluation method and tension evaluation system - Google Patents

Description

Article 30, Paragraph 2 of the Patent Act applies "The 15th Annual Conference of the Japan Society of Maintenology Abstracts" Publisher: Japan Society of Maintenology Publication date: July 10, 2018 Page number: Pages 125 to 130

Application of Article 30, Paragraph 2 of the Patent Law “The Abstracts of the 15th Academic Lecture Meeting of the Japan Society of Maintenology”

Application of Article 30, Paragraph 2 of the Patent Act "The 53rd Geotechnical Research Presentation Collection" Publisher: The Geotechnical Society of Japan Date of publication: July 24, 2018 Page number: Pages 1459 to 1460

Application of Article 30, Paragraph 2 of the Patent Act "The 53rd Geotechnical Research Presentation Collection" Publisher: The Geotechnical Society of Japan Publication date: July 24, 2018 Page number: Pages 1461-1462

Article 30, Paragraph 2 of the Patent Act applies "The 73rd Annual Academic Lecture Meeting of the Japan Society of Civil Engineers 2018 DVD-ROM version of Lecture Summaries" Publisher: Japan Society of Civil Engineers Publication date: August 29, 2018 Page number: Page 237-238

Article 30, Paragraph 2 of the Patent Act applies "The 73rd Annual Academic Lecture Meeting of the Japan Society of Civil Engineers 2018 DVD-ROM version of Lecture Summaries" Publisher: Japan Society of Civil Engineers Publication date: August 29, 2018 Page number: Pages 239 to 240

TECHNICAL FIELD The present invention relates to a ground anchor tension evaluation method and a tension evaluation system used for diagnosing ground anchor construction failures, aged deterioration, and the like.

The ground anchor (hereinafter also referred to as "anchor") construction method involves embedding a plurality of ground anchors in the target slope and applying a predetermined tension to transmit a predetermined prestress to the fixed ground, resulting in a reaction. It is a construction method that integrates a structure and the ground to maintain and manage the condition of the ground on a slope or slope. It is

At this time, it is necessary to properly maintain and manage the construction status of anchors. Along with the observation of the movement of the tension force, the measurement of the residual tensile force, the so-called tension force, is carried out.

As a method for evaluating tension, there is a lift-off test using a center hole type jack or a small and lightweight jack. However, although the lift-off test using such a jack can measure the tension, considering the installation of these devices, especially the center hole type jack and measures to prevent falling, it takes a lot of time and workers. Is required. In addition, if the geology or material (anchor) has deteriorated over time, in the worst case, there is a risk that the anchor may be pulled out or the free length of the anchor may be broken.

Moreover, it is important to evaluate the soundness of individual anchors, but it is also necessary to evaluate the entire slope where many anchors are installed. However, it is difficult to quickly measure a large number of anchors. Small and lightweight jacks require time to install, even if not as long as center hole jacks.

Therefore, various soundness evaluation techniques have been proposed for non-destructively evaluating the tension based on the natural vibration frequency of the anchor. For example, in Patent Document 1, as a technique for diagnosing the tension force from the natural vibration frequency in the direction perpendicular to the axis of the anchor, it is possible to compare the natural vibration frequencies of the measurement target and the healthy anchor over high-order vibration modes. An evaluation technique for diagnosing the damage location has been proposed.

Then, in Patent Document 2, as the vibration frequency, the frequency of the dominant component with the maximum power spectrum amount is used, and the relationship between the head length of the anchor, the vibration frequency, and the introduced load (tension) is obtained in advance, and the target A technique for estimating the introduced load from the head length and vibration frequency of the anchor has been proposed.

In addition, the present inventors used multivariate analysis such as regression analysis to construct a soundness evaluation formula model that shows the relationship between tension and head length, vibration frequency, free length and other multiple evaluation parameters. , by obtaining the tension based on this soundness evaluation formula model, we have developed an evaluation technique for accurately evaluating the tension at the anchor installation site (see Patent Document 3).

Japanese Patent Application Laid-Open No. 2001-074706 JP 2003-121278 A JP 2017-194275 A

However, it cannot be said that the evaluation techniques described in Patent Documents 1 and 2 are highly accurate. For example, the evaluation technique described in Patent Document 1 does not consider the installation conditions of anchors such as the bearing plate and the head length of the anchor, so it is assumed that the evaluation values will vary depending on the installation conditions of the measurement target. be. In addition, since the evaluation focuses only on the natural vibration frequency, there is a risk of erroneous evaluation depending on differences in shape, material, and installation conditions.

In the evaluation technique described in Patent Document 2, the frequency component that maximizes the amount of power spectrum used is greatly affected by the measurement conditions, the shape of the inspection object, and the constraint conditions, so it is not suitable as a criterion for judgment. not In addition, since the reaction force wall and the fixing part are different between the site and the experimental device, it may not be possible to evaluate with the derived relational expression.

Further, in the evaluation technique described in Patent Document 3, in order to achieve high accuracy, it is necessary to acquire a large amount of measurement data at the stage of constructing the soundness evaluation formula model.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a ground anchor tension evaluation technique capable of evaluating the tension of a ground anchor non-destructively, with high accuracy, more efficiently and in a short period of time.

The inventor of the present invention has noticed that the tendon is supported by the head of the anchor to which tension is applied, which is similar to that of the cantilever beam. In addition, in Patent Document 3, the lateral bending vibration obtained in the process of developing an evaluation technique using a soundness evaluation formula model has a correlation with the tension force, and the peak frequency of the vibration indicating the lateral bending vibration is We paid our attention to the knowledge that it can be preferably used as a parameter for evaluating tension. Then, we thought that the theoretical formula showing the relationship between the axial force and the natural frequency of bending vibration in the cantilever beam can be applied to the relationship between the tension force of the anchor and the natural frequency of bending vibration of the anchor head.

However, the fixed state of the anchor head cannot be said to be the same as that of the cantilever beam, and it has been found that the above-mentioned theoretical formula showing the relationship between the axial force and the natural vibration frequency of the bending vibration cannot be applied as it is. Therefore, the present inventors conducted further studies from both the experimental and theoretical perspectives, derived a relational expression between the tension applied to the anchor and the natural vibration frequency of the bending vibration based on the above theoretical formula, and found that this relation It was found that the above problem can be solved when the expression is applied to the anchor to be evaluated.

The inventions according to claims 1 and 2 are based on the above knowledge, and the invention according to claim 1,
A ground anchor tension evaluation method for evaluating the tension of the ground anchor based on vibration characteristics obtained by hitting the ground anchor from the outside,
The natural vibration frequency of bending vibration is used as a reference from the vibration waveform obtained by hitting multiple ground anchors with different head shapes determined by the head length and head outer diameter under a constant tension force. a first formula derivation step of obtaining the frequency and deriving a relational expression between the reference frequency and the head shape as a first formula;
A natural vibration frequency of bending vibration is obtained from each vibration waveform obtained by striking a plurality of ground anchors with known head shapes under different tensions, and the tension and the first a second formula derivation step of deriving a relational expression of the ratio of the natural vibration frequency of the bending vibration to the reference frequency in the formula as a second formula;
A reference frequency calculation step of measuring the head shape of the ground anchor to be evaluated and calculating the reference frequency of the ground anchor to be evaluated based on the first formula;
an evaluation natural vibration frequency acquiring step of acquiring a natural vibration frequency of bending vibration of the ground anchor to be evaluated by hitting the head of the ground anchor;
The calculated reference frequency of the ground anchor to be evaluated and the acquired natural vibration frequency of the bending vibration of the ground anchor to be evaluated are substituted into the second formula to obtain the tension force of the ground anchor to be evaluated. and a tension calculation step of calculating
In the first formula derivation step, in the first formula shown below, by multivariate analysis with the head length L and the head outer diameter D as independent variables with respect to the reference frequency f ₀ (L, D) , regression coefficients α and β .

In addition, the invention according to claim 2 ,
In the second formula derivation step, in the second formula shown below, the tension T is (f'/f ₀ (L, D)) ² by a simple regression analysis with 2 as an independent variable, the regression coefficient γ and δ .

Both the natural vibration frequency in the primary bending vibration mode and the secondary or higher order bending vibration mode can be applied to the evaluation of the tension force. It is more sensitive to tension than the following vibration modes, and tension can be evaluated with higher resolution. On the other hand, from the viewpoint of efficiency, the second-order vibration mode is preferable because the evaluation can be sufficiently performed using the natural vibration frequency of the second-order vibration mode.

That is, the invention according to claim 3 is
3. The method for evaluating tension force of a ground anchor according to claim 1, wherein a natural vibration frequency of a primary bending vibration mode is used as the natural vibration frequency of the bending vibration .

And the invention according to claim 4 ,
3. The method for evaluating tension force of a ground anchor according to claim 1, wherein a natural vibration frequency of a secondary or higher bending vibration mode is used as the natural vibration frequency of the bending vibration.

In addition, the invention according to claim 5 ,
5. The method for evaluating tension force of a ground anchor according to claim 4 , wherein a natural vibration frequency of a secondary bending vibration mode is used as the natural vibration frequency of the bending vibration.

Next, when the measurement point of the vibration waveform is set on the side surface of the anchor head closer to the tendon fixture, the vibration of higher-order bending vibration modes including the second order can be measured with higher sensitivity.

That is, the invention according to claim 6 is
Using the natural vibration frequency of a secondary or higher bending vibration mode as the natural vibration frequency of the bending vibration,
When obtaining the natural vibration frequency of the bending vibration, the measurement point of the vibration waveform of the ground anchor is set on the side surface of the head of the ground anchor near the tendon fixture. It is a tension evaluation method of the described ground anchor.

When measuring the vibration waveform, the head of the ground anchor is hit with a hammer, and the vibration waveform generated thereby is acquired by the vibration detection sensor. The hammer is not particularly limited, and may be a plastic hammer, a rubber hammer, a wooden hammer, a test hammer, or an iron hammer. Also, as the vibration detection sensor, an accelerometer capable of acquiring vibration, a microphone, or the like may be used, but it is possible to directly contact the measurement target and measure it, and it is possible to acquire a vibration signal with high sensitivity. Preferably, an acoustic emission sensor (AE sensor) is used.

That is, the invention according to claim 7 is
7. A ground anchor tension evaluation method according to any one of claims 1 to 6 , wherein the vibration waveform is generated by striking with a hammer and acquired by a vibration detection sensor. is.

And the invention according to claim 8 ,
8. The tension evaluation method of the ground anchor according to claim 7 , wherein an AE sensor is used as the vibration detection sensor.

As a method of obtaining the frequency distribution from the vibration waveform and obtaining the natural vibration frequency of the bending vibration, it is preferable to numerically analyze the vibration waveform, and more preferably to perform fast Fourier transform processing in this numerical analysis.

That is, the invention according to claim 9 is
By numerically analyzing the vibration waveform, the frequency distribution is obtained from the vibration waveform,
The tension evaluation method of the ground anchor according to any one of claims 1 to 8 , wherein the natural vibration frequency of bending vibration is obtained from the obtained frequency distribution.

Further, the invention according to claim 10 ,
10. The ground anchor tension evaluation method according to claim 9 , wherein fast Fourier transform processing is performed in the numerical analysis.

The ground anchor tension evaluation method in each of the above claims can also be understood from the point of view of the system. That is, the invention according to claim 11 ,
A ground anchor tension evaluation system used in the ground anchor tension evaluation method according to any one of claims 1 to 10 ,
vibration waveform acquisition means for acquiring a vibration waveform by hitting the ground anchor;
Head shape measuring means for measuring a head shape defined by the head length and the head outer diameter of the ground anchor;
a natural vibration frequency acquiring means for performing a frequency distribution analysis on the acquired vibration waveform and acquiring a natural vibration frequency of the bending vibration of the ground anchor;
A natural vibration frequency of bending vibration is obtained as a reference frequency from a vibration waveform obtained by hitting a plurality of ground anchors having different head shapes under a constant tension force, and the reference frequency and the head are obtained. a first formula derivation means for deriving a relational expression with a shape as a first formula;
A natural vibration frequency of bending vibration is obtained from each vibration waveform obtained by striking a plurality of ground anchors with known head shapes under different tensions, and the tension and the first a second formula deriving means for deriving a relational expression of the ratio of the natural vibration frequency of the bending vibration to the reference frequency in the formula as a second formula;
Reference frequency calculation means for measuring the head shape of the ground anchor to be evaluated and calculating the reference frequency of the ground anchor to be evaluated based on the first formula;
Evaluation natural vibration frequency acquisition means for acquiring a natural vibration frequency of bending vibration of the ground anchor to be evaluated by hitting the head of the ground anchor;
The calculated reference frequency of the ground anchor to be evaluated and the acquired natural vibration frequency of the bending vibration of the ground anchor to be evaluated are substituted into the second formula to obtain the tension force of the ground anchor to be evaluated. It is a ground anchor tension evaluation system characterized by comprising a tension calculation means for calculating .

Further, the invention according to claim 12 ,
12. The ground anchor tension evaluation system according to claim 11 , wherein the vibration waveform acquiring means includes a hammer as the striking means for the ground anchor.

Further, the invention according to claim 13 ,
13. The tension evaluation system for a ground anchor according to claim 11 or 12 , wherein the vibration waveform acquisition means comprises a vibration detection sensor as means for acquiring the vibration waveform.

Further, the invention according to claim 14 ,
14. The ground anchor tension evaluation system according to claim 13 , wherein the vibration detection sensor is an AE sensor.

Further, the invention according to claim 15 ,
The ground anchor according to any one of claims 11 to 14 , wherein the natural vibration frequency acquisition means comprises numerical analysis means for obtaining a frequency distribution by numerically analyzing the vibration waveform. It is a tension evaluation system.

Further, the invention according to claim 16 ,
16. A ground anchor tension evaluation system according to claim 15 , wherein said numerical analysis means comprises fast Fourier transform processing means.

ADVANTAGE OF THE INVENTION According to this invention, the tension force evaluation technique of a ground anchor which can evaluate the tension force of a ground anchor nondestructively, highly accurately, more efficiently in a short time can be provided.

FIG. 4 is a cross-sectional view schematically showing the structure of a ground anchor; FIG. 2 is a side view schematically showing an example of a specimen used for mockup tests; FIG. 4 is a diagram illustrating a method of measuring a vibration waveform in one embodiment of the present invention; FIG. 4 is a diagram showing an example of a vibration waveform obtained in one embodiment of the present invention; FIG. 4 is a diagram showing an example of frequency distribution obtained in one embodiment of the present invention; It is a figure which shows an example of the frequency distribution acquired by changing tension. FIG. 4 is a diagram showing an example of the relationship between the evaluation value and the degree of tension, using the natural vibration frequency as the evaluation value. FIG. 8 is a diagram showing another example of the relationship between the evaluation value and the degree of tension, using the natural vibration frequency as the evaluation value. FIG. 9 is a diagram showing still another example of the relationship between the evaluation value and the degree of tension, using the natural vibration frequency as the evaluation value. It is a figure which shows an example of the frequency distribution obtained by FEM analysis and a laboratory test. FIG. 4 is a diagram showing an example of changes in natural vibration frequency when tension is changed. FIG. 8 is a diagram showing another example of changes in natural vibration frequency when tension is changed. It is a figure which shows an example of the relationship between natural vibration frequency and tension. FIG. 4 is a diagram showing an example of the relationship between the natural vibration frequency and D/ ^L2 ; FIG. 4 is a diagram showing an example of the relationship between the natural vibration frequency and D/ ^L2 ; It is a figure which shows an example of the relationship between tension|tensile_strength and (f/ _f0 ) ² . It is a figure which shows an example of the relationship between the estimated (calculated) value of tension, and an actual value. It is a figure explaining the examination method of the measuring method of a vibration waveform. FIG. 4 is a diagram showing the positions of measurement points of a vibration waveform and the frequency distribution of bending vibration obtained at each measurement point;

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described based on embodiments with reference to the drawings.

[1] Anchor tension evaluation method The anchor tension evaluation method of the present embodiment is applied to an anchor derived based on a theoretical formula showing the relationship between the axial force of a cantilever beam and the natural vibration frequency of bending vibration. The tension of the anchor is calculated and evaluated by applying the relational expression between the tension and the natural vibration frequency of the bending vibration to the anchor to be evaluated. A specific description will be given below.

1. Derivation of Relational Expression Between Tension Force and Natural Vibration Frequency (1) Anchor Configuration First, a general configuration of an anchor will be described. FIG. 1 is a cross-sectional view schematically showing the structure of a ground anchor. The anchor 1 comprises an anchor head 11 , a tendon 14 and an anchoring portion 16 . The tendon 14 is formed in a cylindrical shape, one end of which is connected to the anchor head 11 and the other end of which is fixed to a fixing portion 16 formed in the boring hole 20 . In the tendon 14, 14a is a free length portion and 11a is an extra length portion. An axial tensile force (tensile force) is applied to the tendon 14 .

The anchor head 11 includes a tendon fixture 11b, a bearing plate (base plate) 11c, and a pressure receiving plate 11d in addition to the excess length portion 11a. The excess length portion 11a is fixed to the bearing plate 11c via the tendon fixture 11b by tension, and the head length is formed from the tip of the excess length portion 11a to the upper surface of the bearing plate 11c. A nut fixing type, a wedge or a wedge/nut combined type, and the like are usually used for fixing on the upper part of the anchor. A tendon 14 is fixed by screwing with a nut, which is a tendon fixing member 11b.

(2) Derivation of tension force evaluation formula In the case of a cantilever beam, the natural vibration frequency f can be obtained by using the following formula (1) as a theoretical formula. Moreover, in the case of a beam with simple support at both ends on which an axial force acts, the natural vibration frequency f' can be obtained by using the following formula (2) as a theoretical formula. By referring to these theoretical formulas, a tension evaluation formula can be derived.

From equation (1), it can be seen that as L increases, f decreases and shifts to the low frequency side.

Then, by transforming equation (2) as (f'/f) ² =1+(T/n ² Pc), if n and Pc are determined, (f'/f) ² and axial force T is in a linear relationship, and it can be seen that if f' can be measured, the axial force T can be calculated based on the value of (f'/f) ² . When n is fixed, as T increases, f' increases and shifts to the high frequency side, and when T is fixed, f' increases as n increases. /f, that is, the shift width increases.

(3) Derivation of Relational Expressions for Anchor With reference to the above-described relational expressions for cantilever beams and simple-supported beams at both ends, the present inventor determined that the natural vibration frequency of the bending vibration of the anchor head to which tension is applied is , is related to the shape of the anchor head, and assuming that the ratio (frequency ratio) between the fundamental frequency and any natural vibration frequency at a given tension is related to the tension, In addition, we conducted experiments and studies in order of "relationship between the shape of the anchor head and the natural vibration frequency of the bending vibration" and "relationship between the tension and the natural vibration frequency", and confirmed that this assumption was correct.

(a) Relationship between the shape of the anchor head and the natural vibration frequency of bending vibration As described above, the fixed state of the anchor head differs from that of the cantilever beam. It cannot be applied as a relational expression in the anchor as it is.

Specifically, in the case of an anchor, unlike a normal cantilever beam, the anchor head 11 is fixed by tension force, and there is no natural vibration frequency f in the state where tension force is not applied. Further, even if one end of the extra length portion 11a is simply cantilevered, the fixing condition of the extra length portion 11a in the anchor cannot be reproduced. Therefore, for example, the boundary conditions of the nut and plate, the modulus of longitudinal elasticity (Young's modulus), etc. are different, and f cannot be obtained.

Therefore, the present inventor referred to the theoretical formula (1) for the cantilever beam, and instead of the natural vibration frequency f when no axial force is applied, the natural vibration frequency (reference After studying the expression of the frequency f ₀ and the head shape (head length L and head outer diameter D) by a relational expression, the following first formula was arrived at.

From the first formula, it can be seen that the reference frequency f ₀ (L, D) is determined by the head length L and the head outer diameter D. need to leave Therefore, in the present embodiment, first, each of a plurality of ground anchors having different head lengths L and head outer diameters D is hit under a state of being fixed with a constant tension, and each vibration Get the waveform. Next, the obtained vibration waveforms are subjected to frequency distribution analysis to obtain natural vibration frequencies f ₀ (L, D). Then, regression analysis is performed on the obtained relationship between f ₀ (L, D) and (D/L ² ) to obtain constants α and β as regression coefficients.

(b) Relationship between tension and natural vibration frequency I came up with the idea that the formula can be created as a tension evaluation formula.

From the second formula, it can be seen that the tension T is determined by the reference frequency f ₀ (L, D) and the actually measured natural vibration frequency f′. need to keep Therefore, in the present embodiment, the anchor for which f ₀ (L, D) is obtained from the first equation is hit by applying different tension T to acquire the vibration waveform, and then the frequency distribution analysis is performed. Then, the natural frequency f' of each is obtained. Then, a regression analysis is performed on the relationship between the tension T and (f'/f ₀ ) ² to obtain constants γ and δ as regression coefficients.

(c) Evaluation of tension In this way, in the present embodiment, it is divided into two stages, the first formula derivation process and the second formula derivation process, and even with a small amount of data, α, β, γ, and δ are determined. Therefore, depending on the type of anchor, the tension and the natural frequency can be correlated with high accuracy, and the natural frequency of the object to be measured can be obtained. can be evaluated in a short period of time.

To obtain specific data, the head length, head outer diameter, and vibration waveform measured for an existing anchor are used. In addition, measurement data obtained by mock-up tests can also be used. An example of a test piece used in the mockup test is shown as a schematic side view in FIG. In FIG. 2, a test piece using a threaded bolt is shown. In FIG. 2, 21 is a threaded bolt, 22 is a nut, 23 is a concrete block, and 24 is a through hole. When measuring the vibration waveform, a vibration detection sensor is set on the side of the head of the bolt 21, and the side of the head is struck with a hammer to generate lateral bending vibration in the head.

The head length L of the anchor in the first formula is the length L from the tip of the excess length portion 11a shown in FIGS. 2 and 3 to the lower end of the screwed nut. The head outer diameter D is the span diameter D of the bolt shown in FIG. Note that FIG. 3 is a diagram for explaining a method of measuring a vibration waveform in this embodiment.

2. Specific calculation of tension force to be evaluated in the present embodiment In the present embodiment, the tension force T to be evaluated is specifically calculated according to the following procedure. First, confirm the head material and fixing method of the anchor to be evaluated. Next, the head length L and head outer diameter D of the anchor to be evaluated are measured. Also, the vibration waveform of the bending vibration is measured, and the frequency of the measured vibration waveform is analyzed to obtain the natural vibration frequency f'. Next, apply the measured head length L and head outer diameter D to the first applicable formula to calculate the reference frequency f ₀ of the anchor to be evaluated, and obtain the natural vibration frequency f′ and Apply the reference frequency f ₀ to the second equation to calculate the tension T of the anchor under evaluation.

Thus, in the present embodiment, tension can be easily calculated with high accuracy.

[2] Tension evaluation system The tension evaluation system of the present embodiment includes the following means.
・Vibration waveform acquisition means for acquiring a vibration waveform by hitting the ground anchor ・Head shape measurement means for measuring the head shape determined by the head length and head outer diameter of the ground anchor ・Acquired Natural vibration frequency acquisition means for acquiring the natural vibration frequency of the bending vibration of the ground anchor by performing frequency distribution analysis on the vibration waveform ・Blowing multiple ground anchors with different head shapes under a certain tension A first formula derivation means for obtaining the natural vibration frequency of the bending vibration as a reference frequency from the vibration waveform obtained by the above, and deriving a relational expression between the reference frequency and the head shape as a first formula. From each vibration waveform obtained by hitting multiple ground anchors under different tensions, the natural vibration frequency of the bending vibration is obtained, and the tension and the bending vibration with respect to the reference frequency in the first equation are Second formula derivation means for deriving the relational expression with the ratio of the natural vibration frequency as the second formula ・Measure the head shape of the ground anchor to be evaluated, and calculate the reference frequency of the ground anchor to be evaluated based on the first formula Reference frequency calculation means for calculation Evaluation natural vibration frequency acquisition means for acquiring the natural vibration frequency of the bending vibration of the ground anchor to be evaluated by hitting the head of the ground anchor Calculated reference of the ground anchor to be evaluated Tension force calculation means for calculating the tension force of the ground anchor to be evaluated by substituting the frequency and the acquired natural vibration frequency of the bending vibration of the ground anchor to be evaluated into the second expression.

Each of the means described above is preferably built in, for example, a personal computer (PC) as a program so as to be processed as a series of systems.

Note that, as specific head shape measuring means, for example, a tape measure, a ruler, a depth gauge, a laser rangefinder, or the like is used.

The vibration waveform acquisition means includes a vibration excitation means for generating a vibration waveform by striking with a hammer and a vibration detection sensor for acquiring the generated vibration waveform.

As described above, FIG. 3 is a schematic diagram showing the method of measuring the vibration waveform in this embodiment. The vibration waveform is measured by installing a vibration detection sensor such as an AE sensor on the side surface of the extra length portion 11a and hitting the side surface of the extra length portion 11a with a hammer. The measured vibration waveform is recorded on the PC. FIG. 4 shows an example of the obtained vibration waveform.

Numerical analysis software, multivariate analysis software such as single regression analysis and multiple regression analysis, and structural analysis software such as finite element method (FEM) are input in advance into the PC. The PC obtains a frequency distribution including natural vibration frequencies by performing a frequency analysis by applying numerical analysis to the recorded vibration waveform using numerical analysis software. The acquired frequency distribution is displayed on the display device. FIG. 5 shows an example of the obtained frequency distribution. A natural vibration peak of the bending vibration is specified from this frequency distribution, and a natural vibration frequency is obtained.

For numerical analysis, known analysis methods such as fast Fourier transform (FFT), autoregressive maximum entropy method (MEM), autoregressive model (AR), and autoregressive-moving average model (ARMA) can be used. Among these, FFT is preferable because it can be processed in a short time.

In addition, using regression analysis software, under a certain tension force, the PC is used to determine the natural vibration frequency (natural vibration peak frequency) of the bending vibration of multiple ground anchors with different head lengths and head outer diameters, the head Determine the values of α and β in the first equation based on the results of the length and head outer diameter measurements.

In addition, the natural vibration frequency calculated as a function of the head length and head outer diameter in the first formula was used as the reference frequency, and the tension was changed for anchors with known head length and head outer diameter. A simple regression analysis is used to identify the values of γ and δ in the second equation based on the sometimes obtained natural frequency of the bending vibration.

In addition, for the anchor to be evaluated, the measured head length and head outer diameter are applied to the first formula to calculate the reference frequency f ₀ , and the measured natural vibration frequency f' was calculated. The tension force T is calculated by applying the reference frequency _f0 to the second formula and displayed on the display device.

In addition, the PC derives a formula showing the relationship between the tension force T and the natural vibration frequency f' using FEM software as necessary, and the first formula and the second formula derived based on the actual measurement data. Verify the tension T calculated using At this time, as described above, since the tension and the natural frequency are associated with high accuracy, the tension can be evaluated and verified with high accuracy in a short time.

EXAMPLES The present invention will now be described more specifically based on examples.

1. Experiment 1
In Experiment 1, it was verified that the mutual relationship between T, f' and L in the theoretical formula (1) for the cantilever beam and the theoretical formula (2) for the simply supported beam at both ends can also be applied to the anchor. At the same time, verification was performed by theoretical analysis using a conventional FEM.

(1) Verification by experiment (laboratory test) As an anchor specimen, an anchor with a head fixing mechanism of a screw fixing type using a nut and a tendon of a multiple PC steel twisted wire type with a total length of 4700 mm was used. At this time, depending on the structure of the tendon and the size of the outer diameter of the head, the anchor is selected according to No. 1 to No. 4, and each type has three levels of head length: 100 mm, 200 mm, and 300 mm (280 mm only for No. 1). Then, a predetermined load was applied as tension force by a tensile tester, and the natural vibration frequency f' was determined for each.

Table 1 summarizes the specifications of each type of anchor and the magnitude of the applied test load (tension). In addition, this test load (tension) is, when the yield point load of each specimen is _TYS , the tensile load is _TUS , and the allowable tension of the anchor _{is Ta} (= 0.6TUS ₎ , It is shown in order from 0.2T _a /0.5T _a /0.8T _a /T _a /T _a to the middle of 0.9T _YS /6 stages of 0.9T _YS .

Fig. 6 shows the frequency distribution of specimen No. 1 with a head length of 100 mm. In FIG. 6, the vertical axis is the intensity (Magnitude) of the vibration signal, and the horizontal axis is the frequency. The numerical values surrounded by squares are the natural vibration frequencies f', and the solid line indicates the natural vibration frequency of the primary mode and the dashed line indicates the natural vibration frequency of the secondary mode.

Also, specimen No. 1 to No. 7 to 9 show the relationship between the natural vibration convergence number f' and the tension in the primary mode and secondary mode of No. 4 head lengths of 100 mm, 200 mm, and 300 mm (No. 1 is 280 mm). 7 to 9, the vertical axis indicates the evaluation value (natural vibration frequency), and the horizontal axis indicates the degree of tension as a ratio (ratio) to 0.6 _Tus .

From FIGS. 7 to 9, both the natural vibration frequency of the primary mode and the natural vibration frequency of the secondary mode shift to the high frequency side as the degree of tension (tension force) increases in any specimen. , and it can be seen that the tension force can be calculated based on the natural vibration frequency.

In addition, the width of the shift of the natural vibration frequency of the secondary mode is larger than that of the natural vibration frequency of the primary mode, and the sensitivity of the natural vibration frequency of the secondary mode to the tension force is higher than that of the primary mode. It was found that the use of the natural vibration frequency of the secondary mode could improve the resolution of tension force evaluation.

In addition, the width of the shift of the natural vibration frequency was large in the low tension region, but showed a slight relaxation tendency in the high tension region. This result agrees with the fact that the natural vibration frequency f' is proportional to the square root of the tension force T in Equation (2). Also, from a comparison of FIGS. 7 to 9, it was confirmed that the natural vibration frequency shifts to the low frequency side as the head length increases, as suggested by equation (2).

(2) Verification by FEM analysis 2, and in order to improve reproducibility, an anchor head model that models a PC cable, a nut, and an apartment (anchor plate: bearing plate and pressure receiving plate) was used. The head length was set to 300 mm, and the lower surface of the anchor and the lower surface of the PC cable were completely fixed. In addition, the Young's modulus at the center of the nut was changed to simulate the reduction in rigidity due to contact between the apartment and the nut.

Concrete FEM analysis was performed by time-series response analysis considering the influence of tension force. Then, the horizontal vibration of the anchor head obtained from the analysis results was subjected to frequency analysis to obtain a frequency distribution.

Table 2 shows the material constants used for the analysis. Table 3 shows analysis conditions. As common analysis conditions, the friction coefficient of sliding contact is 0.1, the contact conditions between the tendon and the apartment and between the apartment and the nut are bonded contact, the impact load is 1000 N, and the duration is 2.0 × 10 ^-4 . A triangular pulse of seconds was input.

First, CASE No. FIG. 10 shows the frequency distributions obtained by the FEM analysis and laboratory tests for 1 to 6. As shown in FIG. 10, when the Young's modulus at the center of the nut is 205 GPa, which is the same as the material constant, the natural vibration peak of the FEM analysis is a value on the higher frequency side than the experimental result. This is thought to be because the lower part of the anchor plate is completely fixed, and the apartment-nut and nut-plate joints are in contact, and the rigidity of the entire analysis model is higher than in the laboratory test.

On the other hand, when the Young's modulus of the central portion of the nut is lowered, the rigidity contributing to the vibration of the anchor head is lowered, so the natural vibration peak shifts to the low frequency side. The Young's modulus at the center of the nut, which is in good agreement with the experimental results, was 4.1 to 8.2 GPa (0.02 to 0.04 times the material constant).

Next, CASE No. The analysis results of CASE No. 7 to 8 are shown in FIG. The analysis results of 9 to 10 are shown in FIG. 11 and 12, the vertical axis is the magnitude of the vibration signal, and the horizontal axis is the frequency. As shown in FIGS. 11 and 12, in both the primary mode and the secondary mode in any CASE, the natural vibration frequency shifts to the high frequency side as the tension increases, especially in the secondary mode. It can be seen that the shift width in vibration is large.

In addition, in order to quantitatively evaluate the tendency of the FEM analysis results described above, FIG. 13 collectively shows the laboratory test results and the FEM analysis results. In FIG. 13, the vertical axis is the natural vibration frequency, and the horizontal axis is the tension force. As shown in FIG. 13, the results of the laboratory test and the results of the FEM analysis are similar, and it was confirmed that the increase in the natural vibration frequency accompanying the increase in tension can be reproduced by the FEM analysis as well.

2. Experiment 2
In experiment 2, the first formula and the second formula were derived by performing regression analysis based on the measured data. In the experiment, the vibration of the anchor head was measured for the anchor constructed on the slope of the expressway. As for the types of anchors, the upper fixing type is the nut fixing type, and the prestressing tendon is of the PC steel wire type. The structure of the tendon was 1×φ21.8. Here, for the newly installed anchor, a hammering diagnosis was performed after the fixing work of the head was completed. On the other hand, for the existing anchors, a lift test was performed after the hammering test, and the tension was measured in advance.

(1) Specifications of Anchor Table 4 shows the specifications of the anchor used in the experiment.

(2) Derivation of the first formula In deriving the first formula, the data of the newly installed anchor at the test point 2, where the tension is almost constant at 319.6 to 339.6 kN, was used. An AE sensor was used to measure the vibration waveform. Also, as the natural vibration frequency for deriving the formula, the natural vibration frequency of the secondary mode, which has excellent resolution, is used.

The results of regression analysis are shown in FIG. In FIG. 14, the vertical axis is frequency, that is, the natural vibration frequency of the secondary mode, and the horizontal axis is D/ ^L2 . As shown in FIG. 14, the correlation coefficient ^R2 is 0.9701, and it can be seen that there is a good correlation between the obtained natural vibration frequency and D/ ^L2 when the tension force is substantially constant. Also, from the regression equation shown in FIG. 14, the regression coefficients of the first equation were determined as α=3794.5 and β=466.93.

In addition, FIG. 15 shows a diagram in which all data including the experimental results of test points 1 and 3 are collectively described. From Fig. 15, when the tensile force is 244 kN, 237 to 273 kN, and test points 1 and 3, which are smaller than test point 2, the natural vibration frequency shifts to the lower frequency side than the regression equation obtained for the anchor at test point 2. It is understood that This agrees with the tendency of the natural vibration frequency to increase/decrease as the tension increases/decreases, which was obtained in previous laboratory tests.

(3) Derivation of the second formula Next, in order to derive the second formula, a diagram was created in which the vertical axis was the tension force and the horizontal axis was (f'/f ₀ ) ² . The created diagram is shown in FIG.

As shown in FIG. 16, the correlation coefficient R ² is 0.4024, indicating that there is a correlation between tension and (f'/f ₀ ) ² . Also, from the simple regression equation shown in FIG. 16, the regression coefficients of the second equation were determined as γ=481.93 and δ=−175.82.

The tension T is obtained by the above simple regression equation. FIG. 17 shows the relationship between the tension force obtained by this simple regression equation and the tension force actually measured. In FIG. 17, the vertical axis is the estimated tension, that is, the tension calculated using the simple regression equation, and the horizontal axis is the actual tension. The solid line rising to the right is a line indicating that the estimated tension = the actual tension, and the upper and lower broken lines parallel to the solid line indicate that the difference between the estimated tension and the actual tension is 10%. is a line. From FIG. 17 it can be seen that 25 out of 44 or 57% of anchors can be estimated within ±10% and all anchors fall within ±20%.

3. Experiment 3
As described above, it was found that the natural vibration frequency of the secondary mode vibration has a higher resolution of the tension force than the natural vibration frequency of the primary mode. Therefore, in Experiment 3, a measurement method for efficiently acquiring a secondary mode vibration signal without being disturbed by other vibration signals, for example, a primary mode vibration signal, was examined.

Specifically, an experiment was conducted using the cut bolt shown in FIG. In FIG. 18, 21 is an M24 threaded bolt, 22 is a nut, 23 is a concrete block, and 24 is a through hole. Also, 25 is a hydraulic jack, S is an AE sensor, and H is a hammer.

FIG. 19 shows the examination results. Fig. 19 shows the vibration waveform measured at the measurement point (1), that is, the side surface of the tip of the thread bolt in Fig. 18, and the measurement point (2), that is, the side surface closer to the nut, which is the tendon fixture. It is a figure which shows the frequency distribution obtained from the measured vibration waveform. From FIG. 19, it can be seen that the first mode vibration component can be effectively picked up at the measurement point (1). On the other hand, when measuring at the measurement point (2), that is, when a vibration detection sensor is set on the side of the excess length portion 11a closer to the tendon fixing tool, and the side of the tendon fixing tool is hit with a hammer to vibrate, It can be seen that the vibration component of the primary mode is small, and the vibration component of the second or higher order vibration mode is easy to grasp.

Although the present invention has been described above based on the embodiments, the present invention is not limited to the above embodiments. Various modifications can be made to the above embodiment within the same and equivalent scope of the present invention.

1 (Ground) Anchor 11 Anchor Head 11a Extra Length 11b Tendon Fixture (Nut)
11c Bearing plate 11d Pressure receiving plate 14 Tendon 14a Free length portion 16 Fixing portion 20 Boring hole 21 Cut bolt 22 Nut 23 Concrete block 24 Through hole 25 Hydraulic jack S AE sensor H Hammer D Head outer diameter L Head length

Claims (16)

A ground anchor tension evaluation method for evaluating the tension of the ground anchor based on vibration characteristics obtained by hitting the ground anchor from the outside,
The natural vibration frequency of bending vibration is used as a reference from the vibration waveform obtained by hitting multiple ground anchors with different head shapes determined by the head length and head outer diameter under a constant tension force. a first formula derivation step of obtaining the frequency and deriving a relational expression between the reference frequency and the head shape as a first formula;
A natural vibration frequency of bending vibration is obtained from each vibration waveform obtained by striking a plurality of ground anchors with known head shapes under different tensions, and the tension and the first a second formula derivation step of deriving a relational expression of the ratio of the natural vibration frequency of the bending vibration to the reference frequency in the formula as a second formula;
A reference frequency calculation step of measuring the head shape of the ground anchor to be evaluated and calculating the reference frequency of the ground anchor to be evaluated based on the first formula;
an evaluation natural vibration frequency acquiring step of acquiring a natural vibration frequency of bending vibration of the ground anchor to be evaluated by hitting the head of the ground anchor;
The calculated reference frequency of the ground anchor to be evaluated and the acquired natural vibration frequency of the bending vibration of the ground anchor to be evaluated are substituted into the second formula to obtain the tension force of the ground anchor to be evaluated. and a tension calculation step of calculating
In the first formula derivation step, in the first formula shown below, by multivariate analysis with the head length L and the head outer diameter D as independent variables with respect to the reference frequency f ₀ (L, D) , a step of determining regression coefficients α and β .

In the second formula derivation step, in the second formula shown below, the tension T is (f'/f ₀ (L, D)) ² by a simple regression analysis with 2 as an independent variable, the regression coefficient γ and δ, the tension evaluation method of the ground anchor according to claim 1 .

3. The ground anchor tension evaluation method according to claim 1, wherein the natural vibration frequency of the first bending vibration mode is used as the natural vibration frequency of the bending vibration. 3. The method for evaluating tension force of a ground anchor according to claim 1, wherein a natural vibration frequency of a secondary or higher bending vibration mode is used as the natural vibration frequency of the bending vibration. 5. The ground anchor tension evaluation method according to claim 4 , wherein the natural vibration frequency of the second bending vibration mode is used as the natural vibration frequency of the bending vibration. Using the natural vibration frequency of a secondary or higher bending vibration mode as the natural vibration frequency of the bending vibration,
When obtaining the natural vibration frequency of the bending vibration, the measurement point of the vibration waveform of the ground anchor is set on the side surface of the head of the ground anchor near the tendon fixture. A ground anchor tension evaluation method as described. 7. A ground anchor tension evaluation method according to any one of claims 1 to 6 , wherein the vibration waveform is generated by striking with a hammer and acquired by a vibration detection sensor. . 8. The ground anchor tension evaluation method according to claim 7 , wherein an AE sensor is used as the vibration detection sensor. By numerically analyzing the vibration waveform, the frequency distribution is obtained from the vibration waveform,
9. The method for evaluating tension force of a ground anchor according to any one of claims 1 to 8 , wherein the natural vibration frequency of the bending vibration is obtained from the obtained frequency distribution. 10. The ground anchor tension evaluation method according to claim 9 , wherein fast Fourier transform processing is performed in said numerical analysis. A ground anchor tension evaluation system used in the ground anchor tension evaluation method according to any one of claims 1 to 10 ,
vibration waveform acquisition means for acquiring a vibration waveform by hitting the ground anchor;
Head shape measuring means for measuring a head shape defined by the head length and the head outer diameter of the ground anchor;
a natural vibration frequency acquiring means for performing a frequency distribution analysis on the acquired vibration waveform and acquiring a natural vibration frequency of the bending vibration of the ground anchor;
A natural vibration frequency of bending vibration is obtained as a reference frequency from a vibration waveform obtained by hitting a plurality of ground anchors having different head shapes under a constant tension force, and the reference frequency and the head are obtained. a first formula derivation means for deriving a relational expression with a shape as a first formula;
A natural vibration frequency of bending vibration is obtained from each vibration waveform obtained by striking a plurality of ground anchors with known head shapes under different tensions, and the tension and the first a second formula deriving means for deriving a relational expression of the ratio of the natural vibration frequency of the bending vibration to the reference frequency in the formula as a second formula;
Reference frequency calculation means for measuring the head shape of the ground anchor to be evaluated and calculating the reference frequency of the ground anchor to be evaluated based on the first formula;
Evaluation natural vibration frequency acquisition means for acquiring a natural vibration frequency of bending vibration of the ground anchor to be evaluated by hitting the head of the ground anchor;
The calculated reference frequency of the ground anchor to be evaluated and the acquired natural vibration frequency of the bending vibration of the ground anchor to be evaluated are substituted into the second formula to obtain the tension force of the ground anchor to be evaluated. A tension force evaluation system for a ground anchor, comprising tension force calculation means for calculating 12. The ground anchor tension evaluation system according to claim 11 , wherein the vibration waveform acquiring means includes a hammer as the striking means for the ground anchor. 13. The ground anchor tension evaluation system according to claim 11 , wherein the vibration waveform acquisition means comprises a vibration detection sensor as means for acquiring the vibration waveform. 14. The ground anchor tension evaluation system according to claim 13 , wherein the vibration detection sensor is an AE sensor. The ground anchor according to any one of claims 11 to 14 , wherein the natural vibration frequency acquisition means comprises numerical analysis means for obtaining a frequency distribution by numerically analyzing the vibration waveform. tension evaluation system. 16. The ground anchor tension evaluation system according to claim 15 , wherein said numerical analysis means comprises fast Fourier transform processing means.

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