JP2018179521A - Tension force measuring apparatus and tension force measuring method for ground anchor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for measuring tension force of a ground anchor with nondestruction and precisely.SOLUTION: A vibration unit 41 gives vibration to an extra length portion 11. The frequency of the vibration given by the vibration unit 41 is swept in a range including the estimated natural vibration frequency in a free length portion 12. The detection unit 42 detects vibration in the extra length portion 11. By analyzing the detected vibration, tension force of a ground anchor can be measured.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a technique for nondestructively measuring the tension of a ground anchor.

  The ground anchor fixes the anchorage to a stable ground, connects the anchorage to the surface structure with a long member (so-called tendon or anchor body), and exerts a tensile force (tensile force) acting on the long member ) Is a system that stabilizes the structure. BACKGROUND ART Conventionally, ground anchors are used, for example, for stability of slopes, prevention of overturning and lifting of ground structures, or temporary suspension or earth retaining.

  In such a ground anchor, the tension in the long member sometimes deviates from the appropriate range due to the aged deterioration, the change in the ground, and the like, and in such a case, a predetermined stability effect can not be expected. Therefore, it is regularly performed to determine the tension in the long member.

  By the way, the long member of the ground anchor is often buried deep in the ground, and it is difficult to accurately determine the tension. The lift-off test using hydraulic jacks can measure the tension relatively accurately, but this not only involves partial failure of the ground anchor, but also has the problem of extensive testing and large cost. Therefore, conventionally, for example, as shown in the following Patent Documents 1 to 3, techniques for nondestructively estimating or diagnosing tension have been proposed.

  In the technique of Patent Document 1, the anchor head exposed to the ground is hit with a hammer or the like, and the vibration detected by the vibration sensor attached to the anchor head is regarded as the natural vibration frequency of the anchor body to estimate tension. . However, in general, the frequency of vibration applied by striking with a hammer or the like and the natural vibration frequency of the anchor body are largely different, and it is estimated that it is quite difficult to vibrate the anchor body at its natural vibration frequency by striking. Therefore, in this technique, there is room for further improvement in estimation accuracy.

  In the technique of Patent Document 2, it is proposed to apply an ultrasonic pulse to the extra-long portion (a part of tendon) exposed to the outside of the anchor head and to measure the tension from the propagation time of the reflected wave. However, the variation of the propagation time based on the difference in tension is generally small, and the influence of noise is large. Therefore, there is room for further improvement in the accuracy of the tension obtained by this method.

  In the technique of Patent Document 3, the propagation time of the ultrasonic signal input to the anchor head is measured to estimate the stress acting on the anchor head, and indirectly to estimate the tension of the anchor body itself from there. is suggesting. However, the variation of the ultrasonic wave propagation time based on the stress variation is generally small, and the estimation accuracy of the stress acting on the anchor head is not expected to be high. Furthermore, there is room for doubt in the estimation accuracy of estimating the tension of the anchor body itself based on the stress of the anchor head, and as a result, the reliability of the estimated tension still has room for improvement. is there.

JP, 2001-74706, A JP 7-113231 A JP, 2002-4274, A

  The present invention has been made in view of the above-described circumstances. The main object of the present invention is to provide a technique for nondestructively and accurately measuring the tension of a ground anchor.

  The present invention can be expressed as the invention described in the following items.

(Item 1)
An apparatus for measuring the tension of a ground anchor having an extra length and a free length, comprising:
It has an excitation unit and a detection unit,
The excitation unit is configured to give vibration to the extra length portion,
The frequency of the vibration given by the vibration unit is swept in a range including the estimated natural vibration frequency in the free length portion,
The tension force measuring device for a ground anchor, wherein the detection unit is configured to detect a vibration in the extra length portion.

(Item 2)
The tension measurement apparatus for a ground anchor according to Item 1, wherein the frequency of the vibration given by the vibration unit is swept in a range further including a frequency lower than the estimated natural vibration frequency.

(Item 3)
The tension measurement apparatus for a ground anchor according to Item 1 or 2, wherein the frequency of the vibration given by the vibration part is swept from a low frequency to a high frequency.

(Item 4)
The estimated natural vibration frequency is calculated using at least a length of the free length portion and a coefficient based on a material of the free length portion. The ground anchor according to any one of items 1 to 3, Tension measuring device.

(Item 5)
An accelerometer which detects an acceleration in the extra length part is used as said detection part. The tension measurement device of the ground anchor according to any one of Items 1 to 4.

(Item 6)
It further comprises a judgment unit,
The determination unit is configured to determine tension in the free length portion based on the frequency of the vibration detected by the detection unit. The tension of the ground anchor according to any one of items 1 to 5. Force measuring device.

(Item 7)
When the low frequency peak and the high frequency peak exist in the vibration detected by the detection unit, the determination unit determines the tension in the free length portion based on the frequency of the low frequency peak. The tension measurement device of the ground anchor according to item 6.

(Item 8)
The determination unit is configured to determine tension in the free length portion by further using the length of the free length portion and a coefficient based on the material of the free length portion. Anchor tension measurement device.

(Item 9)
The tension measurement apparatus for a ground anchor according to any one of Items 1 to 8, wherein the excitation unit and the detection unit are configured integrally.

(Item 10)
A method for measuring the tension of a ground anchor having a surplus length and a free length, comprising:
Applying vibration to the extra length portion, wherein the frequency of the applied vibration is swept in a range including an estimated natural vibration frequency in the free length portion;
Detecting the vibration in the extra length part.

(Item 11)
The tension | tensile_strength measurement method of the ground anchor of item 10 further equipped with the step which determines the tension in the said free length based on the vibration in the said excess length.

  According to the present invention, it is possible to measure the tension of the ground anchor nondestructively and accurately.

It is an explanatory view for showing rough composition of a tension measurement device of a ground anchor concerning one embodiment of the present invention. It is a flow chart for explaining the procedure of the tension measurement method of the ground anchor concerning one embodiment of the present invention. It is a graph which shows the running spectrum of the vibration input, Comprising: A horizontal axis shows time, a vertical axis shows a frequency, and a brightness | luminance shows vibration intensity. It is a graph which shows the running spectrum of the detected vibration. In FIG. 4A, the horizontal axis represents time, the vertical axis represents frequency, and luminance represents the amplitude (signal strength) of vibration. The horizontal axis in FIG. 4B indicates the amplitude (the left side is positive), and the vertical axis indicates the frequency. It is a graph which shows the resonant frequency measured by the Example, Comprising: A horizontal axis shows residual tensile force and a vertical axis shows a resonant frequency.

  Hereinafter, an apparatus for measuring tension of a ground anchor according to an embodiment of the present invention (hereinafter sometimes simply referred to as "measurement apparatus" or "apparatus") will be described with reference to the attached drawings.

(Basic configuration of ground anchor)
First, as a premise of the description, an example of a ground anchor (hereinafter sometimes simply referred to as "anchor") will be described with reference to FIG. The ground anchor 1 includes an extra length portion 11, a free length portion 12, a fixing portion 13, an anchor head 14, and a sheath 15.

  The extra length portion 11 and the free length portion 12 are configured by an integral long member (also referred to as a tendon or a tension member). In this embodiment, for example, a strand of PC steel, a strand of multiple PC steel, a PC steel rod, and a continuous fiber reinforcement (for example, FRP rod) can be used as this long member, but it is not limited thereto. Further, in the example of FIG. 1, only one long member is shown to facilitate the description, but a plurality of long members may be arranged in parallel.

  One end (ground side) of the free length portion 12 is fixed to a stable portion (stable ground) of the ground 2 by the fixing unit 13. The fixing unit 13 can fix one end of the free length portion 12 to the ground by an appropriate method such as grout injection, for example. Further, the other end of the free length portion 12 is continuous with and integrated with the extra length portion 11 and is restrained by the anchor head 14.

  The anchor head 14 is attached to a load support 3 which is a structure fixed to the surface of the ground 2. The anchor head 14 can maintain the tension (tensile force) given to the free length portion 12 by restraining the other end of the free length portion 12. As a specific configuration of the anchor head, for example, one that applies tension to the free length portion 12 by a wedge structure or a bolt and nut structure is known, but is not limited thereto. The part exposed to the outer side of the anchor head 14 among the elongate members which comprise the free length part 12 comprises the surplus length part 11 in this embodiment. The anchor head 14 can be provided with a removable cap (not shown) for protecting the extra length 11.

  The sheath 15 is disposed between the free length portion 12 and the ground 2 and is formed of a tubular member covering the periphery of the free length portion 12, and the free length portion 12 is free from external force and water from the ground 2. It is supposed to protect.

  Such a ground anchor 1 can be configured in the same manner as an existing one, and thus further detailed description will be omitted.

(Configuration of tension force measuring device of the present embodiment)
The tension measurement device 4 of the present embodiment includes an excitation unit 41 and a detection unit 42. Furthermore, the device 4 additionally includes a determination unit 43.

(Excitation unit)
The vibrating portion 41 is attached to the extra length portion 11 and is configured to give vibration to the extra length portion 11. The frequency of the vibration given by the vibration unit 41 is swept in a range including the estimated natural vibration frequency (described later) in the free length portion 12. More preferably, the frequency of the vibration given by the vibration unit 41 of the present embodiment is set to be swept in a range further including a frequency lower than or higher than the estimated natural vibration frequency. That is, the excitation unit 41 of the present embodiment can apply vibration while changing the frequency in a preset range. In addition, the fluctuation range of the frequency may be preset or may be dynamically generated based on some parameter.

  Although the frequency of the vibration given by the vibration unit 41 is swept from low frequency to high frequency in this embodiment, it is not limited to this, and it is configured to sweep in the opposite direction It is also possible. Further, although the vibration waveform given by the excitation unit 41 is a sine wave in the present embodiment, the present invention is not limited to this, for example, it may be any periodic vibration waveform such as a triangular wave or a square wave. .

  Here, the estimated natural vibration frequency is calculated using at least the length of the free length portion 12 and a coefficient based on the material of the free length portion 12. The specific method of this calculation will be described later.

  The direction of the vibration given to the extra length part 11 by the vibration part 41 is set to the direction crossing the axis of the extra length part 11, more preferably the direction orthogonal to it. A commercially available vibrator capable of sweeping the frequency in the necessary range can be used as the vibration unit 41, and thus the detailed description will be omitted.

(Detection unit)
The detection unit 42 is configured to detect a vibration in the extra length portion 11. In the present embodiment, although an accelerometer for detecting the acceleration in the extra length portion 11 is used as the detection portion 42, the present invention is not limited to this, and in summary, the vibration (displacement, velocity of the extra length portion 11 Or what is necessary is just what can acquire the time change of acceleration. Further, the detection unit 42 of this example can detect a component in the same direction as the vibration applied by the vibration excitation unit 41.

  As the detection unit 42, any vibration frequency in the necessary range may be detected, and various commercially available vibration sensors can be used, and thus the detailed description will be omitted.

(Judgment unit)
The determination unit 43 is configured to determine the tension in the free length portion 12 based on the frequency obtained by frequency analysis of the vibration detected by the detection unit 42. Specifically, the determination unit 43 of this example uses the length of the free length portion 12, the coefficient based on the material of the free length portion 12, and the detected vibration frequency to determine the tension in the free length portion 12 It is configured to determine. In addition, when the low frequency peak and the high frequency peak exist in the vibration detected by the detection unit 42, the determination unit 43 of this example determines the tension in the free length unit 12 based on the frequency of the low frequency peak. It is configured to determine. The specific determination method will also be described later. Although the determination unit 43 in the present embodiment is not particularly limited, for example, the output from the detection unit 42 can be implemented by a combination of appropriate computer hardware and a computer program.

(Physical model of ground anchor)
Hereinafter, a physical model of the ground anchor 1 will be described as a premise for explaining the tension measurement method of the present embodiment.

  First, the vibration of the free length portion 12 in the ground 2 can be approximated to the vibration of a string. Even if it is inside the ground 2, the free length portion 12 is considered to be capable of free vibration under its ideal installation state. In this case, assuming that the length of the free length portion 12 is L, the linear density thereof is μ, and the resonance frequency (natural vibration frequency) of the free length portion 12 is f, the following relationship exists among them.

T = 4 L ² f ² μ (1)
Here, T is a tension (tension) acting on the free length portion, and μ can be calculated from the following equation.

μ = (π / 4) d ² ρ
Where d is the diameter of the free length and ρ is the density of the material.

  In the equation (1), the length L and the linear density μ are generally known. The tension T at a specific ground anchor is determined as a reference value or a set value. If these variables are known, the frequency f can be determined. Conversely, it can be said that the frequency f determined in this way is a value when the tension T is in a desired state. So, in this embodiment, f calculated | required in this way is used as presumed natural oscillation frequency. Of course, the above equation is merely an example, and appropriate modifications and parameter changes are possible as needed. In short, as the estimated natural vibration frequency in the present embodiment, a resonance frequency obtained when the vibration of the free length portion 12 is assumed to be a vibration of a string, or a range in the vicinity thereof If it is

(Method of measuring tension in this embodiment)
Next, the tension measurement method in the present embodiment will be described with further reference to FIG.

(Step SA-1 in FIG. 2)
First, the excitation unit 41 and the detection unit 42 are installed in the extra length section 11 (see FIG. 1). Next, vibration is applied to the extra-long portion 11 by the excitation unit 41. The frequency of the given vibration is swept in a range including the estimated natural vibration frequency in the free length section 12. Here, the estimated natural vibration frequency f of this example is calculated in advance by the following equation (2) from the equation (1).

  Here, as the tension T, a design value or a reference value set based on some evaluation standard can be used.

  However, the estimated natural vibration frequency does not have to be calculated separately for each target ground anchor. For example, based on the estimated natural vibration frequency at the standard ground anchor (s), the realistic fluctuation range of the frequency is taken into consideration, and the frequency is swept in a wide range including the fluctuation range It is also possible. This case also corresponds to an example of the sweep in the range including the estimated natural vibration frequency. In short, the range of the sweep may be a range assumed to include the natural vibration frequency, and the specific width thereof can be determined experimentally, for example.

  In addition, the sweep in the present embodiment is not limited to the one that changes the frequency continuously, but may be one that changes the frequency discretely within a range that does not cause any problem in practice.

  The running spectrum of a specific vibration input is shown in FIG. The spectral intensity is represented by the brightness in this figure. In this example, the sweep start frequency is 10 Hz and the sweep end frequency is 200 Hz. Also, the frequency was changed linearly with the sweep time being 50 seconds. Moreover, in this example, a plurality of values in the range of 35.9 Hz to 51.1 Hz are assumed as the estimated natural vibration frequency.

  Here, the running spectrum will be briefly described. The running spectrum is a spectrum generated by arranging a plurality of section spectra measured while shifting a target time (or a target sample period).

(Step SA-2 in FIG. 2)
Next, the detection unit 42 detects the vibration in the extra length portion 11. The detection unit 42 in this example detects a vibration in the same direction as the excitation unit 41 and sends the vibration to the determination unit 43.

(Step SA-3 in FIG. 2)
The determination unit 43 generates a running spectrum of the obtained vibration. FIG. 4 shows the running spectrum of the vibration detected for the input of FIG. In FIG. 4 (a), superposition of the spectral waveform in each section (envelope of the amplitude in FIG. 4 (b)) is shown by luminance, but in FIG. There is.

  Next, the determination unit 43 determines the tension in the free length portion 12 based on the vibration detected by the detection unit 42. A specific method of tension determination will be described with reference to FIG. 4 (b). In this example, as a result of sweeping the excitation frequency, it is possible to detect two peaks: a peak at 37.1 Hz (hereinafter referred to as low frequency peak) and a peak at around 70 Hz (hereinafter referred to as high frequency peak). According to the study of the present inventors, in the ground anchor having the extra length portion, the objects that cause resonance based on the excitation are mainly the extra length portion and the free length portion. And since the length of the free length portion is usually 4 m or more, the resonance frequency of the free length portion is generally lower than the extra length portion exposed to the outside from the anchor head. Therefore, in general, it can be determined that the low frequency peak is due to the resonance in the free length portion 12 and the high frequency peak is due to the resonance in the extra length portion 11. Therefore, the frequency of the low frequency peak can be identified as the resonant frequency of the free length portion 12.

  In the present embodiment, this determination is automatically performed by the determination unit 43. However, the determination can also be performed by the operator using the frequency analysis result of the detected vibration.

  Alternatively, the resonance frequency of the extra length portion 11 can be separately acquired by some method (for example, by light hitting), and the frequency of the other peak can be specified as the resonance frequency of the free length portion 12. Since the extra length portion 11 is exposed to the outside, it is relatively easy to specify its own resonance frequency.

(Step SA-4 in FIG. 2)
Next, the determination unit 43 uses the resonance frequency f of the free length portion 12 to determine the tension T in the free length portion 12 based on the above-described equation (1). Since both the length L of the free length portion 12 and its linear density μ are generally known, the tension in the free length portion 12 can be determined nondestructively by the method of the present embodiment.

  Also in the prior art, it has been proposed to estimate the tension of the free length nondestructively by detecting the vibration of the extra length and the anchor head. However, in these conventional techniques, analysis is performed without considering the estimated natural frequency based on the physical properties of the free length part. On the other hand, in the present embodiment, since the excitation frequency is swept so as to include the estimated natural frequency when the free length 12 is assumed to be a string, the natural frequency of the free length 12 is accurately determined. There is an advantage that the possibility of being acquired is high.

  Then, the detected vibration is subjected to frequency analysis, and a peak in the vicinity of the estimated natural frequency can be estimated to be the natural frequency of the free length portion 12 and tension can be calculated using the natural frequency. . In this way, there is an advantage that the tension of the free length portion 12 can be accurately calculated.

  If no peak is found near the estimated natural vibration frequency, it can be determined that the free length portion 12 has some abnormality. For example, when tension changes abnormally due to breakage of the free length portion 12 or damage to the anchor head 14, it is also assumed that there is no peak in the vicinity of the estimated natural vibration frequency. In such a case, it is possible to determine an abnormality of the ground anchor without calculating tension using another peak of unclear meaning.

  In the case where one ground anchor 1 is configured using a plurality of free lengths 12, the free length 12 may be excessively loaded (for example, damage to other free lengths). . In that case, since the natural vibration frequency rises abnormally, a peak may not be found in the vicinity of the estimated natural vibration frequency. Also in that case, the abnormality of the said ground anchor can be determined by not finding such a peak. Here, the extent to which the peak detection range is set can be determined, for example, experimentally or by application.

  In the present embodiment, sweeping of the excitation frequency is performed in a range including a frequency lower than the estimated natural vibration frequency. Therefore, even in the case where the resonance frequency is lowered due to the slack of the free length portion 12 or the like. The low frequency peak can be detected appropriately to calculate the tension. In addition, if the excitation frequency is swept in a range including a frequency higher than the estimated natural vibration frequency, the low frequency peak is caused even when the resonance frequency is increased due to the increase in load on the free length portion 12. Can be appropriately detected to calculate tension.

  Further, in the present embodiment, since the frequency of the vibration given by the vibration unit 41 is swept from the low frequency to the high frequency, the low frequency peak can be detected quickly. That is, since the low frequency peak is usually used to calculate the tension of the free length portion 12, according to the present embodiment, the sweep operation can be performed efficiently.

(Example)
Next, an example in which a resonant frequency (of a low frequency peak) is acquired for an actual ground anchor by using the method of the above-described embodiment is shown in FIG. In these examples, the tension by the latest lift-off test using a jack was shown as "remaining tension". And the frequency obtained by substituting this value into Formula (2) was shown as a white circle in the figure as a predicted value. In addition, the resonance frequency detected by the method of the present embodiment is indicated by black circles, black triangles, black squares, and black diamonds in the figures for each example. Since each ground anchor has four free lengths 12 (and thus four extra lengths 11), the resonant frequency is detected by the above-described embodiment. Furthermore, the average of the resonant frequency in each free length portion is indicated by a horizontal line. The free anchor length of the ground anchor used in this example was 4.0 m.

From these results, the following can be said.
(1) In any of the embodiments, the predicted value and the average value differ by at most 10 Hz at most, and it can be seen that the vibration of the low frequency peak can be reliably detected if the sweep range is appropriately set. .
(2) Since the predicted value of the resonance frequency calculated based on the estimated tension and the average value of the detected peak frequencies have a clear correlation, the magnitude of the actual tension can be It also shows that you can show at high and low.
(3) In the case where only the measured value of one free length part is largely separated from the average value or the predicted value as in Example 3, there is a possibility that an abnormality of only the free length part is detected, It can be used to assess the soundness of the ground anchor.

  Note that the descriptions of the embodiments and the examples are merely examples, and do not show essential configurations of the present invention. The configuration of each part is not limited to the above as long as the purpose of the present invention can be achieved.

  For example, in the above-described embodiment, the excitation unit 41 and the detection unit 42 are separately provided, but may be integrated. In the embodiment described above, the detection unit 42 is directly attached to the extra length portion 11. However, the detection portion 42 is attached to the excitation portion 41, and the vibration of the excess length portion 11 via the excitation portion 41 Can also be detected. Similarly, a configuration is also possible in which vibration is applied to the excess length portion 11 by the vibration excitation unit 41 via the detection unit 42 attached to the excess length portion 11.

  Further, in the above-described embodiment, the frequency peak is detected using the running spectrum, but the present invention is not limited to this, and the frequency peak can also be detected based on general frequency analysis.

  Furthermore, although the linear density μ is assumed to be constant in the above-described embodiment, μ may vary depending on the tension. In this case, the equations (1) and (2) can be applied in consideration of the fluctuation of the linear density μ. For example, it is possible to experimentally obtain the correspondence between the fluctuation of the linear density μ and the tension, and correct the expressions (1) and (2) using the correspondence.

1 Ground Anchor 11 Excess Length 12 Free Length 13 Anchorage 14 Anchor Head 15 Sheath 2 Ground 3 Load Supporter 4 Tension Force Measuring Device 41 Excitation Unit 42 Detection Unit 43 Determination Unit

Claims (11)

An apparatus for measuring the tension of a ground anchor having an extra length and a free length, comprising:
It has an excitation unit and a detection unit,
The excitation unit is configured to give vibration to the extra length portion,
The frequency of the vibration given by the vibration unit is swept in a range including the estimated natural vibration frequency in the free length portion,
The tension force measuring device for a ground anchor, wherein the detection unit is configured to detect a vibration in the extra length portion. The tension measurement apparatus of the ground anchor according to claim 1, wherein the frequency of the vibration given by the vibration unit is swept in a range further including a frequency lower than the estimated natural vibration frequency. The tension measurement apparatus of the ground anchor according to claim 1 or 2, wherein the frequency of the vibration given by the vibration part is swept from low frequency to high frequency. The ground anchor according to any one of claims 1 to 3, wherein the estimated natural vibration frequency is calculated using at least a length of the free length portion and a coefficient based on a material of the free length portion. Tension measuring device. The tension force measuring device of the ground anchor according to any one of claims 1 to 4, wherein an accelerometer for detecting an acceleration in the extra length portion is used as the detection unit. It further comprises a judgment unit,
The ground anchor according to any one of claims 1 to 5, wherein the determination unit is configured to determine tension in the free length portion based on the frequency of the vibration detected by the detection unit. Tension measuring device. When the low frequency peak and the high frequency peak exist in the vibration detected by the detection unit, the determination unit determines the tension in the free length portion based on the frequency of the low frequency peak. The tension measuring device of the ground anchor according to claim 6 which has become. The configuration according to claim 6, wherein the determination unit is configured to determine the tension in the free length portion by further using the length of the free length portion and a coefficient based on the material of the free length portion. Ground anchor tension measurement device. The tension measurement apparatus for a ground anchor according to any one of claims 1 to 8, wherein the excitation unit and the detection unit are integrally configured. A method for measuring the tension of a ground anchor having a surplus length and a free length, comprising:
Applying vibration to the extra length portion, wherein the frequency of the applied vibration is swept in a range including an estimated natural vibration frequency in the free length portion;
Detecting the vibration in the extra length part. The method for measuring tension of a ground anchor according to claim 10, further comprising the step of determining tension in the free length based on vibration in the extra length.