EFFICIENT SAFTY ASSESSMENT OF EXPRESSWAY SLOPE
REINFORCED BY GROUND ANCHORS BASED ON
RESIDUAL TENSILE LOAD EVALUATED BY VIBRATION METHOD
M. YAMAZAKI & T. KITAMURA
Central Nippon Highway Engineering Nagoya Co., Ltd., Japanm.yamazaki.a@c-nexco-hen.jp, t.kitamura.b@c-nexco-hen.jp
H. SAITO, N. OGAWA & T. NIIBEOYO Corporation, Japansaito-hideki@oyonet.oyo.co.jp, ogawa-naoto@oyonet.oyo.co.jp,
niibe-takamasa@oyonet.oyo.co.jp
A. YASHIMA
Gifu University, Japanyashima@gifu-u.ac.jp
ABSTRACT
Ground Anchoring is one of the popular methods for maintaining the stability of expressway
slope. Expressway companies have carried out the mandatory inspections including theconfirmation of the residual tensile load of 5% of total anchors or at least 5 anchors for eachanchored slope every five years. The residual tensile load of anchor is usually measured by
the lift-off test with the center hole jack, which sometimes requires a temporary scaffold andtypically requires time and costs. The authors have proposed a new method of nondestructive evaluation for residual tensile load of anchor by the “vibration method” based on
the assumption of “string” of anchor tendon tension part. Based on the proposed technique,
it is easily possible to obtain the residual tensile load by the frequency of the free vibrationof tendon tension part, line density and tendon free length. A series of on-site measurement
for various type anchors were conducted by using the proposed technique. It is found outthat the proposed technique can easily evaluate the magnitude of the residual tensile loadof anchor with the error of about 5%. The maintenance engineers can quantitatively evaluate
the safety of anchored slope using the proposed technique.
1. INTRODUCTION
Ground Anchoring is one of the popular methods for maintaining the stability of highway
slope. The anchoring method was first applied to expressways in 1969 in Japan and thenumber increased gradually from around 1985. The present number of anchors constructedalong the expressways in Japan is over 120,000 [1], [2]. To maintain the stabilizing functionof the anchors, it is necessary that the anchors are not corroded and maintain residual
tensile load within the expected range. The soundness of anchor is evaluated based on the
Guideline for the evaluation of residual tensile load [3], [4] (see Table 1). Periodic inspectionsof anchors are important to ensure the slope stability. Expressway companies have carriedout the mandatory inspections including the confirmation of the residual tensile load of 5%
of total anchors or at least 5 anchors for each anchored slope every five years in accordancewith the maintenance manual [5] and survey manual [6].
The residual tensile load of anchor is usually measured by the lift-off test with the center
hole jack, which sometimes requires a temporary scaffold and typically requires time andcosts (see Photograph 1). Expressway is the essential network. Therefore, it is very difficult
to close the road to traffic for on-site inspection work. We should carry out on-site inspection
work under the heavy traffic condition with traffic lane regulation. To satisfy this requirement,[1]

XXVIIth World Road Congressmuch more convenient as well as less expensive technique for evaluating the residual
tensile load of anchor has been long awaited.
Table 1 Guideline for evaluation of residual tensile load [3], [4]Range of residual tensile load

0.9 T ys0.65 T us
0.6 T us
0.3 T us
0.1 T usT y s : Tendon yield strength ,Evaluation

Situation

D

Possibility of failure

C

Possibility of dengerous situation

B

Tendency to drop of soundness

A

Sound

B

Tendency to drop of soundnessCNon-function

D

Pull up, jump out is seen

T us : Tendon ultimate strength

Photograph 1 Center hole jack and scaffold used for lift-off test
In our research project, we have proposed a new method of non-destructive evaluation forresidual tensile load of anchor by the “vibration method” based on the assumption of “string”
of anchor tendon tension part. Based on the proposed technique, it is easily possible toobtain the residual tensile load by the frequency of the free vibration of tendon tension part,line density and tendon free length.
A series of full-scale model experiments and on-site measurements for various type anchors
were conducted by using the proposed technique. It is found out that the proposed technique
can easily evaluate the magnitude of the residual tensile load of anchor with the error of
about 5%. The maintenance engineers can quantitatively evaluate the safety of anchoredslope using the proposed technique.
2. REVIEW OF OTHER STUDIES
To overcome the shortcomings of lift-off test, many researchers and engineers tried todevelop much more convenient as well as less expensive technique for evaluating theresidual tensile load of anchor. Some of the previous studies are summarized in Table 2.
The schematic diagram of anchor head is shown in Figure 1.
[2]

XXVIIth World Road CongressTable 2 Summary of previous research and their characteristics
Auhor(year)

Measurement
part

Measuring
methodMeasuring principle(Physical phenomenon)

Physical quantity of interest

Mitsuishi K. et al .
(2002) [7]

Extra part

Hammer blow
Acceleration sensor

Change in vibration frequency of
extra part (cantilever beam)

Hundres of Hz
Vibration frequency

Takaue Y. et al .
(2010) [8]

Nut orBearing plate

Hammer blow
Acceleration sensor

unknown

Thousands of Hz
Vibration frequency

Harada N. et al .
(2016) [9]Extra part

Hammer blow
Acceleration sensor

unknown

Thousands of Hz
Vibration frequency

Hamasaki T. et al .
(2019) [10]Extra part

Hammer blow
AE sensor

Change in vibration frequency of
extra part (cantilever beam)

Hundres - Thousands of Hz
Vibration frequency

Okubo K. et al .
(2020) [11]TendonMeasurement of
optical fiber
embedded in tendon

Strain measurment
using pulsed light

Amount of strain

Hisada H. et al.
(2022) [12]Fixer part

X-ray stressmeasurment method

Chenges in stress on
bearing plate

Dozens of Mpa
StressPressure receiving structure
Surface

Extra part
Fixer part
(Nut, Wedge)

TendonBearing plate

Small step
Slope

GroundFigure 1 Schematic diagram of anchor head and tendon
Many studies tried to find the relationship between the resonance frequency of the anchor
head and tensile load by hitting the extra part, nut or bearing plate. Frequency of interest
varies from hundreds of Hz to thousands of Hz. Measuring principle and physicalphenomenon of interest are not clear [7], [8], [9], [10]. Therefore, these method have notbeen implemented yet.
Okubo et al. [11] proposed the optical fiber sensing in which the optical fiber was glued tothe anchor strand. Although the proposed technique has been in practice, the optical fibershould be installed in advance during construction. Therefore, it is impossible to apply thistechnique to the existing anchor without optical fiber.
Hisada et al. [12] proposed simple tension evaluation method using X-ray. They tried toestimate the residual tensile load based on the residual stress of the fixer part measured by
X-ray irradiation. However, they concluded that the proposed method cannot replace the liftoff test.
3. PRINCIPLE OF VIBRATION METHOD AND MEASURING PROCEDURE
3.1. Principle of vibration method
The ground anchor consists of anchor body (fixing part), tendon tension part and anchor
head as shown in Figure 2. We first assume that the tendon tension part of the ground[3]

XXVIIth World Road Congressanchor is approximated by a “string” fixed at both ends. If the tendon tension part is thin andlong enough, the assumption of “string” is considered appropriate [13], [14]. Based on thisassumption, the resonance frequency, f (Hz) of the free vibration of “string” in Figure 2 canbe determined by line density, μ (kg/m), tension free length of the PC steel, L (m) and theoperating tension, T (N) by Eq.1). Therefore, the residual tensile load can be determined byEq.2).
f=

1

T

2L

μ

T=4L2 f2μ

・・・1)

・・・2)
Anchor headAnchor body

Tendon tention part = String (Tendon free length)

Tendon (PC steel wire, etc.)

Extra part

GroundSurface

Figure 2 Schematic structure diagram of ground anchor
3.2. Measuring Procedure
The proposed method is performed with a small vibrator and accelerometers attached to theextra part of the anchor head. The schematic diagram of tensile load measuring system isshown in Figure 3.
We perform the proposed method by the following procedure. First, we place a small vibrator
and accelerometers on the extra part of the anchor head. The waveform generated by thevibrator is a sweep waveform whose frequency of a sinusoidal wave increases continuouslywith time as shown in the upper part of Figure 4. The vibration is applied for 60 or 120
seconds with a frequency increase rate of 1 oct/min or less. In anchors with multiple steel
wires, accelerometers are attached to the vibrated steel wire and the adjacent steel wires.
The lower part in Figure 4 shows an example of measurement data. We can easily
understand the resonance frequency from the received waveform. We consider themeasured resonance frequency as the natural frequency of the tendon tension part andcalculate the residual tensile load by using Eq.2).
Measuring instrument
Data logger

Vibrator
Accelerometer

Vibrator controller
Function generator
Trigger switch

Anchor body

Tendon tention part(Tendon free length)

Extra part
Laptop computer

Anchor head

Figure 3 Schematic diagram of tensile load measuring system by vibration method[4]

XXVIIth World Road CongressSweep waveform
Frequency of a sinusoidal wave increases continuously with time.

Excitation waveform

Resonance point
Received waveform
Time (second)

Figure 4 Schematic diagram of excitation / received waveform and resonance point
4. FULL SCALE MODEL EXPERIMENTIn order to confirm the tendon tension part can be approximated as “string”, we carried out
full scale experiments with three different type of anchors such as nut fixed type, wedge-nut
fixed type and wedge fixed type. The basic properties of anchors are summarized in Table
3. The lengths of tendon tension part of different anchors are about 7 m. Its length issufficiently long compared to the diameter of each steel wire. The experimental apparatus
with 4,000 kN reaction force capacity is shown in Photograph 2. The center hole jack was
equipped at one end and the accelerometer with vibrator was mounted on the extra lengthpart of the other end of anchor. The small target was attached on the center part of tendon
tension part to monitor the vibration acceleration of the tendon part of anchor directly byusing the Laser Doppler vibrometer. The schematic view of the measuring system aroundthe anchor head and the center of tendon part is also shown in Photograph 2.
Table 3 Anchors used for full scale anchor model experiment[5]

XXVIIth World Road CongressPhotograph 2 Full scale anchor model (example of VSL E5-3)
The natural frequency vibration of tendon tension part with the length of about 7 m ofdifferent anchor type was first generated by hitting the tendon tension part with a rubber
hammer. The free vibration generated by tapped with a rubber hammer was monitored by
the Laser Doppler vibrometer. Observed and theoretical natural frequencies for different
type of anchors with different tensile load are summarized in Table 4. The comparison
between observed natural frequencies and theoretical natural frequencies calculated by Eq.
1) is shown in Figure 5. It is found that the observed natural frequency for all type of anchors
coincides very well with the theoretical natural frequency over a wide range of tensile load.
This means that the tendon tension part can be approximated as “string”.
Table 4 Observed and theoretical values of tensile load and natural frequency
Anchorstandard
SEEE F60UA

SFL-3

VSL E5-3

Tensile load
P e (kN)
Set value
conditions / set value (kN)

Observed value
P e (kN)

Tendonfree lengthl sf (mm)Observed
naturalfrequency
f m (Hz)

Theoreticalnaturalfrequency
f t (Hz)

set

60.0

59.9

5872

13.1

13.2

1/3 T as

115.0

116.4

5878

17.8

18.5

1/2 T as

170.0

169.7

5883

21.2

22.3

2/3 T as

230.0

230.3

5887

24.4

25.9

T as

345.0

347.1

5898

29.7

31.8

0.9 T y s

445.0

444.9

5908

33.5

36.0

set

41.0

42.7

7225

8.2

7.9

1/3 T as

155.0

154.8

7238

15.7

15.0

1/2 T as

235.0

235.1

7246

19.5

18.4

2/3 T as

310.0

310.3

7253

22.5

21.2

T as

470.0

470.1

7268

27.8

26.0

0.9 T y s

600.0

600.1

7288

31.5

29.4

set

50.0

49.3

6853

10.4

10.6

1/3 T as

100.0

100.0

6862

14.6

15.1

1/2 T as

150.0

150.4

6868

17.8

18.6

2/3 T as

200.0

200.0

6874

20.4

21.4

T as

300.0

300.0

6885

25.0

26.2

0.9 T y s

400.0

100.4

6899

28.9

30.2

T as : tendon allowable strength , T y s : tendon yield strength[6]

XXVIIth World Road CongressFinally, we tried to obtain the natural frequency of the tendon tension part by using a small
vibrator and accelerometers attached to the extra part of the anchor head. An example ofrunning spectrum and Fourier spectrum diagram is shown in Figure 6. It is found that thecorrect natural frequency of the tendon tension part was observed by the accelerometerattached to the extra part.
40

Observed natural frequency fm (Hz)35
y = 1.06 x
R² = 1.00

30

y = 0.94 x
R² = 1.00

25
20
15
10

SEEE F60UA

y = 0.96 x
R² = 1.00

5

SFL-3
VSL E5-3

0
0

5

10

15

20

25

30

35

40

Theoretical natural frequency ft (Hz)Figure 5 Comparison between observed and theoretical natural frequency

Figure 6 Example of running spectrum and Fourier spectrum diagram (VSL E5-3, 2/3 Tas)
[7]

XXVIIth World Road Congress5. ON-SITE MEASUREMENT AND DISCUSSION
In order to confirm the validity of the vibration method, on-site measurements using the liftoff test and vibration method were conducted for existing ground anchors.
A total of 45 anchors were investigated on 14 slopes from A to N in Japan, as shown in
Table 5. The tests were performed on the following seven types of anchors; SEEE, VSL,
SHS, KP, FLO, SFL, and EHD. The tendon free lengths ranged from 4.0 to 16.0 m. Lift-off
tests were performed on all anchors, and residual tensile loads were determined for 43 ofthese anchors. Anchors I-3 and I-4 did not "lift off" after 250 kN of tension loading, and noresidual tensile load was determined because there was a risk that the anchor would break
if we loaded it any further. The evaluation of soundness of anchor introduced in Table 1 isalso shown in Table 5.
The residual tensile loads estimated from the vibration method (P) and measured by the liftoff test (Pe) are shown in Table 5, and the comparison of Pe and P is shown in Figure 7. The
relationship between Pe and P is approximately 1:1, and the error of P with respect to Pe iswithin ±5% for 32 anchors and ±10% for 11 anchors at most. There is no systematicdifference in error with respect to differences in anchor type or tendon free length.

Residual tensile load by vibration method P (kN)In I-3 and I-4, where the residual tensile load could not be determined by the lift-off test, theresidual tensile load higher than the maximum load in lift-off test was estimated by thevibration method. This method is characterized by its ability to safely estimate the residual
tensile force even for such over-tensioned anchors.
500

A-SEEE F20TA
B-SEEE F70UA
B-SEEE F100UA

400

C-VSL E5-2
D-VSL E5-3
E-VSL E5-3

300

F-VSL E5-3
G-VSL E5-4
H-SHS S5-3

200

I-KP5-2
J-FLO-3
K-SFL-1

100

L-SFL-3
M-EHD5-3
N-EHD5-4
±0%

0
0

100

200

300

400

500

Residual tensile load by lift-off test Pe (kN)

±5%
±10%

Figure 7 Comparison of residual tensile load obtained by lift-off test and vibration method[8]

XXVIIth World Road CongressTable 5 Comparison of no-site measurement results of lift-off test and vibration method
Slope
name

AnchornumberAnchorstandard

Fixed typeA

A-1

SEEE F20TA

Nut

A-2

SEEE F20TA

Nut

A-3

SEEE F20TA

Nut

B

C

D

E

Lift-off test

Tendonfree
lengthl sf (m)Residualtensile load
P e (kN)

7.6

Vibration method

Evaluation

Resonance
frequency
f m (Hz)Res idual
tensile load
P (kN)

Evaluation

Error(P -P e)/P e
(%)

57.0

B

15.0

61.0

B

7.0

7.6

98.0

A

19.6

100.0

A

2.0

10.6

142.0

A

17.0

151.0

A

6.3

B-1

SEEE F70UA

Nut

7.2

321.8

A

22.5

320.0

A

-0.6

B-2

SEEE F70UA

Nut

7.2

310.0

A

22.2

306.8

A

-1.0

B-3

SEEE F100UA

Nut

13.1

344.0

A

11.1

359.8

A

4.6

B-4

SEEE F100UA

Nut

13.1

378.7

A

11.6

343.4

A

-9.3

B-5

SEEE F100UA

Nut

13.1

376.6

A

11.6

356.0

A

-5.5

C-1

VSL E5-2

Wedge

8.5

85.0

B

14.1

89.0

B

4.7

C-2

VSL E5-2

Wedge

8.5

79.0

B

13.7

84.0

B

6.3

D-1

VSL E5-3

Wedge

5.5

170.0

A

24.1

163.1

B

-4.1

D-2

VSL E5-3

Wedge

4.0

243.2

A

41.1

251.6

A

3.5

E-1

VSL E5-3

Wedge

5.5

194.7

A

26.4

195.1

A

0.2

E-2

VSL E5-3

Wedge

13.5

283.6

A

13.8

281.0

A

-0.9

E-3

VSL E5-3

Wedge

9.5

253.7

A

17.2

249.1

A

-1.8

E-4

VSL E5-3

Wedge

15.0

291.4

A

11.7

284.1

A

-2.5

F

F-1

VSL E5-3

Wedge

4.0

163.0

B

32.5

157.0

B

-3.7

F-2

VSL E5-3

Wedge

4.5

162.0

B

29.9

168.0

A

3.7

G

G-1

VSL E5-4

Wedge

9.0

250.0

A

15.4

237.0

A

-5.2

G-2

VSL E5-4

Wedge

8.5

274.0

A

17.5

273.0

A

-0.4

G-3

VSL E5-4

Wedge

10.0

250.0

A

14.4

259.0

A

3.6

G-4

VSL E5-4

Wedge

11.5

174.0

B

10.5

182.0

B

4.6

H-1

SHS S5-3

Wedge

13.5

256.0

A

12.0

243.5

A

-4.9

H-2

SHS S5-3

Wedge

15.5

252.0

A

10.8

262.7

A

4.2

H-3

SHS S5-3

Wedge

15.5

253.0

A

10.0

275.2

A

8.8

I-1

KP5-2

Wedge+Nut

7.5

225.0

B

26.0

236.0

B

4.9

I-2

KP5-2

Wedge+Nut

7.5

172.0

A

22.4

174.0

A

1.2

I-3

KP5-2

Wedge+Nut

7.5

>250

-

26.9

251.0

C

-

H

I

J

K

L

M

N

I-4

KP5-2

Wedge+Nut

8.0

>250

-

26.8

284.0

D

-

J-1

FLO-3

Wedge+Nut

6.0

266.0

A

22.7

246.0

A

-7.5

J-2

FLO-3

Wedge+Nut

6.0

265.0

A

22.5

242.0

A

-8.7

J-3

FLO-3

Wedge+Nut

10.5

190.0

B

11.2

184.0

B

-3.2

J-4

FLO-3

Wedge+Nut

13.0

245.0

A

10.3

238.0

A

-2.9

K-1

SFL-1

Wedge+Nut

7.5

52.0

B

15.0

56.0

B

7.7

K-2

SFL-1

Wedge+Nut

13.0

62.0

B

8.8

57.0

B

-8.1

K-3

SFL-1

Wedge+Nut

9.0

35.0

B

10.0

36.0

B

2.9

L-1

SFL-3

Wedge+Nut

8.0

210.0

B

15.4

201.0

B

-4.3

L-2

SFL-3

Wedge+Nut

8.0

221.0

B

16.3

223.0

B

0.9

L-3

SFL-3

Wedge+Nut

8.0

191.0

B

15.2

195.0

B

2.1

M-1

EHD5-3

Wedge+Nut

10.0

202.0

A

14.7

201.0

A

-0.5

M-2

EHD5-3

Wedge+Nut

16.0

140.0

B

7.8

143.0

B

2.1

N-1

EHD5-4

Wedge+Nut

11.0

236.0

A

12.6

240.0

A

1.7

N-2

EHD5-4

Wedge+Nut

11.0

411.0

A

16.3

400.0

A

-2.7

N-3

EHD5-4

Wedge+Nut

9.5

421.0

A

19.1

408.0

A

-3.1

N-4

EHD5-4

Wedge+Nut

14.5

390.0

A

12.5

409.0

A

4.9

[9]

XXVIIth World Road Congress6. ON-SITE INSPECTION WORK UNDER HEAVY TRAFFIC CONDITION
It is very difficult to close the road to traffic for on-site inspection work. We should carry out
on-site inspection work under the heavy traffic condition with traffic lane regulation as shownin Photograph 3. We applied the proposed vibration method to the evaluation for residual
tensile load of ground anchors installed in tunnel. The inspection work was carried out on
the aerial work platform as shown in Photograph 3. If the lift-off test requires the temporary
safety equipment under the heavy traffic condition, the lift-off test is sometimes impossible
to carry out. On the other hand, the proposed technique can evaluate residual tensile loadof anchor without the temporary and large equipment.
A total of 15 SEEE F70TA (nut fixed type) anchors were investigated. The tendon free
lengths ranged from 11.8 to 15.3 m. There are 9 anchors on the driving lane side and 6 on
the passing lane side. The sweep waveform vibration was applied to the extra part of anchor
head for 60 seconds with a frequency increase rate of 1 oct/min or less.

Photograph 3 Application example of vibration method under heavy traffic condition
Table 6 Inspection result of vibration method under heavy traffic condition
Lane

Driving

AnchornumberAnchorstandard

Fixed
type

Tendonultimate
strength
T us (kN)

Tendonyield
strength
T ys (kN)

Settingload
P t (kN)Line
densityμ (kg/cm)

Tendonfree
lengthl sf (m)Observed
resonant
frequency
f m (Hz)

Residualtensileload
P (kN)

P / PtEvaluation

207 L-2D

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.1

347.9

0.89

A

211 L-3D

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

12.0

406.1

1.03

A

216 L-1B

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.4

365.7

0.93

A

217 L-6A

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.9

401.4

1.02

A

218 L-4A

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

10.8

333.6

0.85

A

219 L-1A

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.3

360.0

0.92

A

243 L-5A

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

11.8

15.8

421.4

1.07

A

243 L-5B

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

11.8

16.1

437.5

1.11

B

244 L-1B

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

11.8

15.3

392.6

1.00

A

Passing 201 R-5D

SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.3

361.9

0.92

A

207 R-1B SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.8

392.0

1.00

A

217 R-4B SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.7

387.3

0.99

A

218 R-4B SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

13.2

493.2

1.26

C

220 R-1A SEEE F70TA

Nut

714.0

608.0

392.4

3.04

15.3

11.2

354.2

0.90

A

243 R-4B SEEE F70TA

Nut

714.0

608.0

392.4

3.04

11.8

15.5

404.5

1.03

A

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XXVIIth World Road Congress

Measured resonance frequencies and estimated residual tensile loads for 15 anchors are
summarized in Table 6. Residual tensile loads estimated by the vibration method ranged
from 333.6 to 493.2 kN. Based on the anchor soundness guideline shown in Table 1, 13
anchors are evaluated to be “A” (Sound), 1 anchor to be “B” (Tendency to drop ofsoundness) and 1 anchor to be “C” (Possibility of dangerous situation).
To order to save the inspection time, three processes, “anchor cap removal”, “measurement”
and “restoration work” were divided into three groups. The working time for 1 anchor was
about 40 minutes. The total inspection work for 15 anchors took about 10 hours.
7. CONCLUSIONS
We have proposed a new method of non-destructive evaluation for residual tensile load ofanchor by the “vibration method” based on the assumption of “string” of anchor tendon
tension part. Based on a series of full-scale model experiments and on-site measurementsfor various type anchors conducted by using the proposed technique, the following
conclusions were obtained.
(1) Based on full scale model experiments, the observed natural frequencies of tendon
tension part for all type of anchors are found to coincide very well with the theoreticalnatural frequencies over a wide range of tensile load. This means that the tendon tension
part can be approximated as “string” and the residual tensile load can be estimated by
the equation “T = 4 L2 f 2 μ ”.
(2) The correct natural frequency of the tendon tension part can be observed by theaccelerometer attached to the extra part.
(3) The sweep waveform vibration to the extra part of anchor head for 60 or 120 seconds
with a frequency increase rate of 1 oct/min or less is effective to capture the resonancefrequency of the tendon tension part. The combination of the running spectrum andFourier spectrum diagram of the extra part vibration is recommended to clearly judge
the resonance frequency of the tendon tension part(4) Based on the on-site measurement for 45 anchors, the relationship between the residual
tensile loads measured by the lift-off test (Pe) and the residual tensile loads estimatedfrom the vibration method (P) is approximately 1:1, and the error of P with respect to Pe
is within ±5% for 32 anchors and ±10% for 11 anchors at most.
(5) The lift-off test sometimes requires a temporary scaffold, safety equipment and typicallyrequires time and costs. On the other hand, the proposed technique can evaluate
residual tensile load of anchor without the temporary and large equipment. The
maintenance engineers can quantitatively evaluate the safety of anchored slope using
the proposed technique.
During the field measurement for 45 anchors, the resonance points are clearly seen as sharp
spectral peaks for many anchors. On the other hand, there were some few data in which
multiple peaks were observed and data in which the resonance point was unclear. In the
future, standardization of the method for reading the resonance frequency will be necessary.
Also, we are studying whether it is possible to measure the residual tensile load withaccuracy that does not cause practical problems even if the test is performed using asimplified procedure. If the tensile load can be measured in a short time by improving theprocedure, the survey cost will be reduced and more anchors can be measured. That would
facilitate sustainably maintaining infrastructure.

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XXVIIth World Road CongressACKNOWLEDGEMENT
This research was partially supported by the Grant-in-Aid for Scientific Research of Japan(No.20KK0091).
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XXVIIth World Road Congress