10th Asian Regional Conference of IAEG (2015)

The relation between tunnel supports and elastic wave structures
determined by the TFT system in a tunnel
Masashi NAKAYA(1)*, Kazuhiro ONUMA(1), Hiroyuki YAMAMOTO(1)(1) Hazama Ando Corporation, Tokyo, JAPAN*E-mail: nakaya.masashi@ad-hzm.co.jp
Abstract
The design for a mountain tunnel is based on the results of preliminary surveys
conducted on the surface above the planned tunnel route. One of the most useful surveys isseismic exploration by the refraction method, which is performed along the entire tunnel
length. However, when the tunnel is deep under the ground, the precision of the survey
usually decreases. It is important to conduct seismic exploration within the tunnel as
construction progresses, and to verify the seismic velocity of rock mass near the tunnel.
Accordingly, there is considerable interest in the development of a seismic-wave
exploration system that enables accurate and safe measurements to be taken within thetunnel as it is being driven. We developed the Tunnel Face Tester (TFT), which measures
the occurrence of seismic waves by tunnel excavation blasting. This paper discusses theseismic velocity of rock mass near the tunnel, and the results of in-site measurements.
Keywords: excavation blasting, seismic exploration, refraction method, mountain tunnel
1.

Introduction

There is also the limitation that other work must be
interrupted to conduct the measurement. With theseissues in mind, we developed a seismic wave
survey system, the Tunnel Face Tester (TFT, Fig.1),
which utilizes the tunnel blast as a wave source,
thereby promoting worker safety and enablingother work to progress during measurement.
(Nakaya et al., 2013, 2014).

The new Austrian tunneling method is an
excavation method that relies on the inherent
strength of the surrounding rock as the main
component of tunnel support. The method entails
the prompt spraying of shotcrete (concrete) overeach newly excavated stretch together with thesetting of rock bolts. The support pattern for such
work, such as the thickness of the shotcrete to be
sprayed and the number of rock bolts, is selected tomatch the observed ground conditions.
Preliminary surveys are conducted on theground surface over the full length of the tunnel
route. The results of such surveys are combinedwith the findings of borehole drilling and othersurveys to design a support pattern suitable foreach section of the tunnel. With the surveys
conducted from the ground surface, however,overburden depth and other factors can limit theaccuracy and reliability of the results. Thus, whendriving a tunnel, it is necessary to compare actual
ground conditions as encountered at the cutting
face with inferred ground conditions based on thesurvey results so as to confirm the suitability oftunnel design.
Seismic wave exploration is utilized as a method
to quantitatively estimate the surrounding rockconditions of a tunnel. There are dangers entailed
in utilizing this method at the cutting face,
including the risk of collapse, and it can therefore
be applied only under certain geological conditions.

2. Outline of measurement using TFT system
This development work was conducted to createa system that is easy to handle for tunnel engineers
in the daily tunneling cycle. The system uses a
seismic sensor fixed on a tunnel wall behind thetunnel face, utilizes excavation blasting as a wave
source, and measures seismic waves automatically.
As a result, all blasting can be measured. Figure 2
shows the wave data obtained with the TFT system.
The travel time of elastic wave can be calculated
from the difference between the blasting signal thattriggers the exploration and the rise of the elasticwave.
The TFT system (Figs. 1 and 2) consists of a
portable detector unit (a) and peripheral devices(b)–(d). A current sensor (b) has a noncontact
connection with the blasting lead wire and detects
the ignition current. A geophone (c;GS-20DH,OYO; natural frequency: 28 Hz) of a typecommonly used for seismic exploration ismechanically fixed to the head of a rock bolt (L =3–4 m) in the tunnel wall, which performs as a1

10th Asian Regional Conference of IAEG (2015)

waveguide. The ignition signal and seismic
waveform are acquired by a 2 channel IC recorder(d; DR-05, TASCAM; WAV format; max. sampling
frequency: 96 kHz; max. resolution: 24-bit).
This system can detect acquisition of the signalof the blasting and the waveform of the seismic
wave propagated along the tunnel wall. The traveltime of the wave (ti) is determined from the timedelay between onsets of the blasting signal and theseismic signal. The following equation gives theapparent velocity of seismic waves that traveled
the distance between the geophone and the tunnelface:

V pi  Li ti

Fig. 1 Components of the Tunnel Face Tester(1)

As the tunnel face advances, the change in theseismic velocity is calculated from a combinationof the velocities in the previous measurement
interval and the face advance interval as thefollowing equation:

V pi～i 1  Li 1  Li  ti 1  ti (2)When the geophone position is unchanged, Vp inthe face advance interval can be determined fromthe travel time curve (t: arrival time of P wave; L:
length between geophone and tunnel face), as
shown in Fig. 3. The travel time curve and Vp aredetermined by the following equations:t  aL  b(3)V p  dL dt  1 a(4)Fig. 2 An example of waveform

3. Evaluation of ground around tunnel face byTFT survey
3.1 Measurement method
The TFT survey was conducted for construction
work of Hanabuchiyama Tunnel No. 2 of National
Route 108, as ordered by the Ministry of Land,Infrastructure, Transport and Tourism, located in
Naruko Onsen, Osaki-shi, Miyagi Prefecture. The
total length of the road tunnel is 1194 m and theinternal cross section is 50.4 m2. The geology of thetunnel consists mostly of granodiorite of theCretaceous Period of the Mesozoic Era and a part of
the ground at the end of the tunnel contains fine tuff
from the Hosokura mine formed in the Neogene
period. Also, the refraction elastic wave survey
showed the existence of a fissure zone in the center ofthe tunnel. The elastic wave velocity at1.5D (1.5x10.54 m = 15.8 m) from the tunnel’s ground plane(F.H.) was 0.6-3.8 km/sec when measured with thetomography method and 0.3-5.1 km/sec whenmeasured with the layer stripping method. Figure 5
shows the longitudinal geological profile at theFig. 3 Conceptual diagram of the measurement

Fig. 4 Travel time curve2

10th Asian Regional Conference of IAEG (2015)

Fig. 5 Geological profile of Hanabuchiyama tunnel No.2

face was calculated from the obtained travel timecurves. The travel time curves are obtained from thedistance between the blasting point and the geophone
and the first arrival time, and the tunnel face elastic
wave velocity can be calculated from the slope. The
travel time curves obtained in the measurement of thetunnel are shown in Fig. 6.
An approximate line is calculated for the straightline section of each travel time curve and the elastic
wave velocity was calculated from the slope of theline. Table 1 shows the measurement result of eachmeasurement section. In all the 38 sections where theTFT survey was conducted, the correlation
coefficient of the approximate lines was r=0.920 or
higher, indicating that the calculation of the tunnel
face elastic wave velocity was accurate.

design stage. For the survey, a seismometer wasmechanically set on a rock bolt head on the wall
70-120 m back from the tunnel face. Other equipment
was set near the seismometer after making the area
explosion-resistant. When the drilling work advanced
about 50 m, the survey equipment including theseismometer was moved forward for another survey.
In principle the elastic waves that occurred by every
blasting were measured and the travel time of the firstwaves was calculated.

3.2

Measurement result
As the drilling work advanced, the elastic waves
produced at the working face (blasting point) thatmoved successively were continuously measured.
The ground elastic wave velocity around the tunnel

310th Asian Regional Conference of IAEG (2015)

The average number of the measured blasting
works was 13 and the average measurement section
length was 21.6 m. Therefore, when the tunnel faceadvanced about 20 m, about 13 blasting data setswere obtained to accurately calculate the elastic wavevelocity.
The calculated elastic wave velocity was
Vp=1.2-5.2 km/sec, about the same as the one
obtained in the refraction elastic wave survey madefrom the ground surface. The relation between thetunnel face elastic wave velocity obtained in the TFT
survey and the actual support pattern is shown in Fig.
7. In the section where a heavy pattern was selectedbased on the result of the tunnel face survey, thetunnel face elastic wave velocity was relatively low.
This indicates that the TFT survey can detect achange in the geological features of the tunnel face.
Next, Fig. 8 shows the results of the tunnel faceelastic wave velocity for each actual support pattern.
In the figure, the tunnel face elastic wave velocity
obtained in the TFT survey are plotted for the lower
limit of the elastic wave velocity for each support
pattern (Fig. 5). The correlation coefficient of all thedata is r=0.83 and the actual support pattern and thetunnel face elastic wave velocity have almost thesame tendency. It also shows that the larger the lower
limit of the elastic wave velocity is, the less variation
the tunnel face elastic wave velocity has. This could
be due to the influence of the excavating-caused loose
area. Namely this relation is observed because theinfluence of the loose area would be small in a morerobust rock bed.

Table 1 List of TFT survey resultsNo.

mesurement section(m)number of blasting

(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
(12)
(13)
(14)
(15)
(16)
(17)
(18)
(19)
(20)
(21)(22)
(23)
(24)
(25)
(26)
(27)
(28)
(29)
(30)
(31)
(32)
(33)
(34)
(35)
(36)
(37)
(38)
total
average
max
min

12.0
21.0
11.0
8.0
6.0
18.0
9.0
11.4
9.6
15.6
25.2
22.8
20.4
25.2
24.0
24.0
25.0
40.2
25.2
29.1
7.5
23.5
6.0
25.5
30.0
28.5
37.5
28.5
32.4
26.4
28.8
24.0
30.0
21.0
19.0
35.0
16.0
18.0
820.3
21.6
40.2
6.0

8
13
6
5
68
7
95
11
16
171221
18
1916
17
14
244
123
1213
13
14
13
22
17
19
17
21
151515
16
13
506
13
24
3

elastic wave
velocity(km/sec)correlation
coefficient
r

2.7
2.8
1.2
2.1
2.2
3.2
2.6
2.8
4.9
4.0
4.8
5.1
4.8
4.5
5.1
4.3
4.4
4.6
4.1
4.0
4.7
4.7
4.6
5.2
4.7
4.8
4.8
4.7
4.0
3.9
4.1
4.0
3.0
2.4
2.4
2.7
3.1
3.1

0.991
0.988
0.977
0.986
0.983
0.995
0.987
0.989
0.991
0.971
0.994
0.994
0.995
0.989
0.990
0.990
0.989
0.992
0.990
0.988
0.920
0.987
0.9430.994
0.995
0.997
0.996
0.996
0.995
0.996
0.991
0.981
0.982
0.982
0.987
0.990
0.985
0.992

3.8
5.2
1.2

0.986
0.997
0.920

4. Summary
A tunnel elastic wave survey system Tunnel Face
Tester (TFT) where blasting is used as the vibration
source was developed. With the system, one canaccurately measure the elastic wave velocity aroundthe tunnel face. In this study TFT survey wasconducted for all blasting sections of acurrently-constructed tunnel. The tunnel face elastic
wave velocity calculated in the TFT survey showed
the almost same tendency as the actual support
pattern and reflected the geological change. Also,
good agreement was found in comparison with theelastic wave velocity for the support pattern. There
was a particularly strong correlation in thehigh-velocity region.
In future, the TFT survey method will be applied
under various geological conditions and anevaluation method of loose areas
will beyp studied.
()

Fig. 8 Relation between support patternand elastic wave velocity in faceexcavation blasting (in Japanese), Proceeding
Tunnel Technology Conference of JSCE, Vol. 23,
pp. 209-215.
Nakaya, M., et al. (2014) : Development of ameasurement system of seismic wave generated by
the excavation blasting for evaluating thegeological condition around a tunnel face, XII
International IAEG Congress, Torino, pp.
1031-1034.

References
Nakaya, M., et al. (2013) : Development andillustration of system which evaluating geologicalcondition around tunnel cutting face by the4