AbstractThe health status of a railway tunnel should be regularly inspected during its service period to ensure safe operation. Ground-penetrating radar (GPR) has been used as a key technique for tunnel detection; however, so far, the measurements of GPR are only obtainable in contact mode. Such methods cannot meet the requirements of the operational tunnel disease census and regular inspections. Therefore, a new method—vehicle-mounted GPR with long-range detection—has been developed. It consists of six channels. The distance from its air-launched antenna to the tunnel lining is approximately 0.93 m–2.25 m. The scanning rate of each channel is 976 1/s. When the sampling point interval is 5 cm, the maximum speed can reach up to 175 km/h. With its speed and air-launched antenna, this system has a significant advantage over existing methods. That is, for an electrified railway, there is no need for power outages. Indeed, the proposed system will not interrupt normal railway operation. Running tests were carried out on the Baoji–Zhongwei and Xiangfan–Chongqing railway lines, and very good results were obtained

Li, Zhilin

Su, Guofeng

Zan, Yuewen

Zhang, Xiyuan

Elsevier - Publisher Connector 

Case Studies in Nondestructive Testing and Evaluation 6 (2016) 63–69Contents lists available at ScienceDirect
Case Studies in Nondestructive Testing and 
Evaluation
www.elsevier.com/locate/csndt
An innovative vehicle-mounted GPR technique for fast and 
eﬃcient monitoring of tunnel lining structural conditions
Yuewen Zan a,∗, Zhilin Li a, Guofeng Su a, Xiyuan Zhang b
a State–Province Joint Engineering Laboratory of Spatial Information Technology for High-Speed Railway Safety, Southwest Jiaotong 
University, Chengdu 611756, China
b Xi’an Jiaotong University, Xi’an 710049, China
a r t i c l e i n f o a b s t r a c t
Article history:
Available online 14 October 2016
The health status of a railway tunnel should be regularly inspected during its service period 
to ensure safe operation. Ground-penetrating radar (GPR) has been used as a key technique 
for tunnel detection; however, so far, the measurements of GPR are only obtainable in 
contact mode. Such methods cannot meet the requirements of the operational tunnel 
disease census and regular inspections. Therefore, a new method—vehicle-mounted GPR 
with long-range detection—has been developed. It consists of six channels. The distance 
from its air-launched antenna to the tunnel lining is approximately 0.93 m–2.25 m. The 
scanning rate of each channel is 976 1/s. When the sampling point interval is 5 cm, the 
maximum speed can reach up to 175 km/h. With its speed and air-launched antenna, this 
system has a signiﬁcant advantage over existing methods. That is, for an electriﬁed railway, 
there is no need for power outages. Indeed, the proposed system will not interrupt normal 
railway operation. Running tests were carried out on the Baoji–Zhongwei and Xiangfan–
Chongqing railway lines, and very good results were obtained.
© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC 
BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction
Regular inspections and health assessments are conducted for the repair and maintenance of tunnels. Railway tunnels, 
operating for a certain period of years, are aging and appear to be diseased, which is a worldwide problem. For example, in 
China, there presented the lining block-falling of tunnel vault for many times in 2001. The most signiﬁcant accident occurred 
when a 30-m-long vault block suddenly collapsed and interrupted traﬃc for 10 days [1]. After the Taiwan earthquake of 
1999, 57 tunnels were investigated, and 85.96% exhibited different degrees of disease [2]. According to a study of 302 
operating highway tunnels in Hokkaido conducted by the highway tunnel branch of the Japanese Hokkaido Civil Engineers 
Association in 1986, there were diseases in 46.69% of the tunnels [3]. Undoubtedly, to eliminate these hazards in a timely 
fashion and to ensure operational safety, regular health inspections of tunnels are crucial.
Regular tunnel inspections are challenging. Nowadays, the traditional methods of GPR include a handheld antenna and 
hydraulic supporting antenna, which require power outages during maintenance times. The eﬃciency of both approaches 
is quite low because their antennas are close to the tunnel walls. Moreover, the antenna must go up and down when the 
drop post of the electriﬁcation catenary appears. As a result, the detecting speed is slow. Currently, there seems to be 
no traﬃc-interference-free nondestructive testing (NDT) method used to evaluate tunnel linings. Therefore, ﬁnding a fast 
* Corresponding author.
E-mail address: dhxyzyw@home.swjtu.edu.cn (Y. Zan).http://dx.doi.org/10.1016/j.csndt.2016.10.001
2214-6571/© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
64 Y. Zan et al. / Case Studies in Nondestructive Testing and Evaluation 6 (2016) 63–69Fig. 1. Locations of the six antennas for vehicle-mounted GPR.
and nondestructive inspection method for tunnel health conditions has always been a concern in the ﬁeld of the tunnel 
maintenance.
2. Design and method
2.1. Detection speed
In order to improve the detection speed, two problems need to be solved: the train must operate normally without a 
collision between the antenna and catenary, and the scanning rate of the GPR system should be high enough.
To solve the ﬁrst problem, the GPR antenna must be mounted within the rolling stock gauge, and six sets of antennas 
should be ﬁxed on the train. In China, for an electric single-track tunnel, the distance from the antennas to the tunnel wall 
is 93–133 cm (antenna 5 and antenna 6, in Fig. 1), and the distance from the antenna to the tunnel vault is 175–225 cm 
(antenna 1 and antenna 2). Therefore, the average distance from an air-launched antenna to the lining is 159 cm. Thus, 
a new detecting scheme that uses an air-launched antenna at a long distance from the tunnel wall was proposed.
The solution to the second problem is to increase the scanning rate of the GPR system. The scanning rate must meet the 
requirements of the designed testing speed. Assuming that the designed measuring-point spacing is 5 cm, the single-channel 
scanning rate for the GPR should be at least 444 1/s if we want to test at a speed of 80 km/h. In reality, the scanning rate 
of each channel of this system is 976 1/s, so the highest test speed of 175 km/h can be reached.
2.2. Detection eﬃciency
Improving the detection eﬃciency requires the use of a multichannel GPR system. Therefore, a detecting system with six 
channels and six measuring lines was arranged (Fig. 1). This system can detect a tunnel with a full-section mode.
2.3. 300-MHz air-launched antenna
Because the ground-coupled antenna suffers from a “blind zone” effect, shallow anomalies such as defects within the 
lining will be diﬃcult to distinguish [4,5]. The depth of the blind zone extends from the surface to a depth of 1.5 times 
the wavelength [6], as shown in Fig. 2. The range of the blind zone is almost equal to the near-ﬁeld range. In this region, 
the direct-coupled wave signal is too strong to expose to both the reﬂected wave from the lining surface and the reﬂected 
signal from the surface defect.
We developed a 300-MHz air-launched antenna whose wavelength in the air is 100 cm. A distance of 1.5 times the 
wavelength in the air is close to the average distance between the antenna and the tunnel wall (159 cm). This is the 
Y. Zan et al. / Case Studies in Nondestructive Testing and Evaluation 6 (2016) 63–69 65Fig. 2. Schematic diagram of blind zone [6].
Fig. 3. Radar image for shotcrete lining with rough surface.
distance that makes the air-launched antenna operational in the far-ﬁeld range. Thus, we can see the surface state and 
geometry of the lining, as shown in Fig. 3.
Fig. 3 shows a radar image of the shotcrete lining of Yanshan tunnel. With good tunnel surrounding rock, only the 
spray anchor bracing was constructed after blasting. The radar image can reﬂect the geometry of the spray anchor lining. 
Therefore, for this detection method, by comparing radar images over the years, deformations in the tunnel lining can be 
found.
3. System and parameters
The vehicle-mounted GPR system for tunnel detection consists of six channels, a positioning system, and a data process-
ing system. Photographs of the system are shown in Fig. 4.
Six-channel GPR system: The pulse repetition frequency of each channel is 500 kHz, the antenna center frequency is 
300 MHz, and the scanning rate of each channel is 976 1/s. Each scan has 512 points, and the time window is designed 
with 60 ns/screen. The receiver’s maximum noise is 0.75 mV, and the A/D converter has 16 bits. The size of the antenna is 
800 mm (length), 200 mm (width), and 250 mm (height).
Positioning technique: We adopted a positioning method combined with distance measurement and GIS correction. 
Namely, using GPS and GIS systems, we automatically obtained the mileage outside the tunnel. The mileage sensor measures 
the distance inside the tunnel. This allows the vehicle-mounted GPR system to detect the entire line at once, which is 
continuous and fully automated.
Data processing technique: The post-processing software can process six-channel data simultaneously with a fast pro-
cessing speed and small storage capacity without any transition ﬁles. The multichannel processing software can display the 
six channels of data, or any channel data. The data processing procedure is shown in Fig. 5.
66 Y. Zan et al. / Case Studies in Nondestructive Testing and Evaluation 6 (2016) 63–69Fig. 4. Vehicle-mounted GPR system.
Fig. 5. Flowchart of data processing.
4. Experimental results
The railway vehicle-mounted GPR system was tested in electriﬁed railway tunnels on the Baoji–Zhongwei line and the 
Xiangfan–Chongqing line in 2013. We obtained a signiﬁcant amount of observed data, which has been analyzed and veriﬁed.
Fig. 6 shows that the reinforcing section (with a steel arch and ejector anchor) can be identiﬁed, and that the length of 
the steel arch reinforcement section and steel arch interval can be accurately determined. A section of the lining appears 
disconnected and convex. In addition, crescent-shaped reﬂections are visible. It was veriﬁed afterward that Yanjiashan tun-
nel, built in July 1993, had some diseases after operation. The tunnel has serious longitudinal and transverse cracks with 
a 25 mm–40 mm cracking width and 50 mm–70 mm convex height, as well as local dropping block. From June 2003 to 
July 2005, the railway administration department conducted bolt-grouting reinforcement along a 140-m-length arch of the 
tunnel that contained diseases, setting a P34 at each meter of the steel arch.
Y. Zan et al. / Case Studies in Nondestructive Testing and Evaluation 6 (2016) 63–69 67Fig. 6. Radar image of arch top of Yanjiashan tunnel from k96+ 545 to k96+ 578.
Fig. 7. Radar image of signiﬁcant disease in left vault of Daping tunnel.
Fig. 7 shows a radar image of the vault lining of the Daping tunnel in 2013. From K437 +093.5 to K437 +118.5, diseases 
were signiﬁcant. Repairs were conducted in 2010 by chiseling a groove inside the arch lining at each meter, inserting a 
permanent rail arch, and ﬁlling it with C20 concrete. The length of the serious defect section and the interval of the steel 
arch in the radar image are consistent with the design [7].
Fig. 8 shows a radar image of the arch top in the Qingliangshan tunnel on the Baoji–Zhongwei railway line in 2012. The 
image shows the section from K102 + 137 to K102 + 144, where a large deformation occurred. This area was reinforced 
with a steel arch and spray anchor grouting. It can be seen from the radar image that the deformation of the surrounding 
rock appears to have a curved contour. After this was conﬁrmed, a collapse and large deformation occurred during the 
construction process, and reinforcement was required.
Part of the radar image that shows water seepage in the vault of Guanyinxia tunnel is shown in Fig. 9. The strong 
reﬂecting image on the lining surface indicates water seepage.
68 Y. Zan et al. / Case Studies in Nondestructive Testing and Evaluation 6 (2016) 63–69Fig. 8. Radar image of arch top of Qingliangshan tunnel from K102+ 137 to K102+ 144.
Fig. 9. Radar image for water seepage of vault in Guanyinxia tunnel.
5. Discussion and conclusion
Compared to ordinary GPR, for air-launched antennas of vehicle-mounted GPRs working in the far ﬁeld [8], direct waves 
are separated from reﬂection waves of the lining surface. Then, areas with damaged lining can be clearly seen.
Instead of using the method discussed by A. Wimsatt et al. (2013), our proposed data analysis method adopts an analyti-
cal imaging method. Using the properties of the image, some information can be easily identiﬁed: integrity and deformation, 
disturbance of the surrounding rock, steel arches inside the lining, and the ﬂatness of the lining surface.
The vehicle-mounted GPR system, which can detect an entire railway line in a short amount of time, provides a fast, 
interference-free method for periodic inspections and health assessments of existing tunnels.
Acknowledgements
It is gratefully acknowledged that the work presented in this paper has been sponsored by the MOE’s Program for 
Changjiang Scholars and Innovative Research Team (IRT13092) and the “2011 Collaborative Innovative Research Center.”
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An innovative vehicle-mounted GPR technique for fast and efficient monitoring of tunnel lining structural conditions 

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An innovative vehicle-mounted GPR technique for fast and efficient monitoring of tunnel lining structural conditions

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