Application of transient electromagnetic radar in quality evaluation of tunnel composite lining

Construction and Building Materials 240 (2020) 117958

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Application of transient electromagnetic radar in quality evaluation of tunnel composite lining Ye Zijian a,b, Chengping Zhang a,b,⇑, Ye Ying c, Zhu Wenjun d a

Key Laboratory of Urban Underground Engineering of the Education Ministry, Beijing Jiaotong University, Beijing 100044, China School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China c Beijing Municipal Engineering Research Institute, Beijing Key Laboratory of Underground Engineering Construction Prediction & Precaution, Beijing 100037, China d Shanghai Key Laboratory of Rail Infrastructure Durability and System Safety, Department of Transportation Engineering, Tongji University, Shanghai 201804, China b

h i g h l i g h t s
A new method to detect insufficient thickness and voids behind lining in tunnel is proposed.
Contact status behind the tunnel lining can be determined by the variation of apparent resistivity.
Easier to detect the area of lining disease by transient electromagnetic radar (TER) than ground-penetrating radar (GPR).

a r t i c l e

i n f o

Article history: Received 21 September 2019 Received in revised form 17 December 2019 Accepted 24 December 2019

Keywords: Transient electromagnetic radar Apparent resistivity Composite lining Contact status Ground penetrating radar

a b s t r a c t Quality evaluation of the composite lining structure is very important for the safe operation of the tunnel. This study applied the transient electromagnetic radar (TER) to evaluate the contact status behind the composite lining structure. The feasibility of TER is verified by the variation of the apparent resistivity in the composite lining based on the one-dimensional (1D) forward calculation and physical simulation. The results of TER in the detection of voids and insufficient thickness is validated by the drilling test. A comparative test is conducted at the same location with quality problems using ground-penetrating radar (GPR). The results show that: (i) the TER is able to detect the insufficient lining thickness and voids behind the tunnel composite lining, contact status between composite lining and surrounding rocks can be determined by the variation of apparent resistivity; (ii) due to the difference of apparent resistivity, test image of TER shows clearly the boundary between surrounding rocks, primary and secondary lining, and voids, which made TER easier than GPR to detect the area of insufficient thickness and voids; (iii) combined with the drilling test results, TER and GPR results are generally consistent with the actual situation. The research and development of TER provides a new idea for the detection and evaluation of the tunnel lining diseases. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, tunnels are considered as one of the leading underground transportation structures and have been widely constructed in China. Most of those tunnels have been in service for many years. Due to the influence of poor construction, water erosion, improper backfilling [1], voids happen to the tunnels inevitably behind the lining. The lining cracks and spalling happen gradually, which may threaten the safety of the tunnel. Voids behind tunnel lining have been considered as one of the significant ⇑ Corresponding author at: Key Laboratory of Urban Underground Engineering of the Education Ministry, Beijing Jiaotong University, Beijing 100044, China. E-mail address: [email protected] (C. Zhang). https://doi.org/10.1016/j.conbuildmat.2019.117958 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

reasons for the damage of the lining structure, mainly due to the eccentric loading which can induce undesirable stresses [2–6]. Most of the voids behind the tunnel lining concentrate at the haunch and the vault of the tunnel, and mainly exist at the vault of the tunnel. Meanwhile, since poorly construction of the primary lining, insufficient lining thickness often combines with voids, which seriously affects the quality of the tunnel lining as shown in Fig. 1. The insufficient thickness of composite lining at the vault and the haunch of the railway tunnel needs to be repaired for the safety concern. Therefore, precisely detection and location of the voids and insufficient thickness are important for the timely maintenance and safe operation of the tunnel. The conventional inspection for the voids is conducted by tapping the hammers on the lining surface and then identifying the

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Haunch

Vault

Fig. 1. Composite lining at the vault and the haunch of the tunnel that needs to be repaired.

sound from the hollow drum. Nowadays, with the advanced technology, the ground-penetrating radar (GPR) has been considered as a useful non-destructive inspection for the quality evaluation on the tunnel linings, since it can provide a relatively convenient and fast geophysical technique for the evaluation on the tunnel and civil engineering [7–11]. However, there are still some limitations for the application of GPR in the tunnel lining inspection, such as: (i) signal is easily affected by the power cables, water, fabric reinforcement in the tunnel; (ii) the data interpretation is typically performed manually; (iii) the penetration depth and the resolution to identify voids needs to be improved [12–15]. Due to the limitations, the application of GPR in the evaluation of the contact status behind tunnel composite lining still needs to be properly improved. Therefore, a new approach is required for the non-destructive inspection. Transient electromagnetic method (TEM) is a time domain electromagnetic method, which is based upon the difference of electrical resistivity of the underground layers. TEM system usually consists of a transmitter instrument, transmitter loop or transmitting wire, receiver coil, and receiver instrument. Basic principles of the TEM as shown in Fig. 2 [22]. The transmitter loop (Tx-loop) is placed on the ground. The receiver coil (Rx-coil) is a smaller one in the center of the Tx-loop. The transmitter creates a constant current that passes through the Tx-loop. And then a primary magnetic field is induced. Ground services as a conductor which generates eddy currents under the primary magnetic field. According to Faraday’s law, eddy current is not absent immediately when the current is turned off abruptly. However, the eddy current flows in a closed path below the Rx-loop, which produces a secondary magnetic field. The secondary magnetic field is changed with time and a voltage is induced in the receiver coil. Magnitude and distribution of the current intensity in the secondary field depends on the resistivity of underground layers. The resistivity of underground layers can be deduced by the induced voltage. Meanwhile, the resistivity is highly related to the strength and the porosity of

Tx-loop I

Rx-coil

Geologic anomaly

Fig. 2. Basic principles of the TEM.

rock in underground layers. Therefore, the resistivity of the underground layers can be measured by the TEM method, and the strength and the porosity of rock can be evaluated by the variation of the resistivity. With the advantages of high sensitivity for low resistivity and deep detection, TEM has been applied in many fields [16–19]. It was originally developed for mineral investigations. Nowadays, the application is basically concentrated in the area of geophysical exploration, especially for hydrogeology investigations, such as subsurface target mapping, water-sealed underground storage caverns, mapping coalmine goaf, hydro-geophysical prospecting, water-inrushing structures in coal mines and so on. However, as for engineering inspection, especially for the lining thickness and voids behind of the tunnel lining is a relatively new application. For the purpose: (i) identification of the voids and the area of insufficient thickness; (ii) new way for the tunnel lining inspection by the advantage of TEM (deep detection depth, water and low resistivity object high sensitivity). Based on transient electromagnetic method (TEM), a new self-developed Transient Electromagnetic Radar (TER) is proposed as shown in Fig. 3. TER consists of an electrical transmitter and receiver, antenna, measuring wheel, and processing software. According to the principle of the seismometer, the underdamped central loop device is designed [20]. As for the performance index of TER, the range of detection depth could be 0–20 m and the range of central frequency is 0.25–50 Hz. More detailed information about the index of TER is shown in Table 1 [22]. TER test is to identify contact status based on the difference of apparent resistivity between concrete structure and surrounding rock and voids. By the summarization and analysis on the apparent resistivity of the different contact status behind the shield tunnel lining segment, TER has been applied for the evaluation of the voids behind the shield tunnel lining segment [21,22]. However, due to the differences between the composite lining (including the primary and secondary linings) and the shield segment (reinforced concrete) which mainly focus on distribution of rebars and concrete thickness and other aspects, there are also many differences in detection of contact status behind the tunnel lining. In the composite tunnel lining, the resistivity of reinforced concrete and water is relatively low, while that of void and rock is relatively high. Therefore, further research is still required: (i) summarize and analyze the apparent resistivity in different contact status behind tunnel composite linings; (ii) the effect of TER on the evaluation of lining thickness and voids behind the lining; (iii) compare with the conventional inspection method (GPR) to improve the accuracy of the evaluation. In this study, firstly, based on the TEM calculation, the apparent resistivity patterns of different contact status behind the composite linings are simulated and analyzed, the results are compared with the physical simulations. Secondly, the

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Antenna Electrical transmitter & receiver

Computer

Measuring wheel

(a)

(b)

Fig. 3. TER system: (a) Transmission receiving device with a measuring wheel. (b) Underdamping center loop.

Table 1 Transient electromagnetic radar technical index [22]. Transmitter Transmitting frequency Time of current turned off Current supply Trigger mode Launching mode Current waveform Power supply Connection Equipment dimension Weight

Receiver 0.0625–222 Hz <100 ls 0–50 A Rising edge Continue/measuring wheel/GPS Bipolar square wave, duty circle can be adjusted

TER is applied for the detection of the voids and lining thickness of the composite lining in a railway tunnel, and the drilling test is made to verify the actual effectiveness of TER. At last, GPR is used at the same position of tunnel lining that tested by TER. The results for both GPR and TER tests are compared and verified by the drilling test results. As a new non-destructive detection method, the TER is the first time tested and applied in the detection of the composite lining in railway tunnel. Compared with the study on the safety of the tunnel operation, the safety of tunnel construction has been more fully investigated [25–29]. Therefore, this study is of great significance to enriching the methods for the evaluation on the quality and safety of the operating tunnels.

2. TEM forward calculation and physical simulation 2.1. Test design To verify the effectiveness of TER in the detection of contact status behind the composite lining, as well as study and summarize the variation of apparent resistivity under different contact status behind the lining, the test is designed. Firstly, numerical calculation is conducted based on the one-dimensional (1D) forward calculation of apparent resistivity of TEM and information of model size, apparent resistivity in the dense and voids contact status are calculated respectively by using BETEM software. Secondly, a physical simulation test is designed to detect different contact status along survey lines of the concrete lining model by TER. The test results are extracted and analyzed. At last, numerical calculation and measured physical simulation results are compared to verify the TER test, and the variation of apparent resistivity are summarized.

Sampling frequency A/D resolution ratio Dynamic range Delay time window Synchronous mode Times of superimposition 6–12.6 V WiFi 240  160  103 mm 2 kg

4.096 kHz–52.734 kHz 24Bit high precision 175 dB 1000 ms Cable/GPS 1–9999

Dimensions of the concrete lining model and layouts of the survey line, as shown in Fig. 4. The dimensions of the concrete lining model are 0.8 m in length, 1 m in width, and 0.8 m in height. To simulate the lining structure, the model is poured by plain concrete, compressive strength of concrete is 30 MPa, the thickness of the lining model is 0.3 m. To simulate the contact status behind the lining, grouting soil is filled at the back of the lining, grouting area is 0.35 m in length, 0.6 m in width, 0.4 m in height. There is a 0.2 m width area on both sides of the grouting soil left with voids for the comparison. The position and layout of the survey line are shown in Fig. 4b, L1 is passing through concrete lining (0.3 m thickness), air (0.5 m thickness), respectively. L2 is passing through concrete lining (0.3 m thickness), grouting soil (0.35 m thickness), air (0.15 m/0.5 m thickness), respectively. Test parameters of TER are adopted as follows: transmitting frequency is 6.25 Hz, duty ratio is 20%, receiver frequency is 26 kHz, number of channels is 330. Testing process is shown in Fig. 4c, antenna of TER is densely attached to the surface of the lining structure, moving slowly and uniformly in the direction of a survey line, the test distance is recorded by measuring the wheel as it rotates. 2.2. TEM forward calculation 2.2.1. Apparent resistivity The data obtained by TEM is the induction voltage decays with time. The velocities of the induction voltage decays in different conductors are different. The relationship of the measured induction voltage decays with time can be expressed as:

U ðt Þ ¼

1 1X

s

n¼1

n2 t

e s

ð1Þ

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Z. Ye et al. / Construction and Building Materials 240 (2020) 117958

Plain concrete Grouting soil

Plain concrete

(a)

(b)

L1

L2 (c) Fig. 4. Physical simulation test: (a) Layout of the concrete lining model. (b) Size of the concrete lining model and layout of the survey line. (c) Testing process.

where, s is the time constant of the medium; t is the measuring time; U ðt Þ is the measured induction voltage. The decay velocity of the induction voltage mainly depends on the time constant value. The measured induction voltage should be converted to apparent resistivity, the relationship between the induction voltage and the apparent resistivity can be expressed as [22,23]:

where: UðtÞ is the measured induction voltage; I is the measured current; q is the apparent resistivity; a is the radius of the transmitter coil; t is the measuring time; erf ðuÞ can be written as: Ru 2 erf ðuÞ ¼ p2ffiffiffi 0 et dt; u is defined as the parameter related to the

ð2Þ

4p  107 H=m. As for the coincident-loop of the TEM, the measured

UðtÞ ¼

   2 Iq 2  3erf ðuÞ  pffiffiffiffi u 3 þ 2u2 eu 3 a p

p

electrical resistivity and measuring equipment which can be qffiffiffiffiffiffiffi2ffi deduced by: u ¼ l40qat ; l0 is the magnetic permeability equal to

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induction voltage can be converted to apparent resistivity with the following formula [19]: 5

l20 SI 1 qðtÞ ¼ 4p 5t52 UðtÞ

!23 ð3Þ

where, S is the area of the coil, S ¼ pa2 n; n is number of turns. To get the depth from the time as shown in Eq. (4). Theoretically, the depth corresponding to the time can be obtained by:

h ¼ vt

ð4Þ

where h is the depth; magnetic wave.

v

is the propagation velocity of the electro-

2.2.2. Physical model The variation of apparent resistivity of concrete lining structures is investigated under different contact states. According to the actual size of the simulation test, the parameters of the grouted soil and concrete structure behind lining are estimated as shown in Fig. 4, the physical model of contact status behind the lining is established respectively for 1D forward the modelling calculation. Parameters of the model are shown in Table 2, the concrete is considered homogeneous with a constant apparent resistivity of 1000 Om [24]. Condition of voids behind the concrete lining is simulated in L1, apparent resistivity variations from low (concrete) to high (air). Condition of grouting behind concrete lining is simulated in L2, apparent resistivity variations from relatively high (concrete) to low (grout) to high (air). 2.2.3. Calculation results The principle of 1D forward model uses predetermined parameters to numerically simulate transient geoelectric section according to the electromagnetic field propagation, and then to obtain the appropriate geoelectric section curve. Calculation of 1D forward model is under the frequency domain. According to the electromagnetic response of the geological model, the Helmholtz equation in frequency domain is solved for the transient electromagnetic field. The results of time-varying electromagnetic fields convert the frequency domain to the time domain. 1D forward model is established and calculated as shown in Fig. 5a. The coil is modelled by the input size as one of the parameters in calculation of the apparent resistivity. Results of the forward calculation of the physical model are shown in Fig. 5b. Due to the influence of air and different contact status behind the concrete lining, the position of apparent resistivity value of the increasement in L1 is earlier than L2. With lower resistance behind the concrete lining, the apparent resistivity values of L1 increased more significant than that of L2 during transition process of electromagnetic wave from lower resistance to higher resistance. 2.3. Results analysis of the physical modelling test 2.3.1. Test results Testing process of TER is shown in Fig. 4c. In order to accurately estimate the velocity, the velocity is calibrated during the test as shown in Fig. 6a. The detailed process of the velocity measurement is as follows: (i) measuring the tunnel lining by TER at a position

with the normalized lining thickness; (ii) find the time corresponding to the normalized lining thickness in the image; (iii) calculate the wave velocity by Eq. (4). After the velocity is deduced, the time can be converted to the corresponding depth by Eq. (4), and then the depth can be evaluated. The raw data obtained by TER is the data of the induced voltage as shown in Fig. 6b. Apparent resistivity is converted from the measured induced voltage by Eqs. (2) and (3). To clearly show the distribution of the apparent resistivity, the data is processed by logarithm, and different values are identified by the colors, apparent resistivity images along the survey line can be obtained as shown in Fig. 6c. The TER mainly depends on the variation of apparent resistivity to judge the contact status behind the lining structure. In Fig. 6c, the red area stands for high resistivity and the blue area stands for low resistivity. At the thickness of 0.3 m in yellow line, both L1 and L2 are shown clear boundaries, the thickness of blue area with low resistivity is approximately consistent with the lining designed thickness, it can be concluded that blue area is the area of the concrete structure. Behind the thickness of 0.3 m, L1 is tested under the condition of voids behind the lining (air with extremely high resistivity), the results show uniform red, which can be judged as air, and L2 is grouting soil with relatively low resistivity, due to the influence of grouting soil, test result of L2 is in lighter color comparing at the same position of L1. The apparent resistivity of L2 is significant uniformly reduced. Meanwhile, 0.2 m void position that reserved on both sides of the grouting soil in L2 is affected by the air and has high resistivity, the signal shows in test results are the same with the signal of air behind in L1, which is in red. Behind grouting soil area in L2 is air, due to disturbance by air, the distribution of apparent resistivity is not uniformly, and electromagnetic wave oscillated strongly. 2.3.2. Analysis of contour map of apparent resistivity Test data is extracted and made in a contour map for the validity of TER as shown in Fig. 7. By comparing the apparent resistivity contour map, it can be known that the variation law of apparent resistivity is consistent with the actual situation of the model. The distribution of apparent resistivity of air, grouting body and concrete structure can also be found in the figure. The black area of the curve shows relatively low apparent resistivity. Due to the low apparent resistivity of the grouting soil, the black area with low apparent resistivity of L2 is thicker than L1. 2.3.3. Data extraction and comparison To verify the accuracy of apparent resistivity measured by TER, the results of forward calculation and physical simulation test are compared. The apparent resistivity in physical simulation test is extracted at the representative position of L1 and L2, which is at the middle of L1 and L2 (at the survey line of 0.5 m) as shown in Fig. 8. According to the TER results of L1 and L2 in Fig. 8, the apparent resistivity of voids behind the lining is higher than that of dense contact status which is grouting soil. Moreover, some positions (the initial position of the survey line) are affected by voids between the surface of test antenna and the lining structure, apparent resistivity increased abnormally. The variation trend of apparent resistivity is from low resistance to high resistance and finally tends to be stable. At last, due to influence of voids in L1

Table 2 Parameters of the physical model. Numbers 1 2 3

L1;

Medium

Resistivity (Om)

Thickness (m)

Medium

Resistivity (Om)

Thickness (m)

Concrete (dry) Air /

1000 1E+12 /

0.3 0.5 /

Concrete (dry) Dense contact (grouting soil) Air

1000 100 1E+12

0.3 0.35 0.15

L2;

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Log(Apparent Resitivity/ Depth(m)

Log(V/volt)

Log(Time/s) Log(Apparent Resitivity/ (a) 1x108

8x107

6x107

L1(Air) L2(Grout)

4x107

2x107

0

2x106

4x106

6x106

8x106

1x107

Depth(m)

(b) Fig. 5. Calculation of 1D forward model: (a) Data processing software interface. (b) The apparent resistivity curve changes with the depth.

behind the thickness of 0.3 m, the oscillation amplitude of apparent resistivity is higher than the condition of behind lining with grouting soil. Meanwhile, the resistivity of grouting soil is close to the lining structure at the thickness of 0.4–0.7 m. The apparent resistivity changes relatively gently in L2. The results of the calculation and the TER test has been compared under the same parameter (the depth) as shown in Fig. 8. The value of apparent resistivity in the forward calculation is different from the actual concrete model. The measured data is affected by the air or non-uniform medium. And the apparent resistivity changes from low to high. However, the variation trends of the results are almost the same. 3. Application in the railway tunnel 3.1. Site investigation and data acquisition For further verification of the TER in practical engineering, TER is applied in a double-line railway tunnel located in the Sichuan

Province in southwest of China as shown in Fig. 9. The maximum buried depth of the tunnel is about 890 m. The tunnel has been preliminarily completed for five years. Due to quality problems, repairments are required in some parts of the construction before the operation. According to the geophysical test of the natural field audio magnetotelluric method (AMT), background of the geological structure is presented as follows: the stratigraphic lithology in tunnel area is complex; electrical properties of surrounding rocks are characterized by alternating low, medium and high resistivity, resistivity near the tunnel of some sections is low; the surrounding rocks are broken, weak, water-rich or structurally developed. To evaluate the contact status behind tunnel lining and verify the testing effect of TER, layout of three survey lines in the tunnel are conducted as shown in Fig. 10. These survey lines are respectively arranged in tunnel vault 1#, left and right haunch 2#, and right and left side wall 3#. TER test parameters in this test are the same to the physical simulation test. Representative detection paragraphs at different

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Length(m) 0.0

0.2

0.4

0.6

0.8

1.0

0.0

Depth(m)

0.2

0.4

0.6

0.8

(a) Length(m) Depth(m)

(b) 0.5

1.0

L1 0.09 0.18

0.5 0.00

1.0

L2

0.09

Concrete

0.26

Concrete

0.18 0.26

0.3m

0.35

0.35

0.44

0.44

0.53

Air

Grout

0.53

0.61

0.61

0.70

0.70

0.79

0.79

0.88

0.88

Air

Air

(c) Fig. 6. (a) Electromagnetic wave velocity measurement process. (b) Raw data of the induced voltage test by TER. (c) Apparent resistivity changes with the depth along profiles L1 and L2.

parts of the tunnel are selected to study the application effect of TER in the detection of lining thickness and voids. Then according to the mileage pile number and position provided by the tester, coring machine, air gun, and other tools are used for drilling test verification. 3.2. Detection of lining thickness 3.2.1. Vault TER field testing process is shown in Fig. 11. TER receiver and transmitter are connected to the computer by WIFI. The TER antenna is attached to the surface of the vault lining. The inspection car moves forward along the direction of L1 in a slow and uniform speed. The distance is recorded by measuring the wheels. Vault DK544 + 600-DK544 + 610 of the tunnel is selected as the research object due to the serious lining problems. According to geotechnical investigation and design information, surrounding

rock is interbedded with soft and hard rocks. The tunnel is constructed by the composite lining with the compressive strength of 30 MPa. The thickness of primary lining is designed to be 0.12 m and the secondary lining thickness is 0.4 m. Electromagnetic wave signal contains information about the induction voltage changes with the time sending back to the receiver coil. The apparent resistivity can be converted from the induced voltage by Eqs. (2) and (3). Meanwhile, the velocity is calibrated at the position of the tunnel entrance, the time can be converted to the depth by Eq. (4). The detailed process of the velocity measurement is shown in Fig. 6a. Moreover, to easily distinguish the distribution of the apparent resistivity, the apparent resistivity is processed with logarithm. The value is differed by colors. The TER results are shown in Fig. 12. In Fig. 12, the collected data are made into a contour map as shown in yellow color. The red area represents the surrounding rocks with relatively high resistivity, and blue area represents

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0.8

0.8

0.7

0.7

0.6

Air

0.6

0.5

Depth(m)

Depth(m)

Air 0.4

0.3

0.5

Air

Air

0.4

Grout

0.3

0.2

0.2

Concrete

Concrete

0.1

0.1

L1

L2

0

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Length(m)

Length(m) Fig. 7. Contour map of apparent resistivity test by TER.

L1 (Air) L2 (Grout)

40

ρ/Ω.m

30

1x108 8x107

ρ/Ω.m

20

6x107

L1(Air) L2(Grout)

4x107 2x107

TER Test Results

Calculation Results 0

2x106 4x106 6x106 8x106 1x107

Depth(m)

0.0

0.2

0.4

0.6

0.8

1.0

Depth(m) Fig. 8. Comparison of apparent resistivity changes at the position of 0.5 m of L1 and L2.

the plain concrete lining with relatively low resistivity. Due to the differences in apparent resistivity, the primary and secondary lining and surrounding rock have clear boundaries as shown in the green line. According to the TER results, the thickness of secondary lining of DK544 + 600-DK544 + 607 is about 0.4 m, which satisfies the design requirements. The primary and secondary linings are tightly compacted. The thickness of the secondary lining of DK544 + 607-DK544 + 610 is around 0.35 m, which does not meet the design requirements, it is considered that the irregular construction of primary lining probably causes the secondary lining thinner. However, to verify the test results, the drilling test for verification is carried out on the site where the thickness at the position of DK544 + 607.4 is significantly decreased, drilling test results are shown in figure. The secondary lining thickness is measured to be about 0.34 m, and the secondary lining is not tightly compacted with the primary lining, results are consistent with the TER results. To further analyze the variation of the apparent resistivity in different medium (such as the primary lining, secondary lining, and surrounding rocks), some representative apparent resistivity

data are extracted from the TER results as shown in Fig. 13. Data along the direction of the depth from shallow to deep respectively are extracted for analysis of the apparent resistivity variations, at the depths of 0.25 m (secondary concrete lining), 0.4 m (designed thickness), 0.5 m (primary lining), and 0.8 m (surrounding rock). Besides, overall apparent resistivity test data is extracted and made in a contour map. It can be seen from Fig. 13, according to the contour map of apparent resistivity, lining thickness nearby the position of DK544 + 607.4 is insufficient, which is the same to the TER test results. Apparent resistivity changes abnormally in non-uniform medium. From Fig. 13, blue part of the figure stands for resistivity value at the detection depth of 0.25 m. The inside of the concrete lining, apparent resistivity is relatively low and changes relatively gentle due to the uniform medium of concrete lining material. Red part of the figure, stands for resistivity value at the detection depth of 0.4 m, which is the boundary between the secondary lining and primary lining. Due to poor quality of the shotcrete, the primary lining thickness is more uneven than the secondary lining, so variation of apparent resistivity is higher than that of at a depth of 0.25 m. Black part of the figure is at a depth of 0.5 m, which is primary lining, affected by air inside, value of apparent resistivity is uneven distributed and unregular oscillated. At the depth of 0.8 m, area of surrounding rocks, apparent resistivity abnormal oscillation may be due to the surrounding rock properties changes. 3.2.2. Haunch According to the detection results at the vault of the tunnel, TER was also used to detect the lining quality along L2 at the haunch of the tunnel, as shown in Fig. 14. Insufficient thickness of DK542 + 380-DK542 + 395 at the right haunch of the tunnel is taken as the research object. Based on the geotechnical investigation data and design information, surrounding rock is weathered in loose structure. At the haunch of the tunnel lining, the primary lining is made by shotcrete and the designed thickness is 0.25 m. Secondary lining is concrete lining with the compressive strength of concrete is 30 MPa, and designed thickness is 0.45 m. The TER results are shown in Fig. 15a. Some parts of secondary lining are insufficient in the thickness. Detailed information of results tested by TER is as follows: lining thickness around the position of DK542 + 384-DK542 + 395 is reduced, and the thinnest

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Fig. 9. Location of the tunnel in the Sichuan province, southwest of China.

TER Antenna (with measuring wheel)

1#: Vault 2#: Right haunch

3#: Right side wall

2#: Left haunch

3#:Left side wall

Electrical transmitter and receiver

1#: Vault

3.5m

Computer Drainage trench

Fig. 10. Site condition and layout of the survey line.

thickness is about 0.37 m. Thickness of lining at the position of DK542 + 389-DK542 + 390 is severely decreased, and the thinnest thickness is about 0.28 m. Drilling test also carried out at the position of DK542 + 389.5 for verification. The measured secondary lining thickness is the same as TER results. Contour map of apparent resistivity is shown in Fig. 15b. The thickness of the lining at the position of circular frame as shown in the figure is significantly reduced, which corresponded to the TER results. 3.3. Detection of voids According to the physical simulation, when electromagnetic wave encounters air leading to the apparent resistivity oscillated

Fig. 11. TER field testing process at the vault of the tunnel.

obviously, unusually this feature could be used for the void detection. The voids happen to the vault of the tunnel at DK542 + 560-DK542 + 578. In this area, the surrounding rock is strongly weathered, the designed thickness of the shotcrete at the vault of the tunnel is 0.27 m, and the secondary lining thickness is 0.5 m, the steel frame spacing of grating is 0.6 m, spacing of secondary reinforcing bars is 250 mm. The TER results are shown in Fig. 16. The boundary of the primary and secondary lining is shown in green line, which follows along the edge of the apparent resistivity contour line (as shown

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0.34m

Drilling test

10m

DK544+610

DK544+600

0.35m Secdonary lining

0.4m Primary lining DK544+607 DK544+607.4

Surrounding rocks

Insufficient thickness

Fig. 12. TER detection and drilling test results at the vault of the tunnel (DK544 + 600-DK544 + 610).

in yellow). The black line represents the designed thickness of secondary lining. To easily locate the lining and the voids behind the tunnel, the localization of the boundary line is done by visual analysis of the image. The criterion for the location of boundary line between the primary and secondary linings is based on the apparent resistivity difference in the primary and secondary lining. According to the construction information, the primary lining is

Survey line 2

DK542+395

TER Antenna (with measuring wheel)

DK542+380

DK544+607.4 Fig. 13. Comparison and analysis of the representative data that extracted along the direction of depth of the tunnel.

Fig. 14. TER field testing process at the haunch of the tunnel.

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0.28m

20m

DK542+380

Drilling test DK542+395

0.28m

0.37m

Secondary lining

0.45m Primary lining 0.7m

Insufficient thickness

DK542+389.5

Surrounding rocks DK542+384

DK542+385

DK542+389

DK542+390

(a) DK542+384

DK542+385

DK542+389

DK542+390

(b) Fig. 15. TER detection results at the haunch of the tunnel (DK542 + 380-DK542 + 400): (a) TER detection and drilling test results. (b) Contour map of apparent resistivity.

made of shotcrete, and the secondary lining is made of reinforced concrete. Under the influence of the rebar inside the secondary lining, the apparent resistivity of the secondary lining is lower than the apparent resistivity of the primary lining. Moreover, the different value of apparent resistivity is shown in different colors, such as the red for the high value and the blue for the low value. Therefore, the boundary line could be recognized in the image. Fig. 16 shows that the thickness of the secondary lining in some positions are insufficient. The apparent resistivity has distinguished oscillation by comparing with the compact status (such as DK542 + 576-DK542 + 578 in the figure), three positions with the voids behind the lining are located by TER as follows: DK542 + 563-DK542 + 565 (about 2 m), DK542 + 567-DK542 + 570 (about 3 m), DK542 + 572-DK542 + 574 (about 2 m). As the influence of the voids, the boundary between the primary lining and the surrounding rock is difficult to be determined. Designed construction joints at the position of DK542 + 565 and DK542 + 576 also can be detected by TER.

Drilling test is conducted at two positions to verify TER results as shown in Fig. 16. In the position 1#(DK542 + 566), the secondary lining thickness of the measured value is around 0.42 m, which does not meet the requirement of 0.5 m, uncompacted and loosely contact status behind the lining. But no cavities are found. In the position 2#(DK542 + 569), the secondary lining thickness of the measured value is around 0.5 m, which is loosely contact status behind the lining with the cavities depth of about 0.3 m. The results of the drilling test agree well with the TER results. The apparent resistivity achieved from the TER is made in a contour map as shown in Fig. 17a, the positions of the voids and construction joints are shown in the red corresponding to the TER results in Fig. 16. For further comparison of the variation of apparent resistivity between voids and dense contact status behind the lining, the apparent resistivities at drilling position 1# (dense contact status) and 2# (void) are extracted as shown in Fig. 17b. It can be seen from the figure that the rebar inside the concrete influences the apparent resistivity oscillated obviously inside the range

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Z. Ye et al. / Construction and Building Materials 240 (2020) 117958

0.5m 0.42m

1#Drilling test DK542+560

18m

DK542+566

2#Drilling test DK542+578

DK542+569

Secdondary Lining

0.5m

Primary Lining 0.77m

DK542+563

Cavities reflection

Surrounding rocks

DK542+572

DK542+565 DK542+567

DK542+570

Construction joint

DK542+574 DK542+576

Construction joint

Fig. 16. TER detection and drilling test results at the vault of the tunnel (DK542 + 560-DK542 + 578).

of lining. Meanwhile, affected by the air, the apparent resistivity at position of the voids behind the lining is higher than that of dense contact status without voids.

Meanwhile, the ground within driving route of inspection car on the tunnel detection line shall be cleared before the GPR inspection, such as removing obstacles that stacked near the sidewall in the detection area.

4. Comparison of application test results between GPR and TER 4.1. GPR testing and data processing 4.1.1. GPR testing For further study on the TER, the conventional detection method of GPR is used to measure along the survey line at the position of insufficient thickness and voids so as to compare the detection effects between GPR and TER. The GPR equipment is shown in Fig. 18, which is tested by RAMAC/GPR. The equipment includes X3M host and 500 MHz antenna. The layout of the GPR test line is the same as that of the TER test. The parameters of the GPR test are as follows: sampling frequency: 7000–7500 MHz; sampling number: 512–536 (adjusted by sampling frequency and time window); superposition time: automatically stacking eight times; trigger mode: distance trigger (measuring wheel); time window: 70–75 ns (1 s = 109 ns); sampling interval: 0.02 m. The GPR testing process are shown in Fig. 18. During the process of tunnel lining inspection, inspection car is required to move forward smoothly at a speed of 2–3 km/h to ensure that the antenna could be worked stably and the contact between antenna and lining surface of the tunnel is in good state.

4.1.2. Data processing GPR is an electromagnetic device that transmits highly frequency electromagnetic pulse into the ground. When the electromagnetic wave passes through the non-continues medium with a different dielectric constant, the variations in the reflected signal can be recorded by the radar system. The velocity of the electromagnetic wave is highly dependent on the electromagnetic properties [13,14]. The propagation velocity used during the field test is the calibrated velocity, which is the velocity of the electromagnetic wave propagate in the tunnel composite lining. The propagation velocity is calibrated at the tunnel entrance, a position available to known the lining thickness, as shown in Fig. 19b. The measurement is shown in Fig. 19a. The process is: (i) measure the thickness of the concrete lining to get the depth; (ii) using GPR measuring along the same position of the concrete lining to obtain the radar image, find the propagation time at the position of the lining thickness in the radar image; (iii) the propagation velocity in the field can be calculated by:

v¼

2H Dt

ð5Þ

Z. Ye et al. / Construction and Building Materials 240 (2020) 117958

1# DK542+566

13

2# DK542+569

(a) Contour map of apparent resistivity

Lining design thickness

Rebar reflection

Secdondary lining

Primary lining

Surrounding rock

(b) Comparison and analysis of apparent resistivity data at drilling position of #1 and #2 Fig. 17. (a) Contour map of apparent resistivity. (b) Comparison and analysis of apparent resistivity data at drilling position of #1 and #2.

Fig. 18. GPR equipment and testing process.

where: Dt is the double path time delay, v is the measured velocity of the electromagnetic waves, H is the depth. The relationship between the velocity of the electromagnetic wave and the relative dielectric constant can be expressed as:

er ¼

 c 2

v

ð6Þ

where er is the relative dielectric constant of the medium, c is the velocity of the electromagnetic waves in vacuum.

Due to the complex structure of the composite lining, the signal of the raw data by GPR contains unfavorable noise. To strong target signal, improve SNR (signal to noise ratio) and find useful information from the radar image, the raw data should be properly processed to easily interpretation. The raw data processing is descripted as below [14]: (a) import and edit the display of GPR data; (b) eliminate the bad traces; (c) adjust and edit time window; (d) filtering the import data; (e) time-zero correction; (f) energy decay; (g) subtracting average; (h) band pass butter worth, cutoff frequency; (h) reduce the background noise.

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Z. Ye et al. / Construction and Building Materials 240 (2020) 117958

Fig. 19. Electromagnetic wave velocity measurement: (a) Process of electromagnetic wave velocity measurement. (b) Electromagnetic wave velocity calibration in the field.

4.2. Comparison and discussion of test results 4.2.1. Insufficient thickness GPR test is set along the same survey line with the TER test. The results of GPR and TER tests are compared and analyzed. According to the position of insufficient thickness in Section 3.2 tested by TER, the comparison of the results between GPR and TER at the vault of the tunnel (DK544 + 603-DK544 + 610) are shown in Fig. 20. Fig. 20 shows that the DK544 + 604-DK544 + 608 behind the lining is in loosely contact status and with voids inside in GPR test due to the influence of the voids, the lining thickness is hard to be identified. Due to influenced by artificial operation in the process of detection tests, such as rough, uneven concrete lining surface, instrument testing process of ups and downs or left and right deviation, TER and GPR results show some differences. The TER results are discussed before, the insufficient lining thickness and loosely contact status behind lining are detected at the position of DK544 + 607. However, the voids are only found in the GPR. According to the description of TER and drilling test results, the TER results matches well with the drilling test results. The position of abnormal signal both shows in the GPR and TER tests are still close. The reason for the voids shows in the GPR test may be due to the poor construction and under-excavation of the primary lining, the insufficient thickness of the secondary lining, and loosely contact status accompanied with the voids appeared between the lining and surrounding rocks. Comparison of the GPR and TER results at the haunch of the tunnel (DK542 + 382-DK542 + 393) is shown in Fig. 21. The results of

the TER and GPR tests are similar, especially in the position of insufficient thickness. Fig. 21 shows the GPR results at DK542 + 386-DK542 + 393, the minimum lining thickness is 0.3 m. According to the drilling test results, the secondary lining thickness is about 0.28 m, which is close to both the TER and GPR results. The boundary between the primary, secondary lining and surrounding rocks could be recognized in the TER results, but the primary lining is hard to identify in the GPR results.

4.2.2. Voids detection Comparison of the GPR and TER results at the vault of the tunnel (DK542 + 564-DK542 + 577) is shown in Fig. 22. There are a 6 m and a 1 m length area of voids behind lining according to the GPR results. It can be inferred that the reflection of rebars inside the lining are not straight due to the influence of voids between surrounding rocks and secondary lining. The positions of construction joints for both GPR and TER are similar. However, some differences happen to the results of TER and GPR tests. The drilling test in Fig. 16 at position 1# DK542 + 566 with the measured lining thickness is 0.42 m and found no cavities, which did not match with the GPR results. Drilling test in Fig. 16 at the position 2# DK542 + 569, the lining thickness is 0.5 m, which is loose contact status behind the lining. The measured thickness of the void is 0.2 m, although both the GPR and TER results show that the voids at this position of the drilling test results are closer to the TER results.

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Z. Ye et al. / Construction and Building Materials 240 (2020) 117958

3

DK544+604 4

Distance(m) 5

6

Drilling test DK544+608 7 8 9

10 0.0

0

0.2

0.6 0.8

Depth(m)

Time(ns)

0.4 10

1.0 20 1.2

GPR

1.4

Secondary lining Primary lining

Surrounding rocks

TER Fig. 20. Comparison of test results at the vault of the tunnel (DK544 + 603-DK544 + 610).

Drilling test DK542+386 2

3

4

5

6

DK542+393

Distance(m) 7

8

9

10

11

12

13 0.0

0

0.2

0.6 0.8 1.0

Depth(m)

Time(ns)

0.4 10

1.2

20

GPR

1.4 1.6

Secondary lining

Primary lining

Surrounding rocks

TER Fig. 21. Comparison of test results at the haunch of the tunnel (DK542 + 382-DK542 + 393).

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Z. Ye et al. / Construction and Building Materials 240 (2020) 117958

DK542+564 4

0

Distance(m) 5

6

7

8

1#

9

10

11

12

13

14

15

DK542+577 16

2#

17

0.0 0.2

0.6 0.8 1.0

Construction joint

20

Depth(m)

Time(ns)

0.4 10

1.2

GPR

1.4

Secondary lining

Primary lining

Surrounding rocks

TER Fig. 22. Comparison of test results at the vault of the tunnel (DK542 + 560-DK542 + 580).

5. Summary and conclusions

CRediT authorship contribution statement

The variation of apparent resistivity in the structure of composite lining and the detection of TER are studied according to the TEM forward calculation and physical simulation test. The effect of TER based on the principle of TEM for the quality inspection of the composite lining is explored by comparing results of the TER test with the results of drilling test and GPR test. Conclusions are drawn as follows:

Zijian Ye: Writing - original draft, Writing - review & editing, Formal analysis, Methodology. Chengping Zhang: Conceptualization, Funding acquisition, Project administration, Supervision. Ying Ye: Software, Validation, Resources. Wenjun Zhu: Visualization.

(1) Due to the influence of the low resistivity of grouting soil, apparent resistivity of the dense contact status behind the lining changes gently. The influence of the air, voids behind the lining can cause apparent resistivity fluctuations violently. Therefore, contact status between the lining and surrounding rock can be determined by abnormal variation of the apparent resistivity. (2) The TER in nondestructive testing for railway tunnels is verified by the drilling test, it can be concluded that lining thickness and size and location of voids can be detected by TER. The position of insufficient thickness and voids behind lining can be easily located. (3) Results for both TER and GPR tests are validated by the results of the drilling test in the detection. TER is much more accessible than GPR in the identification of thickness of primary and secondary lining. (4) To avoid the impact of the artificial operation, detect the voids and insufficient thickness more easily, still a lot of essential research and engineering verification test need to be involved. Further study should be progressed in the aspects of the volume detection of the voids.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The study was supported by the National Natural Science Foundation of China (Grant no.: 51978042) and the Research and Development Plan of China Railway Corporation (No. N2018G029, No. J2017G001). References [1] Jifei Wang, Hongwei Huang, Xiongyao Xie, et al., Void-induced liner deformation and stress redistribution, Tunnell. Underground Space Technol. 40 (2014) 263–276, https://doi.org/10.1016/j.tust.2013.10.008. [2] Chengping Zhang, Gang Feng, Zhang Xu, et al., Effect of double voids behind lining on safety state of tunnel structures, Chin. J. Geotech. Eng. 37 (3) (2015) 487–493, https://doi.org/10.11779/CJGE201503012. [3] M.A. Meguid, H.K. Dang, The effect of erosion voids on existing tunnel linings, Tunn. Undergr. Space Technol. 24 (3) (2009) 278–286, https://doi.org/10.1016/ j.tust.2008.09.002. [4] Mohamed A. Meguid, Sherif Kamel, A three-dimensional analysis of the effects of erosion voids on rigid pipes, Tunn. Undergr. Space Technol. 43 (2014) 276– 289.

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