Simulation Model and Detection Method for Defects behind Segments of Shield Tunnels Detected by Ground Penetrating Radar

Abstract

A simulation model for detecting defects behind the segments of a shield tunnel using ground penetrating radar, which relates to a simulation model and a detection method. The model includes a tunnel pipe and a partition wall vertically and coaxially buried underground from the inside out. The tunnel pipe is assembled by multiple segments, and the annular cavity between the tunnel pipe and the partition wall is divided into multiple defect areas by connecting walls. The partition wall is also provided with a notch, and the space between the notch and the outer wall of the tunnel pipe is a forward modeling area for reducing the systematic error caused by the partition wall. The forward modeling area is backfilled with compacted soil for simulating the natural soil compaction condition around the segments, and the top of the compacted soil is covered with a concrete surface layer. The method includes step A: preparation before detection; step B: start detection; step C: determine the position of the segment thickness and the relative dielectric constant of the segment; step D: data processing; step E: data interpretation. The present invention can accurately analyze the types of defects behind the segments of a shield tunnel, solve the problem of being unable to verify a large number of detection results of shield tunnels, has the characteristics of high judgment accuracy and reliable structure, and is easy to promote and use.

Description

Technical Field

[0001] The present invention relates to a simulation model and a detection method, in particular to a simulation model and a detection method for detecting defects behind the segments of a shield tunnel by a ground penetrating radar. Background Art

[0002] With the development of urban transportation, the subway has become the most efficient means of transportation in a city. At present, shield machines are mostly used for tunneling in subway tunnels. During the tunneling process, in order to control uneven settlement, grouting is generally carried out behind the segments. However, due to differences in various performance indicators of the grouting liquid, uneven strata where the tunnel is located, and changes in underground water content, etc., there are defects of imperfect compaction in the grouting behind the segments, which brings a series of problems, such as segment offset, excessive ground settlement, etc. And the ground penetrating radar, as a high-resolution and efficient non-destructive testing technology, has begun to be applied to the detection of defects behind the segments of a shield tunnel. However: (1) The detection results of the ground penetrating radar usually need to be verified by comparative tests or on-site excavations. However, the detection site of the shield tunnel is restricted by construction conditions and cannot be verified in large quantities. (2) Since steel bars (magnetic metals) have a strong shielding effect on electromagnetic waves, the effective reflection of the ground penetrating radar antenna depends on the spacing of the steel bar mesh, the antenna spacing, as well as the size and burial depth of the detection target. In the shield segment, the steel bar mesh is arranged densely, and most of the electromagnetic wave signals emitted by the ground penetrating radar antenna are blocked and absorbed by the dense double-layer steel bar mesh arranged in the shield segment. See Figure 1 、 Figure 2 the schematic structural diagram of the radar antenna transmitting-reflecting model shown. Less electromagnetic waves effectively reflected from the defects behind the segments of the shield tunnel reach the receiving antenna. Therefore, it is difficult to accurately detect the defects behind the segments of the shield tunnel by using the ground penetrating radar. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a simulation model and a detection method for detecting defects behind the segments of a shield tunnel by a ground penetrating radar, so as to solve the deficiencies in the prior art that the detection results of the shield tunnel cannot be verified in large quantities by using the ground penetrating radar and it is difficult to detect the defects behind the segments of the shield tunnel.

[0004] The design principle of the present invention: It can be seen from the reflection coefficient formula that:

[0005]

[0006] r is the reflection coefficient, ε 1 、ε 2 are the dielectric constants of two substances respectively;

[0007] (1) The greater the difference in the electromagnetic properties of the two substances, the higher the reflectivity and the easier it is to receive the reflected signal of the target;

[0008] (2) The properties of the reflection interface can be roughly inferred from the phase attributes of the electromagnetic wave echoes; when the electromagnetic wave enters a low-speed medium from a high-speed medium, and when it enters a medium with a large dielectric constant from a medium with a small dielectric constant, the reflection coefficient is negative. At this time, the phase characteristics of the reflected wave are opposite to those of the incident wave, and the phase of the transmitted wave does not change. That is, when the radar wave enters a medium with a large dielectric constant from a medium with a small dielectric constant, when it enters a low-speed medium from a high-speed medium, and when it enters an optically dense medium from an optically sparse medium, the reflection coefficient is negative, that is, the amplitude of the reflected wave is reversed. Conversely, when entering a high-speed medium from a low-speed medium, the amplitude of the reflected wave is in the same direction as that of the incident wave.

[0009] The technical solution to solve the above technical problems is: a simulation model for detecting defects behind the segments of a shield tunnel by a ground penetrating radar, including a circular tunnel pipe and a partition wall vertically buried coaxially from the inside to the outside underground, and the tops of the tunnel pipe and the partition wall are flush with the ground; the tunnel pipe is assembled by a plurality of segments, and the annular cavity between the tunnel pipe and the partition wall is divided into a plurality of defect areas for simulating various working conditions by a connecting wall; the partition wall is also provided with a notch, and the space between the notch and the outer wall of the tunnel pipe is a forward modeling area for reducing the systematic error caused by the partition wall, and the forward modeling area is backfilled with compacted soil for simulating the dense working condition of the natural soil around the segments, and the top of the compacted soil is covered with a concrete surface layer.

[0010] A further technical solution of the present invention is: the defect areas include a defect area I, a defect area II, a defect area III, and a defect area IV; the defect area I includes a filling area, and a grouting dense area and a grouting cavity area are also arranged between the filling area and the outer wall of the tunnel pipe.

[0011] A further technical solution of the present invention is: I-beams and steel mesh sheets are buried at different positions in the defect area II; irregular cavities, tubular cavities, and metal pipes are also respectively arranged behind the segments in the defect area II.

[0012] A still further technical solution of the present invention is: the defect area III is a large cavity area for simulating the karst cave working condition around the segments.

[0013] A still further technical solution of the present invention is: the defect area IV is a crushed stone backfill area for observing the radar wave reflection characteristics of the crushed stone behind the segments; a cavity working condition is also arranged in the defect area IV.

[0014] A still further technical solution of the present invention is: the distance from the inner wall of the partition wall to the inner wall of the tunnel pipe is 800 - 1000 mm; a drain hole is also arranged at the bottom of the tunnel pipe, and an anti-rust guardrail for preventing people from falling is installed around the top of the tunnel pipe; a circular water grate for preventing people from falling is covered on the top of each defect area.

[0015] Another technical solution of the present invention is: a detection method for a simulation model for detecting defects behind the segments of a shield tunnel by a ground penetrating radar, and the method includes the following steps:

[0016] A. Preparation before testing:

[0017] A1. Select the antenna

[0018] Select 400M~1500M frequency shielded antenna;

[0019] A2. Survey line layout

[0020] The survey lines are laid out by combining longitudinal detection with circumferential detection. Multiple longitudinal survey lines are laid out longitudinally along the inner circumferential surface of the tunnel tube, and multiple circumferential survey lines are laid out circumferentially along the inner circumferential surface of the tunnel tube.

[0021] A3. Adjust the test instrument settings

[0022] Before testing, the instrument's 0-7ns sampling period gain is suppressed by 7-10dB, and the segmented gain of the sampling period after 7ns is increased to 25dB-30dB to enhance the signal strength behind the segment, making it easier for on-site testing personnel to find the defect location and retest and lock it in time;

[0023] B. Start testing

[0024] The inspectors enter the tunnel tube cavity and use the geological radar to inspect along the longitudinal and circumferential lines of the inner circumferential surface of the tunnel tube; the longitudinal lines are inspected from top to bottom;

[0025] C. Determine the thickness of the tube segment and the relative dielectric constant of the tube segment

[0026] (1) Set 0 point and the first wave position;

[0027] (2) According to the reflection waveform characteristics of the radar wave out of the pipe segment, the initial-end double journey time t of the radar wave of the pipe segment and the thickness d of the pipe segment are determined, and then the relative dielectric constant of the pipe segment is calculated according to the relative dielectric constant formula: ε = 0.09t 2 / 4d 2 , ε--relative dielectric constant of the segment, d--actual thickness of the segment, unit: m, t--two-way travel time of the segment, unit: s;

[0028] D. Data processing

[0029] Respectively performing data processing on the collected longitudinal detection data signal and circumferential detection data signal;

[0030] E. Data Interpretation

[0031] The target signal is identified and the target position is calculated based on the signal after data processing, and the type and position of the defect behind the segment are determined.

[0032] A further technical solution of the present invention is as follows: In step C, determining the segment thickness and the relative permittivity of the segment includes the following specific contents: Place a steel plate on the back of the segment and scan it with a radar antenna to detect the signal at the position of the steel plate. When it is determined that the characteristics of the radar wave at this position are such that when there is no steel plate, the radar wave has a positive reflection when exiting the segment, and when there is a steel plate, the radar wave has a negative reflection when exiting the segment. Set the zero point, set the position of the first wave, and according to the reflection waveform characteristics of the radar wave exiting the segment, determine the initial - termination double - travel time t of the radar wave of the segment and the segment thickness d, and then calculate the relative permittivity of the segment according to the relative permittivity formula: ε = 0.09t 2 / 4d 2 , where ε is the relative permittivity of the segment, d is the actual thickness of the segment, unit: m, and t is the two - way travel time of the segment, unit: s.

[0033] A further technical solution of the present invention is as follows: In step D, data processing, the data processing includes the following processes:

[0034] D1. Set the zero - point position and set the zero point at the peak of the positive phase of the first wave;

[0035] D2. Set the correction zero offset to remove fixed interference and center the waveform left and right;

[0036] D3. Set numerical filtering to remove low - frequency and high - frequency interference signals;

[0037] D4. Set background elimination and select the "overall method" to eliminate lateral background interference;

[0038] D5. Set moving average to remove noise;

[0039] D6. Set inter - trace equalization to improve the contrast of effective signals of segments and defects through the correlation of inter - trace data;

[0040] D7. For the segment thickness characteristics, adjust the gain to suppress the signal of the segment part and improve the signal strength of the defect area behind the segment;

[0041] D8. If encountering inclined - layer interference waves, use dip filtering to eliminate inclined - layer interference waves;

[0042] D9. Use time migration or depth migration methods to relocate the inclined - layer reflection wave interface and converge the diffracted waves.

[0043] Due to the adoption of the above - mentioned structure, compared with the prior art, the geological radar detection shield tunnel segment back - defect simulation model and detection method of the present invention have the following beneficial effects:

[0044] 1. It can accurately analyze the types of defects behind the shield tunnel segments

[0045] The simulation model for detecting defects behind the segments of a shield tunnel by ground penetrating radar according to the present invention includes a circular tunnel pipe and a partition wall vertically buried coaxially from the inside out in the ground, and the tops of the tunnel pipe and the partition wall are flush with the ground; the tunnel pipe is assembled by a plurality of segments, and the annular cavity between the tunnel pipe and the partition wall is divided into a plurality of defect areas for simulating various working conditions by a connecting wall; the partition wall is also provided with a notch, and the space between the notch and the outer wall of the tunnel pipe is a forward modeling area for reducing the systematic error brought by the partition wall. The forward modeling area is backfilled with compacted soil for simulating the compacted condition of the natural soil around the segments, and the top of the compacted soil is covered with a concrete surface layer. By constructing a tunnel segment model, the present invention uses a plurality of defect areas to constitute various typical working conditions, simulates the complex backfilling effect behind the actual segments, and uses radar testing to obtain typical data for analysis and comparison, so as to accurately analyze the types of defects behind the segments of the shield tunnel.

[0046] 2. It can solve the problem of being unable to verify a large number of detection results of shield tunnels

[0047] Since the present invention conducts simulation by constructing a tunnel segment model, it can accurately analyze the types of defects behind the segments of the shield tunnel, thereby solving the problem of being unable to verify a large number of detection results of shield tunnels.

[0048] 3. High judgment accuracy

[0049] The present invention is provided with a notch on the partition wall, and the space between the notch and the outer wall of the tunnel pipe is set as a forward modeling area for reducing the systematic error brought by the partition wall. Because different radars have different detection ranges, and the grouting thickness plus the segment thickness will not be too large, a radar with a relatively high frequency and a wide frequency range of electromagnetic waves can be used to detect the grouting density behind the segments of the shield tunnel. The distance from the inner wall of the partition wall to the inner wall of the tunnel pipe set in this application is 800 - 1000 mm, which is much larger than the general grouting thickness plus the segment thickness (except for over-excavation and geological mutation situations). However, there are differences in the dielectric constant, conductivity, and magnetic permeability between the material of the partition wall (ordinary sintered bricks) and different filling materials, which will more or less reflect signals that interfere with judgment, and there will be no such partition wall at the construction site. Therefore, by providing a notch on the partition wall and setting the space between the notch and the outer wall of the tunnel pipe as a forward modeling area, the present invention is used to reduce the systematic error brought by the partition wall, thereby improving the judgment accuracy.

[0050] 4. Reliable structure

[0051] The present invention uses a brick partition wall to construct the outer defect area of the segment and can flexibly replace the filling material in the defect area; a circular water grate is covered on the top of the defect area to prevent personnel from falling, and the structure of the present invention is safe and reliable.

[0052] Next, in combination with the accompanying drawings and embodiments, the technical features of the simulation model and detection method for defects behind the segments of a shield tunnel detected by a ground penetrating radar according to the present invention will be further described. Description of the Drawings

[0053] Figure 1 : One of the schematic structural diagrams of the radar antenna transmission-reflection model in the background technology

[0054] Figure 2 : Another schematic structural diagram of the radar antenna transmission-reflection model in the background technology

[0055] Figure 3: Schematic structural diagram (top view) of the simulation model for defects behind the segments of a shield tunnel detected by the ground penetrating radar according to the present invention in Embodiment 1

[0056] Figure 4: Cross-sectional view A-A of Figure 3

[0057] Figure 5 : Layout diagram of survey lines and unfolded diagram of segments described in step A2 in Embodiment 2

[0058] Figure 6: Radar result diagram of survey line 20# described in step A3 in Embodiment 2

[0059] Figure 7: Detection result diagram after increasing the gain set by the instrument to 25 dB - 30 dB in step A3 in Embodiment 2

[0060] Figure 8: Waveform diagram when setting the first wave position in step C in Embodiment 2

[0061] Figure 9 : Reflection waveform diagram of the radar wave exiting the segment in step C in Embodiment 2

[0062] Figure 10 : Waveform diagram when measuring the actual position of the steel plate in step C in Embodiment 2

[0063] Figure 11 : Waveform diagram when setting the zero point position in step D1 in Embodiment 2

[0064] Figure 12 : Waveform diagram when setting the correction zero offset in step D2 in Embodiment 2

[0065] Figure 13 : Screenshot of setting numerical filtering in step D3 in Embodiment 2

[0066] Figure 14 : Screenshot of adjusting the gain in step D7 in Embodiment 2

[0067] Figure 15 : Image before data processing in step D in Embodiment 2

[0068] Figure 16 : In Example 2, Step D: The image after data processing,

[0069] Figure 17 : In Example 2, Step E: In the data interpretation, the radar wave reflection of the hole defect shows a pattern with the characteristics of blue - red - blue.

[0070] Figure 18 : Schematic diagram of the reflection position of the segment thickness described in Working Condition 1

[0071] Figure 19 : Analysis chart of the results of Working Condition 2

[0072] Figure 20 : Detection results of Working Condition 3 Figure 1 ,

[0073] Figure 21 : Detection results of Working Condition 3 Figure 2 ,

[0074] Figure 22 : Detection results of Working Condition 4 Figure 1 ,

[0075] Figure 23 : Data results of Working Condition 4 Figure 2 ,

[0076] Figure 24 : Detection results of Working Condition 5 Figure 1 ,

[0077] Figure 25 : Data results of Working Condition 5 Figure 2 ,

[0078] Figure 26 : Detection results of Working Condition 6 Figure 1 ,

[0079] Figure 27 : Detection results of Working Condition 6 Figure 2 ,

[0080] Figure 28 : Distribution map of the survey lines of Working Condition 7

[0081] Figure 29 : Detection results of Working Condition 7 Figure 1 ,

[0082] Figure 30 : Detection results of Working Condition 7 Figure 2 ,

[0083] Figure 31 : Data analysis chart of Working Condition 7

[0084] Figure 32 : Detection result chart of Working Condition 8

[0085] Figure 33 : In working condition 9, the reflection diagram of defect area III,

[0086] Figure 34 : In working condition 9, the characteristic diagram of the segment interface,

[0087] Figure 35 : In working condition 10, the detection result diagram of defect area II,

[0088] Figure 36 : In working condition 11, the detection result diagram of defect area I,

[0089] Figure 37 : In working condition 12, the detection result diagram of the connecting bolts,

[0090] Figure 38 : In working condition 12, the detection result diagram of the embedded channels.

[0091] In the above figures, the reference numerals of each part are as follows:

[0092] 1 - Tunnel pipe, 11 - Segment, 12 - Waterproof layer, 13 - Drain hole, 14 - Rust-proof guardrail, 15 - Circular water grate,

[0093] 2 - Partition wall,

[0094] 3 - Connecting wall,

[0095] 4 - Defect area,

[0096] 41 - Defect area I, 411 - Filled soil area, 412 - Grouting dense area, 413 - Grouting cavity area,

[0097] 42 - Defect area II, 421 - I-beam and steel mesh, 422 - Irregular cavity, 423 - Tubular cavity, 424 - Metal pipe,

[0098] 43 - Defect area III, 44 - Defect area IV,

[0099] 5 - Forward modeling area,

[0100] 6 - Concrete surface layer,

[0101] T0 - Transmitting antenna, T1 - Receiving antenna, GJ - Steel mesh, QX - Defect. Detailed implementation method

[0102] Example 1

[0103] A simulation model for detecting defects behind the segments of a shield tunnel using ground penetrating radar, comprising a circular tunnel pipe 1 and a partition wall 2 vertically and coaxially buried underground from the inside out, with the tops of the tunnel pipe 1 and the partition wall 2 flush with the ground surface; the tunnel pipe 1 is assembled by multiple segments 11 through connecting bolts, and the annular cavity between the tunnel pipe 1 and the partition wall 2 is divided into multiple defect areas 4 for simulating various working conditions by a connecting wall 3; the partition wall 2 is also provided with a notch, and the space between the notch and the outer wall of the tunnel pipe 1 is set as a forward modeling area 5 for reducing the systematic error caused by the partition wall. The forward modeling area 5 is backfilled with compacted soil for simulating the natural soil compaction condition around the segments, and the top of the compacted soil is covered with a concrete surface layer 6.

[0104] The defect areas 4 include a defect area I 41, a defect area II 42, a defect area III 43, and a defect area IV 44; among them:

[0105] The defect area I 41 includes a filling area 411 filled with soil with a water content of 12.8% (simulating soil with the same water content as in the actual project for data collection), and a grouting densification area 412 and a grouting cavity area 413 are also arranged between the filling area 411 and the outer wall of the tunnel pipe 1. The grouting cavity area 413 is filled with water or air.

[0106] In the defect area II 42, I-beams and steel mesh sheets 421 are buried at different depths for simulating the initial support steel arch and steel mesh of the mining method to observe the radar wave reflection characteristics of this structure; in the defect area II 42, irregular cavities 422 for observing the radar wave reflection characteristics of this structure, tubular cavities 423 constructed with plastic corrugated pipes, and metal pipes 424 for observing the radar wave reflection characteristics behind the segments are also respectively arranged behind the segments 11.

[0107] The defect area III 43 is a large cavity area for simulating the karst cave condition around the segments and observing its radar wave reflection characteristics.

[0108] The defect area IV 44 is a crushed stone backfilling area for observing the radar wave reflection characteristics of the crushed stone behind the segments; a cavity working condition is also set in the defect area IV 44.

[0109] The distance from the inner wall of the partition wall to the inner wall of the tunnel pipe is 900 mm; a waterproof layer 12 and a drain hole 13 are also arranged at the bottom of the tunnel pipe 1, and a 1.2 m anti-rust guardrail 14 for preventing personnel from falling is installed around the top of the tunnel pipe 1; a circular water grate 15 for preventing personnel from falling is covered on the top of each defect area 4.

[0110] As a variation of the first embodiment, the distance from the inner wall of the partition wall to the inner wall of the tunnel pipe is any value within 800 - 1000 mm.

[0111] Embodiment 2:

[0112] A method for detecting a simulation model of defects behind a shield tunnel segment using a geological radar, the method comprising the following steps:

[0113] A. Preparation before testing:

[0114] A1. Select the antenna

[0115] Since the thickness of the pipe segment is 30cm, for the structure of concrete pipe segment, first choose the 400M~1500M frequency shielded antenna;

[0116] A2. Survey line layout

[0117] The survey lines are laid out by combining longitudinal detection with circumferential detection. Multiple longitudinal survey lines are laid out longitudinally along the inner circumferential surface of the tunnel tube, and multiple circumferential survey lines are laid out circumferentially along the inner circumferential surface of the tunnel tube.

[0118] The survey line layout is shown in Figure 5 The connecting bolts between the segments that make up the tunnel pipe have great interference, so the connecting bolts should be avoided on the detection path; Figure 5 As shown in the figure, the 1# to 18# measuring lines are tested from top to bottom (driving direction in actual engineering), and the 19# to 21# measuring lines are tested along the circumferential direction of the pipe segments. The most suitable testing path is found out through the test results;

[0119] A3. Adjust the test instrument settings

[0120] If you follow the routine test, adjust the instrument gain to 8dB~12dB, and test the 20# test line in a circular direction. Figure 5 Measurement line layout diagram, test results see Figure 6 Due to the shielding effect of the double-layer steel mesh, such test results are almost difficult to distinguish between voids, water-containing areas, and backfill compaction areas. To solve this problem, the present invention first suppresses the gain of the instrument in the 0-7ns period by 7-10dB before testing, and increases the segmented gain to 25dB-30dB after 7ns (see Figure 7 ), this setting enhances the signal strength behind the segment, making it easier for on-site inspection personnel to find the defect location and retest and lock it in time.

[0121] B. Start testing

[0122] The inspectors climbed up the ladder to enter the tunnel tube cavity, and used the geological radar to detect along the longitudinal and circumferential lines of the inner circumferential surface of the tunnel tube. The longitudinal lines were inspected from top to bottom. From the actual inspection results, it can be seen that the 1#~20# inspection lines can collect relatively ideal inspection data, and the bolt hand holes should be avoided during inspection.

[0123] C. Determine the thickness position and relative dielectric constant of the segment

[0124] Step C, determining the segment thickness position and the relative dielectric constant of the segment, includes the following specific contents: Place a steel plate on the back of the segment and scan it with a radar antenna to detect the signal at the position of the steel plate. When it is determined that the characteristics of the radar wave at this position are such that when there is no steel plate, the radar wave has a positive reflection when exiting the segment, and when there is a steel plate, the radar wave has a negative reflection when exiting the segment. Set point 0 and set the position of the first wave as shown in Figure 8 , according to the Figure 9 reflection waveform characteristics of the radar wave exiting the segment as shown, the bar chart shows that the in-phase reflection of the steel plate is manifested as the trough position; measure the actual position where the steel plate is located as shown in Figure 10 , determine the initial - end double - travel time t of the segment radar wave and the segment thickness d, and then calculate the relative dielectric constant of the segment according to the relative dielectric constant formula: ε = 0.09t 2 / 4d 2 , where ε is the relative dielectric constant of the segment, d is the actual thickness of the segment (m), and t is the two - way travel time of the segment (s);

[0125] D. Data processing

[0126] Perform data processing on the longitudinally detected data signals and circumferentially detected data signals collected respectively;

[0127] The data processing described above includes the following processes:

[0128] D1. Set the zero - point position, and set the zero - point at the peak of the positive phase of the first wave, see Figure 11 ;

[0129] D2. Set the correction zero offset to remove fixed interference and center the waveform left and right, see Figure 12 ;

[0130] D3. Set numerical filtering to remove low - frequency and high - frequency interference signals, see Figure 13 ;

[0131] D4. Set background elimination, and select the "overall method" to eliminate horizontal background interference;

[0132] D5. Set moving average, and set the moving window to 3 to remove noise;

[0133] D6. Set inter - trace equalization to improve the contrast of the effective signals of the segment and defects through the correlation of inter - trace data;

[0134] D7. For the segment thickness characteristics, adjust the gain to suppress the signal of the segment part and increase the signal strength of the defect area behind the segment (see Figure 14 );

[0135] D8. If encountering inclined - layer interference waves, use dip filtering to eliminate inclined - layer interference waves;

[0136] D9. Use time migration or depth migration method to migrate the reflection wave interface of dipping layers and converge diffracted waves.

[0137] The pre - processing image is as Figure 15 , and the post - processing image is as Figure 16 , and the double - layer steel mesh and defect areas are clearly visible;

[0138] E. Data interpretation

[0139] Identify the target signal according to the signal after data processing and calculate the target position, and judge the type and position of the defects behind the segment. Among them, since the characteristics of the radar wave of the segment thickness have been determined, add gain control points at 5 ns and set the gain to 8 dB, add gain control points at 10 ns and set the gain to 30 dB. At this time, more abundant detection information will be displayed.

[0140] Due to the shielding effect of the double - layer steel mesh, the radar wave signals of the defects in the segment thickness or embedded parts are interfered and deformed to varying degrees; the reflections of the first - layer steel bar and the second - layer steel bar are obvious; the radar wave reflection of the cavity defect shows a blue - red - blue characteristic, see Figure 17 , and the specific data interpretation can be found in Working Conditions 1 to 12.

[0141] In this step E, existing data interpretation techniques are adopted, and the specific process will not be elaborated.

[0142] Beneficial working conditions:

[0143] Working Condition 1: Determine the reflection characteristics of the radar wave from entering the segment to exiting the segment. Since a whole steel plate can strongly reflect the radar wave completely, the radar wave energy cannot propagate downward anymore. Image characteristics: Place a steel plate on the back of the segment and scan it with a radar antenna (due to the shielding effect of the steel mesh in the segment, the gain set by the instrument needs to be increased to 25 dB - 30 dB during testing), find the signal at the position of the steel plate, determine the reflection position of the segment thickness (see Figure 18 ), and determine the characteristics of the radar wave at this place.

[0144] Working Condition 2: Circumferential survey line, the back of the segment is filled with crushed stones (without grouting). Image characteristics: Since there are gaps in the crushed stones in the crushed - stone filling area, the reflection of the isophase axis on the radar image is disordered, there are many irregular reflections, and the reflection is weak. The effect of crushed - stone filling is obvious, see Figure 19 .

[0145] Working Condition 3: Circumferential survey line, cavity + water behind the segment. The survey line of Working Condition 3 runs along the cavity area - grouting area - cavity area, and the detection results can be seen in Figure 20 , Figure 21, as can be seen from the figure, due to the high dielectric constant (conductivity) of the water in the irrigation area, strong reflection occurs in this area, and at the same time, there is a shielding effect. Under the same gain condition, the in-phase wave amplitude reflection in the irrigation area is twice that of the cavity area; after the electromagnetic wave passes through the segment, obvious reflection occurs in the cavity area. Since the cavity area is strip-shaped, the cavity reflection is also strip-shaped, and the reflection color band is red-blue-red.

[0146] Condition 4: Circumferential survey line, no grouting + grouting for the backfill soil behind the segment, the survey line goes from no grouting to grouting, and the results are shown in Figure 22 、 Figure 23 , Image characteristics: Strong reflection in the cavity area, and due to the certain strength and density of the grouting in the grouting area, the grouting area combines well with the soil, and the reflection is weak.

[0147] Condition 5: Circumferential survey line, dense backfill soil behind the segment, the survey line is taken from the forward modeling (soil backfill area) - defect area IV (crushed stone backfill area), and the results are shown in Figure 24 、 Figure 25 , Image characteristics: The backfill soil is relatively dense, the reflection is weak, and due to the gaps in the crushed stones in the crushed stone filling area, the in-phase axis reflection on the radar image is disordered, with many irregular reflections, but the reflection is weak.

[0148] Condition 6: Circumferential survey line, the survey line goes from defect area IV (crushed stone backfill area) - defect area III (cavity area), and the results are shown in Figure 26 、 Figure 27 , Image characteristics: Weak reflection in the crushed stone area, discontinuous in-phase axis reflection, strong reflection in the cavity area, and the reflected wave is relatively continuous and uniform.

[0149] Condition 7: Longitudinal survey line ( Figure 28 , 1# survey line, 2# survey line, 3# survey line), the survey line is taken from the forward modeling (soil backfill area) (top - bottom), and the results are shown in Figure 29 、 Figure 30 , Image characteristics: Weak radar wave reflection in the densely backfilled area, discontinuous in-phase axis reflection, and relatively strong reflection in the cavity area. In addition, under the same gain condition, the coaxial reflected wave amplitude in the cavity area is about twice that of the densely filled area, as shown in Figure 31 .

[0150] Condition 8: Forward modeling mode, the segment edge section is obvious, and there is no obvious cavity reflection in the backfill soil. See Figure 32 .

[0151] Condition 9: Reflection characteristics of cavity area III: Reflection characteristics of blue-red-blue (gray-scale image: white-black-white), and the waveform characteristics show periodic reflection. See Figure 33 ; The segment interface characteristics are obvious. See Figure 34 .

[0152] Condition 10: Characteristics of defect area II

[0153] 1. Due to the shielding effect of the double-layer steel bar mesh, the radar wave signals of the segment thickness defects or embedded parts are interfered and deformed to varying degrees, as shown in Figure 35 ;

[0154] 2. The reflections of the first-layer steel bars and the second-layer steel bars are obvious; the radar wave reflection of the cavity defect shows the characteristics of blue-red-blue reflection;

[0155] 3. The radar wave reflections of metal I-beams, steel bar meshes, and metal corrugated pipes show the characteristics of blue-red-blue, and the phase is opposite to that of the cavity;

[0156] 4. The coaxial arc of the 300mm diameter plastic corrugated pipe is obvious, showing the reflection characteristics of an arc shape;

[0157] 5. Due to the influence of the double-layer steel bar shielding, the reflection signal of the I-beam is discontinuous, and the crescent arc is not obvious;

[0158] 6. The segment interface characteristics are obvious.

[0159] Condition 11: Characteristics of Defect Area 1

[0160] After grouting, the interface between the segment and the coating is not obvious, and the coaxial reflection of the radar wave in the cavity area is chaotic;

[0161] The grouting age is 60 days, the strength is 12MPa, and the grouting reflection interface is not obvious, as shown in Figure 36 ;

[0162] Condition 12: Connecting Bolts and Embedded Channels

[0163] 1. The radar wave reflection characteristics of the connecting bolts are shown in the figure. The connecting bolts are located between the two-segment linings, with strong reflections, showing the characteristics of red-blue-red reflection, as shown in Figure 37 ;

[0164] 2. The radar wave reflection characteristics of the embedded channels are shown in the figure. The embedded channels are located near the surface of the segment, with strong radar wave reflections, showing the characteristics of red-blue-red reflection, and are almost close to the air layer, as shown in Figure 38 .

Claims

1. A simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar, characterized in that: It includes a circular tunnel pipe (1) and a partition wall (2) vertically and coaxially buried underground from the inside out, and the tops of the tunnel pipe (1) and the partition wall (2) are flush with the ground; the tunnel pipe (1) is assembled by a plurality of segments (11), and the annular cavity between the tunnel pipe (1) and the partition wall (2) is divided into a plurality of defect areas (4) for simulating various working conditions by a connecting wall (3); the partition wall (2) is also provided with a notch, and the space between the notch and the outer wall of the tunnel pipe (1) is a forward modeling area (5) for reducing the systematic error caused by the partition wall. The forward modeling area (5) is backfilled with compacted soil for simulating the natural soil density condition around the segments, and the top of the compacted soil is covered with a concrete surface layer (6).

2. The simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar according to claim 1, characterized in that: The defect area (4) includes a defect area I (41), a defect area II (42), a defect area III (43), and a defect area IV (44); the defect area I (41) includes a backfill area (411), and a grouting compacted area (412) and a grouting cavity area (413) are also provided between the backfill area (411) and the outer wall of the tunnel pipe (1).

3. The simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar according to claim 2, characterized in that: I-beams and steel mesh sheets (421) are buried at different positions in the defect area II (42); irregular cavities (422), tubular cavities (423), and metal pipes (424) are also respectively arranged behind the segments (11) in the defect area II (42).

4. The simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar according to claim 2, characterized in that: The defect area III (43) is a large cavity area for simulating the karst cave condition around the segments.

5. The simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar according to claim 2, characterized in that: The defect area IV (44) is a crushed stone backfill area for observing the radar wave reflection characteristics of the crushed stone behind the segments; a cavity working condition is also set in the defect area IV (44).

6. The simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar according to any one of claims 1 to 5, characterized in that: The distance from the inner wall of the partition wall to the inner wall of the tunnel pipe is 800 - 1000 mm; a drain hole (13) is also provided at the bottom of the tunnel pipe (1), and an anti-rust guardrail (14) for preventing personnel from falling is installed around the top of the tunnel pipe (1); a circular water grate (15) for preventing personnel from falling is covered on the top of each defect area (4).

7. A detection method for the simulation model for detecting defects behind segments of shield tunnels using ground penetrating radar according to claim 1, characterized in that: This method includes the following steps: A. Preparation before detection: A1. Selecting an antenna Select a 400M - 1500M frequency shielded antenna; A2. Layout of survey lines The survey lines are laid out by combining longitudinal detection with circumferential detection. Multiple longitudinal survey lines are laid out longitudinally along the inner circumferential surface of the tunnel tube, and multiple circumferential survey lines are laid out circumferentially along the inner circumferential surface of the tunnel tube. A3. Adjust the test instrument settings Before testing, the instrument's 0-7ns sampling period gain is suppressed by 7-10dB, and the segmented gain of the sampling period after 7ns is increased to 25dB-30dB to enhance the signal strength behind the segment, making it easier for on-site testing personnel to find the defect location and retest and lock it in time; B. Start testing The inspectors enter the tunnel tube cavity and use the geological radar to inspect along the longitudinal and circumferential lines of the inner circumferential surface of the tunnel tube; the longitudinal lines are inspected from top to bottom; C. Determine the thickness of the tube segment and the relative dielectric constant of the tube segment (1) Set 0 point and set the first wave position; (2) According to the reflection waveform characteristics of the radar wave out of the pipe segment, the initial-end double travel time t of the radar wave of the pipe segment and the thickness d of the pipe segment are determined, and then the relative dielectric constant of the pipe segment is calculated according to the relative dielectric constant formula: ε = 0.09t2 / 4d2, ε is the relative dielectric constant of the pipe segment, d is the actual thickness of the pipe segment, unit: m, t is the round-trip travel time of the pipe segment, unit: s; D. Data processing Respectively performing data processing on the collected longitudinal detection data signal and circumferential detection data signal; E. Data interpretation The target signal is identified and the target position is calculated based on the signal after data processing, and the type and position of the defect behind the segment are determined.

8. The detection method of the simulation model of the defect behind the shield tunnel segment by using the geological radar according to claim 7, Features: The step C, determining the thickness of the pipe segment and the relative dielectric constant of the pipe segment, includes the following specific contents: placing a steel plate on the back of the pipe segment, scanning with a radar antenna, and finding the signal of the steel plate position; determining the characteristics of the radar wave at this location: when there is no steel plate, the radar wave is positively reflected when it exits the pipe segment, and when there is a steel plate, the radar wave is negatively reflected when it exits the pipe segment; setting the 0 point, setting the first wave position, and determining the initial-terminal double journey time t of the radar wave of the pipe segment and the thickness d of the pipe segment according to the reflection waveform characteristics of the radar wave exiting the pipe segment, and then calculating the relative dielectric constant of the pipe segment according to the relative dielectric constant formula: ε=0.09t2 / 4d2, ε--relative dielectric constant of the pipe segment, d--actual thickness of the pipe segment, unit: m, t--round-trip travel time of the pipe segment, unit: s.

9. The detection method of the simulation model of the defect behind the shield tunnel segment by using geological radar according to claim 7, Features: In the step D, data processing, the data processing includes the following process: D1. Set the zero point position, and set the zero point at the peak of the first wave positive phase; D2. Set the correction zero bias to remove fixed interference and center the waveform left and right; D3. Set numerical filtering to remove low-frequency and high-frequency interference signals; D4. Set background elimination and select "overall method" to eliminate horizontal background interference; D5. Set sliding average to remove noise; D6. Set up inter-channel equalization to improve the contrast of effective signals of segments and defects through the correlation of inter-channel data; D7. Adjust the gain according to the segment thickness characteristics, suppress the signals of the segment part, and enhance the signal intensity in the defect area behind the segment; D8. If encountering inclined layer interference waves, use dip filtering to eliminate them; D9. Employ time migration or depth migration methods to relocate the inclined layer reflection wave interface and converge the diffracted waves.

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