Test research method for influence of karst on shield tunnel excavation face stability

Abstract

The invention relates to the technical field of shield construction, in particular to an experimental research method for influence of karst on stability of a shield tunnel excavation face. Comprising the following steps that S1, a model test system is constructed, and the model test system comprises a model box, a shield tunnel model arranged in the model box, a data acquisition unit and a water supply and water level control unit; s2, after the model box is filled with water-rich sandy soil to a preset height, a cavity simulating karst is buried; s3, setting an underground water level in the model box through the water supply and water level control unit, and simulating a target seepage condition; s4, gradually reducing the internal air pressure of the air bag at a constant rate, and simulating the gradual collapse process of the karst; s5, in the pressure reduction process of the air bag, continuously collecting soil pressure, pore water pressure and earth surface displacement data in front of the excavation face through the data collection unit; and S6, on the basis of the data acquired in the step S5, analyzing an influence rule of karst progressive collapse on stress distribution of the excavation surface and surface deformation.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of shield construction, and particularly relates to a test research method for the influence of karst on the stability of a shield tunnel excavation face. BACKGROUND

[0002] The geological conditions in karst areas are highly complex, and karst collapse, as a frequently-occurring geological disaster, is often caused by factors such as groundwater activity and cave development, and poses a serious threat to underground engineering. With the acceleration of urbanization, shield tunnels are increasingly widely used in karst areas, for example, in subways, water conservancy and transportation infrastructure, but the influence of karst collapse on the stability of a shield tunnel excavation face has become a focus problem in the engineering field. Water-rich sandy soil layers are more prone to deformation and damage under the action of karst collapse due to their high permeability and low shear strength, and phenomena such as soil liquefaction and intensified settlement occur, which not only affects construction safety, but also may lead to long-term operation risks.

[0003] Through the above analysis, the existing problems are that there is little research on the influence of karst on a water-rich sandy soil layer shield tunnel face. The existing research on excavation face stability focuses on the stability of excavation faces in homogeneous or single-layer conditions, and the research on the stability of a water-rich sandy soil composite layer shield excavation face under the condition of karst collapse is still insufficient, and it is urgently needed to reveal the influence mechanism through test research to provide theoretical support for engineering practice. SUMMARY

[0004] The application provides a test research method for the influence of karst on the stability of a shield tunnel excavation face to solve the above problems.

[0005] The application adopts the following technical scheme: a test research method for the influence of karst on the stability of a shield tunnel excavation face, comprising the following steps: S1: constructing a model test system, wherein the model test system comprises a model box, a shield tunnel model arranged in the model box, a data acquisition unit and a water supply and water level control unit; S2: after filling water-rich sandy soil in the model box to a predetermined height, a cavity body simulating karst is buried, the cavity body is an air bag with controllable internal air pressure; then the soil is continuously filled and monitoring instruments are arranged; S3: setting the groundwater level in the model box through the water supply and water level control unit to simulate target seepage conditions; S4: gradually reducing the internal air pressure of the air bag at a constant rate to simulate the gradual collapse process of karst; S5: during the air bag pressure reduction process, continuously collecting the soil pressure, pore water pressure and ground surface displacement data in front of the excavation face through the data acquisition unit; S6: Based on the data collected in step S5, analyze the influence law of karst progressive collapse on the stress distribution of the excavation face and the ground deformation.

[0006] In step S4, the internal pressure of the air bag is gradually reduced at a constant rate, specifically: at a rate of reducing a fixed air pressure value every 1 second, and after each stage is reduced, it is static for 5-10 seconds, and after the data collection is stable, the next stage of pressure reduction is carried out.

[0007] In step S5, the data collection unit collects data at a frequency of 1 time per second; and when the air bag pressure reduction time reaches a multiple of 5 seconds, the soil collapse area in front of the excavation face is photographed by the image recording device.

[0008] In step S2, before the air bag is buried, it is inflated to a full state, and the initial air pressure is set to 50kPa.

[0009] In step S2, the monitoring instrument includes a plurality of soil pressure cells arranged in the soil layer in the model box, a plurality of displacement meters arranged on the ground, and soil pressure sensors and pore water pressure sensors arranged at the position of the cutter head of the shield tunnel model.

[0010] The data collected by the data collection unit includes vertical and horizontal soil pressure, cutter soil pressure, pore water pressure and ground displacement data; in step S6, the characteristic data at specific time points during the air bag pressure reduction process is extracted, and the specific time points include 15 seconds, 30 seconds, 45 seconds and the final collapse time.

[0011] In step S6, the stress distribution influence law is analyzed, specifically: define the vertical stress proportion coefficient n v and the horizontal stress proportion coefficient n h , respectively representing the ratio of the vertical soil pressure and the horizontal soil pressure at the specific time point to the initial soil pressure, and draw the variation curves of n v and n h with the pressure reduction time.

[0012] In step S6, the ground deformation influence law is analyzed, specifically: based on the ground displacement data, draw the lateral distribution graph of the ground settlement tank, and determine the width and depth development law of the settlement tank by using the "V" shape symmetric graph analysis method.

[0013] After step S6, it further includes: S7: Establish a numerical model consistent with the model test parameters, and verify the rationality of the numerical model using test data; S8: Based on the verified numerical model, extend the analysis of the influence law of different karst sizes and positions on the limit stability of the excavation face.

[0014] The air bag is buried in the stratum directly above the excavation face of the shield tunnel model.

[0015] Compared with the prior art, the present application has the following beneficial effects: The present application simulates karst cavity by introducing controllable air pressure air bag, and creates a test method which can accurately reproduce the dynamic process of karst progressive collapse. This method overcomes the key problem that the traditional technology is difficult to simulate the water-soil-cavity interaction, so as to intuitively and dynamically reveal the real influence mechanism of karst collapse on the stress distribution and stability of the shield tunnel excavation face, and provides an unprecedented effective means for understanding such complex geological problems.

[0016] Based on the system data monitoring and analysis, the present application successfully quantifies the soil pressure redistribution law and the surface 'V' shaped settlement tank characteristics caused by karst collapse, and the research conclusion can provide clear warning parameters and supporting design basis for shield construction in karst area. This helps engineers to optimize the construction scheme in advance, effectively avoids the risks such as excavation face collapse and surface settlement, ensures the safety of the project, significantly saves the cost and construction period, and has great practical value. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 It is a schematic diagram of a shield tunnel model test system in water-rich sandy soil stratum; Figure 2 It is a top view of surface monitoring; Figure 3 It is a top view of stratum soil pressure monitoring; Figure 4 It is a soil pressure layout diagram of A-A section; Figure 5 It is a soil pressure layout diagram of B-B section; Figure 6 It is a soil pressure layout diagram of C-C and D-D sections; Figure 7 It is a cutter soil pressure and pore water pressure monitoring instrument layout diagram; Figure 8 It is a diagram of the change rule of cutter pore water pressure with time; Figure 9 It is a diagram of the change rule of cutter soil pressure with time; Figure 10 It is a diagram of B21-B24 surface deformation rule; Figure 11 It is a diagram of n v ( t =0s) along the shield tunneling direction; Figure 12 It is a diagram of n v ( t =15s) along the shield tunneling direction; Figure 13 It is a diagram of n v( t =30s) graph; Figure 14 For n along the tunnel boring direction v ( t =45s) graph; Figure 15 For n along the tunnel boring direction v ( t =54s) Figure. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are some embodiments of the present invention, but not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] An experimental research method for studying the impact of karst on the stability of the shield tunnel excavation face includes the following steps: S1: Construct a model test system, which includes a model box 1, a shield tunnel model 3 set inside the model box 1, a data acquisition unit, and a water supply and water level control unit; Specifically, such as Figure 1 As shown, the model test system includes: a model box 1, whose front and sides are made of transparent material for easy observation, and whose interior is used to simulate the tunnel excavation environment; a shield tunnel model 3, which is set inside the model box 1; an excavation face model 4, which is set inside the shield tunnel model 3; an excavation face baffle 2, which is set in front of the shield tunnel model 2 to simulate the tunnel face and can move backward at a controllable rate; a data acquisition unit, including multiple earth pressure cells arranged in the soil layer inside the model box 1, earth pressure sensors and pore water pressure sensors arranged on the excavation face baffle 2, and a data acquisition instrument for collecting sensor signals; and a water supply and water level control unit, which includes a water pump 2 and a pressure measuring tube 5; the pressure measuring tube 5 is a transparent tube with precise graduations, whose bottom is connected to the shield tunnel model 3 through a pipeline, and whose top is open to the atmosphere, to reflect the water pressure inside the shield tunnel model 3. The model box 1 is equipped with an inlet pipe 6 and an outlet pipe 7, both of which are connected to the water pump 2.

[0020] The shield tunnel model 3 is cut from a circular aluminum cylinder with a thickness of 20 mm, and has a semicircular cross section, a length of 400 mm, and an inner diameter of 300 mm; the excavation face model 4 is composed of a front plate 4.1 and a back plate 4.2 (support plate) capable of simulating a shield excavation chamber. The front plate 4.1 is made of a semicircular plexiglass plate with a thickness of 20 mm and is uniformly perforated, and is fixed at the right end of the shield tunnel model to simulate the cutter head of the shield machine and provide support to the ground in front of the excavation face. The back plate 4.2 is a support plate composed of two pieces of steel wire mesh with a thickness of 1 mm and a piece of sponge with a thickness of 5 mm in the middle, which is water permeable and soil impermeable, and can generate a seepage field near the excavation face; to prevent excessive deformation of the support plate, a steel bar framework is provided between the steel wire meshes to increase the rigidity thereof; the support plate is connected to the lead screw through a bearing, and can be controlled to move backward by rotating the rotating wheel under the interaction of the lead screw and the lead screw nut, to simulate the case where the support pressure of the excavation face is insufficient.

[0021] The shield tunnel model 3 is firmly fixed to the front plexiglass plate of the model box by a stainless steel hoop. In order to prevent stress concentration and damage at the screw hole of the front plexiglass plate of the model box when the stainless steel hoop is fastened, the following measures are taken: ① A stainless steel threaded pad with a large bottom area is used instead of a nut inside the model box, and 3M double-sided tape is placed between the glass plate and the stainless steel hoop, which can avoid stress concentration on one hand and prevent water from flowing out of the screw hole on the other hand; ② Two aluminum strips with screw holes are directly attached to the corresponding positions of the glass plate outside the model box to disperse the stress transmitted by the screws.

[0022] The layout positions of the earth pressure cells include: The earth pressure cells arranged on the baffle are annularly distributed, and one is arranged at the middle position, totaling 6 earth pressure cells; The earth pressure cells arranged in the ground total 34, wherein: 14 vertical earth pressure cells are arranged on the A-A section, which is a horizontal section at the middle height of the shield tunnel model 3; the vertical positions are arranged in three layers with a distance of 2 times the diameter D of the tunnel, which are the top of the cutter head, planes at positions 2 / 3D and 4 / 3D from the top of the cutter head, and the horizontal intervals are 0.5D, 0.5D, 1D and 1.5D from the back of the cutter head, the cutter head, the front of the cutter head, respectively; 14 horizontal earth pressure cells are arranged on the B-B section, which is a horizontal section above the A-A section; the vertical positions are arranged in three layers with a distance of 2 times the diameter D of the tunnel, which are the top of the cutter head, planes at positions 2 / 3D and 4 / 3D from the top of the cutter head, and the horizontal intervals are 0.5D, 0.5D, 1D and 1.5D from the back of the cutter head, the cutter head, the front of the cutter head, respectively; Three vertical earth pressure cells are arranged in the C-C section, which is a horizontal section above the B-B section; the vertical position is arranged in three layers with 2 times the diameter D of the tunnel, which are the top of the cutter, the planes at 2 / 3D and 4 / 3D from the top of the cutter, and the horizontal interval is at the cutter; Three vertical earth pressure cells are arranged in the D-D section, which is a horizontal section above the C-C section; the vertical position is arranged in three layers with 2 times the diameter D of the tunnel, which are the top of the cutter, the planes at 2 / 3D and 4 / 3D from the top of the cutter, and the horizontal interval is at the cutter.

[0023] The air bag is buried in the stratum directly above the excavation face of the shield tunnel model 3.

[0024] S2: After filling the water-rich sand in the model box 1 to a predetermined height, a cavity body simulating karst is buried, the cavity body is an air bag with controllable internal air pressure; then continue to fill the soil and arrange monitoring instruments; First, clean the model box, and smear vaseline on the inside of the model box to eliminate the friction between the box and the soil. And paste a scale ruler inside the box to ensure accurate filling; to ensure uniform filling, calculate the mass of soil sample required for 10 cm and at the same time tamp and saturate with water. It is worth noting that according to the arrangement scheme of the instruments and the position of the karst, the soil is filled higher than the specific position of the earth pressure cell and the "karst", and then the monitoring instruments are buried; before burying the "karst", the air bag is inflated to a full state, the air pressure is 50kPa, and after the filling is completed, the ground displacement meter is buried; finally, connect the water pump and power supply, and the static strain acquisition instrument and the osmotic pressure meter acquisition instrument.

[0025] S3: Set the groundwater level in the model box 1 through the water supply and water level control unit to simulate the target seepage condition.

[0026] S4: Gradually reduce the internal air pressure of the air bag at a constant rate to simulate the gradual collapse process of the karst; The internal air pressure of the air bag is gradually reduced at a constant rate, specifically: at a rate of reducing a fixed air pressure value every 1 second, after each stage, stand still for 5-10 seconds, and then reduce the pressure of the next stage after the data acquisition is stable.

[0027] Specifically, the water pump switch is turned on after the test starts to ensure that the pressure measuring pipe height is 0 to ensure that the water pressure of the excavation face is 0. It is worth noting that the air bag pressure is reduced in a manner of 1 s time unit to simulate karst progressive collapse, and 10 seconds are allowed to stand at each stage for data stabilization; the influence area is photographed at each stage at a time of 5 s multiples. After completing a set of test conditions, the data storage of the acquisition instrument is clicked and the relevant files are exported, and the power of the acquisition instrument and the related equipment is turned off. Then the drainage hole of the model box is opened, and after the water in the model box is completed, the monitoring equipment and the soil in the model box are removed, and the soil tank and the inside of the model box are cleaned to prepare for the next test.

[0028] S5: During the air bag pressure reduction process, the data acquisition unit continuously acquires the soil pressure, pore water pressure and ground displacement data in front of the excavation face; The data acquisition frequency of the data acquisition unit is 1 per second; and at a time of 5 s multiples, the image recording device is used to take a photo of the soil collapse area in front of the excavation face.

[0029] Based on the test results, the influence of karst on the stress distribution of the stratum in front of the excavation face is quantified. The main implementation process is: the acquisition frequency of the acquisition instrument is set to 1 per second, and after the test is completed, the EXCEL table is exported and the stratum soil pressure values at times of 15 s, 30 s, 45 s and the last moment are extracted, and the values are divided by the initial soil pressure to obtain the corresponding proportion coefficient.

[0030] S6: Based on the data collected in step S5, the influence of karst progressive collapse on the stress distribution of the excavation face and the ground deformation is analyzed.

[0031] The data collected by the data acquisition unit includes vertical and horizontal soil pressure, cutter soil pressure, pore water pressure and ground displacement data; in step S6, the characteristic data at specific time points during the air bag pressure reduction process are extracted, including 15 s, 30 s, 45 s and the final moment of collapse.

[0032] The influence of karst progressive collapse on ground deformation and progressive collapse is analyzed. The main implementation process is: the acquisition frequency of the acquisition instrument is set to 1 per second, and after the test is completed, the EXCEL table is exported and the ground displacement values at times of 15 s, 30 s, 45 s and the last moment are extracted, and the analysis is performed using a "V" shaped symmetric graph. The change law of the stratum and ground deformation area is photographed at 5 times.

[0033] S7: A numerical model consistent with the model test parameters is established, and the rationality of the numerical model is verified by using the test data; S8: Based on the verified numerical model, the influence law of different karst sizes and positions on the limit stability of the excavation face is extended. Embodiments: In order to explore the influence law of karst on the stability of the shield tunnel excavation face in the water-rich sand stratum under seepage conditions, a model test is carried out based on the indoor test device to obtain the stress distribution and surface change law of the earth pressure caused by the instability of the excavation face under the condition of gradual collapse of karst.

[0035] Figure 1 The shield tunnel excavation seepage stability model test system is shown. The inside of the model box is 900 mm long, 1050 mm wide and 1500 mm high. In order to facilitate the observation of the water level and stratum deformation, in addition to the bottom and back of the model box being designed as 5 mm thick steel plates, the front, left and right are designed as 20 mm thick organic glass plates. In order to prevent excessive deformation of the organic glass plate, square aluminum columns are arranged transversely in the middle of the organic glass plate for support.

[0036] The graded soil prepared by the similarity law is used, and the internal friction angle of the test material is 40.4° measured by the direct shear test. 3 The relative density is 0.55.

[0037] Figures 2-6 The arrangement scheme of the stratum earth pressure with a buried depth ratio of 2 is shown. In the stratum model, 34 earth pressure cells are arranged, including 20 vertical earth pressure and 14 horizontal earth pressure. 20 displacement meters are arranged on the ground surface. Static strain acquisition instruments are used to connect the earth pressure cells and displacement meters to obtain the change law of the earth body instability area and the ground surface deformation.

[0038] Figure 7 The monitoring arrangement scheme of the earth pressure cell and the seepage pressure meter in front of the cutter head is shown, including 6 earth pressure cells and 3 seepage pressure meters.

[0039] Firstly, clean the model box, and smear vaseline on the inside of the model box to eliminate the friction between the box and the soil. Then, stick a ruler on the inside of the box to ensure the accuracy of filling. To ensure the uniformity of the filling, calculate the mass of the soil sample required for 10 cm and tamp it with a rammer and saturate it with water. It is worth noting that, according to the arrangement scheme of the instrument and the karst location, the filling is higher than the soil pressure box and the specific location of the "karst" after the monitoring instrument is buried; the air bag is inflated to a full state before the "karst" is buried, with an air pressure of 50 kPa, and the surface displacement meter is buried after the filling is completed; finally, connect the water pump and power supply, as well as the static strain acquisition instrument and the osmotic pressure meter acquisition instrument. Turn on the water pump switch after the test starts to ensure that the pressure tube height is 0 to ensure that the water pressure of the excavation face is 0. It is worth noting that the air pressure of the air bag is reduced in units of 1 s to simulate the gradual collapse of karst, and the data is stable for 10 seconds at each stage; take photos of the affected area at each stage at multiples of 5 seconds. After completing a set of test conditions, click on the data storage of the acquisition instrument and export the relevant files, and turn off the power of the acquisition instrument and related equipment. Then open the drainage hole of the model box, and after the water in the model box is completed, remove the monitoring equipment and the soil in the model box, and clean the soil tank and the inside of the model box to prepare for the next test.

[0040] As shown in Figure 8 , 9 is the change rule of the pore water pressure and soil pressure monitoring data on the front baffle of the excavation face with the increase of time. Since the karst collapse area is located above the excavation face, the pore water pressure and soil pressure maintain a relatively stable level, mainly affected by the structure disturbance of the rock-soil body. In the initial stage, local micro-pressure fluctuation is caused by the expansion of the collapse fissure.

[0041] The lateral distribution of the displacement of different positions on the ground with the gradual collapse of karst is shown in Figure 5 . Taking B21-B24 as an example, in the initial stage of the test, the insufficient support pressure of the excavation face leads to the sliding and loosening of the soil in the karst area, and micro-cracks appear on the ground with local settlement, and the deformation rate is relatively slow; with the advancement of excavation, the fissure gradually expands and forms a wedge-shaped damage zone, and the surface settlement shows an accelerating growth trend, especially in the area of the karst vault.

[0042] In order to more intuitively reflect the stress distribution law in the soil arch, the vertical and horizontal stress proportionality coefficients are defined as n v and n h , respectively. Taking the vertical soil pressure in the direction of the vertical tunnel excavation face as an example, the change rule is shown in Figure 6It can be seen that the slow deformation begins to occur above the excavation face at t=15s; the soil arch above the excavation face continues to develop after t=30s; and the instability region along the tunnel direction is about 2.0D in height and 1.0D in length, while the instability region width perpendicular to the tunnel direction is about 1.0D.

[0043] The present application has reference significance for similar research. On the one hand, a reasonable layout method is provided, so that the monitoring data is comprehensive and reasonable; on the other hand, a data analysis method is provided, which respectively analyzes the phenomenon of each graph at the same interval time, then analyzes the change law as a whole, and finally analyzes the reasons.

[0044] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement to part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. An experimental research method for the influence of karst on the stability of the shield tunnel excavation face, characterized in that, Includes the following steps: S1: Construct a model test system, which includes a model box (1), a shield tunnel model (3) set in the model box (1), a data acquisition unit, and a water supply and water level control unit; S2: After filling the model box (1) with water-rich sand to a predetermined height, a cavity simulating karst is buried. The cavity is an airbag with controllable internal air pressure. Then, the soil is filled and monitoring instruments are installed. S3: Set the groundwater level in the model box (1) through the water supply and water level control unit to simulate the target seepage conditions; S4: Gradually reduce the internal air pressure of the airbag at a constant rate to simulate the gradual collapse process of karst; S5: During the airbag decompression process, the data acquisition unit continuously collects data on soil pressure, pore water pressure and surface displacement in front of the excavation face; S6: Based on the data collected in step S5, analyze the influence of karst progressive collapse on the stress distribution and surface deformation of the excavation face.

2. The experimental research method for the influence of karst on the stability of the shield tunnel excavation face according to claim 1, characterized in that, In step S4, the air pressure inside the airbag is gradually reduced at a constant rate. Specifically, the pressure is reduced by a fixed value every second, and after each stage of reduction, the pressure is allowed to stand still for 5-10 seconds until the data collection is stable before proceeding to the next stage of depressurization.

3. The experimental research method for the influence of karst on the stability of the shield tunnel excavation face according to claim 2, characterized in that, In step S5, the data acquisition unit acquires data once per second; and when the airbag depressurization time reaches a multiple of 5 seconds, it takes a picture of the soil collapse area in front of the excavation face using an image recording device.

4. The experimental research method for the influence of karst on the stability of the shield tunnel excavation face according to claim 1, characterized in that, In step S2, before embedding the airbag, it is first inflated to full capacity, with the initial air pressure set to 50 kPa.

5. The experimental research method for the influence of karst on the stability of the shield tunnel excavation face according to claim 1, characterized in that, In step S2, the monitoring instruments include multiple earth pressure cells installed in the soil layer inside the model box (1), multiple displacement gauges installed on the ground surface, and earth pressure sensors and pore water pressure sensors installed at the cutterhead position of the shield tunnel model (3).

6. The experimental research method according to claim 5, characterized in that, The data collected by the data acquisition unit includes vertical and horizontal earth pressure of the strata, earth pressure of the cutterhead, pore water pressure and surface displacement data; in step S6, feature data at specific time points during the decompression process of the airbag are extracted, including 15 seconds, 30 seconds, 45 seconds and the final moment of collapse.

7. The experimental research method according to claim 6, characterized in that, In step S6, the analysis of the influence of stress distribution specifically involves defining the vertical stress proportionality coefficient n. v and horizontal stress proportionality coefficient n h , representing the ratios of vertical and horizontal earth pressures to the initial earth pressure at specific time points, respectively, and plotting n v and n h The curve showing the change in blood pressure over time.

8. The experimental research method according to claim 6, characterized in that, In step S6, the analysis of the influence of surface deformation specifically involves: drawing a lateral distribution map of the surface subsidence trough based on surface displacement data, and using the "V"-shaped symmetry diagram analysis method to determine the development law of the width and depth of the subsidence trough.

9. The experimental research method according to claim 1, characterized in that, Step S6 is followed by: S7: Establish a numerical model consistent with the parameters of the model experiment, and use experimental data to verify the rationality of the numerical model; S8: Based on the validated numerical model, we further analyze the influence of different karst sizes and locations on the ultimate stability of the excavation face.

10. The experimental research method according to claim 1, characterized in that, The airbag is buried in the stratum directly above the excavation face of the shield tunnel model (3).

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