A simulation method for a shield mud membrane closed gas test

Abstract

The application relates to the technical field of rail transit construction, and discloses a simulation method for shield mud film closed gas test, which comprises the following steps: S1: establishing a simulation system for the shield mud film closed gas test; S2: preparing mud according to requirements; S3: measuring the porosity, density, residual water content, water conductivity coefficient, elastic modulus, Poisson's ratio and water holding capacity parameters of the mud and stratum soil; S4: inputting the material parameters obtained in S3 into the simulation system for the shield mud film closed gas test, and then carrying out model verification by using a small amount of mud film closed gas test; and S5: carrying out simulation calculation and analysis. The application provides an indoor closed gas test simulation model based on the Biot poroelasticity theory and under the fluid-solid coupling condition, considers the different water holding capacity characteristics of mud films and strata, carries out model verification by using the gas migration law and seepage flow evolution data obtained through the indoor test, and realizes rapid evaluation of the mud film closed gas characteristics under complex geological conditions and shield parameter working conditions.

Description

Technical Field

[0001] This invention relates to the field of rail transit construction technology, and in particular to a simulation method for a shield tunneling mud film air tightness test. Background Technology

[0002] With the continuous advancement of urbanization, the development of underground space plays an increasingly important role in modern urban construction. Especially in the field of tunnel engineering, shield tunneling technology, with its high efficiency, safety, and minimal impact on the surrounding environment, has become a key method for underground space development. Slurry shield tunneling technology is particularly suitable for highly permeable soil layers, soft strata, and underground environments with high water pressure. Unlike traditional earth pressure balance (EPB) shield tunnels, slurry shield tunnels can adjust the excavation face pressure in real time through slurry support, avoiding the pressure imbalance and water seepage problems that may occur with EPB shield tunnels in highly permeable environments. This gives slurry shield tunneling a significant advantage under complex geological conditions, and it has been widely used, especially in the construction of underwater tunnels and densely populated urban areas (References: Deng Zongwei, Wu Zhenzhi, Cao Hao, Shen Pinghuan. Surface deformation caused by slurry shield tunneling based on fluid-structure interaction [J]. Journal of Central South University (Natural Science Edition), 2013, 44(2), 785-791. and Yu Baomin, Ji Yuguo. Current status and risk management of shield tunneling hatch opening technology in China [J]. Tunnel Construction (Chinese and English), 2018, 38(4): 683-693.).

[0003] However, the inevitable wear and tear on equipment during tunnel boring machines (TBMs), especially the high-intensity work of the cutterhead and cutting tools, necessitates regular inspection and maintenance. While traditional atmospheric pressure tunneling can ensure normal equipment operation under certain circumstances, it relies on ground reinforcement and is difficult to implement effectively in highly permeable strata. Therefore, pressurized tunneling technology has emerged, which protects the excavation face through pneumatic support and provides a safe, airtight environment for equipment maintenance. The core of this technology is the balance between air pressure and ground pressure, ensuring the stability of the excavation face while preventing potential dangers such as ground instability and groundwater intrusion. However, applying too much or too little air pressure during pressurized tunneling can pose risks. For example, excessively high air pressure may cause mud film rupture and gas leakage, leading to surface heave, changes in building loads, and worker health problems such as decompression sickness. Simultaneously, when gas leaks, the pneumatic loading fails, potentially causing excavation face instability and surface subsidence. Conversely, excessively low air pressure may lead to insufficient pressure at the excavation face, causing ground collapse or water backflow, threatening construction safety. Therefore, accurately balancing air pressure and ground pressure is the key to pressurized tunnel opening technology (References: Wei Daiwei, Zhu Wei, Min Fanlu. Changes in excess pore water pressure over time during mud film formation in slurry shield tunnels [J]. Journal of Hydraulic and Architectural Engineering, 2013, 11(3): 36-40. and Min Fanlu, Wei Daiwei, Jiang Teng, Zhang Chunlei. Experimental study on the permeability characteristics of mud in strata [J]. Rock and Soil Mechanics, 2014, 35(10), 2801-2806. and Zhu Wei, Min Fanlu, Yao Zhanhu, Wang Rui, Wei Daiwei, Jiang Teng. Current status and examples of shield tunnel opening technology [J]. Modern Tunnel Technology, 2015, 52(1), 9-18.).

[0004] The design of pressurized opening air pressure loading generally requires conducting indoor air tightness tests to gain a deeper understanding of the air tightness characteristics of the mud film-stratum combination under the air pressure. However, traditional mud film air tightness tests are often time-consuming and labor-intensive, especially in order to effectively guide field conditions, the test often takes 12 hours or even several months to complete (References: Wang Chao, Zhu Wei, Min Fanlu, Qian Yongjin. Experimental study on measurement of mud film air intake value under compressed air conditions [J]. Tunnel Construction (Chinese and English), 2019, 39(4):626-632. and Sun Jinxin, Zhong Xiaochun, Sun Heming, Zhang Shuxiang, Xi Zhendong. Study on the air tightness capacity of shield tunneling with pressurized opening in gravel strata [J]. Journal of Underground Space and Engineering, 2021, 17(2):445-460.). Therefore, by establishing a shield tunneling mud film air tightness test simulation system, parameter calibration based on a small number of indoor tests can be achieved, thereby quickly evaluating the mud film air tightness characteristics under different geological conditions and shield tunneling parameter conditions. This simulation system will greatly improve testing efficiency, reduce time costs, provide strong theoretical support and technical guarantee for the design of pressurized tunnel boring machine (TBM) opening, and promote the further development of TBM construction technology in underground space development.

[0005] Existing technology 1:

[0006] Existing literature includes Wu Di, Zhou Shunhua, and Li Yaochen. Deformation-seepage-diffusion coupled calculation model of mud infiltration in saturated sand [J]. Acta Mechanica Sinica, 2015, 47(6), 1126-1036. and Mao Jiahua, Yuan Dajun, Yang Jiangxiao, and Zhang Bing. Theoretical study on the characteristics of void variation in mud-water shield excavation in sandy strata [J]. Rock and Soil Mechanics, 2020, 41(7), 2283-2292. The technologies disclosed in these two literatures establish a mud infiltration test simulation model under saturated conditions based on the principle of mass conservation, and then calculate and analyze the deposition and diffusion process of mud into the strata under different construction parameters.

[0007] Existing technology cannot assess the air-tightness characteristics of mud films:

[0008] While both mud permeability tests and mud film air tightness tests involve the interaction between mud and formation, their core focuses differ. Mud permeability tests primarily study the infiltration and deposition process of mud in soil; therefore, the calculation results of mud permeability test simulation models are based on mud deposition concentration or formation porosity after mud deposition, aiming to evaluate the quality (thickness, permeability, etc.) of mud films formed under different mud pressures in complex formations. Mud film air tightness tests, on the other hand, mainly focus on the air tightness characteristics of mud films formed by mud under pressure loading. Therefore, the calculation results of mud film air tightness test simulation models are mainly based on water / vapor saturation in composite formations, aiming to analyze how to maintain formation stability and prevent gas leakage and groundwater intrusion after mud film formation by adjusting air pressure. Therefore, the mud permeability test simulation model proposed in Technical Scheme 1 cannot be used to evaluate the air tightness characteristics of mud films.

[0009] Existing technology 2:

[0010] There is an existing open-source paper titled "Airtightness failure analysis of filter cake during shield tunneling machine hyperbaric intervention" published online by Dalong Jin, Bowen Cai, Xinggao Li, Zheng Mou, and Jicheng Shu in Acta Geotechnica. The technology disclosed in this paper uses the principle of mass conservation to establish a simulation model for shield tunneling machine mud film air tightness test, and then calculates and analyzes the mud film air tightness time under different geological conditions and shield tunneling parameters.

[0011] Existing technology 2 does not fully consider the fluid-structure interaction:

[0012] First, this scheme fails to consider the impact of the mud film's mechanical response on its airtightness. The solid skeleton of the mud film deforms during pneumatic loading. Due to the poor permeability of the mud film and the inability to expel moisture promptly, this mechanical deformation causes a sharp change in local water pressure, affecting the pressure distribution at the excavation face and in the surrounding strata. This water pressure change not only alters the gas flow path but may also affect the air-water interaction and airtightness of the mud film. Therefore, the coupling effect between the mud film's mechanical deformation and water pressure changes is crucial for accurately simulating the airtightness characteristics of the excavation face; neglecting this will lead to an underestimation of the mud film's support capacity and airtightness in actual construction.

[0013] Meanwhile, this scheme is based on the saturation assumption, meaning it does not consider the impact of mud film and formation water loss on the air-tightness characteristics of composite formations. However, during pressurized excavation, the mud film and formation are usually in an unsaturated state. Therefore, the capillary suction and residual saturation of the mud film and formation determine the distribution of moisture and the flow path of gas. When moisture is lost, the gas velocity increases, thereby changing the gas-water distribution and the excavation face pressure. If the hydraulic behavior of unsaturated soil is not considered, the model cannot accurately describe the gas-water interaction, and therefore cannot accurately predict the gas pressure distribution, the air-tightness characteristics of the mud film, and the stability of the excavation face. Summary of the Invention

[0014] To overcome or alleviate one or more of the above-mentioned technical problems, the purpose of this invention is to provide a simulation method for shield tunneling mud film air tightness test. This method provides an indoor air tightness test simulation model based on Biot's pore elasticity theory under fluid-structure interaction conditions, and considers the different water-holding capacity characteristics (soil-water characteristic curves) of mud film and stratum. Then, the model is verified by the gas migration law and leakage flow evolution data obtained from the indoor test. Finally, it realizes the rapid evaluation of mud film air tightness characteristics under complex geological conditions and shield tunneling parameter conditions, and provides important theoretical guidance and technical support for shield tunneling pressurized opening design.

[0015] This invention provides the following technical solution:

[0016] A simulation method for shield tunneling mud film air tightness test includes the following steps:

[0017] S1: Establish the governing equations for the movement of pore water and pore gas in the shield tunneling mud film air-tightness test, as well as the mechanical equilibrium equations of the soil, and embed the equations to solve the required constitutive relations, thereby establishing a simulation system for the shield tunneling mud film air-tightness test.

[0018] S2: Prepare mud as needed and collect soil samples from the site;

[0019] S3: Determine the porosity, density, and residual water content of mud and soil through physical property tests; determine the hydraulic conductivity of mud and soil through permeability tests; determine the elastic modulus and Poisson's ratio of mud and soil through triaxial tests; determine the water holding capacity parameters of mud and soil through water holding capacity tests, and plot soil-water characteristic curves;

[0020] S4: Input the material parameters obtained in S3 into the simulation system of the shield tunnel mud film air tightness test obtained in S1, and then use a small number of mud film air tightness tests to carry out model verification.

[0021] S5: Conduct simulation calculations and analyses under different loading air pressures, mud film thicknesses, mud film soil-water characteristic curves and hydraulic conductivity, and stratum soil-water characteristic curves and hydraulic conductivity to ultimately achieve rapid evaluation of mud film air-tightness characteristics under complex geological conditions and shield tunneling parameter conditions.

[0022] In the above embodiments, porosity is the same as saturated water content.

[0023] According to some implementation methods, the governing equations for the movement of pore water and pore gas in the shield tunneling mud film airtightness test are as follows:

[0024]

[0025] In the formula, n represents porosity, and α = 1 - K d / K s K represents the Biot coefficient. d K represents the bulk modulus of the infill skeleton. s ε represents the stiffness of the solid particles. v p represents the volumetric deformation of the filling material. w and p g Let T and S represent pore water pressure and air pressure, respectively, T represent temperature, and S represent water saturation. K represents the pressure at which water is saturated. w p represents the stiffness of water. c =p g -p w Indicates capillary pressure, v rw and v rg M represents the apparent flow velocities of water and gas relative to the solid phase in the pores, respectively. g R represents the molecular mass of the gas phase, and R represents the universal gas constant.

[0026] The mechanical equilibrium equation of the soil in the shield tunneling mud film air tightness test is as follows:

[0027]

[0028] In the formula, σ represents the total stress tensor, ρ is the total density of the soil, and ρ s ρ w and ρ gLet represent the densities of each phase, and g represent the gravitational acceleration.

[0029] To solve the system of partial differential equations (1), (2), and (3) simultaneously, the constitutive relation described below is used:

[0030] Darcy's law, which describes water vapor transport under unsaturated conditions, is as follows:

[0031]

[0032] In the formula, k rw and k rg Each represents the relative permeability of the phase, μ w and μ g These represent the dynamic viscosity of each phase, and k represents the inherent permeability of the soil.

[0033] The relative permeability of each phase is related to the water content or saturation of the soil and is calculated by the following formula:

[0034]

[0035] In the formula m s The saturation degree S of pore water is calculated using the following formula, which is a parameter of soil water-holding capacity:

[0036]

[0037] In the formula, θ, θ r θ s These are the volumetric water content, residual water content, and saturated water content, respectively. The saturated water content is the same as the porosity value, and the volumetric water content θ is calculated using the following formula:

[0038]

[0039] In the formula α s For another soil water-holding capacity parameter, the capillary head H p and material parameter n s Calculated by the following formula:

[0040] H p =p c / ρg (10)

[0041] n s =1 / (1-m) s (11)

[0042] The total stress of the soil is calculated by the following formula:

[0043] σ=Dε-αpδ ij (12)

[0044] In the formula, D is the stiffness matrix composed of elastic modulus and Poisson's ratio, and ε is the strain tensor.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] The simulation method for shield tunneling mud film air tightness testing provided by this invention overcomes the limitations of existing scheme one, which cannot reproduce the physical process of mud film air tightness, and also addresses the theoretical shortcomings of existing scheme two, which does not consider fluid-structure interaction effects and soil water-holding capacity. Therefore, the shield tunneling mud film air tightness testing simulation system established by this scheme can reasonably explain the hydraulic mechanism of mud film air tightness, a feature not found in other similar schemes. Furthermore, this scheme can achieve rapid evaluation of mud film air tightness characteristics under complex geological conditions and shield tunneling parameter conditions based on parameter calibration from a small number of indoor tests. This improves testing efficiency and reduces time costs, providing strong theoretical guidance and technical support for the pressurized opening design of shield tunnels. Attached Figure Description

[0047] Figure 1 The diagram shows the evolution of leakage flow rate under different air pressure loading conditions in the air closure test, as provided in the embodiments of the present invention.

[0048] Figure 2 The diagram shows the final leakage flow rate under different loading pressure conditions provided in the embodiments of the present invention.

[0049] Figure 3 The evolution law of soil column saturation distribution over time (air pressure) provided in the embodiments of the present invention

[0050] =150kPa).

[0051] Figure 4 The soil column saturation distribution after 36 hours of air pressure loading is provided in an embodiment of the present invention. Detailed Implementation

[0052] The present invention will now be described in detail with reference to embodiments and accompanying drawings. However, it should be understood that the embodiments and drawings are for illustrative purposes only and do not constitute any limitation on the scope of protection of the present invention. All reasonable modifications and combinations included within the inventive spirit of the present invention fall within the scope of protection of the present invention.

[0053] The present invention will be further described below with reference to the accompanying drawings.

[0054] Example 1

[0055] This embodiment uses a mud film air tightness test conducted in a subway pressurized opening project as an example to demonstrate the implementation process of this solution:

[0056] (1) Prepare mud as needed and collect soil samples from the site.

[0057] (2) The porosity, density and residual water content of mud and formation are determined by conventional physical property tests, the hydraulic conductivity of mud and formation is determined by permeability test, the elastic modulus and Poisson's ratio of mud and formation are determined by triaxial test, and the soil-water characteristic curve (water holding capacity parameter) of mud and formation is determined by water holding capacity test.

[0058] (3) Establish a model test device with a diameter of 10cm and a height of 80cm, fill it with 74cm of soil and saturate it, then apply mud pressure on top to finally form a mud film of about 6cm.

[0059] (4) Conduct a mud film air tightness test by opening the bottom valve of the device (air pressure = water pressure = 0) and applying air pressure to the top mud film surface, and monitor the leakage flow rate at the bottom of the device in real time during the test.

[0060] (5) The material parameters obtained in step (2) above are brought into the simulation system of the shield tunnel mud film air tightness test, and the mud film air tightness test described in steps (3) to (4) above is simulated and reproduced. The model is verified by using the leakage flow monitoring data obtained from the monitoring.

[0061] (6) Based on this, simulation calculation and analysis can be carried out under different loading air pressure, mud film thickness, mud film soil-water characteristic curve and water conductivity, stratum soil-water characteristic curve and water conductivity, etc., to realize rapid evaluation of mud film air-tightness characteristics under complex geological conditions and shield tunneling parameter conditions.

[0062] (7) Taking different air pressure loading conditions as an example, the leakage flow rate at the bottom of the air tightness test device can be obtained through calculation and monitoring, such as... Figure 1 As shown in the figure, the calculation results of this scheme are in good agreement with the experimental results of previous researchers.

[0063] Specifically, when the applied air pressure does not exceed 75 kPa, the leakage flow rate increases slowly only under the influence of the hydraulic gradient in the soil column. However, when the air pressure reaches 100 kPa, increasing the air pressure will cause a significant increase in the leakage flow rate. Meanwhile, the temporal evolution of the leakage flow rate under various working conditions is roughly the same and can be divided into three main stages:

[0064] (A) Initial stage: At this stage, the gas-liquid interface has not yet entered the formation (i.e., the mud film has not yet been broken through), the leakage flow rate increases slowly, and the greater the gas pressure, the earlier this stage ends (i.e., the mud film breaks through quickly).

[0065] (B) Drainage stage: After the mud film is broken, gas enters the lower stratum with a higher permeability coefficient, which in turn quickly displaces the pore water, ultimately causing a rapid increase in leakage flow. At the same time, the greater the loading gas pressure, the greater the hydraulic gradient in the soil column, and therefore the faster the leakage flow increases.

[0066] (C) Steady stage: As time goes on, the water vapor migration in the soil column eventually reaches a steady stage in equilibrium with the boundary conditions, so the leakage rate gradually slows down.

[0067] Figure 2 The calculation results show the evolution of leakage flow rate with pressure after 36 hours of continuous pressure application. It can be observed that... Figure 2 The calculation results shown are consistent with Figure 1 The results show good consistency. Specifically, when the air pressure does not exceed 75 kPa, the total leakage flow increases slowly with increasing air pressure. However, when the air pressure continues to rise, the soil column will suddenly experience a rapid increase in leakage flow at around 90 kPa. Based on the above analysis results, the mud film air tightness value under this mud film-stratum combination condition can be determined to be approximately 90 kPa.

[0068] To further illustrate the air-tightness characteristics of mud film under air pressure, the evolution of soil column saturation distribution over time is plotted using an air pressure of 150 kPa as an example. The calculation results are as follows: Figure 3 As shown, existing technical solutions cannot obtain this type of data display. The figure shows that after 0.5 hours of pressure application (… Figure 3 (b) Since the mud film has a strong water-holding capacity and a low permeability coefficient, the gas-liquid interface remains within the mud film, corresponding to stage (A) above. Figure 3 As shown in (c), the mud film under this pressure condition will break down in about 1.5 hours, marking the end of stage (A) above. Subsequently, the gas will enter the more permeable coarse-grained strata (…). Figure 3

[0069] (d) then rapidly removes pore water through displacement (i.e., stage (B) above) until water vapor migration finally reaches equilibrium with the boundary conditions. Figure 3 (e), i.e., the aforementioned stage (C)).

[0070] Figure 4 The calculated saturation distribution of the soil column after 36 hours of continuous air pressure application is shown. The figure shows that when the applied air pressure is lower than the air closure value ( Figure 4 (a) The soil column will eventually approach saturation, with only slight undersaturation at the top of the strata with weak water-holding capacity due to capillary suction gradient. As the air pressure increases and exceeds the occlusion threshold, the gas-liquid interface will penetrate the mud film and eventually enter the lower strata, causing significant water loss through displacement. Figure 4 (b)). As the loading pressure continues to increase, the pore water in the soil column will be more effectively discharged, and the higher the loading pressure, the lower the saturation level in the soil column. Figure 4 (c) Figure 4 (d)).

[0071] The simulation results above demonstrate that the mud film air tightness test simulation system established in this scheme can reasonably explain the hydraulic mechanism of mud film air tightness, a capability not found in other similar schemes. Furthermore, this scheme can achieve rapid evaluation of mud film air tightness characteristics under complex geological conditions and shield tunneling parameters based on parameter calibration from a limited number of indoor tests. This, in turn, improves testing efficiency and reduces time costs, providing strong theoretical guidance and technical support for the pressurized opening design of shield tunnels.

[0072] The above embodiments are merely preferred embodiments of the present invention, and the scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A simulation method for shield tunneling mud film air tightness test, characterized in that, Includes the following steps: S1: Establish the governing equations for the movement of pore water and pore gas in the shield tunneling mud film air-tightness test, as well as the mechanical equilibrium equations of the soil, and embed the equations to solve the required constitutive relations, thereby establishing a simulation system for the shield tunneling mud film air-tightness test. S2: Prepare mud as needed and collect soil samples from the site; S3: Determine the porosity, density, and residual water content of mud and soil through physical property tests; determine the hydraulic conductivity of mud and soil through permeability tests; determine the elastic modulus and Poisson's ratio of mud and soil through triaxial tests; determine the water holding capacity parameters of mud and soil through water holding capacity tests, and plot soil-water characteristic curves; S4: Input the material parameters obtained in S3 into the simulation system of the shield tunnel mud film air tightness test obtained in S1, and then use a small number of mud film air tightness tests to carry out model verification. S5: Conduct simulation calculations and analyses under different loading air pressures, mud film thicknesses, mud film soil-water characteristic curves and hydraulic conductivity, and stratum soil-water characteristic curves and hydraulic conductivity, ultimately achieving rapid evaluation of mud film air-tightness characteristics under complex geological conditions and shield tunneling parameter conditions. The governing equations for the movement of pore water and pore gas in the shield tunneling mud film airtightness test are as follows: ， ， In the formula, n represents porosity, and α = 1 − K d / K s K represents the Biot coefficient. d K represents the bulk modulus of the infill skeleton. s ε represents the stiffness of the solid particles. v p represents the volumetric deformation of the filling material. w and p g Let T and S represent pore water pressure and air pressure, respectively; T represent temperature; and S represent water saturation, K. w p represents the stiffness of water. c =p g -p w Indicates capillary pressure, v rw and v rg M represents the apparent flow velocities of water and gas relative to the solid phase in the pores, respectively. g R represents the molecular mass of the gas phase, and R represents the universal gas constant. The mechanical equilibrium equation of the soil in the shield tunneling mud film air tightness test is as follows: ， In the formula, σ represents the total stress tensor, ρ is the total density of the soil, and ρ s ρ w and ρ g Let represent the densities of each phase, and g represent the gravitational acceleration.

2. The simulation method for the shield tunneling mud film air tightness test according to claim 1, characterized in that, To solve the system of partial differential equations (1), (2), and (3) simultaneously, the following constitutive relations are used: Darcy's law, which describes water vapor transport under unsaturated conditions, is as follows: ， ， In the formula, k rw and k rg Each represents the relative permeability of the phase, μ w and μ g These represent the dynamic viscosity of each phase, and k represents the inherent permeability of the soil. The relative permeability of each phase is related to the water content or saturation of the soil and is calculated by the following formula: ， ， In the formula m s The saturation degree S of pore water is calculated using the following formula, which is a parameter of soil water-holding capacity: ， In the formula, θ, θ r θ s These are the volumetric water content, residual water content, and saturated water content, respectively. The saturated water content is the same as the porosity value, and the volumetric water content θ is calculated using the following formula: ， In the formula α s For another soil water-holding capacity parameter, the capillary head H p and material parameter n s Calculated by the following formula: ， ， The total stress of the soil is calculated by the following formula: ， In the formula, D is the stiffness matrix composed of elastic modulus and Poisson's ratio, and ε is the strain tensor.

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