Method for measuring critical value of stratum porosity in shield earth pressure balance

Abstract

The invention discloses a method for measuring a critical value of stratum porosity in shield earth pressure balance, and relates to the field of shield construction measurement. Comprising the following steps: establishing a pool with a circular bottom area S; high-pressure gas overflows upwards through the tunnel face of the shield tunneling machine, and the amount Q of the overflowed gas is controlled; the pool is observed until scattered bubbles are continuously and uniformly generated at the bottom of the pool, and in the corresponding time period t0-t1, the accumulated gas discharge volume reading V1 is calculated through the flow reading of the pressure maintaining system flowmeter; the shield tunneling machine maintains the stable pressure flow until the total volume of the discharged accumulated gas is close to the pre-estimated pore volume of the soil body, the corresponding time period is tbegin-top, the accumulated gas discharge volume reading V2 is calculated, and the critical value n of the stratum porosity is calculated. The invention further comprises a corresponding device, extra soil sampling experiments are not needed, formula calculation is carried out only according to the gas discharged by the pressure maintaining system, and convenience and rapidness are achieved.

Description

Technical Field

[0001] This invention relates to the field of soil porosity critical value measurement, specifically to a method for measuring the critical value of stratum porosity in shield tunnel earth pressure balance. Background Technology

[0002] Currently, research on soil porosity and porosity measurement largely relies on conventional soil consolidation tests. The core limitation of this traditional method is the necessity of collecting soil samples first, followed by a complex laboratory experimental process for measurement. Especially in shield tunneling operations, obtaining the porosity of the overlying soil presents not only the practical problem of difficult soil collection but also the additional time and resources required for laboratory testing. This process is not only cumbersome and inefficient but also difficult to apply in scenarios where laboratory calibration conditions are inconvenient, severely restricting the real-time acquisition of ground porosity-related parameters during shield tunneling.

[0003] Therefore, there is an urgent need for a measurement scheme that does not require the collection of soil samples, so as to realize the direct on-site measurement of the critical value of the stratum porosity in the shield tunneling earth pressure balance construction scenario, so as to avoid the dependence of traditional methods on indoor soil sampling and complex test procedures, and meet the needs of obtaining key stratum parameters efficiently and conveniently during construction. Summary of the Invention

[0004] To address the shortcomings of the existing technology, this invention provides a method for measuring the critical value of ground porosity in shield tunneling earth pressure balance. By integrating on-site soil sampling, a linked pressure-maintaining system, calibrated bubble observation, and accurate volume conversion, the critical value of ground porosity can be measured in real time during shield tunneling construction.

[0005] The technical solution of this invention is as follows:

[0006] A method for measuring the critical value of ground porosity in shield tunneling earth pressure balance includes:

[0007] Step 1: Construct a seepage-proof, circular pool with a base area of ​​S, directly opposite the shield tunneling site;

[0008] Step 2: Based on the underground shield machine and the tunnel with an excavation depth of H, high-pressure gas is released upwards by the shield machine, and the amount of gas released is controlled Q.

[0009] Step 3: Observe the water tank until scattered bubbles are continuously and evenly generated at the bottom of the tank. During the time period t0~t1, measure the flow rate reading through the flow meter of the pressure-maintaining system. Calculate the cumulative gas discharge volume reading V1.

[0010]

[0011] Step 4: Maintain stable output voltage and flow rate This process involves continuously venting the water tank until the total volume of gas discharged approaches the estimated pore volume of the soil. Ensure that the pore gas is fully replaced; record the cumulative gas discharge volume reading V2 at this time, and record the time period tbegin~tover:

[0012]

[0013] Step 5: Calculate the critical value of formation porosity n:

[0014]

[0015] In the formula, n is the critical value of the formation porosity, V is the volume of the soil passed through, and H is the vertical distance between the shield tunnel face and the ground surface. .

[0016] As a preferred embodiment, the air bubbles overflowing upwards from the excavation side of the tunnel boring machine include: a pressure-holding system that adjusts gradually from low to high pressure, allowing gas to infiltrate and gradually replace pore water in the soil. In this invention, the pressure-holding system adopts a gradual pressure increase method from low to high, which allows gas to smoothly infiltrate into the soil and gradually replace pore water, avoiding high pressure directly breaking through the soil layer to form dominant channels. This ensures that the gas uniformly fills the pore network, laying the foundation for accurate determination of critical states and improving the stability of measurement results.

[0017] As a preferred embodiment, the pressure range of the pressure-holding system includes:

[0018]

[0019] In the formula, This is an empirical value of capillary pressure determined based on soil properties. The density of the water body is given. This invention, by defining a specific formula for the air pressure range, precisely matches the shield tunneling depth with the capillary characteristics of the soil layer. This ensures that the gas can effectively displace pore water and penetrate to the surface, while avoiding measurement failures caused by excessively high or low air pressure. It adapts to the air pressure control requirements under different working conditions, enhancing the versatility and reliability of the method.

[0020] As a preferred embodiment, the measurement method is applied to low-permeability saturated or near-saturated cohesive soil layers that are homogeneous and free of macroscopic cracks or seepage channels. This invention is specifically adapted to low-permeability saturated / near-saturated cohesive soil layers, avoiding the interference of macroscopic cracks and seepage channels on gas migration, ensuring that gas flow is primarily through pores, further improving the accuracy of porosity determination, and making the method more suitable for core engineering applications.

[0021] As a preferred embodiment, the water tank is positioned directly below the shield tunneling site, and further includes positioning the water tank above the shield tunnel axis. In this invention, positioning the water tank above the shield tunnel axis allows for precise focusing on the core area of ​​the rising gas, reducing collection errors caused by lateral gas diffusion, ensuring that all emitted gas is captured, improving the accuracy of volume conversion, and simplifying the observation and data acquisition process.

[0022] As a preferred approach, the process continues until scattered bubbles are continuously and uniformly generated at the bottom, including: observing the continuous and uniform generation of scattered bubbles at the bottom of the pool, and maintaining this bubble state for at least 15 minutes after the pressure-holding system output stabilizes, thus determining that the soil has entered a seepage state. This invention clarifies the quantitative criteria for determining the critical state, standardizes the observation criteria of continuous and uniform bubbles and 15 minutes of stability, avoids subjective judgment errors, and ensures that data is recorded only after the soil has truly entered a stable seepage state, providing a reliable basis for subsequent volume calculations and porosity derivation.

[0023] As a preferred option, this includes: multiple pressure holding calculations. The final fitted average value n is obtained by calculating V2 multiple times and taking the fitted average value. In this invention, by calculating V2 multiple times and taking the fitted average value, the random error of a single measurement is effectively offset, the accuracy of the measurement results is further optimized, and the critical value of formation porosity is made to better fit the actual soil characteristics, providing a more reliable reference for setting construction pressure holding parameters.

[0024] As a preferred embodiment, the measurement is stopped when the cumulative exhaust volume of the tunnel boring machine reaches 130-160 m³. This invention clearly defines the stopping point for the cumulative exhaust volume, simplifies the measurement operation process, avoids inefficiency caused by excessive exhaust, and ensures that the gas in the soil pores is fully replaced. This improves the efficiency of simultaneous construction measurements while ensuring the effectiveness of the measurement.

[0025] A measuring device for determining the critical value of formation porosity in earth pressure balance during shield tunneling, comprising the following methods for determining the critical value of formation porosity:

[0026] Water tank device: Directly above the shield tunneling site, with seepage prevention treatment, the bottom area of ​​the water tank is S;

[0027] Tunnel boring machine and its pressure holding system: The tunnel boring machine face is used to release air bubbles at a depth of H, and the tunnel boring machine is used to control the amount of gas released, Q.

[0028] Cumulative exhaust volume measuring device: used to measure the real-time gas flow rate of the pressure holding system within the time period t0~tk. To calculate the cumulative gas discharge volume reading

[0029]

[0030] Cumulative exhaust volume measurement device: used to maintain stable output pressure and flow rate. This process involves continuously venting the water tank until the total volume of gas discharged approaches the estimated pore volume of the soil. During the recording period tbegin~tover, record the cumulative gas discharge volume reading V2 at this time:

[0031]

[0032] Formation porosity critical value calculation device: used to calculate the critical value of formation porosity.

[0033]

[0034] In this invention, the various components of the device are coordinated and adapted to the measurement method. The water tank device ensures clear observation and complete collection of bubbles, the pressure holding system achieves precise venting control, and the volume measurement and calculation device is efficiently linked. The structure is simple and easy to deploy. It does not require complex additional components and can achieve direct on-site measurement without soil sampling, thus meeting the practical needs of shield tunneling construction scenarios.

[0035] As a preferred embodiment, the pressure range of the pressure-holding system includes:

[0036]

[0037] In the formula, This is an empirical value of capillary pressure determined based on soil properties. Where H is the density of the water body, and H is the depth of the tunnel boring machine or the depth of the overflow outlet. In this invention, the air pressure control range of the measuring device is clearly defined to ensure that the air pressure of the device is accurately matched with the soil characteristics and burial depth, to ensure effective gas penetration and stable exhaust, to avoid device failure due to improper air pressure, and to further enhance the device's adaptability to operating conditions and the reliability of measurement.

[0038] According to the above-described solution, the beneficial effects of this invention are as follows:

[0039] This invention integrates surface water tank observation, shield tunneling pressure-maintaining system venting, and volume conversion, eliminating the need for soil sampling and indoor testing. This allows for direct on-site determination of the critical porosity value of the stratum during shield tunneling construction. The device only requires a seepage-proof water tank above the tunnel axis, relying on the shield's built-in pressure-maintaining system, making it simple and easy to operate. Through clear time period divisions and flow rate acquisition, coupled with intuitive calculation formulas, the measurement results are reliable and accurate. Furthermore, measurements can be conducted simultaneously with shield tunneling, efficiently meeting the real-time acquisition needs on-site and providing direct basis for setting construction pressure-maintaining parameters, thus contributing to safe and stable construction. Attached Figure Description

[0040] Figure 1This is a diagram illustrating an application scenario of the present invention. Detailed Implementation

[0041] To better understand the purpose, technical solution, and technical effects of this invention, the invention will be further explained and described below in conjunction with the accompanying drawings and embodiments. It should be noted that similar reference numerals and letters in the following drawings indicate similar items; therefore, once an item is defined in one drawing, it does not need to be further defined and explained in subsequent drawings. It is also stated that the embodiments described below are only for explaining this invention and are not intended to limit this invention.

[0042] It should be noted that when a component is referred to as "fixed to" or "set on" another component, it can be directly on the other component or there may be an intermediate component. When a component is referred to as "connected to" another component, it can be directly connected to the other component or there may be an intermediate component.

[0043] The indicated orientation or positional relationship is based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship in which the product is usually placed when in use, or the orientation or positional relationship in which a person skilled in the art would normally understand it, or the orientation or positional relationship in which the product is usually placed when in use. It is only for the purpose of facilitating the description of this application and simplifying the description, and is not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0044] Example 1

[0045] like Figure 1 As shown, a method for measuring the critical value of stratum porosity in shield tunnel earth pressure balance is presented. This embodiment is applied to a shield tunnel excavation project of a subway in a certain city. As a demonstration example of the specific implementation of the present invention, the stratum in the construction section is a uniform silty clay layer without obvious macroscopic cracks and seepage channels, that is, a low-permeability saturated or near-saturated cohesive soil layer, which is consistent with the applicable scenario of the present invention.

[0046] The vertical burial depth H from the tunnel boring machine face to the ground surface is 22m. The critical value of the porosity of the overlying soil needs to be measured in real time on site to provide a basis for optimizing the pressure-holding parameters of the tunnel boring machine. However, the conditions for conducting indoor soil sampling tests are not available or are difficult to meet.

[0047] I. Required Equipment and Parameter Presets:

[0048] core equipment

[0049] Water tank device: The sidewalls are formed by circular steel formwork with a height of 0.45m, leaving a water depth of 0.35m. The inner diameter of the water tank is 5.2m, and the bottom area S is approximately 21.237m². The center of the water tank is strictly aligned with the axis of the shield tunnel. The sidewalls only serve to contain the water, and the bottom is not additionally sealed and is directly in contact with the soil. The bottom seepage prevention adopts simple compaction treatment: a 0.3m deep foundation pit is excavated, the surface loose soil is removed, and clay is laid in layers and compacted. Each layer is 10cm thick and the compaction degree is ≥96%. The compacted clay is used to block the water seepage while ensuring that air bubbles can pass through without obstruction.

[0050] Tunnel Boring Machine and its Pressure Holding System: The tunnel boring machine is equipped with a high-pressure gas pressure holding system, which includes an air filter, a pressure reducing valve, a pneumatic valve controller, a pressure sensor, and a high-precision flow meter with a measurement range of 0-8 m³ / s. The pressure holding system dries the high-pressure air through the air filter and delivers it to the pneumatic valve controller via the pressure reducing valve. The excavation chamber pressure is fed back to the controller in real time. The controller maintains stable excavation chamber pressure by opening and closing the pneumatic valve to replenish air or by controlling the pneumatic ball valve to release pressure through the pilot valve. Simultaneously, the flow meter monitors the discharge gas flow rate in real time.

[0051] Auxiliary equipment: Three high-definition cameras are evenly arranged around the circumference of the pool, with the lenses facing the soil contact surface at the bottom of the pool, to assist manual observation of the bubble state from above; the data recording and processing terminal is linked with the flow meter and pressure sensor, and can automatically collect instantaneous flow rate Q(t), pressure and time data, and has the function of flow-time integration calculation.

[0052] II. Preset Parameter Settings

[0053] The pressure range of the pressure-holding system includes:

[0054]

[0055] The lower limit of the pressure holding system is calculated based on the above, where: water density =1000kg / m³, gravitational acceleration g=9.8m / s², burial depth H=22m; empirical value of capillary pressure in silty clay layer =7kPa, P≥1.3×1000×9.8×22+7000=294680Pa≈294.7kPa, the actual initial pressure is set at 270kPa, with space reserved for step-up pressure adjustment;

[0056] The empirical value of capillary pressure in silty clay layers is Pc = 7 kPa, which can be determined by combining local experience with similar geological engineering.

[0057] The calculation yields: P≥1.3×1000×9.8×22+7000=294680Pa≈294.7kPa. The actual initial pressure setting is 270kPa, with room for step-by-step pressure adjustment.

[0058] Estimated soil pore volume: Based on geological survey data, the natural porosity of the silty clay layer here is estimated to be 26%. The soil volume is calculated using the formula V=1 / 3×S×H, V=1 / 3×21.237m²×22m≈155.32m³. The estimated pore volume Vpredicted=155.32m³×26%≈40.38m³, which provides a reference for the venting stop point.

[0059] Exhaust stop point description: In this embodiment, based on the estimated pore volume, 150m³ is selected as the reference value for exhaust stop to ensure that the pore gas is fully replaced.

[0060] III. Specific Implementation Steps

[0061] Step 1: Construction and Seepage Prevention Acceptance of Water Tank Equipment

[0062] Excavate a foundation pit on the ground surface above the shield tunnel axis according to the design dimensions, lay clay in layers and compact it to the design elevation, ensuring uniform compaction without any loose areas; use steel formwork to form a circular sidewall, fix it firmly to prevent deformation after water injection, and seal the joints to prevent lateral water leakage; inject clean water into the pool to a depth of 0.35m, and observe the seepage prevention effect after standing for 24 hours: if the water level drops by ≤7mm / 24h, it meets the water stability requirements during the measurement process, and the pool is deemed qualified; adjust the camera equipment to ensure full coverage of the soil contact surface at the bottom of the pool, and clearly capture the state of the air bubbles.

[0063] Step 2: Start-up of the pressure holding system and step-by-step pressure increase

[0064] The tunnel boring machine's pressure-maintaining system is activated, with an initial air pressure of 270 kPa. Pressure is increased by 35 kPa at each stage, and maintained for 7 minutes after each stage. Simultaneously, the state of air bubbles at the bottom of the water tank is observed via video equipment and manually. During the pressure increase process, pressure sensors provide real-time feedback on the excavation chamber pressure, and the pneumatic valve controller automatically adjusts the valve opening and closing to maintain stable pressure in the excavation chamber, preventing high pressure from breaching the soil and creating a superior channel.

[0065] Step 3: Critical State Determination and V1 Recording

[0066] After the pressure was increased to 330 kPa and stabilized, scattered air bubbles began to appear at the soil contact surface at the bottom of the pool. The pressure was increased to 350 kPa and stabilized. Scattered bubbles were observed to continuously and evenly seep out from most areas of the bottom of the pool. The distribution of bubbles was confirmed to be uniform by video playback. This state remained unchanged for 15 minutes after the pressure stabilized, which met the criteria for critical state judgment. The soil was judged to have entered a stable seepage state.

[0067] At this point, the critical state start time t0=09:10 is recorded, and observation continues until t1=09:25, maintaining this stable state for 15 minutes. The instantaneous flow rate Q1(t) during this period is integrated over time using the data terminal to calculate the cumulative gas discharge volume. The integral result is 1352m³, which was automatically calculated and recorded via the terminal.

[0068] Step 4: Continuous Exhaust and V2 Recording

[0069] Maintain a constant pressure in the pressure-holding system and continuously ventilate at a steady flow rate of 1.5 m³ / s, while simultaneously monitoring the cumulative vent volume. When the cumulative flowmeter reading shows a vent volume of 150 m³ (a specific example within the range of 130-160 m³), ​​and based on geological data, it is determined to be close to the estimated pore volume, record the stop time tover=11:08 and the start time tbegin=09:25 (the time period tbegin~tover, duration 103 minutes, during which the cumulative flowmeter reading is recorded).

[0070] .

[0071] Calculated using the formation porosity critical value formula n=(V2-V1) / V×100%:

[0072] The gas volume in the displaced pores = V2 - V1 = 7900 m³ - 1350 m³ = 6550 m³;

[0073] The volume of the soil is V = 1 / 3 × S × H = 1 / 3 × 21.237 m² × 22 m ≈ 155.32 m³;

[0074] The critical value of porosity in a single measurement is n1 = 6550 / 155.32 × 100% ≈ 42.17%.

[0075] Step 6: Multiple measurements and fitting average value

[0076] Repeat steps 1 to 5 above a total of 3 times, with a 2.5-hour interval between each measurement (to ensure that the formation gas dissipates and returns to its initial state), and obtain 3 sets of porosity data: n1=42.17%, n2=42.33%, n3=42.09%.

[0077] The average value calculated by data statistical fitting is n=(42.17%+42.33%+42.09%) / 3≈42.19%, which is the critical value of the formation porosity in this construction section.

[0078] During this implementation, the simple compacted seepage-proof water tank showed no significant leakage. Direct contact between the bottom and the soil ensured unobstructed airflow, resulting in clear and interference-free observation. The pressure-maintaining system maintained stable pressure, and flow rate acquisition was accurate. The error in three repeated measurements was ≤0.24%, indicating good repeatability and reliable measurement accuracy. The calculated critical porosity value of 42.19% provided a direct basis for setting the shield tunneling pressure-maintaining parameters. In subsequent construction, the pressure-maintaining system was adjusted according to the calculated lower limit of 294.7 kPa. During shield tunneling, the tunnel face remained stable, and surface settlement was controlled within allowable limits, verifying the engineering practicality and reliability of this invention.

[0079] V. Precautions

[0080] The clay at the bottom of the pool must be compacted evenly and densely to avoid water leakage due to insufficient compaction in certain areas. At the same time, the original soil structure must not be damaged to prevent affecting the gas migration path.

[0081] During the stepped pressurization process, it is necessary to strictly control the pressurization amplitude and stabilization time of each stage to avoid rapid injection of high pressure that could break through the soil layer and form a dominant channel, leading to distortion of the critical state determination.

[0082] When observing bubbles, focus on the contact surface between the soil and water, distinguish between the floating of bubbles and soil particles, and ensure accurate determination of critical states. When the pool area is large, the number of cameras can be increased or a zoned observation method can be adopted.

[0083] The interval between multiple measurements needs to be adjusted according to the permeability of the formation. Sufficient time should be allowed for low-permeability cohesive soil layers to ensure that the formation gas after the previous venting is completely dissipated, and to avoid cross-interference with the measurement results.

[0084] The exhaust stop point can be flexibly adjusted based on the estimated pore volume and the range of 130-160m³. The core is to ensure that the original gas in the soil pores is fully replaced, without strictly limiting a single value.

[0085] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0086] The above embodiments merely illustrate several implementation methods of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the protection scope of the present invention. Therefore, the protection scope of this invention patent should be determined by the appended claims.

Claims

1. A method for measuring the critical value of ground porosity in shield tunneling earth pressure balance, characterized in that, include: Step 1: Construct a seepage-proof, circular pool with a base area of ​​S, directly above the shield tunneling site; Step 2: High-pressure gas is released from the tunnel boring machine face, and the amount of gas released, Q, is controlled. Step 3: Observe the water tank above until scattered bubbles are continuously and evenly generated at the bottom of the tank. During the corresponding time period t0~t1, measure the flow rate reading through the flow meter of the pressure-maintaining system. Calculate the cumulative gas discharge volume reading V1. Step 4: The tunnel boring machine maintains a stable flow rate. This process involves continuously venting the water tank until the total volume of gas discharged approaches the estimated pore volume of the soil. Record the cumulative gas discharge volume reading V2 at this time, corresponding to the time period tbegin~tover: Step 5: Calculate the critical value of formation porosity n: In the formula, n is the critical value of the formation porosity, V is the volume of the soil passed through, and H is the vertical distance between the shield tunnel face and the ground surface. .

2. The method for measuring the critical value of stratum porosity in shield tunneling earth pressure balance according to claim 1, characterized in that, Bubbles overflow from the excavation side of the tunnel boring machine upwards, including through a pressure-holding system that adjusts gradually from low to high, allowing gas to infiltrate and gradually replace the pore water in the soil.

3. The method for measuring the critical value of stratum porosity in shield tunneling earth pressure balance according to claim 2, characterized in that, The pressure range of the pressure-holding system includes: In the formula, This is an empirical value of capillary pressure determined based on soil properties. This represents the density of the water.

4. The method for measuring the critical value of stratum porosity in shield tunneling earth pressure balance according to claim 1, characterized in that, The measurement method is applied to low-permeability saturated or near-saturated cohesive soil layers that are uniform and free from macroscopic cracks or leakage channels.

5. The method for measuring the critical value of stratum porosity in shield tunneling earth pressure balance according to claim 1, characterized in that, The water tank is positioned directly below the shield tunneling site, and also includes: positioning the water tank above the shield tunnel axis.

6. The method for measuring the critical value of stratum porosity in shield tunneling earth pressure balance according to claim 1, characterized in that, Until scattered bubbles are continuously and evenly generated at the bottom, including: observing that scattered bubbles are continuously and evenly generated at the bottom of the pool, and maintaining the bubble state for at least 15 minutes after the pressure holding system output stabilizes, it is determined that the soil has entered the seepage state.

7. The method for measuring the critical value of ground porosity in shield tunneling earth pressure balance according to claim 1, characterized in that, include: Multiple pressure holding calculations And n, to obtain the final fitted average value n.

8. The method for measuring the critical value of stratum porosity in shield tunneling earth pressure balance according to claim 1, characterized in that, include: Measurements were stopped when the cumulative exhaust volume of the tunnel boring machine reached 130-160 m³.

9. A measuring device for determining the critical value of ground porosity in shield tunneling earth pressure balance, characterized in that, The method for measuring the critical value of ground porosity in shield tunnel earth pressure balance as described in any one of claims 1-8 includes: Water tank device: Directly above the shield tunneling site, with seepage prevention treatment, the bottom area of ​​the water tank is S; Tunnel boring machine and its pressure holding system: The tunnel boring machine face is used to release air bubbles at a depth of H, and the tunnel boring machine is used to control the amount of gas released, Q. Cumulative exhaust volume measuring device: used to measure the real-time gas flow rate of the pressure holding system within the time period t0~tk. To calculate the cumulative gas discharge volume reading Cumulative exhaust volume measurement device: used to maintain stable output pressure and flow rate. This process involves continuously venting the water tank until the total volume of gas discharged approaches the estimated pore volume of the soil. During the recording period tbegin~tover, record the cumulative gas discharge volume reading V2 at this time: Formation porosity critical value calculation device: used to calculate the critical value of formation porosity.

10. A method for measuring the critical value of formation porosity in shield tunneling earth pressure balance according to claim 9, characterized in that, The pressure range of the pressure-holding system includes: In the formula, This is an empirical value of capillary pressure determined based on soil properties. Where H is the density of the water, and H is the vertical distance from the tunnel face to the ground surface.

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