Model test device and method for stability of shield tunnel frozen face opening

Abstract

This application provides a model test device and method for the stability of the tunnel face during the freezing method in shield tunnels, relating to the field of tunnel and underground engineering technology. The device includes: a model box; a water level control system comprising a first and second water tank with independent adjustment on both sides, used to establish static water level or dynamic water seepage conditions; a ground freezing and thawing system comprising a buried temperature control pipe and connected refrigerant and heat supply units, achieving freezing and thawing through the same pipeline; a shield tunnel system with a tunnel face opening at one end; a tunnel face pressure control system comprising an airbag and a pressure regulating unit, simulating support pressure and pressure relief during opening by inflation and deflation; and a monitoring and data acquisition system for collecting data on ground temperature, stress, pore water pressure, and deformation. This device can completely simulate the entire process of freezing, pressure relief during opening, and thawing, providing a test platform for studying the stability of the tunnel face under the coupling of multiple factors.

Description

Technical Field

[0001] This application relates to the field of tunnel and underground engineering technology, and in particular to a model test device and method for the stability of the tunnel face opened by the freezing method in shield tunnels. Background Technology

[0002] When constructing shield tunnels in water-rich soft soil strata, controlling the stability of the tunnel face is a core issue for construction safety. When the tunnel boring machine (TBM) experiences mud cake formation on the cutterhead or severe cutter wear requiring pressurized opening for maintenance, conventional dewatering or grouting reinforcement methods have drawbacks such as significant environmental impact and uncertain reinforcement effects. Ground freezing, through artificial refrigeration to form a frozen soil curtain, can effectively improve soil strength and water-tightness, providing temporary reinforcement for opening operations.

[0003] Currently, research on the freezing method for opening tunnel face in shield tunnels mainly relies on numerical simulation and field monitoring. Numerical simulation can consider multi-physics coupling, but the model parameters deviate from actual engineering conditions. While field monitoring is realistic, the number of monitoring points is limited, and the influencing factors are complex, making it difficult to conduct systematic parameter sensitivity analysis. Indoor physical model testing, as a research method between the two, has the advantages of strong controllability, good repeatability, and relatively low cost. However, existing model testing devices still have limitations in simulation capabilities, failing to realistically reflect the complex process of the tunnel face stability evolution under freezing conditions. Furthermore, the test conditions of existing devices are relatively simple, unable to meet the needs of comparative analysis under various working conditions. Therefore, a more comprehensive model testing scheme is urgently needed. Summary of the Invention

[0004] The purpose of this application is to provide a model test device and method for the stability of the tunnel face during the freezing method of shield tunnels. This method can solve the problems in related technologies, such as the difficulty of model test devices in systematically simulating the entire freezing method opening process, the single groundwater condition, the lack of flexibility in controlling the tunnel face pressure, the inadequacy of monitoring methods, and the lack of multi-factor coupling analysis capabilities.

[0005] To solve the above-mentioned technical problems, this application is implemented as follows: A first aspect of this application discloses a model test apparatus for the stability of the tunnel face during the shield tunnel freezing method, comprising: Model box, used to hold the strata; The water level control system includes a first water tank and a second water tank located on both sides of the model box. The water levels of the first water tank and the second water tank can be adjusted independently to establish static water level or dynamic water seepage conditions in the stratum. A formation freezing and thawing system includes a temperature control pipe buried in the formation, and a refrigerant supply unit and a heat supply unit connected to the temperature control pipe; the refrigerant supply unit is used to inject refrigerant into the temperature control pipe to freeze the formation, and the heat supply unit is used to inject heat medium into the same temperature control pipe to thaw the formation. A shield tunneling system is embedded in the stratum, and one end of the shield tunneling system has a tunnel face opening; The face pressure control system includes an airbag disposed at the opening of the face and an air pressure regulating unit connected to the airbag; the air pressure regulating unit is used to inflate or deflate the airbag to simulate the face support pressure and the opening and depressurization process. A monitoring and data acquisition system is used to acquire at least the temperature, stress, pore water pressure and deformation data of the formation.

[0006] Optionally, the refrigerant supply unit is a liquid nitrogen tank, and the heat supply unit is a hot water tank; the liquid nitrogen tank and the hot water tank are respectively connected to the temperature control pipe through flow control valves to independently control the freezing rate and the thawing rate.

[0007] Optionally, the water level control system includes: The water pump includes a first water pump for injecting water into the first water tank and a second water pump for injecting water into the second water tank; The drain valve includes a first drain valve disposed at the bottom of the first water tank and a second drain valve disposed at the bottom of the second water tank; Multiple water level control holes are arranged along the height direction on the side walls of the first water tank and the second water tank for controlling the water level height; A water level gauge is used to monitor the water levels of the first and second water tanks in real time. The first water tank and the second water tank are connected to the stratum inside the model box through a permeable medium connection structure. By adjusting the water level difference between the first water tank and the second water tank, a hydraulic gradient is formed in the stratum to simulate the influence of different seepage conditions on the freezing effect and the stability of the working face.

[0008] Optionally, the shape of the airbag matches the cross-sectional shape of the shield tunnel system, and the outer surface of the airbag is in direct contact with the soil at the opening of the tunnel face; the air pressure regulating unit includes an air pump, a pressure gauge and a flow control valve, used to realize the staged or continuous unloading of the airbag pressure.

[0009] Optionally, the monitoring and data acquisition system includes: A MEMS sensor matrix is ​​embedded in the soil in front of the tunnel face. Each MEMS sensor integrates an earth pressure gauge, strain gauge, thermometer and pore water pressure gauge to monitor the multi-physics spatial distribution and dynamic changes of the soil in front of the tunnel face. An infrared thermometer non-contactly collects temperature distribution cloud maps of the soil surface through the observation window of the model box, which is used to monitor the formation and expansion of the frozen soil curtain and the recovery of the temperature field during the thawing process. The surface deformation monitoring device is used to acquire speckle images of the soil surface and calculate the displacement and strain across the entire field in order to monitor surface deformation. The data acquisition and processor is connected to the MEMS sensor matrix, the infrared thermometer, and the surface deformation monitoring device for multi-channel synchronous data acquisition.

[0010] Optionally, the MEMS sensor matrix is ​​arranged in a grid pattern at different depths in front of the face of the tunnel boring machine to form a three-dimensional monitoring array.

[0011] Optionally, the model box adopts a composite structure of rigid and transparent materials, and the joints are provided with waterproof seals; the ground layer is filled with standard sand through layered compaction.

[0012] A second aspect of this application discloses a model test method for the stability of the tunnel face during the shield tunnel freezing method, applied to the apparatus described in the first aspect of this application. The method includes: The model box was filled with soil layers, and a shield tunnel system, temperature control pipes, airbags, and monitoring and data acquisition system were installed. Initial groundwater conditions are established in the strata using a water level control system. The airbag is inflated by the air pressure regulating unit so that the airbag pressure reaches the preset initial support pressure of the working face. Refrigerant is injected into the temperature control pipe through the refrigerant supply unit to freeze the strata in front of the tunnel face, forming a frozen soil curtain, and the temperature, stress and pore water pressure changes during the freezing process are monitored in real time. After the freezing reaches the design requirements, the airbag is deflated through the air pressure regulating unit to simulate the depressurization process and monitor the response of the soil at the working face during the depressurization process. Heat medium is injected into the same temperature control pipe through the heat medium supply unit, and the temperature, stress and pore water pressure changes of the soil at the working face are monitored during the thawing process.

[0013] Optionally, after freezing reaches the design requirements, the airbag is deflated via a pressure regulating unit, including: The airbags are deflated using either staged or continuous unloading methods, and the stress, deformation, and pore water pressure data of the soil at the working face are recorded under different depressurization levels to determine the critical depressurization ratio or safe depressurization range.

[0014] Optionally, the method further includes: Multiple sets of comparative tests were conducted by changing at least one of the following test parameters: water level conditions, water level difference, freezing rate, thawing rate, temperature control pipe arrangement, and airbag depressurization rate, to analyze the influence of each parameter on the stability of the working face.

[0015] The embodiments of this application have the following advantages: In this embodiment of the application, by setting the same temperature control tube that connects both the refrigerant supply unit and the heat supply unit, the device can sequentially achieve the freezing and thawing of the stratum in the same test. Combined with the airbag inflation and deflation operation of the face pressure control system, it can completely simulate the entire process of opening the shield tunnel freezing method of "freezing-opening and depressurizing-thawing", making up for the shortcomings of related technologies that can only simulate a single stage.

[0016] By setting up a water level control system on both sides of the model box, static water level conditions or dynamic water seepage conditions under different hydraulic gradients can be established in the stratum according to the needs of the experiment, thereby truly reflecting the influence of groundwater on the freezing effect and the stability of the working face, and solving the problem of the single groundwater condition in related technologies.

[0017] By using airbags as the pressure actuators at the tunnel face, and coordinating with the inflation and deflation operations of the air pressure regulating unit, flexible loading of the tunnel face support pressure and gradual unloading during the depressurization process can be achieved. Compared with rigid loading methods, airbag loading provides more uniform pressure distribution, a smooth and controllable pressure loading and unloading process, and less interference with the soil stress field, thus more realistically simulating the stress state and depressurization behavior of the tunnel face in actual engineering projects.

[0018] The monitoring and data acquisition system can collect data on at least the temperature, stress, pore water pressure and deformation of the strata, providing data support for analyzing the evolution of the multi-physics field of the soil during freezing, decompression and thawing, helping to reveal the instability mechanism of the tunnel face, and providing quantitative basis for stability evaluation.

[0019] Thus, the device integrates water level control, freezing and thawing, face pressure control, and multi-parameter monitoring into one unit, enabling the study of face stability under multi-field coupling conditions of water, soil, heat, and force, and providing a reliable experimental platform for comparative analysis of multiple working conditions. Attached Figure Description

[0020] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is a schematic diagram of the structure of a model test device for the stability of the tunnel face under the freezing method of a shield tunnel, provided in an embodiment of this application. Figure 2 This is a schematic diagram of the model box and water level control system structure in the embodiments of this application; Figure 3 This is a schematic diagram of the formation freezing and thawing system in the embodiments of this application; Figure 4 This is a schematic diagram of the shield tunnel system and face pressure control system in the embodiments of this application; Figure 5 This is a schematic diagram of the MEMS sensor in the embodiments of this application; Figure 6 This is a schematic diagram of the formation multi-field coupling response monitoring matrix in the embodiments of this application. Figure 7 This is a schematic diagram of the surface subsidence monitoring matrix in the embodiments of this application; Figure 8 This is a schematic diagram of data acquisition and processing in an embodiment of this application; Figure 9 This is a schematic diagram of the structure of another model test device for the stability of the tunnel face under the freezing method provided in this application embodiment; Figure 10 This is a flowchart illustrating the steps of a model test method for the stability of the tunnel face during the freezing method in a shield tunnel, as provided in an embodiment of this application. Figure 11 This is a flowchart of another model test method for the stability of the tunnel face under the freezing method provided in this application embodiment. Detailed Implementation

[0022] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the technical solutions in the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0023] In related technologies, model test studies on the freezing method for shield tunnels mainly focus on the formation law of the freezing curtain or the freezing reinforcement effect itself, and generally suffer from the following shortcomings: First, incomplete test conditions. Most existing devices are only designed for conventional excavation processes and cannot simulate the impact of freezing reinforcement and thawing processes on the stability of the tunnel face, making it difficult to conduct complete condition studies from freezing to opening and depressurization to thawing. Second, low freezing control precision. Existing freezing systems mostly use refrigeration units or refrigerant circulation methods, making it difficult to accurately control the freezing rate. Furthermore, the equipment is complex and costly, hindering multi-condition comparative tests. Third, limited groundwater conditions. Existing devices are mostly set up under static water level conditions, making it difficult to simulate the seepage and flowing water level conditions commonly encountered in actual engineering projects, and thus unable to study the impact of seepage on the freezing effect and tunnel face stability. Fourth, unreasonable tunnel face pressure loading methods. Tunnel face pressure control mostly uses rigid loading methods, which cannot simulate the gradual changes during the opening and depressurization process. Moreover, rigid loading devices occupy a large space and easily interfere with the soil stress field, affecting the accuracy of the test results. Fifth, insufficient monitoring methods. Current monitoring methods primarily employ point sensors, resulting in low monitoring density. This makes it difficult to obtain the dynamic spatial distribution patterns of soil stress, deformation, temperature, and pore water pressure ahead of the tunnel face, hindering a deeper understanding of the tunnel face instability mechanism. Sixth, there is a lack of systematic experimental methods. Existing technologies cannot quantify the coupled effects of multiple factors such as water level conditions, seepage characteristics, freezing pipe arrangement, and freezing-thawing rates on tunnel face stability, making it difficult to establish a systematic experimental evaluation system.

[0024] Therefore, how to systematically simulate the entire process of freezing-opening depressurization-thawing under controlled groundwater conditions, and quantitatively evaluate the stability of the tunnel face when the shield tunnel is opened by freezing method, is an urgent technical problem to be solved.

[0025] Therefore, this application provides a model test device and method for the stability of the tunnel face under the frozen method in shield tunnels. The technical concept is as follows: by constructing an integrated model test system, a full-process simulation under multi-field coupling conditions of water, soil, heat, and force is achieved. A single set of temperature control pipes is used to connect the refrigerant supply unit and the heat supply unit respectively, realizing the same pipeline control for ground freezing and thawing; independent water level control systems are set on both sides of the model box, simulating static water level or dynamic water seepage conditions with different hydraulic gradients by adjusting the water level difference; airbags are used as the pressure actuators at the tunnel face, working in conjunction with a pressure regulating unit to achieve flexible loading of support pressure and gradual unloading during opening and depressurization; simultaneously, a monitoring system capable of collecting temperature, stress, pore water pressure, and deformation data is configured to provide multi-physics field data support for tunnel face stability analysis. The various systems work together to completely reproduce the entire process of "freezing-opening and depressurization-thawing" under controllable boundary conditions, and support multi-condition comparative tests, thereby quantitatively evaluating the influence of various factors on tunnel face stability.

[0026] Reference Figure 1 As shown, Figure 1 This is a schematic diagram of a model test device for the stability of the tunnel face under the shield tunnel freezing method, provided in an embodiment of this application. Specifically, the device includes: Model box, used to hold the strata; The water level control system includes a first water tank and a second water tank located on both sides of the model box. The water levels of the first water tank and the second water tank can be adjusted independently to establish static water level or dynamic water seepage conditions in the stratum. A formation freezing and thawing system includes a temperature control pipe buried in the formation, and a refrigerant supply unit and a heat supply unit connected to the temperature control pipe; the refrigerant supply unit is used to inject refrigerant into the temperature control pipe to freeze the formation, and the heat supply unit is used to inject heat medium into the same temperature control pipe to thaw the formation. A shield tunneling system is embedded in the stratum, and one end of the shield tunneling system has a tunnel face opening; The face pressure control system includes an airbag disposed at the opening of the face and an air pressure regulating unit connected to the airbag; the air pressure regulating unit is used to inflate or deflate the airbag to simulate the face support pressure and the opening and depressurization process. A monitoring and data acquisition system is used to acquire at least the temperature, stress, pore water pressure and deformation data of the formation.

[0027] In this embodiment, the model box is used to contain the strata, providing space for soil filling and boundary constraints throughout the experiment. The model box can be made of materials with certain strength and rigidity to ensure that it does not deform during soil filling and water pressure application.

[0028] The water level control system includes a first water tank and a second water tank located on either side of the model tank. By independently controlling the filling and draining of the first and second water tanks, the water levels in each tank can be raised to preset heights. When the water levels on both sides are the same, the formation is in a static water condition. When there is a height difference between the two water levels, a hydraulic gradient is formed, resulting in seepage in the formation, simulating the dynamic water seepage conditions in actual engineering. By independently adjusting the water level difference between the first and second water tanks, the strength of the seepage gradient can be controlled to meet the needs of different experimental conditions.

[0029] The ground freezing and thawing system is the core component for achieving artificial ground freezing and subsequent thawing. The temperature control pipe is buried at a predetermined location in the ground, typically positioned in the area requiring reinforcement ahead of the tunnel face. The temperature control pipe serves as a channel for the flow of refrigerant and heat transfer fluid. A refrigerant supply unit is connected to the temperature control pipe to inject refrigerant. As the refrigerant flows within the pipe, it absorbs heat from the surrounding soil, lowering the soil temperature below freezing. Pore water in the soil gradually freezes, forming a frozen soil curtain, thus reinforcing the ground and preventing water seepage. A heat transfer fluid supply unit is connected to the same temperature control pipe to inject heat transfer fluid. After freezing and depressurization tests, heat transfer fluid can be injected into the same pipeline through the heat transfer fluid supply unit to gradually thaw the frozen soil curtain, restoring the soil to its original state, thereby simulating the ground thawing process in actual engineering projects. By using the same set of temperature control tubes to connect the refrigerant supply unit and the heat supply unit respectively, this device can sequentially complete the two stages of freezing and thawing in the same experiment, realizing the simulation of the entire process of opening the chamber by freezing method.

[0030] The shield tunneling system is embedded in the ground to simulate the lining structure of an actual shield tunnel. One end of the system has a face opening, corresponding to the face location during actual shield tunneling. The system can employ a segmented, assembled tunnel segment structure, facilitating simultaneous installation during soil filling and allowing adjustment of tunnel depth and orientation as needed. The face opening is used to install components of the face pressure control system.

[0031] The airbags in the tunnel face pressure control system are positioned at the tunnel face opening of the shield tunnel system, with their outer sides in direct contact with the soil at the opening. The airbags possess excellent flexibility and airtightness, allowing them to deform under external forces while maintaining stable internal air pressure. A pressure regulating unit is connected to the airbags and is used to inflate or deflate them. During the initial testing phase and freezing process, the airbags are inflated through the pressure regulating unit to reach a preset internal pressure. This pressure acts on the tunnel face soil, simulating the support pressure provided by the shield machine in actual engineering. When simulating the decompression process, the airbags are gradually deflated through the pressure regulating unit, gradually reducing the airbag pressure and achieving progressive decompression of the tunnel face. The airbag loading method provides uniform pressure distribution, a smooth and controllable decompression process, and minimal interference with the stress field of the surrounding soil, enabling a more realistic simulation of the stress state and decompression behavior at the tunnel face in actual engineering projects.

[0032] The monitoring and data acquisition system may include multiple sensors and a data acquisition and processor. Sensors are embedded in the strata at designed locations, particularly in key areas ahead of the tunnel face, to monitor various physical parameters in real time during the test. The data acquisition and processor is connected to each sensor, enabling simultaneous acquisition, recording, and display of monitoring data, providing data support for subsequent tunnel face stability analysis. This system can also be used to obtain the dynamic evolution of multi-physics fields in the soil during freezing, decompression, and thawing processes.

[0033] The six systems described above work together to form a model test device for the stability of the tunnel face under the frozen method in shield tunnels. The water level control system independently adjusts the water levels in the first and second water tanks to provide controllable static or dynamic water seepage conditions for the test; the ground freezing and thawing system enables active freezing and controlled thawing of the ground; the shield tunnel system and the face pressure control system jointly simulate the tunnel structure and the face support and depressurization process; and the monitoring and data acquisition system records the changes in various physical parameters in real time during the test. This device can completely reproduce the entire process of "freezing-opening depressurization-thawing" under indoor conditions, providing a reliable test platform for studying the stability of the tunnel face under the frozen method in water-rich soft soil strata.

[0034] The following describes in further detail the model test apparatus for the stability of the tunnel face under the freezing method of this application through some optional embodiments.

[0035] In one optional embodiment, the model box adopts a composite structure of rigid and transparent materials, and the joints are provided with waterproof seals; the stratum is filled with standard sand through layered compaction.

[0036] In this embodiment, the model box structure adopts a composite structure of rigid materials (such as stainless steel plates) and transparent materials (such as plexiglass). The main frame and load-bearing parts of the model box use rigid materials to ensure overall strength and rigidity, while the transparent materials located on the side walls or observation windows facilitate visualization during the experiment. All joints are equipped with waterproof sealants (such as waterproof sealant) to ensure that the model box does not leak under water injection and seepage conditions, thus guaranteeing the stability of groundwater conditions and the reliability of experimental data.

[0037] Standard sand was used as the simulated soil material in the strata. Standard sand is characterized by stable physical and mechanical properties and uniform particle size distribution, ensuring consistency of soil properties across different test batches and facilitating reuse. A layered compaction method was employed during filling, where a certain thickness (e.g., 50-80 mm) of sand was laid each time, followed by compaction with the same amount of work, and this process was repeated layer by layer until the designed total height was reached. This method ensures uniform density distribution throughout the strata, reduces test errors caused by uneven filling, and improves test repeatability.

[0038] Thus, through the above-mentioned combined design and sealing treatment, the model box in this embodiment not only meets the requirements of strength and water tightness, but also creates conditions for visual observation during the test process, making it easy to observe phenomena such as the expansion of the frozen soil curtain and soil deformation; the standard sand has stable physical properties, and the layered compaction filling ensures the uniformity and repeatability of the strata, making it easy to carry out multi-condition comparative tests and improving the reliability and comparability of test data.

[0039] In one optional embodiment, the water level control system further includes: The water pump includes a first water pump for injecting water into the first water tank and a second water pump for injecting water into the second water tank; The drain valve includes a first drain valve disposed at the bottom of the first water tank and a second drain valve disposed at the bottom of the second water tank; Multiple water level control holes are arranged along the height direction on the side walls of the first water tank and the second water tank for controlling the water level height; A water level gauge is used to monitor the water levels of the first and second water tanks in real time. The first water tank and the second water tank are connected to the stratum inside the model box through a permeable medium connection structure. By adjusting the water level difference between the first water tank and the second water tank, a hydraulic gradient is formed in the stratum to simulate the influence of different seepage conditions on the freezing effect and the stability of the working face.

[0040] In this embodiment, the first and second water tanks are respectively located on both sides of the model box, serving as storage containers for groundwater supply. The two water tanks are independent of each other, and their respective water levels can be controlled independently. The water tanks are connected to the strata inside the model box through a permeable medium communication structure (such as a permeable plate or permeable stone with holes), allowing water in the water tanks to enter or seep out of the strata in a controllable manner, forming a stable seepage boundary.

[0041] The first water pump is used to fill the first water tank, and the second water pump is used to fill the second water tank. The first drain valve is located at the bottom of the first water tank, and the second drain valve is located at the bottom of the second water tank, used to drain the water in their respective tanks. By independently controlling the start / stop and flow rate of the water pumps and drain valves corresponding to each water tank, the water levels in the first and second water tanks can be independently and precisely adjusted.

[0042] Water level control holes are arranged along the height direction on the side walls of the first and second water tanks, respectively, with each control hole corresponding to a specific water level. By opening or closing control holes at different heights, the target water level of the corresponding water tank can be quickly set. Excess water will flow out from the control hole, thus maintaining a stable water level at the preset height. Water level gauges are set on the side or inside of the water tanks to read and monitor the current water level height of the first and second water tanks in real time, facilitating precise control of the water level difference by operators.

[0043] like Figure 2 As shown, by adjusting the water levels in the first and second water tanks respectively, different water levels are created on both sides, resulting in a hydraulic head difference at both ends of the formation. Driven by this hydraulic head difference, water will seep from the higher water level side through the formation to the lower water level side, forming a stable seepage field. The magnitude of the seepage gradient is directly proportional to the water level difference between the two sides and inversely proportional to the length of the seepage path in the formation. By adjusting the specific value of the water level difference, dynamic water seepage conditions under different hydraulic gradients can be simulated; when the water levels on both sides are the same, static water conditions are simulated.

[0044] This water level control system allows for dynamic adjustment of seepage conditions during the experiment, thereby studying the effects of different groundwater seepage environments on the formation of the frozen curtain (such as the rate of frozen soil expansion, the thickness and uniformity of the frozen soil curtain) and the stability of the working face during the opening and depressurization process.

[0045] The technical solution of this embodiment, through independently controlled dual water tanks and their corresponding injection pumps and drainage valves, enables flexible switching and precise control of static water level and dynamic water seepage conditions with different hydraulic gradients, thus overcoming the deficiency of the single groundwater condition in existing devices. Multiple water level control holes arranged along the height make water level setting simpler and more repeatable, and the water level gauge facilitates real-time monitoring. By dynamically adjusting the water level difference during the experiment, the seepage field can be changed in real time, simulating the impact of groundwater level changes or seepage condition evolution on the stability of the tunnel face in actual engineering, providing a reliable boundary condition control method for studying the multi-field coupling of water-soil-freeze-force.

[0046] In one optional embodiment, the refrigerant supply unit is a liquid nitrogen tank, and the heat supply unit is a hot water tank; the liquid nitrogen tank and the hot water tank are respectively connected to the temperature control pipe through flow control valves to independently control the freezing rate and the thawing rate.

[0047] In this embodiment, the refrigerant supply unit is a liquid nitrogen tank. Liquid nitrogen has an extremely low temperature (approximately -196°C at atmospheric pressure) and a large latent heat of vaporization, enabling it to rapidly absorb heat from the soil and achieve rapid freezing of the strata. Compared to traditional refrigeration units, using liquid nitrogen as the refrigerant offers advantages such as faster cooling rates, sufficient cooling capacity, and the elimination of complex compression and circulation equipment, making it suitable for model tests requiring the formation of a distinct frozen soil curtain. The liquid nitrogen tank is connected to the temperature control pipe via cryogenic piping and is equipped with appropriate safety valves and pressure gauges to ensure operational safety.

[0048] The heat transfer unit is a warm water tank, which stores and provides the warming medium (usually water). Its temperature can be controlled within a range of, for example, 20°C to 40°C, depending on the experimental requirements. The warm water is injected into the temperature control pipe via a circulating pump and pipeline, transferring heat to the surrounding frozen soil and gradually thawing it. Using warm water as the heat transfer medium allows for controllable adjustment of the thawing rate, avoiding the problems of excessively slow or uncontrollable natural thawing.

[0049] like Figure 3 As shown, a first flow control valve (freezing valve) is installed on the inlet pipe between the liquid nitrogen tank and the temperature control pipe, and a second flow control valve (thawing valve) is installed on the inlet pipe between the warm water tank and the temperature control pipe. The two flow control valves are independent and do not interfere with each other. By adjusting the opening of the first flow control valve, the injection flow rate of liquid nitrogen can be controlled, thereby affecting the amount of cold released per unit time and achieving precise control of the freezing rate. By adjusting the opening of the second flow control valve, the injection flow rate of warm water can be controlled, thereby achieving precise control of the thawing rate. Furthermore, the liquid passing through the first and second flow control valves is distributed to multiple temperature control pipes buried at different geological formations via a main distributor. Pressure gauges and thermometers are also installed on the inlet and return pipes to monitor fluid pressure and temperature. This independent control method allows different rate parameters to be used during the freezing and thawing stages according to the experimental design requirements, such as rapid freezing and slow thawing, or vice versa, to study the impact of rate matching on the stability of the tunnel face.

[0050] The outlet pipes of the liquid nitrogen tank and the warm water tank can be connected to the inlet of the same temperature control pipe via a three-way valve or in parallel connection, while the return pipes return to their respective recovery or discharge systems. During freezing tests, the liquid nitrogen path is opened and the warm water path is closed; during thawing tests, the warm water path is opened and the liquid nitrogen path is closed. The two supply units share the same temperature control pipe buried underground, eliminating the need for separate piping for freezing and thawing, simplifying the system structure, ensuring consistency in the freezing and thawing processes, and facilitating comparative analysis.

[0051] The technical solution of this embodiment uses liquid nitrogen as a refrigerant, which has strong freezing ability and rapid cooling, and can realistically simulate the formation process of artificial freezing curtain in actual engineering. Warm water is used as a heat medium, which can actively and controllably accelerate the thawing process and avoid the time cost of waiting for natural thawing. By independently adjusting the injection flow rate of liquid nitrogen and warm water through the flow control valve, the freezing rate and thawing rate can be accurately and independently controlled, which is convenient for conducting comparative tests under different freezing / thawing rate schemes.

[0052] In one optional embodiment, the shape of the airbag matches the cross-sectional shape of the shield tunnel system, and the outer surface of the airbag is in direct contact with the soil at the opening of the tunnel face; the air pressure regulating unit includes an air pump, a pressure gauge and a flow control valve, used to realize the staged or continuous unloading of the airbag pressure.

[0053] like Figure 4 As shown, the shield tunnel system is constructed by splicing segments, with each segment connected by flanges or sleeves, ensuring watertightness while facilitating assembly and disassembly. Airbags are installed at the tunnel face opening, their shape designed to match the cross-sectional shape of the shield tunnel system. For example, if the shield tunnel cross-section is circular, the airbags are also circular. This shape-matching design ensures that the airbags, after inflation, can uniformly conform to the entire inner wall of the tunnel face opening, avoiding localized pressure concentrations or support blind spots caused by shape mismatch, thereby improving the uniformity and effectiveness of pressure transmission.

[0054] The outer surface of the airbag is in direct contact with the soil at the tunnel face opening, without any rigid force transmission components in between. This direct contact method can transfer the air pressure inside the airbag to the soil surface without loss, and due to the flexibility of the airbag itself, it can adapt to the slight unevenness of the soil surface, making the pressure distribution more uniform, while avoiding additional disturbance to the soil stress field by the rigid loading plate.

[0055] The air pressure regulating unit includes an air pump, a pressure gauge, and a flow control valve. The air pump is used to inflate or deflate the airbag, providing a pressure source; the pressure gauge is used to monitor the pressure inside the airbag in real time, so that operators or control systems can accurately grasp the current support pressure; the flow control valve is located on the connecting pipeline between the air pump and the airbag, and is used to regulate the inflation or deflation flow rate.

[0056] Two pressure relief modes can be achieved by combining the control of the flow control valve opening and the air pump operating status. Staged unloading gradually reduces the airbag pressure in a step-by-step manner, for example, by reducing the pressure by a fixed value (e.g., 20% of the initial pressure each time) or a fixed percentage, maintaining pressure stability for a period after each stage of unloading to observe the soil response at the tunnel face under that pressure. Continuous unloading smoothly and continuously reduces the airbag pressure at a constant rate (e.g., a few kilopascals per minute), simulating the gradual pressure decay process at the tunnel face during actual opening. The two unloading methods can be flexibly selected according to the experimental objectives. Staged unloading helps identify the critical pressure value for tunnel face instability, while continuous unloading more closely resembles the gradual pressure relief process in actual engineering.

[0057] The technical solution adopted in this embodiment matches the shape of the airbag with the tunnel cross-section, ensuring uniform distribution of support pressure across the entire face and avoiding simulation distortion caused by local stress concentration. The airbag is in direct contact with the soil, resulting in a short pressure transmission path and minimal loss. Furthermore, the flexible material exhibits strong self-adaptability, causing far less interference to the original stress field of the soil compared to rigid loading methods. The combination of an air pump, pressure gauge, and flow control valve enables precise adjustment and real-time monitoring of the airbag pressure. It supports both staged unloading and continuous unloading modes, allowing for precise study of the critical conditions for face instability and realistic simulation of the gradual decompression process in actual engineering projects. This provides a flexible experimental means for developing safe decompression control standards.

[0058] In one optional embodiment, the monitoring and data acquisition system includes: A MEMS sensor matrix is ​​embedded in the soil in front of the tunnel face. Each MEMS sensor integrates an earth pressure gauge, strain gauge, thermometer and pore water pressure gauge to monitor the multi-physics spatial distribution and dynamic changes of the soil in front of the tunnel face. An infrared thermometer non-contactly collects temperature distribution cloud maps of the soil surface through the observation window of the model box, which is used to monitor the formation and expansion of the frozen soil curtain and the recovery of the temperature field during the thawing process. The surface deformation monitoring device is used to acquire speckle images of the soil surface and calculate the displacement and strain across the entire field in order to monitor surface deformation. The data acquisition and processor is connected to the MEMS sensor matrix, the infrared thermometer, and the surface deformation monitoring device for multi-channel synchronous data acquisition.

[0059] In this embodiment, multiple MEMS (Micro Electro Mechanical System) sensors are embedded at different positions in front of the tunnel face according to a certain spatial arrangement scheme (such as a matrix), forming a MEMS sensor matrix. Figure 5As shown, each MEMS sensor integrates a strain gauge, a thermometer sensor, and a pore pressure sensor, which can monitor the multi-physics spatial distribution of the soil in front of the tunnel face in real time (i.e., the differences in soil pressure, strain, temperature, and pore pressure at different locations) and the dynamic changes of each parameter over time (i.e., the evolution law during freezing, decompression, and thawing).

[0060] In one optional embodiment, the MEMS sensor matrix is ​​arranged in a grid pattern at different depths in front of the face of the tunnel boring machine, forming a three-dimensional monitoring array. For example... Figure 6 As shown, the sensors are arranged in a horizontal plane (parallel to the tunnel face) at equal or unequal row and column spacings, forming a checkerboard-like distribution. The sensors are not only arranged in the same depth plane, but also have multiple monitoring sections set up longitudinally (perpendicular to the tunnel face, i.e., the tunnel excavation direction), with a set of grid sensors arranged on each section. Through the combination of the horizontal grid and longitudinal sections, the MEMS multi-parameter sensors form a three-dimensional monitoring network within the soil in front of the tunnel face. This array can capture the physical field distribution of the soil in front of the tunnel face in three-dimensional space. For example, along the tunnel axis (depth), it can monitor the advance distance of the freezing front; perpendicular to the axis, it can monitor the lateral expansion range and uniformity of the frozen soil curtain; and it can also acquire the three-dimensional response of soil pressure and pore pressure at different locations during the decompression process.

[0061] An infrared thermometer is a non-contact temperature measurement device that scans the soil surface using a transparent viewing window (such as an plexiglass window) on a model box. The soil surface experiences temperature changes due to internal freezing and thawing, which are transmitted outwards as infrared radiation. The infrared thermometer receives this radiation and converts it into a temperature distribution cloud map, thus displaying the temperature field of the soil surface intuitively and in real time. This cloud map allows for dynamic monitoring of the formation location, expansion range, and rate of expansion of the frozen soil curtain during freezing, as well as the uniformity of the frozen soil shape. During thawing, it monitors the gradual recovery of the temperature field, determining whether the frozen soil curtain has completely thawed. Infrared thermometry offers advantages such as being non-contact, interference-free, and providing full-field coverage, overcoming the limitations of point-based temperature sensors that can only measure temperatures at limited points.

[0062] The surface deformation monitoring device employs Digital Image Correlation (DIC) technology. First, a randomly distributed speckle pattern (usually black with white dots or white with black dots) needs to be manually sprayed onto the soil surface (especially the ground surface). This forms the surface settlement monitoring matrix, such as... Figure 7As shown in the diagram, during the experiment, a high-resolution camera continuously captures images of the soil surface at regular time intervals. The surface deformation monitoring device calculates the displacement vector of each speckle region by comparing the positional changes of the speckle pattern in images from adjacent time points, thereby obtaining the displacement and strain fields of the entire field. This device is mainly used to monitor surface deformation, including frost heave during freezing and surface settlement, cracking, or collapse that may be caused by face instability during decompression and thawing. The DIC can provide full-field, quantitative deformation data to help identify potential hazardous areas and instability modes.

[0063] like Figure 8 As shown, the data acquisition and processing unit is the central control unit of the entire monitoring system, connected to the MEMS sensor matrix, infrared thermal imager, and surface deformation monitoring device via data cable or wirelessly. Data from all sensors is acquired simultaneously under the same time reference, ensuring strict temporal correspondence between data from different sources, facilitating subsequent coupled analysis. The final output includes surface deformation cloud maps, surface temperature cloud maps, soil pressure distribution, ground deformation distribution, lower-level pore pressure distribution, and ground temperature distribution.

[0064] The technical solution adopted in this embodiment integrates multiple parameters using a single MEMS sensor, eliminating measurement errors caused by differences in the installation positions of different sensors. This enables synchronous acquisition of multi-physics field data at the same measuring point, providing high-quality data for coupled analysis. The MEMS three-dimensional monitoring array achieves omnidirectional coverage of the soil in front of the tunnel face in both horizontal and depth directions, enabling precise characterization of the spatial distribution and dynamic migration patterns of stress, strain, temperature, and pore pressure. The MEMS sensor provides the internal response of the soil, while the infrared thermometer and surface deformation monitoring device provide information on soil surface temperature and deformation. This combination of internal and external data allows for multi-view, multi-scale assessment of the tunnel face stability. Furthermore, the data acquisition and processing achieve multi-channel synchronous acquisition, ensuring strict correspondence of each physical quantity on the time axis, providing a reliable data foundation for revealing the transient response and multi-field coupling mechanisms during freezing, decompression, and thawing processes.

[0065] like Figure 9 As shown, Figure 9 This is a schematic diagram of a model test device for the stability of the tunnel face under the shield tunnel freezing method, provided in an embodiment of this application. The device includes a model box, a water level control system, a ground freezing and thawing system, a shield tunnel system, a tunnel face pressure control system, and a monitoring and data acquisition system.

[0066] The model box is used to contain the standard sand strata. The model box uses a composite structure of rigid and transparent materials. The rigid material forms the main frame to ensure strength, while the transparent material forms the observation window, facilitating visual observation throughout the experiment. All joints of the model box are waterproofed to ensure watertightness.

[0067] Independent first and second water tanks are installed on the left and right sides of the model box, respectively, forming a water level control system. The first and second water tanks are connected to the stratum inside the model box via a permeable medium connection structure. The first water tank is connected to a first water pump and a first drain valve, and the second water tank is connected to a second water pump and a second drain valve. Multiple water level control holes are provided along the height direction on the side walls of both the first and second water tanks, and water level gauges are installed. By independently adjusting the water level difference between the two tanks, static water level or dynamic water seepage conditions with different hydraulic gradients can be established in the stratum.

[0068] The formation freezing and thawing system includes a temperature control pipe buried in the formation, and a refrigerant supply unit and a heat transfer medium supply unit connected to the temperature control pipe. In this embodiment, the refrigerant supply unit is a liquid nitrogen tank, and the heat transfer medium supply unit is a hot water tank. The liquid nitrogen tank is connected to the inlet of the temperature control pipe through a first flow control valve, and the hot water tank is connected to the inlet of the same temperature control pipe through a second flow control valve. The return end of the temperature control pipe is connected to a liquid nitrogen recovery pipeline or a discharge port, respectively. By switching the first and second flow control valves, liquid nitrogen or hot water can be injected into the temperature control pipe to achieve formation freezing or thawing, and the freezing and thawing rates can be independently controlled by adjusting the opening of the flow control valves.

[0069] The shield tunnel system is buried in the strata and uses segmented PVC pipes to simulate the tunnel lining structure. One end of the shield tunnel system has a face opening, which corresponds to the face position in actual shield tunneling.

[0070] The tunnel face pressure control system includes an airbag located at the tunnel face opening and a pressure regulating unit connected to the airbag. The airbag is made of high-strength rubber and its shape matches the cross-sectional shape of the shield tunnel system. The outer surface of the airbag is in direct contact with the soil at the tunnel face opening. The pressure regulating unit includes an air pump, a pressure gauge, and a flow control valve. The air pump is connected to the airbag via pipeline and is used to inflate or deflate the airbag. The pressure gauge is used to monitor the pressure inside the airbag in real time. The flow control valve is used to regulate the inflation and deflation rates, enabling staged or continuous unloading of the airbag pressure, thereby simulating the tunnel face support pressure and the depressurization process.

[0071] The monitoring and data acquisition system includes a MEMS sensor matrix, an infrared thermometer, a surface deformation monitoring device, and a data acquisition and processing unit. The MEMS sensor matrix comprises multiple MEMS sensors, arranged in a grid pattern and buried at different depths in the soil ahead of the tunnel face. Each MEMS sensor integrates an earth pressure gauge, strain gauge, thermometer, and pore water pressure gauge to monitor the multi-physics spatial distribution and dynamic changes of the soil ahead of the tunnel face. The infrared thermometer non-contactly acquires temperature distribution cloud images of the soil surface through the observation window of the model box, used to monitor the formation and expansion of the frozen soil curtain and the temperature field recovery during thawing. The surface deformation monitoring device includes a high-resolution camera and a DIC analysis module, used to acquire speckle images of the soil surface and calculate the overall displacement and strain to monitor surface deformation. The data acquisition and processing unit is connected to each MEMS sensor, infrared thermal imager, and surface deformation monitoring device via data cables for multi-channel synchronous data acquisition.

[0072] The aforementioned systems work together to form a complete model test device for the stability of the tunnel face under the frozen method. This device allows for the complete simulation of the entire process of "freezing-opening and depressurization-thawing" under controlled indoor conditions, acquiring real-time multi-dimensional dynamic data on ground temperature, stress, pore water pressure, and deformation. This provides a reliable test platform for evaluating tunnel face stability and optimizing the freezing and opening scheme.

[0073] This application also provides a model test method for the stability of the tunnel face during the freezing method in shield tunnels, applied to the apparatus described in the above embodiments. (Refer to...) Figure 10 As shown, Figure 10 This is a flowchart illustrating the steps of a model test method for the stability of the tunnel face during the freezing method in a shield tunnel, as provided in an embodiment of this application. Figure 10 As shown, the method may include steps S1001 to S1006: Step S1001: Fill the model box with soil and install the shield tunnel system, temperature control pipe, airbag and monitoring and data acquisition system.

[0074] In this step, based on the test plan design, the installation positions of each component in the model box are determined, including the burial depth and axial direction of the shield tunnel system, the arrangement range of the temperature control pipe (usually located in the area to be reinforced in front of the tunnel face), the installation position of the airbag (at the opening of the tunnel face), and the burial points of monitoring sensors (such as temperature, stress, and pore water pressure sensors).

[0075] Then, the geological material (such as standard sand) is filled into the model box in layers. Each layer is compacted after reaching the predetermined thickness to ensure uniform density. During the filling process, shield tunnel segments, temperature control pipes, airbags, and monitoring sensors are installed simultaneously to ensure that all components are in their designed positions and reliably fixed. After filling to the designed total height, the geological preparation is complete.

[0076] Step S1002: Establish initial groundwater conditions in the stratum using a water level control system.

[0077] In this step, the water level control system is activated, and water is injected into the tanks on both sides of the model box. The water levels in both tanks are adjusted according to the initial groundwater conditions required for the experiment. For example, to simulate static water conditions, the water levels on both sides are kept consistent; to simulate dynamic water seepage conditions, a preset water level difference is created. Water slowly enters or seeps into the formation through the permeable medium connecting structure until a stable pore water pressure distribution and seepage field are formed throughout the formation. After establishing the initial groundwater conditions, the mixture is allowed to stand for a period of time (e.g., 10-15 minutes) to allow the seepage field to fully stabilize, and the initial state data is recorded.

[0078] Step S1003: Inflate the airbag through the air pressure regulating unit to make the airbag pressure reach the preset initial support pressure of the working face.

[0079] In this step, the air pump of the air pressure regulating unit is started to inflate the airbag installed at the tunnel face opening. Simultaneously, the internal pressure of the airbag is monitored in real time using a pressure gauge. When the airbag pressure reaches a preset value (this preset value is usually determined based on the sum of the soil pressure and water pressure at the tunnel face, or calculated proportionally based on the actual tunnel face support pressure of the project), the air pump or regulating valve is turned off to maintain pressure stability. At this point, the airbag applies uniform support pressure to the outer tunnel face soil, simulating the tunnel face support force provided by an actual tunnel boring machine, thus keeping the soil stable under initial stress.

[0080] Step S1004: Inject refrigerant into the temperature control pipe through the refrigerant supply unit to freeze the strata in front of the tunnel face, forming a frozen soil curtain, and monitor the changes in temperature, stress and pore water pressure in real time during the freezing process.

[0081] In this step, the passage between the refrigerant supply unit and the temperature control pipe is opened, and refrigerant (e.g., liquid nitrogen) is injected into the temperature control pipe buried in the stratum. The refrigerant flows through the pipe, absorbing heat from the surrounding soil, gradually lowering the soil temperature below freezing. As heat continues to be carried away, the pore water in the soil gradually freezes, forming a frozen soil curtain. During the freezing process, the monitoring and data acquisition system collects real-time data on soil temperature, stress, and pore water pressure, recording the expansion process of the freezing front and changes in soil stress. When the frozen soil curtain reaches the designed thickness or range (e.g., determined based on the monitored temperature field), the refrigerant injection is stopped, completing the freezing stage.

[0082] Step S1005: After the freezing reaches the design requirements, the airbag is deflated through the air pressure regulating unit to simulate the opening and depressurization process, and the response of the soil at the working face is monitored during the depressurization process.

[0083] In this step, after the freezing construction is completed (i.e., the frozen soil curtain has formed and meets the reinforcement requirements), the gas inside the airbag is gradually released through the air pressure regulating unit, causing the airbag pressure to gradually decrease. This process simulates the depressurization operation at the working face before opening the tunnel for maintenance in actual engineering.

[0084] During the decompression process, the monitoring and data acquisition system continuously monitors the stress, deformation, and pore water pressure changes of the soil near the working face, observing whether the soil exhibits signs of instability (such as sudden stress drop, accelerated deformation, etc.). By recording the soil response under different decompression levels, the stability of the working face under frozen reinforcement conditions can be assessed.

[0085] Step S1006: Inject heat medium into the same temperature control pipe through the heat medium supply unit, and monitor the changes in temperature, stress and pore water pressure of the soil at the working face during the thawing process.

[0086] In this step, after completing the pressure relief observation (or after completing the opening simulation), the passage between the heat medium supply unit and the same temperature control pipe is opened, and heat medium (e.g., warm water) is injected into the pipe. The heat medium flows in the pipe, transferring heat to the surrounding frozen soil, causing the frozen soil curtain to gradually thaw.

[0087] During the thawing process, the monitoring and data acquisition system continues to collect data on soil temperature, stress, and pore water pressure, recording the temperature field recovery process and the possible stress redistribution and deformation of the soil that may occur during thawing. The thawing phase ends when the soil temperature recovers to near its initial value or reaches the designed thawing time, completing one full test cycle.

[0088] The technical solution of this embodiment fully reproduces the entire process of opening the tunnel face using the freezing method by sequentially executing steps such as stratum filling, groundwater establishment, initial support, freezing, depressurization, and thawing, thus overcoming the limitation of existing methods that can only simulate a single stage. Utilizing a water level control system and a refrigerant / heating medium supply unit, groundwater conditions, freezing rate, and thawing rate can be precisely controlled, providing a repeatable experimental method for studying the stability of the tunnel face under different boundary conditions. The gradual unloading of pressure at the tunnel face through airbag deflator more realistically reflects the mechanical behavior of opening and depressurizing in actual engineering. Simultaneous acquisition of temperature, stress, and pore water pressure data at each stage of freezing, depressurization, and thawing provides direct experimental evidence for revealing the instability mechanism and multi-field coupling laws of the tunnel face.

[0089] In an optional embodiment, step S1005 above, "after freezing reaches the design requirements, the airbag is deflated by the air pressure regulating unit", specifically includes: deflating the airbag by a graded unloading or continuous unloading method, and recording the stress, deformation and pore water pressure data of the soil at the working face under different pressure relief levels, so as to determine the critical pressure relief ratio or safe pressure relief range.

[0090] The staged unloading method involves gradually reducing the pressure inside the airbag according to pre-set step values. For example, each unload may reduce the pressure by 20% of the current pressure or a fixed pressure value. After each stage of unloading, the pressure is maintained at a stable level for a period of time (e.g., 5-10 minutes) to observe and record the response of the soil at the tunnel face under that pressure level. Staged unloading helps to accurately identify the critical pressure point at which the tunnel face transitions from a stable to an unstable state. When a sudden drop in soil stress, a significant increase in deformation rate, or abnormal fluctuations in pore water pressure are observed after a certain stage of unloading, it can be determined that the tunnel face has reached or is close to an unstable state. The pressure value or unloading ratio at this point can serve as an important basis for the critical unloading ratio.

[0091] Continuous unloading refers to smoothly and continuously reducing the airbag pressure at a constant rate (e.g., 2 kPa per minute), simulating the gradual process of opening the airbag for pressure relief in actual engineering. Continuous unloading is closer to actual engineering conditions and can reflect the dynamic response of the soil during the continuous pressure decay process. By continuously recording the stress, deformation, and pore pressure data throughout the entire pressure relief process, complete curves of each physical quantity changing with pressure can be plotted, thereby analyzing the evolution law of the tunnel face stability.

[0092] Understandably, regardless of whether staged or continuous unloading is used, it is necessary to simultaneously record the stress, deformation, and pore water pressure data of the soil at the working face under different levels of pressure relief. This data includes: pressure values ​​at each level or continuous pressure values, corresponding soil pressure readings, displacement or strain values ​​measured by sensors, and changes in pore water pressure.

[0093] Using the technical solution of this embodiment, the graded unloading method can accurately capture the critical pressure point of the tunnel face instability, which is convenient for establishing quantitative stability evaluation indicators; the continuous unloading method is closer to the actual engineering pressure relief process and can truly reflect the dynamic response characteristics of the soil; the two methods can be flexibly selected or combined to meet the needs of different research purposes and improve the adaptability and practicality of the test method.

[0094] In an optional embodiment, the method further includes: Multiple sets of comparative tests were conducted by changing at least one of the following test parameters: water level conditions, water level difference, freezing rate, thawing rate, temperature control pipe arrangement, and airbag depressurization rate, to analyze the influence of each parameter on the stability of the working face.

[0095] In this embodiment, water level conditions include two basic types: static water level and dynamic water level. Under dynamic water level, the water level elevation can be further changed (e.g., high water level, low water level). Under dynamic water seepage conditions, the seepage gradient can be changed by adjusting the water level difference between the two water tanks, and the influence of different seepage intensities on the freezing effect and the stability of the tunnel face can be studied. The freezing rate can be controlled by adjusting the injection flow rate of the cooling medium (e.g., liquid nitrogen), and the influence of rapid freezing and slow freezing on the formation quality of the frozen soil curtain and the development of internal soil stress can be studied. The thawing rate can be controlled by adjusting the injection flow rate of the heating medium (e.g., warm water), and the influence of the thawing rate on the recovery of soil strength and changes in tunnel face stability can be studied. The temperature control pipe arrangement includes uniform arrangement across the entire cross-section, locally dense arrangement, ring arrangement, or linear arrangement, etc., to study the morphology and reinforcement effect of the frozen soil curtain under different freezing schemes. The airbag decompression rate includes the magnitude of the stage difference and residence time of staged unloading, as well as the rate of continuous unloading, and is used to study the influence of different decompression methods on the critical conditions for instability of the tunnel face.

[0096] After completing a full test (including all steps such as formation filling, groundwater establishment, freezing, depressurization, and thawing), keep all other conditions constant and change only at least one of the above parameters to conduct a new round of tests. For example, keep the freezing rate, thawing rate, and temperature pipe arrangement constant, and only change the water level difference to conduct two sets of comparative tests; or change both the freezing rate and depressurization rate simultaneously to study their coupled effects. To ensure data comparability, each test should use the same formation materials, filling method, and initial conditions, and be conducted strictly according to the same operating procedures.

[0097] Thus, by comparing data such as temperature distribution, stress evolution, pore water pressure changes, surface deformation, and critical pressure for instability of the tunnel face monitored in multiple sets of experiments, the influence of each individual parameter on the stability of the tunnel face can be quantitatively assessed, and the coupling mechanism between multiple parameters can be revealed (such as the weakening effect of seepage on freezing and the influence of depressurization rate on critical pressure for instability). This allows for the selection of parameter combinations that are conducive to the stability of the tunnel face, providing a scientific basis for the design of freezing schemes and the control of depressurization in actual engineering projects.

[0098] like Figure 11 As shown, Figure 11 This is a flowchart illustrating the steps of a model test method for the stability of the tunnel face during the freezing method, as provided in this application embodiment. Specifically, the method includes the following steps: Step 1: Pre-experiment preparation and model establishment.

[0099] Step 1-1: Model Preparation. Based on the experimental design, determine the placement of the freezing pipes, tunnel depth, monitoring point locations, and the installation location of the face airbags. Lay standard sand at the bottom of the model box and fill the soil using a layered compaction method. During each layer of filling, simultaneously install monitoring devices such as shield tunnel segments, temperature control pipes, and MEMS sensors to ensure all components are in their designed positions. After filling the soil to the face height, install the face airbags and continue filling to the designed total height. Spray a speckle pattern onto the soil surface for use by a digital image correlation monitoring system to monitor surface deformation.

[0100] Steps 1-2: Establishing the underground seepage field. Water is slowly injected into the tanks on both sides of the model box using a water pump to prevent soil erosion. According to the experimental design, a target water level is set through a water level control hole to achieve either static conditions (same water level on both sides) or seepage conditions (different water levels on both sides). After reaching the target water level, the mixture is left to stand for a period of time to allow a stable initial seepage field and pore water pressure field to form in the soil.

[0101] Steps 1-3: Initial Stress Field Equilibrium. Start the data acquisition system and record various monitoring data related to the initial state of the soil. Inflate the airbags at the tunnel face using an air pump to balance the airbag pressure with the sum of the soil and water pressures at the tunnel face, maintaining stability. Maintain constant airbag pressure and observe the monitoring data from the MEMS sensor matrix to ensure that the soil stress field has reached equilibrium.

[0102] Step 2: Stability of bottom-level freezing / thawing and opening positions.

[0103] Step 2-1: Ground Freezing Process. According to the design scheme, liquid nitrogen is injected into the temperature control pipe through a flow control valve to begin the freezing process. The liquid nitrogen injection flow rate and time are controlled to achieve the designed freezing rate and freezing range. During the freezing process, the pressure in the airbags at the working face is maintained constant to simulate the stable state of the working face during freezing. Soil temperature, stress, pore water pressure, and deformation changes are monitored in real time using MEMS sensors, infrared thermometers, and surface deformation monitoring devices. Once the designed freezing time has elapsed or the soil has reached the designed freezing temperature, liquid nitrogen injection is stopped, completing the freezing stage. Step 2-2: Pressure Relief and Stability Assessment. After freezing, the gas inside the airbag is gradually released through a flow control valve to progressively unload the pressure at the tunnel face. Unloading can be done in stages or continuously and slowly, with the unloading rate set according to experimental requirements. During unloading, MEMS sensors monitor the stress, deformation, temperature, and pore water pressure changes of the soil in front of the tunnel face in real time, while surface deformation and settlement are monitored using a surface deformation monitoring device. The stability of the tunnel face is recorded, and the criteria for judgment include sudden changes in soil stress, continuous deformation development, and significant surface settlement or collapse. Based on the monitoring data, the stability of the tunnel face under different pressure relief levels is evaluated to determine the safe pressure relief range.

[0104] Steps 2-3: Ground Thawing Process. After completing the depressurization observation, the airbag pressure at the working face is restored to the initial or set pressure using an air pump to simulate the working face support state after opening the chamber. Warm water is injected into the temperature control pipe to begin ground thawing. During the thawing process, soil temperature, stress, pore water pressure, and deformation data are continuously collected in real time through various monitoring systems. A complete test cycle is completed when the soil temperature returns to the initial temperature or the designed thawing time is reached.

[0105] Step 3: Multi-condition analysis and comparison test.

[0106] Step 3-1: Data Acquisition and Processing Analysis. Based on the collected soil temperature, stress, pore water pressure, and deformation data, stress-deformation-temperature-pore pressure coupled response analysis is performed to summarize the evolution law of tunnel face stability.

[0107] Steps 3-4: Comparative Experiments and Parameter Analysis. Based on the experimental research objectives, multiple sets of comparative experiments were conducted by changing parameters such as water level conditions, water level difference, freezing rate, thawing rate, temperature control pipe arrangement, and airbag depressurization rate. The differences in tunnel face stability under different working conditions were compared and analyzed, and the influence of each factor on tunnel face stability was quantified. Based on the experimental data, tunnel face stability evaluation indicators (such as critical depressurization ratio, safety factor, etc.) were established, and the stability evolution law of the tunnel face under the shield tunnel freezing method considering water-soil-heat coupling was derived.

[0108] Thus, this method can realistically simulate the entire process of opening the chamber using the freezing method. Through pressure relief and stability observation, the safe pressure relief range of the working face can be quantitatively assessed. Furthermore, the multi-condition test design can reveal the law of multi-factor coupling influence, providing a scientific basis for engineering practice.

[0109] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0110] This application describes embodiments of methods and apparatus according to flowchart illustrations and / or block diagrams. It should be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing terminal device to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing terminal device, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes.

[0111] These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing terminal device to operate in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes.

[0112] These computer program instructions can also be loaded onto a computer or other programmable data processing terminal equipment, causing a series of operational steps to be performed on the computer or other programmable terminal equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable terminal equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0113] Although preferred embodiments of the present application have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the embodiments of the present application.

[0114] Finally, it should be noted that in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or terminal device that includes said element.

[0115] The above provides a detailed description of the model test device and method for the stability of the tunnel face under the freezing method of shield tunneling. Specific examples have been used to illustrate the principle and implementation of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​this application. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A model test device for the stability of the tunnel face during the freezing method of a shield tunnel, characterized in that, include: Model box, used to hold the strata; The water level control system includes a first water tank and a second water tank located on both sides of the model box. The water levels of the first water tank and the second water tank can be adjusted independently to establish static water level or dynamic water seepage conditions in the stratum. A formation freezing and thawing system includes a temperature control pipe buried in the formation, and a refrigerant supply unit and a heat supply unit connected to the temperature control pipe; the refrigerant supply unit is used to inject refrigerant into the temperature control pipe to freeze the formation, and the heat supply unit is used to inject heat medium into the same temperature control pipe to thaw the formation. A shield tunneling system is embedded in the stratum, and one end of the shield tunneling system has a tunnel face opening; The working face pressure control system includes an airbag disposed at the opening of the working face and an air pressure regulating unit connected to the airbag; The air pressure regulating unit is used to inflate or deflate the airbag to simulate the support pressure at the working face and the depressurization process. A monitoring and data acquisition system is used to acquire at least the temperature, stress, pore water pressure and deformation data of the formation.

2. The apparatus according to claim 1, characterized in that, The refrigerant supply unit is a liquid nitrogen tank, and the heat supply unit is a hot water tank; the liquid nitrogen tank and the hot water tank are respectively connected to the temperature control pipe through flow control valves to independently control the freezing rate and the thawing rate.

3. The apparatus according to claim 1, characterized in that, The water level control system also includes: The water pump includes a first water pump for injecting water into the first water tank and a second water pump for injecting water into the second water tank; The drain valve includes a first drain valve disposed at the bottom of the first water tank and a second drain valve disposed at the bottom of the second water tank; Multiple water level control holes are arranged along the height direction on the side walls of the first water tank and the second water tank for controlling the water level height; A water level gauge is used to monitor the water levels of the first and second water tanks in real time. The first water tank and the second water tank are connected to the stratum inside the model box through a permeable medium connection structure. By adjusting the water level difference between the first water tank and the second water tank, a hydraulic gradient is formed in the stratum to simulate the influence of different seepage conditions on the freezing effect and the stability of the working face.

4. The apparatus according to claim 1, characterized in that, The shape of the airbag matches the cross-sectional shape of the shield tunnel system, and the outer surface of the airbag is in direct contact with the soil at the opening of the tunnel face; the air pressure regulating unit includes an air pump, a pressure gauge and a flow control valve, which are used to realize the staged or continuous unloading of the airbag pressure.

5. The apparatus according to claim 1, characterized in that, The monitoring and data acquisition system includes: A MEMS sensor matrix is ​​embedded in the soil in front of the tunnel face. Each MEMS sensor integrates an earth pressure gauge, strain gauge, thermometer and pore water pressure gauge to monitor the multi-physics spatial distribution and dynamic changes of the soil in front of the tunnel face. An infrared thermometer non-contactly collects temperature distribution cloud maps of the soil surface through the observation window of the model box, which is used to monitor the formation and expansion of the frozen soil curtain and the recovery of the temperature field during the thawing process. The surface deformation monitoring device is used to acquire speckle images of the soil surface and calculate the displacement and strain across the entire field in order to monitor surface deformation. The data acquisition and processor is connected to the MEMS sensor matrix, the infrared thermometer, and the surface deformation monitoring device for multi-channel synchronous data acquisition.

6. The apparatus according to claim 5, characterized in that, The MEMS sensor matrix is ​​arranged in a grid pattern at different depths in front of the face of the tunnel boring machine, forming a three-dimensional monitoring array.

7. The apparatus according to claim 1, characterized in that, The model box adopts a composite structure of rigid and transparent materials, and the joints are waterproof and sealed; the stratum is filled with standard sand through layered compaction.

8. A model test method for the stability of the tunnel face during the freezing method of a shield tunnel, characterized in that, Applied to the apparatus of any one of claims 1-7, the method comprises: The model box was filled with soil layers, and a shield tunnel system, temperature control pipes, airbags, and monitoring and data acquisition system were installed. Initial groundwater conditions are established in the strata using a water level control system. The airbag is inflated by the air pressure regulating unit so that the airbag pressure reaches the preset initial support pressure of the working face. Refrigerant is injected into the temperature control pipe through the refrigerant supply unit to freeze the strata in front of the tunnel face, forming a frozen soil curtain, and the temperature, stress and pore water pressure changes during the freezing process are monitored in real time. After the freezing reaches the design requirements, the airbag is deflated through the air pressure regulating unit to simulate the depressurization process and monitor the response of the soil at the working face during the depressurization process. Heat medium is injected into the same temperature control pipe through the heat medium supply unit, and the temperature, stress and pore water pressure changes of the soil at the working face are monitored during the thawing process.

9. The method according to claim 8, characterized in that, After freezing reaches the design requirements, the airbag is deflated via the air pressure regulating unit, including: The airbags are deflated using either staged or continuous unloading methods, and the stress, deformation, and pore water pressure data of the soil at the working face are recorded under different depressurization levels to determine the critical depressurization ratio or safe depressurization range.

10. The method according to claim 8, characterized in that, The method further includes: Multiple sets of comparative tests were conducted by changing at least one of the following test parameters: water level conditions, water level difference, freezing rate, thawing rate, temperature control pipe arrangement, and airbag depressurization rate, to analyze the influence of each parameter on the stability of the working face.

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