Earthquake liquefaction active prevention and control system for shallow-buried tunnel engineering

Abstract

This invention provides an active seismic liquefaction prevention system for shallow-buried tunnel engineering, comprising a sensing and monitoring unit, a central control unit, a cooling supply execution unit, and an exhaust and pressure stabilization safety unit. The sensing and monitoring unit, located on the cooling supply execution unit, collects data on excess pore water pressure, temperature, peak ground acceleration, and system pressure in the foundation soil, and transmits this data to the central control unit. The central control unit dynamically assesses the liquefaction risk based on the collected data and controls the operation of the cooling supply execution unit and the exhaust and pressure stabilization safety unit according to the assessment results. Under the control of the central control unit, the cooling supply execution unit delivers liquid nitrogen to the foundation soil to freeze the easily liquefiable soil and exhausts the vaporized nitrogen to the exhaust and pressure stabilization safety unit. The exhaust and pressure stabilization safety unit releases the vaporized nitrogen into the atmosphere and regulates the system pressure. This system effectively solves the problems of slow response and limited protective effect in existing technologies.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of tunnel engineering, and particularly relates to a shallow-buried tunnel engineering earthquake liquefaction active prevention and control system. BACKGROUND

[0002] With the continuous advancement of infrastructure construction, the construction range of tunnel engineering gradually extends to regions with complex geological conditions such as coastal areas and river alluvial plains. These regions are widely distributed with saturated sand soil and silt soil layers. Tunnel engineering, as the core infrastructure in the fields of underground transportation, water transportation and urban pipe gallery, plays an irreplaceable role in transportation, energy transmission and urban development. In particular, in the densely populated and economically developed regions along the coastal economic belt and delta plain, the safety and stability of tunnels are directly related to regional economic development and people's life and property safety. The so-called saturated sand soil refers to a sand soil layer in which the pores between soil particles are completely filled with water. The silt soil layer refers to a soil layer with a particle size between sand and clay. These two types of soil layers have certain bearing capacity under natural conditions and can meet the basic needs of tunnel construction and operation, but they are prone to a special engineering geological problem, i.e., earthquake liquefaction, under the action of sudden loads such as earthquakes.

[0003] When tunnels are constructed and operated in the above-mentioned complex geological regions, the problem of earthquake liquefaction has become a key bottleneck restricting engineering safety. When an earthquake occurs, seismic waves will repeatedly vibrate saturated sand soil and silt soil layers, which will cause the excess pore water pressure in the soil layer to rise sharply in a short time. The excess pore water pressure refers to the part of the pressure borne by the pore water in the soil body that exceeds the static pore water pressure. The sharp rise of the excess pore water pressure will rapidly reduce or even eliminate the effective stress between soil particles. The effective stress refers to the stress transmitted between soil particles and is the core of maintaining the strength and stability of the soil body. Once the effective stress is sharply reduced, the soil body will lose its original bearing capacity and shear strength and will present a liquid-like state, which is called instantaneous liquefaction. After the soil body liquefies, it will directly cause a series of serious diseases in the tunnel structure, such as large settlement of the tunnel, upward floating of the tunnel, cracking of the lining structure, and even complete instability and collapse of the tunnel in severe cases. This will not only cause huge economic losses but also seriously threaten the operation safety of personnel and vehicles in the tunnel, bringing great difficulties to the later repair and maintenance.

[0004] To address the seismic liquefaction problem of saturated sandy and silty soil strata in tunnel engineering, the industry has developed several conventional anti-liquefaction treatment technologies. Currently, the most widely used methods are replacement, reinforcement, and drainage. Replacement is the most direct approach. The core idea is to excavate and remove the saturated sandy or silty soil strata around the tunnel that pose a liquefaction risk, replacing them with non-liquefiable soil layers with good liquefaction resistance, such as cohesive soil or gravelly soil. This fundamentally eliminates the liquefaction hazard by changing the soil properties. Reinforcement is also commonly used, with grouting reinforcement being the most frequently employed. This involves injecting grout into the liquefiable soil layer, allowing the grout to bind with the soil particles, thereby increasing the soil's density and liquefaction resistance, and enhancing the stability of the strata. Drainage involves constructing drainage channels around the tunnel, such as drainage ditches and drainage boards, to drain excess pore water from the strata, gradually dissipating excess pore water pressure, thus inhibiting liquefaction and ensuring the safety of the tunnel structure.

[0005] While existing anti-liquefaction technologies can mitigate the risk of tunnel liquefaction to some extent, they still have many shortcomings in practical engineering applications and fail to fundamentally solve the problem. Replacement methods, although offering direct treatment, require large-scale excavation of the strata, resulting in a massive workload, long construction period, and significantly increased costs. More importantly, this excavation-replacement method is difficult to implement for tunnels already built and in operation, making subsequent reinforcement impossible. Grouting reinforcement is highly dependent on geological conditions. In deep sandy soil strata, the grout cannot penetrate evenly to all areas, easily leading to uneven reinforcement and leaving some areas vulnerable to liquefaction, failing to achieve comprehensive anti-liquefaction effects. The core limitation of drainage methods lies in their limited drainage efficiency. Conventional drainage channels can only slowly dissipate pore water pressure. Under sudden conditions such as strong earthquakes, they cannot respond quickly enough to drain pore water in time to suppress the sudden rise in excess pore water pressure, thus failing to prevent liquefaction. In addition, all existing anti-liquefaction technologies share a common shortcoming—the lack of an anti-liquefaction system capable of responding in real time to changes in ground stress. Most of these technologies are passive protective measures, only providing some mitigation after or during liquefaction, and cannot quickly activate protective mechanisms when earthquake warnings are issued or abnormal excess pore water pressure occurs. Their timeliness is extremely poor, making it difficult to effectively prevent damage to tunnel structures. In summary, the tunnel engineering field urgently needs a tunnel anti-liquefaction system and construction method that is simple in structure, convenient to construct, responds quickly, and provides reliable protection to solve the many problems existing in current technologies and ensure the operational safety of tunnels in complex geological areas. Summary of the Invention

[0006] To address the problems existing in the prior art, this invention provides an active earthquake liquefaction prevention and control system for shallow-buried tunnel engineering. It constructs a rapid response system integrating perception, decision-making, and execution, realizing a fundamental shift from passive protection to active suppression, and solving the problems of delayed response and limited protection effect in the prior art.

[0007] To achieve the above objectives, the present invention provides the following technical solution: an active seismic liquefaction prevention and control system for shallow-buried tunnel engineering, comprising a sensing and monitoring unit, a central control unit, a cooling supply execution unit, and an exhaust pressure stabilization safety unit, wherein: The sensing and monitoring unit is installed on the cooling supply execution unit to collect the excess pore water pressure, temperature, peak ground acceleration, and air pressure in the foundation soil and transmit them to the central control unit. The central control unit is used to dynamically assess the liquefaction risk based on the data collected by the sensing and monitoring unit, and to control the operation of the cooling supply execution unit and the exhaust pressure stabilization safety unit based on the assessment results. The cooling supply execution unit is used to deliver liquid nitrogen to the foundation soil under the control of the central control unit, freeze the earthquake-liquefied foundation soil, and discharge the vaporized nitrogen to the exhaust pressure stabilization safety unit. The exhaust pressure stabilizing safety unit is used to release vaporized nitrogen into the atmosphere and regulate system pressure.

[0008] Furthermore, the sensing and monitoring unit includes a pore water pressure sensor, an accelerometer, a temperature sensor, and a pressure sensor. The pore water pressure sensor is used to monitor the excess pore water pressure of the foundation soil, the accelerometer is used to monitor the peak ground acceleration, the temperature sensor is used to monitor the temperature of the foundation soil, and the pressure sensor is used to monitor the air pressure within the system. The pore water pressure sensor, accelerometer, temperature sensor, and pressure sensor are all connected to the central control unit.

[0009] Furthermore, the cooling supply execution unit includes a liquid nitrogen storage tank, an emergency pressurization injection pump set, a main liquid supply pipe, a refrigerant distribution unit, a circumferential connecting freezing branch pipe, a high-speed response solenoid valve, and a composite freezer. The outlet of the liquid nitrogen storage tank is sealed and connected to the inlet of the emergency pressurization injection pump set, and the outlet of the emergency pressurization injection pump set is connected to the inlet of the main liquid supply pipe. The outlet of the main liquid supply pipe is connected to multiple refrigerant distribution units in sections, and the refrigerant distribution units are fixed at intervals in the passage on the lower side of the tunnel. The circumferential connecting freezing branch pipe is horizontally buried below the tunnel floor structure layer, and multiple composite freezers are vertically installed in the liquefiable sand foundation. The bottom end of the composite freezer is sealed and connected to the circumferential connecting freezing branch pipe, and the top end of the composite freezer is connected to the branch outlet of the refrigerant distribution unit. The high-speed response solenoid valve is installed at the branch outlet of each refrigerant distribution unit. The emergency pressurization injection pump set and the high-speed response solenoid valve are connected to the central control unit.

[0010] Furthermore, the planar hole spacing of multiple composite freezers is 0.8m to 1.2m, and the row spacing is the same as the hole spacing to form a square grid. The grid density is dynamically adjusted according to the liquefaction risk level. The composite freezers penetrate the liquefiable layer and enter the lower stable bearing layer. Their longitudinal protection range extends to 1.5 times the tunnel diameter on both sides of the outer edge of the tunnel structure.

[0011] Furthermore, the composite freezer includes an outer protective steel pipe and a high-efficiency freezing inner pipe placed inside the outer protective steel pipe. The annular gap between the high-efficiency freezing inner pipe and the outer protective steel pipe is connected to the inlet of the exhaust pressure stabilization safety unit. The pore water pressure sensor, accelerometer, and temperature sensor of the sensing and monitoring unit are installed on the outer protective steel pipe, and the air pressure sensor is set at the top of the high-efficiency freezing inner pipe.

[0012] Furthermore, the liquid nitrogen storage tank and emergency pressurization injection pump set are installed in a dedicated heat-insulated equipment compartment on one side of the tunnel.

[0013] Furthermore, the exhaust pressure stabilization safety unit includes an exhaust pressure stabilization channel, a safety relief valve, and an exhaust pipe. One end of the exhaust pressure stabilization channel is connected to the nitrogen outlet of the cooling supply execution unit. A safety relief valve is installed on the exhaust pressure stabilization channel. The outlet of the exhaust pressure stabilization channel is connected to the exhaust pipe inlet. The exhaust pipe outlet extends out to the ground surface and connects to the atmosphere.

[0014] Furthermore, the criteria for the central control unit to conduct dynamic assessments of liquefaction risks are as follows: When 0.3≤excess pore water pressure ratio<0.5, or 0.08g≤peak ground acceleration<0.15g and vibration duration exceeds 10s, it is a Class I state, and the flow rate of the cooling supply execution unit is 5%-10% of the design reference flow rate; When 0.5≤excess pore water pressure ratio<0.7, or 0.15g≤peak ground acceleration<0.25g and accompanied by excess pore water pressure ratio≥0.3, it is a Class II state, and the flow rate of the cooling supply to the execution unit is 30%-50% of the design reference flow rate; When 0.7≤excess pore water pressure ratio<0.8, or 0.25g≤peak ground acceleration<0.3g and accompanied by excess pore water pressure ratio≥0.5, it is a Class III state, and the flow rate of the cooling supply to the execution unit is 70%-100% of the design reference flow rate; When the excess pore water pressure ratio is ≥0.8, or the peak ground acceleration is ≥0.3g, it is a Class IV response, and the flow rate of the cooling supply unit is 100%-150% of the design reference flow rate.

[0015] Furthermore, the baseline design flow rate is calculated as follows:

[0016]

[0017]

[0018] in, This represents the total heat that needs to be removed from the soil. The effective cooling capacity provided for liquid nitrogen; For the volume of soil; This refers to the soil density. The specific heat capacity of the soil; The degree of soil cooling; The latent heat of the water-ice phase transition, w This refers to the soil moisture content. η For system thermal efficiency; The latent heat of vaporization of liquid nitrogen; The specific heat capacity at constant pressure of nitrogen; This represents the temperature rise of nitrogen gas from its boiling point to the target temperature. This refers to the duration of the cooling effect.

[0019] Furthermore, when the central control unit controls the cold supply execution unit to deliver liquid nitrogen to freeze the earthquake-induced liquefied foundation, if the excess pore water pressure ratio is less than 0.3 and remains stable for more than 10 minutes, or the foundation soil temperature drops below 0℃ and remains below -5℃, or the peak ground acceleration weakens, it indicates that the liquefaction risk has been eliminated.

[0020] Compared with the prior art, the present invention has at least the following beneficial effects: This invention provides an active seismic liquefaction prevention system for shallow-buried tunnel engineering. By integrating a sensing and monitoring unit, a central control unit, a cooling supply execution unit, and an exhaust and pressure stabilization safety unit, a complete active seismic liquefaction prevention system is constructed, achieving multi-unit collaborative operation. Specifically, the sensing and monitoring unit comprehensively collects key parameters of the foundation soil and the system, providing accurate data support for risk assessment; the central control unit dynamically assesses liquefaction risks based on the collected data and precisely controls the operation of each execution unit, achieving real-time response and intelligent prevention of liquefaction risks, achieving a fundamental shift from passive protection to active suppression, and solving the problem of delayed response in existing technologies; the cooling supply execution unit efficiently delivers liquid nitrogen to the foundation soil, achieving rapid freezing of liquefiable soil and physically eliminating liquefaction hazards; the exhaust and pressure stabilization safety unit safely discharges vaporized nitrogen and regulates system pressure, ensuring stable and reliable system operation, and comprehensively improving the system's controllability and safety. Furthermore, this system is suitable for shallow-buried tunnel engineering, providing structural safety guarantees throughout tunnel construction and operation, combining applicability and reliability.

[0021] This invention clearly defines the composition of the sensing and monitoring unit and the functions of each sensor. The pore water pressure sensor, accelerometer, temperature sensor, and air pressure sensor accurately collect liquefaction risk-related parameters and system operating parameters. All sensors are connected to the central control unit, providing accurate and timely data support for dynamic assessment of liquefaction risk, improving the system's intelligent and rapid response, ensuring the scientific nature of the assessment, and laying the foundation for precise control of cooling supply.

[0022] This invention clarifies the structure and connection relationship of the cold supply execution unit. Through the coordinated operation of components such as liquid nitrogen storage tank and emergency pressurization injection pump set, it achieves efficient and accurate delivery of liquid nitrogen and freezing of soil. The emergency pressurization pump set meets the needs of long-distance cooling. The refrigerant distribution unit and high-speed response solenoid valve realize precise regional control. The combination of circumferential connecting freezing branch pipe and composite freezer provides the foundation for the grid-like three-dimensional freezing structure, ensuring the complete and uniform freezing range. All execution components are connected to the central control unit, improving the system integration and controllability.

[0023] This invention clarifies the planar hole spacing, grid layout, and protection range of the composite freezer. The hole spacing and row spacing of 0.8m to 1.2m form a square grid, and the grid density can be dynamically adjusted according to the liquefaction risk level, achieving precise adaptation and flexible control of the freezing range. The composite freezer deeply penetrates the liquefiable layer and enters the lower stable bearing layer. The longitudinal protection range extends to 1.5 times the tunnel diameter on both sides of the outer edge of the tunnel structure, fully covering the liquefiable strata around the tunnel, effectively preventing problems such as tunnel settlement, uplift, and lining cracking. At the same time, the combined design of its outer steel pipe and high-efficiency freezing inner pipe not only ensures the structural stability of the freezer but also efficiently transmits liquid nitrogen and improves the efficiency of cold energy transfer. The annular gap between the two is connected to the exhaust pressure stabilization safety unit, which can promptly discharge the nitrogen gas after liquid nitrogen vaporization and achieve system pressure balance. The pore water pressure sensor, accelerometer, and temperature sensor are installed on the outer steel pipe, and the air pressure sensor is set at the top of the high-efficiency freezing inner pipe. The reasonable layout of the sensors further improves the monitoring accuracy and system response capability.

[0024] The liquid nitrogen storage tank and emergency pressurization injection pump set of this invention are installed in a dedicated heat-insulated equipment compartment on one side of the tunnel. This not only reduces the loss of liquid nitrogen cooling capacity and improves utilization efficiency, but also facilitates equipment installation, maintenance and management, reduces costs, avoids external interference, ensures stable and safe equipment operation, and improves the overall reliability and practicality of the system.

[0025] This invention clarifies the composition and connection relationship of the exhaust pressure stabilization safety unit. Through the coordinated operation of the exhaust pressure stabilization channel, safety relief valve, and exhaust pipe, vaporized nitrogen is safely and pollution-free discharged (aligning with green environmental protection advantages). The safety relief valve regulates system pressure and prevents safety hazards, while the exhaust pipe ensures unobstructed discharge, avoids nitrogen accumulation, and guarantees long-term stable system operation. The central control unit accurately classifies liquefaction risk levels (Level I, II, III, and IV) by combining the ultrapore water pressure ratio, peak ground acceleration, and vibration duration. It then sets the flow range of the cooling supply execution unit for each risk level, achieving precise, graded control that balances operational economy and risk prevention effectiveness, improving system controllability and response speed. Simultaneously, it provides a specific calculation formula for the benchmark design flow rate, covering key parameters such as soil volume, density, specific heat capacity, temperature drop, water content, latent heat of vaporization of liquid nitrogen, and system thermal efficiency, providing a reliable theoretical basis for flow rate setting and improving the accuracy and energy efficiency of cooling supply.

[0026] This invention clarifies the specific criteria for determining the elimination of liquefaction risks. By comprehensively judging three core indicators—excess pore water pressure ratio, foundation soil temperature, and peak ground acceleration—the risk elimination can be accurately identified. When the excess pore water pressure ratio is less than 0.3 and remains stable for more than 10 minutes, the foundation soil temperature drops below 0°C and remains below -5°C, or the peak ground acceleration weakens, the liquefaction risk is determined to be eliminated. The operating status of the cooling supply execution unit can be adjusted in a timely manner to avoid energy waste and increased costs, prevent unnecessary impacts on the foundation and tunnel structure, improve the system's active prevention and control closed loop, and enhance the system's economy and reliability. Attached Figure Description

[0027] Figure 1 This is a cross-sectional structural design drawing of the tunnel anti-liquefaction system provided by the present invention.

[0028] Figure 2 This is a diagram showing the sensor arrangement on the outer protective steel pipe and the high-efficiency freezing inner pipe provided by the present invention.

[0029] In the attached diagram: 1. Central control unit; 2. Liquid nitrogen storage tank; 3. Emergency pressurization injection pump set; 4. Main liquid supply pipe; 5. Refrigerant distribution unit; 6. Circumferential connecting freezing branch pipe; 7. High-speed response solenoid valve; 8. Exhaust pressure stabilizing channel; 9. Safety pressure relief valve; 10. Exhaust pipe; 11. Outer protective steel pipe; 12. High-efficiency freezing inner pipe; 13. Liquefiable sandy soil foundation; 14. Pore water pressure sensor; 15. Accelerometer; 16. Temperature sensor; 17. Air pressure sensor. Detailed Implementation

[0030] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0031] This invention provides an active seismic liquefaction prevention system for shallow-buried tunnel engineering. Utilizing engineering-grade durable components, it is adapted to the complex working conditions of underground tunnels, including dampness and vibration, and can stably meet the seismic liquefaction prevention requirements of liquefiable sandy soil foundations. The system is corely divided into four functional units: a sensing and monitoring unit, a central control unit, a cooling supply execution unit, and an exhaust and pressure stabilization safety unit. The central control unit serves as the core control hub. The sensing and monitoring unit completes on-site data acquisition, the cooling supply execution unit implements liquid nitrogen delivery and soil freezing, and the exhaust and pressure stabilization safety unit is responsible for the discharge of vaporized nitrogen and the system's safe pressure stabilization. These four units are connected via pipelines, electrical connections, and signal transmission to form a complete closed-loop system of sensing, judgment, execution, and feedback. The sensing and monitoring unit includes a pore water pressure sensor 14, an accelerometer 15, a temperature sensor 16, and a pressure sensor 17, which are responsible for collecting formation and equipment operating parameters. Central Control Unit 1, the core control hub of the intelligent active earthquake liquefaction prevention and control system, is located in a dedicated tunnel management room; The cooling supply execution unit includes a liquid nitrogen storage tank 2, an emergency pressurization injection pump group 3, a main liquid supply pipe 4, a refrigerant distribution unit 5, a high-speed response solenoid valve 7, a circumferential connecting freezing branch pipe 6, and a composite freezer composed of an outer protective steel pipe 11 and a high-efficiency freezing inner pipe 12. The exhaust pressure stabilization safety unit includes an exhaust pressure stabilization channel 8, a safety relief valve 9, and an exhaust pipe 10, which is responsible for discharging vaporized nitrogen and ensuring pipeline pressure safety.

[0032] Furthermore, the central control unit 1 receives real-time data from all sensors, runs a built-in dynamic liquefaction risk assessment algorithm, and monitors, analyzes, and makes decisions regarding the status of the entire system. When the risk is determined to exceed a threshold, precise control commands are issued to the actuators (such as solenoid valves and booster pumps), and adaptive adjustments can be made based on feedback data.

[0033] Furthermore, the emergency pressurization injection pump set 3 is used to start when the system requires the maximum flow rate or the delivery distance is extremely long, instantly increasing the pressure of the liquid nitrogen delivery pipeline, ensuring that the liquid nitrogen can overcome the pipeline resistance and quickly and in sufficient quantity reach each freezer at the far end, which is the key power guarantee for achieving rapid response.

[0034] Furthermore, the refrigerant distribution unit 5 is used to precisely distribute the liquid nitrogen from the main pipe to each composite freezer within its jurisdiction, according to the instructions of the central control unit 1.

[0035] Furthermore, the circumferential connecting freezing branch pipe 6 is used to connect the bottoms of all composite freezers in the horizontal direction, forming a grid-like overall freezing structure. It itself acts as a secondary freezing pipe, directly freezing the surrounding soil. Together with the composite freezers, it forms a spatial grid structure, greatly improving the overall stiffness and bearing capacity of the frozen soil.

[0036] Furthermore, a high-speed response solenoid valve 7 is installed at the inlet of each composite freezer, receiving second-level electrical signal commands from the central control unit 1 to quickly open or close the liquid nitrogen passage. It is the final actuator for achieving independent and precise control of a single freezer or a group of freezers.

[0037] Furthermore, the exhaust pressure stabilizing channel 8 is used to collect vaporized nitrogen from each outer protective steel pipe 11 and to further stabilize the system pressure using its volume.

[0038] Furthermore, the safety relief valve 9 is used to automatically open and relieve pressure when the system pressure exceeds the safety limit due to unexpected circumstances (such as valve closure or vent blockage), protecting pipelines and equipment from overpressure damage.

[0039] Furthermore, the exhaust pipe 10 is used to safely discharge the collected and stabilized nitrogen gas into the Earth's surface atmosphere, completing the entire phase change cycle.

[0040] Furthermore, the outer protective steel pipe 11 is the final interface for transferring cold energy from the inside of the freezer to the external soil, and its outer surface directly contacts the soil and exchanges heat. The nitrogen passage structure is as follows: the nitrogen gas formed after liquid nitrogen is vaporized in the high-efficiency freezing inner pipe 12 of the composite freezer flows upward through the annular gap between the outer protective steel pipe 11 and the high-efficiency freezing inner pipe 12, and flows into the exhaust pressure stabilizing channel 8 through each branch pipeline; the exhaust pressure stabilizing channel 8 collects the gaseous nitrogen from each outer protective steel pipe 11 and uses its volume to buffer system pressure fluctuations; the end of the exhaust pressure stabilizing channel is connected to an exhaust pipe, which leads the nitrogen gas to the ground surface for safe discharge; a safety pressure relief valve is installed on the exhaust pressure stabilizing channel, which automatically opens to relieve pressure when the system pressure exceeds the set safety threshold, protecting the pipeline system.

[0041] Furthermore, the direct source of the freezing effect is the process of introducing liquid nitrogen into the high-efficiency freezing inner tube 12 and eventually causing it to vaporize and undergo a phase change, transferring the cold energy to the outer protective steel tube 11 through the tube wall.

[0042] Furthermore, the pore water pressure sensor 14 is installed in a pre-embedded groove on the outer wall of the outer protective steel pipe 11 to monitor changes in excess pore water pressure in real time. An abnormal rise in pore water pressure is the most direct precursor to formation liquefaction, and its data is the primary basis for the central control unit 1 to make risk assessments.

[0043] Furthermore, the accelerometer 15 is used to monitor peak ground acceleration (PGA) to provide earthquake early warning signals and ground motion intensity data. When fused with pore water pressure data, it can trigger the system in advance or simultaneously to address dynamic liquefaction caused by earthquakes.

[0044] Furthermore, the temperature sensor 16 is installed in a pre-embedded groove on the outer wall of the outer protective steel pipe 11. The groove is filled with a heat-conducting medium to ensure good thermal contact. A protective cover is installed on the outside. The sensor lead is passed through a stainless steel flexible hose for protection and led along the pipe wall to the ground surface; it directly monitors the temperature at the frozen soil boundary. The data is used to determine whether the freezing range has reached the design expectation, the freezing development rate, and to provide feedback to adjust the liquid nitrogen flow rate.

[0045] Furthermore, the pressure sensor 17 is used to monitor the pressure of the vaporized nitrogen in the high-efficiency freezing inner tube 12. The pressure value directly reflects the vaporization rate of liquid nitrogen and the degree of venting, and is a key feedback parameter for the central control unit 1 to adjust the valve opening and maintain the efficient and stable operation of the system.

[0046] Furthermore, the connection relationships of the above-mentioned devices are as follows: Liquid nitrogen storage tank 2 and emergency pressurization injection pump set 3 are installed in a dedicated heat-insulated equipment compartment on one side of the tunnel. The outlet of liquid nitrogen storage tank 2 is sealed and connected to the inlet of emergency pressurization injection pump set 3. The outlet of emergency pressurization injection pump set 3 is connected to the main liquid supply pipe 4 via a flange. The main liquid supply pipe 4 is laid in the tunnel's lower passage, running along the dedicated passage inside the tunnel and connecting in sections to multiple refrigerant distribution units 5. The refrigerant distribution units 5 are fixed at intervals in the tunnel's lower passage. The branch outlet of each refrigerant distribution unit 5 is connected to the inlet of the composite freezer via a high-pressure hose. Each branch is equipped with a high-speed response solenoid valve 7 to achieve independent control of different areas. The circumferential connecting freezing branch pipe 6 is horizontally buried below the tunnel floor structure layer. Multiple composite freezing devices (composed of an outer protective steel pipe 11 and a high-efficiency freezing inner pipe 12 placed inside the outer protective steel pipe 11) are vertically driven into the liquefiable sandy soil foundation 13 and distributed in a grid pattern. The planar hole spacing is 0.8m to 1.2m, and the row spacing is the same as the hole spacing to form a square grid. The grid density is dynamically adjusted according to the liquefaction risk level. In low-risk areas, the spacing can be arranged at 1.2m, and in high-risk areas, the spacing is increased to 0.8m. The composite freezing device penetrates the liquefiable layer and enters the lower stable bearing layer. Its longitudinal protection range extends to 1.5 times the tunnel diameter on both sides of the outer edge of the tunnel structure, thereby forming a continuous and uniform overall frozen soil curtain below the tunnel floor. The bottom end of the composite freezing device is sealed and connected to the circumferential connecting freezing branch pipe 6, and the top end is connected to the refrigerant distribution unit 5 through a branch pipeline. Each branch pipeline is equipped with a high-speed response solenoid valve 7, forming a grid-like three-dimensional freezing pipeline. The annular gap between the outer protective steel pipe 11 and the high-efficiency freezing inner pipe 12 of the composite freezer is connected to one end of the exhaust pressure stabilizing channel 8. The exhaust pressure stabilizing channel 8 is located at the bottom of the refrigerant distribution unit and collects the gas from each outer protective steel pipe 11. A safety pressure relief valve 9 is installed on the exhaust pressure stabilizing channel 8. The other end of the exhaust pressure stabilizing channel 8 is connected to the exhaust pipe 10 and leads directly to an open area on the ground surface. The central control unit 1 is located in the tunnel management room. The signal transmission lines of the pore water pressure sensor 14, accelerometer 15, temperature sensor 16, and air pressure sensor 17 are all connected to the central control unit 1 through shielded cables. The control lines of the central control unit 1 are electrically connected to the emergency pressurization injection pump group 3, the refrigerant distribution unit 5, and the high-speed response solenoid valve 7, forming a two-way closed-loop connection for signal acquisition and command issuance.

[0047] Furthermore, when the system is working: The pore water pressure sensor 14 and accelerometer 15 collect monitoring data in real time and upload it to the central control unit 1. The central control unit 1 performs fusion processing on the multi-source sensor data, including data filtering, feature extraction, and comprehensive liquefaction risk assessment. When the assessment result exceeds the preset risk threshold, it immediately sends an opening command to the refrigerant distribution unit 5 in the target area and the high-speed response solenoid valve 7 of the corresponding branch, and can also activate the emergency pressurization injection pump group 3. Under pressure drive (the required driving pressure increases with the increase of the target flow rate, and is also affected by pipeline resistance and exhaust back pressure), liquid nitrogen is rapidly injected into the high-efficiency freezing inner tube 12 through the main liquid supply pipe 4, the distribution unit, and the valve.

[0048] After liquid nitrogen injection, the system verifies the liquefaction inhibition effect through real-time monitoring data: 1) When the excess pore water pressure ratio ru monitored by the pore water pressure sensor 14 continuously decreases from a high level to below 0.3 and remains stable for more than 10 minutes, it indicates that the excess pore water pressure has been significantly dissipated, the effective stress of the soil has been restored, and the risk of liquefaction has been eliminated. 2) The temperature monitored by temperature sensor 16 dropped below 0℃ and remained below -5℃, confirming that a permafrost curtain had formed; 3) If the seismic response monitored by accelerometer 15 decreases synchronously, it can be used as an auxiliary verification.

[0049] The central control unit 1 dynamically adjusts the liquid nitrogen injection rate based on the feedback data mentioned above: when liquefaction is determined to be suppressed, the flow rate is gradually reduced or injection is stopped, achieving closed-loop intelligent control with on-demand cooling and verifiable effects. Vaporized nitrogen is discharged into the exhaust pressure stabilization channel 8 through the outer protective steel pipe 11, and finally safely discharged through the exhaust pipe 10. Pressure and temperature sensors provide real-time feedback to assist the system in adjustment.

[0050] Preferably, the liquefaction risk comprehensive judgment algorithm built into the central control unit 1 is as follows: The system uses the excess pore water pressure ratio (ru) as the primary and most direct criterion, and peak ground acceleration (PGA) as an auxiliary early warning and verification signal to construct a four-level graded response mechanism: When 0.3≤ru<0.5, or 0.08g≤PGA<0.15g and the vibration duration exceeds 10s, the system enters Level I state and starts a low-flow precooling cycle. When 0.5≤ru<0.7, or 0.15g≤PGA<0.25g and accompanied by ru≥0.3, the system enters Level II state and the partition starts medium flow freeze; When 0.7≤ru<0.8, or 0.25g≤PGA<0.3g and accompanied by ru≥0.5, the system enters Level III state and initiates high-volume partition enhanced freezing; When ru≥0.8 or PGA≥0.3g, the system enters Level IV regardless of the pore water pressure and starts operating at maximum power.

[0051] The system pre-calculates the design baseline flow rate Q required to suppress liquefaction in the target region based on thermodynamic principles. During the response process: Under Level I conditions, the system initiates pipeline precooling and circulation, with the flow rate controlled at 5%-10% Q, primarily to maintain the pipeline in a low-temperature standby state. In Level II status, the system initiates proactive freezing for localized risk areas, increasing traffic to 30%-50% Q; Under Level III conditions, the system enters an enhanced freezing mode, with the flow rate increasing to 70%-100% Q to rapidly form a continuous permafrost barrier; Under Level IV conditions, the system activates the emergency pressurization mode and activates all available refrigerant reserves, with the flow rate reaching 100%-150% Q in a short time to ensure rapid solidification of the soil under extreme conditions.

[0052] Furthermore, the specific indicators of the system response mechanism can be adjusted according to the importance of the project and the level of seismic fortification, and the actual values ​​of flow rates at each level can be finely adjusted in a closed loop based on the real-time feedback of the permafrost development rate and temperature field data.

[0053] The formula for calculating the baseline design flow rate is as follows: 1. Total heat to be removed from the soil (kJ):

[0054] in, This represents the volume of the soil (m3). Soil density (kg / m³) 3 ), That is, the mass of the soil; Specific heat capacity of soil (kJ / (kg)) ℃)), The temperature drop of the soil (°C). Cooling the sensible heat of the soil; The latent heat of the water-ice phase transition is 334 kJ / kg, with a standard value of 334 kJ / kg. w represents the soil moisture content (dimensionless or mass fraction). The latent heat of water freezing in the soil (determined by geotechnical testing).

[0055] 2. Effective cooling capacity provided by liquid nitrogen :

[0056] in, ηThis represents the system thermal efficiency (0.6~0.8). It is an engineering empirical coefficient that covers factors such as pipe cooling loss, uneven heat transfer, and frozen pipe wall thermal resistance. This coefficient is crucial for simplifying the process; its precise value needs to be determined through experiments or numerical simulations. The latent heat of vaporization of liquid nitrogen is (kJ / kg, standard value 199.3 kJ / kg). The specific heat capacity at constant pressure of nitrogen (kJ / (kg)) ℃)); This represents the temperature rise (°C) of nitrogen gas from its boiling point (-196°C) to the target temperature. This is the duration of the cooling effect (h or s, which must be matched with the specific heat capacity unit).

[0057] 3. Design baseline flow rate Q (mass flow rate per unit time, i.e., the mass of liquid nitrogen injected into the freezing pipe through the pipeline per unit time):

[0058] in, The total heat (kJ) that needs to be removed from the soil. Effective cooling capacity (kJ) provided for liquid nitrogen.

[0059] Example 1 like Figure 1 , 2 As shown, this invention provides an active seismic liquefaction prevention system for shallow-buried tunnel engineering. The system comprises a central control unit 1, a liquid nitrogen storage tank 2, an emergency pressurization injection pump set 3, a main liquid supply pipe 4, a refrigerant distribution unit 5, a circumferential connecting freezing branch pipe 6, a composite freezer, a high-speed response solenoid valve 7, a sensing and monitoring unit, an exhaust pressure stabilizing channel 8, a safety pressure relief valve 9, and an exhaust pipe 10. The composite freezer consists of an outer protective steel pipe 11 and a high-efficiency freezing inner pipe 12 placed inside the outer protective steel pipe 11. The sensing and monitoring unit includes a pore water pressure sensor 14, an accelerometer 15, a temperature sensor 16 installed on the outer protective steel pipe 11, and a pressure sensor 17 located at the top of the high-efficiency freezing inner pipe 12. Liquid nitrogen storage tank 2 and emergency pressurization injection pump set 3 are installed in a dedicated heat insulation equipment compartment on one side of the tunnel; central control unit 1 is located in the tunnel management room; main liquid supply pipe 4 is laid in the lower tunnel passage; refrigerant distribution unit 5 is fixed at intervals in the lower tunnel passage; circumferential connecting freezing branch pipe 6 is horizontally buried below the tunnel floor structure layer; multiple composite freezers are vertically driven into the liquefiable sand foundation 11 and distributed in a grid pattern, with their bottom ends connected to the circumferential connecting freezing branch pipe 6 and their top ends connected to the refrigerant distribution unit 5 through branch pipes, and each branch pipe is equipped with a high-speed response solenoid valve 7; exhaust pressure stabilizing channel 8 is located below the refrigerant distribution unit, collects gas from each outer protective steel pipe 11, and leads it to the ground surface through exhaust pipe 10, and safety pressure relief valve 9 is installed on exhaust pressure stabilizing channel 8.

[0060] After the system is powered on, it enters a normal standby monitoring state. The sensing and monitoring unit collects the operating parameters of the liquefiable sandy soil foundation 13 and the equipment in real time and uploads them to the central control unit 1 continuously. The central control unit 1 receives and integrates various real-time data and compares them with the preset liquefaction risk threshold to complete the risk level determination. When the liquefaction risk is determined to exceed the corresponding preset threshold, a control command is immediately issued to accurately open the high-speed response solenoid valve 7 of the designated zone and simultaneously start the emergency pressurization injection pump group 3.

[0061] The cryogenic liquid nitrogen in the liquid nitrogen storage tank 2 is pressurized by the emergency pressurization injection pump group 3 and then transported to each refrigerant distribution unit 5 through the main liquid supply pipe 4. It is then precisely injected into the target high-efficiency freezing inner pipe 12 through the branch pipeline. The liquid nitrogen rapidly vaporizes and absorbs heat in the high-efficiency freezing inner pipe 12. The cold energy is transferred to the surrounding liquefiable sandy soil foundation 13 through the outer protective steel pipe 11, so that the saturated sandy soil freezes and solidifies rapidly, blocking the pore water pressure transmission path and inhibiting the development of earthquake liquefaction from the source. The nitrogen gas formed after the liquid nitrogen vaporizes is collected step by step through the annular gap between the outer protective steel pipe 11 and the high-efficiency freezing inner pipe 12 and the exhaust pressure stabilization channel 8. Finally, it is safely discharged to the ground surface through the exhaust pipe 10, ensuring the stability of pipeline pressure throughout the process.

[0062] This system uses the excess pore water pressure ratio (ru) and peak ground acceleration (PGA) as core indicators, and classifies the risk response into four levels based on the duration of the vibration. The thresholds, response actions, and corresponding measured data for each level are as follows. All measured data are taken from the field measurements of liquefiable sandy soil foundation 13 in a conventional shallow-buried tunnel, which are consistent with the actual working conditions of the project: 1. Level I state Judgment criteria: 0.3≤ru<0.5, or 0.08g≤PGA<0.15g and vibration duration exceeds 10s; Response action: Initiate low-flow precooling circulation, do not carry out large-scale soil freezing, only maintain the pipeline in a low-temperature standby state; Actual test data: The liquid nitrogen supply flow rate was controlled at 45~60 kg / h, the temperature monitored by temperature sensor 16 was stable at -5~-10℃, the pore water pressure sensor 14 monitored the ru value maintained in the range of 0.32~0.45, the accelerometer 15 monitored the PGA value stable at 0.08~0.12g, and the gas pressure sensor 17 monitored the internal pressure of the pipeline maintained at 0.2~0.3MPa, with no risk of overpressure.

[0063] 2. Level II Judgment criteria: 0.5 ≤ ru < 0.7, or 0.15g ≤ PGA < 0.25g and accompanied by ru ≥ 0.3; Response actions: Initiate medium-flow freezing in designated areas and carry out preventative freezing of soil in high-risk sections; Measured detection data: The liquid nitrogen supply flow rate is increased to 90 - 120 kg / h, the pipe wall temperature drops to -15 - -20 °C, the temperature of the 13 soil bodies in the liquefiable sandy soil foundation gradually drops below 0 °C, the rising rate of pore water pressure significantly slows down, the ru value stabilizes in the range of 0.50 - 0.65, the internal pressure in the pipeline stabilizes at 0.3 - 0.4 MPa, and the safety relief valve 9 has no action.

[0064] 3. Level III status Judgment conditions: 0.7 ≤ ru < 0.8, or 0.25g ≤ PGA < 0.3g and accompanied by ru ≥ 0.5; Response action: Start large-flow partition enhanced freezing, rapidly expand the frozen soil range, and block liquefaction conduction; Measured detection data: The liquid nitrogen supply flow rate is increased to 180 - 220 kg / h, the pipe wall temperature stabilizes at -25 - -30 °C, the surrounding liquefiable sandy soil is frozen and solidified within 30 minutes, the ru value gradually drops back to the range of 0.60 - 0.70, there is no obvious displacement of the soil body under the influence of seismic motion, and the pipeline operation parameters are stable.

[0065] 4. Level IV status Judgment conditions: ru ≥ 0.8, or PGA ≥ 0.3g, directly trigger regardless of the real-time state of pore water pressure; Response action: Start the maximum power operation of the whole system, freeze synchronously in the whole area, and rapidly form a complete frozen soil curtain; Measured detection data: The liquid nitrogen supply flow rate reaches 240 - 260 kg / h (benchmark design flow rate), the pipe wall temperature rapidly drops below -35 °C, a continuous frozen soil curtain is formed under the tunnel floor within 15 minutes, the ru value rapidly drops below 0.5, the internal pressure in the pipeline stabilizes within the safe range, the gasified nitrogen is smoothly discharged, and there is no abnormal pressure.

[0066] Combined with the parameters of the 13 liquefiable sandy soil foundation in this embodiment (soil density 2000 kg / m 3 , water content 30%, freezing range 120 m 3 ), the benchmark design flow rate of liquid nitrogen is calculated to be 240 kg / h, and the response flow rates at all levels are adjusted proportionally based on this as the benchmark to ensure that the cold quantity supply matches the liquefaction risk and avoid cold quantity waste or insufficient freezing.

[0067] When the local vibration signal completely disappears, the PGA value monitored by the accelerometer 15 drops below 0.05g, the ru value monitored by the pore water pressure sensor 14 remains stable below 0.3 for 15 minutes, and the temperature monitored by the temperature sensor 16 gradually rises above -5 °C, the central control unit 1 gradually closes the high-speed response solenoid valve 7 and the emergency pressurization injection pump group 3, and the system returns to the standby monitoring state, completing a prevention and control closed loop.

[0068] In summary, this system addresses the technical challenges of liquefaction in saturated sandy and silty soil strata surrounding shallow tunnels under seismic loading, which can lead to tunnel settlement, uplift, and lining cracking. Specifically designed for shallow tunnel projects in typical geological conditions such as coastal plains and alluvial river areas, the system employs liquid nitrogen freezing pipelines deployed within the target soil layer. This rapidly lowers the soil temperature to below -80°C, completely freezing pore water to form an ice skeleton structure that binds discrete soil particles together, creating a continuous, uniform, high-strength frozen soil curtain. Simultaneously, the system uses cryogenic sensors to monitor soil temperature in real-time and control the liquid nitrogen injection rate, ensuring sufficient freezing under various geological conditions, including dense sand and cohesive soils, achieving a uniaxial compressive strength of 3-5 MPa for the frozen soil. a. The elastic modulus is increased by 40% to 60% compared to the unfrozen state, and because the pore water is completely converted into solid ice, the liquefaction conditions caused by the accumulation of excess pore water pressure are physically eliminated. The compressive strength of the frozen soil formed by this system is increased by 30% to 50% and the elastic modulus is increased by 40% to 60% compared to the existing brine circulation freezing technology. At the same time, it avoids the technical defects of the brine method, which may leave unfrozen water due to the freezing temperature of only -25℃ to -35℃ and still poses a risk of liquefaction under dynamic loads. This system can be pre-embedded and installed simultaneously during the construction of new tunnels, and can also be retrofitted to existing tunnels in the later stage, realizing the active prevention and control of seismic liquefaction risks, replacing the traditional passive reinforcement methods such as replacement and grouting, and ensuring the structural safety of the tunnel throughout the construction and operation process.

Claims

1. An active seismic liquefaction prevention system for shallow-buried tunnel engineering, characterized in that, It includes a sensing and monitoring unit, a central control unit (1), a cooling supply execution unit, and an exhaust pressure stabilization and safety unit, wherein: The sensing and monitoring unit is set on the cold supply execution unit to collect the excess pore water pressure, temperature, peak ground acceleration and air pressure in the foundation soil and transmit them to the central control unit (1). The central control unit (1) is used to dynamically assess the liquefaction risk based on the data collected by the sensing and monitoring unit, and to control the operation of the cold energy supply execution unit and the exhaust pressure stabilization safety unit based on the assessment results; The cold supply execution unit is used to deliver liquid nitrogen to the foundation soil under the control of the central control unit (1), freeze the earthquake-liquefied foundation soil, and discharge the vaporized nitrogen to the exhaust pressure stabilization safety unit. The exhaust pressure stabilizing safety unit is used to release vaporized nitrogen into the atmosphere and regulate system pressure.

2. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 1, characterized in that, The sensing and monitoring unit includes a pore water pressure sensor (14), an accelerometer (15), a temperature sensor (16), and a pressure sensor (17). The pore water pressure sensor (14) is used to monitor the excess pore water pressure of the foundation soil. The accelerometer (15) is used to monitor the peak ground acceleration. The temperature sensor (16) is used to monitor the temperature of the foundation soil. The pressure sensor (17) is used to monitor the air pressure in the system. The pore water pressure sensor (14), accelerometer (15), temperature sensor (16), and pressure sensor (17) are all connected to the central control unit (1) via signal.

3. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 1, characterized in that, The cooling supply execution unit includes a liquid nitrogen storage tank (2), an emergency pressurization injection pump group (3), a main liquid supply pipe (4), a refrigerant distribution unit (5), a circumferential connecting freezing branch pipe (6), a high-speed response solenoid valve (7), and a composite freezer; the outlet of the liquid nitrogen storage tank (2) is sealed and connected to the inlet of the emergency pressurization injection pump group (3), and the outlet of the emergency pressurization injection pump group (3) is connected to the inlet of the main liquid supply pipe (4); the outlet of the main liquid supply pipe (4) is connected to multiple refrigerant distribution units (5) in sections, and the refrigerant distribution units (5) are spaced at fixed intervals. Inside the passage on the lower side of the tunnel; the circumferential connecting freezing branch pipe (6) is horizontally buried below the tunnel bottom slab structure layer, and multiple composite freezers are vertically installed in the liquefiable sand foundation (13). The bottom end of the composite freezer is sealed and connected to the circumferential connecting freezing branch pipe (6), and the top end of the composite freezer is connected to the branch outlet of the refrigerant distribution unit (5). The high-speed response solenoid valve (7) is installed at the branch outlet of each refrigerant distribution unit (5). The emergency pressurization injection pump group (3) and the high-speed response solenoid valve (7) are connected to the central control unit (1).

4. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 3, characterized in that, The planar hole spacing of multiple composite freezers is 0.8m to 1.2m, and the row spacing is the same as the hole spacing to form a square grid. The grid density is dynamically adjusted according to the liquefaction risk level. The composite freezers penetrate the liquefiable layer and enter the lower stable bearing layer. Their longitudinal protection range extends to 1.5 times the tunnel diameter on both sides of the outer edge of the tunnel structure.

5. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 3, characterized in that, The composite freezer includes an outer steel pipe (11) and a high-efficiency freezing inner pipe (12) placed inside the outer steel pipe (11). The annular gap between the high-efficiency freezing inner pipe (12) and the outer steel pipe (11) is connected to the inlet of the exhaust pressure stabilization safety unit. The pore water pressure sensor (14), accelerometer (15), and temperature sensor (16) of the sensing and monitoring unit are installed on the outer steel pipe (11), and the air pressure sensor (17) is set on the top of the high-efficiency freezing inner pipe (12).

6. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 3, characterized in that, The liquid nitrogen storage tank (2) and the emergency pressurization injection pump set (3) are installed in a special heat insulation equipment compartment on one side of the tunnel.

7. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 1, characterized in that, The exhaust pressure stabilization safety unit includes an exhaust pressure stabilization channel (8), a safety relief valve (9), and an exhaust pipe (10). One end of the exhaust pressure stabilization channel (8) is connected to the nitrogen outlet of the cold energy supply execution unit. The exhaust pressure stabilization channel (8) is equipped with a safety relief valve (9). The outlet of the exhaust pressure stabilization channel (8) is connected to the inlet of the exhaust pipe (10). The outlet of the exhaust pipe (10) extends out of the ground and connects with the atmosphere.

8. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 1, characterized in that, The criteria for the central control unit (1) to conduct dynamic assessment of liquefaction risk are as follows: When 0.3≤excess pore water pressure ratio<0.5, or 0.08g≤peak ground acceleration<0.15g and vibration duration exceeds 10s, it is a Class I state, and the flow rate of the cooling supply execution unit is 5%-10% of the design reference flow rate; When 0.5≤excess pore water pressure ratio<0.7, or 0.15g≤peak ground acceleration<0.25g and accompanied by excess pore water pressure ratio≥0.3, it is a Class II state, and the flow rate of the cooling supply to the execution unit is 30%-50% of the design reference flow rate; When 0.7≤excess pore water pressure ratio<0.8, or 0.25g≤peak ground acceleration<0.3g and accompanied by excess pore water pressure ratio≥0.5, it is a Class III state, and the flow rate of the cooling supply to the execution unit is 70%-100% of the design reference flow rate; When the excess pore water pressure ratio is ≥0.8, or the peak ground acceleration is ≥0.3g, it is a Class IV response, and the flow rate of the cooling supply unit is 100%-150% of the design reference flow rate.

9. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 8, characterized in that, The baseline design flow rate is calculated as follows: in, This represents the total heat that needs to be removed from the soil. The effective cooling capacity provided for liquid nitrogen; For the volume of soil; This refers to the soil density. The specific heat capacity of the soil; The degree of soil cooling; The latent heat of the water-ice phase transition, w This refers to the soil moisture content. η For system thermal efficiency; The latent heat of vaporization of liquid nitrogen; The specific heat capacity at constant pressure of nitrogen; This represents the temperature rise of nitrogen gas from its boiling point to the target temperature. This refers to the duration of the cooling effect.

10. The active seismic liquefaction prevention system for shallow-buried tunnel engineering according to claim 8, characterized in that, When the central control unit (1) controls the cold supply execution unit to deliver liquid nitrogen to freeze the earthquake liquefied foundation, if the excess pore water pressure ratio is less than 0.3 and remains stable for more than 10 minutes, or the temperature of the foundation soil drops below 0℃ and remains below -5℃, or the peak ground acceleration weakens, it indicates that the liquefaction risk has been eliminated.

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