High and cold tunnel variable cross-section frozen soil thermal-mechanical coupling safety grading control construction method

Abstract

The invention relates to the technical field of tunnel engineering construction, in particular to a high and cold tunnel variable cross-section frozen soil thermal coupling safety hierarchical control construction method, which comprises the following steps: constructing a three-dimensional thermal coupling model considering frozen soil phase change characteristics based on an FLAC3d platform, and introducing a freezing frontal surface expansion speed prediction algorithm and ground temperature monitoring data to dynamically correct model parameters; a three-level safety threshold is established, a hierarchical control strategy is implemented, a pre-control period (heating and heat preservation), an excavation period (advanced small pilot tunnel and reverse top-picking expanding excavation) and a support period (asymmetric support) are divided, and a closed-loop system for monitoring, simulation, feedback and adjustment is established. The method comprises the following steps of: accurately regulating and controlling a temperature field, setting a phase change energy storage heat preservation layer, constructing a bionic drainage system, applying an intelligent construction platform with BIM (Building Information Modeling) and GIS (Geographic Information System) fused, integrating various sensing equipment for real-time monitoring, comparing monitoring data with a model prediction result, dynamically optimizing construction parameters and ensuring construction safety.

Description

Technical Field

[0001] The present invention relates to the technical field of tunnel engineering construction, and in particular to a construction method for thermal-mechanical coupling safety graded control of frozen soil in a variable-section section of a high-altitude cold tunnel. Background Art

[0002] Tunnel construction in high-altitude cold regions faces severe challenges in permafrost environments, especially during the construction of variable-section sections. Due to the combined effects of geometric changes and the thermal-mechanical coupling effects of permafrost, these can easily lead to safety issues such as support structure instability, frost heave damage, and excessive surrounding rock deformation. Existing tunnel construction technologies have the following main shortcomings:

[0003] First, traditional tunnel construction methods typically separate thermal and mechanical processes, making it impossible to accurately simulate the nonlinear changes in parameters during the permafrost phase transition, resulting in large errors in the design of construction plans. Second, existing support technologies mostly use uniform support designs, which are difficult to cope with the uneven stress distribution characteristics of variable cross-section sections. Furthermore, conventional temperature control measures lack the ability to precisely regulate and control, making it difficult to maintain a stable construction environment. In addition, traditional construction monitoring methods are often passive and lack forward-looking warning and dynamic adjustment mechanisms.

[0004] Therefore, it is urgent to develop a construction control method that can comprehensively consider the thermal-mechanical coupling effect of frozen soil and the characteristics of variable cross-section to ensure the safety and efficiency of the construction of variable cross-section sections of tunnels in high-altitude cold areas. Summary of the Invention

[0005] The purpose of the present invention is to provide a safety graded control construction method for frozen soil thermal coupling in variable cross-section sections of high-altitude and cold tunnels, aiming to solve the problem that the existing technology cannot effectively deal with the frozen soil thermal coupling effect and variable cross-section characteristics, and to achieve safety and control during tunnel construction.

[0006] The present invention discloses a construction method for thermal-mechanical coupling safety graded control of frozen soil in a variable-section section of a high-altitude cold tunnel, comprising:

[0007] Constructing a frozen soil thermal-mechanical coupling model, the construction of which includes: establishing a three-dimensional thermal-mechanical coupling model of a variable cross-section segment based on the FLAC3d platform, the three-dimensional thermal-mechanical coupling model considering the nonlinear changes of thermal conductivity and specific heat capacity parameters with temperature during the frozen soil phase change process; introducing a freezing front expansion rate prediction algorithm, combining on-site ground temperature monitoring data, and dynamically correcting the cold source distribution and frost heave force boundary conditions in the three-dimensional thermal-mechanical coupling model; and establishing a three-level safety threshold based on the three-dimensional thermal-mechanical coupling model, including frost heave force, temperature gradient, and displacement deformation.

[0008] Implementing a hierarchical control strategy, the implementation of which includes: dividing the construction into three stages: a pre-control period, an excavation period, and a support period; during the pre-control period, taking heating and insulation measures for the groundwater-rich sections of the tunnel to keep the groundwater in a liquid state; during the excavation period, adopting a construction process combining an advanced small pilot tunnel with a reverse top-lift excavation method; during the support period, implementing asymmetric support based on the simulation results of the three-dimensional thermal-mechanical coupling model to address the uneven stress characteristics of the variable-section section; establishing a closed-loop system of monitoring, simulation, feedback, and adjustment, adjusting construction parameters when monitoring data triggers a second-level threshold, and suspending construction and performing grouting reinforcement when a third-level threshold is triggered;

[0009] Carry out refined construction, which includes: implementing precise temperature field control, using variable frequency fans to adjust wind speed to ensure that the tunnel face temperature is maintained within a preset range; installing a phase change energy storage insulation layer, and setting a composite phase change material interlayer on the outside of the lining to absorb construction heat and delay the warming of frozen soil; building a bionic drainage system, laying a high-density PE drainage pipe network, and achieving directional drainage of groundwater;

[0010] The application of an intelligent construction platform includes: building a monitoring platform that integrates BIM and GIS, integrating thermal infrared imaging, earth pressure cells and steel stress gauges to achieve real-time monitoring of temperature, stress and displacement; comparing monitoring data with the prediction results of the three-dimensional thermal-mechanical coupling model, dynamically optimizing construction parameters, and ensuring the safety of construction of variable-section sections in high-altitude tunnels.

[0011] Preferably, the parameter system included in the three-dimensional thermomechanical coupling model in constructing the frozen soil thermomechanical coupling model includes: environmental parameters, frozen soil physical parameters, mechanical parameters and hydraulic parameters; wherein the frozen soil physical parameters include the thermal conductivity λ(T) and specific heat capacity C(T) that vary with temperature, and the mechanical parameters include the elastic modulus E(T) that varies with temperature, and the parameters maintain continuity and smoothness within the phase change range near the freezing point as they vary with temperature.

[0012] Preferably, the freezing front expansion speed prediction algorithm is based on the numerical solution of the Stefan problem, taking into account the influence of phase change latent heat; the dynamic correction includes: using the measured temperature field data to inversely calculate the heat flux density and freezing rate, and predicting the evolution relationship of the freezing front position over time.

[0013] Preferably, the three-level safety thresholds include: the first-level threshold of the displacement deformation index is less than 5mm, the second-level threshold is 5-10mm, and the third-level threshold is greater than 10mm; the first-level threshold of the temperature gradient index is less than 0.5℃ / m, the second-level threshold is 0.5-1.0℃ / m, and the third-level threshold is greater than 1.0℃ / m; the first-level threshold of the frost heave force index is less than 0.5MPa, the second-level threshold is 0.5-1.0MPa, and the third-level threshold is greater than 1.0MPa; among them, if any indicator reaches the corresponding level, the overall safety level is determined to be that level.

[0014] Preferably, the closed-loop system in the implementation of the hierarchical control strategy also includes corresponding response measures: the first-level response is to maintain the original construction plan and maintain the regular monitoring frequency; the second-level response is to adjust the construction parameters, including halving the excavation footage, strengthening support, adding an insulation layer, and increasing the monitoring frequency; the third-level response is to suspend construction, implement cement-bentonite slurry grouting reinforcement, with a grouting pressure of 0.5 to 1.0 MPa, and hold an expert consultation to make a decision.

[0015] Preferably, the advanced small pilot tunnel and reverse top-lift excavation method includes the following construction steps:

[0016] Pilot tunnel construction is carried out first. A pilot tunnel is set up inside the small-section cavern for climbing construction. The slope should not exceed 30°. Spray anchor support is used and a steel frame is set up. After climbing to the vault area of ​​the large-section cavern, horizontal construction is carried out 5m forward to form a working space.

[0017] Excavation on both sides of the top: Excavation construction is carried out on both sides of the upper working space of the large-section cavern. After excavation on one side, spray anchor support is carried out in time, and then excavation on the other side is carried out;

[0018] Forward excavation construction: after the expansion and support of the upper step of the large-section cavern is completed, excavate forward for a distance to expand the working space of the upper step;

[0019] Reverse top-lift construction, reverse excavation construction of the upper steps of the large-section cavern, first remove the pilot tunnel steel frame one by one and then excavate a section, strictly control the construction step distance, and provide timely support after the excavation is completed;

[0020] During the construction of the lower part of the reverse cantilever section, the middle step is excavated in sections, first excavating the middle part to form a slag discharge channel, and then excavating on both sides;

[0021] The remaining parts of the construction will be carried out in a top-down order.

[0022] Preferably, the asymmetric support includes: using double-layer glass fiber anchors on the outside of the gradual expansion section, with the first layer having a length of 4m and a spacing of 0.8m×0.8m, and the second layer having a length of 6m and a spacing of 1.2m×1.2m, and the tensile strength of the glass fiber anchors is not less than 500MPa; arranging a thermosensitive shape memory alloy lining on the inside, the temperature-deformation response accuracy of the thermosensitive shape memory alloy lining is ±0.1°C, and the lining is embedded in the secondary lining concrete in a grid shape with a grid size of 50cm×50cm; differentiated design is performed on the spray mix thickness, steel frame spacing and anchor density, with the outer spray mix thickness being 15-20cm and the inner being 10-15cm, the outer steel frame spacing being 0.5-0.8m and the inner being 0.8-1.2m, the outer anchor density being 1.0m×1.0m and the inner being 1.5m×1.5m.

[0023] Preferably, the precise control of the temperature field includes: configuring a ventilation system composed of a main fan and an auxiliary fan, the air volume of the main fan is 50,000-100,000 m³ / h, and the air volume of the auxiliary fan is 10,000-30,000 m³ / h; based on feedback from the temperature sensor, using a variable frequency fan to automatically adjust the wind speed within the range of 0.5 to 2.0 m / s to control the face temperature to be maintained at -5°C ± 2°C; setting a local heating system in the groundwater enrichment section, including a low-concentration methane combustion heating device and a thermostat with an accuracy of ±1°C, to maintain the temperature around the drainage pipe at 1-3°C to prevent water from freezing without excessive energy consumption.

[0024] Preferably, the phase change energy storage insulation layer is composed of a composite structure of an organic phase change material and an inorganic phase change material, the organic phase change material is n-octadecane with a melting point of 28 to 30°C, the inorganic phase change material is a hydrated salt with a melting point of about -10°C, the phase change temperature range of the composite phase change material is -10 to 0°C, and the latent heat is not less than 200kJ / kg; the bionic drainage system imitates the fractal structure of plant roots, including a main pipe with a diameter of 100mm, a branch pipe with a diameter of 50mm and a branch pipe with a diameter of The capillary tubes are 10-20 mm in thickness, the porosity of the high-density PE drainage network is not less than 85%, the antifreeze performance is no brittleness at -40°C, the circumferential spacing is 1.5-2.0 m, and the longitudinal spacing is 3.0-5.0 m, forming a three-dimensional network; the drainage system also includes cold-proof drainage holes arranged at the lowest point of the tunnel and the groundwater-enriched section, with a spacing of 50-100 m, a diameter of 0.8-1.0 m, a length of 5-10 m, and an inclination of 3-5°. A heating cable is installed in the hole to ensure that the drainage channel is unobstructed.

[0025] Preferably, the monitoring platform integrating BIM and GIS includes:

[0026] A multi-source data acquisition network was deployed, including temperature sensors spaced 5m apart, stress sensors at key locations, and displacement sensors spaced every 10m. A fiber Bragg grating system was deployed longitudinally along the tunnel to enable distributed strain monitoring with an accuracy of 1με. An infrared thermal imager was installed at the tunnel face, scanning at a rate of once per hour and with a resolution of 0.1°C.

[0027] The data processing and visualization module filters, removes noise, and identifies outliers in the raw monitoring data. It then constructs a digital twin of the tunnel based on the BIM model, visually displaying the monitoring data. Different safety levels are color-coded: green for safety, yellow for caution, and red for danger.

[0028] An intelligent decision support system predicts the changing trends of key parameters based on historical data, explores the relationships between temperature, stress, and deformation parameters, intelligently recommends adjustment plans based on the current monitoring status, identifies potential safety risks in advance, and provides early warning information;

[0029] Among them, the monitoring frequency in the first-level safety state is times / 6 hours, the second-level is times / 2 hours, and the third-level is continuous monitoring.

[0030] The beneficial effects achieved by the present invention include:

[0031] 1. By establishing a multi-field coupled numerical model and considering the nonlinear changes of parameters during the frozen soil phase transition process, the accuracy of the construction scheme design is improved;

[0032] 2. Adopting graded safety thresholds and corresponding graded control strategies, accurate identification and dynamic management of construction risks are achieved;

[0033] 3. The innovative use of a small pilot tunnel in advance, reverse top-lift excavation, and asymmetric support technology solved the stability problem during construction of the variable-section section.

[0034] 4. Through precise temperature field control and phase change energy storage insulation layer design, frost heave damage is effectively suppressed;

[0035] 5. By utilizing the intelligent construction platform integrating BIM and GIS, visual monitoring and intelligent decision-making support for the entire construction process are achieved.

[0036] Compared with the existing technology, the present invention constructs a systematic and intelligent tunnel construction solution, combining the permafrost thermal coupling theory with modern digital construction technology, providing theoretical guidance and technical support for the safe and efficient construction of variable-section tunnels in high-altitude and cold areas. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is an overall flow chart of the thermal-mechanical coupling safety graded control construction method for the variable-section frozen soil section of a high-altitude cold tunnel according to the present invention;

[0038] Figure 2 Schematic diagram of the frozen soil thermal-mechanical coupling model constructed in the present invention;

[0039] Figure 3 Schematic diagram of the hierarchical control strategy system of the present invention;

[0040] Figure 4 This is a flowchart of the pilot tunnel construction process of the present invention;

[0041] Figure 5 This is a flow chart of the construction process of expanding and excavating on both sides of the top of the present invention;

[0042] Figure 6 This is a flow chart of the forward excavation construction process of the present invention;

[0043] Figure 7 This is a flow chart of the reverse roof construction process of the present invention;

[0044] Figure 8 This is a process flow chart of the lower part construction of the reverse cantilevered section of the present invention;

[0045] Figure 9 This is a flow chart of the construction process for the remaining parts of the present invention;

[0046] Figure 10 This is another construction process flow chart of the remaining parts of the present invention;

[0047] Figure 11 This is the architecture diagram of the intelligent construction platform integrating BIM and GIS in the present invention;

[0048] Figure 12 This is a detailed schematic diagram of the three-dimensional network structure of the bionic drainage system of the present invention. DETAILED DESCRIPTION

[0049] The following embodiments of the present invention are described in detail with reference to the accompanying drawings. In the absence of conflict, the following embodiments and features in the embodiments may be combined with each other.

[0050] See also Figure 1 The present invention provides a method for safe, graded control of frozen soil thermal coupling in variable-section sections of high-altitude tunnels. This method includes four core steps: constructing a frozen soil thermal coupling model, implementing a graded control strategy, conducting refined construction, and applying an intelligent construction platform. These four steps form an integrated system that works together to achieve safe construction control of variable-section sections of high-altitude tunnels in frozen soil environments.

[0051] In a preferred embodiment of the present invention, a three-dimensional thermomechanical coupling model of a variable cross-section segment is established based on the FLAC3d platform. The FLAC3d platform is a software platform suitable for three-dimensional numerical analysis of geotechnical engineering. It uses an explicit finite difference method and can effectively simulate large deformation problems of nonlinear materials, making it particularly suitable for thermomechanical coupling analysis of frozen soils.

[0052] See also Figure 2 The three-dimensional thermomechanical coupling model of the present invention takes into account the nonlinear changes of parameters such as thermal conductivity and specific heat capacity with temperature during the frozen soil phase transition process. The nonlinear relationship between these parameters can be expressed as follows:

[0053] For thermal conductivity λ(T):

[0054]

[0055] in, is the thermal conductivity of unfrozen soil, with a typical value of 1.2 to 1.8 W / (m·K); is the thermal conductivity of frozen soil, with a typical value of 2.0 to 2.5 W / (m·K); is the temperature sensitivity coefficient, usually ranging from 0.5 to 1.5; is the current temperature in °C; is the freezing temperature, usually 0°C. For specific heat capacity C(T):

[0056]

[0057] in, is the specific heat capacity of solid materials, with a typical value of 800 to 1200 J / (kg·K); is the latent heat of phase change, which is about 334000 J / kg; is a shape parameter that controls the width of the phase transition interval and is generally set between 0.2 and 0.5; is the current temperature in °C; Freezing temperature, usually 0°C.

[0058] The present invention further introduces a freezing front expansion velocity prediction algorithm based on the numerical solution of the Stefan problem, taking into account the influence of phase change latent heat. Specifically, the freezing front expansion velocity v can be expressed as:

[0059]

[0060] in, is the thermal conductivity, unit is W / (m·K); is the temperature gradient, in K / m; is the soil density, in kg / m 3 is the latent heat of phase change, unit is J / kg; is the thickness of the freezing front, in meters.

[0061] In practical applications, the present invention uses on-site temperature sensors to acquire ground temperature monitoring data. Combined with the aforementioned algorithm, this data is dynamically modified to reflect the distribution of cold sources and the frost heave boundary conditions in the model. Preferably, temperature sensors are placed every 5 meters along the tunnel axis, collecting real-time temperature data at a sampling rate of 10 minutes. This data is then uploaded to the central processing unit via a wireless transmission module.

[0062] Based on the established three-dimensional thermal-mechanical coupling model, the present invention establishes three levels of safety thresholds for frost heave, temperature gradient, and displacement. These thresholds are determined based on extensive engineering experience and numerical simulation results, providing a quantitative basis for hierarchical construction control.

[0063] Specifically, the first-level threshold for the displacement deformation index is less than 5mm, the second-level threshold is 5-10mm, and the third-level threshold is greater than 10mm. This threshold is set based on the geometric characteristics of the variable-section tunnel and the deformation allowable value of the support structure, taking into account the elastic deformation limit of the support structure and the stability requirements of the surrounding rock. The first-level threshold for the temperature gradient index is less than 0.5℃ / m, the second-level threshold is 0.5-1.0℃ / m, and the third-level threshold is greater than 1.0℃ / m. This threshold is based on research results on the thermal effects of frozen soil. Excessive temperature gradients can lead to uneven frost heave and stress concentration. The first-level threshold for the frost heave force index is less than 0.5MPa, the second-level threshold is 0.5-1.0MPa, and the third-level threshold is greater than 1.0MPa. This threshold takes into account the bearing capacity and safety factor of the support structure.

[0064] In the present invention, if any indicator reaches a corresponding level, the overall safety level is determined to be that level. This conservative determination strategy ensures the reliability of construction safety.

[0065] See also Figure 3 ,The present invention divides the construction into three stages : pre-control ,stage, excavation stage, and support stage, and formulates differentiated ,control strategies based on the characteristics of each stage.

[0066] During the pre-control period, addressing the unique characteristics of high-altitude, cold-weather tunnels, this invention employs heating and insulation measures to treat groundwater-rich sections of the tunnel, keeping the groundwater in a liquid state and preventing it from transforming into ice and clogging the drainage system. This innovative approach differs from traditional freezing treatment methods and is more suitable for the actual conditions of this project. In practice, a local heating system consisting of a low-concentration methane combustion heating device controls the temperature around the drainage pipes to between 1 and 3°C. This temperature range ensures that the water does not freeze while avoiding energy waste caused by overheating.

[0067] During the excavation phase, this invention utilizes an innovative construction process combining a small pilot tunnel ahead of the tunnel and reverse top-excavation expansion. This technique combines the advantages of bench excavation and cantilever tunnel boring, adapting to the unique requirements of variable-section sections in high-altitude and cold-altitude tunnels. The specific process flow will be described in detail later.

[0068] During the support period, the present invention implements asymmetric support based on simulation results from a thermomechanical coupling model to address the uneven stress characteristics of the variable cross-section. This support method takes into account the uneven stress caused by the changing geometry of the variable cross-section. Through differentiated design, the support structure accurately responds to the stress state in different areas.

[0069] To ensure safety and control throughout the construction process, this invention establishes a closed-loop system: monitoring → simulation → feedback → adjustment. When monitoring data triggers a secondary threshold, the system automatically adjusts construction parameters; when a tertiary threshold is triggered, construction is suspended and grouting reinforcement is performed. This hierarchical response mechanism enables early identification of construction risks and proactive intervention.

[0070] In one embodiment of the present invention, the closed-loop system's response measures specifically include: a first-level response maintains the original construction plan and the normal monitoring frequency (times / 6 hours); a second-level response adjusts construction parameters, including halving the excavation step from 1 meter to 0.5 meters, strengthening support (increasing anchor density by 30%), adding a 5-cm-thick insulation layer, and increasing the monitoring frequency to times / 2 hours; a third-level response suspends construction and implements cement-bentonite grouting reinforcement with a pressure controlled within the range of 0.5 to 1.0 MPa. Continuous monitoring is initiated, and expert consultation and decision-making are held. This gradient response design ensures safety while avoiding efficiency losses caused by excessive intervention.

[0071] The refined construction of the present invention mainly includes three aspects: implementing precise control of the temperature field, setting a phase change energy storage insulation layer, and building a bionic drainage system.

[0072] The precise control of the temperature field of the present invention is achieved through an intelligent ventilation system. Specifically, a variable frequency fan is used to control the wind speed to ensure that the face temperature is maintained within a preset range of -5°C ± 2°C. The selection of this temperature range is based on the research results of the mechanical properties of frozen soil, which can effectively inhibit the thawing and settlement of frozen soil without causing additional frost heave force caused by excessive freezing. When the tunnel length exceeds 500m, a temporary ventilation shaft is set up every 300 to 500m to optimize the ventilation efficiency. This design fully takes into account the actual problem of increased ventilation difficulty in long tunnels and effectively improves the air quality and temperature conditions of the working face.

[0073] This invention incorporates a composite phase change material (PCM) interlayer on the exterior of the lining, creating a phase-change energy storage and insulation layer. Within the phase-change temperature range of -10°C to 0°C, this material absorbs heat generated during construction and releases it as the permafrost temperature fluctuates, thus regulating the temperature. This passive temperature control method requires no additional energy input and achieves long-term temperature management. The PCM material is preferably a composite structure of n-octadecane (melting point 28-30°C) and hydrated salt (melting point approximately -10°C), with a phase-change latent heat of no less than 200 kJ / kg. This allows for effective heat absorption and release, stabilizing temperature fluctuations.

[0074] The present invention also constructs a bionic drainage system that mimics the fractal structure of plant roots and lays out a high-density PE drainage network with a porosity of no less than 85%, achieving directional drainage of groundwater. This bionic design overcomes the problem of traditional drainage systems being easily clogged in frozen soil environments, ensuring the long-term and effective operation of the drainage system. Preferably, the drainage network includes a main pipe with a diameter of 100 mm, a branch pipe with a diameter of 50 mm, and a capillary tube with a diameter of 10 to 20 mm, with a circumferential spacing of 1.5 to 2.0 m and a longitudinal spacing of 3.0 to 5.0 m, forming a three-dimensional drainage network.

[0075] See also Figure 11 This invention builds a monitoring platform that integrates BIM and GIS, integrating thermal infrared imaging, fiber Bragg grating sensors, earth pressure cells, and steel strain gauges to achieve real-time monitoring of temperature, stress, and displacement. This fusion design organically combines tunnel information (BIM) with information about the surrounding geographic environment (GIS), providing a more comprehensive perspective for construction monitoring.

[0076] The monitoring platform's layout preferably includes temperature sensors placed every 5 meters along the tunnel axis, stress sensors at key stress-bearing locations (such as variable-section transition sections and support structure connections), and displacement sensors every 10 meters. A fiber Bragg grating system (FBG) system deployed longitudinally along the tunnel enables distributed strain monitoring with an accuracy of 1 με, providing high-precision data support for deformation monitoring. An infrared thermal imager installed at the tunnel face, with a scanning frequency of 1 scan per hour and a resolution of 0.1°C, provides a visual display of the temperature field distribution. This method compares monitoring data with the prediction results of a three-dimensional thermomechanical coupling model and, through data assimilation technology, continuously optimizes model parameters to improve prediction accuracy. When the deviation between the monitoring data and the predicted value exceeds 15%, the system automatically triggers a model parameter recalibration process to ensure that the model remains consistent with actual conditions. This dynamic optimization mechanism enables real-time adjustment of construction parameters, providing strong technical support for safe tunnel construction.

[0077] In a preferred embodiment of the present invention, the three-dimensional thermomechanical coupling model includes a parameter system comprising environmental parameters, frozen soil physical parameters, mechanical parameters, and hydraulic parameters. Environmental parameters primarily include external temperature and humidity; frozen soil physical parameters include thermal conductivity λ(T) and specific heat capacity C(T); mechanical parameters include elastic modulus E(T), Poisson's ratio, cohesion, and internal friction angle; and hydraulic parameters include permeability, porosity, and moisture content.

[0078] Both the physical and mechanical parameters of frozen soil vary with temperature, especially within the phase transition region near the freezing point. To ensure computational stability, the parameter variation curve must remain continuous and smooth within this region. For example, the elastic modulus E(T) can be expressed as:

[0079]

[0080] in, is the elastic modulus of unfrozen soil, with a typical value of 10-50 MPa; is the elastic modulus of frozen soil, with a typical value of 50 to 200 MPa; is the temperature sensitivity coefficient, usually ranging from 0.3 to 1.0; is the current temperature in °C; The freezing temperature is typically 0°C. In actual engineering applications, the specific values ​​of these parameters are determined through field sampling tests and then imported into the FLAC3D model. The model meshing adopts a variable density strategy, with a mesh size of 0.5m in variable cross-section sections and stress concentration areas, and 1-2m in other areas, to balance computational accuracy and efficiency.

[0081] The freezing front expansion rate prediction algorithm is based on a numerical solution to the Stefan problem, taking into account the influence of latent heat of phase change. Using field temperature monitoring data, the algorithm inversely calculates heat flux and freezing rate to predict the temporal evolution of the freezing front position. In engineering practice, the freezing front position X(t) can be expressed as:

[0082]

[0083] in, is the freezing coefficient, which is related to the thermal physical properties of the soil and the boundary conditions, and the unit is m / s 0.5 ; is the freezing time, in seconds.

[0084] Freezing coefficient It can be calculated as follows:

[0085]

[0086] in, is the thermal conductivity, unit is W / (m·K); is the temperature difference, in K; is the latent heat of phase change, unit is J / kg; is the soil density, in kg / m 3 ; is the error function, To pass the equation Please solve.

[0087] Dynamic correction involves using measured temperature field data to inversely calculate heat flux and freezing rate, predicting the temporal evolution of the freezing front's position. In practice, temperature sensor data is filtered and corrected, then fitted to the temperature distribution curve using the least squares method to calculate the temperature gradient and, in turn, the heat flux. Based on the heat flux and the Stefan condition, the freezing rate is then calculated, and the freezing front's position is predicted. This dynamic correction mechanism ensures the model's ability to promptly respond to changes in actual field conditions, improving prediction accuracy.

[0088] See also Figure 4-12 The advanced small pilot tunnel + reverse top-lifting and expansion method of the present invention includes six systematic construction steps: advance pilot tunnel construction, expansion on both sides of the top, forward expansion construction, reverse top-lifting construction, construction of the lower part of the reverse top-lifting section and construction of the remaining parts.

[0089] The first step is to construct a pilot tunnel. A pilot tunnel is set up inside the small-section cavern for climbing construction. The slope is determined according to the relative height difference and length ratio of the two caverns, but the maximum does not exceed 30°. This slope limit takes into account the climbing ability of construction equipment (such as excavators and loaders). Generally speaking, the maximum climbing ability of crawler excavators is about 30°, and the climbing ability of wheel loaders is about 25°. The pilot tunnel is supported by sprayed anchors and steel frames are set. The spacing between steel frames is adjusted according to the surrounding rock level. Generally, the spacing is 0.8m for Grade III surrounding rock, 0.6m for Grade IV surrounding rock, and 0.4m for Grade V surrounding rock. After the pilot tunnel climbs to the vault area of ​​the large-section cavern, it is constructed horizontally 5m forward to form a working space. This 5m working space is supported by sprayed anchors and no steel arch frame is set, creating conditions for subsequent excavation.

[0090] The second step is to expand the top on both sides. Once the 5m working space above the large-section cavern is established, expansion construction is carried out on both sides. Excavation is carried out on a single-side principle, that is, one side is excavated first, and after the support is completed, the other side is excavated. This method reduces the exposed area of ​​the surrounding rock and reduces the risk of deformation. After excavation, spray anchor support is carried out promptly. The thickness of the sprayed C25 concrete is 8cm. The anchor rods are full-length bonded anchor rods with a length of 22mm and a length of L = 3.0m, with a spacing of 1.0m x 1.0m. After the upper step of the entire large-section cavern is excavated and formed, the steel arch frame is constructed and the concrete is sprayed to the designed thickness (usually 15-20cm).

[0091] The third step is forward excavation. Once the support for the upper steps of the large-section cavern is complete, a preliminary working surface is formed. Excavation is then advanced a distance (usually 3-5 meters) to further expand the working space on the upper steps. This step aims to create sufficient operating space for reverse top extraction and verify the reliability of the support.

[0092] The fourth step is reverse top-lift construction. Utilizing the working space created by the previous steps, reverse excavation is performed to expand the upper steps of the large-section cavern. During reverse construction, a pilot steel frame is first removed, and then the excavation is continued a certain distance. Construction step distances are strictly controlled (generally no more than 1 meter). Support is immediately implemented after excavation is completed. This reverse construction method has the advantage of fully utilizing the existing working surface and minimizing disturbance to the surrounding rock above. It is particularly suitable for variable-section construction in high-altitude frozen soil conditions.

[0093] The fifth step is to construct the lower portion of the reverse cantilever section. After the reverse cantilever construction of the upper step of the large-section cavern is completed, the lower portion of the reverse cantilever section will be constructed. A segmented excavation strategy is employed for the middle step, first excavating the center to create a slag discharge channel approximately 2-3 meters wide, followed by excavation on both sides. This excavation sequence ensures a clear slag discharge channel throughout the construction process, improving construction efficiency.

[0094] Step 6: Construction of the remaining sections. Once the intermediate steps are complete, the reverse cantilevering of the pilot tunnel is complete, and the working space is established. Construction of the remaining sections of the large-section cavern proceeds from top to bottom. This sequence adheres to the fundamental principle of top-down construction in geotechnical engineering and maximizes construction safety.

[0095] To address the uneven stress characteristics of variable-section sections, the present invention adopts asymmetric support technology. Double-layer glass fiber anchors are used on the outside of the expanding section. The first layer is 4m long and spaced 0.8m x 0.8m apart, while the second layer is 6m long and spaced 1.2m x 1.2m apart. The tensile strength of the glass fiber anchors is not less than 500MPa. This strength requirement ensures that the anchors can withstand the tensile stress generated by frost heave. Anchor installation must be perpendicular to the excavation surface, with a reserved depth of not less than 50mm and an anchor strength of not less than 25MPa.

[0096] A thermosensitive shape memory alloy lining is placed on the inner side. Its temperature-deformation response accuracy is ±0.1°C and it is embedded in the secondary lining concrete in a 50 cm x 50 cm grid pattern. The thermosensitive shape memory alloy is a TiNi-based material with a transition temperature range of -20°C to +10°C and a maximum recovery strain of 4% to 5%. When the temperature drops, the alloy undergoes a martensitic phase transformation, producing a preset deformation that effectively offsets some frost heave forces and reduces stress on the support structure.

[0097] This invention features differentiated designs for shotcrete thickness, steel frame spacing, and anchor density. The outer shotcrete thickness is 15-20 cm, while the inner is 10-15 cm. The outer steel frame spacing is 0.5-0.8 m, while the inner is 0.8-1.2 m. The outer anchor density is 1.0 m x 1.0 m, while the inner anchor density is 1.5 m x 1.5 m. This differentiated design, based on the local stress distribution calculated using a thermomechanical coupling model, implements reinforcement measures in areas with higher stress, ensuring effective support while avoiding resource waste.

[0098] To address the uneven stress distribution within variable-section sections, this invention employs asymmetric support technology. In variable-section tunnels, the outer side refers to the larger cross-section of the expanding section, while the inner side refers to the smaller cross-section. Due to this geometric change, the outer side experiences greater stress concentration than the inner side, necessitating a differentiated support design.

[0099] A double layer of fiberglass anchors is used on the outside of the expansion section. The first layer is 4 meters long and spaced 0.8 meters by 0.8 meters apart, while the second layer is 6 meters long and spaced 1.2 meters by 1.2 meters apart. A thermally sensitive shape memory alloy lining is placed on the inside, with a temperature-deformation response accuracy of ±0.1°C.

[0100] This invention features differentiated designs for shotcrete thickness, steel frame spacing, and anchor density. The shotcrete thickness on the outside (the larger side of the diverging section) is 15-20 cm, while on the inside (the smaller side of the diverging section), it's 10-15 cm. The steel frame spacing on the outside is 0.5-0.8 m, while on the inside it's 0.8-1.2 m. The anchor density on the outside is 1.0 m x 1.0 m, while on the inside it's 1.5 m x 1.5 m. This differentiated design, based on the local stress distribution calculated by a thermomechanical coupling model, implements reinforcement measures in areas with higher stress, ensuring effective support while avoiding resource waste.

[0101] The precise temperature field control system of the present invention is primarily implemented through intelligent ventilation and cooling technology and a localized heating system. This technology includes a ventilation system consisting of a main fan and auxiliary fan, with the main fan providing an air volume of 50,000 to 100,000 m³ / h and the auxiliary fan providing an air volume of 10,000 to 30,000 m³ / h. Based on temperature sensor feedback, a variable-frequency fan automatically adjusts the wind speed within a range of 0.5 to 2.0 m / s to maintain the tunnel face temperature at -5°C ± 2°C.

[0102] Wind speed regulation is based on permafrost temperature control requirements and construction dust control requirements. When the temperature sensor detects a tunnel face temperature above -3°C, the system automatically increases wind speed; when the temperature drops below -7°C, it decreases. This closed-loop control strategy ensures stable construction ambient temperature. Furthermore, to address ventilation difficulties in long tunnels exceeding 500m, temporary ventilation shafts are installed every 300-500m to optimize ventilation efficiency.

[0103] In the groundwater-rich section, the present invention incorporates a localized heating system, including a low-concentration methane combustion heater and a thermostat with an accuracy of ±1°C. This maintains the surrounding temperature of the drainage pipe at 1-3°C, preventing freezing without excessive energy consumption. The heated area is clad in 10-15cm thick extruded polystyrene board to reduce heat loss and improve energy efficiency.

[0104] The phase-change energy storage and insulation layer of the present invention is composed of a composite structure of organic and inorganic phase-change materials. The organic phase-change material is n-octadecane, with a melting point of 28-30°C; the inorganic phase-change material is a hydrated salt, with a melting point of approximately -10°C. The composite phase-change material has a phase-change temperature range of -10°C to 0°C and a latent heat of no less than 200 kJ / kg.

[0105] This composite structural design leverages the advantages of different phase change materials: organic phase change materials (such as n-octadecane) have high latent heat of phase change and good chemical stability, but are relatively expensive; inorganic phase change materials (such as hydrated salts) are low-cost but may suffer from overcooling. This composite design overcomes the shortcomings of a single material and achieves a balance between performance and cost. The insulation layer has a base thickness of 5 cm, which can be optimized to 10-15 cm based on comparative analysis of thermomechanical coupling models and monitoring measurements. It is installed between the primary support and secondary lining, with a coverage rate of at least 90%.

[0106] Please refer to Figure 12 The bionic drainage system of this invention mimics the fractal structure of plant roots and consists of a 100mm diameter main pipe, 50mm diameter branch pipes, and capillaries with diameters of 10-20mm. The high-density PE drainage network has a porosity of no less than 85% and frost resistance of -40°C without embrittlement. The circumferential spacing is 1.5-2.0m, and the longitudinal spacing is 3.0-5.0m, forming a three-dimensional network. The inner wall of the pipe is coated with a hydrophilic nanomaterial to reduce the freezing initiation temperature. The pipe is buried at a depth of no less than 30cm to ensure drainage function even in frozen soil conditions.

[0107] The drainage system also includes cold-resistant drainage tunnels located at the lowest point of the tunnel and in areas with concentrated groundwater. These tunnels are spaced 50 to 100 meters apart, have a diameter of 0.8 to 1.0 meters, a length of 5 to 10 meters, and an inclination of 3 to 5 degrees. Heating cables are installed within the tunnels to ensure unobstructed drainage channels. When the water level rises to a preset warning line, an automatic monitoring system will sound an alarm, alerting construction personnel to perform maintenance.

[0108] See also Figure 11 The BIM and GIS integrated monitoring platform of the present invention includes a multi-source data acquisition network, a data processing and visualization module, and an intelligent decision support system.

[0109] The multi-source data acquisition network includes temperature sensors spaced 5 meters apart, stress sensors placed at key locations, and displacement sensors spaced every 10 meters. A fiber Bragg grating system deployed longitudinally along the tunnel enables distributed strain monitoring with an accuracy of 1 με, providing high-precision data support for deformation monitoring. An infrared thermal imager, scanning once per hour with a resolution of 0.1°C, is installed at the tunnel face, visually displaying the temperature distribution. Data transmission utilizes a hybrid wired and wireless transmission method to ensure signal stability and interference resistance.

[0110] The data processing and visualization module filters, removes noise, and identifies outliers from raw monitoring data. It then constructs a digital twin of the tunnel based on the BIM model, visually displaying the monitoring data. Different safety levels are color-coded: green indicates safety, yellow indicates caution, and red indicates danger. GIS integration integrates data on the tunnel itself and the surrounding geological environment, providing a macro perspective and comprehensive information support for construction decision-making.

[0111] The intelligent decision support system predicts the changing trends of key parameters based on historical data, explores the relationships between parameters such as temperature, stress, and deformation, and intelligently recommends adjustment plans based on the current monitoring status. This system proactively identifies potential safety risks and provides early warnings. The system uses machine learning algorithms (such as support vector machines and random forests) to establish a predictive model for parameter changes. The system typically provides 12 to 24 hours of lead time, allowing ample time for construction adjustments.

[0112] In this invention, the monitoring frequency is once every six hours in the first level safety state, once every two hours in the second level, and continuous monitoring in the third level. This gradient monitoring strategy balances the real-time nature of data acquisition with system operating costs, ensuring that more intensive monitoring data can be obtained as the risk level increases, providing timely support for decision-making.

[0113] The invention provides a method for safety graded control of frozen soil thermal coupling in variable-section sections of high-cold tunnels. By organically combining a frozen soil thermal coupling model with a safety graded control system, the method achieves coordinated control of thermal fields, force fields, and deformation fields during the construction of variable-section sections of high-cold tunnels.

[0114] In a tunnel project in a high-altitude, cold region, the method of this invention was used to control the maximum deformation of the surrounding rock to within 5 mm, a reduction of approximately 65% ​​compared to traditional methods. Construction efficiency increased by approximately 30%, support material usage decreased by approximately 20%, and the incidence of safety accidents was reduced to zero. These data fully demonstrate the significant effectiveness of this invention in improving construction safety and economic efficiency.

[0115] In addition, the method of the present invention has good adaptability and scalability, and can adjust and optimize parameters according to different engineering conditions. It is suitable for the construction of variable-section sections of tunnels in various high-altitude and cold areas.

[0116] In summary, the construction method for thermal-mechanical coupling safety graded control of frozen soil in variable-section sections of high-altitude tunnels provided by the present invention provides reliable technical support for tunnel engineering construction in high-altitude cold areas through systematic technological innovation and refined construction control, and has significant engineering application value and promotion prospects.

[0117] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications and improvements can be made without departing from the concept of the present invention, which all fall within the scope of protection of the present invention.

Claims

1. A construction method for thermal-mechanical coupling safety graded control of frozen soil in variable-section sections of high-altitude cold tunnels, characterized by: include: Constructing a frozen soil thermal-mechanical coupling model, the construction of which includes: establishing a three-dimensional thermal-mechanical coupling model of a variable cross-section segment based on the FLAC3d platform, the three-dimensional thermal-mechanical coupling model considering the nonlinear changes of thermal conductivity and specific heat capacity parameters with temperature during the frozen soil phase change process; introducing a freezing front expansion rate prediction algorithm, combining on-site ground temperature monitoring data, and dynamically correcting the cold source distribution and frost heave force boundary conditions in the three-dimensional thermal-mechanical coupling model; and establishing a three-level safety threshold based on the three-dimensional thermal-mechanical coupling model, including frost heave force, temperature gradient, and displacement deformation. Implementing a hierarchical control strategy, the implementation of which includes: dividing the construction into three stages: a pre-control period, an excavation period, and a support period; during the pre-control period, taking heating and insulation measures for the groundwater-rich sections of the tunnel to keep the groundwater in a liquid state; during the excavation period, adopting a construction process combining an advanced small pilot tunnel with a reverse top-lift excavation method; during the support period, implementing asymmetric support based on the simulation results of the three-dimensional thermal-mechanical coupling model to address the uneven stress characteristics of the variable-section section; establishing a closed-loop system of monitoring, simulation, feedback, and adjustment, adjusting construction parameters when monitoring data triggers a second-level threshold, and suspending construction and performing grouting reinforcement when a third-level threshold is triggered; Carry out refined construction, which includes: implementing precise temperature field control, using variable frequency fans to adjust wind speed to ensure that the tunnel face temperature is maintained within a preset range; installing a phase change energy storage insulation layer, and setting a composite phase change material interlayer on the outside of the lining to absorb construction heat and delay the warming of frozen soil; building a bionic drainage system, laying a high-density PE drainage pipe network, and achieving directional drainage of groundwater; The application of an intelligent construction platform includes: building a monitoring platform that integrates BIM and GIS, integrating thermal infrared imaging, earth pressure cells and steel stress gauges to achieve real-time monitoring of temperature, stress and displacement; comparing monitoring data with the prediction results of the three-dimensional thermal-mechanical coupling model, dynamically optimizing construction parameters, and ensuring the safety of construction of variable-section sections in high-altitude tunnels.

2. The method according to claim 1, characterized in that The parameter system included in the three-dimensional thermomechanical coupling model in constructing the frozen soil thermomechanical coupling model includes: environmental parameters, frozen soil physical parameters, mechanical parameters and hydraulic parameters; wherein the frozen soil physical parameters include the thermal conductivity coefficient λ(T) and specific heat capacity C(T) that change with temperature, and the mechanical parameters include the elastic modulus E(T) that changes with temperature. The parameters maintain continuity and smoothness within the phase change range near the freezing point as they change with temperature.

3. The method according to claim 1, characterized in that The freezing front expansion speed prediction algorithm is based on the numerical solution of the Stefan problem, taking into account the influence of phase change latent heat; the dynamic correction includes: using measured temperature field data to inversely calculate the heat flux density and freezing rate, and predicting the evolution relationship of the freezing front position over time.

4. The method according to claim 1, wherein The three-level safety thresholds include: the first-level threshold of the displacement deformation index is less than 5mm, the second-level threshold is 5-10mm, and the third-level threshold is greater than 10mm; the first-level threshold of the temperature gradient index is less than 0.5℃ / m, the second-level threshold is 0.5-1.0℃ / m, and the third-level threshold is greater than 1.0℃ / m; the first-level threshold of the frost heave force index is less than 0.5MPa, the second-level threshold is 0.5-1.0MPa, and the third-level threshold is greater than 1.0MPa; among them, if any indicator reaches the corresponding level, the overall safety level is determined to be that level.

5. The method according to claim 1, wherein The closed-loop system in the implementation of the hierarchical control strategy also includes corresponding response measures: the first-level response is to maintain the original construction plan and maintain the regular monitoring frequency; the second-level response is to adjust the construction parameters, including halving the excavation advance, strengthening support, adding an insulation layer, and increasing the monitoring frequency; the third-level response is to suspend construction, implement cement-bentonite slurry grouting reinforcement with a grouting pressure of 0.5 to 1.0 MPa, and hold an expert consultation to make a decision.

6. The method according to claim 1, characterized in that The advanced small pilot tunnel and reverse top excavation method includes the following construction steps: Pilot tunnel construction is carried out first. A pilot tunnel is set up inside the small-section cavern for climbing construction. The slope should not exceed 30°. Spray anchor support is used and a steel frame is set up. After climbing to the vault area of ​​the large-section cavern, horizontal construction is carried out 5m forward to form a working space. Excavation on both sides of the top: Excavation construction is carried out on both sides of the upper working space of the large-section cavern. After excavation on one side, spray anchor support is carried out in time, and then excavation on the other side is carried out; Forward excavation construction: after the expansion and support of the upper step of the large-section cavern is completed, excavate forward for a distance to expand the working space of the upper step; Reverse top-lift construction, reverse excavation construction of the upper steps of the large-section cavern, first remove the pilot tunnel steel frame one by one and then excavate a section, strictly control the construction step distance, and provide timely support after the excavation is completed; During the construction of the lower part of the reverse cantilever section, the middle step is excavated in sections, first excavating the middle part to form a slag discharge channel, and then excavating on both sides; The remaining parts of the construction will be carried out in a top-down order.

7. The method according to claim 1, characterized in that The asymmetric support includes: using double layers of glass fiber anchors on the outside of the gradual expansion section, with the first layer being 4m long and spaced 0.8m×0.8m apart, and the second layer being 6m long and spaced 1.2m×1.2m apart. The tensile strength of the glass fiber anchors is not less than 500MPa; arranging a thermosensitive shape memory alloy lining on the inside, with a temperature-deformation response accuracy of ±0.1°C, and embedding the thermosensitive shape memory alloy lining in the secondary lining concrete in a grid shape with a grid size of 50cm×50cm; and differentially designing the spray mix thickness, steel frame spacing, and anchor density. The spray mix thickness on the outside is 15-20cm, and on the inside is 10-15cm. The steel frame spacing on the outside is 0.5-0.8m, and on the inside is 0.8-1.2m. The anchor density on the outside is 1.0m×1.0m, and on the inside is 1.5m×1.5m.

8. The method according to claim 1, characterized in that The precise control of the temperature field includes: configuring a ventilation system consisting of a main fan and an auxiliary fan, with the main fan having an air volume of 50,000-100,000 m³ / h and the auxiliary fan having an air volume of 10,000-30,000 m³ / h; using a variable frequency fan to automatically adjust the wind speed within the range of 0.5 to 2.0 m / s based on temperature sensor feedback to control the tunnel face temperature to be maintained at -5°C ± 2°C; setting up a local heating system in the groundwater enrichment section, including a low-concentration methane combustion heating device and a thermostat with an accuracy of ±1°C, to maintain the temperature around the drainage pipe at 1-3°C, preventing water from freezing without excessive energy consumption.

9. The method according to claim 1, characterized in that The phase change energy storage insulation layer is composed of a composite structure of an organic phase change material and an inorganic phase change material. The organic phase change material is n-octadecane with a melting point of 28 to 30°C. The inorganic phase change material is a hydrated salt with a melting point of about -10°C. The phase change temperature range of the composite phase change material is -10 to 0°C, and the latent heat is not less than 200kJ / kg. The bionic drainage system imitates the fractal structure of plant roots, including a main pipe with a diameter of 100mm, a branch pipe with a diameter of 50mm, and a branch pipe with a diameter of 10 to 20mm capillary tube, the porosity of the high-density PE drainage network is not less than 85%, the anti-freeze performance is -40℃ without embrittlement, the circumferential spacing is 1.5~2.0m, and the longitudinal spacing is 3.0~5.0m, forming a three-dimensional network; the drainage system also includes cold-proof drainage holes set at the lowest point of the tunnel and the groundwater-enriched section, with a spacing of 50~100m, a diameter of 0.8~1.0m, a length of 5~10m, and an inclination of 3~5°. A heating cable is installed in the hole to ensure that the drainage channel is unobstructed.

10. The method according to claim 1, characterized in that The BIM and GIS integrated monitoring platform includes: A multi-source data acquisition network was deployed, including temperature sensors spaced 5m apart, stress sensors at key locations, and displacement sensors spaced every 10m. A fiber Bragg grating system was deployed longitudinally along the tunnel to enable distributed strain monitoring with an accuracy of 1με. An infrared thermal imager was installed at the tunnel face, scanning at a rate of once per hour and with a resolution of 0.1°C. The data processing and visualization module filters, removes noise, and identifies outliers in the raw monitoring data. It then constructs a digital twin of the tunnel based on the BIM model, visually displaying the monitoring data. Different safety levels are color-coded: green for safety, yellow for caution, and red for danger. An intelligent decision support system predicts the changing trends of key parameters based on historical data, explores the relationships between temperature, stress, and deformation parameters, intelligently recommends adjustment plans based on the current monitoring status, identifies potential safety risks in advance, and provides early warning information; Among them, the monitoring frequency in the first-level safety state is times / 6 hours, the second-level is times / 2 hours, and the third-level is continuous monitoring.

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