Simulation method for thermal probe static sounding permafrost penetration based on discrete element

Abstract

The invention discloses a simulation method for thermal probe static sounding permafrost penetration based on discrete elements. The simulation method comprises the following steps: establishing a multi-scale permafrost particle geometrical shape template database; constructing a cylindrical frozen soil three-dimensional discrete element model; setting particle attribute parameters, a contact model and parameters; establishing a thermal probe static sounding model and setting parameters; setting temperature boundary conditions of the thermal probe and solving the model; and heat conduction in the temperature static sounding penetration process is simulated, and conical tip resistance, side wall friction force and pore pressure are obtained through calculation. According to the method, a frozen soil heat-force-water-phase change four-field full-coupling simulation system under a discrete element framework is established, and the limitation of traditional continuous medium hypothesis is broken through; the dynamic association of the microscopic phase change and the macroscopic penetration resistance is realized, a theoretical support is provided for the application of a thermal probe static sounding technology in cold region engineering, the problem of multi-physical field coupling in a frozen soil environment is effectively solved, and a technical path is provided for related simulation researches.

Description

Technical Field

[0001] The present invention relates to the technical field of numerical simulation in geotechnical engineering, and particularly to a simulation method for the static penetration of a thermal probe into frozen soil based on the discrete element method. Background Art

[0002] As a key medium in cold region engineering, the mechanical and thermodynamic parameters obtained from in-situ tests of frozen soil are crucial for engineering safety. However, the traditional method of taking drilled samples to the laboratory for testing in in-situ tests of frozen soil has problems such as sample disturbance, scale effect, and low cost efficiency, and it is difficult to reflect the true characteristics of frozen soil under natural stress conditions. The thermal probe static penetration testing (H-CPT) technology, by integrating a heating module and multi-physical field sensors, can not only effectively penetrate hard frozen soil layers, but also dynamically study the soil response under freeze-thaw cycles, synchronously obtain multi-dimensional data such as mechanical, thermal, and electrical data, showing significant technical advantages, and has currently been popularized and applied in frozen soil areas. However, the penetration mechanism of H-CPT in frozen soil is still not clear, and key scientific issues such as the influence law of the ice-water phase change in frozen soil on the measured parameters of the thermal probe static penetration testing under thermo-mechanical coupling need to be urgently solved.

[0003] Traditional continuum models are difficult to accurately characterize the complex interactions of thermo-mechanical-hydraulic multi-field coupling in frozen soil. Although the existing discrete element method (DEM) simulations can better describe the mechanical behavior at the particle scale, they have not fully considered the synergistic action mechanism of heat conduction-hydraulic coupling-mechanical response. It is particularly worth noting that the existing DEM research mainly focuses on the static analysis of the mechanical properties of frozen soil specimens under preset temperature conditions, and fails to achieve a complete simulation of the dynamic penetration process of static penetration testing during real-time heat exchange. This research status has led to insufficient understanding of the mechanical response mechanism of frozen soil in a real thermo-mechanical coupling environment, and there is an urgent need to develop a dynamic penetration simulation method that can couple the heat exchange process. Therefore, carrying out research on the simulation of the static penetration of a thermal probe into frozen soil based on DEM and establishing a multi-field coupling model integrating heat conduction-phase change-mechanical failure have important theoretical value and engineering significance for revealing the microscopic response mechanism of frozen soil particles during the penetration process, optimizing the probe design, and improving the test accuracy. Summary of the Invention

[0004] Object of the Invention: The object of the present invention is to provide a simulation method for the static penetration of a thermal probe into frozen soil based on the discrete element method.

[0005] Technical Solution: The simulation method for the static penetration of a thermal probe into frozen soil based on the discrete element method according to the present invention includes the following steps:

[0006] Step 1: Establish a multi-scale geometric shape template database of frozen soil particles;

[0007] Step 2: Construct a three-dimensional discrete element model of cylindrical frozen soil;

[0008] Step 3: Set the particle property parameters, contact models and parameters;

[0009] Step 4: Establish a static cone penetration model of the thermal probe and set parameters;

[0010] Step 5: Set the temperature boundary conditions of the thermal probe and solve the model;

[0011] Step 6: Simulate the heat conduction during the static cone penetration of temperature, and obtain the tip resistance, sidewall friction and pore pressure through calculation.

[0012] Furthermore, the said Step 1 includes:

[0013] Use three-dimensional laser scanning technology to perform high-precision morphology scanning on soil particles in typical frozen soil areas, obtain particle geometric characteristics, and establish a multi-scale geometric shape template database of frozen soil particles, providing a basic template for the particle morphology reconstruction in the subsequent discrete element model.

[0014] Furthermore, the said Step 2 includes:

[0015] Based on the particle geometric database in Step 1, use irregular polyhedron particles to simulate the soil skeleton, spherical particles to simulate ice crystals, and droplet particles to simulate unfrozen water, construct a three-dimensional discrete element model of cylindrical frozen soil, and the volume ratios of soil, ice and water phases in the model are dynamically adjusted according to the actual ice content of the frozen soil, and the spatial distribution of the three-phase medium is ensured to conform to the structural characteristics of the frozen soil through the porosity control algorithm.

[0016] Furthermore, the said Step 3 includes:

[0017] Set corresponding physical parameters for soil particles, ice particles and water particles in the three-dimensional discrete element model of frozen soil constructed in Step 2, and set corresponding contact models and parameters for between soil particles, between ice particles, between water particles, between soil-ice particles, between soil-water particles, between ice-water particles, between soil particles and boundaries, between ice particles and boundaries, and between water particles and boundaries respectively.

[0018] Furthermore, the static cone penetration model of the thermal probe in the said Step 4 includes a thermal probe, a friction sleeve and a static cone penetration rod. Set the physical parameters of the probe, the friction sleeve and the static cone penetration rod respectively, and set the contact models and parameters of different frozen soil particles and the static cone penetration of the thermal probe. The probe, the friction sleeve and the static cone penetration rod form a whole and move downward at a uniform speed. When the probe sinks to the specified depth and stops, heat the surrounding soil, and establish a heat conduction simulation path with the frozen soil particles through the heat pipe algorithm.

[0019] Furthermore, the heat conduction simulation path includes the simulation of heat conduction between soil-ice-water particles in frozen soil media and the simulation of ice-water phase change caused by temperature changes.

[0020] Furthermore, the simulation of heat conduction between soil-ice-water particles in the frozen soil media includes:

[0021] Regarding the entire frozen soil particle system as a heat transfer network to consider the heat transfer between frozen soil particles, each particle of the system represents a heat storage device, and a virtual heat pipe connects the centers of mass of two contacting disks to transfer heat energy between two connected heat storage devices through the virtual heat pipe;

[0022] The heat conduction equation of a single reservoir is measured by the equation:

[0023]

[0024] where Q p represents the heat power flowing out of the heat storage tank in the actual pipeline p, represents the total heat power in N pipelines, Q v represents the heat source intensity of the heating wall, m represents the heat mass, and C v represents the specific heat at constant volume.

[0025] Furthermore, the heat power Q flowing out of the heat storage tank in the actual pipeline p p is expressed as:

[0026]

[0027] where ΔT represents the temperature difference between two heat storage devices at both ends of the pipeline, η represents the thermal resistance per unit length, and l p represents the length of the heat pipe;

[0028] The thermal resistance η per unit length is expressed as:

[0029]

[0030] where n represents the porosity of the soil mass, k represents the macroscopic electro-thermal coefficient, V b represents the volume of the particle, n b represents the sphere containing the center of mass within the measurement sphere, and N p represents the number of heat transfer pipes.

[0031] Furthermore, the simulation of ice-water phase change caused by temperature changes includes:

[0032] In the initial modeling stage, the system is constructed as a four-phase coupling system including rigid ice particles, soil particles, liquid water, and air medium;

[0033] When the environmental temperature is lower than the phase change critical point, the ice particles maintain a stable crystal structure, and the contact behavior with surrounding particles exhibits typical elastic characteristics. The contact overlap is controlled within a very small range to ensure numerical stability. At this time, heat conduction mainly depends on the contact heat transfer between solid-phase particles.

[0034] When the system is subjected to an external heat source, the ice particles begin to absorb heat, and their temperature field shows a gradient distribution characteristic. After reaching the phase change temperature threshold, a significant physical property transformation occurs in the particle system: on the one hand, the geometric shape changes from a regular crystal structure to a liquid cluster with a free surface; on the other hand, the material mechanics parameters change by an order of magnitude, and significant penetration overlap occurs between the liquid-phase particles and the surrounding medium.

[0035] Further, step 6 includes:

[0036] The tip resistance represents the normal contact force vector of the probe cone surface in real-time integration, and is expressed as:

[0037]

[0038] where q c represents the tip resistance, A c represents the surface area of the cone tip, θ i represents the angle between the cone wall and the vertical direction, F n represents the normal force between the particle and the cone wall, F t represents the tangential force between the particle and the cone wall, and i represents the total number of ice, soil, and water particles;

[0039] The side friction resistance fs is calculated by statistically analyzing the time history curve of the tangential force component on the surface of the casing;

[0040] A monitoring spherical domain with a radius of 50 mm is set around the probe, and the excess pore pressure is calculated through the stress tensor of water particles:

[0041]

[0042] where u is the pore pressure within the measurement circle, N w is the number of water particles within the measurement circle, P j is the internal pressure of the jth water particle, which is calculated by dividing the resultant normal contact force received by the jth water particle by the surface area, V j is the volume of the water particle, V w is the total volume of water particles.

[0043] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: By using a thermal probe to controllably heat the frozen soil around the penetration path, inducing ice-water phase change, and simultaneously monitoring key parameters such as tip resistance, sidewall friction, and pore water pressure during the penetration process in real time, thereby revealing the frozen soil penetration response mechanism under the coupling action of heat, force, and water; Initiating a fully coupled simulation system of heat, force, water, and phase change for frozen soil under the discrete element framework, breaking through the limitations of the traditional continuum hypothesis; Proposing an ice particle phase transformation algorithm under the heating conditions of thermal static cone penetration, realizing the dynamic correlation between microscopic phase change and macroscopic penetration resistance, providing theoretical support for the application of thermal probe static cone penetration technology in cold region engineering, effectively solving the problem of multi-physical field coupling in the frozen soil environment, and providing a technical path for related simulation research. Description of the Drawings

[0044] Figure 1 is the construction flow chart of the present invention;

[0045] Figure 2 is the discrete element model diagram of the thermal probe static cone penetration into frozen soil, where (a) is the diagram of the frozen soil sample; (b) is the diagram of the static cone penetration rod sample. Detailed Embodiment

[0046] The technical solution of the present invention will be further described below in conjunction with the drawings.

[0047] The simulation method of the thermal probe static cone penetration into frozen soil based on discrete element according to the present invention includes the following steps:

[0048] Step 1: Establish a multi-scale frozen soil particle geometry template database: Use three-dimensional laser scanning technology to perform high-precision morphology scanning on soil particles in typical frozen soil areas, obtain particle geometric characteristics (including angularity, flatness, and surface roughness), and establish a multi-scale frozen soil particle geometry template database to provide a basic template for particle morphology reconstruction in the subsequent discrete element model.

[0049] Step 2: Construct a three-dimensional discrete element model of cylindrical frozen soil: Based on the particle geometric data in Step 1, use irregular polyhedron particles to simulate the soil skeleton, spherical particles to simulate ice crystals, and droplet particles to simulate unfrozen water, and construct a three-dimensional discrete element model of cylindrical frozen soil with a radius of 5.0 meters and a height of 10.0 meters. The volume ratios of the soil, ice, and water phases in the model are dynamically adjusted according to the actual ice content (5%-40%) of the frozen soil, and the spatial distribution of the three-phase medium is ensured to conform to the structural characteristics of the frozen soil through the porosity control algorithm.

[0050] Step 3: Set the particle property parameters, contact model and parameters: Set the corresponding physical parameters for the soil particles, ice particles, and water particles in the three-dimensional discrete element model of frozen soil constructed in Step 2, and set the corresponding contact models and parameters for between soil particles, between ice particles, between water particles, between soil-ice particles, between soil-water particles, between ice-water particles, between soil particles and the boundary, between ice particles and the boundary, and between water particles and the boundary, respectively.

[0051] Step 4: Establish a thermal probe static cone penetration model and set parameters: The thermal probe static cone penetration model consists of three parts, the thermal probe, the friction sleeve, and the static cone penetration rod. The conical thermal probe (apex angle 60°, diameter 35.7 mm) and the cylindrical friction sleeve (surface area of \(150 cm^2\)) are assembled with rigid polyhedron particles, and virtual heat source nodes are embedded inside. The friction sleeve and the static cone penetration rod are cylinders and are both simulated with polyhedron particles. As shown in 2 Figure (a), the static cone penetration rod penetrates into the frozen soil sample, the yellowish-brown and brown represent two soil samples, and the blue represents ice particles. Figure (b) is a perspective view to show the situation of the static cone penetration rod in the frozen soil sample. Set the physical parameters of the probe, the friction sleeve, and the static cone penetration rod respectively, and set the contact models and parameters for the thermal probe static cone penetration between different frozen soil particles (including ice-soil-water three-phase frozen soil particles). Establish a heat conduction simulation path with the frozen soil particles through the heat pipe algorithm. Figure 2 The heat conduction simulation path during the thermal probe static cone penetration into frozen soil consists of two parts: The first part is the heat transfer simulation between soil-ice-water particles in the frozen soil medium, and the second part is the ice-water phase change simulation caused by temperature changes.

[0052] First part, calculate the heat transfer between frozen soil particles: Consider the entire frozen soil particle system as a heat transfer network to consider the heat transfer between frozen soil particles. Each particle in the system represents a heat storage device, and a virtual heat pipe connects the centers of mass of two contacting disks. Transfer heat energy between two connected heat storage devices through the virtual heat pipe.

[0053] The heat conduction equation of a single reservoir is measured by the equation:

[0054]

[0055]

[0056] where \(Q\) p represents the heat power flowing out of the heat storage tank in the actual pipeline \(p\), represents the total heat power in \(N\) pipelines, \(Q\) v represents the heat source intensity of the heating wall, \(m\) represents the heat mass, and \(C\) v represents the specific heat at constant volume.

[0057] The heat power \(Q\) flowing out of the heat storage tank in the actual pipeline \(p\)p Expressed as:

[0058]

[0059] Where ΔT represents the temperature difference between two heat storage devices at both ends of the pipeline, η represents the thermal resistance per unit length, and l p represents the length of the heat pipe;

[0060] The thermal resistance η per unit length is expressed as:

[0061]

[0062] Where n represents the porosity of the soil, k represents the macroscopic electro-thermal coefficient, and V b represents the volume of the particles, and n b represents the sphere containing the centroid within the measurement sphere, and N p represents the number of heat transfer tubes.

[0063] Second part, numerical simulation of the ice-water phase change process in frozen soil: achieved through a discrete system of multiphase media. In the initial modeling stage, the system is constructed as a four-phase coupled system including rigid ice particles, soil particles, liquid water, and air medium. When the environmental temperature is lower than the phase change critical point, the ice particles maintain a stable crystal structure, and their contact behavior with surrounding particles shows typical elastic characteristics, with the contact overlap controlled within a very small range to ensure numerical stability. At this time, heat conduction mainly depends on the contact heat transfer between solid-phase particles, and the air phase has a negligible contribution to the overall heat transfer due to its extremely low thermal conductivity. When the system is subjected to an external heat source, the ice particles start to absorb heat, and their temperature field shows a gradient distribution characteristic. After reaching the phase change temperature threshold, significant physical property changes occur in the particle system: on the one hand, the ice particles undergo solid-liquid phase change and are accompanied by a volume contraction effect, and their geometric shape changes from a regular crystal structure to a liquid cluster with a free surface; on the other hand, the material mechanics parameters change by several orders of magnitude, especially the sharp decrease in the elastic modulus leads to significant penetration overlap between the liquid-phase particles and the surrounding medium. This phase change process strictly follows the law of conservation of energy, and the heat accumulation of each discrete unit must meet the dual requirements of sensible heat and latent heat required to heat up from the initial temperature to the phase change point. Through this discretized modeling method of multi-physical field coupling, a unified description of the microstructural evolution and macroscopic mechanical response of the frozen soil system under thermal disturbance is achieved.

[0064] Step 5: Set the temperature boundary conditions of the thermal probe and solve the model; Set the temperature boundary conditions of the thermal probe and solve the model: First, set the constant temperature condition of the thermal probe at 60 °C to simulate the heating of the frozen soil medium for 30 s, and then cancel the constant temperature condition of the thermal probe to simulate the free dissipation of heat between the probe and the frozen soil medium for 300 s. During the heating process, the ice-water phase change process caused by temperature changes is considered in real time during the thermal transport simulation of the frozen soil medium.

[0065] Step 6: Simulate the heat conduction during the penetration process of the temperature static cone penetration test. At each time step, traverse the frozen soil particles (including soil, ice, and water particles) in contact with the cone tip, and calculate the tip resistance, sidewall friction, and pore pressure.

[0066] The tip resistance represents the real-time integral of the normal contact force vector on the cone surface of the probe, expressed as:

[0067]

[0068] where q c represents the tip resistance, A c represents the surface area of the cone tip, θ i represents the angle between the cone wall and the vertical direction, F n represents the normal force between the particle and the cone wall, F t represents the tangential force between the particle and the cone wall, and i represents the total number of ice, soil, and water particles;

[0069] The side friction resistance fs is calculated by statistically analyzing the time history curve of the tangential force component on the surface of the casing;

[0070] A monitoring spherical domain with a radius of 50 mm is set around the probe, and the excess pore pressure is calculated through the stress tensor of water particles:

[0071]

[0072] where u is the pore pressure within the measurement circle, N w is the number of water particles within the measurement circle, P j is the internal pressure of the j-th water particle, calculated by dividing the resultant normal contact force received by the j-th water particle by the surface area, V j is the volume of the water particle, V w is the total volume of water particles.

Claims

1. A simulation method for the static cone penetration of a thermal probe into frozen soil based on the discrete element method, characterized in that The steps include: Step 1: Establish a multi-scale frozen soil particle geometry template database; Step 2: Construct a three-dimensional discrete element model of cylindrical frozen soil; Step 3: Set particle property parameters, contact model and parameters; Step 4: Establish a thermal probe static penetration model and set parameters; Step 5: Set the thermal probe temperature boundary conditions and solve the model; Step 6: Simulate heat conduction during the penetration process of the temperature static penetration test and obtain the cone tip resistance, side wall friction and pore pressure through calculation.

2. The simulation method for the static cone penetration test of a thermal probe into frozen soil based on the discrete element method according to claim 1, characterized in that The step 1 comprises: Three-dimensional laser scanning technology is used to perform high-precision morphological scanning of soil particles in typical permafrost areas, obtain the geometric characteristics of the particles, and establish a multi-scale permafrost particle geometric shape template database, providing a basic template for the subsequent particle morphology reconstruction in the discrete element model.

3. The simulation method for the static cone penetration of frozen soil by a thermal probe based on the discrete element method according to claim 1, wherein The step 2 includes: Based on the particle geometry database from step 1, irregular polyhedron particles are used to simulate the soil skeleton, spherical particles are used to simulate ice crystals, and droplet particles are used to simulate unfrozen water. A three-dimensional discrete element model of cylindrical frozen soil is constructed. The volume ratio of the three phases of soil, ice, and water in the model is dynamically adjusted according to the actual ice content of the frozen soil, and a porosity control algorithm is used to ensure that the spatial distribution of the three-phase medium conforms to the structural characteristics of the frozen soil.

4. The simulation method for the static cone penetration test of a thermal probe into frozen soil based on the discrete element method according to claim 1, wherein, The step 3 comprises: Set corresponding physical parameters for the soil particles, ice particles, and water particles in the three-dimensional discrete element model of frozen soil constructed in step 2, and set corresponding contact models and parameters between soil particles, between ice particles, between water particles, between soil and ice particles, between soil and water particles, between ice and water particles, between soil particles and boundaries, between ice particles and boundaries, and between water particles and boundaries.

5. The simulation method for a thermal probe static cone penetration test into frozen soil based on the discrete element method according to claim 1, characterized in that The thermal probe static penetration model in step 4 includes a thermal probe, a friction sleeve and a static penetration rod. The physical parameters of the probe, friction sleeve and static penetration rod are set respectively, and the contact model and parameters of different frozen soil particles and the thermal probe static penetration are set. The probe, friction sleeve and static penetration rod form a whole and move downward at a uniform downward speed. When the probe sinks to a specified depth and stops, it heats the surrounding soil and establishes a heat conduction simulation path with the frozen soil particles through the heat pipe algorithm.

6. The simulation method for the static cone penetration of frozen soil by a thermal probe based on the discrete element method according to claim 5, wherein The heat conduction simulation path includes heat conduction simulation between soil-ice-water particles in the frozen soil medium and ice-water phase change simulation caused by temperature change.

7. The simulation method for the static cone penetration test of a thermal probe into frozen soil based on the discrete element method according to claim 6, characterized in that The heat conduction simulation between soil-ice-water particles in the frozen soil medium includes: The entire frozen soil particle system is considered as a heat transfer network to consider the heat transfer between frozen soil particles. Each particle in the system represents a heat reservoir. A virtual heat pipe connects the mass centers of the two contacting disks, and heat energy is transferred between the two connected heat reservoirs through the virtual heat pipe. The heat conduction equation for a single reservoir is measured by the equation: Among which Q p represents the heat power flowing out of the heat storage tank in the actual pipeline p, represents the total heat power in N pipelines, Q v represents the heat source intensity of the heating wall, m represents the heat mass, C v represents the specific heat at constant volume.

8. The simulation method for the static cone penetration test of a thermal probe into frozen soil based on the discrete element method according to claim 7, wherein, The thermal power Q flowing out of the heat storage tank in the actual pipeline p p is expressed as: where ΔT represents the temperature difference between two heat storage devices at both ends of the pipeline, η represents the thermal resistance per unit length, and l p represents the length of the heat pipe; The thermal resistance per unit length η is expressed as: where n represents the porosity of the soil mass, k represents the macroscopic electro-thermal coefficient, and V b represents the volume of the particles, and n b represents the sphere containing the centroid within the measurement sphere, and N p represents the number of heat transfer tubes.

9. The simulation method for the static cone penetration of a thermal probe into frozen soil based on the discrete element method according to claim 6, wherein The ice-water phase transition simulation caused by the temperature change includes: In the initial modeling stage, the system was constructed as a four-phase coupled system consisting of rigid ice particles, soil particles, unfrozen water, and air medium; When the ambient temperature is lower than the phase transition critical point, the ice particles maintain a stable crystal structure and contact with the surrounding particles. The contact overlap is controlled within a very small range to ensure numerical stability. At this time, heat conduction mainly relies on contact heat transfer between solid phase particles. When the system is affected by an external heat source, the ice particles begin to absorb heat, and their temperature field shows a gradient distribution characteristic. After reaching the phase change temperature threshold, a significant physical property transformation occurs in the particle system: on the one hand, the geometric morphology changes from a regular crystal structure to a liquid cluster with a free surface; on the other hand, the material mechanics parameters change by several orders of magnitude, and there is a significant penetration overlap between the liquid-phase particles and the surrounding medium.

10. The simulation method for the static cone penetration of a thermal probe into frozen soil based on the discrete element method according to claim 6, characterized in that, The said step 6 includes: The tip resistance represents the normal contact force vector of the probe cone surface in real-time integration, which is expressed as: Among them, q c represents the cone tip resistance, A c represents the cone head surface area, θ i represents the angle between the cone wall and the vertical direction, F n represents the normal force between the particle and the cone wall, F t represents the tangential force between the particle and the cone wall, and i represents the total number of ice, soil, and water particles; The side friction resistance fs is calculated by statistically analyzing the time history curve of the tangential force component on the casing surface; A monitoring spherical domain with a radius of 50 mm is set around the probe, and the excess pore pressure is calculated through the water particle stress tensor: where u is the measured pore pressure inside the circle, N w is the number of water particles inside the measured circle, P j is the internal pressure of the j-th water particle, calculated by dividing the normal contact resultant force received by the j-th water particle by the surface area, V j is the volume of the water particle, V w is the total volume of water particles.

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