Multi-field coupling control unsaturated soil direct shear test system and method

Abstract

The invention discloses a multi-field coupling control unsaturated soil direct shear test system and method, and belongs to the technical field of geotechnical engineering. The system comprises a high-pressure sealing pressure-bearing loading host, a shear box assembly integrating a pottery clay plate and a temperature control flow channel, a dual-channel environment control system and a servo loading and acquisition control system. Accurate control of matrix suction is realized by combining a high-pressure closed cavity with an axial translation technology, wide-temperature-range wrapped temperature control is realized through a shear box side wall integrated flow channel, active control of three complex normal boundary conditions of constant load, constant rigidity and constant volume is realized by adopting a normal servo driving module with a built-in sensor, and meanwhile, the normal servo driving module is used for driving the shear box side wall integrated flow channel to control the temperature of the shear box side wall integrated flow channel. Monotonic shearing and cyclic shearing of unsaturated soil are achieved through the horizontal shearing servo driving module. The problem that traditional equipment cannot accurately test static and dynamic characteristics of unsaturated soil in a multi-field coupling environment is solved, and a high-simulation test platform is provided for soil behavior research under complex working conditions.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering testing equipment technology, specifically to a multi-field coupled control system and method for direct shear testing of unsaturated soil. Background Technology

[0002] Unsaturated soils are widely used in practical engineering projects such as slopes, roadbeds, embankments, and geological disposal sites. Their mechanical properties are significantly affected by the coupling effects of multiple physical fields, including stress, matrix suction, and temperature. In-depth research on the shear strength and deformation behavior of unsaturated soils under multi-field coupling conditions is of great significance for assessing the long-term stability of engineering projects, preventing geological disasters, and ensuring the safety of major infrastructure.

[0003] Direct shear testing is a fundamental method for obtaining soil shear strength parameters. However, traditional direct shear apparatuses are typically open structures, making it impossible to control and measure the key state variable of unsaturated soil—matrix suction—during the test. This makes the test results difficult to reflect the soil mechanical behavior under real hydrological conditions, greatly limiting its application in unsaturated soil research. To control suction, the axis translation technique has been introduced, which requires the independent application of pore air pressure and pore water pressure within a closed cavity. While some existing improved devices have constructed pressure chambers to achieve suction control, they still have significant limitations: First, within the confined, high-pressure space, it is difficult to integrate an efficient and uniform temperature control system. Temperature control piping often interferes with the bottom drainage and suction passages, leading to spatial or thermal interference, making it impossible to accurately control suction while simulating wide-temperature environments (such as freeze-thaw cycles in seasonally frozen regions or high-temperature evaporation processes). Second, normal loading often relies on external weights, air pressure, or simple electric loading. The loading rod must penetrate the high-pressure chamber wall, and the resulting sealing friction resistance and the additional force exerted by the internal air pressure on the rod severely interfere with the accuracy of load measurements. Furthermore, the single loading method cannot simulate complex boundary conditions in actual engineering, such as elastic constraints (constant stiffness) or strict displacement limitations (constant volume) on soil. In addition, most existing direct shear apparatuses use unidirectional loading and cannot perform cyclic shear tests. However, in actual engineering projects, the subgrade soil is often subjected to traffic cyclic loads, and the reservoir bank slope soil is often subjected to wave cyclic loads. The soil stiffness degradation, fatigue failure, and liquefaction characteristics under these working conditions cannot be revealed by monotonic shear tests.

[0004] In recent years, although some studies have attempted to consider multi-field coupling effects in geotechnical tests, existing technologies often only allow for single-field control or simple two-field coupling. They lack a comprehensive system capable of high-precision, independent, and coordinated control of the "temperature-suction-complex mechanical boundary" on the same test platform. This technological bottleneck makes it difficult for laboratory tests to realistically reproduce the actual response of soil and rock masses in extreme or complex environments such as deep-earth energy storage, geological disposal of high-level radioactive waste, and lunar soil development, thus hindering the verification of relevant theoretical models and the optimization of engineering designs.

[0005] Therefore, developing a multi-field coupled direct shear test system that can integrate high-precision suction control, wide-temperature range temperature control, and active control of complex normal boundary conditions has become a key issue that urgently needs to be addressed in the field of geotechnical engineering testing technology. Summary of the Invention

[0006] (a) Purpose of the invention This invention aims to overcome the shortcomings of the prior art and provide a system and method for direct shear testing of unsaturated soil that can accurately, independently and collaboratively control matrix suction, temperature and complex normal boundary conditions, and realize multi-field coupling of heat, water and force.

[0007] (II) Technical Solution To achieve the above objectives, the present invention adopts the following technical solution: A multi-field coupled controlled direct shear test system for unsaturated soil includes: A high-pressure sealed pressure-bearing loading host includes a high-pressure resistant sealed pressure-bearing cavity and a precision chamber-penetrating transmission mechanism disposed on the sealed pressure-bearing cavity for the normal loading rod and the horizontal shearing rod to pass through. A shear box assembly integrating a clay plate and a temperature-controlled flow channel is disposed in the sealed pressure-bearing cavity. A high-air-intake clay plate is embedded in the center of the base of the shear box assembly. A drainage channel communicating with the clay plate is provided below it and connected to an external water pressure control system. A closed fluid circulation channel is provided inside the side wall of the shear box assembly. The closed fluid circulation channel is connected to an external temperature control device through an independent pipeline. A dual-channel environmental control system includes a suction control channel and a temperature control channel. The suction control channel includes a pneumatic control circuit connected to the sealed pressure-bearing cavity to apply pore gas pressure and a water pressure control circuit connected to the drainage channel to apply pore water pressure. The temperature control channel includes an external hot and cold air circulator connected to the closed fluid circulation channel. The servo loading and acquisition control system includes a normal servo drive module, a horizontal shear servo drive module, a built-in force sensor, a built-in displacement sensor, and a control unit. The built-in force sensor and the built-in displacement sensor are disposed in the sealed pressure-bearing cavity and are respectively connected to the normal loading rod and the shear box assembly. The end of the horizontal shear rod is connected to the shear box assembly through a rigid connector to realize horizontal push-pull reciprocating loading. The control unit is used to receive signals from the sensors and control the servo drive module to actively control the normal boundary conditions and horizontal shearing mode of the sample during the shearing process.

[0008] Furthermore, the sealed pressure-bearing cavity is made of stainless steel and is designed to withstand a pressure of not less than 2MPa; the precision through-chamber transmission mechanism can achieve low-friction movement of the normal loading rod while maintaining the airtightness of the sealed pressure-bearing cavity.

[0009] Furthermore, the built-in force sensor includes a normal force sensor and a horizontal shear force sensor, which are directly fixed to the end of the normal loading rod and the end of the horizontal shear rod, respectively.

[0010] Furthermore, the closed fluid circulation channel is a spiral flow channel surrounding the sidewall of the shear box.

[0011] Furthermore, the temperature control range of the external heating and cooling circulation unit is -30℃ to 70℃.

[0012] Furthermore, the water pressure control loop includes a high-precision pressure volume controller, which is connected to the drainage channel via a high-pressure resistant pipeline.

[0013] Furthermore, the control unit is configured to execute at least one of the following three normal boundary condition control modes and at least one of the following horizontal shear control modes: Normal boundary condition control mode: A constant normal load mode, wherein the normal servo drive module is controlled by the feedback signal of the normal force sensor to maintain a constant normal force acting on the specimen; A constant normal stiffness mode is used, in which the target normal force is calculated based on a preset stiffness value and the change in normal displacement collected by the displacement sensor, and the normal servo drive module is controlled to make dynamic adjustments. A constant volume mode, wherein the normal servo drive module is controlled by the feedback signal of the displacement sensor to maintain the normal deformation of the specimen at zero; Horizontal shear control mode: Monotonic shear mode: Shear displacement is applied monotonically according to a preset shear rate; Cyclic shearing mode: The horizontal shearing servo drive module is controlled to apply cyclic shear stress or shear displacement to the sample according to the preset waveform, amplitude and frequency.

[0014] Furthermore, in the constant normal stiffness (CNS) mode, the control unit calculates the theoretical normal stress variation value based on Hooke's law or a user-defined nonlinear stiffness model.

[0015] Furthermore, it also includes a removable insulation layer that wraps around the outside of the sealed pressure-bearing cavity.

[0016] This invention also provides a method for direct shear testing of unsaturated soil, performed on a multi-field coupled control system for direct shear testing of unsaturated soil. The system includes a sealed pressure chamber, a shear box assembly disposed within the chamber, a dual-channel environmental control system, and a servo loading and acquisition control system. The method includes the following steps: Sample loading and system packaging steps: The unsaturated soil sample is loaded into the shear box assembly. The base of the shear box assembly is embedded with a high air intake value clay plate, and the inside of the side wall is provided with a closed fluid circulation channel; the closed fluid circulation channel is connected to an external hot and cold circulation machine, and the sealed pressure-bearing cavity is closed and sealed. Multi-field coupling environment application and equilibration steps: The sample is adjusted to the set temperature by the external hot and cold circulation machine connected to the closed fluid circulation channel. At the same time, the sample is balanced under the set temperature and matrix suction by applying pore gas pressure to the sealed pressure chamber and pore water pressure to the bottom of the clay plate. Consolidation and shear test procedure: A normal consolidation stress is applied to the specimen through a normal servo drive module. The normal servo drive module drives a normal loading rod through a chamber-penetrating mechanism. The end of the normal loading rod is equipped with a built-in sensor to directly contact the specimen. After consolidation, a boundary condition control mode is selected from constant normal load, constant normal stiffness, or constant volume. The horizontal shear mode is set to monotonic shear or cyclic shear. The horizontal shear servo drive module is started to perform shearing. During the shearing process, the normal servo drive module is adjusted in real time based on the feedback signal of the built-in sensor according to the selected mode. Test completion and removal steps: After reaching the preset shear displacement, stop loading, remove the pore gas pressure and pore water pressure in sequence, turn off the temperature control, open the sealed pressure-bearing cavity and take out the sample.

[0017] Furthermore, the step of real-time control of the normal servo drive module includes: Real-time acquisition of sample normal force and normal displacement data collected by built-in force and displacement sensors; The following controls are executed based on the selected mode: When the constant normal load mode is selected, the normal force data is used as the control target, and displacement compensation is performed by the normal servo drive module to maintain the constant force value. When the constant normal stiffness mode is selected, the required normal force adjustment is calculated based on the change in the normal displacement data and the preset stiffness value, and the corresponding load is applied through the normal servo drive module. When the constant volume mode is selected, the normal displacement data is set to zero as the control target, and the reaction force is applied by the normal servo drive module to lock the normal deformation of the sample.

[0018] (III) Beneficial Effects Compared with the prior art, the present invention has the following significant advantages: 1. By constructing a high-pressure sealed pressure environment and integrating axis translation technology (a technology that achieves precise control of matrix suction (i.e., the difference between air pressure and water pressure) by independently controlling the pore air pressure and pore water pressure inside the soil sample), precise active control of unsaturated soil matrix suction is achieved. It can simulate the dry-wet cycle process under the full suction path, significantly improving the authenticity and reliability of the test data.

[0019] 2. An innovative closed-loop fluid circulation channel is integrated into the side wall of the shear box, which enables wide-range, uniform temperature control of the sample from -30℃ to 70℃ in a high-pressure, sealed space. This effectively solves the problem of mutual interference between temperature field and moisture field control, making it possible to simulate multi-field coupled environments such as freeze-thaw cycles and high-temperature evaporation.

[0020] 3. The normal loading is achieved by using a servo motor driven by a built-in sensor, which significantly reduces the impact of cross-chamber friction and air pressure interference on load measurement. It can also actively and accurately execute three complex boundary conditions: constant load (CNL), constant stiffness (CNS), and constant volume (CV), filling the technical gap in the study of shear characteristics of unsaturated soil under specific constraints.

[0021] 4. By deeply integrating suction control, temperature control and complex boundary condition control, the comprehensive simulation and coordinated regulation of the multi-field coupling effect of "thermal-water-mechanical" are realized, providing a highly simulated experimental verification method for complex geotechnical problems in fields such as deep earth engineering, lunar soil research and nuclear waste disposal. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 A schematic diagram of the overall structure of the multi-field coupled control unsaturated soil direct shear test system provided by the present invention.

[0023] Figure 2 A cross-sectional structural diagram of the sealed pressure-bearing cavity and the loading transmission mechanism provided by the present invention.

[0024] Figure 3 A schematic diagram of the shear box assembly structure of the integrated clay plate and temperature-controlled flow channel provided by the present invention.

[0025] Figure 4This is a cross-sectional structural diagram of the terracotta slab and drainage channel provided by the present invention.

[0026] Figure 5 This is a top view of the drainage channel provided by the present invention.

[0027] Figure 6 This is a schematic diagram of the arrangement structure of the closed fluid circulation channel provided by the present invention inside the shear box.

[0028] Figure 7 This is a schematic diagram of the arrangement structure of the closed fluid circulation channel in the upper shear box provided by the present invention. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] The terms "first," "second," etc., used in the specification and claims of this invention are used to distinguish similar objects and not to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that embodiments of the invention can be implemented in orders other than those illustrated or described herein, and the objects distinguished by "first," "second," etc., are generally of the same class and the number of objects is not limited; for example, a first object can be one or more. Furthermore, in the specification and claims, "and / or" indicates at least one of the connected objects, and the character " / " generally indicates that the preceding and following objects are in an "or" relationship.

[0031] Please see Figure 1 and Figure 2 As shown, the present invention provides a multi-field coupled control system for direct shear testing of unsaturated soil, which mainly consists of four parts: a high-pressure sealed pressure-bearing loading host, a shear box assembly integrating a clay plate and a temperature-controlled flow channel, a dual-channel environmental control system, and a servo loading and acquisition control system.

[0032] The high-pressure sealed pressure-bearing loading host includes a sealed pressure-bearing cavity 1 made of stainless steel with a design pressure resistance of 2MPa. This sealed pressure-bearing cavity 1 provides a stable, high-pressure, and sealed physical environment for the entire system, which is the basis for implementing axial translation technology to control suction. Its high pressure resistance ensures safety under a wide range of suction control. Precision through-chamber transmission mechanisms 2 are installed on the top and side walls of the sealed pressure-bearing cavity 1 to guide the normal loading rod 3 and the horizontal shearing rod 4 through the cavity wall. This mechanism can achieve low-friction movement of the normal loading rod 3 while maintaining the cavity seal. These mechanisms, while ensuring the airtightness of the sealed pressure-bearing cavity 1, greatly reduce the frictional damping of the rod movement, thereby ensuring smooth and precise loading action.

[0033] Combined Figures 3 to 5 As shown, the shear box assembly is disposed inside the sealed pressure-bearing cavity 1, including an upper shear box 5 and a lower shear box 6. A high-air-entry-value clay plate 7 is embedded in the center of the base of the lower shear box 6. This clay plate 7 is one of the key components of the invention; it utilizes the capillary effect of micropores to allow water molecules to pass freely while blocking gas, thereby establishing a reliable gas-water pressure interface at the bottom of the sample. A drainage channel 8 (in a preferred embodiment, a spiral water trough) is machined below the clay plate 7, which converges to an interface 80 at the bottom for connecting to an external water pressure control system. This design enables independent application of pore water pressure and precise monitoring of drainage volume. By cooperating with the cavity air pressure, a precise and stable matrix suction can be established inside the sample based on the principle of axial translation, completely solving the fundamental problem of uncontrollable suction in open-type direct shear apparatuses.

[0034] Finally combined Figure 6 and Figure 7 As shown, a closed fluid circulation channel 9 is machined inside the metal sidewalls of the upper shear box 5 and the lower shear box 6. This closed fluid circulation channel 9 is an independent closed loop, physically isolated from the bottom drainage channel 8, and connected to the external high-precision hot and cold circulation machine 11 through a high-pressure resistant flexible pipe 10 passing through the cavity wall. The circulating medium flows within the closed fluid circulation channel 9, exchanging heat efficiently with the sample through the metal box wall, achieving direct and uniform "enveloping" temperature control of the sample. This sidewall integrated flow channel design cleverly avoids spatial conflicts and functional interference between the temperature control pipeline and the bottom suction drainage system in a confined high-pressure space, achieving for the first time on a direct shear apparatus a wide temperature range (-30℃ to 70℃) precise temperature control in a high-pressure closed environment.

[0035] The dual-channel environmental control system includes a suction control channel and a temperature control channel. The suction control channel consists of an air pressure control loop and a water pressure control loop. The air pressure control loop injects dry compressed air into the sealed pressure chamber 1 through an air source 12 and a pressure regulating valve to control the pore air pressure. The water pressure control loop, through a high-precision pressure / volume controller 13, is connected to the interface 80 of the drainage channel 8 via a pipeline to apply back pressure to the water cavity below the clay plate 7 to control the pore water pressure, and can monitor water volume changes in real time to calculate the sample saturation. The temperature control channel consists of a high-precision hot and cold circulation machine 11, a flexible pipeline 10, and a closed fluid circulation channel 9 forming a closed loop. The independent control of water, air, and temperature channels achieves decoupling and precise regulation of the moisture field, stress field, and temperature field, laying the foundation for multi-field coupling experiments.

[0036] The servo loading and acquisition control system is the core of this invention for controlling complex boundary conditions. It includes a normal servo drive module, a horizontal shear servo drive module, a built-in force sensor, a built-in displacement sensor, and a control unit 18. Normal loading is driven by a servo motor 14, which moves the normal loading rod 3 through a chamber-penetrating mechanism. The key innovation lies in directly embedding the normal force sensor 15 and the normal displacement sensor 16 within the sealed pressure-bearing cavity 1, and installing them between the end of the normal loading rod 3 and the loading plate. Similarly, the horizontal shear force sensor 17 is also embedded and installed at the end of the horizontal shear rod 4. The end of the horizontal shear rod 4 is rigidly connected to the lower shear box 6 using a gapless pin or bolt. This rigid connection ensures that when the servo motor reverses, a reverse tensile force can be applied to the sample, thereby achieving cyclic shearing. This built-in sensor arrangement allows direct measurement of the actual load and deformation acting on the sample, completely avoiding interference from the frictional resistance of the chamber-penetrating rods and the "lifting force" generated by the high-pressure gas inside the cavity on the normal loading rod 3, ensuring the original accuracy of the data acquisition. This is a prerequisite for achieving high-precision closed-loop control.

[0037] The control unit 18 receives signals from the sensor and controls the servo drive module to actively control the normal boundary conditions and horizontal shearing modes of the sample during the shearing process. The servo drive module is the servo motor 14. Based on this hardware, the software algorithm of the control unit 18 can implement at least one of three advanced normal boundary condition control modes and at least one of the following horizontal shearing modes: Normal boundary condition control mode: Constant Normal Load (CNL) mode: Using the reading of the normal force sensor 15 as the feedback target, when the shearing of the specimen causes the force value to fluctuate, the servo motor 14 immediately fine-tunes the position of the normal loading rod 3 to compensate, thereby maintaining the total normal force constant, providing traditional but more precise shear test conditions.

[0038] Constant Normal Stiffness (CNS) mode: The user presets a stiffness value. During the experiment, the system monitored the change in normal displacement in real time. And according to the Hooke's Law algorithm formula (Or use a custom nonlinear model) Calculate the theoretical change in normal force. Then, drive servo motor 14 to actively adjust the load. This allows for precise simulation of the elastic constraints on soil in actual engineering projects, such as tunnel surrounding rock and pile perimeter soil, revealing the shear characteristics of soil under stiffness constraints—something traditional equipment cannot achieve.

[0039] Constant Volume (CV) Mode: The system uses the reading of the normal displacement sensor 16 as the control target, driving the servo motor 14 to apply sufficient reaction force to force the normal deformation of the specimen to be locked at zero. This allows direct study of the shear stress-strain relationship and strength characteristics of soil under conditions of no volumetric deformation (i.e., isochoric), and is suitable for analyzing engineering problems subject to strict displacement constraints.

[0040] Horizontal shear control mode: Monotonic shear mode: Shear displacement is applied monotonically according to a preset shear rate.

[0041] Cyclic Shearing Mode: Based on the high-response characteristics of the servo motor 14, the control unit 18 has a built-in waveform generator. Users can input sine wave, triangular wave, and other command signals to control the horizontal shearing rod 4 to perform reciprocating motion. Combined with the feedback from the built-in horizontal shear force sensor 17, cyclic shearing under different boundary conditions can be realized to simulate the dynamic evolution of unsaturated soil under cyclic loading.

[0042] In addition, a removable rubber-plastic sponge insulation layer 19 can be wrapped around the outside of the sealed pressure-bearing cavity 1. This insulation layer is fixed by Velcro or straps, making it easy to install, remove, and maintain. Its function is to further block heat exchange between the cavity and the external environment, improving the efficiency and stability of the temperature control system, especially during extreme high and low temperature tests.

[0043] This invention also provides a method for direct shear testing of unsaturated soil, performed on a multi-field coupled control system for direct shear testing of unsaturated soil. The system includes a sealed pressure chamber, a shear box assembly disposed within the chamber, a dual-channel environmental control system, and a servo loading and acquisition control system. The method includes the following steps: Sample loading and system packaging steps: The unsaturated soil sample is loaded into the shear box assembly. The base of the shear box assembly is embedded with a high air intake value clay plate 7. The side wall is provided with a closed fluid circulation channel 9. The closed fluid circulation channel 9 is connected to the external hot and cold circulation machine, and the sealed pressure chamber 1 is closed and sealed. Multi-field coupling environment application and equilibration steps: The sample is adjusted to the set temperature by the external hot and cold circulation machine 11 connected to the closed fluid circulation channel 9. At the same time, the sample is balanced under the set temperature and matrix suction by applying pore gas pressure to the sealed pressure chamber 1 and pore water pressure to the bottom of the clay plate 7. Consolidation and shear test procedure: A normal consolidation stress is applied to the specimen through a normal servo drive module. The normal servo drive module drives a normal loading rod 3 through a chamber-penetrating mechanism. The end of the normal loading rod 3 is equipped with a built-in sensor to directly contact the specimen. After consolidation, a boundary condition control mode is selected from constant normal load, constant normal stiffness, or constant volume. The horizontal shear mode is set to monotonic shear or cyclic shear. The horizontal shear servo drive module is started to perform shear. During the shearing process, the normal servo drive module is adjusted in real time based on the feedback signal of the built-in sensor according to the selected mode. Test completion and removal steps: After reaching the preset shear displacement, stop loading, remove the pore gas pressure and pore water pressure in sequence, turn off the temperature control, open the sealed pressure-bearing cavity and take out the sample.

[0044] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A multi-field coupling controlled unsaturated soil direct shear test system, characterized in that, The application relates to a high-pressure sealed pressure-bearing loading host machine, a shear box assembly integrated with a clay plate and a temperature-controlled flow channel, a double-channel environment control system, and a servo loading and collection control system. The high-pressure sealed pressure-bearing loading host machine comprises a high-pressure-resistant sealed pressure-bearing cavity and a precision cabin-penetrating transmission mechanism arranged on the sealed pressure-bearing cavity for the penetration of a normal loading rod and a horizontal shear rod. The shear box assembly integrated with a clay plate and a temperature-controlled flow channel is arranged in the sealed pressure-bearing cavity, the center of the base of the shear box assembly is embedded with a high-air-intake-value clay plate, a drainage flow channel is arranged below the clay plate and connected to an external water pressure control system, and a closed fluid circulation channel is arranged in the side wall of the shear box assembly and connected to an external temperature control device through an independent pipeline. The double-channel environment control system comprises a suction force control channel and a temperature control channel, the suction force control channel comprises an air pressure control loop communicated with the sealed pressure-bearing cavity to apply a pore air pressure and a water pressure control loop communicated with the drainage flow channel to apply a pore water pressure, and the temperature control channel comprises an external cooling and heating circulation machine communicated with the closed fluid circulation channel. The servo loading and collection control system comprises a normal servo driving module, a horizontal shear servo driving module, an internal force sensor, an internal displacement sensor and a control unit, the internal force sensor and the internal displacement sensor are arranged in the sealed pressure-bearing cavity and connected with the normal loading rod and the shear box assembly respectively, the end of the horizontal shear rod is connected with the shear box assembly through a rigid connecting piece to realize horizontal push-pull reciprocating loading, and the control unit is used for receiving signals of the sensors and controlling the servo driving module to realize active control of a normal boundary condition and a horizontal shear mode during shearing.

2. The multi-field coupled controlled unsaturated soil direct shear test system according to claim 1, wherein, The sealed pressure-bearing cavity is made of stainless steel and has a pressure resistance not less than 2 MPa; and the precision cabin-penetrating transmission mechanism can realize low-friction movement of the normal loading rod while maintaining air tightness of the sealed pressure-bearing cavity.

3. The multi-field coupled controlled unsaturated soil direct shear test system of claim 1, wherein, The internal force sensor comprises a normal force sensor and a horizontal shear force sensor which are directly fixed to the end of the normal loading rod and the end of the horizontal shear rod respectively.

4. The multi-field coupled controlled unsaturated soil direct shear test system of claim 1, wherein, The closed fluid circulation channel is a spiral flow channel surrounding the side wall of the shear box.

5. The multi-field coupling controlled unsaturated soil direct shear test system according to claim 1 or 4, characterized in that, The temperature control range of the external cooling and heating circulation machine is-30 DEG C to 70 DEG C.

6. The multi-field coupled controlled unsaturated soil direct shear test system of claim 1, wherein, The water pressure control loop comprises a high-precision pressure volume controller connected to the drainage flow channel through a high-pressure-resistant pipeline.

7. The multi-field coupled controlled unsaturated soil direct shear test system of claim 3, wherein, The control unit is configured to be capable of executing at least one of the following three normal boundary condition control modes and at least one of the following horizontal shear control modes: The normal boundary condition control modes are as follows: A constant normal load mode in which a feedback signal of the normal force sensor is used to control the normal servo driving module to maintain a constant normal acting force on the sample; A constant normal stiffness mode in which a target normal force is calculated according to a preset stiffness value and a normal displacement change amount collected by the displacement sensor, and the normal servo driving module is controlled to be dynamically adjusted; A constant volume mode in which a feedback signal of the displacement sensor is used to control the normal servo driving module to maintain a zero normal deformation amount of the sample. Horizontal shear control mode: Monotonic shear mode: monotonic shear displacement is applied according to a preset shear rate; Cyclic shear mode: the horizontal shear servo drive module is controlled to apply cyclic reciprocating shear stress or shear displacement to the sample according to a preset waveform, amplitude and frequency.

8. The multi-field coupled controlled unsaturated soil direct shear test system of claim 7, wherein, In the constant normal stiffness mode, the control unit calculates the theoretical normal stress change value according to Hooke's law or a user-defined nonlinear stiffness model.

9. The multi-field coupled controlled unsaturated soil direct shear test system of claim 1, wherein, It also includes a detachable thermal insulation layer wrapped outside the sealed pressure chamber.

10. A method of non-saturated soil direct shear test, characterized by, The method is performed on a multi-field coupling controlled unsaturated soil direct shear test system as claimed in any one of claims 1-9, which comprises a sealed pressure chamber, a shear box assembly arranged in the chamber, a dual-channel environment control system, and a servo loading and acquisition control system; the method comprises the following steps: Sample loading and system packaging step: unsaturated soil sample is loaded into the shear box assembly, the center of the base of the shear box assembly is embedded with a high air intake value ceramic plate, and the inside of the side wall is provided with a closed fluid circulation channel; the closed fluid circulation channel is connected to an external cold and hot circulation machine, and the sealed pressure chamber is closed and sealed; Multi-field coupling environment application and balancing step: the sample is adjusted to a set temperature by the external cold and hot circulation machine connected to the closed fluid circulation channel, and at the same time, the sample is balanced at the set temperature and matrix suction by applying pore air pressure to the sealed pressure chamber and pore water pressure to the bottom of the ceramic plate; Consolidation and shear test step: a normal servo drive module is used to apply a normal consolidation stress to the sample, the normal servo drive module drives a normal loading rod through a through-chamber mechanism, and the end of the normal loading rod is provided with a built-in sensor to directly contact the sample; after consolidation, one of the boundary condition control modes of constant normal load, constant normal stiffness or constant volume is selected, the horizontal shear mode is set to monotonic shear or cyclic shear, the horizontal shear servo drive module is started for shearing, and during the shearing process, the normal servo drive module is real-time regulated based on the feedback signal of the built-in sensor according to the selected mode; Test end and removal step: stop loading after reaching the preset shear displacement, remove the pore air pressure, pore water pressure and turn off the temperature control in sequence, and open the sealed pressure chamber to take out the sample.

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