A multi-field coupling soil-geotextile interface direct shear test device and method

Abstract

The present application relates to a kind of muscle soil interface direct shear test device and method of multi-field coupling, the device includes base platform, bearing platform, shear box, environment simulation module, mechanical loading module and measurement and control system, base platform is equipped with environment simulation station and direct shear test station, it is installed linear slide on it.Movable bearing platform can be accurately along linear slide, realize seamless switching of sample between station.Shear box uses unique double-layer side wall structure, micro-hole is set on inner layer transparent plate, and cavity is formed between the two.Environment simulation module supports the rapid simulation of dry-wet cycle and freeze-thaw cycle in conjunction with shear box.Mechanical loading module can be observed by high-speed camera in real time when loading muscle soil interface shear zone evolution.The present application is through multi-station sequential operation mode, and traditional test period is shortened to several hours from several days, realizes the high-efficiency, high-precision muscle soil interface multi-field coupling test, and provides reliable technical means for geotechnical engineering and geological engineering design and research.

Description

Technical Field

[0001] This invention relates to the field of reinforced soil engineering technology, and in particular to a multi-field coupled direct shear test device and method for the reinforced soil interface. Background Technology

[0002] The shear strength parameters (cohesion and internal friction angle) of soil and rock masses are key indicators in the engineering design of slopes, roadbeds, tunnels, retaining walls, and other structures. In the field of reinforced soil engineering, the mechanical properties of the reinforced-soil interface directly control the overall safety and stability of reinforced soil structures. In actual engineering, the reinforced-soil interface, like the surrounding soil and rock mass, is subjected to harsh environmental conditions involving multiple coupled fields such as wet-dry cycles (e.g., seasonal rainfall evaporation) and freeze-thaw cycles (e.g., in cold regions). This leads to significant deterioration of the interface's mechanical properties, resulting in reduced shear strength and potentially causing increased deformation or even instability in the reinforced structure. Therefore, developing experimental techniques and equipment that can efficiently and accurately simulate these environmental effects and are specifically designed for testing the mechanical properties of the reinforced-soil interface plays an irreplaceable and crucial role in revealing the interface deterioration mechanism and improving the design level and long-term safety of reinforced soil engineering.

[0003] Chinese patent document CN116465758A discloses a direct shear test apparatus and method for determining reinforced soil interface parameters under freeze-thaw cycles. The apparatus includes a large shear system, a freeze-thaw cycle system, and a track system connecting the large shear system and the freeze-thaw cycle system. The freeze-thaw cycle system includes an insulated chamber, an insulated layer outside the shear chamber, a cooling and heating circulation pump, a freezing pipe and a temperature sensor inside the shear chamber, and inlet and outlet pipes. The freeze-thaw cycle system freezes and thaws the reinforced soil sample in the shear chamber. The track system then transfers the frozen-thawed sample into the large shear system for a direct shear test to obtain the interface mechanical parameters. This test apparatus and method are convenient to operate when determining the interface mechanical parameters of reinforced soil under freeze-thaw cycles. The freeze-thaw cycle system has good insulation properties, low heat loss, and high freeze-thaw efficiency, enabling reliable determination of the interface mechanical parameters of reinforced soil under freeze-thaw cycles.

[0004] However, the existing environmental simulation and direct shear test coupling technology, especially when applied to the testing of reinforced-soil interfaces where the integrity of the specimen and the consistency of test conditions are extremely important, reveals its core shortcomings, severely restricting scientific research progress and engineering applications. These shortcomings are mainly reflected in the following aspects: (1) The environmental simulation process is time-consuming and inefficient. Current environmental simulation technologies suffer from significant bottlenecks. During freeze-thaw cycles, the high thermal inertia and low heat transfer efficiency of the temperature control system typically require over 24 hours, sometimes even up to 48 hours, to complete a single freeze-thaw cycle. Similarly, during saturation, traditional methods (such as top water spraying, simple immersion, or constant-head permeation) result in long and uneven water penetration paths, often requiring tens of hours of immersion time to ensure complete saturation of large-sized samples. This extremely low efficiency makes it difficult to conduct systematic experiments involving multiple cycles and various operating conditions, failing to meet the need for rapid data acquisition.

[0005] (2) The operation process is fragmented and cumbersome, causing significant disturbance to the sample. Existing technologies physically separate environmental simulation equipment from large direct shear apparatuses in terms of both space and process, resulting in lengthy and fragmented operational procedures. Samples need to be loaded, unloaded, transported, and repositioned multiple times between the environmental chamber and the direct shear apparatus. This process not only introduces significant non-experimental waiting and preparation time, leading to low equipment utilization, but more seriously, for samples such as those at the reinforced-soil interface, which are extremely sensitive to minute displacements and disturbances, repeated handling can easily alter the reinforced-soil contact state, causing pre-debonding or structural pre-damage, resulting in severely distorted final test results that fail to accurately reflect the material properties after environmental effects.

[0006] (3) Simulation technology itself has limitations, which restricts the improvement of efficiency. The inefficiency stems from the limitations of the simulation technology itself. Saturation techniques rely on traditional methods such as permeable stones, which easily create dominant seepage channels in large samples, leading to uneven saturation and making it impossible to precisely control or accelerate the process. Temperature control techniques suffer from low heat transfer efficiency, poor uniformity of the internal temperature field of the sample, and slow state transitions. These technical bottlenecks not only result in inefficiency but also affect the realism of the simulation and the reliability of the data.

[0007] In summary, existing direct shear test environment simulations are inefficient, making it difficult to conduct systematic tests involving multiple cycles and operating conditions, and failing to meet the need for rapid data acquisition. Furthermore, the physical separation of the environmental simulation equipment from the large direct shear apparatus in terms of space and process results in lengthy and fragmented operational procedures. Therefore, there is an urgent need to provide a direct shear test device with high environmental simulation efficiency, good test continuity, and high accuracy. Summary of the Invention

[0008] The purpose of this invention is to overcome the defects of the prior art by providing a multi-field coupled direct shear test device and method for the reinforced soil interface, which solves or partially solves the problems of low efficiency in environmental simulation, fragmented operation process leading to easy disturbance of the sample and long test cycle in the current direct shear test.

[0009] The objective of this invention can be achieved through the following technical solutions: On the one hand, this solution provides a multi-field coupled direct shear test device for reinforced soil interface, including a base platform, a bearing platform and a shear box, wherein the shear box is fixed on the bearing platform; the base platform is provided with an environmental simulation station and a direct shear test station, and the base platform is fixed with two parallel linear slide rails, wherein the bearing platform is slidably fixed on the linear slide rails, and the linear slide rails are used to transfer the shear box between the environmental simulation station and the direct shear test station; The sidewall of the shearing box has a double-layer structure, including an outer support plate and an inner transparent plate. The outer support plate and the inner transparent plate enclose a closed cavity. The sidewall of the shearing box is provided with an air inlet and an air outlet that penetrate the cavity. The inner transparent plate is provided with multiple micropores, which are evenly distributed around the circumference of the shearing box. The shearing box includes an upper shearing box and a lower shearing box.

[0010] Furthermore, the inner transparent plate is made of polycarbonate sheet, and the thickness of the inner transparent plate is 8-12mm. The pore diameter of the micropores is 0.5-1.5mm, and the micropores are used to connect the cavity and the inner cavity of the shear box.

[0011] Furthermore, the outer supporting plate and the inner transparent plate are distributed in parallel to each other, and the thickness of the cavity is 8-15mm.

[0012] Furthermore, the bearing platform is provided with positioning pins and locking blocks, and the environmental simulation station and the direct shear test station are respectively provided with positioning holes that cooperate with the positioning pins. The environmental simulation station and the direct shear test station are respectively provided with wedge-shaped locking grooves that cooperate with the locking blocks, for positioning and temporary locking of the bearing platform.

[0013] Furthermore, the environmental simulation workstation is equipped with an environmental simulation component, which includes a constant temperature water circulation unit, a refrigerant tank, a multi-way valve-controlled pipeline unit, a blower unit, and a vacuum pump. The shearing box is equipped with a replaceable top cover structure, which includes a porous pipe network cover plate and a rigid loading cover plate. The porous pipe network cover plate contains a porous pipe network. The constant temperature water circulation unit and the refrigerant tank are both connected to the porous pipe network through the multi-way valve-controlled pipeline unit. The porous pipe network has dual functions as a fluid channel and a heat exchange channel that adapts to different operating conditions: during the humidification or saturation stage, the porous pipe network acts as a fluid distribution channel, uniformly distributing water to the top of the sample under the supply pressure for top-down immersion of the sample; during the freezing stage, the porous pipe network switches to a liquid nitrogen circulation channel, introducing liquid nitrogen from the coolant tank; the liquid nitrogen flows within the porous pipe network and cavities, absorbing heat from the soil and rock sample mainly through indirect heat exchange via the pipe and cavity walls, and utilizing the boiling phase change of liquid nitrogen (latent heat of vaporization of approximately 199 kJ / kg) to achieve highly efficient cryogenic freezing; the nitrogen gas generated after the liquid nitrogen vaporizes is discharged through the outlet, leaving no liquid residue or chemical change in the sample.

[0014] Furthermore, the water output from the constant temperature water source circulation unit enters the porous pipe network after being switched by the multi-way valve control pipeline unit, and immerses the sample in the shearing chamber from top to bottom; the vacuum pump is used to generate a negative pressure of -1~-8 kPa in the cavity, for dehumidification and defrosting during the freezing and thawing and dry-wet cycle process, and to assist in removing residual liquid water in the porous pipe network (51) and cavity (44) before freezing and thawing.

[0015] On the other hand, this solution also provides a test method using the aforementioned multi-field coupled reinforced-soil interface direct shear test device. The environmental simulation station is equipped with an environmental simulation module, and the direct shear test station is equipped with a mechanical loading module. The test method includes the following steps: Step S1: Prepare soil and rock samples in a shear box fixed on a support platform, and place sensors and visual markers in the soil and rock samples. Move the support platform and lock it in the environmental simulation position. Step S2: Connect the environmental simulation module to the interface of the shear box, start the environmental simulation module, and perform dry-wet cycle or freeze-thaw cycle treatment on the soil and rock sample in the shear box. Step S3: After the environmental simulation is completed, separate the environmental simulation component and the shear box. Move the shear box along the linear slide rail using the support platform, and transfer and lock the shear box on the direct shear test station. Step S4: The actuator of the mechanical loading module is coupled with the shear box. The mechanical loading module applies normal stress according to preset parameters and conducts a direct shear test. The mechanical response and deformation field data are recorded synchronously with the data acquisition system through the DIC module. Step S5: Post-process the collected stress-displacement curves and DIC deformation field data to analyze the degradation law of strength parameters and shear failure mechanism of soil and rock after environmental action.

[0016] Furthermore, the shear box has a transparent observation window on its side wall, and a DIC module is located directly opposite the transparent observation window; in step S1, the step of visual marking in the soil and rock sample includes, during the preparation of the soil and rock sample, inserting an aluminum rod into the shear box near the transparent observation window, removing the aluminum rod after the soil and rock sample is compacted, and injecting red quartz sand into the pores formed in the soil and rock sample for tracer marking, and the DIC module is used to analyze the shear band evolution of the reinforcement-soil interface.

[0017] Furthermore, the environmental simulation components include a constant temperature water circulation unit, a refrigerant tank, a blower unit, and a vacuum pump; the top of the shear box is provided with a perforated pipe network cover plate, and a perforated pipe network is arranged inside the perforated pipe network cover plate; the constant temperature water circulation unit and the refrigerant tank are both connected to the perforated pipe network through a multi-way valve-controlled pipeline unit. In step S2, the freeze-thaw cycle process includes, During the saturation stage, the constant temperature water circulation unit performs dual-path saturation treatment on the soil and rock samples through a porous pipe network and cavity micropore infiltration. During the freezing stage, after the soil and rock sample is fully saturated, the multi-way valve-controlled pipeline unit is switched to drain and purge the residual liquid water in the porous pipe network and cavity; liquid nitrogen stored in the coolant tank is injected into the porous pipe network and cavity; the liquid nitrogen flows in the porous pipe network and cavity, mainly absorbing the heat of the soil and rock sample through indirect heat exchange between the pipe walls and cavity walls, and rapidly absorbing the heat of the sample by utilizing the boiling phase change of liquid nitrogen, so as to achieve rapid and uniform freezing from the outside to the inside with high efficiency; the nitrogen gas generated after the liquid nitrogen vaporizes is discharged through the gas outlet; at the same time, the blower unit delivers dry and cold air to the outer space of the shear box to help maintain the low temperature of the environment; During the melting stage, the multi-way valve-controlled pipeline unit is switched to stop the liquid nitrogen injection; the constant temperature water source circulation unit delivers room temperature water to the shearing chamber, the blower unit delivers hot air into the shearing chamber, and a vacuum pump applies negative pressure to the cavity; when the soil and rock sample returns to the specified temperature, one freeze-thaw cycle is completed.

[0018] Furthermore, in step S2, the dry-wet cycle treatment includes, During the humidification stage, the constant temperature water circulation unit inputs room temperature water into the porous pipe network and cavity, and applies positive pressure to the cavity until the moisture content of the soil sample reaches the preset value. During the drying stage, the blower unit blows hot air exceeding 50 degrees Celsius into the upper part of the shear box and applies negative pressure to the cavity through the vacuum pump; when the moisture content of the soil sample drops to the specified value, one dry-wet cycle is completed.

[0019] Compared with the prior art, the present invention has the following advantages: (1) In this design, the sidewall of the shear chamber is a functional double-layer cavity consisting of an outer load-bearing steel plate and an inner transparent rigid plate with micropores. By switching pipelines, hot water or liquid nitrogen can be circulated to achieve rapid heating and cooling; water can be introduced to achieve lateral uniform humidification or saturation; hot air can be introduced for drying; and efficient dehumidification can be achieved through micro-negative pressure with the assistance of a vacuum pump. This shear chamber structure integrates multiple functions such as temperature control, humidity control, drying, and saturation. Existing technologies have different functions in the internal freezing tube. This design greatly increases the area of ​​heat exchange and moisture exchange through the micropores on the entire sidewall plane, shortens the transfer path, ensures rapid homogenization of the temperature and humidity fields inside the sample, and greatly improves the efficiency of wet-dry or freeze-thaw cycles.

[0020] (2) This scheme applies deep cryogenic temperatures from the outer boundary of the sample through sidewall cavities and top pipe networks. The freezing front advances from the outside to the inside, which is consistent with the actual heat transfer direction at the engineering site. The simulation results have higher reference value for engineering design. During the saturation process, in addition to the slow seepage of the top porous pipe network, the sidewall cavities can also drive water to synchronously infiltrate from the lateral boundary. The dual-path saturation method, which proceeds from the outside to the inside and is synchronous from top to bottom, can ensure that large samples reach high uniformity saturation in a short time.

[0021] (3) In the environmental simulation stage, a porous pipe network cover is installed on the top of the shear chamber to provide fluid channels for processes such as saturation and humidification. During the direct shear test, the pipe network cover is replaced with a rigid loading top cover to meet the normal loading requirements of the direct shear test. The two top covers are connected to the upper shear chamber through a quick-change structure. The replacement process does not touch the sample, making replacement convenient and ensuring test accuracy. This architecture allows the same device to carry out various test types such as dry-wet, freeze-thaw, and constant temperature and humidity. Compared with the single nature of existing shear chamber and environmental simulation structure tests, it improves the applicability and efficiency of the test.

[0022] (4) This scheme uses a support platform with linear guide rails, which can be precisely moved to the test position and locked by the control system. From sample preparation in the shear chamber to environmental simulation and finally mechanical testing, the sample is rigidly fixed on the support platform throughout the test cycle, without any disassembly, handling or reinstallation. Only the support platform moves, while the movement of the platform vehicle on the linear guide rails is precisely controllable. This fundamentally ensures the original state of the reinforcement-soil interface and the high reliability of the test results.

[0023] (5) The transparent rigid plate on the inner side of the shear box in this design provides a large-area, distortion-free optical window for the DIC module. Simultaneously, this invention includes the step of pre-embedding colored sand tracer particles during sample preparation. Combined with a high-speed industrial camera, the displacement of the tracer particles during shearing can be recorded in real time. The DIC software can calculate the full-field displacement and strain fields within the entire observation area. Researchers can observe in real time and quantitatively when and where the shear band is generated, how it develops and widens, and how the reinforcing material restricts its expansion. Attached Figure Description

[0024] Figure 1 A schematic diagram of a multi-field coupled direct shear test device for the reinforced-soil interface provided in an embodiment of the present invention; Figure 2 This is a schematic diagram of the clipboard and visualization system provided in an embodiment of the present invention; Figure 3 This is a schematic diagram illustrating the connection between the constant temperature water circulation unit and the shear box provided in an embodiment of the present invention. Figure 4A schematic diagram illustrating the principle of the environment simulation module provided in an embodiment of the present invention; Figure 5 This is a three-dimensional structural diagram of the base platform provided in an embodiment of the present invention; Figure 6 for Figure 2 A partially enlarged schematic diagram of the double-layer sidewall structure of the shear box.

[0025] Reference numerals: 1. Central control computer; 2. Base platform; 21. Reinforcing rib; 22. Vibration damping support; 3. Linear slide rail; 4. Shear box; 5. Constant temperature water circulation unit; 6. Refrigerant tank; 7. Mechanical loading module; 8. Blower unit; 41. Upper shear box; 42. Lower shear box; 43. Transparent observation window; 44. Cavity; 45. Air inlet; 46. Air outlet; 47. Outer bearing plate; 48. Inner transparent plate; 49. Micropores; 51. Porous pipe network; 52. Quick connector; 81. Blower outlet. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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 embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0027] Example 1 like Figure 1 and Figure 6 As shown, this embodiment provides a multi-field coupled direct shear test device for the reinforced soil interface. The device includes a base platform 2, a bearing platform, and a shear box 4. The shear box 4 is rigidly fixed to the bearing platform. The base platform 2 is provided with an environmental simulation station and a direct shear test station, and two parallel linear slide rails 3 are fixed to the base platform 2. The bearing platform is slidably fixed on the linear slide rails 3, which are used to guide the bearing platform to accurately transfer the shear box 4 between the environmental simulation station and the direct shear test station. The side wall of the shear box 4 has a double-layer structure, including an outer bearing plate 47 and an inner transparent plate 48. The outer bearing plate 47 and the inner transparent plate 48 enclose a closed cavity 44. The side wall of the shear box 4 is provided with an air inlet 45 and an air outlet 46 that penetrate the cavity 44. The inner transparent plate 48 is provided with multiple micropores 49, which are evenly distributed around the circumference of the shear box 4. The micropores 49 are used to connect the cavity 44 and the inner cavity of the shear box 4. The shear box 4 includes an upper shear box 41 and a lower shear box 42.

[0028] Working Principle: The soil and rock sample is encapsulated and rigidly fixed within the shear chamber 4 throughout the process, moving along the linear slide rail 3 with the support platform. When environmental treatment is required, the support platform is driven to the environmental simulation station on the base platform 2 and locked, allowing the quick-connect interface 52 on the shear chamber 4 to quickly connect with the environmental simulation component fixed at the station. The environmental simulation component can supply different media to the porous pipe network 51 at the top of the shear chamber 4 and the cavity 44 on the side wall by switching pipelines, and can regulate moisture migration by controlling the pressure within the cavity 44. During this process, heat and moisture act rapidly and uniformly on the soil and rock sample from the outer boundary through the micropores 49 distributed throughout the side wall of the shear chamber, greatly shortening the heat and mass transfer path. After the environmental simulation is completed, the interface automatically separates, the support platform moves to the direct shear test station and locks again, allowing the shear chamber 4 to automatically couple with the mechanical loading module 7 fixed at the station for the direct shear test. Therefore, the test samples do not need to be disassembled or transported during the entire test process, which fundamentally solves the problem of interface state changes and pre-disturbance caused by process fragmentation, and ensures the originality and high reliability of the test results.

[0029] In this embodiment, the inner transparent plate 48 is made of polycarbonate sheet with a thickness of 8-12 mm, and the micropores 49 on it have a pore size of 0.5-1.5 mm. The outer supporting plate 47 and the inner transparent plate 48 are distributed parallel to each other, and the cavity 44 formed between them has a thickness of 8-15 mm. Specifically, the cavity 44 design ensures sufficient medium circulation space to achieve rapid heat and moisture exchange, while avoiding the medium response lag that may be caused by an excessively large cavity volume. The uniformly distributed micropores 49 ensure the uniformity of heat and mass transfer at the lateral boundary of the sample. Through the micropore array on the entire sidewall plane, the effective area for heat and moisture exchange with the sample is greatly increased. Combined with the rapidly flowing medium, this ensures the rapid homogenization of the temperature and humidity fields inside the sample.

[0030] It is important to note that the cavity thickness and the inner transparent plate thickness must be designed in tandem, as both affect heat and moisture transfer efficiency and structural strength. An excessively thick cavity increases flow resistance and slows response; an excessively thin cavity restricts flow rate, reducing heat exchange and wetting uniformity. This range balances flow velocity and coverage, ensuring uniform and controlled lateral boundary coverage of the sample. The inner transparent plate thickness must balance light transmittance, pressure resistance, and micropore processing precision; insufficient thickness leads to deformation, affecting micropore permeation uniformity; excessive thickness increases thermal resistance and slows temperature conduction. The cavity and micropores together form a lateral permeation path. Under positive pressure, water permeates through the micropores to form a uniform thin layer, working in conjunction with the top porous network to achieve dual-path saturation from the outside in, significantly improving efficiency and uniformity.

[0031] In this embodiment, the carrying platform is provided with positioning pins and locking blocks. The environmental simulation station and the direct shear test station are respectively provided with positioning holes that cooperate with the positioning pins, and the environmental simulation station and the direct shear test station are respectively provided with wedge-shaped locking grooves that cooperate with the locking blocks. These are used to accurately position and temporarily lock the carrying platform after the station is switched, thereby improving the positioning accuracy and stability of the platform.

[0032] In this embodiment, an environmental simulation component is installed at the environmental simulation station. The environmental simulation component includes a constant temperature water circulation unit 5, a refrigerant tank 6, a multi-way valve-controlled pipeline unit, a blower unit 8, and a vacuum pump. The top of the shear box 4 is equipped with a replaceable top cover structure, which includes a porous pipe network cover plate and a rigid loading cover plate. A porous pipe network 51 is arranged inside the porous pipe network cover plate. The constant temperature water circulation unit 5, the refrigerant tank 6, and the vacuum pump are all connected to the environmental medium inlet and outlet of the porous pipe network 51 through the multi-way valve-controlled pipeline unit, and are connected to the air inlet 45 and air outlet 46 of the side wall cavity 44 of the shear box 4 through pressure-resistant hoses, so as to realize dynamic switching of medium type and pressure control.

[0033] In actual connection, the physical connection between the environmental simulation component and the shear chamber 4 is mainly achieved by two sets of piping systems: one is to connect the multi-way valve-controlled piping unit to the porous pipe network 51 through the quick interface 52, which is used to supply the medium to the top of the sample; the other is to connect the multi-way valve-controlled piping unit, the vacuum pump and the air inlet 45 and the air outlet 46 on the side wall of the shear chamber 4 through the pressure-resistant hose, which is used to introduce the medium into the cavity 44 and control its pressure state.

[0034] In this embodiment, as Figure 3 As shown, the water output from the constant temperature water circulation unit 5 can be switched through a multi-way valve-controlled pipeline unit and enter the porous pipe network 51, immersing the sample in the shear chamber 4 from top to bottom at an extremely low flow rate. The pore size of the porous pipe network 51 is preferably 0.5-1.0 mm, and the pore spacing is 50-80 mm to ensure the uniformity of the outflow. In addition, during the above-mentioned saturation or humidification process, the constant temperature water circulation unit 5 can provide a controllable positive pressure water source of 5-20 kPa to the cavity 44 on the side wall of the shear chamber 4. Under the positive pressure drive, the water seeps into the side of the sample through the micropores 49 on the inner transparent plate 48, forming a dual-path combined saturation mode of slow seepage from the top and thin-layer seepage from the side wall, which significantly improves the saturation efficiency and uniformity of large samples. During the drying stage of freeze-thaw or wet-dry cycle, the vacuum pump can generate a slight negative pressure of -1 to -8 kPa in the cavity 44 to accelerate dehumidification and defrosting or remove water vapor, maintaining the dryness and clarity of the side wall cavity and observation window. By adopting the above configuration of applying controllable positive or negative pressure to cavity 44, the flow of medium or the migration of water vapor can be actively driven, further optimizing the heat and mass transfer efficiency and shortening the environmental simulation cycle.

[0035] In this embodiment, as Figure 5As shown, to improve the overall rigidity and stability of the base platform 2, longitudinal and transverse reinforcing ribs 21 are provided below the upper load-bearing panel of the base platform 2 to improve the bending and torsional rigidity of the platform. Specifically, the upper load-bearing panel can be made of steel plate with a thickness of 12-20mm, and CNC machined and ground to ensure that the flatness of the mounting surface of the linear slide rail 3 is controlled within the range of 0.05-0.1mm, so as to ensure the smooth movement of the load-bearing platform and the final positioning accuracy. Multiple height-adjustable shock-absorbing feet 22 are installed at the bottom of the base platform 2 to adjust the levelness of the platform and absorb external vibration interference.

[0036] In this embodiment, the environmental parameter monitoring unit includes temperature and humidity sensors that can be embedded inside the sample to monitor changes in the sample's internal state in real time. The central control unit is integrated into the central control computer 1 and is used to automatically control the movement of the support platform, the switching of environmental simulation parameters, and the mechanical loading process, while simultaneously collecting data from all sensors to achieve fully automated operation throughout the entire test process.

[0037] In this embodiment, as Figure 2 As shown, the outer support plate 47 has a window at the position corresponding to the inner transparent plate 48 to form a high-strength transparent observation window 43, and a stiffening frame is welded around the window to enhance local rigidity. The inner transparent rigid plate of the shear box 4 provides a large-range, distortion-free optical window for the DIC module.

[0038] In this embodiment, the thickness of the inner transparent plate 48, the pore size of the micropores 49, and the thickness of the cavity 44 need to be comprehensively designed based on factors such as the sample size and the target heat and moisture exchange rate. All parameters are within the optimal ranges verified through extensive practical experience. The porous pipe network cover plate and the upper shear box 41 can be detachably connected via quick-change clamps or a quick-locking pressure plate structure. Similarly, the rigid loading cover plate can be sealed and fixed to the upper shear box 41 via a similar quick-locking structure, making replacement convenient and without touching the sample. The drive device, servo motor, and other power and control components of the entire testing apparatus are all conventional technologies in the field, and their specific models and connection methods can be adapted according to actual load and accuracy requirements.

[0039] This embodiment also provides a test method using the above-described apparatus. For example... Figures 1 to 6 As shown, the method includes the following sequential steps: Step S1: Prepare a soil and rock sample in a shear box 4 fixed on a support platform, and place sensors and visual markers in the soil and rock sample. Move and lock the support platform in the environmental simulation position through the central control unit.

[0040] Step S2: Connect the environmental simulation module to the interface of the shear box 4, start the preset program, and perform dry-wet cycle or freeze-thaw cycle treatment on the soil and rock sample in the shear box 4.

[0041] Step S3: After the environmental simulation is completed, the interface between the environmental simulation module and the shear box 4 is automatically separated. The central control system drives the bearing platform to move along the linear slide rail 3, transferring the shear box 4 and locking it on the direct shear test station.

[0042] Step S4: The actuator of the mechanical loading module 7, which is fixed on the direct shear test station, automatically couples with the shear box 4, applies normal stress according to the set parameters and conducts a direct shear test. At the same time, the DIC module and the data acquisition system synchronously record the mechanical response and deformation field data.

[0043] Step S5: Post-process the collected stress-displacement curves and DIC deformation field data to analyze the degradation law of strength parameters and shear failure mechanism of soil and rock after environmental action.

[0044] In this embodiment, in step S1, the placement of the visualization markers can be done in the following specific way: During the preparation of the soil and rock sample, an aluminum rod is pre-inserted at the position of the shear box 4 near the transparent observation window 43. After the soil and rock sample is compacted in layers, the aluminum rod is carefully removed and red quartz sand is poured into the pores formed in the sample to form a tracer marker for DIC analysis. The DIC module is mainly used to analyze the shear band evolution process of the reinforcement-soil interface.

[0045] The step of pre-embedding colored sand tracer particles during sample preparation, combined with a high-speed industrial camera, allows for real-time recording of the tracer particle displacement during shearing. DIC software can then calculate the full-field displacement and strain fields across the entire observation area. Compared to existing technologies that only provide macroscopic mechanical parameters such as peak strength and residual strength, offering no direct observation of the shear failure process at the soil-reinforcement interface, this invention enables researchers to observe in real-time, quantitatively, when and where shear bands originate, how they develop and widen, and how the reinforcement limits their expansion.

[0046] In this embodiment, as Figure 4 As shown, the freeze-thaw cycle treatment specifically includes three stages. The first is the saturation stage: the constant-temperature water circulation unit 5, through the combined action of the porous pipe network 51 and the cavity 44, employs a dual-pathway approach (top slow infiltration and sidewall microporous infiltration) to achieve high-uniformity saturation of the soil and rock samples. In this stage, the porous pipe network 51 and the cavity 44 are filled with room-temperature water. By applying a positive pressure of 5–20 kPa to the cavity 44, the water is uniformly infiltrated into the lateral boundary of the sample through the micropores 49 on the inner transparent plate 48, achieving rapid and uniform saturation from the outside in and from top to bottom simultaneously. After saturation, the multi-way valve-controlled pipeline unit is switched to drain the remaining room-temperature water from the porous pipe network 51 and the cavity 44, and dry high-pressure gas of 0.1–0.3 MPa is introduced for purging, ensuring that no liquid water remains in the pipes and cavities, preparing for subsequent liquid nitrogen injection and freezing.

[0047] The next stage is freezing: After purging and emptying, the multi-way valve-controlled pipeline unit is switched, the purging gas path is closed, and the automatic pressurization output function of the coolant tank 6 is activated. Liquid nitrogen stored in the coolant tank 6 is injected into the porous pipe network 51 and the side wall cavity 44 through insulated pipes. The liquid nitrogen flows within the porous pipe network 51 and the cavity 44, primarily absorbing heat from the soil and rock sample through indirect heat exchange via the pipe and cavity walls. Utilizing the boiling phase change of liquid nitrogen (latent heat of vaporization approximately 199 kJ / kg), it rapidly absorbs heat from the sample, achieving efficient and rapid uniform freezing from the outside in. The nitrogen gas generated after vaporization is discharged through the outlet 46. Simultaneously, the blower unit 8 delivers dry, cold air to the outer space of the shear chamber 4 to help maintain the low ambient temperature.

[0048] Finally, the thawing stage: After the freezing stage, the multi-channel valve-controlled pipeline unit is switched to stop liquid nitrogen injection and the dry, cold air supply from the blower unit 8. The valves are switched so that the constant-temperature water circulation unit 5 supplies room-temperature water (20℃~30℃) to the porous pipe network 51 and the side wall cavity 44. The blower unit 8 switches to hot air mode, supplying hot air (25℃~35℃) to the outer space of the shear chamber 4. A slight negative pressure of -1 to -6 kPa is applied to the cavity 44 via a vacuum pump to assist in dehumidification until the sample warms to the specified temperature, completing one freeze-thaw cycle. Using this process, combined with the device's rapid heat exchange capability, the traditional freeze-thaw test cycle can be significantly shortened.

[0049] In this embodiment, the wet-dry cycle treatment can also be divided into two stages. Humidification stage: The constant temperature water circulation unit 5 inputs room temperature water into the porous pipe network 51 and the side wall cavity 44, and applies a positive pressure of 5-20 kPa to the cavity 44 to accelerate water infiltration from the side wall until the moisture content of the soil sample reaches a preset value. Drying stage: The blower unit 8 blows hot air exceeding 50 degrees Celsius into the upper end of the shear box 4, and simultaneously starts the vacuum pump to apply a negative pressure of -1 to -8 kPa to the cavity 44 to accelerate the removal of water vapor evaporated from the sample. When the moisture content of the soil sample drops to a specified value, one wet-dry cycle is completed.

[0050] In conjunction with the above embodiments, this embodiment provides specific steps for a direct shear test of a large loess sample under freeze-thaw cycles, including: 1. Sample preparation and system initialization Sample preparation: Loess soil from a construction site was dried to constant weight in an oven at 105℃, crushed, and sieved through a 2mm sieve. The soil-water ratio was calculated based on the target dry density and moisture content. Samples were prepared in a shear chamber 4 using a layered compaction method, with roughening treatment between layers.

[0051] System deployment: A porous pipe network 51 is pre-embedded at the bottom of the shear chamber and connected to the constant temperature water circulation unit 5 via a quick interface 52. A temperature sensor is embedded in the center of the sample. An aluminum rod is inserted into the corresponding position of the transparent observation window 43, backfilled with soil and compacted, then pulled out and filled with red quartz sand to form a tracer mark.

[0052] Workstation positioning: The central control computer 1 controls the carrier platform to move along the linear slide rail 3, and precisely positions the shearing box 4 at the environmental simulation workstation.

[0053] 2. Environmental simulation (freeze-thaw cycle) Saturation Process: In this embodiment, the sample is uniformly saturated using a dual-path approach of slow top infiltration and thin-layer sidewall infiltration. First, the high-uniformity saturation system is activated. Constant-temperature water from the constant-temperature water circulation unit 5 is slowly infiltrated into the sample from top to bottom through a porous pipe network 51 at a constant flow rate. The porous pipe network uses a microporous array with a diameter of 0.5 mm to avoid localized scouring and ensure uniform infiltration. Simultaneously, water at the same temperature is introduced into the double-layer sidewall cavity of the shear chamber, and a controllable positive pressure of 5–20 kPa is applied to the cavity 44 through the pressure control unit, forming a thin layer of water flowing along the sidewall direction within the cavity. Driven by positive pressure, the constant-temperature water infiltrates the outer edge of the sample through the microporous array on the inner transparent plate via micro-infiltration, thereby achieving synchronous and uniform wetting of the sample's lateral boundaries. This process continues until a free water surface appears at the top of the sample, maintained for 24 hours to ensure complete saturation.

[0054] Freezing Stage: After purging and evacuation, the multi-way valve-controlled pipeline unit is switched, the purging gas path is closed, and the automatic pressurization output function of the coolant tank 6 is activated. Liquid nitrogen stored in the coolant tank 6 is injected into the porous pipe network 51 and the side wall cavity 44 via insulated pipes. As the liquid nitrogen flows within the porous pipe network 51, it primarily removes heat from the top of the sample through indirect heat exchange via the pipe walls. The liquid nitrogen in the side wall cavity 44 primarily absorbs heat from the sample's periphery through indirect heat exchange via the inner transparent plate 48 wall. During flow, the liquid nitrogen absorbs heat from the wall surface and undergoes a boiling phase change, utilizing its approximately 199 kJ / kg latent heat of vaporization to remove a large amount of heat from the sample. This forces heat flow unidirectionally from the outside to the inside of the sample, resulting in a very rapid and uniform freezing front advance.

[0055] It should be noted that the openings on the porous pipe network 51 and the micropores 49 on the inner transparent plate 48 are designed for water permeation during the saturation stage. During the freezing stage, liquid nitrogen does not actively discharge into the shear chamber through the micropores when flowing in the pipes and cavities; freezing is mainly achieved through indirect heat exchange. Even if a trace amount of liquid nitrogen seeps into the shear chamber through the micropores 49 due to factors such as pressure fluctuations, liquid nitrogen is extremely chemically stable, being a colorless, odorless, non-toxic, and non-corrosive inert medium. Upon contact with the soil sample, it rapidly boils and vaporizes into nitrogen gas, which is then discharged through the outlet 46, leaving no liquid residue or chemical change in the sample and causing no chemical contamination. Therefore, the heat transfer during the freezing stage of this invention is primarily based on indirect heat exchange, and even if a trace amount of leakage occurs, there is no risk of contamination. This approach ensures both high-efficiency freezing and extremely high safety and reliability.

[0056] Meanwhile, the blower unit 8 delivers dry, cold air at -20℃ to -25℃ to the outer space of the shear box 4 to help accelerate cooling and reduce external heat intrusion.

[0057] Melting Stage: After the freezing stage, the switching valve redirects the medium flow to the constant temperature water circulation unit, allowing the unit to output 20℃~30℃ warm water, which then enters the porous pipe network 51 at the top of the shear chamber and the double-layer sidewall cavity 44. To accelerate the melting process and avoid excessive temperature gradients on the sample surface, the blower unit 8 is activated, continuously supplying 25℃~35℃ hot air to the cavity and the surrounding space of the shear chamber. To prevent localized condensation caused by water vapor accumulation in the sidewall cavity, the vacuum pump can be activated as needed to apply a slight negative pressure of -1~-6kPa to the sidewall cavity, promptly removing thawing water vapor and maintaining a dry and clean state within the cavity. Once the sample has warmed to the specified temperature, one cycle is complete.

[0058] 3. Mechanical Testing and Data Acquisition Workstation switching: After the environmental simulation is completed, the platform vehicle moves along the linear slide rail 3 to the direct shear test workstation and automatically couples with the mechanical loading module 7.

[0059] Loading test: Normal stress is applied, and after consolidation, the material is sheared at a constant rate to a specified displacement. The stress-displacement curve is recorded simultaneously, and the deformation is recorded by capturing images of tracer particles using a high-speed camera.

[0060] 4. Results Analysis The strain field distribution within the shear band was analyzed using DIC software, and the influence of freeze-thaw cycles on cohesion and internal friction angle was obtained by combining experimental data.

[0061] This embodiment eliminates intermediate disturbances, and the multi-station sequential operation mode ensures that the sample does not need to be disassembled or transported from loading to the end of the test. Optimal configuration of function and performance: the two core modules of environmental simulation and mechanical loading are independently fixed and can be designed and manufactured according to their functional requirements. It combines large-scale, versatility, and high performance; its modular design makes it adaptable to testing various soils, jointed rock masses, geosynthetic composites, and other geotechnical materials. The environmental simulation is realistic and highly accurate; the high-uniformity saturation system achieves uniform saturation of large geotechnical samples through microporous seepage technology. It has a high degree of automation and intelligence; the entire process is integrated and controlled by a central control system, realizing seamless connection and data synchronization from environmental simulation to mechanical testing.

[0062] Example 2 This embodiment also provides a direct shear test method for the interface of geogrid-reinforced soil under simulated wet-dry cycles, which is basically the same as that in Embodiment 1. The difference is that the specific steps of the direct shear test method in this embodiment include: 1. Sample preparation The sample was prepared according to the method in Example 1. When the soil was filled to half the height, the geogrid was laid and fixed to both sides of the shear box with a special clamp.

[0063] 2. Dry and wet cycle Humidification Stage: The switching valve redirects the medium flow to the constant-temperature water circulation unit, which outputs clean water at approximately 20°C, which then enters the porous pipe network 51 and the sidewall cavity 44. To improve the wetting efficiency in the sidewall direction, a micro-positive pressure of 5–10 kPa is applied to the cavity 44 via the pressure control unit, generating a stable micro-permeation driving force for the water in the sidewall direction. Under these conditions, the sample gradually becomes humidified from the outside in, and humidity changes are monitored by the temperature and humidity sensor group at the top of the sample. The humidification process typically lasts 6–12 hours until the sample moisture content reaches the predetermined target.

[0064] Drying Stage: Hot air at 60°C is continuously blown into the upper space of the shear chamber using blower unit 8. To accelerate moisture migration and prevent condensation on the transparent sidewalls, a vacuum pump can be activated simultaneously to maintain a slight negative pressure of -1 to -5 kPa in the double-layer sidewall cavity of the shear chamber. This negative pressure effectively removes moisture evaporated from the outer edge of the sample, ensuring a uniform humidity gradient along the sidewalls during drying and guaranteeing clear imaging through the observation window.

[0065] 3. Observation of the shear interface During the shearing process, the evolution of the shear band at the reinforcement-soil interface and the deformation of the grid are analyzed in detail using the DIC module.

[0066] By connecting the hollow micropores and porous network pipes with the constant temperature water circulation unit, the rock and soil samples can be humidified. With the help of the blower unit and vacuum pump, the rock and soil samples can be efficiently dehumidified through micro negative pressure, thereby enabling the simulation of the dry and wet cycle conditions.

[0067] In this embodiment, considering that although liquid nitrogen has no risk of chemical pollution, its ultra-low temperature characteristics (-196℃) and the possibility of local hypoxia after vaporization, the device is equipped with the following safety measures: (1) A forced ventilation system is set up in the test area to ensure that the vaporized nitrogen is discharged in time and to maintain the oxygen concentration at no less than 19.5%; (2) An oxygen concentration sensor is set up around the shear box 4. When the oxygen concentration is lower than 19.5%, an audible and visual alarm is automatically triggered and the liquid nitrogen supply is cut off; (3) A safety pressure relief valve is set up in the cavity 44 and the porous pipe network 51 to prevent the liquid nitrogen vaporization from causing the pipe pressure to rise abnormally; (4) The operating area is equipped with antifreeze gloves, goggles and other low temperature protection equipment to ensure the safety of the operators.

[0068] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A multi-field coupled direct shear test device for reinforced soil interface, characterized in that, The system includes a base platform (2), a support platform, and a shear box (4), with the shear box (4) fixed on the support platform. The base platform (2) is provided with an environmental simulation station and a direct shear test station. The base platform (2) is fixed with two parallel linear slide rails (3), and the support platform is slidably fixed on the linear slide rails (3). The linear slide rails (3) are used to transfer the shear box (4) between the environmental simulation station and the direct shear test station. The sidewall of the shear box (4) has a double-layer structure, including an outer support plate (47) and an inner transparent plate (48). The outer support plate (47) and the inner transparent plate (48) enclose a closed cavity (44). The sidewall of the shear box (4) is provided with an air inlet (45) and an air outlet (46) that penetrate the cavity. The inner transparent plate (48) is provided with a plurality of microholes (49). Each microhole (49) is evenly distributed around the shear box (4). The shear box (4) includes an upper shear box (41) and a lower shear box (42).

2. The multi-field coupled direct shear test device for the reinforced-soil interface according to claim 1, characterized in that, The inner transparent plate (48) is made of polycarbonate, and the thickness of the inner transparent plate (48) is 8-12mm, and the pore size of the micropores (49) is 0.5-1.5mm. The micropore (49) has a working condition adaptive function switching structure: During the saturation or humidification stage, when liquid fluid is filled into the cavity (44) and positive pressure is applied, the micropores (49) serve as liquid fluid infiltration channels, allowing fluid to seep into the inner cavity of the shear box (4) through the micropores (49) for lateral uniform humidification of the soil and rock samples. During the freezing stage, the liquid water in the cavity (44) is drained and replaced by liquid nitrogen supply; the liquid nitrogen flows in the cavity (44) and the porous pipe network (51), and absorbs the heat of the soil and rock sample through the pipe wall and cavity wall in the form of inter-wall heat exchange, thereby freezing the soil and rock sample.

3. The multi-field coupled direct shear test device for the reinforced-soil interface according to claim 1, characterized in that, The outer supporting plate (47) and the inner transparent plate (48) are distributed in parallel to each other, and the thickness of the cavity (44) is 8-15mm.

4. The multi-field coupled direct shear test device for the reinforced-soil interface according to claim 1, characterized in that, The bearing platform is provided with positioning pins and locking blocks. The environmental simulation station and the direct shear test station are respectively provided with positioning holes that cooperate with the positioning pins. The environmental simulation station and the direct shear test station are respectively provided with wedge-shaped locking grooves that cooperate with the locking blocks, which are used for positioning and temporary locking of the bearing platform.

5. The multi-field coupled direct shear test device for the reinforced-soil interface according to claim 1, characterized in that, The environmental simulation station is equipped with an environmental simulation component, which includes a constant temperature water circulation unit (5), a refrigerant tank (6), a multi-way valve-controlled pipeline unit, a blower unit (8), and a vacuum pump. The shear box (4) is equipped with a replaceable top cover structure, which includes a porous pipe network cover plate and a rigid loading cover plate. A porous pipe network (51) is arranged inside the porous pipe network cover plate. The constant temperature water circulation unit (5) and the refrigerant tank (6) are both connected to the porous pipe network (51) through the multi-way valve-controlled pipeline unit.

6. The multi-field coupled direct shear test device for the reinforced-soil interface according to claim 5, characterized in that, The constant temperature water source circulation unit (5) is connected to the porous pipe network (51) through the multi-way valve control pipeline unit, which is used to supply water to the shear box (4) and immerse the soil and rock sample in the shear box (4) from top to bottom; the vacuum pump is used to generate a negative pressure of -1~-8kPa in the cavity (44), and is used for dehumidification and defrosting during the freezing and thawing and dry-wet cycle process, as well as to assist in removing the residual liquid fluid in the porous pipe network (51) and cavity (44) before freezing and thawing.

7. A test method using a multi-field coupled direct shear test apparatus for the reinforced-soil interface as described in any one of claims 1-6, characterized in that, The environmental simulation station is equipped with an environmental simulation module, and the direct shear test station is equipped with a mechanical loading module. The test method includes the following steps: Step S1: Prepare a soil and rock sample in a shear box (4) fixed on a bearing platform, and place sensors and visual markers in the soil and rock sample. Move the bearing platform and lock it in the environmental simulation position. Step S2: Connect the environmental simulation module to the interface of the shear box (4), start the environmental simulation module, and perform dry-wet cycle or freeze-thaw cycle treatment on the soil and rock sample in the shear box (4); Step S3: After the environmental simulation is completed, separate the environmental simulation component and the shear box (4), and move the shear box (4) along the linear slide rail (3) by the support platform, and transfer and lock the shear box (4) on the direct shear test station; Step S4: The actuator of the mechanical loading module is coupled with the shear box (4). The mechanical loading module applies normal stress according to the preset parameters and performs a direct shear test. The DIC module records the mechanical response and DIC deformation field data simultaneously. Step S5: Post-process the recorded mechanical response data and DIC deformation field data to analyze the degradation law of strength parameters and shear failure mechanism of soil and rock samples after environmental simulation.

8. The test method according to claim 7, characterized in that, The shear box (4) is provided with a transparent observation window (43) on its side wall, and a DIC module is provided in the position directly opposite the transparent observation window (43). In step S1, the step of visual marking in the soil and rock sample includes: during the preparation of the soil and rock sample, an aluminum rod is pre-inserted in the shear box (4) near the transparent observation window (43). After the soil and rock sample is compacted, the aluminum rod is removed and red quartz sand is injected into the pores formed by the soil and rock sample for tracer marking. The DIC module is used to analyze the shear band evolution of the reinforcement-soil interface.

9. The test method according to claim 7, characterized in that, The environmental simulation components include a constant temperature water circulation unit (5), a refrigerant tank (6), a blower unit (8), and a vacuum pump; the top of the shear box (4) is provided with a porous pipe network cover plate, and a porous pipe network (51) is arranged inside the porous pipe network cover plate. The constant temperature water circulation unit (5) and the refrigerant tank (6) are both connected to the porous pipe network (51) through a multi-way valve-controlled pipeline unit. In step S2, the freeze-thaw cycle process includes, During the saturation stage, the constant temperature water source circulation unit (5) performs dual-path saturation treatment on the soil and rock samples through a porous pipe network (51) and cavity micropore infiltration; During the freezing stage, after the soil and rock sample is fully saturated, the multi-way valve-controlled pipeline unit is switched to drain the residual liquid water in the porous network (51) and cavity (44); the liquid nitrogen stored in the coolant tank (6) is injected into the porous network (51) and cavity (44) through the insulated pipeline; the liquid nitrogen flows in the porous network (51) and cavity (44) and absorbs the heat of the soil and rock sample through the pipe wall and cavity wall in the form of indirect heat exchange; at the same time, the blower unit (8) delivers dry and cold air to the outer space of the shear box (4) to help maintain the low temperature of the environment; During the melting stage, the multi-way valve control pipeline unit is switched, the liquid nitrogen supply is turned off, the constant temperature water source circulation unit (5) delivers room temperature water to the shear box (4), the blower unit (8) delivers hot air into the shear box (4), and applies negative pressure to the cavity (44) through the vacuum pump; when the soil and rock sample returns to the specified temperature, one freeze-thaw cycle is completed.

10. The test method according to claim 9, characterized in that, In step S2, the wet-dry cycle treatment includes the following steps: During the humidification stage, the constant temperature water source circulation unit (5) inputs room temperature water into the porous pipe network (51) and cavity (44) and applies positive pressure to the cavity (44) until the moisture content of the soil sample reaches the preset value. During the drying stage, the blower unit (8) blows hot air of more than 50 degrees Celsius into the upper end of the shear box (4) and applies negative pressure to the cavity (44) through the vacuum pump; when the moisture content of the soil sample drops to the specified value, one dry-wet cycle is completed.

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