Servo pressurization bidirectional pneumatic permeation test device and method

Abstract

The application discloses a servo pressurized bidirectional pneumatic permeation test device and method, and belongs to the field of geotechnical engineering test equipment.The device comprises a support frame system, and a sample assembly, air pressure control, servo mechanical pressurization and data acquisition system arranged on the support frame system; the sample assembly system is composed of a threaded upper pressure chamber, a half-split middle mold assembly and a lower suction chamber; the middle mold is provided with a longitudinal sealing rubber strip; the upper pressure chamber is embedded with a self-tightening U-shaped sealing ring; the middle part is internally provided with a 3D printing honeycomb filter screen with a customized aperture; the lower suction chamber is provided with a support column and a flow sensor; the air pressure control system realizes bidirectional air pressure driving through double servo air pumps; and the servo mechanical pressurization system realizes axial stress closed-loop control.The method comprises the following steps: sample air-tight assembly, axial stress loading, solution injection, bidirectional pneumatic accelerated permeation, data acquisition and permeability calculation; a positive and negative pressure coupling mode is adopted to form a high hydraulic gradient, so that undisturbed, high-precision and rapid permeation test is realized, and the method is suitable for detecting the permeation characteristics of geotechnological soil.

Description

Technical Field

[0001] This invention relates to the field of precision testing instruments for geotechnical engineering, and in particular to a servo-pressurized bidirectional pneumatic permeability testing device and method. Background Technology

[0002] In geotechnical engineering, geological disaster prevention, and environmental geotechnical engineering, soil permeability is a key indicator for evaluating soil strength, deformation, and stability. Especially in areas like the Three Gorges Reservoir area, where large amounts of slip zone soil and expansive soil are distributed, research on the permeability evolution of low-permeability cohesive soil under deep, high-stress environments has extremely important practical guiding significance for engineering design, disaster early warning, and the formulation of remediation plans. Currently, the main devices for measuring the permeability coefficient of soil and rock in the laboratory include constant-head permeameters, variable-head permeameters, and various improved triaxial permeameters. With the development of servo control technology, some permeability devices capable of simulating complex stress environments have emerged. For example, CN111693441A discloses a test device and method for simulating rock seepage, which can, to a certain extent, simulate the seepage law of rocks under dynamic load disturbance.

[0003] However, existing permeability testing devices reveal numerous technical problems that urgently need to be addressed when testing low-permeability soft soils such as slip zone soils. Regarding sample loading and unloading, most triaxial permeameters use integral cylindrical molds or fixed support structures, making sample loading and unloading difficult and prone to secondary disturbance. While demolding is relatively easy for hard materials like rocks, for highly cohesive and easily expansive soft soils such as slip zone soils and compacted clay, the soil sample tends to adhere tightly to the inner wall of the mold after testing or become stuck due to water absorption and expansion. During removal, the soil sample may be compressed, deformed, fractured, or even broken, not only destroying the original structure of the soil sample but also preventing subsequent studies such as microstructural observations, resulting in the loss of crucial experimental data.

[0004] In terms of testing efficiency, existing devices mostly apply positive pressure to one end of the sample to drive permeation. For cohesive soils with extremely low permeability, the time for water to penetrate the soil sample under unidirectional positive pressure is extremely long. Completing a set of permeation tests often takes several days or even weeks, resulting in very low testing efficiency. Moreover, simply and blindly increasing the injection pressure can easily lead to soil sample splitting or sidewall flow, making it difficult to achieve rapid permeation testing while ensuring the structural integrity of the sample.

[0005] The filtration system also has significant drawbacks. Traditional permeability testing devices generally use standard permeable stones or sintered metal plates as the filter boundary. These filtration systems have fixed pore sizes and lack adaptability and flexibility. For soils such as slip zone soils containing a large number of fine particles, the pores of the standard permeable stones are easily and quickly clogged by these particles, resulting in the measured permeability coefficient being the permeability coefficient of the permeable stone rather than the true permeability coefficient of the soil sample, severely affecting the accuracy of the test data. Furthermore, existing technologies lack a filter structure that can flexibly adjust the pore size according to the particle size distribution of the soil sample and is easy to clean and replace, making it difficult to adapt to the testing needs of different types of soil.

[0006] In simulating deep, high-stress environments, the insufficient reliability of dynamic sealing structures is also a prominent issue. Existing devices often use traditional O-rings or packing seals where the pressure rod passes through the top cover of the high-pressure vessel, resulting in poor dynamic seal reliability. During tests involving frequent up-and-down movement of the pressure rod or long-term pressure holding, the dynamic seal is prone to leakage due to wear or excessive internal pressure, leading to fluctuations in confining pressure or permeation pressure, further affecting the stability of the test pressure and the accuracy of the test data.

[0007] Furthermore, while some existing permeability testing devices can simulate the seepage patterns of rocks under dynamic load disturbances, they still cannot simultaneously solve the problems of large sample loading and unloading disturbances, low testing efficiency, easy clogging of the filtration system, and unreliable dynamic sealing structure in the testing of low-permeability soft soil. Moreover, it is difficult to achieve decoupled control and coupled loading of axial mechanical stress and high permeability water pressure in the same device, and it is impossible to accurately reproduce the permeability evolution of deep soil under complex stress environments, thus limiting the in-depth development of research on the permeability characteristics of low-permeability soft soil.

[0008] In summary, developing a permeability testing device and method that enables undisturbed loading and unloading of low-permeability soft soil, efficient and accurate testing, and adaptability to multi-field coupling tests has become an urgent technical problem to be solved in the field of geotechnical engineering testing equipment, and has significant practical significance and necessity. Summary of the Invention

[0009] The technical problem to be solved by this invention is to provide a servo-pressurized bidirectional pneumatic permeability testing device and method, which solves the technical problems of existing permeability testing devices when testing low-permeability soft soil, such as large sample loading and unloading disturbance, long testing cycle, insufficient reliability of high-pressure dynamic seal, easy clogging of the filtration system, and inability to achieve decoupling and coupling loading of stress and permeability pressure. Through the testing device and method of this invention, high-precision, automated, and rapid permeability testing of low-permeability soil can be achieved, while ensuring the integrity of the soil sample structure and accurately simulating the permeability law of soil under deep high ground stress environment.

[0010] To achieve the above technical objectives, the present invention adopts the following technical solution: This invention first provides a servo-pressurized bidirectional pneumatic permeability testing device, including a support frame system, and a sample assembly system, a pneumatic control system, a servo mechanical pressurization system, and a data acquisition system installed on the system. The sample assembly system is the core test chamber, comprising an upper pressure chamber, a middle sample mold assembly, and a lower depressurization chamber connected in sequence. A water inlet valve is installed at the top of the upper pressure chamber, and a honeycomb filter is installed inside the middle sample mold assembly to achieve soil sample bearing and permeate filtration. The pneumatic control system includes a set of servo air pumps, which are connected to the upper pressure chamber and the lower depressurization chamber respectively, and can create a bidirectional pressure difference. The servo mechanical pressurization system includes an electric push rod, whose pressure rod passes through a sealing ring, extends into the upper pressure chamber, and connects to a pressure plate to achieve precise loading of axial stress on the soil sample. The data acquisition system includes a main control box and a servo pressure controller to complete the acquisition, control, and calculation analysis of test data.

[0011] Furthermore, the support frame system includes a top plate, a bottom plate, and a set of support columns vertically connecting the two, forming a rigid load-bearing frame that provides a stable installation and load-bearing foundation for each system. To further ensure the stability of the test device, the bottom plate of the support frame system is fixed to the ground. The sample assembly system is installed inside the support frame system and adopts a top-to-bottom sealed threaded connection structure. The middle sample mold assembly is a split structure consisting of two semi-cylindrical shells joined together. Both ends of the mold have external threads that engage with the internal threads at the bottom of the upper pressure chamber and the top of the lower pressure chamber, respectively. The longitudinal splicing surfaces of the two semi-cylindrical shells have mutually matching sealing grooves, and longitudinal sealing strips are embedded in the sealing grooves. Under the axial extrusion force of the external threads at the top and bottom ends, the longitudinal joints are watertightly sealed. A self-tightening shaft U-shaped sealing ring is embedded in the central perforation of the top cover of the upper pressure chamber, with its open end facing the inside of the upper pressure chamber. The internal pressure of the chamber is used to achieve self-tightening sealing, solving the problem of high-pressure dynamic seal leakage.

[0012] The middle sample mold assembly has filter screens arranged at both the top and bottom. Several rigid support columns are vertically installed in the lower pressure chamber, with their tops abutting against the filter screen below to prevent the filter screen from collapsing under high hydraulic gradients. A solution collection pipe is installed at the center of the bottom of the lower pressure chamber, with its end connected to a micro flow sensor, which is also connected to the data acquisition system. The filter screen is densely covered with honeycomb-shaped pores and is made using 3D printing. The pore size of the honeycomb-shaped pores can be customized according to the particle size distribution of the soil sample to be tested, adapting to different soil sample testing needs and avoiding clogging of the filtration system. The servo-driven electric actuator of the servo mechanical pressurization system is mounted on top of the support frame system. Its pressure rod extends into the upper pressure chamber through a U-shaped sealing ring on the shaft. The servo mechanical pressurization system, in conjunction with the servo pressure controller, forms a closed-loop feedback loop, enabling independent adjustment of the axial mechanical stress of the soil sample. The servo air pump of the pneumatic control system is mounted on the support frame system. The upper servo air pump is connected to the side wall interface of the upper pressure chamber via a pressurization pipe, while the lower servo air pump is connected to the side wall interface of the lower suction chamber via a suction pipe, allowing for positive pressure application and negative pressure suction respectively. The data acquisition system also includes a user terminal, which communicates with a micro-flow sensor to receive real-time flow data of the permeate solution, enabling real-time transmission and visualization analysis of experimental data.

[0013] This invention also provides a permeation test method using the above-mentioned servo-pressurized bidirectional pneumatic permeation test device, specifically including the following steps: Step 1, Sample preparation and airtight assembly: Customize 3D printed honeycomb filters according to the particle size distribution of the soil sample to be tested, place the soil sample into the inner cavity of the middle sample mold assembly, place the 3D printed honeycomb filters on the upper and lower surfaces of the soil sample, and screw the middle sample mold assembly with the upper pressure chamber and the lower pressure chamber to form an airtight test chamber. Step 2, Axial Stress Servo Loading and Simulation: Start the servo pressure controller, and apply a preset vertical axial pressure to the soil sample by driving the pressure rod and pressure plate through the servo electric push rod. The self-tightening effect of the shaft U-shaped sealing ring is used to ensure the seal, and the axial pressure is kept constant through closed-loop feedback. Step 3, Permeate solution injection and system closure: Open the inlet valve to inject the permeate solution into the cavity, and close the valve after the predetermined liquid level is reached to form a closed fluid circulation channel; Step 4, Two-way pneumatic coupling to accelerate infiltration: The main control box of the control system starts two servo air pumps. The upper servo air pump applies positive pressure to the upper pressure chamber, and the lower servo air pump draws negative pressure to the lower pressure chamber, forming a hydraulic gradient at both ends of the soil sample. The support column provides rigid support for the lower 3D printed honeycomb filter. Step 5: Real-time data acquisition and permeability calculation: The volumetric flow rate of the permeate solution is acquired by a micro-flow sensor and transmitted to the user terminal. The user terminal calculates and outputs the permeability coefficient of the soil sample based on Darcy's law.

[0014] In this test method, the soil sample in step 1 is undisturbed soil or remolded soil. The middle sample mold assembly can achieve undisturbed filling of the soil sample by separating two semi-cylindrical shells, thus ensuring the integrity of the original structure of the soil sample.

[0015] The vertical axial pressure applied in step 2 is used to simulate the geostress environment of deep soil. The servo electric actuator achieves precise setting and constant pressure maintenance of the axial pressure through displacement closed-loop adjustment.

[0016] In step 4, a bidirectional pneumatic drive mode is adopted that couples the upper constant positive pressure drive with the lower constant negative pressure suction to form a high hydraulic gradient at both ends of the soil sample, thereby achieving accelerated permeability testing of low-permeability soil.

[0017] The formula for calculating the permeability coefficient in step 5 is: (1); In the formula, The soil sample permeability coefficient; Volumetric flow rate measured by a micro-flow sensor; The axial height of the soil sample inside the central sample mold assembly; This represents the cross-sectional area of ​​the soil sample. The density of the osmotic solution; It is the acceleration due to gravity; This represents the total air pressure difference between the upper and lower ends of the soil sample.

[0018] The servo-pressurized bidirectional pneumatic permeation testing device and method provided by this invention have the following beneficial effects: 1. This invention effectively solves the technical problems of difficult sample loading and unloading, low testing efficiency, easy clogging of the filtration system, and insufficient reliability of dynamic sealing structure in the permeability testing of low-permeability soft soil such as slip zone soil in the field of geotechnical engineering. It successfully overcomes many limitations of the existing technology and realizes efficient, accurate and undisturbed testing of soil permeability.

[0019] 2. The middle sample mold assembly of the present invention adopts a split structure and an axial sealing design with longitudinal sealing strips and threaded engagement. This can achieve undisturbed loading, unloading and filling of undisturbed soil and remolded soil, and effectively ensure the water tightness of the test chamber, thereby improving the overall sealing effect.

[0020] 3. Thanks to the design advantage of the semi-split mold, the soil sample can be completely removed after the test, which can provide high-quality test samples for subsequent research such as soil microstructure observation and CT scanning, and enrich the research dimensions of geotechnical engineering tests.

[0021] 4. This invention uses a self-tightening U-shaped sealing ring for shafts, which can utilize the pressure inside the test chamber to form a self-tightening effect, achieving a sealing effect of "the higher the pressure, the tighter the seal", effectively ensuring zero leakage of the high-pressure dynamic seal during the servo pressurization process.

[0022] 5. The self-tightening shaft U-shaped sealing ring of the present invention, combined with the force sensor, realizes servo closed-loop force control, which can effectively ensure the stability of test pressure, greatly reduce the error caused by pressure fluctuation, and improve the accuracy of penetration test data.

[0023] 6. The present invention adopts a bidirectional pneumatic drive mode that couples the upper constant positive pressure and the lower constant negative pressure, which can build an ultra-high hydraulic gradient at both ends of the soil sample that far exceeds that of conventional devices, and significantly shorten the permeability test cycle of low-permeability soil.

[0024] 7. The bidirectional pneumatic drive mode of the present invention breaks through the technical bottleneck of traditional unidirectional drive, greatly improves the permeability testing efficiency of low-permeability soft soil, and effectively solves the pain point of long testing time for low-permeability soil in the industry.

[0025] 8. The filter screen of the present invention adopts a honeycomb structure made by 3D printing. Its pore size can be flexibly customized according to the particle size distribution of the soil sample to be tested, which can accurately adapt to the testing requirements of different types of soil and effectively avoid the filtration system being clogged by fine particles.

[0026] 9. The present invention has a rigid support column installed in the lower pressure chamber to abut against the lower filter screen, which can effectively resist the pressure brought by the high hydraulic gradient, prevent the filter screen from collapsing or deforming, and ensure the stability of the filtration structure and the smooth conduct of the test.

[0027] 10. The devices of the present invention adopt modular design and can be operated independently, which can realize decoupled control and coupled loading of axial mechanical stress and pore water pressure, perfectly adapting to the testing requirements of multi-field coupled permeability tests in geotechnical engineering.

[0028] 11. This invention can independently control axial mechanical stress, pore water pressure and seepage path through different systems, accurately simulate deep high ground stress environment, and truly restore the seepage evolution law of soil under complex stress conditions.

[0029] 12. This invention provides accurate and reliable experimental data support for geotechnical engineering research on deep soil permeability and rock and soil stability. Its technical advantages and practical applicability have been verified by experiments, and it has real engineering application value.

[0030] 13. This invention is equipped with a dedicated permeation test method. This method is standardized, easy to operate, and highly compatible with the test equipment. It can reduce the difficulty of operation for test personnel and improve the convenience and standardization of test operation.

[0031] 14. This invention combines the real-time data transmission and automated calculation functions of the data acquisition system to achieve high-precision and automated penetration testing, significantly reducing errors caused by manual operation and improving the accuracy of test data.

[0032] 15. All core components of the device of the present invention are connected by threaded sealing, which makes disassembly and assembly convenient, facilitates equipment cleaning, daily maintenance and parts replacement after the test, and effectively reduces the operation and maintenance cost of the equipment.

[0033] 16. The test device of the present invention has a compact overall structure and excellent sealing performance. It can stably withstand the impact of high-pressure fluids and can adapt to the working conditions of geotechnical engineering permeability tests of different pressure levels, with strong adaptability.

[0034] 17. The test device and test method of the present invention are highly versatile and can be widely used for permeability characteristic testing of various low-permeability soft soils such as slip zone soil and expansive soil, covering multiple test scenarios in geotechnical engineering.

[0035] 18. The experimental device of the present invention has a high degree of integration, integrating functions such as mechanical pressurization, pneumatic permeation, and data acquisition into one design, which greatly reduces the footprint of the equipment and effectively improves the space utilization of the laboratory.

[0036] 19. The entire device of the present invention realizes automated intelligent control, greatly reduces the number of manual intervention steps in the experiment, improves the repeatability and consistency of the experiment, and ensures the reliability and comparability of the test results of different batches.

[0037] 20. This invention achieves high-precision soil permeability testing while also ensuring testing efficiency, significantly reducing the time and labor costs of the test, and has extremely high application value in the field of geotechnical engineering permeability testing. Attached Figure Description

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments: Figure 1 This is a main sectional view of the overall structure of the device of the present invention; Figure 2 This is an assembly diagram of the pneumatic servo drive system and device body of the present invention; In the diagram: Top plate 1, Support column 2, Push rod 3, Sealing ring 4, Inlet valve 5, Pressurization pipe 6, Upper pressure chamber 7, Pressure rod 8, Pressure plate 9, Filter screen 10, Middle sample mold assembly 11, Air pump holder 12, Servo air pump 13, Main control box 14, Servo pressure controller 15, Lower pressure chamber 16, Support column 17, Solution collection pipe 18, Micro flow sensor 19, Base plate 20, User terminal 21, Internal thread 22, External thread 23, Housing 24. Detailed Implementation

[0039] The technical solutions of the present invention will be further described below with reference to the embodiments and accompanying drawings: Example 1 like Figures 1 to 2 As shown, this embodiment provides a servo-pressurized bidirectional pneumatic permeability testing device, which is applied to the permeability characteristic testing of low-permeability soft soils such as slip zone soils and expansive soils. It can accurately simulate the deep high-stress environment and achieve efficient and undisturbed permeability testing. The specific structure of the device is as follows: The device includes a support frame system, and a sample assembly system, a pneumatic control system, a servo mechanical pressurization system, and a data acquisition system mounted on the support frame system. These systems are modularly designed and work together to ensure the accuracy and automation of the experiment. Figure 1 As shown, the support frame system is a rigid load-bearing structure, consisting of a top plate 1, a bottom plate 20, and three support columns 2 that vertically connect the two. It provides a stable installation and load-bearing foundation for the other systems, can withstand high-pressure fluid impact, and is suitable for high ground stress simulation test conditions. In order to further ensure the stability of the test device, the bottom plate 20 of the support frame system is fixed to the ground.

[0040] The sample assembly system is installed inside the support frame system and serves as the core cavity for the test. It consists of an upper pressure chamber 7, a middle sample mold assembly 11, and a lower pressure chamber 16, which are connected by sealed threads from top to bottom. The middle sample mold assembly 11 is a split structure consisting of two shells 24 joined together. Both its upper and lower ends are provided with external threads 23, which engage with the internal threads 22 at the bottom of the upper pressure chamber 7 and the top of the lower pressure chamber 16, respectively. The longitudinal joint surfaces of the two shells 24 are provided with mutually cooperating sealing grooves, and longitudinal sealing strips are embedded in the grooves. Under the axial compression force of the threaded engagement, the longitudinal joint is sealed with watertight seals, which not only ensures the cavity is sealed but also allows for undisturbed loading and unloading of soil samples. A self-tightening shaft seal ring 4 is embedded in the central perforation of the top cover of the upper pressure chamber 7, with its open end facing the inside of the upper pressure chamber 7. The internal pressure of the cavity can be used to form a self-tightening effect to achieve a high-pressure dynamic seal with zero leakage. A water inlet valve 5 is also provided at the top of the upper pressure chamber 7, which can flexibly control the injection volume and timing of the permeation solution.

[0041] The middle sample mold assembly 11 has 3D-printed filter screens 10 arranged at both the top and bottom. The pore size of the filter screens 10 is customized according to the particle size distribution of the low-permeability soil sample to be tested, which can effectively avoid fine particles clogging the filtration system. Three rigid support columns 17 are vertically installed in the lower pressure chamber 16. The top of the support columns 17 abuts against the lower filter screens 10 to prevent the filter screens 10 from collapsing or deforming under high hydraulic gradients and to ensure the stability of the filtration structure. A solution collection tube 18 is set at the bottom center of the lower pressure chamber 16, and its end is connected to a micro flow sensor 19, which can collect the volumetric flow rate of the permeation solution in real time and accurately.

[0042] The servo mechanical pressurization system is installed at the center of the top plate 1 of the support frame system. It includes a push rod 3, whose pressure rod 8 extends into the upper pressure chamber 7 through a self-tightening shaft U-shaped seal ring 4. The end of the pressure rod 8 is connected to the pressure plate 9, which can apply vertical axial pressure to the soil sample. The system works with the servo pressure controller 15 to form a closed-loop feedback. The push rod 3 achieves precise setting and constant pressure maintenance of axial pressure through displacement closed-loop adjustment. It can independently adjust the axial mechanical stress of the soil sample and achieve decoupled control from the seepage pressure.

[0043] like Figure 2 As shown, the air pressure control system includes two servo air pumps 13 mounted on the air pump holder 12. The air pump holder 12 is mounted on the support frame system. The upper servo air pump 13 is connected to the side wall interface of the upper pressure chamber 7 via the pressurization pipe 6, which can apply a constant positive pressure to the upper pressure chamber 7. The lower servo air pump 13 is connected to the side wall interface of the lower suction chamber 16 via the suction pipe, which can perform constant negative pressure suction on the lower suction chamber 16. The two work together to create an ultra-high hydraulic gradient at both ends of the soil sample, realizing accelerated permeability testing of low-permeability soil.

[0044] The data acquisition system includes a main control box 14, a servo pressure controller 15, and a user terminal 21. The main control box 14 is used to control the positive pressure application and negative pressure suction of the servo air pump 13. The servo pressure controller 15 realizes closed-loop control of axial pressure. The user terminal 21 is connected to the micro flow sensor 19 and can receive the flow data of the permeate solution in real time. Combined with Darcy's law, it realizes the automatic calculation and result output of permeability, and completes the real-time transmission, visualization analysis and automatic processing of experimental data.

[0045] Among them, the micro-flow sensor 19 is a Coriolis mass flow or thermal microfluidic sensor with a range of 0.1 μL / min to 100 μL / min, a measurement accuracy of ±1% mv, a wetted material of PEEK (Polyether Ether Ketone) / quartz glass, a pressure rating of ≥5 MPa, and a recommended selection of Sensirion SLI-0430 or Bronkhorst L01 series. Due to the extremely low permeability coefficient of the slip zone soil, conventional rotor or turbine flow meters have a dead zone at startup flow, making it impossible to detect extremely weak seepage. Therefore, a microfluidic-grade sensor must be used.

[0046] The self-tightening shaft U-shaped seal 4 is filled with carbon fiber polytetrafluoroethylene (PTFE). The spring material is a 316L stainless steel V-type energy storage spring. The coefficient of friction is ≤0.04 (dynamic friction coefficient). The working pressure is 0~5MPa. It is an internal pressure self-tightening one-way seal, conforming to ISO5597 such as Trelleborg Turcon Variseal. The pressure rod 8 needs to pass through the top cover for dynamic displacement. If a conventional nitrile butadiene rubber (NBR) U-shaped seal is used, its huge friction will severely offset the axial mechanical stress measured by the sensor, leading to distortion of the test force. A low-friction PTFE (polytetrafluoroethylene) plug seal must be used.

[0047] The main control box has 14 compartments. The core processing architecture utilizes an embedded real-time controller based on an FPGA (Field Programmable Gate Array) or an EtherCAT (Ethernet for Control Automation Technology) industrial bus PLC (Programmable Logic Controller), equipped with a hard real-time operating system (RTOS). The signal acquisition module includes a ≥16-bit high-precision analog-to-digital converter with a maximum sampling frequency ≥1 kHz, enabling real-time, delay-free acquisition of minute air pressure fluctuations in the upper pressure chamber and lower depressurization chamber. The signal output module features a 6-bit analog output module (0~10V or 4~20mA) with a step response time ≤1 ms, outputting high-precision analog drive signals to the servo air pump for smooth control of its speed and exhaust volume. Dual-channel synchronization accuracy is maintained; the synchronization delay of the action commands for the upper positive pressure channel and the lower negative pressure channel is ≤1 ms, establishing a core physical guarantee of "bidirectional pneumatic coupling" to prevent soil sample shear failure caused by sudden pressure changes. The communication bus and interface include Gigabit Ethernet (GigE). The RS-485 physical layer supports Modbus TCP / IP (Transmission Control Protocol / Internet Protocol) or EtherCAT communication protocols, ensuring real-time data handshake with micro-flow sensors, servo pressure controllers, and user terminals. The core control algorithm incorporates a dual-channel decoupling algorithm and anti-windup discretized PID (Proportional-Integral-Derivative) control logic to eliminate volumetric dynamic changes caused by permeate loss and maintain a constant differential pressure.

[0048] The automated penetration drive process of the main control box 14 is strictly divided into three stages: (1) Soft-start synchronous pressure building stage: After receiving the target pressure difference command from the user terminal 21, the main control box synchronously sends ramp drive signals to the upper and lower servo air pumps, controlling the positive and negative pressures to synchronously approach the target value at a preset extremely low rate (e.g., 1 kPa / s). Step loading is strictly prohibited to prevent sudden changes in air pressure from causing fluid splitting and mechanical disturbance to the slip zone soil sample; (2) Dynamic decoupling adjustment stage: During the pressure building process, if the volume of any cavity changes instantaneously due to soil sample compression, which in turn causes air pressure fluctuation, the microprocessor in the main control box will calculate the compensation amount within a single control cycle, immediately adjust the speed of the corresponding servo air pump, and at the same time feedforward to adjust the action of the other air pump to eliminate cross-coupling interference in the bidirectional pneumatic process. (3) High-precision steady-state pressure holding stage: When the air pressure at both ends reaches the target set value, the algorithm automatically switches to steady-state pressure holding mode. The main control box uses high-frequency fine adjustment of the output power of the servo air pump to offset the small volume change caused by the loss of permeate, and strictly controls the static pressure fluctuation within the range of ±0.1kPa, providing a constant hydraulic gradient boundary for the stable data acquisition of the micro flow sensor 19.

[0049] The servo pressure controller 15 features DSP (Digital Signal Processor) core closed-loop control, supports EtherCAT bus, and offers a control mode of displacement / load dual closed-loop seamless switching. Load range matching is 0~50kN, control resolution is ±0.1% FS (Full Scale) for static force control, and sampling frequency is ≥1kHz. A recommended selection is an integrated geotechnical controller based on a Panasonic A6 servo drive and HBM high-precision tension / compression sensors. This application uses a "servo electric actuator 3" to apply axial force. Essentially, this controller is an integrated algorithm board combining motor servo drive and force feedback, rather than a fluid pump controller.

[0050] Example 2 In another preferred embodiment, based on Embodiment 1, this embodiment provides a servo-pressurized bidirectional pneumatic permeability testing method. This method uses the servo-pressurized bidirectional pneumatic permeability testing device described in Embodiment 1 to conduct permeability tests. Using slip zone soil from the Three Gorges Reservoir area as the low-permeability soft soil to be tested, the permeability characteristics under deep high-stress environment are tested. The specific steps are as follows: Step 1, Sample Preparation and Airtight Assembly: First, based on the particle size distribution curve of the slip zone soil in the Three Gorges Reservoir area, a filter screen 10 with a suitable pore size is customized using 3D printing technology; then, the undisturbed slip zone soil sample is stably placed into the inner cavity of the middle sample mold assembly 11, which is composed of two shells 24. The customized filter screen 10 is placed horizontally on the top and bottom surfaces of the soil sample, respectively; then, through the external threads 23 at both ends of the middle sample mold assembly 11, the upper end is tightened to the internal thread 22 at the bottom of the upper pressure chamber 7, and the lower end is tightened to the internal thread 22 at the top of the lower pressure chamber 16. The thread compression action makes the connection interfaces fit tightly, and at the same time, the sealing strips on the longitudinal splicing surfaces of the shells 24 achieve watertight sealing, finally constructing an overall airtight sealed test chamber that can withstand the impact of high-pressure fluid.

[0051] Step 2, Axial Stress Servo Loading and Simulation: The servo pressure controller 15 of the data acquisition system is activated, and a command is sent to the push rod 3 to execute a downward movement. The power is transmitted to the pressure plate 9 via the pressure rod 8, applying a preset vertical axial pressure to the sliding soil sample in the cavity to simulate the actual in-situ stress environment of the deep soil. During this process, the self-tightening shaft seal ring 4 at the perforation of the pressure rod 8 generates a self-tightening effect under the pressure inside the container, ensuring that the dynamic sealing performance is leak-free. The servo control system adjusts the output displacement of the push rod 3 in a closed loop through real-time feedback from the force sensor until the axial pressure reaches the set value and enters a constant pressure maintenance state.

[0052] Step 3, Injection of Permeation Solution and System Sealing: Under the premise of maintaining constant axial pressure of soil sample, manually open the water inlet valve 5 at the top of the upper pressure chamber 7, and inject the preset clean water permeation solution into the test chamber through the water inlet. After the solution completely submerges the pressure plate 9 and reaches the predetermined liquid level, immediately close the water inlet valve 5 to ensure that the upper pressure chamber 7, the middle sample mold assembly 11 and the lower pressure chamber 16 form a completely sealed fluid circulation channel, providing a sealed environment for subsequent permeation tests.

[0053] Step 4, Bidirectional Pneumatic Coupling Accelerates Infiltration: The two servo air pumps 13 on the air pump holder 12 are simultaneously activated by the main control box 14. The upper servo air pump 13 applies a constant positive air pressure to the solution surface in the upper pressure chamber 7 through the pressurization pipe 6. At the same time, the lower servo air pump 13 performs vacuuming operation on the lower vacuum chamber 16 through the suction pipe to maintain a constant negative pressure environment. Through the bidirectional pneumatic drive mode of "upper constant positive pressure drive coupled with lower constant negative pressure suction", an ultra-high hydraulic gradient is formed at both ends of the slip zone soil sample, forcing the infiltration solution to accelerate through the soil pores of the low-permeability slip zone soil. At this time, the three rigid support columns 17 in the lower vacuum chamber 16 provide stable rigid support for the bottom filter screen 10, ensuring that the filter screen 10 does not collapse, deform or suffer physical damage under extreme pressure difference.

[0054] Step 5: Real-time Data Acquisition and Permeability Calculation: The permeate solution penetrating the slip zone soil sample is filtered through the lower filter screen 10 and collects in the funnel-shaped solution collection pipe 18 at the center of the lower pressure chamber 16. It then continuously flows through the micro-flow sensor 19 at the end. The micro-flow sensor 19 captures the seepage volumetric flow rate signal in real time and transmits the data to the user terminal 21. The analysis software built into the user terminal 21, combined with Darcy's law, automatically calculates and outputs the permeability data and evolution curve of the slip zone soil sample in real time based on the real-time collected flow data, the geometric dimensions of the slip zone soil sample, and the dynamic total air pressure difference between the upper and lower ends of the soil sample. The permeability coefficient calculation formula is: (1); In the formula, The soil sample permeability coefficient; Volumetric flow rate measured by the micro-flow sensor 19; The axial height of the soil sample inside the central sample mold assembly 11; This represents the cross-sectional area of ​​the soil sample. The density of the osmotic solution; It is the acceleration due to gravity; This represents the total air pressure difference between the upper and lower ends of the soil sample.

[0055] After the test, loosen the sealing threads between the components and separate the two shells 24 of the central sample mold assembly 11 to completely remove the slip zone soil sample, providing a high-quality test sample for subsequent research such as soil microstructure observation and CT (Computed Tomography) scanning.

[0056] In a preferred embodiment, the support frame system includes a top plate 1 and a bottom plate 20, and a set of support columns 2 vertically connecting the two. This configuration provides a stable and robust support structure for the entire test device, ensuring that the device will not shake or deform under stress during the test, thus guaranteeing the accuracy and stability of the test. It also enables precise coordination of the components, providing a reliable foundation for subsequent operations such as permeation tests, and effectively avoiding test errors caused by device instability.

[0057] In a preferred embodiment, the sample assembly system is installed inside the support frame system and includes an upper pressure chamber 7, a middle sample mold assembly 11, and a lower pressure chamber 16 connected sequentially from top to bottom with sealed threaded connections. The middle sample mold assembly 11 is a split structure consisting of two semi-cylindrical shells 24 joined together, with external threads 23 at both its upper and lower ends, which engage with the internal threads 22 at the bottom of the upper pressure chamber 7 and the top of the lower pressure chamber 16, respectively. This configuration facilitates the installation and disassembly of the sample, allowing the soil sample to be quickly placed inside the middle sample mold assembly 11. The sealed threaded connections ensure the airtightness between the components, preventing solution leakage during the test and ensuring that the test is conducted in a closed environment, thereby improving the reliability of the test data.

[0058] In the preferred embodiment, a self-tightening shaft U-shaped sealing ring 4 is embedded in the central perforation of the top cover of the upper pressure chamber 7, with its open end facing the inside of the upper pressure chamber 7. With this configuration, during the pressurization process of the test, as the pressure increases, the lip of the sealing ring 4 automatically expands, which can better fit the contact surface, achieve zero leakage sealing, effectively prevent gas or liquid leakage in the upper pressure chamber 7, ensure the stability of the test pressure, and thus ensure the accuracy of the test data, providing a reliable guarantee for the accurate analysis of the permeability characteristics of the soil sample.

[0059] In a preferred embodiment, the longitudinal splicing surfaces of the two semi-cylindrical shells 24 are provided with mutually cooperating sealing grooves, and longitudinal sealing strips are embedded in the sealing grooves. Under the axial extrusion force of the external threads 23 at the upper and lower ends, the longitudinal joint is sealed with water tightness. The above configuration further enhances the sealing performance of the middle sample mold assembly 11, prevents the solution from leaking from the longitudinal joint during the test, avoids inaccurate test data due to leakage, ensures that the soil sample is tested in a stable and sealed environment, and enables the test results to truly reflect the permeability performance of the soil sample.

[0060] In a preferred embodiment, filter screens 10 are arranged at both the upper and lower ends of the middle sample mold assembly 11, and several support columns 17 are vertically arranged in the lower pressure chamber 16, with their tops abutting against the filter screens 10 below. A solution collection tube 18 is arranged at the center of the bottom of the lower pressure chamber 16, with its end connected to a micro flow sensor 19, which is communicatively connected to the data acquisition system. With the above arrangement, the filter screens 10 can filter soil particles to prevent them from clogging the infiltration path, the support columns 17 provide support for the filter screens 10 to ensure their stability, and the solution collection tube 18 and the micro flow sensor 19 can accurately collect and measure the flow rate of the infiltrated solution, providing accurate data for analyzing the infiltration characteristics of the soil sample.

[0061] In the preferred embodiment, the filter screen 10 is densely covered with honeycomb-shaped pores and is manufactured using 3D printing. The pore size of the honeycomb-shaped pores is customized according to the particle size distribution of the soil sample to be tested. The number of support columns 17 is at least three, forming a rigid structure. With the above configuration, the filter screen 10 with pore size customized according to the particle size distribution of the soil sample can more effectively filter soil sample particles, prevent clogging, and ensure unobstructed permeation paths. The three or more rigid support columns 17 can provide stable support for the filter screen 10, ensuring that the filter screen 10 does not deform during the test, guaranteeing the accuracy and stability of the test, and making the test results more reliable.

[0062] In a preferred embodiment, the data acquisition system includes a main control box 14 and a servo pressure controller 15 mounted on a support frame system; it also includes a user terminal 21, which is communicatively connected to a micro flow sensor 19 for receiving real-time flow data of the permeation solution. With these settings, the main control box 14 and the servo pressure controller 15 can precisely control parameters such as test pressure, and the user terminal 21 can receive flow data in real time, facilitating timely monitoring of the test situation by test personnel, adjustment of test parameters based on the data, improvement of test efficiency, and provision of comprehensive data support for accurate analysis of soil sample permeability characteristics.

[0063] In the preferred embodiment, the servo mechanical pressurization system, in conjunction with the servo pressure controller 15, forms a closed-loop feedback to achieve independent adjustment of the axial mechanical stress of the soil sample. With the above configuration, the closed-loop feedback mechanism can monitor and adjust the axial mechanical stress in real time, ensuring its accuracy and stability. It can independently adjust the stress according to the test requirements, simulate the geostress environment of soil at different depths, provide reliable conditions for studying the permeability characteristics of soil under different stress conditions, and make the test results more valuable for practical applications.

[0064] In a preferred embodiment, the servo mechanical pressurization system includes a servo electric push rod 3 installed on top of the support frame system. Its pressure rod 8 extends into the upper pressure chamber 7 through a U-shaped sealing ring 4, and the end of the pressure rod 8 is connected to a pressure plate 9. With this configuration, the servo electric push rod 3 can precisely control the extension and retraction of the pressure rod 8, thereby applying stable axial pressure to the soil sample through the pressure plate 9. Furthermore, the pressure rod 8 passing through the sealing ring 4 ensures the sealing of the device, preventing pressure leakage and ensuring that the axial pressure is accurately applied to the soil sample, thus improving the accuracy and reliability of the test.

[0065] In a preferred embodiment, the air pressure control system includes a set of servo air pumps 13 installed on the support frame system. The upper servo air pump 13 is connected to the side wall interface of the upper pressure chamber 7 via a pressurization pipe 6, and the lower servo air pump 13 is connected to the side wall interface of the lower depressurization chamber 16 via a depressurization pipe. With this configuration, by pressurizing the upper pressure chamber 7 and depressurizing the lower depressurization chamber 16 with the two sets of servo air pumps 13 respectively, the pressure at both ends of the soil sample can be flexibly controlled to form different pressure differences, simulate various permeability environments, provide convenience for studying the permeability characteristics of soil samples under different pressure conditions, and expand the application range of the test.

[0066] In the preferred embodiment, the soil sample in step 1 is undisturbed soil or remolded soil. The central sample mold assembly 11 achieves undisturbed filling of the soil sample by separating two semi-cylindrical shells 24. The above settings are applicable to both undisturbed soil and remolded soil. Undisturbed filling can maintain the original structure and properties of the soil sample to the greatest extent, so that the test results can truly reflect the permeability characteristics of the soil sample in the actual environment, improve the accuracy and reliability of the test, and provide a more accurate reference for engineering practice.

[0067] In the preferred embodiment, the vertical axial pressure applied in step 2 is used to simulate the geostress environment of deep soil. The servo electric push rod 3 achieves precise setting and constant pressure maintenance of the axial pressure through displacement closed-loop adjustment. The above settings can accurately simulate the geostress environment of deep soil, and the displacement closed-loop adjustment can ensure the stability of the axial pressure, so that the soil sample can be subjected to permeability test under stable stress conditions, improve the fit between the test results and the actual engineering conditions, and provide reliable data support for the study of the permeability characteristics of deep soil.

[0068] In the preferred embodiment, step 4 employs a bidirectional pneumatic drive mode that couples upper constant positive pressure drive with lower constant negative pressure suction to create a high hydraulic gradient at both ends of the soil sample, thereby accelerating the permeability test of low-permeability soil. This bidirectional pneumatic drive mode can quickly create a high hydraulic gradient at both ends of the soil sample, accelerating the permeability process, shortening the test time, and ensuring the accuracy of the test. It can more efficiently study the permeability characteristics of low-permeability soil, providing timely test data support for related engineering projects.

[0069] The servo-pressurized bidirectional pneumatic permeability testing device and method proposed in this invention effectively overcome many difficulties faced by existing permeability testing devices when testing low-permeability soft soils. These problems include large sample loading and unloading disturbances, long testing cycles, poor reliability of high-pressure dynamic seals, easy clogging of the filtration system, and the inability to flexibly decouple and couple stress and permeability pressure. With this device and method, high-precision, automated, and rapid permeability testing of low-permeability soils can be achieved. While ensuring the structural integrity of the soil sample, it accurately simulates the permeability characteristics of soil under deep high-stress environments, providing strong support for related research.

[0070] The device boasts numerous design highlights. The semi-split central sample mold assembly 11, composed of two assembled shells, enables undisturbed and rapid loading and unloading of soil samples, solving the problems inherent in traditional molds. The external threads 23 at its upper and lower ends engage with the internal threads 22 of the upper and lower pressure chambers, ensuring structural stability. The bidirectional pneumatic drive mode, coupled with constant positive pressure drive at the top and constant negative pressure suction at the bottom, establishes a significant hydraulic gradient at both ends of the soil sample via a servo air pump 13 and a vacuum pump, substantially shortening testing time. The self-tightening U-shaped sealing ring 4 utilizes the characteristic of automatic lip expansion under pressure to achieve zero leakage, ensuring accurate test data. The 3D-printed honeycomb filter 10 can adjust its pore size according to the soil sample particle size distribution, preventing clogging and ensuring unobstructed permeation paths. Furthermore, the support columns within the lower pressure chamber 16 provide support for the filter.

[0071] By organically combining a servo-mechanical pressurization system and a pneumatic control system, this device can accurately simulate the complex stress environment of deep soil, providing a reliable platform for studying the permeability characteristics of soil under real stress conditions. This device and method are not only applicable to low-permeability soft soils such as slip zones, but can also be extended to other types of geotechnical materials by adjusting parameters and configurations, demonstrating broad application prospects and good adaptability. Simultaneously, this invention balances ease of experimental operation with accurate data acquisition; its overall structure is reasonable and easy to operate, reflecting high engineering practicality and providing new perspectives and experimental evidence for theoretical research in related fields.

Claims

1. A servo-pressurized bidirectional pneumatic permeation testing device, characterized in that, The system includes a support frame system and a sample assembly system, a pneumatic control system, a servo mechanical pressurization system and a data acquisition system installed on the support frame system. The sample assembly system includes an upper pressure chamber (7), a middle sample mold assembly (11) and a lower pressure chamber (16) connected in sequence. A water inlet valve (5) is provided at the top of the upper pressure chamber (7), and a filter screen (10) is provided inside the middle sample mold assembly (11). The pneumatic control system includes a set of servo air pumps (13), which are connected to the upper pressure chamber (7) and the lower pressure chamber (16) respectively. The servo mechanical pressurization system includes an electric push rod (3), whose pressure rod (8) passes through the sealing ring (4) and extends into the upper pressure chamber (7) to connect to the pressure plate (9). The data acquisition system includes a main control box (14) and a servo pressure controller (15).

2. The servo-pressurized bidirectional pneumatic permeation test apparatus according to claim 1, characterized in that: The support frame system includes a top plate (1) and a bottom plate (20), and a set of support columns (2) that vertically connect the two.

3. The servo-pressurized bidirectional pneumatic permeation test apparatus according to claim 1, characterized in that: The sample assembly system is installed inside the support frame system and includes an upper pressure chamber (7), a middle sample mold assembly (11), and a lower pressure chamber (16) connected by a sealed thread from top to bottom. The middle sample mold assembly (11) is a split structure consisting of two semi-cylindrical shells (24) joined together. Both its upper and lower ends are provided with external threads (23), which are screwed into the internal threads (22) at the bottom of the upper pressure chamber (7) and the top of the lower pressure chamber (16), respectively. A self-tightening shaft U-shaped sealing ring (4) is embedded in the central through hole of the top cover of the upper pressure chamber (7), with its open end facing the inside of the upper pressure chamber (7). The longitudinal splicing surfaces of the two semi-cylindrical shells (24) are provided with mutually cooperating sealing grooves. The sealing grooves are embedded with longitudinal sealing strips. Under the axial extrusion force of the external threads (23) screwed together at the upper and lower ends, a detachable radial fastening clamp is added to the outside of the two semi-cylindrical shells (24) to provide radial locking force, so as to achieve watertight sealing of the longitudinal joint.

4. The servo-pressurized bidirectional pneumatic permeation test apparatus according to claim 1, characterized in that: The middle sample mold assembly (11) has filter screens (10) arranged at both the upper and lower ends. Several support columns (17) are vertically arranged in the lower pressure chamber (16), with their tops abutting against the filter screens (10) below. A solution collection tube (18) is arranged at the center of the bottom of the lower pressure chamber (16), with its end connected to a micro flow sensor (19), and the micro flow sensor (19) is connected to the data acquisition system. The filter screen (10) is densely covered with honeycomb-shaped holes and is made by 3D printing. The pore size of the honeycomb-shaped holes is customized according to the particle size distribution of the soil sample to be tested. The number of support columns (17) is at least three, forming a rigid structure.

5. The servo-pressurized bidirectional pneumatic permeation test apparatus according to claim 4, characterized in that: The data acquisition system includes a control system main control box (14) and a servo pressure controller (15) installed on the support frame system; it also includes a user terminal (21), which is connected to a micro flow sensor (19) to receive the flow data of the permeation solution in real time; the servo mechanical pressurization system works with the servo pressure controller (15) to form a closed-loop feedback to achieve independent adjustment of the axial mechanical stress of the soil sample.

6. The servo-pressurized bidirectional pneumatic permeation test apparatus according to claim 1, characterized in that: The servo mechanical pressurization system includes a servo electric push rod (3) installed on top of the support frame system. Its pressure rod (8) extends into the upper pressure chamber (7) through a shaft U-shaped seal ring (4). The end of the pressure rod (8) is connected to a pressure plate (9).

7. The servo-pressurized bidirectional pneumatic permeation test apparatus according to claim 1, characterized in that: The pneumatic control system includes a set of servo air pumps (13) installed on the support frame system. The upper servo air pump (13) is connected to the side wall interface of the upper pressure chamber (7) via a pressurization pipe (6), and the lower servo air pump (13) is connected to the side wall interface of the lower pressure chamber (16) via a pressure extraction pipe.

8. A servo-pressurized bidirectional pneumatic permeability testing method, comprising a method for conducting soil permeability tests using the servo-pressurized bidirectional pneumatic permeability testing device described in any one of claims 1 to 7, characterized in that, Includes the following steps: Step 1, Sample preparation and airtight assembly: Customize a 3D printed honeycomb filter (10) according to the particle size distribution of the soil sample to be tested, place the soil sample into the inner cavity of the middle sample mold assembly (11), place the 3D printed honeycomb filter (10) on the upper and lower surfaces of the soil sample, and screw the middle sample mold assembly (11) with the upper pressure chamber (7) and the lower pressure chamber (16) to form an airtight test chamber; Step 2, Axial stress servo loading and simulation: Start the servo pressure controller (15), drive the pressure rod (8) and pressure plate (9) through the servo electric push rod (3) to apply the preset vertical axial pressure to the soil sample, use the self-tightening effect of the shaft U-shaped sealing ring (4) to ensure the seal, and maintain the constant axial pressure through closed-loop feedback; Step 3, Permeation Solution Injection and System Closure: Open the inlet valve (5) to inject the permeation solution into the cavity. After reaching the predetermined liquid level, close the valve to form a closed fluid circulation channel. Step 4, bidirectional pneumatic coupling to accelerate infiltration: The two servo air pumps (13) are started by the main control box (14) of the control system. The upper servo air pump (13) applies positive pressure to the upper pressure chamber (7), and the lower servo air pump (13) draws negative pressure to the lower pressure chamber (16), forming a hydraulic gradient at both ends of the soil sample. The support column (17) provides rigid support to the lower 3D printed honeycomb filter (10). Step 5: Real-time data acquisition and permeability calculation: The volumetric flow rate of the permeation solution is acquired by a micro flow sensor (19) and transmitted to the user terminal (21). The user terminal (21) calculates and outputs the permeability coefficient of the soil sample in combination with Darcy's law.

9. The servo-pressurized bidirectional pneumatic permeation test method according to claim 8, characterized in that: The soil sample in step 1 is undisturbed soil or remolded soil. The middle sample mold assembly (11) achieves undisturbed filling of the soil sample by separating two semi-cylindrical shells (24). The vertical axial pressure applied in step 2 is used to simulate the geostress environment of deep soil. The servo electric push rod (3) achieves precise setting and constant pressure maintenance of axial pressure through displacement closed-loop adjustment. In step 4, the bidirectional pneumatic drive mode of "upper constant positive pressure drive and lower constant negative pressure suction" is used to form a high hydraulic gradient at both ends of the soil sample, thereby achieving accelerated permeability testing of low-permeability soil.

10. The servo-pressurized bidirectional pneumatic permeation test method according to claim 8, characterized in that, The formula for calculating the permeability coefficient in step 5 is as follows: （1）； In the formula, The soil sample permeability coefficient; Volumetric flow rate measured by the micro-flow sensor (19); The axial height of the soil sample inside the central sample mold assembly (11); This represents the cross-sectional area of ​​the soil sample. The density of the osmotic solution; It is the acceleration due to gravity; This represents the total air pressure difference between the upper and lower ends of the soil sample.

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