Multifunctional automated non-saturated soil triaxial permeability test device

Abstract

The utility model relates to a kind of multifunctional automated non-saturated soil triaxial permeability test device, including triaxial chamber, confining pressure exerting system, axial force exerting system, gas pressure exerting system and water pressure exerting system and control acquisition system;Triaxial chamber includes loading frame and glass cylinder, loading frame blocks and packs glass cylinder upper and lower ends, and the loading frame in glass cylinder is set up sample on, and the upper and lower ends of sample are provided with pottery plate;Confining pressure exerting system respectively through loading frame bottom and top, and with glass cylinder inner cavity communication;Axial force exerting system is tightly pressed to the pottery plate above sample downwards;Gas pressure exerting system passes through loading frame bottom, and respectively with the upper and lower ends of sample communication;Water pressure exerting system passes through loading frame top, with glass cylinder inner cavity communication, and pass through loading frame bottom, respectively with two pottery plate communication;The utility model realizes the multifunctionalization of non-saturated test instrument, is conducive to the study of non-saturated soil physical mechanics property.

Description

Technical Field

[0001] This utility model relates to the field of geotechnical testing technology, and in particular to a multifunctional automated triaxial permeability testing device for unsaturated soil. Background Technology

[0002] Unsaturated soil refers to soil in which air and water (or partial saturation) are present simultaneously in the pores. Unsaturated soil is widely distributed in my country. The natural sedimentary soils widely distributed on the Earth's surface and the soil problems encountered in engineering construction are almost all unsaturated soil problems. Truly saturated soil is rarely seen in engineering practice, which makes the study of unsaturated soil mechanics very practical and significant.

[0003] Hydraulic hysteresis is a key characteristic of unsaturated soils, typically manifested in the lag of the soil-water characteristic curve. This lag is influenced by various factors, such as soil structure, temperature, and especially stress state. Two important parameters describing the moisture characteristics of unsaturated soils are the unsaturated permeability coefficient and the soil-water characteristic curve, which represents the relationship between soil matrix suction and water content. Currently, there are no automated instruments or test methods for determining the unsaturated permeability coefficient and soil-water characteristic curve of unsaturated soils under triaxial loading. Therefore, researching instruments and test methods for determining the unsaturated permeability coefficient and soil-water characteristic curve of unsaturated soils under triaxial loading is of great significance for research in the field of unsaturated soils.

[0004] Because unsaturated soils contain a gas phase, they have an additional stress variable: suction. Suction arises from the interaction between the surface of the solid phase within the soil and the liquid and gas phases within the pores, increasing the soil's strength and stiffness. Therefore, the theories and experimental techniques used for saturated soils are not applicable to unsaturated soils, often underestimating the soil's safety factor.

[0005] When analyzing unsaturated soil hazards such as rainfall-induced landslides, soil strength parameters are among the most important. These parameters are related to suction, so the contribution of suction to strength must be considered. Currently, few studies have addressed the testing of the mechanical properties of unsaturated soils under triaxial loading conditions. Different matric suction leads to different strength parameters; therefore, it is necessary to study the influence of matric suction on unsaturated soils under triaxial loading conditions when analyzing such unsaturated soil problems.

[0006] Triaxial testing is an existing technique for studying the shear strength characteristics of soils. However, traditional triaxial testing apparatuses are not suitable for studying unsaturated soils because: 1. Both pore air pressure and pore water pressure are applied from one end, resulting in the following drawbacks: uneven stress distribution; applying pressure from only one end leads to greater axial stress at the top than at the bottom, especially for specimens with large height-to-diameter ratios where the confining pressure at the bottom is significantly lower than the design value, causing an overestimation of shear strength and distortion of the shear zone position. High risk of seal failure. Distortion in pore pressure control and measurement; unidirectional drainage significantly prolongs the saturation time of low-permeability clay, and single-point sensors cannot capture sudden changes in pore pressure in the shear zone. In unsaturated soil tests, matrix suction is distributed along a height gradient, doubling the equilibration time. End constraints interfere with deformation; the rigid base inhibits free deformation, induces frictional resistance, and forces the shear zone inclination angle to deviate from the theoretical value.

[0007] 2. Pore air pressure and pore water pressure will change during the experiment, presenting the following drawbacks: Manual adjustment may not respond promptly to pressure changes during the experiment, leading to unstable matrix suction; volume changes in the sample during water absorption or drainage, without automatic compensation, will cause pore water pressure or air pressure to fail to maintain the set value, affecting data accuracy. Frequent manual adjustments are required, increasing experiment time and easily introducing human error. At high suction ranges, manual compensation is difficult to accurately maintain pressure, leading to experiment failure or data deviation. During long-term experiments, pressure may not be corrected in time, affecting the consistency of results.

[0008] 3. The use of a single terracotta plate without a device to detect whether the terracotta plate has reached saturation presents the following drawbacks: Uneven suction distribution and delayed equilibrium. When the terracotta plate is placed on only one side, the matrix suction decreases gradually along the sample height, resulting in a non-uniform moisture content distribution. Low water exchange efficiency. Water only seeps in from one end, easily forming a dry zone at the top of the sample, leading to a large deviation in the measured moisture content. Insufficient reliability in the high suction range. The single terracotta plate bears the entire matrix suction. When the suction is very high, the terracotta plate-sample interface is prone to preferential flow channels due to pressure differential peeling. Frequent high pressure differential impacts accelerate the development of microcracks in the terracotta plate, significantly reducing its lifespan. When the terracotta plate is not saturated, it will lose water and leak air, leading to unstable matrix suction inside the sample. Summary of the Invention

[0009] To address the problem that conventional triaxial testing devices are difficult to accurately conduct research tests on unsaturated soil, this invention provides a multifunctional automated triaxial permeability testing device for unsaturated soil. Based on existing triaxial testing devices, the device is optimized to meet the triaxial stress requirements for unsaturated soil research and to allow for controllable axial loads. It can efficiently measure the permeability coefficient, soil-water characteristic curve, and shear strength of unsaturated soil. The measurement method is simple to apply and has high measurement accuracy.

[0010] To achieve the above objectives, the technical solution adopted by this utility model is as follows: A multifunctional automated triaxial permeability testing device for unsaturated soil includes a triaxial chamber, a confining pressure application system, an axial force application system, an air pressure application system, a water pressure application system, and a control and acquisition system. The triaxial chamber includes a loading frame and a glass cylinder. The loading frame seals the upper and lower ends of the glass cylinder. The sample is placed on the loading frame inside the glass cylinder. Clay plates are placed at the upper and lower ends of the sample. The loading frame holds the sample by two clay plates. The two clay plates help to ensure that the matrix suction of the soil sample is constant during the test. The use of saturated clay plates at both the upper and lower ends of the sample can maintain a constant pore air pressure inside the sample. A rubber mold for wrapping the sample is also provided on the loading frame. The confining pressure application system passes through the bottom and top of the loading frame and communicates with the inner cavity of the glass cylinder. The confining pressure application system facilitates the injection of liquid into the glass cylinder to apply pressure around the sample. The axial force application system presses downward against the clay plate above the sample, facilitating the application of axial load on the sample. There are two sets of air pressure application systems, each of which passes through the bottom of the loading frame and communicates with the upper and lower ends of the sample. The water pressure application system passes through the top of the loading frame, communicates with the inner cavity of the glass cylinder, and passes through the bottom of the loading frame, communicating with the two clay plates.

[0011] Furthermore, the loading frame includes a base, a top cover, mounting rings, fixing rods, and a sample cap. The base has mounting rings on its edge, and each mounting ring has multiple fixing rods. The top cover is detachably installed between the tops of the fixing rods. The diameter of the circle formed by the multiple fixing rods is larger than the diameter of the glass cylinder. The upper and lower ends of the glass cylinder are tightly pressed against the top cover and the base to facilitate sealing of the glass cylinder.

[0012] Furthermore, a clay plate is embedded in the middle of the base, and a clay plate is also embedded in the top cover. The sample caps are arranged at intervals above the middle of the base. The base and the sample caps clamp the sample from top to bottom, and the clay plate is in contact with the upper and lower end faces of the sample. The rubber mold is a rubber tube that is fitted between the upper end of the sample cap and the base. A rubber ring is also fitted on the upper end of the sample cap and the base. The rubber ring tightens the rubber mold to ensure the seal of the sample.

[0013] Furthermore, the control and acquisition system is electrically connected to the confining pressure application system, the axial force application system, the air pressure application system, and the water pressure application system, respectively, to facilitate the control of each system and the reception of monitoring data from each system; The control and acquisition system includes a pressure sensor, a data acquisition device, a computer, and a pressure and volume control device. The computer, the pressure and volume control device, and the data acquisition device are connected in sequence via data cables. The computer has built-in software. The pressure and volume control device is connected to the confining pressure application system, the axial force application system, the air pressure application system, and the water pressure application system via data cables. The data acquisition device is connected to the pressure sensor via a data cable to receive data from the sensor.

[0014] Furthermore, the confining pressure application system includes a confining pressure device, a confining pressure pipe, and a water injection pipe. The confining pressure device is a confining pressure pump. The confining pressure device is connected to a pressure and volume control device. The confining pressure device is connected to the confining pressure pipe. The confining pressure pipe passes through the bottom of the loading frame and connects to the inner cavity of the glass cylinder. The water injection pipe passes through the top of the loading frame and connects to the inner cavity of the glass cylinder.

[0015] Furthermore, a confining pressure channel is provided in the base, and a confining pressure pipe is connected to the confining pressure channel, which in turn connects to the inner cavity of the glass cylinder. A water injection channel is provided on the top cover, and a water injection pipe is connected to the water injection channel, which in turn connects to the inner cavity of the glass cylinder. Valves are provided on both the water injection pipe and the confining pressure pipe, and the pressure sensor is also provided on the confining pressure pipe.

[0016] Furthermore, the axial force application system includes an axial pressure device, a hydraulic cylinder, and a shaft. The axial pressure device is an axial pressure pump, which is connected to a pressure volume control device. The axial pressure device and the hydraulic cylinder are connected through an oil pipe. The hydraulic cylinder is mounted on the top cover of the loading frame, and the piston rod of the hydraulic cylinder is connected to the shaft. The shaft passes downward through the top cover and abuts against the clay plate above the sample.

[0017] Furthermore, each of the aforementioned air pressure application systems includes an air pressure control device and a pore air pressure pipe. The air pressure control device is an air pressure pump, which is connected to a pressure-volume control device and is connected to the pore air pressure pipe. The pore air pressure pipe is equipped with a valve and a pressure sensor. The pore air pressure pipes in the two air pressure application systems pass through the bottom of the loading frame and are connected to the upper and lower ends of the sample, respectively. Both the sample cap and the base are provided with air pressure channels, and the pore air pressure pipe is connected to the air pressure channels. The pore air pressure pipe is connected to the upper and lower ends of the sample through the air pressure channels.

[0018] Furthermore, the water pressure application system includes a water head pressure device, an air-water exchange cylinder, a pore water pressure pipe, and a flushing pipe. The water head pressure device is an air pump, which is connected to a pressure volume control device and is connected to an air-water exchange cylinder containing liquid. A water supply pipe connects the air-water exchange cylinder and the water injection pipe. Two pore water pressure pipes and two flushing pipes are connected in parallel to the water supply pipe, each passing through the loading frame and the glass cylinder before connecting to the upper and lower clay plates. Valves are installed on both the pore water pressure pipe and the flushing pipe, and a pressure sensor is also installed on the pore water pressure pipe. The flushing pipe facilitates flushing the clay plates, saturating them and making them permeable to water but not air.

[0019] The beneficial effects of this utility model through the above technical solution are: This utility model has a reasonable structural design. By improving the triaxial pressure chamber structure, it can safely and efficiently realize three functions: automated triaxial shear test, permeability coefficient determination, and soil-water characteristic curve generation for unsaturated soil. It solves the pain point of the existing technology, namely that the traditional method requires the separate use of a triaxial apparatus, a permeameter, and a pressure plate apparatus, which leads to large differences in sample conditions and poor data correlation.

[0020] This invention relates to a confining pressure application system that works in conjunction with a triaxial chamber. By controlling the valves in the confining pressure pipe and the top water injection pipe, liquid can be easily injected into the glass cylinder to apply pressure around the sample, achieving automatic and controllable confining pressure loading. The magnitude of the confining pressure is sensed by sensors built into the pipes, ultimately enabling the determination of the permeability coefficient and shear strength parameters of unsaturated soil samples under two-dimensional controllable stress.

[0021] This invention allows for real-time adjustment of air pressure via a control and acquisition system. The air pressure application system, in conjunction with a triaxial chamber, enables saturation detection of the upper and lower clay plates to determine if they have reached saturation. Using saturated clay plates at both ends of the sample maintains a constant pore air pressure within the sample. Flushing pipes on the sample cap and base facilitate the flushing of air bubbles from both clay plates, ensuring the plates are saturated and permeable only to water, not air. The placement of clay plates at both ends of the sample guarantees a constant matrix suction during the test, avoiding the problem of a single clay plate failing to maintain stable matrix suction.

[0022] This invention applies air pressure to both ends of the sample through two sets of air pressure application systems, which can improve sealing reliability and reduce leakage risk. The synchronous pressure application at both ends balances the pressure on both sides of the sample, significantly reducing the pressure difference at the sealing interface. Even if the sample undergoes shear deformation, the sealing system can still remain stable, ensuring the integrity of long-term high-pressure tests.

[0023] This invention allows for real-time adjustment of water pressure via a control and acquisition system. The water pressure application system, in conjunction with a triaxial chamber, enables saturation detection of the upper and lower clay slabs to determine if they have reached saturation. A pore water pressure pipe on the base provides permeation head and pore water pressure. An air pressure application system, working in conjunction with the water pressure application system, controls the valves in the pore water pressure pipe and pore air pressure pipe of the sample to automatically and controllably manage different matrix suction forces on unsaturated soil samples. Built-in sensors sense and calculate the specific values ​​of matrix suction, thereby determining the soil-water characteristic curves, permeability coefficient, and shear strength parameters under different matrix suction forces.

[0024] This invention relates to an axial force application system that works in conjunction with a triaxial chamber. The system applies an automatically controllable axial force to precisely center an unsaturated soil cylindrical sample. An internal sensor detects the magnitude of the axial pressure, thereby determining the permeability coefficient and shear strength of the unsaturated soil under one-dimensional controllable stress. By cooperating with the triaxial chamber and confining pressure application system, the axial force application system uses internal sensors to feedback the axial load and confining pressure applied to the sample, thus determining the permeability coefficient and shear strength parameters of the unsaturated soil sample under three-dimensional controllable stress under controlled axial loading at a certain rate.

[0025] This invention can determine the permeability coefficient and shear strength of unsaturated soil under triaxial stress, which is more consistent with the actual stress state of the soil. It can accurately measure and control the suction, confining pressure and axial pressure of unsaturated soil samples without the need for axial translation technology, and can reproduce the negative pore pressure state of unsaturated soil under actual conditions. Attached Figure Description

[0026] Figure 1 This is a front view of a multifunctional automated unsaturated soil triaxial permeability testing device according to this utility model.

[0027] Figure 2 This utility model relates to a multifunctional automated triaxial permeability testing device for unsaturated soil. Figure 1 A magnified view of a portion of the image.

[0028] Figure 3 This is a front view of the triaxial chamber of a multifunctional automated triaxial permeability testing device for unsaturated soil according to this utility model.

[0029] The reference numerals in the drawings are: 1 triaxial chamber, 2 loading frame, 21 base, 22 top cover, 23 mounting ring, 24 fixing rod, 25 specimen cap, 26 nut, 27 clay board, 28 rubber mold, 29 rubber ring, 3 glass cylinder, 4 specimen, 51 confining pressure device, 52 confining pressure pipe, 53 water injection pipe, 54 confining pressure channel, 55 water injection channel, 61 axial pressure device, 62 hydraulic cylinder, 63 axial rod, 64 oil pipe, 71 air pressure control device, 72 pore air pressure pipe, 73 air pressure channel, 81 water head pressure device, 82 air-water exchange cylinder, 83 pore water pressure pipe, 84 scouring pipe, 85 water supply pipe, 86 water pressure channel, 87 scouring channel, 91 pressure sensor, 92 data acquisition device, 93 computer, 94 pressure-volume control device, 10 valve. Detailed implementation manners

[0030] The following makes a detailed description of the specific implementation manners of the present utility model in conjunction with the drawings: As Figures 1-3 shown, a multifunctional automated unsaturated soil triaxial permeability test device includes a triaxial chamber 1, a confining pressure application system, an axial force application system, an air pressure application system, a water pressure application system, and a control and acquisition system. The confining pressure application system, the axial force application system, the air pressure application system, and the water pressure application system all act inside the triaxial chamber 1, and the control and acquisition system is used to control the operation of each of the above systems and collect the operation data of each system.

[0031] The triaxial chamber 1 includes a loading frame 2 and a glass cylinder 3. The loading frame 2 includes a base 21, a top cover 22, a mounting ring 23, a fixing rod 24, and a specimen cap 25. The cross-section of the base 21 is "convex" shaped. An installation ring 23 is bolted to the edge of the base 21. The installation ring 23 is a circular ring body. At least four fixing rods 24 are provided on the installation ring 23. The fixing rods 24 are arranged vertically, and the four fixing rods 24 enclose a circle. A top cover 22 is detachably arranged between the tops of the four fixing rods 24. The top cover 22 is a disc body. The top cover 22 is sleeved on the fixing rods 24. Nuts 26 are provided above and below the top cover 22. The nuts 26 are bolted to the fixing rods 24 to achieve convenient disassembly and assembly of the top cover 22.

[0032] The glass cylinder 3 is arranged inside the loading frame 2. The loading frame 2 can seal the upper and lower ends of the glass cylinder 3 to realize the closure of the inner cavity of the glass cylinder 3. Specifically, the diameter of the circle enclosed by the four fixing rods 24 is larger than the diameter of the glass cylinder 3. Thus, the fixing rods 24 are arranged on the outer side of the glass cylinder 3. The upper and lower ends of the glass cylinder 3 are tightly abutted against the top cover 22 and the base 21, and the base 21 and the top cover 22 are used to seal the glass cylinder 3.

[0033] To improve the sealing performance of the glass cylinder 3, a sealing ring (not shown in the figure) can be installed at the end of the glass cylinder 3. The sealing ring has a circular cross-section and a U-shaped longitudinal section, allowing it to be secured to the end of the glass cylinder 3. The glass cylinder 3 then maintains a tight seal with the top cover 22 and the base 21 through the sealing ring, ensuring a reliable seal.

[0034] A sample 4, a cylindrical soil mass, is placed on a loading frame 2 inside a glass cylinder 3. Clay plates 27 are placed at both ends of the sample 4. Specifically, sample caps 25 are spaced apart above the middle of a base 21, with clay plates 27 embedded in the middle of the base 21 and on the top cover 22. The loading frame 2 holds the sample 4 between the two clay plates 27; that is, the base 21 and sample caps 25 hold the sample 4 vertically, with the clay plates 27 in contact with the upper and lower end faces of the sample 4. During installation, the clay plates 27 are inserted into circular grooves created on the base 21 and sample caps 25. Compared to traditional testing devices where a single clay plate 27 cannot maintain stable matrix suction for the sample 4, the design of two clay plates 27 ensures that the matrix suction of the sample 4 remains constant during the test.

[0035] The loading frame 2 is also equipped with a rubber mold 28 for wrapping the sample 4. The rubber mold 28 is a rubber tube that is fitted between the sample cap 25 and the upper end of the base 21, thus wrapping the sample 4. To fix the rubber mold 28, a rubber ring 29 is also fitted on the upper end of the sample cap 25 and the base 21. The elasticity of the rubber ring 29 is used to tighten the rubber mold 28, fixing it between the sample cap 25 and the base 21. To prevent the rubber ring 29 from shifting position, annular grooves are provided on the edges of the sample cap 25 and the base 21, allowing the rubber ring 29 to be placed within the annular grooves, thereby limiting the vertical displacement of the rubber ring 29.

[0036] The control and acquisition system is electrically connected to the confining pressure application system, axial force application system, air pressure application system, and water pressure application system, respectively. The control and acquisition system can control the operation of these systems and collect operational data. The control and acquisition system includes a pressure sensor 91, a data acquisition device 92, a computer 93, and a pressure-volume control device 94. The computer 93, pressure-volume control device 94, and data acquisition device 92 are connected sequentially via data cables.

[0037] The pressure and volume control device 94 is housed in a control cabinet. It is connected to the confining pressure application system, axial force application system, air pressure application system, and water pressure application system via data cables, controlling the operation of these systems. The data acquisition device 92 is connected to the pressure sensor 91 via a data cable. Pressure sensors 91 are installed in each of the above systems, and the data from the pressure sensors 91 is fed back to the data acquisition device 92.

[0038] After installing sample 4 in triaxial chamber 1, the computer 93 selects the appropriate module according to the test requirements. Available modules include: permeability tests under one-dimensional, two-dimensional, and three-dimensional controllable stress states; soil-water characteristic curve (SWCC) determination tests; and shear tests under multi-dimensional stress states with stress-controlled axial loading. After selecting the appropriate module, the test parameters are preset. The computer 93 then automatically initializes the built-in sensors and verifies the equipment status based on the preset parameters. During the test, the computer 93 maintains the matrix suction under the preset state by real-time adjustment of the water pressure application system and the air pressure application system, and maintains accurate tracking of the stress / strain path by real-time adjustment of the axial pressure application system and the confining pressure application system. Simultaneously, the data acquisition device 92 collects stress, strain, pore pressure, and other data synchronously through multiple channels. The operation is simple, the entire test process is optimized, and precise control and informatization and automation of the entire test process are achieved.

[0039] On the other hand, during the test operation, the computer 93 will dynamically monitor the response characteristics of sample 4, automatically trigger the test stage transition or termination conditions, and complete data verification, storage and report generation. It automatically and efficiently obtains the permeability coefficient and soil-water characteristic curve of unsaturated soil under one-dimensional, two-dimensional and three-dimensional controllable stress states corresponding to the matrix suction value; and the deformation curve and shear strength parameters under multi-dimensional stress states under stress-controlled axial loading, which is beneficial for carrying out different types of tests.

[0040] The confining pressure application system passes through the bottom and top of the loading frame 2 and communicates with the inner cavity of the glass cylinder 3. Specifically, the confining pressure application system includes a confining pressure device 51, a confining pressure pipe 52, and a water injection pipe 53. The confining pressure device 51 is a confining pressure pump and is connected to a pressure and volume control device 94. The operation of the confining pressure device 51 is controlled by the pressure and volume control device 94. The confining pressure device 51 is connected to the confining pressure pipe 52, which is equipped with a valve 10 and a pressure sensor 91. The confining pressure pipe 52 passes through the bottom of the loading frame 2 and communicates with the inner cavity of the glass cylinder 3, thereby allowing the confining pressure device 51 to pump out the gas inside the glass cylinder 3.

[0041] During installation, the confining pressure tube 52 has a confining pressure channel 54 inside the base 21. The confining pressure channel 54 is bent into an "L" shape, and the confining pressure tube 52 is connected to the confining pressure channel 54. That is, one end of the confining pressure channel 54 is directly connected to the inner cavity of the glass cylinder 3, and the other end is connected to the confining pressure tube 52 through a pipe joint. In this way, the confining pressure tube 52 is connected to the inner cavity of the glass cylinder 3 through the confining pressure channel 54.

[0042] A valve 10 is also installed on the water injection pipe 53. The water injection pipe 53 passes through the top of the loading frame 2 and connects to the inner cavity of the glass cylinder 3. Specifically, a water injection channel 55 is opened on the top cover 22, and the water injection pipe 53 is connected to the water injection channel 55. That is, the water injection channel 55 connects directly downward to the inner cavity of the glass cylinder 3, and the water injection channel 55 connects upward to the water injection pipe 53 through a pipe joint. Thus, the water injection pipe 53 connects to the inner cavity of the glass cylinder 3 through the water injection channel 55. The water injection pipe 53 needs to be used in conjunction with a water pressure application system. Water can be supplied into the glass cylinder 3 through the water injection pipe 53. At the same time, the confining pressure pipe 52 discharges the air inside the glass cylinder 3. The water inside the glass cylinder 3 can be used to apply pressure around the sample 4.

[0043] The confining pressure application system works in conjunction with the triaxial chamber 1 and achieves automatic and controllable confining pressure loading by controlling the valves 10 of the confining pressure pipe 52 and the top water injection pipe 53. The system senses the magnitude of the confining pressure through built-in sensors and ultimately achieves the function of determining the permeability coefficient and shear strength parameters of the unsaturated soil sample 4 under two-dimensional controllable stress state.

[0044] The axial force application system presses downward against the clay plate 27 above the specimen 4, thereby applying an axial load to the specimen 4. The axial force application system includes an axial pressure device 61, a hydraulic cylinder 62, and a shaft 63. The axial pressure device 61 is an axial pressure pump, connected to and controlled by a pressure volume control device 94. The axial pressure device 61 and the hydraulic cylinder 62 are connected via an oil pipe 64, on which a pressure sensor 91 is installed. The axial pressure device 61 draws hydraulic oil and pressurizes it to supply power to the hydraulic cylinder 62, providing power for its operation. The hydraulic oil is supplied through a hydraulic oil tank, and the hydraulic oil tank, axial pressure device 61, and hydraulic cylinder 62 are connected in sequence.

[0045] The hydraulic cylinder 62 is arranged vertically and is mounted on the top cover 22 of the loading frame 2. The piston rod of the hydraulic cylinder 62 is connected to the shaft 63, and the hydraulic cylinder 62 drives the shaft 63 to move up and down. The shaft 63 passes downward through the top cover 22, abuts against the sample cap 25, and presses against the clay plate 27 above the sample 4.

[0046] The axial force application system works in conjunction with the triaxial chamber 1 to apply automatically controlled axial force, precisely centering the unsaturated soil cylindrical specimen 4. Built-in sensors detect the magnitude of the axial pressure, thereby determining the permeability coefficient and shear strength of the unsaturated soil under one-dimensional controllable stress. Furthermore, through automated coordination with the triaxial chamber 1 and the confining pressure application system, the axial load and confining pressure applied to the specimen 4 are fed back via built-in sensors. This allows for the determination of the permeability coefficient and shear strength parameters of the unsaturated soil specimen 4 under three-dimensional controllable stress under controlled axial loading at a certain rate.

[0047] There are two sets of air pressure application systems. Each set of air pressure application systems passes through the bottom of the loading frame 2 and is connected to the upper and lower ends of the sample 4 respectively. Specifically, each set of air pressure application system includes an air pressure control device 71 and a pore air pressure pipe 72. The air pressure control device 71 is an air pressure pump. The air pressure control device 71 is connected to the pressure and volume control device 94 and can control the operation of the air pressure control device 71.

[0048] The air pressure control device 71 is connected to the pore air pressure pipe 72, thereby supplying gas at a certain pressure to the pore air pressure pipe 72. The pore air pressure pipe 72 is equipped with a valve 10 and a pressure sensor 91. The pore air pressure pipes 72 in the two air pressure application systems pass through the bottom of the loading frame 2 and are connected to the upper and lower ends of the sample 4.

[0049] Specifically, air pressure channels 73 are provided on both the sample cap 25 and the base 21. The sample cap 25 has one air pressure channel 73, which is in a straight line shape; the base 21 has two air pressure channels 73, both bent into an "L" shape. A pore air pressure pipe 72 connects to the air pressure channel 73, connecting the upper and lower ends of the sample 4. That is, one pore air pressure pipe 72 directly connects to one air pressure channel 73 on the base 21, thus connecting to the bottom of the sample 4; another pore air pressure pipe 72 connects to the other air pressure channel 73 on the base 21 and extends to connect to the air pressure channel 73 on the sample cap 25, thus connecting to the top of the sample 4.

[0050] The control and acquisition system can adjust the air pressure at any time. The air pressure application system, in conjunction with the triaxial chamber 1, can detect whether the upper and lower clay plates 27 have reached saturation. Using saturated clay plates 27 on both the upper and lower sides of the sample 4 can maintain a constant pore air pressure inside the sample 4. Preferably, in traditional test devices, the pore air pressure is applied only at the bottom, which has the drawback of single-end pressurization: when pressure is input from only one end, the sealing device at the other end bears all the pressure, which can easily lead to pressure leakage due to deformation of the sample 4 or failure of the seal. Compared with the traditional single-end pressurization, the method of applying air pressure to both ends of the sample 4 has the following core advantages: it improves the reliability of the seal, reduces the risk of leakage, and the simultaneous pressure application at both ends balances the pressure on both sides of the sample 4, significantly reducing the pressure difference at the sealing interface. Even if the sample 4 undergoes shear deformation, the sealing system can still maintain stability, ensuring the integrity of long-term high-pressure tests.

[0051] The water pressure application system passes through the top of the loading frame 2 and communicates with the inner cavity of the glass cylinder 3. It also passes through the bottom of the loading frame 2 and communicates with two clay plates 27 respectively. Specifically, the water pressure application system includes a water head pressure device 81, an air-water exchange cylinder 82, a pore water pressure pipe 83, and a flushing pipe 84. The water head pressure device 81 is an air pump and is connected to a pressure volume control device 94, which controls the operation of the water head pressure device 81. The water head pressure device 81 is connected to the air-water exchange cylinder 82, which contains liquid. The air-water exchange cylinder 82 has graduations, and the water head pressure device 81 can pump pressurized gas into the air-water exchange cylinder 82 to expel the liquid.

[0052] A water supply pipe 85 connects the air-water exchange cylinder 82 and the water injection pipe 53, allowing the discharged liquid to flow into the water injection pipe 53 and then into the glass cylinder 3. A pore water pressure pipe 83 and a flushing pipe 84 are connected in parallel to the water supply pipe 85, allowing the discharged liquid to flow into both the pore water pressure pipe 83 and the flushing pipe 84 respectively. Both the pore water pressure pipe 83 and the flushing pipe 84 are equipped with valves 10, and a pressure sensor 91 is also installed on the pore water pressure pipe 83.

[0053] Here, there are two pore water pressure pipes 83 and two flushing pipes 84, that is, there are two pore water pressure pipes 83 and two flushing pipes 84. Both the pore water pressure pipes 83 and the flushing pipes 84 pass through the loading frame 2 and the glass cylinder 3 and then connect to the upper and lower clay plates 27. Specifically, two water pressure channels 86 and two flushing channels 87 are opened in the base 21. Both the water pressure channels 86 and the flushing channels 87 are bent into an "L" shape. The sample cap 25 also has one water pressure channel 86 and one flushing channel 87. The water pressure channel 86 and the flushing channel 87 are in a straight line.

[0054] One pore water pressure pipe 83 is directly connected to a flushing channel 87 on the base 21, thus directly connecting to the clay plate 27 located below. Another pore water pressure pipe 83 connects to another flushing channel 87 on the base 21, extends out, and connects to a flushing channel 87 on the sample cap 25, thus connecting to the clay plate 27 located above. The flushing pipe 84 has the same connection structure, which will not be described further here. It should be noted that the valves 10 on each pipeline are electrically controlled valves 10, which are connected to the pressure and volume control device 94 and controlled by the pressure and volume control device 94 to control the opening and closing of the pipeline.

[0055] Two flushing pipes 84 flush away air bubbles from the upper and lower clay plates 27 and the chamber of the test apparatus, saturating the clay plates 27 so that they are only permeable to water and not air. The control and acquisition system can adjust the water pressure at any time. The water pressure application system, in conjunction with the triaxial chamber 1, can detect whether the upper and lower clay plates 27 have reached saturation. The pore water pressure pipe 83 connecting to the base 21 provides the permeation head and pore water pressure.

[0056] The air pressure application system works in conjunction with the water pressure application system to automatically control different matrix suction forces on the unsaturated soil sample 4 by controlling the valves 10 of the pore water pressure pipe 83 and the pore air pressure pipe 72. Under the control and data acquisition of the control and acquisition system, the specific values ​​of matrix suction forces are obtained by sensing and calculating through built-in sensors, thereby determining the soil-water characteristic curves, permeability coefficients and shear strength parameters under different matrix suction forces.

[0057] In traditional experimental setups, pore water pressure is applied only from the bottom, which has several drawbacks: when pressure is applied only from one end of the sample 4, a stress gradient difference exists, with the stress at the top being higher than at the bottom, leading to uneven stress distribution within the sample 4 and distortion of localized boundary deformation; with traditional single-end pressure application, the bottom of the sample 4 is constrained by the rigid base 21, limiting local strain. This approach, which applies water pressure to both ends of the sample 4 simultaneously, offers the following key advantages compared to traditional single-end pressure application: Simultaneous pressure application at both ends can offset the axial stress gradient, achieving a uniform stress distribution throughout the sample 4, more closely resembling the hydrostatic pressure state in a real formation; it reduces end friction and stress concentration, making the deformation of the sample 4 more consistent with theoretical assumptions; and the independent connection of the air pressure control device 71 at both ends of the sample 4 allows for independent adjustment of the pore water pressure at each end. This shortens the saturation time and avoids trapped air bubbles caused by single-end drainage. It also allows for precise control of the hydraulic gradient, simulating a real seepage path. Sensors at both ends can cross-verify data and identify abnormal pore pressure distribution inside sample 4; the air pressure and water pressure pipelines at both ends of sample 4 can independently control matrix suction, achieving bidirectional air-water synchronous regulation.

[0058] This apparatus can be used to conduct tests on the soil-water characteristic curves of unsaturated soil, the shear strength of unsaturated soil, and the permeability coefficient of unsaturated soil. The test method includes the following steps: Step 1, Pre-test Preparation: Prepare the necessary auxiliary materials for the test, including the test soil and soil sample preparation tools. Check that the connections of the triaxial chamber 1 and each component of each system are normal, that there are no leaks in each valve 10, and that the air pressure and hydraulic pressure are stable to ensure that each system is working properly.

[0059] Prepare soil sample 4 according to the test requirements, control the density of soil sample 4 to a certain value, use a three-layer compaction method to prepare sample 4 with various moisture contents and perform air saturation, start the water pressure application system, open the valves 10 of the flushing pipes 84 at the top and bottom of sample 4 to flush the upper and lower clay plates 27 and base 21, saturate the clay plates 27, and use the water head pressure device 81 and air pressure control device 71 to detect the saturation. After confirming that saturation is complete, close the valves 10 on the flushing pipes 84 and stop the water pressure application system.

[0060] Step 2, Sample 4 Installation: Place sample 4 in the middle of base 21, then fasten sample cap 25 on top of the saturated sample 4, ensuring both ends of sample 4 are in contact with the clay plate 27. A rubber mold 28 is fitted over sample 4 and secured by rubber rings 29. The rubber rings tightly fix the rubber molds at the upper and lower ends of sample 4 to sample cap 25 and base 21, respectively. Sample 4 is covered by glass cylinder 3, then top cover 22 is installed, and the clay plate 27 is pressed down by hydraulic cylinder 62 of the axial force application system, with shaft 63 contacting the top of sample cap 25. After connecting triaxial chamber 1 to the corresponding system via pipelines, open pore water pressure pipe 83 and pore air pressure pipe 72 to remove air bubbles from the pipelines.

[0061] Step 3: Water is injected into the glass cylinder 3 from the top of the loading frame 2 through the water pressure application system, that is, water is injected into the glass cylinder 3 through the water injection pipe 53. During water injection, air is discharged from the bottom of the glass cylinder 3 through the confining pressure pipe 52. After the cylinder is full of water, water injection and air discharge are stopped, and the confining pressure pipe 52 is closed. Then, the ends of the confining pressure pipe 52, the pore water pressure pipe 83, and the pore air pressure pipe 72 are connected to the confining pressure device 51, the water head pressure device 81, and the air pressure control device 71, respectively. The pore water pressure pipe 83 connected to the bottom of the air-water exchange cylinder 82 is opened. This activates the confining pressure application system, the air pressure application system, and the water pressure application system, making them ready for immediate operation.

[0062] Step 4: According to the test requirements, operate the computer 93 of the control and acquisition system, select the corresponding soil-water characteristic curve measurement test module, or select the corresponding module from the shear test under stress-controlled axial controllable loading mode and multi-dimensional stress state, or select the corresponding module from the permeability test under one-dimensional, two-dimensional, and three-dimensional controllable stress state, and preset the test parameters, including confining pressure, strain rate, axial pressure, and matrix suction. The type of test will change depending on the selected module.

[0063] The computer 93 of the control and acquisition system automatically controls the operation of the confining pressure application system, the air pressure application system, and the water pressure application system according to the preset test parameters. During the test, the computer 93 adjusts the water pressure application system and the air pressure application system in real time to maintain the matrix suction under the preset state; at the same time, it adjusts the axial pressure application system and the confining pressure application system in real time to maintain the preset stress state and the precise tracking of the stress / strain path.

[0064] Firstly, the data acquisition device 92 collects test data simultaneously, and the computer 93 dynamically monitors the response characteristics of the sample 4, automatically triggering test phase transition or termination conditions. After the test data stabilizes, the computer 93 calculates the matrix suction value of the soil sample 4 at each moisture content level based on the pore air pressure and pore water pressure. Then, it plots the moisture content and corresponding matrix suction value under a given controllable stress state into a soil-water characteristic curve. At the same time, it plots the saturated moisture content, minimum moisture content, and corresponding matrix suction value as upper and lower limit points on the soil-water characteristic curve, completing data verification, storage, and report generation.

[0065] Secondly, the data acquisition device 92 collects test data, and the computer 93 dynamically monitors the response characteristics of the sample 4. The computer 93 in the control acquisition system dynamically monitors the response characteristics of the sample 4, automatically triggers the test stage conversion or termination conditions, and completes data verification, storage and report generation to obtain the deformation curve and strength parameters under the corresponding matrix suction and stress states.

[0066] Thirdly, the data acquisition device 92 collects test data, and the computer 93 dynamically monitors the response characteristics of the sample 4. The computer 93 in the control acquisition system dynamically monitors the response characteristics of the sample 4. When the outflow and inflow of water from the sample 4 are equal, the system automatically triggers the test stage transition and uses the collected water pressure value and the outflow value in the air-water exchange cylinder 82 to calculate and solve the permeability coefficient of the unsaturated soil under the set matrix suction and stress state. The system also completes data verification, storage and report generation to obtain the permeability coefficient under the corresponding matrix suction and stress state.

[0067] Step 5: Repeat steps 1-4 according to the test requirements to obtain soil-water characteristic curves under different matrix suction and different controllable stress states, or to obtain shear strength under different matrix suction and different stress states, or to obtain permeability coefficient of unsaturated soil under different matrix suction and different stress states.

[0068] The embodiments described above are merely preferred embodiments of this utility model and are not intended to limit the scope of implementation of this utility model. Therefore, all equivalent changes or modifications made to the structure, features and principles described in the patent claims of this utility model should be included within the scope of the patent application of this utility model.

Claims

1. A multifunctional automated triaxial permeability testing device for unsaturated soil, characterized in that, It includes a triaxial chamber (1), a control and acquisition system, and a confining pressure application system, an axial force application system, a pneumatic pressure application system, a hydraulic pressure application system, and a control and acquisition system controlled by the control and acquisition system; The triaxial chamber (1) includes a loading frame (2) and a glass cylinder (3). The loading frame (2) seals the upper and lower ends of the glass cylinder (3). A sample (4) is placed on the loading frame (2) inside the glass cylinder (3). Clay plates (27) are placed on the upper and lower ends of the sample (4). The loading frame (2) clamps the sample (4) through the two clay plates (27). A rubber mold (28) for wrapping the sample (4) is also provided on the loading frame (2). The confining pressure application system passes through the bottom and top of the loading frame (2), communicates with the inner cavity of the glass cylinder (3), and injects water into the inner cavity of the glass cylinder (3); the axial force application system presses down against the clay plate (27) above the sample (4) and applies an axial load to the sample (4); The number of air pressure application systems is two sets. Each air pressure application system passes through the bottom of the loading frame (2) and is connected to the upper and lower ends of the sample (4) respectively to ensure that the pressure at both ends of the sample (4) is balanced. The water pressure application system passes through the top of the loading frame (2) and is connected to the inner cavity of the glass cylinder (3). It also passes through the bottom of the loading frame (2) and is connected to the two clay plates (27) respectively to ensure that the clay plates (27) are saturated.

2. The multifunctional automated unsaturated soil triaxial permeability testing device according to claim 1, characterized in that, The loading frame (2) includes a base (21), a top cover (22), a mounting ring (23), a fixing rod (24), and a sample cap (25). The base (21) has a mounting ring (23) on its edge. Each mounting ring (23) has multiple fixing rods (24). The top cover (22) is detachably installed between the tops of the fixing rods (24). The diameter of the circle formed by the multiple fixing rods (24) is larger than the diameter of the glass cylinder (3). The upper and lower ends of the glass cylinder (3) are tightly pressed against the top cover (22) and the base (21).

3. The multifunctional automated unsaturated soil triaxial permeability testing device according to claim 2, characterized in that, A clay plate (27) is embedded in the middle of the base (21), and a clay plate (27) is also embedded in the top cover (22). The sample cap (25) is arranged at intervals above the middle of the base (21). The base (21) and the sample cap (25) clamp the sample (4) from top to bottom. The clay plate (27) is in contact with the upper and lower end faces of the sample (4). The rubber mold (28) is a rubber tube. The rubber mold (28) is fitted between the upper end of the sample cap (25) and the base (21). The upper end of the sample cap (25) and the base (21) is also fitted with a rubber ring (29). The rubber ring (29) tightens the rubber mold (28).

4. The multifunctional automated unsaturated soil triaxial permeability testing device according to claim 2, characterized in that, The control and acquisition system includes a pressure sensor (91), a data acquisition device (92), a computer (93), and a pressure and volume control device (94). The computer (93), the pressure and volume control device (94), and the data acquisition device (92) are connected in sequence via data cables. The pressure and volume control device (94) is connected to the confining pressure application system, the axial force application system, the air pressure application system, and the water pressure application system via data cables. The data acquisition device (92) is connected to the pressure sensor (91) via a data cable.

5. A multifunctional automated unsaturated soil triaxial permeability testing device according to claim 4, characterized in that, The confining pressure application system includes a confining pressure device (51), a confining pressure pipe (52), and a water injection pipe (53). The confining pressure device (51) is a confining pressure pump. The confining pressure device (51) is connected to a pressure volume control device (94). The confining pressure device (51) is connected to the confining pressure pipe (52). The confining pressure pipe (52) passes through the bottom of the loading frame (2) and connects to the inner cavity of the glass cylinder (3). The water injection pipe (53) passes through the top of the loading frame (2) and connects to the inner cavity of the glass cylinder (3).

6. A multifunctional automated unsaturated soil triaxial permeability testing device according to claim 5, characterized in that, The base (21) has a confining pressure channel (54) and a confining pressure pipe (52) connected to the confining pressure channel (54). The confining pressure pipe (52) is connected to the inner cavity of the glass cylinder (3) through the confining pressure channel (54). The top cover (22) has a water injection channel (55) and a water injection pipe (53) connected to the water injection channel (55). The water injection pipe (53) is connected to the inner cavity of the glass cylinder (3) through the water injection channel (55). A valve (10) is provided on both the water injection pipe (53) and the confining pressure pipe (52). The pressure sensor (91) is also provided on the confining pressure pipe (52).

7. A multifunctional automated unsaturated soil triaxial permeability testing device according to claim 4, characterized in that, The axial force application system includes an axial pressure device (61), a hydraulic cylinder (62), and a shaft (63). The axial pressure device (61) is an axial pressure pump. The axial pressure device (61) is connected to a pressure volume control device (94). The axial pressure device (61) and the hydraulic cylinder (62) are connected through an oil pipe (64). The hydraulic cylinder (62) is installed on the top cover (22) of the loading frame (2). The piston rod of the hydraulic cylinder (62) is connected to the shaft (63). The shaft (63) passes downward through the top cover (22) and abuts against the clay plate (27) above the sample (4).

8. A multifunctional automated unsaturated soil triaxial permeability testing device according to claim 4, characterized in that, Each of the aforementioned air pressure application systems includes an air pressure control device (71) and a pore air pressure pipe (72). The air pressure control device (71) is an air pressure pump. The air pressure control device (71) is connected to the pressure volume control device (94). The air pressure control device (71) is connected to the pore air pressure pipe (72). A valve (10) and a pressure sensor (91) are provided on the pore air pressure pipe (72). The pore air pressure pipe (72) in the two air pressure application systems passes through the bottom of the loading frame (2) and is connected to the upper and lower ends of the sample (4). Both the sample cap (25) and the base (21) are provided with air pressure channels (73), and the pore air pressure pipe (72) is connected to the air pressure channel (73). The pore air pressure pipe (72) is connected to the upper and lower ends of the sample (4) through the air pressure channel (73).

9. A multifunctional automated unsaturated soil triaxial permeability testing device according to claim 6, characterized in that, The water pressure application system includes a water head pressure device (81), an air-water exchange cylinder (82), a pore water pressure pipe (83), and a flushing pipe (84). The water head pressure device (81) is an air pump. The water head pressure device (81) is connected to a pressure volume control device (94). The water head pressure device (81) is connected to an air-water exchange cylinder (82) containing liquid. A water supply pipe (85) is connected between the air-water exchange cylinder (82) and the water injection pipe (53). The water supply pipe (85) is connected in parallel with the pore water pressure pipe (83) and the flushing pipe (84). There are two pore water pressure pipes (83) and two flushing pipes (84), and they all pass through the loading frame (2) and the glass cylinder (3) and then connect to the upper and lower clay plates (27). A valve (10) is provided on both the pore water pressure pipe (83) and the flushing pipe (84). A pressure sensor (91) is also provided on the pore water pressure pipe (83).

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