Flexible wall apparatus and method for measuring gas permeability and diffusivity of soil

Abstract

This invention discloses a flexible wall device and method for measuring the gas permeability and diffusion coefficients of unsaturated soil. It mainly consists of a gas washing device, a confining pressure-volume change system, a wet-dry cycle control system, and a gas measuring device. The wet-dry cycle control system includes a humidity controller and valves. The gas measuring device includes a top cover, a cylindrical wall, a base, an upper cavity, and a lower cavity. The top cover, cylindrical wall, and base are connected by bolts and nuts. The base outlet is connected to the gas washing device, a gas supply device, the wet-dry cycle control system, and the gas measuring device. From bottom to top, the lower cavity, the sample, the upper cavity, and the control rod are arranged between the base and the top cover. This device offers high precision, overcoming the measurement errors of gas permeability and diffusion coefficients caused by the sidewall dominant flow in rigid wall permeameters. It can measure the gas permeability, diffusion coefficient, and volume change of samples under wet-dry cycles and different stress conditions, providing a reliable measurement tool and method for assessing gas migration in unsaturated soil.

Description

Technical Field

[0001] This invention relates to a flexible wall device and method for measuring the gas permeability and diffusion coefficient of unsaturated soil under wet-dry cycles. Background Technology

[0002] Shallow soils in nature are often unsaturated, and quantifying gas transport in unsaturated soils is crucial in many fields. Gas transport in unsaturated soils mainly occurs through two mechanisms: convection and diffusion. Convection is influenced by the soil's gas permeability coefficient (k). a The diffusion is influenced by the gas diffusion coefficient (D) of the soil and the gas pressure gradient. p The effect of concentration gradient on soil. Determining the k-value of the soil. a With D p This has significant implications for soil and air pollution control, as well as agriculture. For example, landfill gases (methane and carbon dioxide) produced by the anaerobic degradation of organic matter in landfills are mainly emitted into the atmosphere through convection under the pressure difference between the inside and outside of the cover layer system. Similarly, in agriculture and vegetated slope protection, the oxygen consumption of plant roots is typically transported from the atmosphere to the soil primarily through diffusion. Furthermore, under alternating rainfall and drying conditions, most topsoil undergoes multiple wet-dry cycles. During these cycles, the soil expands and contracts, resulting in significant changes in its volume and microstructure, thus significantly affecting the soil's kJ / kE ratio. a With D p .

[0003] Currently, the measurement of gas permeability and diffusion coefficient of soil in the laboratory is mainly done through rigid wall permeameters, that is, the side of the sample is connected to a rigid sleeve or rigid sidewall, such as patent CN108344668A "Experimental device for testing gas diffusion coefficient and permeability coefficient of unsaturated medium", CN 109883892A "A device and method for measuring gas diffusion coefficient in unsaturated soil", CN106053317A "Two-way gas permeability coefficient measuring instrument for unsaturated garbage soil", and CN109883892B "A device and method for measuring gas diffusion coefficient in unsaturated soil". Soil column devices are also used to measure the gas permeability and diffusion coefficient of unsaturated soil, such as patent CN111458274B "Soil column device and method for measuring gas permeability and diffusion coefficient of unsaturated soil". When using rigid wall permeameters and soil column devices to measure the gas permeability and diffusion coefficient of unsaturated soil, the following limitations exist: (1) There is a gap between the sample and the rigid wall, which leads to the sidewall dominant flow of gas along the interface between the sample and the rigid wall. Since the sidewall dominant flow is much higher than the permeability through the sample, the gas transport parameters of the sample are significantly overestimated; (2) Compacted clay, due to its low permeability and good air-tightness, is often used as an impermeable material in the capping layer system. However, under the action of wet-dry cycles, clay samples are prone to shrinkage deformation, which not only leads to the sidewall dominant flow of gas between the sample and the rigid wall, but also affects the pore structure of the soil, thus leading to k a and D p The changes in permeability are significant. Because rigid walls cannot prevent dominant flow on the sidewalls and are difficult to quantify the volumetric strain induced by soil wetting-drying cycles, they are unsuitable for measuring the gas permeability and diffusion coefficient of fine-grained soils under wetting-drying cycles. These drawbacks can be overcome through more controllable methods, such as using flexible wall permeameters. Currently, flexible wall permeameters exist for measuring the saturated permeability coefficient (k) of soils. s Examples include patents CN113758850A "Flexible Wall Permeameter for Integrated Temperature-Stress Control under Wet-Dry Cycles", CN1312466C "A Flexible Wall Permeameter for Measuring Permeability Coefficient", and CN105806766B "A Flexible Wall Permeameter for Measuring Volumetric Deformation"; flexible wall permeameters are also used to measure the k-value of unsaturated soils. a Examples include Chen et al. (2021, Soil and Tillage Research, 213, 105083) and Li et al. (2021, Arabian Journal of Geosciences, 14:732), but these flexible-wall permeameters are not suitable for measuring D. p This is mainly due to D p The measurement principle and k aThis is quite different, requiring multiple chambers and tracer gases, and the opening and closing of different chambers must be controlled during the experiment to regulate the tracer gas concentration in each chamber. Currently, there is no method to measure D. p Flexible wall devices. Furthermore, current k a and D p Most measurements are performed using two separate devices. Loading and unloading samples in different instruments can easily cause sample breakage and other disturbances, affecting the measurement results. The two sets of equipment also increase costs, are cumbersome to operate, and have long testing cycles. Compared to existing methods for measuring k in unsaturated soil... a and D p Rigid-wall permeameters currently lack the capability to overcome the dominant flow on the sidewalls and thus measure the k-value of unsaturated soil. a and D p Flexible wall device. Summary of the Invention

[0004] To overcome the problems of existing rigid walls, this invention provides a flexible wall device and method for measuring the gas permeability and diffusion coefficient of unsaturated soil.

[0005] The technical solution of this invention is:

[0006] A flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil: The flexible wall device consists of a gas washing device, a confining pressure-volume variation system, a wet-dry cycle control system, and a gas measuring device. The gas washing device includes a high-purity nitrogen cylinder, a pressure reducing valve, a pressure stabilizing valve, and a flow stabilizing valve. The confining pressure-volume variation system includes an air compressor, a pressure regulating valve one, a pressure regulating valve two, and a volume variation measuring tube. The wet-dry cycle control system includes a humidity controller, a humidity controller valve one, and a humidity controller valve two. The gas measuring device includes a top cover, a cylinder wall, a base, an upper cavity, and a lower cavity. The top cover, cylinder wall, and base are connected by bolts and nuts. The base has four channels, which are respectively connected to the gas washing device, the confining pressure-volume variation system, the wet-dry cycle control system, and the gas measuring system. From bottom to top, the structure between the base and the top cover consists of the lower cavity, a lower perforated plate, a lower permeable stone, a sample, an upper permeable stone, an upper perforated plate, the upper cavity, and a control rod.

[0007] The high-purity nitrogen cylinder is equipped with a pressure reducing valve, which is connected to a pressure regulating valve and a flow regulating valve. The flow regulating valve is connected to the air inlet on the base.

[0008] The dry and wet cycle control system includes a humidity controller, a humidity controller valve one, and a humidity controller valve two. One end of the humidity controller is connected to the base interface, and the other end is connected to the air outlet.

[0009] The air compressor output is connected to two pressure regulating valves. The output of pressure regulating valve one is connected to one end of the volume change measuring tube, and the other end of the volume change measuring tube is connected to the water inlet of the base. The water inlet leads to the pressure chamber. The output of pressure regulating valve two is connected to the base interface of the base after passing through a U-shaped pressure measuring tube, a gas sampling port, and a capillary tube two. The base interface leads to the base.

[0010] The top cover includes an exhaust valve and a control rod connected to a perforated loading plate at one end. The control rod seals the sealing ring one to the top cover with a nut one, and seals the sealing ring two to the upper cavity with a nut two.

[0011] The upper cavity has an opening on one side, connecting an air inlet pipe and an air outlet pipe. The air inlet pipe connects to the air inlet of the base, and the air outlet pipe connects sequentially to a humidity controller, an electronic soap film flow meter, and a capillary tube. Liquid droplets are injected into the capillary tube. A control rod passes through the top of the upper cavity. The bottom of the upper cavity is equipped with an upper perforated plate and an upper permeable stone. A perforated silicone gasket is located between the upper perforated plate and the perforated loading plate. The silicone gasket has the same perforation specifications as the upper perforated plate, and the two are overlapped according to their perforation positions, without obstructing each other. The top of the lower cavity is equipped with a lower perforated plate and a lower permeable stone. The internal net height of the upper and lower cavities is the same.

[0012] II. A method for measuring the gas permeability coefficient and diffusion coefficient of unsaturated soil, comprising the following steps:

[0013] The first step is to prepare the sample: dry the soil to be measured, crush it and pass it through a 2mm sieve, adjust the soil to the target moisture content, and compact it in layers using a mold according to the target dry density, and roughen the interface between each layer.

[0014] The second step is to install the sample: Place the lower cavity, sample, and upper cavity sequentially from bottom to top on the base surface. Apply Vaseline to the outer walls of the base, lower cavity, and upper cavity. Then, cover the sample base, lower cavity, sample, and upper cavity sidewalls with a latex film. Finally, secure the sample base and upper cavity with O-rings. Next, use nuts and bolts to fix and seal the cylinder wall, base, and top cover. Open the exhaust valve and adjust pressure regulating valve one to apply a pressure of 5 kPa to the volumetric strain measuring tube. Water enters the pressure chamber through the pipe. When water exits the exhaust valve and no air bubbles are visible, close the exhaust valve. Adjust pressure regulating valve one to apply a confining pressure of 20 kPa to the sample.

[0015] The third step is to flush the upper cavity: close all valves, rotate the control lever so that the holes on the porous loading plate and the upper porous plate are just misaligned, preventing the gas in the upper cavity from entering the sample through the upper porous plate. Open the valves of the high-purity nitrogen cylinder and the electronic soap film flow meter, and control the gas flow rate by adjusting the pressure regulating valve and the flow regulating valve. The gas flows into the upper cavity through the gas inlet pipe and is discharged into the atmosphere through the gas outlet pipe via the electronic soap film flow meter. After 8-10 minutes, close the pressure reducing valve of the high-purity nitrogen cylinder and the valve of the electronic soap film flow meter to stop the nitrogen input.

[0016] Fourth step: Open capillary valve one and capillary valve two, rotate the control lever to connect the porous loading plate and the upper porous plate through their respective openings, allowing gas in the upper cavity to enter the sample through the upper porous plate. Immediately start the timer; at this point, time t=0. Measure the change in oxygen (O2) concentration in the lower cavity over time through the gas sampling port. For samples with a large diffusion coefficient (saturation <85%), the concentration should be measured on a minute timescale. For samples with a small diffusion coefficient (saturation ≥85%), the concentration in the lower cavity should be measured on an hour timescale.

[0017] Fifth, based on Fick's law, the following theoretical formula is derived to fit the relationship between O2 concentration and time in the lower cavity, and the diffusion coefficient D is determined. p :

[0018] (1)

[0019] in:

[0020] (2)

[0021] (3)

[0022] Where: t is time (s); L is half the height of the sample (m); H is the net height inside the upper chamber (m); and H is the height of the lower chamber.

[0023] (Equal net height); C and C t These represent the O2 concentration in the lower cavity at the beginning and the O2 concentration in the lower cavity at time t (m 3 m -3 ); ε is the volumetric gas content of the sample (m³). 3 m -3 ).

[0024] Step 6, Measurement of gas permeability coefficient: Close capillary valve one and capillary valve two, open the U-shaped pressure gauge.

[0025] Pipe valves and electronic soap film flow meter valves, open the air compressor, adjust the intake pressure through pressure regulating valve two, and direct the air to the lower cavity.

[0026] Introduce air at a constant pressure. Once the reading of the U-shaped pressure measuring tube stabilizes, record the readings of the U-shaped pressure measuring tube and the electronic soap film flow meter.

[0027] Step 7: Change the intake pressure to obtain the gas flow rate at three different pressures. The flow rate can then be calculated using Darcy's law.

[0028] Gas permeability coefficient k of the soil sample a :

[0029] (4)

[0030] In the formula, S is the cross-sectional area of ​​the sample; μ is the viscosity coefficient of the gas; ΔP is the pressure difference between the two ends of the sample; and h is the sample height.

[0031] Degree; Q is the air flow rate.

[0032] Step 8: Close the U-shaped pressure gauge valve and the electronic soap film flow meter valve, and open the humidity control valve and humidity control valve.

[0033] Control valve two, adjust the humidity regulator according to the target moisture content, and control the gas flow rate to maintain a constant humidity. The gas flow rate passes through the test valve.

[0034] The soil samples were equilibrated for 8-10 days, and steps four through seven were repeated to obtain the k values ​​for soil samples at different moisture contents. a and D p After each wet-dry cycle, the volumetric strain measurement tube reading is recorded to obtain the volumetric strain of the sample.

[0035] The beneficial effects of this invention are:

[0036] This invention develops a flexible wall device for measuring the gas diffusion coefficient of unsaturated soil, overcoming the following shortcomings of traditional rigid wall devices: the D-value caused by the dominant flow on the sidewalls. p The previous method suffered from measurement errors, inability to simulate the stress state of field soil samples, and inability to measure the volumetric deformation of the sample under wet-dry cycles. This invention, by applying confining pressure, avoids the dominant flow along the sample sidewall caused by sample shrinkage during drying, thus better simulating the stress state of field soil samples. It can measure the volumetric deformation of the sample induced by wet-dry cycles and the k-value of the sample after this deformation. a and D p This greatly improves the sample k a and D p The measurement range and accuracy.

[0037] The device of this invention conducts wet-dry cycle tests using humidity control technology. Based on a single sample, it can measure its k at different moisture contents. a D pIn conjunction with the aforementioned flexible wall system, the soil deformation under wet-dry cycle conditions (k) is addressed. a and D p It overcomes the difficulty of measurement and can perform multiple wet and dry cycles, better simulating real-world conditions and overcoming the limitations of previous devices that required multiple samples for measurement.

[0038] Currently k a and D p Most measurements are performed using two separate devices, which are costly, cumbersome to operate, and have long testing cycles. The device of this invention can complete the measurement of soil k on the same sample. a and D p The measurement method avoids disturbance to the soil caused by multiple sample disassemblies, and can effectively assess the volumetric strain of the sample and k. a and D p The correlation. Attached Figure Description

[0039] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments:

[0040] Figure 1 This is a schematic diagram of the overall structure of the measuring device of the present invention;

[0041] Figure 2 This is a structural diagram of the upper cavity and the lower cavity;

[0042] Figure 3 This is a schematic diagram of the openings of the porous loading plate and the silicone pad; the opening specifications of both are the same.

[0043] In the diagram: 1-High-purity nitrogen cylinder; 2-Pressure reducing valve; 3-Pressure regulating valve; 4-Flow regulating valve; 5-Capillary tube 1; 6-Capillary valve

[0044] 1. Electronic soap film flow meter valve; 7. Electronic soap film flow meter; 9. Air inlet; 10. Air outlet; 11. Nut; 12. Bolt; 13. Control lever; 14. Nut 1; 15. Sealing ring 1; 16. Exhaust valve; 17. Top cover; 18. Cylinder wall; 19. Pressure chamber; 20. Nut 2; 21. Sealing ring 2; 22. Air inlet pipe; 23. Air outlet pipe; 24. Upper cavity; 25. O-ring; 26. Perforated loading plate; 27. Perforated silicone gasket; 28. Upper perforated plate; 29. ​​Upper permeable stone; 30. Test Sample; 31- Lower permeable stone; 32- Lower perforated plate; 33- Latex membrane; 34- Lower cavity; 35- Base; 36- O-ring; 37- Base; 38- Humidity controller valve one; 39- Humidity controller; 40- Water inlet; 41- Base interface; 42- Humidity controller valve two; 43- Gas sampling port; 44- Capillary valve two; 45- Capillary two; 46- U-shaped pressure measuring tube valve; 47- U-shaped pressure measuring tube; 48- Air compressor; 49- Pressure regulating valve one; 50- Pressure regulating valve two; 51- Volumetric deformation measuring tube. Detailed Implementation

[0045] like Figure 1-3 As shown, a flexible wall device and method for measuring the gas permeability and diffusion coefficient of unsaturated soil is characterized by mainly consisting of a gas washing device, a confining pressure-volume variation system, a wet-dry cycle control system, and a gas measuring device. The gas washing device includes a high-purity nitrogen cylinder 1, a pressure reducing valve 2, a pressure stabilizing valve 3, and a flow stabilizing valve 4 (range 0-100mL / min); the confining pressure-volume variation system includes an air compressor 48, a pressure regulating valve 49, a pressure regulating valve 50, and a volume variation measuring tube 51; the wet-dry cycle control system includes a humidity controller 39, a humidity controller valve 38, and a humidity controller valve 42; the gas measuring device includes a top cover 17, a cylinder wall 18, a base 37, an upper cavity 24, and a lower cavity 34, wherein the top cover 17, the cylinder wall 18, and the base 37 are connected by bolts 12 and nuts 11. The base 37 has four channels, which are respectively connected to the gas washing device (occupying two channels: air inlet 9 and air outlet 10), the confining pressure-volume change system (through channel: water inlet 40), the dry and wet circulation control system (through channel: air outlet 10 and base interface 41) and the gas measuring system. From bottom to top, the base 37 and the top cover 17 are arranged as follows: lower cavity 34, lower perforated plate 32, lower permeable stone 31, sample 30, upper permeable stone 29, upper perforated plate 28, upper cavity 24 and control rod 13.

[0046] The high-purity nitrogen cylinder 1 is equipped with a pressure reducing valve 2, which is connected to a pressure regulating valve 3 and a flow regulating valve 4. The flow regulating valve 4 is connected to the air inlet 9 on the base 37.

[0047] The wet-dry cycle control system includes a humidity controller 39, a humidity controller valve 38, and a humidity controller valve.

[0048] Door 2 42, the humidity controller 39 is connected to the base interface 41 at one end and to the air outlet 10 at the other end.

[0049] The output end of the air compressor 48 is connected to two pressure regulating valves. The output end of the first pressure regulating valve 49 is connected to one end of the volume change measuring tube 51. The other end of the volume change measuring tube 51 is connected to the water inlet 40 of the base 37. The water inlet 40 leads to the pressure chamber 19. The output end of the second pressure regulating valve 50 is connected to the base interface 41 of the base 37 after passing through the U-shaped pressure measuring tube 47 (range 0-5kPa, accuracy 0.01kPa), the gas sampling port 43, and the second capillary tube 45. The base interface 41 leads to the base 35.

[0050] The top cover 17 includes an exhaust valve 16 and a control rod 13 connected to a perforated loading plate 26 at one end. The control rod 13 seals the sealing ring 15 to the top cover 17 with a nut 14 and seals the sealing ring 21 to the upper cavity 24 with a nut 20.

[0051] An opening is formed on one side of the upper cavity 24, connecting an air inlet pipe 22 and an air outlet pipe 23. The air inlet pipe 22 is connected to...

[0052] The air inlet 9 of the base 37 is connected to the air outlet 10 via the air outlet pipe 23, which is sequentially connected to the humidity controller 39, the electronic soap film flow meter 8, and the capillary tube 5. Liquid droplets are injected into the capillary tube 5. The top of the upper cavity 24 is penetrated by the control rod 13. The bottom of the upper cavity 24 is equipped with an upper perforated plate 28 and an upper permeable stone 29 from top to bottom. A perforated silicone gasket 27 is located between the upper perforated plate 28 and the perforated loading plate 26. The silicone gasket 27 has the same perforation specifications as the upper perforated plate 28, and they are overlapped according to the perforation position, without obstructing each other. The top of the lower cavity 34 is equipped with a lower perforated plate 32 and a lower permeable stone 31 from bottom to top. The internal net height of the upper cavity 24 and the lower cavity 34 is the same.

[0053] The implementation process of this invention includes the following steps:

[0054] Step 1, Sample Preparation 30: Dry the soil (silt) to be measured, crush it and pass it through a 2mm sieve, adjust the soil to the target mass moisture content of 22%, and determine the target dry density (1.56 g / cm³). 3 The material is compacted in three layers inside the mold, and the interfaces of each layer are roughened.

[0055] The second step is to install the sample 30: Place the lower cavity 34, sample 30, and upper cavity 24 sequentially from bottom to top on the surface of the base 35. Apply Vaseline to the outer walls of the base 35, lower cavity 34, and upper cavity 24. Then, cover the sample base 35, lower cavity 34, sample 30, and upper cavity 24 with a latex film 33. Finally, use O-rings numbered 25 and 36 to fix them to the sample base 35 and upper cavity 24 respectively. Next, use nuts 11 and bolts 12 to fix and seal the cylinder wall 18, base 37, and top cover 17. Open the exhaust valve 16 and adjust the pressure regulating valve 49 to apply a 5 kPa air pressure to the volumetric strain measuring tube 51. Water enters the pressure chamber 19 through the pipe. When water comes out of the outlet of the exhaust valve 16 and there are no air bubbles, close the exhaust valve 16. Adjust the pressure regulating valve 49 to apply a 20 kPa confining pressure to the sample 30.

[0056] The third step is to flush the upper cavity 24: close all valves, rotate the control lever 13 so that the holes on the porous loading plate 26 and the upper porous plate 28 are just misaligned, preventing the gas in the upper cavity 24 from entering the sample 30 through the upper porous plate 28. Open the valves 1 and 7 of the high-purity nitrogen cylinder and the electronic soap film flow meter, and control the inlet flow rate to 100 ml / min by adjusting the pressure regulating valve 3 and the flow regulating valve 4. The gas flows into the upper cavity 24 through the inlet pipe 22 and is discharged to the atmosphere through the outlet pipe 23 and the electronic soap film flow meter 8. After 8-10 minutes, close the pressure reducing valve 2 of the high-purity nitrogen cylinder 1 and the valve 7 of the electronic soap film flow meter to stop the nitrogen input.

[0057] Fourth step, open capillary valve 6 and capillary valve 44, and rotate control lever 13 to allow the porous loading plate to open.

[0058] The upper porous plate 26 and the upper perforated plate 28 are connected through their respective openings, allowing gas in the upper cavity 24 to enter the sample 30 through the upper perforated plate 28. The timer is immediately started, at time t=0. The oxygen (O2) concentration in the lower cavity 34 is measured over time through the gas sampling port 43. For samples with a mass moisture content of 22%, the concentration in the lower cavity 34 should be measured on an hourly timescale. For samples with a mass moisture content of 9% and 18% obtained after subsequent drying using a humidity regulator, the O2 concentration should be measured on a minute-scale.

[0059] Fifth, based on Fick's Law, the following theoretical formula is derived to illustrate the relationship between O2 concentration and time in the lower cavity 34.

[0060] Perform fitting to determine the diffusion coefficient D p :

[0061] (1)

[0062] in:

[0063] (2)

[0064] (3)

[0065] Where: t is time (s); L is half the height of the sample (m); H is the net height inside the upper chamber (m); and H is the height of the lower chamber.

[0066] (Equal net height); C and C t These represent the O2 concentration in the lower cavity at the beginning and the O2 concentration in the lower cavity at time t (m -3 m -3 ); ε is the volumetric gas content of the sample (m³). -3 m -3 ).

[0067] Step 6, Measurement of gas permeability coefficient: Close capillary valve 1 (6) and capillary valve 2 (44), open U-shaped pressure measuring tube valve 46 and electronic soap film flow meter valve 7, turn on air compressor 48, adjust the inlet pressure through pressure regulating valve 2 (50), and introduce air at a constant pressure into the lower cavity 34. After the reading of U-shaped pressure measuring tube 47 stabilizes for 2-3 minutes, record the readings of U-shaped pressure measuring tube 47 and electronic soap film flow meter 8.

[0068] Step 7: Change the intake pressure to obtain gas flow rates (2×10⁻⁶) at three different pressures. -6 -7×10 -6 m 3 / s), root

[0069] The gas permeability coefficient k of the soil sample can be calculated using Darcy's law. a :

[0070] (4)

[0071] Where: S is the cross-sectional area of ​​the sample (m²) 2 μ is the viscosity coefficient of the gas (Pa·s); ΔP is the pressure at both ends of the sample.

[0072] Difference (Pa); h is the sample height (m); Q is the outlet flow rate (m³ / s). 3 / s).

[0073] Step 8: Close the U-shaped pressure measuring tube valve 46 and the electronic soap film flow meter valve 7, and open the humidity control valve 38.

[0074] Using humidity control valve 242, adjust the humidity regulator according to the target moisture content (9% and 18%) to control the flow rate of "drying" gas through sample 30. After equilibration for several days, the moisture content of the sample is reduced from the initial 22% to 18% and then 9%. Repeat steps four to seven to obtain the k of the soil sample at different moisture contents. a and D p After each wet-dry cycle, the volumetric strain measurement tube 51 is recorded to obtain the volumetric strain of the sample.

[0075] By measuring the moisture content of silt samples with different mass concentrations (dry density 1.56 g / cm³), 3 The gas permeability and diffusion coefficients of unsaturated soil obtained by the device of this invention and the rigid wall measurement method were compared (Table 1). As can be seen from the table, the measurement results of both the rigid and flexible walls gradually increased as the sample's initial moisture content decreased from 22% to 9%. This is because the volume shrinkage caused by sample drying gradually increases the influence of the dominant flow on the rigid wall sidewall and the sample. The device of this invention can overcome the influence of the dominant flow on the sidewall and achieve accurate measurement of the gas permeability and diffusion coefficients of unsaturated soil.

[0076] Table 1 Comparison of gas transport parameters measured by the device of the present invention and by a rigid wall

[0077]

[0078] a. Measurement difference is defined as:

Claims

1. A flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil, characterized in that, The flexible wall device consists of a gas scrubbing device, a confining pressure-volume variation system, a dry-wet cycle control system, and a gas measuring device. The gas scrubbing device includes a high-purity nitrogen cylinder (1), a pressure reducing valve (2), a pressure stabilizing valve (3), and a flow stabilizing valve (4). The confining pressure-volume variation system includes an air compressor (48), a pressure regulating valve one (49), a pressure regulating valve two (50), and a volume variation measuring tube (51). The dry-wet cycle control system includes a humidity controller (39), a humidity controller valve one (38), and a humidity controller valve two (42). The gas measuring device includes a top cover (17), a cylinder wall (18), and a base (37). The upper cavity (24) and the lower cavity (34) are connected by bolts (12) and nuts (11). The base (37) has four channels, which are respectively connected to the gas washing device, the confining pressure-volume change system, the dry and wet circulation control system and the gas measuring system. The lower cavity (34), the lower perforated plate (32), the lower permeable stone (31), the sample (30), the upper permeable stone (29), the upper perforated plate (28), the upper cavity (24) and the control rod (13) are connected from bottom to top between the base (37) and the top cover (17). The upper cavity (24) has an opening on one side, connecting the air inlet pipe (22) and the air outlet pipe (23). The air inlet pipe (22) is connected to the air inlet (9) of the base (37). The air outlet pipe (23) is connected in sequence to the humidity controller (39), the electronic soap film flow meter (8), and the first capillary tube (5) via the air outlet (10). Droplets are injected into the first capillary tube (5). The top of the upper cavity (24) is pierced by the control rod (13). The bottom of the upper cavity (24) is equipped with an upper... There is a perforated silicone pad (27) between the perforated plate (28) and the upper permeable stone (29), and between the upper perforated plate (28) and the perforated loading plate (26); the silicone pad (27) and the upper perforated plate (28) have the same opening specifications, and the two are placed overlapping according to the opening position, and the openings of the two do not block each other; the top of the lower cavity (34) is equipped with a lower perforated plate (32) and a lower permeable stone (31) from bottom to top; the internal net height of the upper cavity (24) and the lower cavity (34) is the same; Based on the target moisture content, the humidity regulator was adjusted to control the flow rate of gas with constant humidity through the sample (30). After equilibration for 8-10 days, soil samples with different moisture contents were obtained.

2. The flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil according to claim 1, characterized in that: The high-purity nitrogen cylinder (1) is equipped with a pressure reducing valve (2), which is connected to a pressure regulating valve (3) and a flow regulating valve (4). The flow regulating valve (4) is connected to the air inlet (9) on the base (37).

3. The flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil according to claim 1, characterized in that: The dry and wet cycle control system includes a humidity controller (39), a humidity controller valve one (38) and a humidity controller valve two (42). One end of the humidity controller (39) is connected to the base interface (41), and the other end is connected to the air outlet (10).

4. The flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil according to claim 1, characterized in that: The output end of the air compressor (48) is connected to two pressure regulating valves. The output end of the first pressure regulating valve (49) is connected to one end of the volume change measuring tube (51). The other end of the volume change measuring tube (51) is connected to the water inlet (40) of the base (37). The water inlet (40) leads to the pressure chamber (19). The output end of the second pressure regulating valve (50) is connected to the base interface (41) of the base (37) after passing through the U-shaped pressure measuring tube (47), the gas sampling port (43), and the second capillary tube (45). The base interface (41) leads to the base (35).

5. The flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil according to claim 1, characterized in that: The top cover (17) includes an exhaust valve (16) and a control rod (13) with one end connected to a perforated loading plate (26). The control rod (13) seals the sealing ring (15) with the top cover (17) through a nut (14), and seals the sealing ring (21) with the upper cavity (24) through a nut (20).

6. A method for measuring the gas permeability coefficient and diffusion coefficient of unsaturated soil, characterized in that: The method is implemented using the flexible wall device for measuring the gas permeability and diffusion coefficient of unsaturated soil as described in any one of claims 1 to 5. Includes the following steps: Step 1, Sample preparation (30): Dry the soil to be measured, crush it and pass it through a 2mm sieve, adjust the soil to the target moisture content, and compact it in layers according to the target dry density using a mold, and roughen the interface between the layers. The second step is to install the sample (30): Place the lower cavity (34), sample (30), and upper cavity (24) sequentially from bottom to top on the surface of the base (35). Apply Vaseline to the outer walls of the base (35), lower cavity (34), sample (30), and upper cavity (24). Then, put the latex film (33) on the side walls of the sample base (35), lower cavity (34), sample (30), and upper cavity (24). Finally, fix it to the sample base (35) and upper cavity (24) with O-rings respectively. On the cavity (24), the cylinder wall (18), base (37) and top cover (17) are fixed and sealed with nuts (11) and bolts (12); open the exhaust valve (16), adjust the pressure regulating valve (49), apply a certain air pressure to the volume change measuring tube (51), and water enters the pressure chamber (19) through the pipe. When water comes out of the outlet of the exhaust valve (16) and there are no bubbles, close the exhaust valve (16); adjust the pressure regulating valve (49) to apply a certain confining pressure to the sample (30); The third step is to flush the upper cavity (24): close all valves, rotate the control lever (13) so that the holes on the porous loading plate (26) and the upper porous plate (28) are just misaligned to prevent the gas in the upper cavity (24) from entering the sample (30) through the upper porous plate (28), open the valve (7) of the high-purity nitrogen cylinder (1) and the electronic soap film flow meter, and control the gas flow rate by adjusting the pressure regulating valve (3) and the flow regulating valve (4). The gas flows into the upper cavity (24) through the gas inlet pipe (22) and is discharged to the atmosphere through the gas outlet pipe (23) and the electronic soap film flow meter (8). After 8-10 minutes, close the pressure reducing valve (2) of the high-purity nitrogen cylinder (1) and the valve (7) of the electronic soap film flow meter to stop the nitrogen input. Fourth step, open capillary valve one (6) and capillary valve two (44), rotate control rod (13) so that the porous loading plate (26) and the upper porous plate (28) are connected through their respective openings, and the gas in the upper cavity (24) can enter the sample (30) through the upper porous plate (28). Start the timer immediately. At this time, the time t = 0. Measure the change of oxygen concentration in the lower cavity (34) over time through the gas sampling port (43) of the lower cavity (34). For samples with saturation < 85%, the concentration is measured in minutes. For samples with saturation ≥ 85%, the concentration in the lower cavity (34) is measured in hours. Fifth, based on Fick's law, the following theoretical formula is derived to fit the relationship between O2 concentration and time in the lower cavity (34) to determine the diffusion coefficient D. p : （1） in: （2） （3） In the formula: t is time; L is half the height of the sample; H is the net height inside the upper chamber, which is equal to the net height inside the lower chamber; C0 and C t ε represents the O2 concentration in the lower cavity at the beginning and at time t, respectively; ε is the volumetric gas content of the sample. Step 6, Measurement of gas permeability coefficient: Close capillary valve one (6) and capillary valve two (44), open U-shaped pressure measuring tube valve (46) and electronic soap film flow meter valve (7), turn on air compressor (48), adjust the inlet pressure through pressure regulating valve two (50), control the constant air pressure in the lower cavity (34), and after the reading of U-shaped pressure measuring tube (47) stabilizes, record the readings of U-shaped pressure measuring tube (47) and electronic soap film flow meter (8); Step 7: Change the inlet pressure to obtain the gas flow rate at three different pressures, and calculate the gas permeability coefficient k of the soil sample according to Darcy's law. a : （4） In the formula, S is the cross-sectional area of ​​the sample; μ is the viscosity coefficient of the gas; ΔP is the pressure difference between the two ends of the sample; h is the sample height; and Q is the gas flow rate. Step 8: Close the U-shaped pressure measuring tube valve (46) and the electronic soap film flow meter valve (7), open the humidity controller valve one (38) and the humidity controller valve two (42), adjust the humidity regulator according to the target moisture content, and control the gas flow rate of constant humidity through the sample (30). Equilibrate for 8-10 days, and repeat steps four to seven to obtain the k of the soil sample under different moisture contents. a and D p After each wet-dry cycle, the volume change measurement tube (51) reading is recorded to obtain the volume change of the sample.

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