A pre-buried air-filled ball for measuring the gas permeability and diffusion coefficient of soil

Abstract

The present invention provides a pre-buried ball injection device and method for measuring the gas permeability coefficient and diffusion coefficient of soil, which is composed of a ball injection device and a gas supply device. The ball injection device includes a nylon net, a spherical bracket, crushed stone, a diameter expansion interface, a gas injection pipe, a gas extraction interface and a gas extraction pipe, and the gas supply device includes an inert gas cylinder, a pressure regulating valve, a flow regulating valve, a U-shaped pressure measuring tube, a flow meter and a gas transmission pipe. The present invention is a single-point measurement and does not require any form factor. It only needs to measure the gas pressure value, gas concentration value and corresponding gas flow value at a point of target depth in the unsaturated soil layer, and the gas diffusion coefficient and permeability coefficient of the unsaturated soil can be obtained by simple manual calculation. In addition, the present invention has no limitation on the measurement depth and is small in size (the diameter of the ball injection device is about 2 cm), and has good applicability in indoor tests such as soil columns and model tanks.

Description

Technical Field

[0001] The invention belongs to the field of environmental rock and soil technology, and in particular relates to a pre-buried ball injection device and method for measuring the gas permeability coefficient and diffusion coefficient of unsaturated soil at any depth on site. Background Art

[0002] Quantifying the migration of gas in unsaturated soils has important scientific research and engineering application value in the fields of environmental geotechnical engineering, agronomy, environmental engineering, and ecology. The main mechanisms of gas migration in unsaturated soils are convection and diffusion, which are controlled by the gas permeability and diffusion coefficient, respectively. The gas permeability and diffusion coefficient of unsaturated soils are affected by many factors, including but not limited to the pore size, pore connectivity, saturation, porosity, etc. of unsaturated soils. Therefore, it is difficult to accurately estimate the gas permeability and diffusion coefficient of unsaturated soils in situ through empirical models. At present, there are also devices and methods suitable for measuring the gas permeability and diffusion coefficient of unsaturated soils in the laboratory, but the specimens of these tests are small, usually about 50 mm in diameter and about 50 mm in height. Since the soil on site is highly non-uniform and has different dry-wet cycle histories, a large number of undisturbed soil samples need to be collected on site before the diffusion coefficient and permeability coefficient of the soil on site can be obtained through laboratory measurement devices and methods, such as patents CN109883892B "A gas diffusion coefficient measurement device in unsaturated soil", CN205157395U "A gas permeability coefficient measuring instrument for unsaturated soil at normal pressure", CN 108344668A "Experimental device for testing gas diffusion coefficient and permeability coefficient of unsaturated medium", etc. However, collecting a large number of undisturbed soil samples and conducting laboratory measurements will consume a lot of manpower, material resources and financial resources, and it is difficult to avoid disturbing the soil samples during the collection of undisturbed soil samples, making it difficult to characterize the variability of the gas permeability coefficient and diffusion coefficient of the soil on site in time and space through laboratory measurement devices and methods. Therefore, it is urgent to develop devices and methods for in-situ measurement of gas diffusion coefficient and permeability coefficient of unsaturated soil on site.

[0003] Many scholars have developed different methods for measuring the gas diffusion coefficient / permeability coefficient of porous media on site. The measurement of on-site gas permeability coefficient is mainly obtained through gas permeation devices (such as patent CN102053054A "A method for measuring the gas permeability coefficient of landfill on site") or large-scale on-site gas injection / exhaust tests (such as Shan et al., 1992 (Water Resour. Res., 28 (4): 1105-1120), Olson et al., 2001 (J. Contam. Hydrol., 53 (1-2): 1-19)). Gas permeation devices are usually composed of hollow cylinders or hollow probes, most of which are short in size (20-200 mm in diameter and 25-200 mm in length). Due to the size of the measuring device, most measuring devices are limited to the measurement of surface soil or a certain depth. Large-scale on-site gas injection / exhaust tests can determine the gas permeability coefficient in a larger range, but their construction cost is high and it is difficult to be widely used. The on-site measurement of gas diffusion coefficient usually involves pumping tracer gas into the surface soil and determining the diffusion coefficient by detecting the tracer gas concentration at the injection point or a certain distance from the injection point (Laemmel et al., 2017 (Eur. J. Soil Sci., 68 (2): 156–166), Werner et al., 2003 (Environ. Sci. Technol., 37 (11): 2502-2510), Tick et al., 2007 (Water Air Soil Pollut., 184 (1-4): 355-362)). Some of the current field measurement devices require multiple measurement points to determine the gas diffusion coefficient (Laemmel et al., 2017 (Eur. J. Soil Sci., 68 (2): 156-166)), while others have a longer measurement period of about 4 days (Nioct et al., 1998 (J. Environ. Eng.-ASCE, 124 (11): 1038-1046)), which is easily affected by weather changes and thus affects the accuracy of the measurement results. At the same time, the calculation of the diffusion coefficient is complex, and it is necessary to use very complex analytical solutions or numerical simulations to fit the test data. In addition, these analytical solutions or numerical simulations mostly require various parameters of the soil to be known, such as the aerated porosity, which is very difficult to obtain, especially when the measurement point is deep.

[0004] In addition to the above limitations, among the current field measurement devices, very few devices can simultaneously measure the gas diffusion coefficient and permeability coefficient of the soil, such as patent CN110455673B "Device and method for in-situ measurement of gas migration parameters in unsaturated soil layers" and CN202210413988.9 "Device and method for in-situ measurement of gas diffusion coefficient and permeability coefficient of unsaturated soil". However, the CN110455673B patent has the following disadvantages: its theoretical calculation part needs to assume the distribution of spherical pressure contours and gas concentration contours, but the porous air pipe used for gas injection in the device is actually a cylinder, and the gas concentration and pressure are measured in the porous air pipe, resulting in the gas concentration and pressure distribution not being spherical, and its theoretical assumptions are inconsistent with the actual situation, resulting in certain measurement errors. Both CN110455673B and CN202210413988.9 require penetration operations. Since the casing or steel pipe has a certain diameter, it is easy to cause the surrounding soil to be compacted during the penetration process. In addition, due to soil shrinkage in dry weather, it is easy to cause gas dominant flow along the side wall of the casing or steel pipe, reducing the measurement accuracy. However, this device uses a gas injection pipe and a gas collection pipe with a smaller diameter (such as 2-3mm outer diameter), and pre-buries a gas injection ball with a diameter of about 2cm at the target depth in the soil, and then pulls the gas injection pipe and the gas collection pipe to the surface to avoid the side wall dominant flow in dry conditions and the extrusion of the surrounding soil by the casing. Therefore, in addition to on-site in-situ measurements, the present invention is also well applicable to model tests such as soil columns and model trenches: by selecting a smaller diameter injection ball (such as 1 to 2 cm), the injection pipe and the gas collection pipe are pulled out from the side walls of the soil column, model trench, etc. along the horizontal direction, and real-time measurement of the gas permeability coefficient and diffusion coefficient of the soil in the model test can be achieved, avoiding the disturbance of the model test caused by traditional sampling measurements.

[0005] In order to solve the limitations of existing measurement devices and methods, the present invention has developed a device and method that can accurately and quickly measure the gas diffusion coefficient and permeability coefficient of unsaturated soil at different depths in situ. The device is a single-point measurement and does not require any form factor, avoiding the measurement error caused by multiple measurement points in conventional methods. Compared with previous on-site measurement devices, the size of the balloon device can be smaller (such as 1 to 2 cm in diameter), greatly reducing the disturbance to the on-site soil. Secondly, the measurement data of the present invention can be obtained by simple manual calculation of the gas permeability and diffusion coefficient of the soil, without the need for complex curve fitting or numerical simulation back analysis, thereby improving the measurement efficiency. In addition, the present device can not only be used for on-site measurement of the gas diffusion coefficient and permeability coefficient of unsaturated soil, but can also be used for model tests such as soil columns and model trenches. Summary of the invention

[0006] In order to overcome the above-mentioned deficiencies in the prior art, the present invention provides a pre-buried device and method for measuring gas migration parameters in unsaturated soil layers, wherein the migration parameters include diffusion coefficient and permeability coefficient to meet the needs of engineering applications.

[0007] The technical solution of the present invention is:

[0008] 1. A pre-buried air-filled balloon for measuring the gas permeability and diffusion coefficient of soil:

[0009] It is mainly composed of a ball injection device and an air supply device. The ball injection device includes a nylon net, a spherical bracket (stainless steel or plastic material), gravel, an expansion interface, an air injection pipe, an air extraction interface and an air extraction pipe. One end of the air injection pipe is connected to the expansion interface, and the expansion interface is placed at the center of the spherical bracket. The cavity is filled with gravel, and then the nylon net is wrapped on the surface of the spherical bracket to form an injection ball. The air extraction interface is connected to one end of the air extraction pipe, and the other end of the air extraction pipe is 1 cm away from the surface of the injection ball; the air supply device includes an inert gas cylinder, a pressure regulating valve, a flow regulating valve, a U-shaped pressure measuring tube, a flow meter and an air supply pipe. The output end of the inert gas cylinder is connected to one end of the air supply pipe through the pressure regulating valve, the flow regulating valve, the U-shaped pressure measuring tube and the flow meter in sequence, and the other end of the air supply pipe is connected to the air injection pipe.

[0010] The ball center device of the injection ball is located at the depth of the required measuring point in the unsaturated soil layer.

[0011] The inert gas cylinder is used to generate the gas pressure required for introducing gas and is used as a tracer gas.

[0012] The pressure stabilizing valve and flow stabilizing valve are used to adjust the air intake flow and air pressure of the gas pipeline.

[0013] The U-shaped pressure measuring tube and flow meter are used to monitor the air intake flow and air pressure of the gas pipeline to keep them within a safe range.

[0014] The spherical cavity ball, the lower part of the air injection pipe and the lower part of the air extraction pipe of the air injection ball device are placed in the unsaturated soil layer, and the upper part of the air injection pipe, the upper part of the air extraction pipe and the air extraction interface extend out of the ground surface.

[0015] The gas injection pipe can be adjusted according to the depth of the measuring point, including but not limited to using one gas injection pipe, and a longer gas injection pipe can be used to measure deep soil.

[0016] 2. A method for filling a balloon with gas permeability coefficient and diffusion coefficient, the method of use comprising the following steps:

[0017] The first step is to pre-bury the center of the spherical cavity ball of the balloon device to the depth of the unsaturated soil layer to be measured, or to drill a hole (the hole diameter is slightly larger than the spherical cavity ball), and then install the center of the spherical cavity ball to the depth of the unsaturated soil to be measured, and then backfill and compact the drilled hole after the soil taken out from the drilled hole is sieved with 2 mm;

[0018] The second step is to connect the inert gas cylinder, the pressure regulating valve, the flow regulating valve, the U-shaped pressure measuring tube and the flow meter through the gas transmission pipe to form a gas supply device, and the outlet end of the gas supply device is connected to the gas injection pipe through the gas transmission pipe;

[0019] The third step is to extract 1-3 mL of gas at the gas extraction interface to measure the background concentration of inert gas in the unsaturated soil layer, and record the ambient atmospheric pressure and ambient temperature at that time;

[0020] The fourth step is to open the inert gas cylinder, adjust the intake flow rate to a constant volume flow value through the pressure regulating valve and the flow regulating valve, and then deliver the inert gas to the balloon injection device through the gas supply device. The U-shaped pressure measuring tube and the flow meter monitor the pressure and flow value of the inert gas, and extract 1-3mL of gas at the gas extraction interface every 30 minutes to measure the concentration of the inert gas;

[0021] Step 5: After the measured inert gas concentration is stable, record the measured inert gas concentration value C1 and the corresponding constant volume flow value q1;

[0022] Step 6: Increase the intake flow rate to a higher constant volume flow rate value through the pressure regulating valve and the flow regulating valve, so that the gas pressure measured by the U-shaped pressure measuring tube is higher than the atmospheric pressure and there is a significant difference (≥0.1 kPa). After the reading of the U-shaped pressure measuring tube is stable, record the absolute gas pressure value P2 and the corresponding constant volume flow value q2 measured by the U-shaped pressure measuring tube and the flow meter respectively.

[0023] Step 7: Substitute the measured inert gas concentration value C1 and the corresponding constant volume flow value q1 into the following formula to obtain the gas diffusion coefficient D: g :

[0024]

[0025] Where D g is the gas diffusion coefficient of the unsaturated soil layer (m 2 s -1 ); C1 is the tracer gas concentration in the injection ball when the gas migrates stably (m 3 m -3 );C hg is the background concentration of tracer gas in the soil layer (m 3 m -3 ); r is the radius of the multi-balloon (m);

[0026] Step 8: Substitute the measured absolute gas pressure value P2 and the corresponding constant volume flow value q2 into the following formula to obtain the gas permeability coefficient K: g :

[0027]

[0028] In the formula, K g is the gas permeability coefficient of the unsaturated soil layer (ms -1 ); P2 is the absolute pressure of the soil at 1 cm from the surface of the spherical cavity ball (Pa); ρ a is the air density (kgm -3 ); g is the acceleration due to gravity (9.8ms -2 );P hg is the ambient atmospheric pressure (Pa).

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] The present invention is a single-point measurement and does not require any form factor. It only needs to measure the gas pressure value, gas concentration value and corresponding gas flow value at a point of target depth in the unsaturated soil layer, and the gas diffusion coefficient and permeability coefficient of the unsaturated soil can be obtained by simple calculation, thus avoiding the measurement of multiple points in conventional methods.

[0031] The present invention can measure the gas diffusion coefficient / permeability coefficient of unsaturated soil in any depth, thus overcoming the limitation that the previous measuring devices can only measure the surface soil.

[0032] The present invention is small in size (the diameter of the injection ball is about 2 cm), and can be pre-buried and then pulled out with a smaller diameter injection pipe and gas collection pipe, avoiding the squeezing of the surrounding soil and the influence of the side wall dominant flow by the previous penetration device, and reducing the disturbance of the on-site soil during installation. In addition, the present invention has good applicability in indoor tests such as soil columns and model tanks.

[0033] The present invention can directly calculate the measurement result manually without the need for complex indoor curve fitting or numerical simulation back analysis. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0035] Figure 1 It is a schematic diagram of the overall structure of the measuring device of the present invention;

[0036] Figure 2 It is a schematic structural diagram of the balloon injection device of the present invention;

[0037] Figure 3 It is a schematic diagram of one-dimensional steady-state gas migration corresponding to a spherical cavity ball in an unsaturated soil layer;

[0038] In the figure: 1- inert gas cylinder; 2- valve; 3- gas delivery pipe; 4- pressure regulating valve; 5- flow regulating valve; 6- U-type pressure measuring tube; 7- flow meter; 8- gas injection pipe; 9- expansion interface; 10- nylon net; 11- gravel; 12- spherical cavity ball; 13- gas extraction pipe; 14- gas extraction interface; 15- spherical bracket. DETAILED DESCRIPTION

[0039] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be noted that the present embodiment is intended to further explain the present invention, but is not intended to limit the present invention.

[0040] like Figure 1-3 As shown, a pre-buried air-injection ball for measuring the gas permeability coefficient and diffusion coefficient of soil is mainly composed of an air-injection ball device and an air supply device. The air-injection ball device includes a nylon net 10, a spherical bracket 15, crushed stone 11, a diameter expansion interface 9, an air-injection pipe 8, an air extraction interface 14 and an air extraction pipe 13. One end of the air-injection pipe 8 is connected to the diameter expansion interface 9. The diameter expansion interface 9 is placed at the center of the spherical bracket 15 to form a spherical cavity ball 12. The cavity is filled with crushed stone 11 and covered with a nylon net 10. On the surface of the spherical bracket 15, the gas intake interface 14 is connected to one end of the gas intake pipe 13, and the other end of the gas intake pipe 10 is located 1 cm from the surface of the spherical cavity ball 12; the gas supply device includes an inert gas cylinder 1, a pressure regulating valve 4, a flow regulating valve 5, a U-shaped pressure measuring tube 6, a flow meter 7 and a gas supply pipe 3, and the output end of the inert gas cylinder 1 is connected to one end of the gas supply pipe 3 through the pressure regulating valve 4, the flow regulating valve 5, the U-shaped pressure measuring tube 6, and the flow meter 7 in sequence, and the other end of the gas supply pipe is connected to the gas injection pipe 8.

[0041] The inert gas cylinder 1 provides the gas and air pressure delivered to the ball injection device; the pressure regulating valve 4 and the flow regulating valve 5 accurately control the intake flow rate; the gas transmission pipe 3 and the gas injection pipe 8 play the role of conveying gas; the spherical cavity ball 12 provides a gas migration channel, and the spherical cavity ball 12 is filled with gravel 11, so that the gas can be evenly and stably transferred to the surrounding soil; the spherical bracket 15 makes the filled gravel 11 spherical; the flow meter 7 and the U-shaped pressure measuring tube 6 respectively monitor the size of the intake flow rate and the air pressure.

[0042] The gas transmission pipe 3 adopts a wear-resistant and pressure-resistant polytetrafluoroethylene (PTFE) tube with a diameter of 6 mm; the gas injection pipe 8 and the gas extraction pipe 13 adopt wear-resistant and pressure-resistant PTFE tube with a diameter of 3 mm; the specification of the nylon mesh 10 is 100 mesh; the diameter of the crushed stone 11 is 3-4 mm; the inner diameter of the expansion interface 9 is 6 mm; the diameter of the spherical cavity ball 12 is 40 mm; the inert gas cylinder 1 adopts a high-purity helium cylinder; the pressure regulating valve 4, the flow regulating valve 5, the U-shaped pressure measuring tube 6, and the flow meter 7 all adopt conventional parameter instruments.

[0043] Example

[0044] In the first step, the spherical cavity ball 12 of the balloon injection device is pre-buried into the soil layer, with its center located at the depth of the unsaturated soil layer to be measured, or a hole is drilled (the hole diameter is slightly larger than the spherical cavity ball), and then the center of the spherical cavity ball is installed at the depth of the unsaturated soil to be measured, and then the soil taken out of the drill hole is passed through a 2-mm sieve and backfilled and compacted.

[0045] The second step is to connect the helium cylinder 1, the pressure regulating valve 4, the flow regulating valve 5, the U-shaped pressure measuring tube 6, and the flow meter 7 through the gas transmission pipe 3 to form a gas supply device, and the outlet end of the gas supply device is connected to the gas injection pipe 8 through the gas transmission pipe 3;

[0046] The third step is to extract 1-3 mL of gas at the gas extraction interface 14 to measure the background concentration of helium in the unsaturated soil layer (0 m 3 m -3 ), and record the ambient atmospheric pressure (100500Pa) and ambient temperature (298.15K) at that time;

[0047] The fourth step is to open the gas cylinder valve 2, adjust the intake flow rate to a constant volume flow value (1 mL / min) through the pressure regulating valve 4 and the flow regulating valve 5, and then deliver helium to the balloon filling device through the gas supply device. The U-shaped pressure measuring tube 6 and the flow meter 7 monitor the pressure and flow value of the helium, and extract 1-3 mL of gas at the gas extraction interface 14 every 30 minutes to measure the concentration of helium;

[0048] Step 5: After the measured helium concentration is stable, record the measured helium concentration value C1 (0.099m 3 m -3 ) and the corresponding constant volume flow value q1 (1 mL / min);

[0049] Step 6: Increase the intake flow rate to a higher constant volume flow rate value through the pressure regulating valve 4 and the flow regulating valve 5, so that the gas pressure measured by the U-shaped pressure measuring tube 6 is higher than the atmospheric pressure and there is a significant difference (≥0.1 kPa). After the reading of the U-shaped pressure measuring tube 6 is stable, record the absolute gas pressure value P2 (100700 Pa) measured by the U-shaped pressure measuring tube 6 and the flow meter 7 and the corresponding constant volume flow value q2 (200 mL / min);

[0050] Step 7: Substitute the measured inert gas concentration value C1 and the corresponding constant volume flow value q1 into the following formula to obtain the gas diffusion coefficient D: g :

[0051]

[0052] Where D g is the gas diffusion coefficient of the unsaturated soil layer (m2 s -1 ); C1 is the tracer gas concentration in the injection ball when the gas migrates stably (m 3 m -3 );C hg is the background concentration of tracer gas in the soil layer (m 3 m -3 ); r is the radius of the multi-balloon (m);

[0053] Step 8: Substitute the measured absolute gas pressure value P2 and the corresponding constant volume flow value q2 into the following formula to obtain the gas permeability coefficient K: g :

[0054]

[0055] In the formula, K g is the gas permeability coefficient of the unsaturated soil layer (ms -1 ); P2 is the absolute pressure of the soil at 1 cm from the surface of the spherical cavity ball (Pa); ρ a is the air density (kgm -3 ); g is the acceleration due to gravity (9.8ms -2 );P hg is the ambient atmospheric pressure (Pa).

[0056] After the above in-situ measurement, the gas permeability coefficient and diffusion coefficient of the unsaturated soil obtained by the device of the present invention and the laboratory measurement method were compared by collecting the original soil samples on site and conducting laboratory measurements (Table 1). It can be seen that the device of the present invention can accurately measure the gas permeability coefficient and diffusion coefficient of the soil.

[0057] Table 1 Comparison of gas migration parameters measured by the device of the present invention and laboratory

[0058]

[0059] a. The measurement difference is defined as:

Claims

1. A pre-buried air-filled ball for measuring the gas permeability and diffusion coefficient of soil, characterized in that: The invention comprises a ball injection device and a gas supply device. The ball injection device comprises a nylon net (10), a spherical bracket (15), crushed stones (11), a diameter expansion interface (9), a gas injection pipe (8), a gas extraction interface (14) and a gas extraction pipe (13). One end of the gas injection pipe (8) is connected to the diameter expansion interface (9). The diameter expansion interface (9) is placed at the center of the spherical bracket (15) to form a spherical cavity ball (12). The cavity is filled with crushed stones (11). The surface of the spherical bracket (15) is covered with a nylon net (10). The gas extraction interface (14) is connected to the diameter expansion interface (9). 14) is connected to one end of the gas extraction pipe (13), and the other end of the gas extraction pipe (13) is located 1 cm from the surface of the spherical cavity ball (12); the gas supply device comprises an inert gas cylinder (1), a pressure regulating valve (4), a flow regulating valve (5), a U-shaped pressure measuring tube (6), a flow meter (7) and a gas transmission pipe (3); the output end of the inert gas cylinder (1) is connected to one end of the gas transmission pipe (3) through the pressure regulating valve (4), the flow regulating valve (5), the U-shaped pressure measuring tube (6) and the flow meter (7) in sequence, and the other end of the gas transmission pipe (3) is connected to the gas injection pipe (8).

2. The pre-buried gas-filled ball for measuring the gas permeability and diffusion coefficient of soil according to claim 1, characterized in that: The center of the spherical hollow ball (12) is located at the depth of the required measuring point in the unsaturated soil layer.

3. The pre-buried air-filled ball for measuring the gas permeability and diffusion coefficient of soil according to claim 1, characterized in that: The spherical cavity ball (12), the lower part of the air injection pipe (8) and the lower part of the air extraction pipe (13) of the air injection ball device are placed in the unsaturated soil layer, and the upper part of the air injection pipe (8), the upper part of the air extraction pipe (13) and the air extraction interface (14) extend out of the ground surface.

4. A method for applying the pre-buried ball injection method for measuring the gas permeability and diffusion coefficient of soil as claimed in any one of claims 1 to 3, characterized in that: The method comprises the following steps: The first step is to pre-bury the spherical cavity ball (12) of the balloon injection device into the soil layer, with the center of the ball located at the depth of the unsaturated soil to be measured, or to install the center of the spherical cavity ball (12) at the depth of the unsaturated soil to be measured by drilling a hole, and then backfill and compact the drilled hole after the soil is taken out through a 2 mm sieve; In the second step, the inert gas cylinder (1), the pressure regulating valve (4), the flow regulating valve (5), the U-shaped pressure measuring tube (6), and the flow meter (7) are connected through the gas transmission pipe (3) to form a gas supply device, and the outlet end of the gas supply device is connected to the gas injection pipe (8) through the gas transmission pipe (3); The third step is to extract 1-3 mL of gas at the gas extraction interface (14) to measure the background concentration of the inert gas in the unsaturated soil layer, and record the ambient atmospheric pressure and ambient temperature at that time; The fourth step is to open the inert gas cylinder (1), adjust the intake flow rate to a constant volume flow value through the pressure regulating valve (4) and the flow regulating valve (5), and then deliver the inert gas to the balloon injection device through the gas supply device. The U-shaped pressure measuring tube (6) and the flow meter (7) monitor the pressure and flow value of the inert gas, and extract 1-3 mL of gas at the gas extraction interface (14) every 30 minutes to measure the concentration of the inert gas. Step 5: After the measured inert gas concentration is stable, record the measured inert gas concentration value C1 and the corresponding constant volume flow value q1; Step 6: The intake air flow rate is increased to a higher constant volume flow rate value by means of the pressure regulating valve (4) and the flow regulating valve (5), so that the gas pressure measured by the U-shaped pressure measuring tube (6) is higher than the atmospheric pressure and has a significant difference, i.e., more than 0.1 kPa. After the reading of the U-shaped pressure measuring tube (6) is stable, the absolute gas pressure value P2 and the corresponding constant volume flow rate value q2 measured by the U-shaped pressure measuring tube (6) and the flow meter (7) are recorded respectively; Step 7: Substitute the measured inert gas concentration value C1 and the corresponding constant volume flow value q1 into the following formula to obtain the gas diffusion coefficient D: g : Where D g is the gas diffusion coefficient of the unsaturated soil layer; C1 is the tracer gas concentration in the injection ball when the gas migrates stably; C hg is the background concentration of tracer gas in the soil layer; r is the radius of the injection balloon; Step 8: Substitute the measured absolute gas pressure value P2 and the corresponding constant volume flow value q2 into the following formula to obtain the gas permeability coefficient K of the unsaturated soil: g : K g is the gas permeability coefficient of the unsaturated soil layer; P2 is the absolute air pressure of the soil at 1 cm from the surface of the spherical cavity ball; ρ a is the air density; g is the acceleration due to gravity; P hg is the ambient atmospheric pressure.

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