Device and method for measuring volume gas content of unsaturated soil in situ

Abstract

The invention discloses a device and a method for in-situ measurement of volume gas content of unsaturated soil, and belongs to the technical field of geotechnical engineering, the device comprises a gas supply unit, a measurement unit and a collection unit; the acquisition unit comprises a gas injection ball, and a gas injection hole is formed in the gas injection ball; the air supply unit comprises an air conveying pipeline, the air inlet end of the air conveying pipeline is connected with a tracing air source and an air source, and the air outlet end extends to the center of the air injection ball. The measuring unit comprises a pressure measuring device, a flow meter and an exhaust pipeline, and an exhaust connector is arranged at the air outlet end of the exhaust pipeline. The method comprises the following steps: determining a gas diffusion coefficient and a permeability coefficient according to the steady-state tracer gas concentration and the mixed gas pressure, substituting the coefficients into a migration equation, and inversely calculating the volume gas content in combination with the actually measured transient tracer gas concentration so as to realize in-situ monitoring.

Description

Technical Field

[0001] This invention relates to the field of geotechnical engineering technology, and in particular to an apparatus and method for in-situ measurement of the volumetric air content of unsaturated soil. Background Technology

[0002] Unsaturated soil primarily exists above the groundwater level, containing both moisture and gas within its pores. The volumetric gas content of unsaturated soil refers to the volume of gas per unit volume of unsaturated soil, a crucial parameter for studying unsaturated soil and significantly influencing gas transport parameters. Measuring the gas content of unsaturated soil has significant scientific and engineering application value. For example, in my country's landfill soil cover layers, the volumetric gas content of the impermeable layer must be sufficiently low to ensure good airtightness and promote landfill gas emission reduction. When using soil vapor extraction to remediate organic contaminated sites, determining the volumetric gas content of the unsaturated soil is necessary to determine the appropriate extraction rate for remediating volatile organic compounds in the vadose zone. In agricultural aerated irrigation, real-time monitoring of the volumetric gas content of unsaturated soil is essential for effectively guiding aeration, improving soil aeration, and increasing crop yields. For transient numerical simulations simulating gas transport in unsaturated soil, the volumetric gas content is a necessary input parameter.

[0003] Currently known methods for measuring the volumetric air content of unsaturated soil primarily involve collecting undisturbed soil samples in situ, followed by indoor geotechnical experiments to measure mass water content, dry density, and particle specific gravity, thereby calculating the volumetric air content of the unsaturated soil. However, this method causes significant disturbance to the unsaturated soil during sampling, easily damaging its original structure, especially in deeper layers (e.g., below 3-4 meters). This not only results in a huge workload for sampling but also makes it difficult to collect complete soil samples, leading to distorted test results. Furthermore, it is impossible to conduct long-term monitoring at the same location. Therefore, there is an urgent need for a technology that can monitor changes in the volumetric air content of unsaturated soil in situ. Summary of the Invention

[0004] The purpose of this invention is to solve the above-mentioned technical problems and provide an apparatus and method for in-situ measurement of the volumetric gas content of unsaturated soil. An injection balloon is buried in unsaturated soil at a target depth. Air and tracer gas are injected through an air supply unit. Combined with a collection unit and a measurement unit, the volumetric gas content of undisturbed unsaturated soil samples can be effectively monitored without the need to collect unsaturated soil. This avoids the technical difficulties of subsequent damage to the unsaturated soil and undisturbed soil collection. It achieves in-situ measurement of the volumetric gas content of unsaturated soil at a target depth, and only requires the injection of a small amount of tracer gas or air. The disturbance to the unsaturated soil is negligible, ensuring the accuracy of the measurement values. Furthermore, measurements can be performed at any time as needed, achieving the goal of long-term in-situ monitoring.

[0005] To achieve the above objectives, the present invention provides the following solution: The present invention discloses an in-situ device for measuring the volumetric gas content of unsaturated soil, comprising a gas supply unit, a measurement unit, and a data acquisition unit; The acquisition unit includes an injection balloon, which is used to be buried in unsaturated soil. The center of the injection balloon is used to correspond to the measurement depth. The injection balloon is provided with an injection hole. The gas supply unit includes a gas pipeline, the gas pipeline inlet is connected to a tracer gas source and an air source, and the gas pipeline outlet extends to the center of the injection balloon. The measuring unit includes a pressure measuring device, a flow meter, and an air extraction pipeline. The pressure measuring device and the flow meter are installed on the air supply pipeline. The air inlet of the air extraction pipeline is located above the balloon and close to the surface of the balloon. The air outlet of the air extraction pipeline is provided with an air extraction connector.

[0006] Preferably, the injection balloon includes a spherical cage, the spherical cage is wrapped with an elastic mesh, and the spherical cage is filled with a filler capable of forming a porous structure.

[0007] Preferably, the filler is crushed stone, and the elastic mesh is nylon mesh.

[0008] Preferably, the outlet end of the gas transmission pipeline is provided with a first enlarged diameter port, and the inlet end of the gas extraction pipeline is provided with a second enlarged diameter port.

[0009] Preferably, the pressure measuring device is a U-shaped pressure measuring tube, and the flow meter is a soap film flow meter.

[0010] Preferably, the tracer gas source includes a tracer gas cylinder, which is connected to the gas supply pipeline via a tracer gas pipeline. A first pressure regulating valve and a first flow regulating valve are sequentially provided on the tracer gas pipeline from the inlet end to the outlet end. The air source includes an air cylinder, which is connected to the gas supply pipeline via an air pipeline. A second pressure regulating valve and a second flow regulating valve are sequentially provided on the air pipeline from the inlet end to the outlet end.

[0011] This invention also discloses a method for in-situ measurement of the volumetric air content of unsaturated soil, which employs the aforementioned device for in-situ measurement of the volumetric air content of unsaturated soil, and includes the following steps: S1. The injection balloon is buried in unsaturated soil, with the center of the injection balloon located at the measurement depth; S2. After the device is connected, a portion of the gas is pre-extracted through the extraction pipeline to prevent residual air in the extraction pipeline from affecting the initial concentration of the tracer gas in the unsaturated soil. C hg The measurement was performed; subsequently, the initial concentration of the tracer gas at the measurement depth was measured. C hg And measure the concentration of tracer gas in the atmosphere. C atm Record ambient atmospheric pressure P atm And ambient temperature; S3. The tracer gas source delivers tracer gas to the injection balloon through the gas supply pipeline, controls the gas flow rate, and extracts an amount of gas equal to the amount collected during the initial concentration measurement of the tracer gas in step S2 through the extraction pipeline. The concentration change of the tracer gas at the measurement depth over time is collected. When the difference between two consecutive measured tracer gas concentrations is a preset stable value, the steady-state concentration of the tracer gas is recorded. C 0, and the corresponding intake air flow rate is measured using a flow meter. q ; S4. The gas diffusion coefficient of unsaturated soil at the measurement depth is obtained according to the following formula. D g : ①; In the formula, D g It is the gas diffusion coefficient of unsaturated soil; C 0 represents the steady-state concentration of the tracer gas; C hg This is the initial concentration value of the tracer gas; It is the distance between the air inlet end of the air extraction pipe and the center of the injection ball; q It is the inlet flow rate corresponding to the steady-state concentration of the tracer gas; S5. After the diffusion coefficient measurement is completed, air is supplied to the gas pipeline through an air source to increase the air intake flow rate. After the pressure measuring device reading stabilizes, the steady-state air pressure is recorded. P The formula for calculating the gas permeability coefficient of unsaturated soil at the measurement depth is as follows: ②; In the formula, K g To measure the gas permeability coefficient at a depth; P It is the absolute air pressure value inside the balloon; g It is the acceleration due to gravity; It is air density; π It is pi; P atm Atmospheric pressure; S6. Based on the law of conservation of mass, the governing equation for the transport of tracer gas in unsaturated soil is established as follows: ③; In the formula, i g This refers to the volumetric gas content of unsaturated soil, which is the volume of gas per unit volume of unsaturated soil. C g The molar concentration of the tracer gas; i w This refers to the volumetric water content of unsaturated soil. H w The molar concentration of the tracer gas dissolved in water; P g This refers to the gas pressure in unsaturated soil. c g The unit weight of gas in unsaturated soil; The governing equation for the mixture of tracer gas and air in unsaturated soil is shown below: ④; In the formula, V g Let be the convection velocity of the gas mixture, calculated using the following formula: ⑤; The finite element method is used to solve the control equations for the tracer gas transport described by formula ③ and the control equations for the tracer gas-air mixture described by formula ④. An axisymmetric model is established, and a spherical balloon is used to simulate the balloon at the measurement depth. The radius of the axisymmetric model is at least 20 times the radius of the balloon, and the height is at least 10 times the measurement depth. The boundary conditions are set as follows: for the tracer gas transport process described by formula ③, the edge of the balloon is the tracer gas flow boundary, which should be equal to the molar flow rate per unit surface area of ​​the balloon. For the tracer gas-air mixture transport process described by formula ④, the edge of the balloon is also the gas flow boundary, which should be equal to the volumetric flow rate of the tracer gas input per unit surface area of ​​the balloon. The bottom and sides of the axisymmetric model are set as zero flow boundaries. The tracer gas concentration on the unsaturated soil surface is set to the concentration of the tracer gas in the atmosphere. C atm The initial concentration of tracer gas in unsaturated soil is C hg The air pressure at the surface of the unsaturated soil is set to the ambient atmospheric pressure. P atm Input the gas diffusion coefficients measured in steps S4 and S5 respectively. D g Gas permeability coefficient K g As in formulas ③ and ④ D g and K g Adjust the volumetric air content of unsaturated soil in formula ③. i g The value is calculated until the ratio of the instantaneous concentration to the steady-state concentration of the tracer gas calculated by the finite element method over time reaches the best fit with the measured value. At this point, the volumetric gas content is obtained. i g This refers to the volumetric air content of unsaturated soil at the measurement depth.

[0012] Preferably, in step S1, a hole with a diameter larger than the radius of the injection balloon is formed by a drilling tool. After drilling to the required depth, the injection balloon is placed in the hole, and the unsaturated soil extracted during the drilling process is sieved, compacted, and backfilled.

[0013] Preferably, in step S2, the amount of gas pre-extracted should be greater than the volume in the extraction pipeline, and the gas collection volume when measuring the tracer gas concentration is 1 mL to 4 mL; during the measurement of the change in the concentration of tracer gas in unsaturated soil over time, the sampling interval is 5 min to 10 min for the first 70 min, 20 min for 70 min to 120 min, and 30 min after 120 min; when the difference between two consecutive measurements of tracer gas concentration is within 5% of its average value, the concentration of tracer gas is considered to have reached stability.

[0014] Preferably, in step S3, when the tracer gas is supplied to the injection balloon, the inlet flow rate is 2 mL / min to 6 mL / min; in step S5, when the air is supplied to the injection balloon, the inlet flow rate is increased to 200 mL / min to 400 mL / min.

[0015] The present invention achieves the following technical effects compared to the prior art: In this invention, an injection balloon is buried in unsaturated soil at the target depth. Air and tracer gas are injected through an air supply unit, and the volumetric gas content of the undisturbed soil sample can be effectively monitored in conjunction with a measurement unit. This eliminates the need to collect unsaturated soil, avoiding subsequent damage to the unsaturated soil and the technical difficulties of undisturbed unsaturated soil sample collection. This invention achieves in-situ measurement of the volumetric gas content of unsaturated soil at the target depth. The volumetric gas content of unsaturated soil at the target depth can be determined by injecting a small amount of tracer gas or air, ensuring the accuracy of the measurement values. Furthermore, this invention allows for measurement at any time as needed, achieving the goal of long-term in-situ monitoring. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained by analyzing these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the device for in-situ measurement of the volumetric gas content of unsaturated soil in an embodiment of the present invention. Figure 2 This is a schematic diagram showing the positional relationship between the injection balloon, the gas delivery pipeline, and the gas extraction pipeline in an embodiment of the present invention. Figure 3 This is a schematic diagram of the one-dimensional steady-state transport of tracer gas in an embodiment of the present invention.

[0018] Explanation of reference numerals in the attached drawings: 1. Tracer gas cylinder; 2. Tracer valve; 3. Tracer gas pipeline; 4. First pressure regulating valve; 5. First flow regulating valve; 6. Air cylinder; 7. Air valve; 8. Air pipeline; 9. Second pressure regulating valve; 10. Second flow regulating valve; 11. Gas delivery pipeline; 12. Pressure measuring device; 13. Flow meter; 14. Injection ball; 15. Extraction pipeline; 16. Extraction connector; 17. First expansion port; 18. Spherical cage; 19. Elastic mesh; 20. Packing material; 21. Second expansion port. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments analyzed and obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] The purpose of this invention is to provide an apparatus and method for in-situ measurement of the volumetric gas content of unsaturated soil, so as to solve the problems existing in the prior art. It can realize long-term in-situ measurement of the volumetric gas content of unsaturated soil, thereby meeting the needs of engineering applications and scientific research for continuous and real-time in-situ measurement of the volumetric gas content of unsaturated soil.

[0021] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0022] Example 1 like Figure 1 to Figure 3 As shown, this embodiment provides an in-situ device for measuring the volumetric gas content of unsaturated soil, including a gas supply unit, a measurement unit, and a data acquisition unit. The data acquisition unit includes an injection balloon 14, which is used to be buried in unsaturated soil. The center of the injection balloon 14 corresponds to the measurement depth. The injection balloon 14 is equipped with an injection hole.

[0023] The gas supply unit includes a gas pipeline 11, the inlet of which is connected to a tracer gas source and an air source, and the outlet of which extends to the center of the injection balloon 14, serving as the point gas source during measurement. The tracer gas can be, but is not limited to, an inert gas.

[0024] The measuring unit includes a pressure measuring device 12, a flow meter 13, and an extraction pipeline 15. The pressure measuring device 12 and the flow meter 13 are mounted on the gas supply pipeline 11, preferably arranged sequentially from the inlet to the outlet of the pipeline 11; however, the reverse arrangement is also possible. The inlet of the extraction pipeline 15 is located above and near the surface of the injection balloon 14. The outlet of the extraction pipeline 15 is located above the unsaturated soil and is equipped with an extraction connector 16 for connecting an extraction device to collect gas and determine the concentration of tracer gas in the unsaturated soil.

[0025] The method for using the device for in-situ measurement of the volumetric gas content of unsaturated soil mainly involves supplying tracer gas and air through a gas supply unit, and then measuring the change in the concentration of tracer gas in the unsaturated soil at the burial depth over time through the air extraction connector 16, the steady-state concentration of the tracer gas under steady-state conditions, and the steady-state pressure of the air. Based on the steady-state concentration and steady-state pressure of the tracer gas, the gas diffusion coefficient and gas permeability coefficient of the unsaturated soil are calculated, and then substituted into the control equations describing the transport of tracer gas and mixed gas in the soil, respectively. The volumetric gas content of the unsaturated soil is obtained through numerical inversion. For details, please refer to the method disclosed in Example 2.

[0026] The advantages of this device are: 1. Effectively monitor the volumetric air content of undisturbed unsaturated soil samples without damaging the soil structure. The volumetric air content of unsaturated soil at the target depth can be measured in situ without the need to collect soil samples from the field and bring them back to the laboratory. II. In the process of measuring the volumetric gas content of unsaturated soil in situ, only a small amount of tracer gas or air is injected, so as not to disturb the unsaturated soil. Third, the device is simple to manufacture and portable, meets the testing conditions for unsaturated soil in the field, and can be applied to field tests.

[0027] In one embodiment, the air injection balloon 14, the outlet of the air supply pipeline 11, and the inlet of the air extraction pipeline 15 are all placed in unsaturated soil. The inlet of the air supply pipeline 11, the outlet of the air extraction pipeline 15, and the air extraction connector 16 are all located above the ground surface.

[0028] In one embodiment, the air inlet of the air extraction pipe 15 is 0.5cm to 1.5cm away from the surface of the injection ball 14, and preferably, the distance is 1cm.

[0029] In one embodiment, the injection sphere 14 includes a spherical cage 18, which is wrapped with an elastic mesh 19. The spherical cage 18 contains filler material 20 with pores to form a porous structure. The open areas of the spherical cage 18 and the mesh openings of the elastic mesh 19 together form injection holes. The mesh openings of the elastic mesh 19 prevent particulate soil from entering the spherical cage 18. The filler material 20, on the one hand, prevents external soil (especially small particles) from entering the outlet end of the gas delivery pipe 11 inside the spherical cage 18; on the other hand, the porous structure facilitates the uniform diffusion of the tracer gas.

[0030] In one embodiment, the filler 20 is crushed stone. Crushed stone is chosen because of its economic efficiency and good applicability, and also because of its suitable particle size. Better filler materials, such as polyethylene foam balls, can be used as substitutes. The elastic mesh 19 is nylon mesh, a readily available material.

[0031] In one embodiment, the outlet end of the gas supply line 11 is provided with a first enlarged port 17 to facilitate the smooth discharge of the tracer gas. The first enlarged port 17 of the gas supply line 11 is located at the center of the spherical cage 18. The inlet end of the extraction line 15 is provided with a second enlarged port 21 to facilitate the measurement of the initial concentration of the tracer gas at the measurement depth.

[0032] In one embodiment, the pressure measuring device 12 is a U-shaped pressure measuring tube, used to measure the relative pressure of the gas to be measured within the gas transmission pipeline 11. The advantage of choosing a U-shaped pressure measuring tube compared to pressure sensors is its convenience; it can be handheld and used for field measurements. The flow meter 13 is a soap film flow meter. The soap film flow meter is used to measure the flow rate of the gas to be measured within the gas transmission pipeline 11.

[0033] In one embodiment, the tracer gas source includes a tracer gas cylinder 1 and a tracer gas pipeline 3. The tracer valve 2 of the tracer gas cylinder 1 is connected to the inlet end of the tracer gas pipeline 3, and the outlet end of the tracer gas pipeline 3 is connected to the inlet end of the gas delivery pipeline 11. A first pressure regulating valve 4 and a first flow regulating valve 5 are sequentially provided on the tracer gas pipeline 3 from the inlet end to the outlet end. The air source includes an air cylinder 6 and an air pipeline 8. The air valve 7 of the air cylinder 6 is connected to the inlet end of the air pipeline 8, and the outlet end of the air pipeline 8 is connected to the inlet end of the gas delivery pipeline 11. A second pressure regulating valve 9 and a second flow regulating valve 10 are sequentially provided on the air pipeline 8 from the inlet end to the outlet end. The length of the gas delivery pipeline 11 can be adjusted according to the depth of the measurement point, and is not limited to using a single gas delivery pipeline.

[0034] Open the air valve 7 of air cylinder 6 and close the tracer valve 2 of tracer cylinder 1. Air can then be supplied to the air supply line 11 through the air line 8. The second pressure regulating valve 9 and the second flow regulating valve 10 are used to regulate the air intake flow rate and air pressure.

[0035] By opening the tracer valve 2 of the tracer gas cylinder 1 and closing the air valve 7 of the air cylinder 6, tracer gas can be supplied to the gas supply line 11 through the tracer gas line 3. The first pressure regulating valve 4 and the first flow regulating valve 5 are used to regulate the gas pressure and the inlet flow rate of the tracer gas, respectively.

[0036] In one embodiment, relevant numerical studies have shown that the smaller the diameter of the injection balloon 14, the less impact it has on the experiment. However, if the diameter is too small, it may be inconvenient to place and remove the injection balloon 14. Therefore, the reasonable diameter range of the injection balloon 14 is 2cm to 3cm, which can greatly reduce the disturbance to the in-situ soil and provide a solution for continuous and real-time in-situ measurement of the volumetric gas content of unsaturated soil.

[0037] Example 2 like Figure 1 to Figure 3 As shown, this embodiment provides a method for in-situ measurement of the volumetric air content of unsaturated soil, using the device for in-situ measurement of the volumetric air content of unsaturated soil in Embodiment 1, including the following steps: S1. The balloon 14 is buried in unsaturated soil, with the center of the balloon 14 located at the measurement depth. S2. After the device is connected, a portion of the gas is pre-extracted through the extraction pipe 15 to prevent residual air in the extraction pipe 15 from affecting the initial concentration of the tracer gas in the unsaturated soil. C hg The initial concentration of the tracer gas at the measurement depth was measured; subsequently, gas was extracted through the extraction line 15. C hg And measure the concentration of tracer gas in the atmosphere. C atm Record ambient atmospheric pressure P atm And ambient temperature; S3. The tracer gas source delivers tracer gas to the injection balloon 14 through the gas supply line 11, controlling the gas flow rate. An equal amount of gas as collected in step S2 (when measuring the initial concentration of the tracer gas) is extracted through the extraction line 15. The concentration change of the tracer gas at the measurement depth over time is recorded. When the difference between two consecutive measurements of the tracer gas concentration is a preset stable value, the steady-state concentration of the tracer gas is recorded. C 0, and the corresponding flow rate value is measured by flow meter 13. q ; S4. The gas diffusion coefficient of unsaturated soil at the measurement depth is obtained according to the following formula. D g : ①; In the formula, D g It is the gas diffusion coefficient (m) of unsaturated soil.2 / s), C 0 is the steady-state concentration (m) of the tracer gas. 3 / m 3 ); C hg It is the initial concentration value of the tracer gas (m 3 / m 3 ); It is the distance (m) between the air inlet end of the air extraction pipe 15 and the center of the injection balloon 14; q It is the inlet flow rate (m³) corresponding to the steady-state concentration of the tracer gas. 3 / s); S5. After the diffusion coefficient measurement is completed, air is supplied to the gas pipeline through the air cylinder to increase the air intake flow. After the reading of the pressure measuring device 12 stabilizes, the steady-state air pressure is recorded. P The formula for calculating the gas permeability coefficient of unsaturated soil at the measurement depth is as follows: ②; In the formula, K g To measure the gas permeability coefficient (m / s) at the depth; P It is the absolute air pressure value inside the balloon; g The acceleration due to gravity (9.8 m / s²) 2 ); It is the density of air (kg / m³) 3 π is the mathematical constant of a circle (3.14). P atm Atmospheric pressure (Pa); S6. Based on the law of conservation of mass, the governing equation for the transport of tracer gas in unsaturated soil is established as follows: ③; In the formula, i g The volumetric air content of unsaturated soil (m³) 3 / m 3 This refers to the volume of gas per unit volume of unsaturated soil. C g The molar concentration of the tracer gas (mol / m³) 3 ); i w This refers to the volumetric water content of unsaturated soil. H w The molar concentration (mol / m³) of the tracer gas dissolved in water. 3 ); P g The pressure of gas in unsaturated soil (Pa); c gThe unit weight of gas in unsaturated soil (N / m³) 3 ); The governing equation for the mixture of tracer gas and air in unsaturated soil is shown below: ④; In the formula, V g Let be the convective velocity of the gas mixture (m / s), calculated using the following formula: ⑤; The finite element method was used to solve the control equations for the tracer gas transport described by formula ③ and the control equations for the tracer gas-air mixture described by formula ④. An axisymmetric model was established, and a spherical balloon 14 was used to simulate the balloon at the measurement depth. The radius of the axisymmetric model was at least 20 times the radius of the balloon 14, and the height was at least 10 times the measurement depth. The boundary conditions were set as follows: for the tracer gas transport process described by formula ③, the edge of the balloon 14 was the tracer gas flow boundary, which should be equal to the molar flow rate per unit surface area of ​​the balloon 14. For the tracer gas-air mixture transport process described by formula ④, the edge of the balloon was also the gas flow boundary, which should be equal to the volumetric flow rate of the tracer gas input per unit surface area of ​​the balloon. The bottom and sides of the axisymmetric model were set as zero flow boundaries. The tracer gas concentration on the unsaturated soil surface was set to the concentration of the tracer gas in the atmosphere. C atm The initial concentration of tracer gas in unsaturated soil is C hg The air pressure at the surface of the unsaturated soil is set to the ambient atmospheric pressure. P atm Input the gas diffusion coefficients measured in steps S4 and S5 respectively. D g Gas permeability coefficient K g As in formulas ③ and ④ D g and K g Adjust the volumetric air content of unsaturated soil in formula ③. i g The value is calculated until the ratio of the instantaneous concentration to the steady-state concentration of the tracer gas calculated by the finite element method over time reaches the best fit with the measured value. At this point, the volumetric gas content is obtained. i g This refers to the volumetric air content of unsaturated soil at the measurement depth.

[0038] In one embodiment, in step S1, a borehole with a diameter larger than the radius of the injection balloon 14 is formed using a drilling tool (e.g., a handheld electric drill). After drilling to the required measurement depth, the injection balloon 14 is placed inside the borehole. The unsaturated soil extracted during drilling is then sieved, compacted, and backfilled. The purpose of sieving is to remove large particles from the in-situ soil. During the experiment, it was found that if the particle distribution under the injection balloon 14 is uneven, large pores may be created due to the deformation of the in-situ soil, causing the tracer gas to preferentially flow along the sidewall of the gas delivery pipeline 11 and the outer wall of the injection balloon 14. The backfill soil after sieving is a thin layer laid at the bottom of the borehole, thus having little impact on the in-situ soil structure and minimal impact on the experimental measurement results. Preferably, after sieving, particles smaller than 2 mm are left for backfilling.

[0039] In one embodiment, in step S2, the amount of gas pre-extracted should be greater than the volume within the extraction pipeline 15, and the gas collection volume when measuring the tracer gas concentration is 1 mL to 4 mL. During the measurement of the tracer gas concentration in unsaturated soil over time, the sampling interval is 5 to 10 minutes for the first 70 minutes, 20 minutes for 70 to 120 minutes, and 30 minutes after 120 minutes. When the difference between two consecutive tracer gas concentration measurements is within 5% of its average value, the tracer gas concentration is considered to have reached stability. Generally, the total sampling time for the tracer gas is around 300 minutes. If the gas concentration reaches stability before 70 minutes, the experiment can be stopped early, but it is best to continue observing for another 30 to 40 minutes to confirm that the gas diffusion has indeed reached a steady state without a subsequent decrease in concentration.

[0040] In one embodiment, in step S3, when the tracer gas is delivered to the injection balloon 14, the inlet flow rate is 2 mL / min to 6 mL / min; in step S5, when the air is delivered to the injection balloon 14, the inlet flow rate is increased to 200 mL / min to 400 mL / min.

[0041] In one embodiment, in step S2, the air valve 7 of the air cylinder 6 is opened, and the second pressure regulating valve 9 and the second flow regulating valve 10 are adjusted to regulate the air flow and air pressure.

[0042] In one embodiment, in step S2, the tracer valve 2 of the tracer gas cylinder 1 is opened, and the first pressure regulating valve 4 and the first flow regulating valve 5 are adjusted to regulate the inlet flow rate and pressure of the tracer gas so that the tracer gas flow rate reaches a constant low volume flow rate value.

[0043] Example 3 This embodiment provides a verification test to verify the feasibility of the device for in-situ measurement of the volumetric air content of unsaturated soil in Embodiment 1 and the method for in-situ measurement of the volumetric air content of unsaturated soil in Embodiment 2.

[0044] Step 1: Prepare two sets of earthen bucket devices, namely earthen bucket one and earthen bucket two.

[0045] The dimensions of the soil bucket are 0.58m in height and 0.40m in inner diameter. Injection balloon 14 is buried at a depth of 0.35m. The soil sample compaction degree is 90%, and it is compacted in 13 layers, with each layer having a compaction height of 0.04m. The height of the compacted soil sample is 0.52m. The moisture content of the prepared soil sample is 22%. The volumetric air content of the compacted soil is... i g The value is 0.174; the dimensions of the soil bucket are 0.52m in height and 0.40m in inner diameter. Injection balloon 14 is buried at 0.30m. The soil sample compaction degree is 90%, compacted in 13 layers, with each layer compacted to a height of 0.04m. The height of the compacted soil is 0.52m, the mass moisture content is 14.4%, and the volumetric air content of the compacted soil is... i g It is 0.252; Methane is selected as the tracer gas cylinder 1 because methane is present in low concentrations in the air and is poorly soluble in water. The crushed stone (filler 20) filling the spherical cage 18 ensures that the gas is transported evenly and stably to the surrounding soil.

[0046] Step 2: Compact the sieved soil in layers to the target measurement depth (0.35m for soil bucket 1; 0.30m for soil bucket 2). Stop compaction at this point and bury the balloon 14 of the measuring device along the axis of symmetry of the soil bucket at the measurement depth. After the balloon 14 is placed stably, continue compaction in layers. Step 3: Based on the volume of the corresponding extraction pipeline 15, pre-extract 5mL~6mL of gas from the extraction pipeline 15, then extract 1mL~4mL of gas at the extraction connector 16, and measure the initial concentration of the tracer gas in the pre-buried soil bucket. C hg Measuring the concentration of tracer gases in the atmosphere C atm and the atmospheric pressure at this time P atm And ambient temperature; Step 4: Open tracer valve 2 to allow airflow, and adjust the first pressure regulating valve 4 and the first flow regulating valve 5 to maintain a constant flow rate. Based on the measurement of flow meter 13, control the inlet flow rate of the tracer gas to achieve a constant low flow rate value. q =4mL / min; then 1mL~4mL of gas was extracted through the extraction connector 16, and the volume concentration (m³) of the tracer gas in the earthen bucket was measured at regular intervals. 3 / m 3The sampling intervals are as follows: 5-10 minutes for the first 70 minutes, 20 minutes for the next 70-120 minutes, and 30 minutes thereafter. When the difference in tracer gas concentration within 5% of its average value over a period of 240-300 minutes, the concentration is considered to have reached stability, and the final steady-state concentration of the tracer gas is recorded. C 0; Step 5: Calculate the steady-state concentration of the tracer gas. C 0, the corresponding constant volumetric flow rate value q (m 3 Substituting ( / s) into the following formula, we obtain the gas diffusion coefficient of the soil bucket at the target measurement depth. D g : ①; In the formula, D g It is the gas diffusion coefficient (m) of the unsaturated soil inside the soil bucket. 2 / s), C hg The initial concentration of the tracer gas (m 3 / m 3 ); is the distance (m) from the air inlet of the air extraction pipe 15 buried in the unsaturated soil to the center of the injection balloon 14, which is set to 2cm in this soil bucket test; q It is a controlled constant volume intake flow rate (m³) 3 / s); Step Six: After the diffusion coefficient measurement is completed, switch to air cylinder 6 as the gas source, open air valve 7, and adjust the second pressure regulating valve 9 and the second flow regulating valve 10. Based on the measurement value of flow meter 13, adjust the flow rate to increase to 200 mL / min. After the reading of pressure measuring device 12 stabilizes, record the steady-state air pressure at 5-minute intervals. P The gas permeability coefficient of the soil at the measurement depth is then determined. K g The calculation formula is as follows: ②; In the formula, K g The gas permeability coefficient (m / s) of the unsaturated soil inside the soil bucket; P It is the absolute air pressure inside the balloon. P a ); g The acceleration due to gravity (9.8 m / s²) 2 ); Gas density (kg / m³) 3 ); P atm Atmospheric pressure ( Pa ); Step 7: Based on the law of conservation of mass, establish the governing equations for the transport of tracer gas within the earthen bucket: ③; In the formula, C g The molar concentration of the tracer gas (mol / m³) 3 ); i g The volumetric air content (m³) of unsaturated soil within the soil bucket. 3 / m 3 ); i w Water content by volume; H w The molar concentration of the tracer gas dissolved in water (mol / m³) 3 ); K g The gas permeability coefficient (m / s); P g Gas pressure (P) a ); c g The unit weight of gas in unsaturated soil (N / m³) 3 ); D g The gas diffusion coefficient (m) 2 / s); The steady-state governing equation for the tracer gas-air mixture in unsaturated soil is as follows: ④; In the formula, V g Let be the gas convection velocity (m / s), calculated using the following formula: ⑤; Subsequently, the finite element method was used to solve the transport control equations for the tracer gas and the control equations for the tracer gas-air mixture. An axisymmetric model was established, and a spherical surface with a constant flow rate was set at the target depth to simulate the ventilation of the injection balloon 14. The boundary conditions were set as follows: the boundary of the injection balloon 14 was a flow boundary, and for the tracer gas transport control equation and the mixture control equation, their values ​​were equal to the molar flow rate and volumetric flow rate of the tracer gas per unit surface area of ​​the injection balloon 14, respectively; the bottom and side edges were set as zero flow boundaries. The concentration of the tracer gas on the unsaturated soil surface was set to the concentration of the tracer gas in the atmosphere. C atm The initial concentration of tracer gas in unsaturated soil is C hg The air pressure at the surface of the unsaturated soil is set to atmospheric pressure. The diffusion coefficients measured in steps five and six are then input. Dg and permeability coefficient K g and the volumetric air content of unsaturated soil i g Arbitrary estimated values ​​are obtained to obtain tracer gas concentration-time data calculated by the finite element method; Determine the goodness of fit between the ratio of instantaneous to steady-state concentration of tracer gas calculated by the finite element method and the time data obtained from field measurements, and adjust the input volumetric gas content of the unsaturated soil. i g The process continues until the optimal fit between the two is achieved, at which point the volumetric air content of the input unsaturated soil is determined. i g This refers to the measured value of the air content in the soil volume. After the above in-situ measurements were completed, undisturbed soil samples were collected from the soil bucket using a ring cutter and laboratory measurements were conducted to verify the accuracy of the volumetric air content of the unsaturated soil measured by the device and method of this patent: First, the mass of the cleaned and dried ring cutter was weighed and recorded as follows. b 1 (g), the volume is denoted as V (cm 3 ); then the original ring sample was roughened to make its surface smooth, and weighed and recorded as ); a 1 (g); The original ring sample was dried in an oven at 105℃ for 24 hours. After drying, it was removed and weighed again, and the result was recorded as follows. a 2 (g); combined with the known specific gravity of the soil mixed in the soil bucket ds The volumetric air content of the unsaturated soil in the soil bucket can be obtained. i g for: ⑥; By comparing the results of indoor measurements and finite element fitting, the volumetric air content of the unsaturated soil in the soil bucket was obtained (Table 1). It can be seen that this device can accurately measure the volumetric air content of unsaturated soil.

[0047] Table 1 compares the gas transport parameters measured by this device with those measured in the laboratory.

[0048] Measurement difference is defined as: .

[0049] Before actual in-situ testing, it is necessary to verify the device's sensitivity, whether the gas transport law conforms to our theoretical derivation, and the feasibility of the finite element inversion method under controlled conditions. Furthermore, in the bucket test, various parameters of the compacted unsaturated soil (such as volumetric gas content) can be controlled, directly verifying the feasibility of the device of this invention. The bucket test also involves in-situ measurement (i.e., no need to collect undisturbed unsaturated soil samples for volumetric gas content measurement). Therefore, the bucket test is a transitional step in verifying the feasibility, accuracy, and repeatability of the in-situ measurement method, providing technical basis for field testing.

[0050] Specific examples have been used to illustrate the principles and implementation methods of this invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of this invention. Furthermore, those skilled in the art will recognize that, based on the ideas of this invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A device for in-situ measurement of the volumetric air content of unsaturated soil, characterized in that, Includes a gas supply unit, a measurement unit, and a data acquisition unit; The acquisition unit includes an injection balloon, which is used to be buried in unsaturated soil. The center of the injection balloon is used to correspond to the measurement depth. The injection balloon is provided with an injection hole. The gas supply unit includes a gas pipeline, the gas pipeline inlet is connected to a tracer gas source and an air source, and the gas pipeline outlet extends to the center of the injection balloon. The measuring unit includes a pressure measuring device, a flow meter, and an air extraction pipeline. The pressure measuring device and the flow meter are installed on the air supply pipeline. The air inlet of the air extraction pipeline is located above the balloon and close to the surface of the balloon. The air outlet of the air extraction pipeline is provided with an air extraction connector.

2. The device for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 1, characterized in that, The injection balloon includes a spherical cage, which is wrapped with an elastic mesh, and the spherical cage contains filler capable of forming a porous structure.

3. The device for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 2, characterized in that, The filler is crushed stone, and the elastic mesh is nylon mesh.

4. The device for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 1, characterized in that, The gas supply pipeline has a first enlarged diameter port at the outlet end and the gas extraction pipeline has a second enlarged diameter port at the inlet end.

5. The device for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 1, characterized in that, The pressure measuring device is a U-shaped pressure measuring tube, and the flow meter is a soap film flow meter.

6. The device for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 1, characterized in that, The tracer gas source includes a tracer gas cylinder, which is connected to the gas supply pipeline via a tracer gas pipeline. A first pressure regulating valve and a first flow regulating valve are sequentially installed on the tracer gas pipeline from the inlet end to the outlet end. The air source includes an air cylinder, which is connected to the gas supply pipeline via an air pipeline. A second pressure regulating valve and a second flow regulating valve are sequentially installed on the air pipeline from the inlet end to the outlet end.

7. A method for in-situ measurement of the volumetric air content of unsaturated soil, characterized in that, The device for in-situ measurement of the volumetric air content of unsaturated soil as described in any one of claims 1-6 includes the following steps: S1. The injection balloon is buried in unsaturated soil, with the center of the injection balloon located at the measurement depth; S2. After the device is connected, a portion of the gas is pre-extracted through the extraction pipeline to prevent residual air in the extraction pipeline from affecting the initial concentration of the tracer gas in the unsaturated soil. C hg The measurement was performed; subsequently, the initial concentration of the tracer gas at the measurement depth was measured. C hg And measure the concentration of tracer gas in the atmosphere. C atm Record ambient atmospheric pressure P atm And ambient temperature; S3. The tracer gas source delivers tracer gas to the injection balloon through the gas supply pipeline, controls the gas flow rate, and extracts an amount of gas equal to the amount collected when measuring the initial concentration of the tracer gas in step S2 through the extraction pipeline. The change in the concentration of the tracer gas at the measurement depth over time is collected. When the difference between two consecutive measurements of the tracer gas concentration is a preset stable value, the steady-state concentration of the tracer gas is recorded. C 0, and the corresponding intake air flow rate is measured using a flow meter. q ; S4. The gas diffusion coefficient of unsaturated soil at the measurement depth is obtained according to the following formula. D g : ①； In the formula, D g It is the gas diffusion coefficient of unsaturated soil; C 0 represents the steady-state concentration of the tracer gas; C hg This is the initial concentration value of the tracer gas; It is the distance between the air inlet end of the air extraction pipe and the center of the injection ball; q It is the inlet flow rate corresponding to the steady-state concentration of the tracer gas; S5. After the diffusion coefficient measurement is completed, air is supplied to the gas pipeline through an air source to increase the air intake flow rate. After the pressure measuring device reading stabilizes, the steady-state air pressure is recorded. P The formula for calculating the gas permeability coefficient of unsaturated soil at the measurement depth is as follows: ②； In the formula, K g To measure the gas permeability coefficient at a depth; P It is the absolute air pressure value inside the balloon; g It is the acceleration due to gravity; It is the density of air; π is the mathematical constant pi. P atm Atmospheric pressure; S6. Based on the law of conservation of mass, the governing equation for the transport of tracer gas in unsaturated soil is established as follows: ③； In the formula, θ g This refers to the volumetric gas content of unsaturated soil, which is the volume of gas per unit volume of unsaturated soil. C g The molar concentration of the tracer gas; θ w This refers to the volumetric water content of unsaturated soil. H w The molar concentration of the tracer gas dissolved in water; P g This refers to the gas pressure in unsaturated soil. γ g The unit weight of gas in unsaturated soil; The governing equation for the mixture of tracer gas and air in unsaturated soil is shown below: ④； In the formula, V g Let be the convection velocity of the gas mixture, calculated using the following formula: ⑤ ； The finite element method is used to solve the control equations for the tracer gas transport described by formula ③ and the control equations for the tracer gas-air mixture described by formula ④. An axisymmetric model is established, and a spherical balloon is used to simulate the balloon at the measurement depth. The radius of the axisymmetric model is at least 20 times the radius of the balloon, and the height is at least 10 times the measurement depth. The boundary conditions are set as follows: for the tracer gas transport process described by formula ③, the edge of the balloon is the tracer gas flow boundary, which should be equal to the molar flow rate per unit surface area of ​​the balloon. For the tracer gas-air mixture transport process described by formula ④, the edge of the balloon is also the gas flow boundary, which should be equal to the volumetric flow rate of the tracer gas input per unit surface area of ​​the balloon. The bottom and sides of the axisymmetric model are set as zero flow boundaries. The tracer gas concentration on the unsaturated soil surface is set to the concentration of the tracer gas in the atmosphere. C atm The initial concentration of tracer gas in unsaturated soil is C hg The air pressure at the surface of the unsaturated soil is set to the ambient atmospheric pressure. P atm Input the gas diffusion coefficients measured in steps S4 and S5 respectively. D g Gas permeability coefficient K g As in formulas ③ and ④ D g and K g Adjust the volumetric air content of unsaturated soil in formula ③. θ g The value is calculated until the ratio of the instantaneous concentration to the steady-state concentration of the tracer gas calculated by the finite element method over time reaches the best fit with the measured value. At this point, the volumetric gas content is obtained. θ g This refers to the volumetric air content of the soil at the measurement depth.

8. The method for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 7, characterized in that, In step S1, a hole with a diameter larger than the radius of the injection balloon is formed by a drilling tool. After drilling to the required depth, the injection balloon is placed in the hole. Then, the unsaturated soil extracted during the drilling process is sieved, compacted, and backfilled.

9. The method for in-situ measurement of the volumetric air content of unsaturated soil according to claim 7, characterized in that, In step S2, the amount of gas pre-extracted should be greater than the volume in the extraction pipeline. When measuring the concentration of tracer gas, the gas collection volume is 1 mL to 4 mL. During the measurement of the concentration of tracer gas in unsaturated soil over time, the sampling interval is 5 min to 10 min for the first 70 min, 20 min for 70 min to 120 min, and 30 min after 120 min. When the difference between two consecutive measurements of tracer gas concentration is within 5% of its average value, the concentration of tracer gas is considered to have reached stability.

10. A method for in-situ measurement of the volumetric air content of unsaturated soil according to claim 7, characterized in that, In step S3, when the tracer gas is supplied to the injection balloon, the inlet flow rate is 2 mL / min to 6 mL / min; in step S5, when the air is supplied to the injection balloon, the inlet flow rate is increased to 200 mL / min to 400 mL / min.

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