CN-121977968-A - Device and method for in-situ measurement of volume air content of unsaturated soil

CN121977968ACN 121977968 ACN121977968 ACN 121977968ACN-121977968-A

Abstract

The invention discloses a device and a method for in-situ measurement of the volumetric air content of unsaturated soil, which belong to the technical field of geotechnical engineering, wherein the device comprises an air supply unit, a measurement unit and an acquisition unit; the acquisition unit comprises an injection ball, an air injection hole is formed in the injection ball, the air supply unit comprises an air transmission pipeline, the air inlet end of the air transmission pipeline is connected with a trace air source and an air source, the air outlet end of the air transmission pipeline extends to the center of the injection ball, the measurement unit comprises a pressure measuring device, a flowmeter and an air extraction pipeline, and the air outlet end of the air extraction pipeline is provided with an air extraction joint. According to the method, a gas diffusion coefficient and a permeation coefficient are determined according to the steady-state trace gas concentration and the mixed gas pressure, and then the coefficients are substituted into an migration equation and combined with the actual measurement of the transient trace gas concentration, and the volumetric gas content is calculated in an inversion mode, so that in-situ monitoring is realized.

Inventors

FENG SONG
GUAN RENQIU
LIU HONGWEI
WANG HUAJUN
ZHANG LIYONG
QIU RONG
WANG ZHIHAI
WANG YUXIN
JIANG JIANLIANG

Assignees

浙江省海洋资源发展有限公司
福州大学

Dates

Publication Date: 20260505
Application Date: 20260206

Claims (10)

1. The device for measuring the volume air content of the unsaturated soil in situ is characterized by comprising an air supply unit, a measuring unit and an acquisition unit; The collecting unit comprises an air injection ball, wherein the air injection ball is used for being embedded into unsaturated soil, the center of the air injection ball is used for corresponding to the measurement depth, and an air injection hole is formed in the air injection ball; The air supply unit comprises an air transmission pipeline, the air inlet end of the air transmission pipeline is connected with a tracing air source and an air source, and the air outlet end of the air transmission pipeline extends to the sphere center of the air injection sphere; the measuring unit comprises a pressure measuring device, a flowmeter and an air extraction pipeline, wherein the pressure measuring device and the flowmeter are arranged on the air transmission pipeline, the air inlet end of the air extraction pipeline is positioned above the air injection ball and close to the surface of the air injection ball, and the air outlet end of the air extraction pipeline is provided with an air extraction joint.
2. The device for in-situ measurement of the volumetric air content of unsaturated soil according to claim 1, wherein the air injection ball comprises a spherical cage, an elastic net is wrapped outside the spherical cage, and a filler capable of forming a porous structure is arranged in the spherical cage.
3. The apparatus for in situ measurement of volumetric air content of unsaturated soil according to claim 2, wherein said filler is crushed stone and said elastic net is nylon net.
4. The device for in-situ measurement of the volumetric air content of unsaturated soil according to claim 1, wherein the air outlet end of the air delivery pipeline is provided with a first expanding port, and the air inlet end of the air extraction pipeline is provided with a second expanding port.
5. The apparatus for in-situ measurement of volumetric air content of unsaturated soil according to claim 1, wherein said pressure measuring device is a U-shaped pressure measuring tube and said flowmeter is a soap film flowmeter.
6. The device for in-situ measurement of the volumetric air content of unsaturated soil according to claim 1, wherein the trace air source comprises a trace air cylinder, the trace air cylinder is connected with the air transmission pipeline through a trace air pipeline, a first pressure stabilizing valve and a first flow stabilizing valve are sequentially arranged on the trace air pipeline from an air inlet end to an air outlet end, the air source comprises an air cylinder, the air cylinder is connected with the air transmission pipeline through an air pipeline, and a second pressure stabilizing valve and a second flow stabilizing valve are sequentially arranged on the air pipeline from the air inlet end to the air outlet end.
7. A method for in situ measurement of the volumetric gas content of unsaturated soil, characterized in that an in situ measurement device of the volumetric gas content of unsaturated soil according to any one of claims 1-6 is used, comprising the following steps: S1, burying an air injection ball in an unsaturated soil body, wherein the center of the air injection ball is positioned at a measuring depth; S2, after the connection of the device is finished, extracting part of gas in advance through an air extraction pipeline, preventing the residual air in the air extraction pipeline from affecting the measurement of the initial concentration C hg of the tracer gas in the unsaturated soil body, then measuring the initial concentration C hg of the tracer gas at the measurement depth, measuring the concentration C atm of the tracer gas in the atmosphere, and recording the ambient atmospheric pressure P atm and the ambient temperature; s3, conveying trace gas to the gas injection ball through the gas conveying pipeline by using a trace gas source, controlling the gas inlet flow, extracting the same amount of gas as the gas acquired during the initial concentration measurement of the trace gas in the step S2 through a gas extraction pipeline, acquiring the time-dependent change condition of the concentration of the trace gas at the measurement depth, recording the steady-state concentration C 0 of the trace gas when the difference between the concentration of the trace gas measured continuously twice is a preset steady value, and measuring the corresponding gas inlet flow value q through a flowmeter; S4, obtaining a gas diffusion coefficient D g of the unsaturated soil body with the measured depth according to the following formula: ①; Wherein D g is the gas diffusion coefficient of the unsaturated soil body, C 0 is the steady-state concentration of the tracer gas, and C hg is the initial concentration value of the tracer gas; The distance between the air inlet end of the air suction pipeline and the sphere center of the air injection sphere is equal to the air inlet flow corresponding to the steady-state concentration of the trace gas; S5, after the diffusion coefficient is measured, air is provided for the air transmission pipeline through an air source, the air inlet flow is increased, after the reading of the pressure device to be measured is stable, the steady-state air pressure P is recorded, and then the calculation formula of the gas permeability coefficient of the unsaturated soil body at the measuring depth is as follows: ②; Wherein K g is the gas permeability coefficient at the measuring depth, P is the absolute air pressure value in the air injection ball, g is the gravity acceleration; air density, pi is circumference ratio, P atm is ambient atmospheric pressure; s6, based on mass conservation, establishing a migration control equation of trace gas in the unsaturated soil body as follows: ③; Wherein, theta g is the volume air content of the unsaturated soil body, namely the volume of the gas in the unsaturated soil body in unit volume, C g is the molar concentration of the tracer gas, theta w is the volume water content of the unsaturated soil body, H w is the molar concentration of the tracer gas dissolved in water, P g is the gas pressure in the unsaturated soil body, gamma g is the gravity of the gas in the unsaturated soil body; the control equation of the mixture of the trace gas and the air in the unsaturated soil body is as follows: ④; Wherein V g is the convection velocity of the mixed gas, and the calculation formula is as follows: ⑤ ; solving a trace gas migration control equation described by a formula ③ by a finite element method, The control equation of the tracer gas and air mixture described by the formula ④ is used for establishing an axisymmetric model, the spherical surface is used for simulating an injection ball at the measuring depth, the radius of the axisymmetric model is at least 20 times of the radius of the injection ball, the height is at least more than 10 times of the measuring depth, boundary conditions are set as follows, for the migration process of the tracer gas described by the formula ③, the edge of the injection ball is a tracer gas flow boundary, the value is equal to the mole flow value of the gas of the unit surface area of the injection ball, for the mixed gas migration process of the tracer gas and air described by the formula ④, the edge of the injection ball is also a gas flow boundary, the value is equal to the volume flow value of the tracer gas input by the unit surface area of the injection ball, the bottom edge and the side edges of the axisymmetric model are set as zero flow boundaries, the tracer gas concentration of the unsaturated soil body surface is C atm of the tracer gas in the atmosphere, the initial concentration of the tracer gas in the unsaturated soil body is C hg , the air pressure of the unsaturated soil body surface is set as the ambient atmospheric pressure P atm , and the gas diffusion coefficients D g measured respectively in the input steps S4 and S5 are set, The gas permeability coefficient K g is used as the numerical value of D g and K g in a formula ③ and a formula ④, the numerical value of the volume gas content theta g of the unsaturated soil body in a formula ③ is adjusted until the time-dependent change relation of the ratio of the instantaneous concentration of the trace gas to the steady-state concentration calculated by the finite element method and the measured value reach the best fit, and the obtained volume gas content theta g is the volume gas content of the soil body at the measuring depth.
8. The method for in-situ measurement of the volumetric air content of unsaturated soil according to claim 7, wherein in step S1, a drilling hole with a diameter larger than the radius of the balloon is formed by a drilling tool, the balloon is placed in the drilling hole after the drilling hole is drilled into a depth to be measured, and the unsaturated soil taken out in the drilling process is sieved and compacted and backfilled.
9. The method for in-situ measurement of the volumetric gas content of unsaturated soil according to claim 7, wherein in the step S2, the gas pre-extraction amount is larger than the volume in the air extraction pipeline, the gas collection amount is 1 mL-4 mL when the concentration of the tracer gas is measured, the sampling interval of the first 70min is 5 min-10 min, the sampling interval is 30min after the sampling interval is 20min and 120min in the process of measuring the change of the concentration of the tracer gas in the unsaturated soil along with time, and the concentration of the tracer gas is considered to be stable when the difference of the concentration of the tracer gas measured twice is less than 5% of the average value.
10. The method for in-situ measurement of the volumetric air content of unsaturated soil according to claim 7, wherein in the step S3, when the tracer gas is conveyed to the air injection ball, the air inflow rate is 2-6 mL/min, and in the step S5, when the air is conveyed to the air injection ball, the air inflow rate is enlarged to 200-400 mL/min.

Description

Device and method for in-situ measurement of volume air content of unsaturated soil Technical Field The invention relates to the technical field of geotechnical engineering, in particular to a device and a method for in-situ measurement of the volumetric air content of unsaturated soil. Background The unsaturated soil body mainly exists above the ground water level, and water and gas exist in the pores of the unsaturated soil body. The volume air content of the unsaturated soil body refers to the volume of gas in the unsaturated soil body in unit volume, is an important parameter for researching the unsaturated soil body, and obviously affects the gas migration parameter in the unsaturated soil body. The method has important scientific research and engineering application values in measuring the air content of the unsaturated soil body, for example, the volume air content of an impermeable layer in a soil covering layer of a refuse landfill in China must be low enough to ensure that the impermeable layer has a good air-tight function and promote the emission reduction of landfill air, when the organic pollution site is restored by adopting soil vapor extraction, the proper air extraction rate can be determined only by determining the volume air content of the unsaturated soil body in the site, thereby restoring volatile organic compounds in a gas-covering zone, and in agricultural aeration irrigation, if the volume air content of the unsaturated soil body can be monitored in real time, aeration operation can be effectively guided, thereby improving the aeration of wet soil and improving the crop yield, and the volume air content of the unsaturated soil body is a necessary input parameter for simulating transient numerical simulation of air migration in the unsaturated soil body. The method for measuring the volume air content of the unsaturated soil body known by the inventor at present mainly comprises the steps of collecting an undisturbed soil sample on site, then carrying out an indoor geotechnical experiment, measuring the mass water content, the dry density, the particle specific gravity and the like of the soil body, and calculating to obtain the volume air content of the unsaturated soil body. However, in the sampling process of the method, the disturbance to the unsaturated soil body is large, the original structure of the unsaturated soil body is easily damaged in the sampling process, particularly the unsaturated soil body in a deeper layer (such as the depth of 3-4 meters or less), the workload of the sampling is huge, the complete soil sample is difficult to collect, the test result is distorted, and the long-term monitoring on the same position is also impossible. Therefore, a technology capable of in-situ monitoring the change of the volumetric gas content of the unsaturated soil body is needed. Disclosure of Invention The invention aims to solve the technical problems, and provides a device and a method for in-situ measurement of the volume air content of unsaturated soil, wherein an air injection ball is buried in the unsaturated soil of a target depth, air and trace gas are injected through an air supply unit, the volume air content of an undisturbed unsaturated soil sample can be effectively monitored by matching an acquisition unit and a measurement unit, the unsaturated soil is not required to be acquired, the subsequent technical problems of damage to the unsaturated soil and undisturbed acquisition of the soil are avoided, the volume air content of the unsaturated soil of the target depth is measured in situ, the volume air content of the unsaturated soil of the target depth can be realized by injecting a small amount of trace gas or air, disturbance to the unsaturated soil can be ignored, the accuracy of measurement values is ensured, and the purposes of long-term in-situ monitoring are realized as required. The invention provides a scheme for realizing the aim, which comprises an in-situ measurement device for the volume air content of unsaturated soil, and comprises an air supply unit, a measurement unit and an acquisition unit; The collecting unit comprises an air injection ball, wherein the air injection ball is used for being embedded into unsaturated soil, the center of the air injection ball is used for corresponding to the measurement depth, and an air injection hole is formed in the air injection ball; The air supply unit comprises an air transmission pipeline, the air inlet end of the air transmission pipeline is connected with a tracing air source and an air source, and the air outlet end of the air transmission pipeline extends to the sphere center of the air injection sphere; the measuring unit comprises a pressure measuring device, a flowmeter and an air extraction pipeline, wherein the pressure measuring device and the flowmeter are arranged on the air transmission pipeline, the air inlet end of the air extraction pipeline is positioned above the air injection ball and