CN112161897B - Negative pressure boundary control method for large-scale soil water movement experiment system - Google Patents
Negative pressure boundary control method for large-scale soil water movement experiment system Download PDFInfo
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Abstract
The invention discloses a negative pressure boundary control method for a large-scale soil water movement experiment system, which comprises experiment soil and an experiment soil negative pressure measuring sensor; negative pressure boundary control device is in including setting up vacuum chamber, the setting of experiment soil below are in the pottery of vacuum chamber top contacts the membrane, negative pressure chamber vacuum can be adjusted, pottery contacts membrane connection experiment soil and vacuum chamber and is used for carrying out the negative pressure transmission by vacuum chamber to soil. Under the experimental condition of simulating the soil water movement by the control boundary, for the first class boundary condition (Dirichlet boundary) or the second class boundary condition (Newman boundary), flux is set based on the soil negative pressure measured by the soil negative pressure sensor buried in the boundary layer and the boundary layer, and the control of the first class or the second class boundary condition is realized by controlling and adjusting the vacuum air chamber negative pressure in the whole experimental period based on the functional relation of the vacuum air chamber negative pressure-boundary flux-soil negative pressure determined in the step one.
Description
Technical Field
The invention belongs to the technical field of soil-water boundary condition control, and particularly relates to a negative pressure boundary control method for a large-scale soil-water movement experiment system.
Background
Large soil water movement experiment systems such as an lysimeter, a slope flow experiment system including a soil seepage experiment and the like inevitably need to cut off soil by setting boundaries, boundary condition changes have obvious influence on soil water flow, and flux and processes measured by different boundaries (such as a seepage surface boundary, a gravity drainage boundary and a low-permeability water blocking boundary) show obvious differences. The boundary condition control is one of the unsolved core problems of the large-scale soil water movement experiment system for carrying out in-situ simulation experiments and control experiments.
The current common method for solving the boundary control problem is to enlarge the scale of the soil water movement experiment system, for example, the experiment system with large burial depth (more than 4m) is adopted, although the method reduces the influence of the boundary condition on the measurement result to a certain extent, the size of the corresponding experiment system is very large, and very high requirements are provided for monitoring the soil water movement and the accompanying and accompanying processes thereof. More importantly, the problem is not solved fundamentally in mechanism.
In addition, large soil water movement experimental systems typically have boundary conditions set to either watertight boundaries or positive pressure boundaries. The unsaturated soil water movement is divided into bounded and unbounded conditions, and the bounded unsaturated soil water movement is determined when the boundary is a saturated soil layer (underground water) or an impermeable interlayer; when the unsaturated zone is infinitely deep (or the groundwater burial depth is quite large) and the supply of soil water in the unsaturated zone to groundwater (or the supply of groundwater to the aeration zone) is not researched, the research is often focused on the soil water movement and the accompanying and accompanying processes within a certain depth range, for the unbounded soil water movement problem, an experimental area needs to be selected within a soil layer range with a certain depth, and the boundary is a negative pressure boundary in the case. However, all the existing large-scale soil water experimental devices cannot meet the requirements for developing the experiment for controlling the negative pressure boundary.
Disclosure of Invention
Aiming at the problems in the background art, the invention provides a negative pressure boundary control method for a large-scale soil water movement experiment system, establishes a method for realizing the negative pressure boundary control of the large-scale soil water movement experiment system through air chamber vacuum negative pressure control, and solves the key theory and technical problems of the reliability improvement of the large-scale soil water movement experiment.
In order to solve the technical problems, the invention adopts the following technical scheme:
a negative pressure boundary control method for a large-scale soil water movement experiment system comprises a control device and a control method for carrying out an in-situ simulation experiment by using the control device and controlling a negative pressure boundary under the condition of a boundary simulation soil water movement experiment, wherein the control device comprises:
the experimental soil body is used for developing the in-situ simulation soil water flow experiment of the large-scale soil water movement system;
the experimental soil body negative pressure measuring sensor is buried at the boundary position of the experimental soil body and is used for measuring the negative pressure at the boundary position of the experimental soil body;
the negative pressure boundary control device is used for realizing in-situ soil negative pressure boundary reduction and comprises a vacuum air chamber arranged below the experimental soil body and a ceramic contact membrane arranged at the top end of the vacuum air chamber, wherein the vacuum degree of the vacuum air chamber can be adjusted, and the ceramic contact membrane is connected with the experimental soil body and the vacuum air chamber and is used for transmitting negative pressure from the vacuum air chamber to soil.
The invention also provides a negative pressure boundary control method of the large-scale soil water movement experiment system, which utilizes the control device to carry out the in-situ simulation experiment and control the negative pressure boundary under the condition of the boundary simulation soil water movement experiment and comprises the following steps:
s1, establishing a relation of vacuum air chamber negative pressure-soil water flux through parameter calibration of a ceramic contact membrane;
s2, measuring the negative pressure at the in-situ boundary position through an in-situ soil negative pressure measuring sensor embedded at the in-situ soil boundary position;
s3, according to the negative pressure measurement result at the position of the in-situ soil boundary in the step S2, adopting the negative pressure-soil negative pressure-boundary water flux function relation of the vacuum air chamber determined in the step S1, and adjusting the negative pressure of the vacuum air chamber to enable the experimental soil body negative pressure at the position of the boundary of the experimental system to be consistent with the in-situ lower boundary soil negative pressure;
s4, in the experiment process, continuously measuring the negative pressure change of the in-situ soil boundary through the in-situ soil negative pressure measuring sensor embedded in the in-situ soil boundary position, and dynamically regulating and controlling the simulation experiment boundary through the step S3 to meet the condition that the simulation experiment boundary condition is consistent with the in-situ condition;
s5, collecting and discharging a water body entering a vacuum air chamber from a soil boundary;
s6, under the experimental condition of simulating the soil water movement by the control boundary, for the first kind of boundary condition, namely Dirichlet boundary or the second kind of boundary condition, namely Newman boundary, on the basis of the soil negative pressure measured by the experimental soil negative pressure measuring sensor for embedding the experimental soil boundary layer and the set flux of the boundary layer, the control of the first kind or second kind of boundary condition is realized by controlling and adjusting the negative pressure of the vacuum air chamber in the whole experimental period on the basis of the functional relation of the vacuum air chamber negative pressure-the soil negative pressure-the boundary water flow flux determined in the step S1.
Further, under the conditions of the in-situ simulation experiment and the control boundary experiment, the relationship of vacuum chamber negative pressure, soil negative pressure and boundary water flux under the condition that the ceramic contact membrane established by negative pressure boundary regulation and control is subjected to negative pressure transmission is as follows:
wherein t is a time dimension, Q is a boundary water flux continuously flowing in the experimental soil body-ceramic contact membrane-vacuum air chamber, and delta h is the negative pressure difference between the experimental soil body and the vacuum air chamber;
wherein R reflects the relative permeability of the ceramic contact membrane, K (h) is the soil hydraulic conductivity, and K is a function of the water potential h under unsaturated conditionsp，lpRespectively the permeability coefficient of the ceramic contact membrane, namely the water flux and the thickness under unit negative pressure gradient; b is the area of the negative pressure regulating boundary surface, a1Are first order coefficients. Further, in step S1, the ceramic membrane layer has a permeability coefficient KpAnd a first order coefficient a1The parameters to be calibrated are parameters calibrated by a method of controlling a vacuum air chamber to form different negative pressures and measuring the boundary negative pressure and flux of the corresponding experimental soil, namely the second type of boundary condition controls the flux Q set according to the boundary and the experimental soil negative pressure measured by a negative pressure measuring sensor buried in the experimental soil, the negative pressure of the vacuum air chamber is determined and regulated, the constant flux control of the boundary is realized during the whole experiment, and the permeability coefficient K of the ceramic contact membrane is fitted according to the test resultpAnd a first order coefficient a1。
In step S1, in the continuum of soil-ceramic contact membrane-vacuum chamber, the negative pressure generated by the vacuum chamber is transmitted to the soil through the ceramic contact membrane, forming a continuous negative pressure transmission process, in which the soil water motion equation is:
at the boundary position, unsaturated flux formed in the soil penetrates through the ceramic contact membrane to form continuous flow, and under balanced conditions, the flow equation in the ceramic contact membrane can be expressed as follows:
or
In the formula: z and t are the position and time dimensions, respectively, Kp，lpThe permeability coefficient and the thickness of the ceramic contact membrane are respectively; h, negative pressure; c and K are the soil water content and the hydraulic conductivity respectively and are a function of h, namely c (h) and K (h) are functions of water potential h under unsaturated conditions, and c (h) can be used as a constant (average value in a variation range of h) under the continuous flowing condition of the soil-ceramic contact membrane.
Under the condition of continuous flow, the Lappas transformation is carried out on the formula (1),
where p is the Laplace change constant, the general solution is:
wherein c is1And c2For the integration constant, Δ h is the negative pressure (water potential) difference between the soil boundary and the ceramic contact membrane, and the derivative of equation (3) on the position dimension z is obtained:
and (3) substituting the Laplace transform of the equation (2) into the equation (4) to obtain:
due to the interface position of the ceramic contact membrane and the air chamber
Therefore, the following equations (5) and (6) show that:
thus determining the integration constant c1＝c2＝c
Substituting the formula (7) into the formula (5) to obtain:
performing inverse Laplace transform on the formula (8), and obtaining the following result according to a residue theorem:
wherein, according to the relationship between the hyperbolic function and the complex variable trigonometric function:
order:
then (9) is:
then further according to the decomposition theorem, get:
wherein the content of the first and second substances,
d is the diffusivity of soil water, and is substituted into the formula (11) to obtain:
wherein, negative pressure regulation and control boundary surface area is B, then soil boundary and ceramic contact membrane continuous water flux Q is:
that is, when the difference between the negative pressure at the soil boundary and the negative pressure at the air chamber is Δ h, the boundary flux Q at the time t is
When in use
The first term of the series can be taken (with an error of no more than 5%).
Formula (14) can be represented as
Thus, the boundary layer water flux Q generated under the condition that the soil boundary and the ceramic contact membrane continuously flow and the negative pressure difference between the soil and the air chamber is delta h is determined. (15) In the formula, because
The parameters comprise soil hydrodynamic parameters, including hydraulic conductivity K (h) and diffusivity D; ceramic contact membrane parameters including the permeability coefficient (water flux per negative pressure gradient) K of the ceramic contact membranepAnd a thickness lpAnd a first order coefficient a1. Wherein the soil hydrodynamic parameters can be directly measured by a conventional method, and the thickness l of the ceramic contact filmpDirectly measured by geometric method. Permeability coefficient K of ceramic membrane layerpAnd a first order coefficient a1Are parameters that need to be calibrated. As shown in fig. 3, the parameter calibration device performs parameter calibration by controlling the air chamber to form different negative pressures and measuring the corresponding soil boundary negative pressure and flux.
According to the technical scheme, the second type (Newman boundary) boundary control can determine the negative pressure of the vacuum air chamber according to the flux Q set by the set boundary and the soil negative pressure measured by the negative pressure sensor buried in the soil according to the (16) and regulate and control, and the constant flux control of the boundary is realized during the whole experiment. Wherein formula (16) is directly obtainable from formula (15):
under the experimental condition of simulating the soil water movement by the control boundary, for the first kind of boundary condition, namely Dirichlet boundary or the second kind of boundary condition, namely Newman boundary, based on the soil negative pressure measured by the experimental soil negative pressure measuring sensor for burying the experimental soil boundary layer and the set flux of the boundary layer, based on the functional relation of the vacuum air chamber negative pressure-boundary flux-soil negative pressure determined in the step S1, the control of the first kind or second kind of boundary condition is realized by controlling and adjusting the vacuum air chamber negative pressure in the whole experimental period.
Further, under the condition that the experimental soil is the viscous soil, an experimental soil negative pressure measuring sensor based on the soil vacuum degree galvanic couple induction as the measuring principle is adopted.
Further, ceramic contact membrane below is equipped with tipping bucket formula flow measuring device and soil water receiving flask in proper order among the vacuum air chamber, and it is used for collecting soil water in the experimental soil body, be equipped with vacuum air chamber negative pressure measuring transducer in the vacuum air chamber and be used for measuring negative pressure in the vacuum air chamber, the vacuum air chamber lateral wall is equipped with the connection the connecting device of vacuum air chamber and vacuum pump regulates and control vacuum chamber internal vacuum through the vacuum pump and realizes the regulation and control of air chamber negative pressure, the vacuum pump is connected with negative pressure control regulator.
Compared with the prior art, the invention has the beneficial effects that:
the invention provides a method for realizing negative pressure boundary control of a large-scale soil water movement experiment system by air chamber vacuum negative pressure control, which is completely based on a soil negative pressure-flux-air chamber negative pressure function relation in a soil-ceramic contact membrane-vacuum air chamber continuous flow process, solves the key theory and technical problem of reliability improvement of the large-scale soil water movement experiment, and avoids the problem that high-requirement monitoring on soil water movement and accompanying processes thereof in the boundary control problem in the prior art by enlarging the scale of the soil water movement experiment system.
Drawings
FIG. 1 is a schematic view of a negative pressure boundary regulating device of a large soil water movement experiment system according to the present invention;
FIG. 2 is a negative pressure boundary regulating device of the large soil water movement experiment system of the invention;
FIG. 3 is a graphical representation of the parameter rate of the boundary negative pressure regulating device of an example of the present invention;
the device comprises an experimental soil body 1, a negative pressure boundary control device 2, an experimental soil body negative pressure measuring sensor 3, a negative pressure control regulator 4, a vacuum pump 5, a ceramic contact membrane 6, a tipping bucket type flow measuring device 7, a soil water collecting bottle 8, a vacuum air chamber 9, a vacuum air chamber 10, a vacuum air chamber negative pressure measuring sensor 11 and a connecting device.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
It is to be noted that the experimental methods described in the following embodiments are all conventional methods unless otherwise specified, and the reagents and materials, if not otherwise specified, are commercially available; in the description of the present invention, the terms "lateral", "longitudinal", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", etc., indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are only for convenience in describing the present invention and simplifying the description, and do not indicate or imply that the device or element being referred to must have a particular orientation, be constructed and operated in a particular orientation, and thus, should not be construed as limiting the present invention.
Furthermore, the terms "horizontal", "vertical", "overhang" and the like do not imply that the components are required to be absolutely horizontal or overhang, but may be slightly inclined. For example, "horizontal" merely means that the direction is more horizontal than "vertical" and does not mean that the structure must be perfectly horizontal, but may be slightly inclined.
In the description of the present application, it is further noted that, unless expressly stated or limited otherwise, the terms "disposed," "mounted," "connected," and "connected" are to be construed broadly, e.g., as meaning either a fixed connection, a removable connection, or an integral connection; can be mechanically or electrically connected; they may be connected directly or indirectly through intervening media, or they may be interconnected between two elements. The specific meaning of the above terms in the present application can be understood in a specific case by those of ordinary skill in the art.
The present invention will be further explained with reference to the accompanying drawings and embodiments, where the embodiment provides a negative pressure boundary control method for a large soil water movement experiment system, including: the system comprises an experimental soil body, an experimental soil body negative pressure measuring sensor and a negative pressure boundary control device, wherein the experimental soil body is used for carrying out an in-situ simulation soil water flow experiment of a large-scale soil water movement system; the experimental soil body negative pressure measuring sensor is buried at the boundary position of the experimental soil body and is used for measuring the negative pressure at the boundary position of the experimental soil body; the edge pressing boundary control device comprises a vacuum air chamber arranged below the experimental soil body and a ceramic contact film arranged at the top end of the vacuum air chamber, the vacuum degree of the vacuum air chamber can be adjusted, and the ceramic contact film is connected with the experimental soil body and the vacuum air chamber and is used for transmitting negative pressure to soil from the vacuum air chamber.
Further preferred embodiment in the vacuum air chamber ceramic contact membrane below be equipped with tipping bucket formula flow measuring device and soil water receiving flask in proper order, it is used for collecting soil water in the experimental soil body, be equipped with vacuum air chamber negative pressure measuring transducer in the vacuum air chamber and be used for measuring negative pressure in the vacuum air chamber, the vacuum air chamber lateral wall is equipped with the connection device who connects vacuum air chamber and vacuum pump, adjusts and controls the regulation and control that vacuum chamber internal vacuum degree realized the air chamber negative pressure through the vacuum pump, the vacuum pump is connected with negative pressure control regulator.
When a large soil water movement system is adopted to carry out an in-situ simulation soil water flow experiment, the reduction of an in-situ boundary is realized through the following steps:
s1, establishing a relation of vacuum air chamber negative pressure-soil water flux through parameter calibration of a ceramic contact membrane;
s2, measuring the negative pressure at the in-situ boundary position through an in-situ soil negative pressure measuring sensor embedded at the in-situ soil boundary position;
s3, according to the negative pressure measurement result at the position of the in-situ soil boundary in the step S2, adopting the negative pressure-soil negative pressure-boundary water flux function relation of the vacuum air chamber determined in the step S1, and adjusting the negative pressure of the vacuum air chamber to enable the experimental soil body negative pressure at the position of the boundary of the experimental system to be consistent with the in-situ lower boundary soil negative pressure;
s4, in the experiment process, continuously measuring the negative pressure change of the in-situ soil boundary through the in-situ soil negative pressure measuring sensor embedded in the in-situ soil boundary position, and dynamically regulating and controlling the simulation experiment boundary through the step S3 to meet the condition that the simulation experiment boundary condition is consistent with the in-situ condition;
s5, collecting and discharging a water body entering a vacuum air chamber from a soil boundary;
s6, under the experimental condition of simulating the soil water movement by the control boundary, for the first kind of boundary condition, namely Dirichlet boundary or the second kind of boundary condition, namely Newman boundary, on the basis of the soil negative pressure measured by the experimental soil negative pressure measuring sensor for embedding the experimental soil boundary layer and the set flux of the boundary layer, the control of the first kind or second kind of boundary condition is realized by controlling and adjusting the negative pressure of the vacuum air chamber in the whole experimental period on the basis of the functional relation of the vacuum air chamber negative pressure-the soil negative pressure-the boundary water flow flux determined in the step S1.
According to the technical scheme, the functional relation of the negative pressure of the vacuum air chamber, the negative pressure of the soil and the flow flux of the boundary water under the ceramic contact membrane negative pressure transfer condition is established in the step S1 as follows:
in the continuum formed by the soil-ceramic contact membrane-vacuum air chamber, the negative pressure formed by the vacuum air chamber is transmitted to the soil through the ceramic contact membrane to form a continuous negative pressure transmission process, and in the continuous process, the soil water motion equation is as follows:
at the boundary position, unsaturated flux formed in the soil penetrates through the ceramic contact membrane to form continuous flow, and under balanced conditions, the flow equation in the ceramic contact membrane can be expressed as follows:
or
In the formula: z and t are the position and time dimensions, respectively, Kp，lpThe permeability coefficient and the thickness of the ceramic contact membrane are respectively; h, negative pressure; c and K are the soil water content and the hydraulic conductivity respectively and are a function of h, namely c (h) and K (h) are functions of water potential h under unsaturated conditions, and c (h) can be used as a constant (average value in a variation range of h) under the continuous flowing condition of the soil-ceramic contact membrane.
Under the condition of continuous flow, the Lappas transformation is carried out on the formula (1),
where p is the Laplace change constant, the general solution is:
wherein c is1And c2For the integration constant, Δ h is the negative pressure (water potential) difference between the soil boundary and the ceramic contact membrane, and the derivative of equation (3) on the position dimension z is obtained:
and (3) substituting the Laplace transform of the equation (2) into the equation (4) to obtain:
due to the interface position of the ceramic contact membrane and the air chamber
Therefore, the following equations (5) and (6) show that:
thus determining the integration constant c1＝c2＝c
Substituting the formula (7) into the formula (5) to obtain:
performing inverse Laplace transform on the formula (8), and obtaining the following result according to a residue theorem:
wherein, according to the relationship between the hyperbolic function and the complex variable trigonometric function:
order:
then (9) is:
then further according to the decomposition theorem, get:
wherein the content of the first and second substances,
d is the diffusivity of soil water, and is substituted into the formula (11) to obtain:
wherein, negative pressure regulation and control boundary surface area is B, then soil boundary and ceramic contact membrane continuous water flux Q is:
that is, when the difference between the negative pressure at the soil boundary and the negative pressure at the air chamber is Δ h, the boundary flux Q at the time t is
When in use
The first term of the series can be taken (with an error of no more than 5%).
Formula (14) can be represented as
Thus, the boundary layer water flux Q generated under the condition that the soil boundary and the ceramic contact membrane continuously flow and the negative pressure difference between the soil and the air chamber is delta h is determined. (15) In the formula, because
The parameters comprise soil hydrodynamic parameters, including hydraulic conductivity K (h) and diffusivity D; ceramic contact membrane parameters including the permeability coefficient (water flux per negative pressure gradient) K of the ceramic contact membranepAnd a thickness lpAnd a first order coefficient a1. Wherein the soil hydrodynamic parameters can be directly measured by a conventional method, and the thickness l of the ceramic contact filmpDirectly measured by geometric method. In this example, k (h) is 4.6 × 10 according to the experimental soil properties-6exp(0.0478h)m/s,D＝2.18×10-2m2S; thickness l of ceramic contact filmp0.24mm, and the negative pressure regulating boundary surface area B of 78.5cm2. Permeability coefficient K of ceramic membrane layerpAnd a first order coefficient a1The parameters needing calibration are determined by setting calibration experiments under the condition of water potential difference deltah of different fluxes Q. As shown in fig. 3, the parameter calibration device performs parameter calibration by controlling the air chamber to form different negative pressures and measuring the corresponding soil boundary negative pressure and flux.
According to the technical scheme, the second type (Newman boundary) boundary control can determine the negative pressure of the vacuum air chamber according to the flux Q set by the set boundary and the soil negative pressure measured by the negative pressure sensor buried in the soil according to the (16) and regulate and control, and the constant flux control of the boundary is realized during the whole experiment. Wherein formula (16) is directly obtainable from formula (15):
under the experimental condition of simulating the soil water movement by controlling the boundary, for the first kind of boundary condition, namely Dirichlet boundary or the second kind of boundary condition, namely Newman boundary, based on the soil negative pressure measured by the experimental soil negative pressure measuring sensor for burying the boundary layer of the experimental soil and the set flux of the boundary layer, based on the functional relation of the vacuum air chamber negative pressure, the boundary flux and the soil negative pressure determined in the step S1, the control of the first kind or the second kind of boundary condition is realized by controlling and adjusting the vacuum air chamber negative pressure in the whole experimental period, as shown in FIG. 3, the relation of the vacuum air chamber negative pressure and the soil boundary negative pressure under the same condition of two groups of control Q fluxes is adopted, and the measurement results of all delta h and Q satisfy the control error of the formula (16)
Minimum, determine the permeability coefficient K of the ceramic contact membranepAnd first order coefficient a1Are respectively 7.87 multiplied by 10-8m/s and 0.0554.
According to the above technical scheme, the soil negative pressure sensor for measuring the soil negative pressure needs to have sufficient sensitivity, the soil negative pressure sensor using the potential energy balance principle as the measurement principle can generate larger control error due to the balance time process under the condition of cohesive soil quality, and the soil negative pressure sensor using the soil vacuum degree galvanic couple induction as the measurement principle, such as the soil negative pressure sensor
A sensor.
The foregoing examples are provided for illustration and description of the invention only and are not intended to limit the invention to the scope of the described examples. Furthermore, it will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, and that many variations and modifications may be made in accordance with the teachings of the present invention, all of which fall within the scope of the invention as claimed.
Claims (5)
1. A negative pressure boundary control method for a large-scale soil water movement experiment system is characterized by comprising a control device and a control method for carrying out an in-situ simulation experiment by using the control device and controlling a negative pressure boundary under the condition of a boundary simulation soil water movement experiment, wherein the control device comprises:
the experimental soil body is used for developing the in-situ simulation soil water flow experiment of the large-scale soil water movement system;
the experimental soil body negative pressure measuring sensor is buried at the boundary position of the experimental soil body and is used for measuring the negative pressure at the boundary position of the experimental soil body;
the negative pressure boundary control device is used for realizing in-situ soil negative pressure boundary reduction and comprises a vacuum air chamber arranged below the experimental soil body and a ceramic contact film arranged at the top end of the vacuum air chamber, wherein the vacuum degree of the vacuum air chamber can be adjusted, and the ceramic contact film is connected with the experimental soil body and the vacuum air chamber and is used for transmitting negative pressure from the vacuum air chamber to soil;
the control method specifically comprises the following steps:
s1, determining the permeability coefficient K of a ceramic membrane layer of a ceramic contact membranepAnd a first order coefficient a1Establishing a relation of negative pressure of a vacuum air chamber, negative pressure of soil and water flux of a boundary;
s2, measuring the negative pressure at the in-situ boundary position through an in-situ soil negative pressure measuring sensor embedded at the in-situ soil boundary position;
s3, according to the negative pressure measurement result at the position of the in-situ soil boundary in the step S2, adopting the negative pressure-soil negative pressure-boundary water flux function relation of the vacuum air chamber determined in the step S1, and adjusting the negative pressure of the vacuum air chamber to enable the experimental soil body negative pressure at the position of the boundary of the experimental system to be consistent with the in-situ lower boundary soil negative pressure;
s4, in the experiment process, continuously measuring the negative pressure change of the in-situ soil boundary through the in-situ soil negative pressure measuring sensor embedded in the in-situ soil boundary position, and dynamically regulating and controlling the simulation experiment boundary through the step S3 to meet the condition that the simulation experiment boundary condition is consistent with the in-situ condition;
s5, collecting and discharging a water body entering a vacuum air chamber from a soil boundary;
s6, under the experimental condition of simulating the soil water movement by the control boundary, for the first kind of boundary condition, namely Dirichlet boundary or the second kind of boundary condition, namely Newman boundary, on the basis of the soil negative pressure measured by the experimental soil negative pressure measuring sensor for embedding the experimental soil boundary layer and the set flux of the boundary layer, the control of the first kind or second kind of boundary condition is realized by controlling and adjusting the negative pressure of the vacuum air chamber in the whole experimental period on the basis of the functional relation of the vacuum air chamber negative pressure-the soil negative pressure-the boundary water flow flux determined in the step S1.
2. The negative pressure boundary control method of the large-scale soil water movement experiment system according to claim 1, wherein under the conditions of the in-situ simulation experiment and the control boundary experiment, the relationship of vacuum air chamber negative pressure-soil negative pressure-boundary water flux under the condition that the ceramic contact membrane established by negative pressure boundary regulation and control is subjected to negative pressure transmission is as follows:
wherein t is a time dimension, Q is a boundary water flux continuously flowing in the experimental soil body-ceramic contact membrane-vacuum air chamber, and delta h is the negative pressure difference between the experimental soil body and the vacuum air chamber;
wherein R reflects the relative permeability of the ceramic contact membrane, K (h) is the soil hydraulic conductivity, and K is a function of the water potential h under unsaturated conditionsp，lpRespectively the permeability coefficient of the ceramic contact membrane, namely the water flux and the thickness under unit negative pressure gradient; b is the area of the negative pressure regulating boundary surface, a1Is the first order coefficient, D is the soil water diffusivity.
3. The negative pressure boundary control method of the large soil water movement experiment system as claimed in claim 2, wherein in step S1, the method comprises the steps of controlling the vacuum air chamber to form different negative pressures, and measuring the corresponding experimental soil boundary negative pressure and flux to perform parameter calibration, i.e. the second kind of boundary conditions controls the flux Q set according to the boundary and the experimental soil negative pressure measured by the experimental soil negative pressure measuring sensor buried in the experimental soil to determine the vacuum air chamber negative pressure and regulate, so as to realize the boundary constant flux control during the whole experiment, and the ceramic contact membrane permeability coefficient K is fitted according to the test resultpAnd a first order coefficient a1。
4. The method as claimed in claim 1, wherein the experimental soil negative pressure measuring sensor based on the soil vacuum degree couple induction is adopted under the condition that the experimental soil is viscous soil.
5. The negative pressure boundary control method of the large-scale soil water movement experiment system according to claim 1, characterized in that: ceramic contact membrane below is equipped with tipping bucket formula flow measuring device and soil water receiving flask in proper order among the vacuum air chamber, and it is used for collecting soil water in the experiment soil body, be equipped with vacuum air chamber negative pressure measuring transducer in the vacuum air chamber and be used for measuring negative pressure in the vacuum air chamber, the vacuum air chamber lateral wall is equipped with the connection the vacuum air chamber is connected with the connecting device of vacuum pump, regulates and control vacuum chamber interior vacuum through the vacuum pump and realizes the regulation and control of air chamber negative pressure, the vacuum pump is connected with negative pressure control regulator.
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