CN113820248B - Landfill body dominant flow testing device and method based on tracer migration - Google Patents
Landfill body dominant flow testing device and method based on tracer migration Download PDFInfo
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Abstract
The invention discloses a landfill body preferential flow testing device and method based on tracer migration. The device mainly comprises a pressurized water supply device, a landfill column convection-diffusion device, a sampling device and a loading device. The pressurized water supply device comprises a pressurizer for controlling injection pressure so as to simulate the migration of tracers in the landfill body under different water heads; the landfill column convection-diffusion device is used for simulating the dominant flow migration phenomenon of the tracer in a landfill site; the loading device is used for controlling the overlying pressure applied to the garbage; the sampling device is used for sampling tracer solutions at different points in the filling column convection-diffusion device; the dominant flow determination method mainly considers the characteristic of strong heterogeneity of saturated landfill garbage, establishes a double-medium heterogeneous flow migration mathematical model of the tracer in the model, and obtains parameters such as fracture-pore ratio and the like representing the dominant flow by inverting the experimental result of concentration distribution of the tracer.
Description
Technical Field
The invention belongs to the field of fluid migration simulation experiment methods and devices for municipal solid waste landfill sites, and particularly relates to a landfill body dominant flow testing device and method based on tracer migration, which can simulate the actual situation of a landfill site at a certain water head and measure related parameters of liquid-phase tracer migration.
Technical Field
The risk of polluting soil and underground water by leachate leakage commonly exists in more than 1600 urban solid waste sanitary landfill sites and more than twenty thousand simple landfill sites in China, and the urban environmental safety is seriously threatened. The pollutants identified in the landfill site comprise 5 types of organic matters, inorganic salts, heavy metals and the like, and the conditions of the landfill site in meteorology, terrain and hydrogeology are different, so that the pollution prevention, control and treatment of the landfill site are greatly challenged.
Under the influence of the thought of 'heavy construction and light maintenance' in China, the problem of leachate leakage caused by stress damage of an antifouling liner of a sanitary landfill during the landfill process is very prominent, the burial depth of a leakage point is usually more than 10m, and a deep leakage point detection repair or vertical separation technology is urgently needed. At present, the leakage detection of foreign landfill sites mainly adopts an electric leakage detection positioning technology. However, the core requirement of direct current method positioning is that the medium on and under the HDPE film should have good conductivity, but GCL replaces clay layers with high conductivity in many newly-built waste landfill sites in China, so that on one hand, a current field cannot be formed at a leakage part, and on the other hand, electrodes under the film cannot be laid; the time domain reflection method requires that the leachate contains hydrocarbons; magnetic detection requires a dielectric layer on the film to have higher magnetic conductivity; the positioning precision of the ground penetrating radar method is not ideal enough; the positioning precision of the transmission line model is greatly influenced by the components of the landfill leachate. Tracer-based leak localization methods have begun to be of interest in recent years and have been successfully applied to dam depth leak detection. The tracer technology can be used for monitoring in different areas and different stages, and the tracer agent can accurately track the migration direction and flow velocity of fluid and identify the leakage direction, and is successfully applied to positioning of oil field leakage channels.
Some scholars regard garbage as a uniform and isotropic porous medium, and describe the convection-diffusion rule of liquid in a landfill body by using a single-pore model. However, a large number of in-situ experiments and engineering applications show that the simulation result of the single-pore model has a large error with the measured data. The reason is that the landfill garbage in each region has different compositions, and the uneven seepage channels are caused by the complicated particle shape, particle size, arrangement structure and the like, and the large-pore dominant potential flow is adopted. Therefore, it is necessary to consider the dominant flow of the garbage, study the convection-diffusion rule of the high-efficiency liquid phase tracer in the saturated landfill body, and determine parameters such as the permeability coefficient of the garbage.
Disclosure of Invention
The invention aims to provide a landfill body dominant flow testing device and method based on tracer migration, which can simulate the actual situation of a landfill under different water heads and different pressures, measure related parameters of liquid-phase tracer migration, obtain a dominant flow channel through tracer inversion and guide a field experiment.
In order to achieve the purpose, the invention adopts the following technical scheme:
a landfill body potential flow testing device based on tracer migration comprises a pressurized water supply device, a loading device, a landfill column convection-diffusion device and a sampling device; the pressurizing water supply device is connected with the bottom of the landfill column convection-diffusion device, the loading device is used for pressurizing the landfill column convection-diffusion device, and the sampling device is installed on the landfill column convection-diffusion device and used for sampling liquid in the landfill column diffusion convection device;
the landfill column convection-diffusion device comprises a column-shaped landfill chamber, a base plate, a liquid injection pipe, a liquid discharge pipe and a collection container; the cylindrical landfill cavity is fixed on the backing plate, the bottom of the cylindrical landfill cavity is provided with a water injection hole, and a water injection channel is arranged inside the backing plate; one end of the liquid injection pipe penetrates through the water injection channel and the liquid injection hole and extends into the cylindrical landfill chamber, the other end of the liquid injection pipe is positioned outside the water injection channel, and a first valve is arranged at the other end of the liquid injection pipe;
the cylindrical landfill cavity is internally provided with a bottom gravel layer, a bottom geotextile protective layer, garbage, a top geotextile filter layer and a top gravel layer from bottom to top in sequence; one end of the liquid discharge pipe is arranged at the gravel layer at the top, and the other end of the liquid discharge pipe is communicated with a water discharge barrel; and a second valve and a flowmeter are also arranged on the liquid discharge pipe.
A test method of a landfill body dominant flow test device based on tracer migration is characterized in that in consideration of the current situation that a saturated landfill body is strong in heterogeneity, gaps in the saturated landfill body are divided into pores and fractures, a convection-diffusion equation of a tracer in the saturated landfill body is established according to Darcy's law and Fick's law, appropriate initial conditions and boundary conditions are determined according to experimental conditions, a control equation, the boundary conditions and the like are input into COMSOL, the concentration distribution conditions of the tracer in a landfill column under different parameter combinations are calculated, and appropriate parameter groups are selected according to a least square method and experimental results, namely parameters of the saturated landfill body;
the invention has the following beneficial effects (innovation points):
the device of the invention uses a pressurizer to control pore water in the landfill column to allow a tracer solution to move in the landfill column under the condition of keeping a certain water head, and simulates the convection-diffusion phenomenon of the tracer in a landfill body under the condition of a constant water head. By adjusting the pressurizer, the condition that tracers are injected into the landfill body under different water heads can be simulated. Through setting up sampling device, can realize the arbitrary point location sampling of arbitrary time, satisfy all-round full period monitoring demand.
The invention considers that the garbage is strong in heterogeneity, the gap in the garbage is divided into pores and cracks, the cracks are large in volume, the flowing speed of liquid in the cracks is high, the liquid flowing through the cracks is large, the pores are opposite, and the pores and the cracks carry out mass exchange on the interface. The convection-diffusion equation of the tracer in the pores and fissures is established. The invention can calculate parameters such as permeability coefficient, diffusion dilution and the like of garbage according to a control equation and an experimental result, and can obtain the fracture-to-void ratio wfAccording to wfThe spatial distribution of (a) infers the location of the dominant stream within the landfill.
Drawings
FIG. 1 is a schematic structural diagram of an experimental apparatus in an embodiment of the present invention;
FIG. 2 is a schematic top view of an experimental setup configuration in an embodiment of the present invention;
FIG. 3 is a schematic diagram of a sampling device according to an embodiment of the present invention;
FIG. 4 is a parameter signature of an experimental set-up in an embodiment of the present invention;
FIG. 5 is a graph showing the results of concentration monitoring experiments at different sites in an example of the present invention;
FIG. 6 is a graph of concentration value fitting results in an example of the present invention;
in the figure: 1-1 water injection container, 1-2 third valves, 1-3 pressurizers, 1-4 flow rate meters, 1-5 pressure gauges, 1-6 fourth valves and 1-7 external connecting pipes; 2-1 of a base, 2-2 of a support, 2-3 of an I-shaped pressing plate and 2-4 of weights; 3-1 cylindrical landfill chamber, 3-2 bottom gravel layers, 3-3 bottom geotextile protective layers, 3-4 solid wastes, 3-5 top geotextile filter layers, 3-6 top gravel layers, 3-7 first valves, 3-8 liquid injection pipes, 3-9 backing plates, 3-10 second valves, 3-11 flow meters, 3-12 liquid discharge pipes and 3-13 drainage containers; 4 sampling device, 4-1 sampling tube, 4-2 end geotextile layer, 4-3 end gravel layer, 4-4 ribbon, 4-5 sealing layer and 4-6 fifth valve.
Detailed Description
The invention is further illustrated by the following figures and examples.
The invention discloses an experimental device for researching the migration rule of a liquid phase tracer in a saturated landfill garbage body. The pressurized water supply device comprises a pressurizer, a water tank, a water inlet pipe, a water outlet pipe, a water inlet pipe, a water outlet pipe and a water outlet pipe, wherein the pressurizer is used for controlling injection pressure, simulating the condition that tracers are injected into a landfill body under different water heads, and simulating the convection-diffusion phenomenon of the tracers in the landfill body under the condition of a constant water head; the landfill column convection-diffusion device is used for injecting and simulating landfill garbage; the loading device is used for providing different pressures for the garbage in the convection-diffusion device of the landfill column and simulating the pressures at different depths of the landfill site; the sampling device is used for sampling different point positions in the flow-diffusion device of the embedded column. And detecting the sample by using a liquid chromatograph, and obtaining the permeability coefficient and the diffusion coefficient of the garbage according to the result and the relevant model.
FIG. 1 is a schematic structural diagram of an experimental apparatus in an embodiment of the present invention, including a pressurized water supply apparatus, a loading apparatus, a landfill column convection-diffusion apparatus, and a sampling apparatus; the pressurizing water supply device is connected with the bottom of the landfill column convection-diffusion device, the loading device is used for pressurizing the landfill column convection-diffusion device, and the sampling device is installed on the landfill column convection-diffusion device and used for sampling liquid in the landfill column convection-diffusion device;
the landfill column convection-diffusion device is used for carrying out convection-diffusion experiments on landfill column samples and comprises a cylindrical landfill chamber 3-1, a backing plate 3-9, a liquid injection pipe 3-8, a liquid discharge pipe 3-12 and a water discharge barrel 3-11; the cylindrical landfill cavity 3-1 is fixed on the backing plate 3-9, the bottom of the cylindrical landfill cavity 3-1 is provided with a water injection hole, and the inside of the backing plate 3-9 is provided with a water injection channel; one end of the liquid injection pipe 3-8 penetrates through the water injection channel and the liquid injection hole and extends into the cylindrical landfill chamber 3-1, the other end of the liquid injection pipe 3-8 is positioned outside the water injection channel, and the end is provided with a first valve 3-7.
The cylindrical landfill chamber 3-1 is internally provided with a bottom gravel layer 3-2, a bottom gauze layer 3-3, garbage 3-4, a top geotextile filter layer 3-5 and a top gravel layer 3-6 in sequence from bottom to top, wherein gauze is used for separating the garbage and gravel, the gravel has the function of enabling leachate to uniformly flow out, and in the embodiment, the thickness of paving the gravel is 5 cm. The outer wall of the cylindrical landfill chamber 3-1 is provided with a through hole for installing a sampling device. One end of the liquid discharge pipe 3-12 is arranged at the top gravel layer 3-6, and the other end is communicated with a water discharge barrel 3-13; the liquid discharge pipe 3-12 is also provided with a second valve 3-10 and a flowmeter 3-11.
The loading device comprises a base 2-1, a support 2-2, an I-shaped pressing plate 2-3 and weights 2-4; the landfill column convection-diffusion device is fixed on the base, the I-shaped pressing plate is installed above the landfill column convection-diffusion device through a support, and the weight is located on the I-shaped pressing plate. The support is fixed on the base, and the upper plate of the I-shaped pressing plate is sleeved on the support and can slide on the support; the lower plate of the I-shaped pressing plate is positioned above the cylindrical landfill cavity 3-1, and the outer diameter of the lower plate is equal to the inner diameter of the cylindrical landfill cavity 3-1.
The injection pipe positioned in the cylindrical landfill chamber 3-1 is of a telescopic structure, and the injection position can be changed through telescopic.
The pressurized water supply device comprises a water injection container 1-1, an external connecting pipe 1-7, a third valve 1-2 arranged on the external connecting pipe 1-7, a pressurizer 1-3, a flow rate meter 1-4 and a pressure gauge 1-5; the third valve 1-2 is positioned at one end of the external connecting pipe 1-7 close to the water injection container 1-1, and the other end of the external connecting pipe is communicated with the liquid injection pipe 3-8 of the landfill column convection-diffusion device. The flow meter and the pressure gauge are used for monitoring the flow rate and the pressure in the external pipe.
In order to better control the water injection flow, the other end of the external connecting pipe is provided with a fourth valve 1-6, and the water flow is controlled by the two valves on the external connecting pipe together.
As shown in fig. 3, the sampling device comprises a sampling tube 4-1, a fifth valve 4-6, a sealing layer 4-5, an end gauze layer 4-2 and an end gravel layer 4-3.
One end of the sampling tube 4-1 extends into the cylindrical landfill chamber 3-1 from the side wall of the cylindrical landfill chamber 3-1, the extending end is wrapped by two end gauze layers 4-2 and one end gravel layer 4-3, the end gravel layer 4-3 is positioned between the two end gauze layers 4-2 and is bound and fixed through a binding belt, the diameter of gravel is 2.5mm-5mm, on one hand, gravel in the cylindrical landfill chamber 3-1 is prevented from flowing out along with water flow, on the other hand, the sampling area is enlarged, and meanwhile, garbage is prevented from blocking the sampling tube. The other end of the sampling tube is provided with a fifth valve 4-6, and the outer wall of the sampling tube 4-1 between the fifth valve and the gauze layer at the end part is provided with a sealing layer 4-5 to prevent the liquid in the cylindrical landfill chamber 3-1 from permeating out along the outer wall of the sampling tube 4-1. In this embodiment, the sealing layers 4 to 5 are formed of structural adhesive.
The sampling devices are distributed at equal intervals along the circumferential direction and the axial direction of the cylindrical landfill chamber 3-1; the sampling devices in two rows are distributed in 2n rows at equal intervals along the circumferential direction, n is more than or equal to 2, the lengths of the sampling tubes 4-1 extending into the cylindrical landfill chamber 3-1 of the sampling devices in two opposite rows are the same, and the lengths of the sampling tubes 4-1 extending into the cylindrical landfill chamber 3-1 of the sampling devices in two adjacent rows are different. As shown in fig. 2, in this embodiment, n is 2, that is, 4 rows are equally spaced in the circumferential direction, wherein the length of the sampling tube 4-1 extending into the cylindrical landfill chamber 3-1 from one pair of sampling devices in opposite rows is longer, and the length of the other pair is shorter, so as to meet the sampling requirements at different depths and different radial directions.
In one embodiment of the invention, the method further comprises adjusting the pressurizers 1-3 to achieve the head change.
The working process of the device is as follows:
introducing a prepared tracer solution into a water injection container 1-1, setting a required injection water head in a pressurizer, opening a fourth valve 1-6 and a third valve 1-2 to ensure that the solution is injected into a cylindrical landfill chamber 3-1 through an external connecting pipe 1-7 and an injection pipe 3-8 at a certain constant speed, opening a fifth valve 4-6 at different times for sampling, testing the concentration of the tracer in the sample by using a special instrument, inputting the result into comsol software, and calculating the migration mode of the tracer in the landfill and the diffusion coefficient and permeability coefficient of the landfill according to a convection-diffusion equation.
In this embodiment, the calculation processes of the parameters such as diffusion coefficient, permeability coefficient, fracture-to-void ratio, and the like are as follows:
according to the size of the gaps, the gaps in the saturated garbage are divided into pores and cracks. The pores (matrix) are smaller in volume, with slower liquid diffusion and less liquid migration through the pores; the fractures (fractures) are larger in volume, with more fluid diffusing through them and migrating through them.
Establishing a two-dimensional axisymmetric model of tracer migration in the landfill column, wherein the two-dimensional axisymmetric model comprises a water flow control equation and a solute transport control equation, and the equations are assumed as follows:
1. the migration direction of the liquid is parallel to a horizontal axis (r axis) and a vertical axis (y axis);
2. the migration behavior of the tracer in the leachate only considers the convection-diffusion operation, neglects the effects of dispersion, adsorption/desorption and the like
3. The flow of liquid obeys darcy's law;
4. the liquid flows out from the crack and the pores together to be buried, the liquid in the crack and the pores is subjected to mass exchange at the interface of the crack pores, and the anisotropy values of the two regions are the same;
the water flow control equation is as follows:
h＝wf×hf+(1-wf)×hm (7)
in the formula, KfrIs the axial permeability coefficient of the fracture, hfIs the water head of the crack, r is the horizontal axis direction, KfzIs the radial permeability coefficient of the fracture, z is the vertical axis direction, beta is the convective exchange coefficient, K is the permeability coefficient of the pore fracture exchange, hmIs the head of the pores ufzRadial velocity of the liquid in the fracture ufrAxial velocity of the liquid in the fracture, KmrIs the axial permeability coefficient of the pores, KmzIs the radial permeability coefficient of the pores, umzThe radial velocity of the liquid in the pores umrThe velocity of the liquid in the pore axis, wfThe crack occupying space ratio is defined, and h is a percolate water head in the landfill body;
the solute transport control equation is as follows:
c＝wf×cf+(1-wf)×cm (10)
in the formula, cfThe concentration of tracer in the fracture, t is time, DfrAs axial diffusion coefficient of tracer in fracture, DfzThe radial diffusion coefficient of the tracer in the fracture, gamma is the diffusion exchange coefficient, D is the diffusion coefficient of the pore fracture exchange, cmAs concentration of tracer in the fracture, DmrAs axial diffusion coefficient of tracer in pores, DmzThe radial diffusion coefficient of the tracer in the pores, and c is the concentration of the tracer in the landfill body;
the right boundary is an injection boundary, and the right boundary conditions are as follows:
cf(r＝0,z1≤z≤z1+zb,t＞0)＝c0 (15)
cm(r＝0,z1≤z≤z1+zb,t＞0)＝c0 (16)
hf(r＝0,z1≤z≤z1+zb,t＞0)＝h0 (17)
hm(r＝0,z1≤z≤z1+zb,t＞0)＝h0 (18)
in the formula, z1The distance from the lower end of the inlet to the bottom of the stack, zbIs the width of the injection port; h is a total of0Is the initial head, c0Is the initial concentration of the tracer, H is the height of the cylindrical landfill chamber;
the left boundary is an external environment boundary, and the left boundary conditions are as follows:
wherein R is the radius of the cylindrical landfill cavity;
the upper boundary is an outflow boundary, and the upper boundary condition is as follows:
cf(0≤r≤R,z-H→ 0+,t＞0)＝0 (29)
cm(0≤r≤R,z-H→ 0+,t＞0)＝0 (30)
hf(0≤r≤R,z＝H,t＞0)＝0 (31)
hm(0≤r≤R,z＝H,t＞0)＝0 (32)
the lower boundary is a bottom boundary, and the lower boundary conditions are as follows:
initial conditions set as
t＝0,cf＝0,cm＝0,hf＝h0,hm＝h0
To demonstrate the effectiveness of the invention, the bottom of the landfill, z, was filled at 25.5kPa11000mg/L of 2, 3, 4, 5-tetrafluorobenzoic acid solution was injected at 0, and the injection was continued for 5.5 h. The sampling device is n 2, samples are taken every 1h, and the sampling device surrounds the landfill column for a circle, 4 types of sampling ports are arranged, wherein the sampling ports 1-2, 1-3, 1-4, 1-5, 3-2, 3-3, 3-4 and 3-5 extend into the landfill column for 14.3cm, the sampling ports 2-2, 2-3, 2-4, 2-5, 4-2, 4-3, 4-4 and 4-5 sample at the inner wall of the landfill column, and the experimental specific device is shown in fig. 4 (only the sampling ports 1-2, 1-3, 1-4, 1-5, 3-2, 3-3, 3-4 and 3-5 are shown). Early experiments show that the inner diameter of a landfill column is 57cm, the height of garbage is 1.04m, the garbage is old garbage more than one year in the old harbor of Shanghai, the diameter of the garbage is 1-10cm, and the relative density of the garbage is 600kg/m3Initial water content of 56.5% and degree of compaction of 5.7kN/m3。
The results of the experiment are shown in FIG. 5 below. The experimental result shows that the monitoring concentration of each point is between 0 and 1300 mg/L; the concentration at the same point rises along with the increase of time, which shows that the phenomenon of liquid convection in the landfill body is obvious; the characteristic that the concentration of the same radius changes along with the height at the same time is not obvious, which shows that the garbage has strong nonuniformity and the liquid diffusion operation in the landfill is strong; the data of 2-3 groups are larger in total, which shows that a dominant flow channel exists near the point 2-3, so that a large amount of tracers are transported to the vicinity of the point; the first set of data is generally smaller, indicating that there are more small pores near the first set.
In addition, the outflow speed of the liquid is monitored in the experimental process, the radial average seepage coefficient of the landfill column is obtained according to a two-dimensional axisymmetric model and boundary conditions of the migration of the tracer in the landfill column, and the result is shown in table 1:
TABLE 1 Experimental results Table
| Experiment number | Q(mL/s) | Kz(cm/s) | V(cm/s) |
| 1 | 7.2 | 0.0046 | 0.0028 |
| 2 | 7.1 | 0.0046 | 0.0028 |
| 3 | 8.6 | 0.0056 | 0.0034 |
According to the model and the experimental result, parameters of the landfill body are obtained through inversion, the parameter values are shown in the table 2, and the numerical fitting result is shown in the figure 6 (only taking the sampling ports 1-2, 1-3, 1-4 and 1-5 as examples). The numerical simulation result shows that the fitting result is good, and the model can better reflect the concentration distribution condition of the tracer in the landfill body; w at different pointsfThe difference is large and can be according to wfThe space distribution of the (A) roughly judges the dominant flow condition in the landfill body, and the w is positioned at 1-4 points under the same radiusfIs w at 1-2f1 times of the total flow rate, which indicates that a dominant flow channel exists near 1-4, and similarly, a dominant flow channel also exists near 1-3, but the pore channel near 1-2 is smaller; the diffusion coefficient of the tracer in the landfill is large, and is 10-6To 10-7Between orders of magnitude.
TABLE 2 parameter value-taking table
The foregoing lists merely illustrate specific embodiments of the invention. It is obvious that the invention is not limited to the above embodiments, but that many variations are possible. All modifications which can be derived or suggested by a person skilled in the art from the disclosure of the present invention are to be considered within the scope of the invention.
Claims (8)
1. A landfill body potential flow testing device based on tracer migration is characterized by comprising a pressurized water supply device, a loading device, a landfill column convection-diffusion device and a sampling device; the pressurizing water supply device is connected with the bottom of the landfill column diffusion convection device, the loading device is used for pressurizing the landfill column convection-diffusion device, and the sampling device is installed on the landfill column convection-diffusion device and used for sampling liquid in the landfill column convection-diffusion device;
the landfill column convection-diffusion device comprises a cylindrical landfill chamber (3-1), a backing plate (3-9), a liquid injection pipe (3-8), a liquid discharge pipe (3-12) and a collection container (3-11); the cylindrical landfill cavity (3-1) is fixed on the backing plate (3-9), the bottom of the cylindrical landfill cavity (3-1) is provided with a water injection hole, and a water injection channel is arranged inside the backing plate (3-9); one end of the liquid injection pipe (3-8) penetrates through the water injection channel and the liquid injection hole and extends into the cylindrical landfill chamber (3-1), the other end of the liquid injection pipe (3-8) is positioned outside the water injection channel, and the other end is provided with a first valve (3-7);
the cylindrical landfill chamber (3-1) is internally provided with a bottom gravel layer (3-2), a bottom geotextile protective layer (3-3), garbage (3-4), a top geotextile filter layer (3-5) and a top gravel layer (3-6) from bottom to top in sequence; one end of the drain pipe (3-12) is arranged at the top gravel layer (3-6), and the other end is communicated with a drain barrel (3-13); the liquid discharge pipe (3-12) is also provided with a second valve (3-10) and a flowmeter (3-11);
the pressurized water supply device comprises a water injection container (1-1), an external connecting pipe (1-7), a third valve (1-2), a pressurizer (1-3), a flow rate meter (1-4) and a pressure gauge (1-5), wherein the third valve is arranged on the external connecting pipe (1-7); the third valve (1-2) is positioned at one end of the external connecting pipe (1-7) close to the water injection container (1-1), and the other end of the external connecting pipe is communicated with a liquid injection pipe (3-8) of the landfill column convection-diffusion device;
the sampling device comprises a sampling tube (4-1), a fifth valve (4-6), a sealing layer (4-5), an end gauze layer (4-2) and an end gravel layer (4-3);
one end of the sampling tube (4-1) extends into the cylindrical landfill chamber (3-1) from the side wall of the cylindrical landfill chamber (3-1), and the extending end is wrapped with two end gauze layers (4-2) and one end gravel layer (4-3), and the end gravel layer (4-3) is positioned between the two end gauze layers (4-2) and is bound and fixed through a binding belt; the other end of the sampling tube is provided with a fifth valve (4-6), and a sealing layer (4-5) is arranged on the outer wall of the sampling tube (4-1) between the fifth valve and the gauze layer at the end part to prevent liquid in the cylindrical landfill chamber (3-1) from permeating along the outer wall of the sampling tube (4-1).
2. The tracer transport-based landfill body preferential flow testing device is characterized by comprising a base (2-1), a bracket (2-2), an I-shaped pressing plate (2-3) and weights (2-4); the landfill column convection-diffusion device is fixed on the base, the I-shaped pressing plate is installed above the landfill column convection-diffusion device through a support, and the weight is located on the I-shaped pressing plate.
3. The tracer transport-based landfill body preferential flow testing device is characterized in that the bracket is fixed on the base, and the upper plate of the I-shaped pressing plate is sleeved on the bracket and can slide on the bracket; the lower plate of the I-shaped pressing plate is positioned above the cylindrical landfill cavity (3-1), and the outer diameter of the lower plate is equal to the inner diameter of the cylindrical landfill cavity (3-1).
4. Tracer transport based preferential flow test apparatus for landfills according to claim 1, characterized in that the injection pipe inside the cylindrical landfill chamber (3-1) is of a telescopic structure.
5. The tracer transport based landfill body preferential flow testing device according to claim 1, wherein the other end of the external connection pipe is provided with a fourth valve (1-6).
6. Tracer migration based preferential flow testing device for landfill bodies according to claim 1, characterized in that several sampling devices are equally spaced circumferentially and axially along the cylindrical landfill chamber (3-1); the sampling devices in two rows are distributed at equal intervals along the circumferential direction and are 2n rows, n is more than or equal to 2, the lengths of the sampling pipes (4-1) extending into the cylindrical landfill chamber (3-1) of the sampling devices in the two rows are the same, and the lengths of the sampling pipes (4-1) extending into the cylindrical landfill chamber (3-1) of the sampling devices in the two adjacent rows are different.
7. A testing method of a tracer migration-based landfill body preferential flow testing device is characterized in that in consideration of the current situation that a saturated landfill body is strong in heterogeneity, gaps in the saturated landfill body are divided into pores and fractures, a convection-diffusion equation of the tracer in the saturated landfill body is established according to Darcy's law and Fick's law, initial conditions and boundary conditions are determined according to experimental conditions, the concentration distribution conditions of the tracer in a landfill column under different parameter combinations are calculated, and a proper parameter set is selected according to a least square method and experimental results, namely parameters of the saturated landfill body.
8. The test method according to claim 7, comprising the steps of:
(1) establishing a two-dimensional axisymmetric model and boundary conditions for migration of the tracer in the embedded column, wherein the two-dimensional axisymmetric model comprises a water flow control equation and a solute transport control equation, namely a convection-diffusion equation; the boundary conditions comprise a right boundary condition, a left boundary condition, an upper boundary condition and a lower boundary condition;
the water flow control equation is as follows:
h＝wf×hf+(1-wf)×hm (7)
in the formula, KfrIs the axial permeability coefficient of the fracture, hfIs the water head of the crack, r is the horizontal axis direction, KfzIs the radial permeability coefficient of the fracture, z is the vertical axis direction, beta is the convective exchange coefficient, K is the permeability coefficient of the pore fracture exchange, hmIs the head of the pores ufzRadial velocity of the liquid in the fracture ufrAxial velocity of the liquid in the fracture, KmrIs the axial permeability coefficient of the pores, KmzIs a poreRadial permeability coefficient of (u)mzThe radial velocity of the liquid in the pores umrThe velocity of the liquid in the pore axis, wfThe crack occupying space ratio is defined, and h is a percolate water head in the landfill body;
the solute transport control equation is as follows:
c＝wf×cf+(1-wf)×cm (10)
in the formula, cfThe concentration of tracer in the fracture, t is time, DfrAs axial diffusion coefficient of tracer in fracture, DfzThe radial diffusion coefficient of the tracer in the fracture, gamma is the diffusion exchange coefficient, D is the diffusion coefficient of the pore fracture exchange, cmConcentration of tracer in the fracture, DmrIs the axial diffusion coefficient of the tracer in the pores, DmzThe radial diffusion coefficient of the tracer in the pores, and c is the concentration of the tracer in the landfill body;
the right boundary is an injection boundary, and the right boundary conditions are as follows:
cf(r＝0,z1≤z≤z1+zb,t＞0)＝c0 (15)
cm(r＝0,z1≤z≤z1+zb,t＞0)＝c0 (16)
hf(r＝0,z1≤z≤z1+zb,t＞0)＝h0 (17)
hm(r＝0,z1≤z≤z1+zb,t＞0)＝h0 (18)
in the formula, z1The distance from the lower end of the inlet to the bottom of the stack, zbIs the width of the injection port; h is0Is the initial head, c0Is the initial concentration of the tracer, H is the height of the cylindrical landfill chamber;
the left boundary is an external environment boundary, and the left boundary conditions are as follows:
wherein R is the radius of the cylindrical landfill cavity;
the upper boundary is an outflow boundary, and the upper boundary condition is as follows:
cf(0≤r≤R,z-H→0+,t＞0)＝0 (29)
cm(0≤r≤R,z-H→0+,t＞0)＝0 (30)
hf(0≤r≤R,z＝H,t＞0)＝0 (31)
hm(0≤r≤R,z＝H,t＞0)＝0 (32)
the lower boundary is a bottom boundary, and the lower boundary conditions are as follows:
(2) introducing a prepared tracer solution into a water injection container, arranging a required injection water head in a pressurizer, opening a fourth valve and a third valve to ensure that the solution is injected into a cylindrical landfill chamber through an external connecting pipe and an injection pipe at a certain constant speed, opening a fifth valve at different times for sampling, and testing the concentration of the tracer in the sample by using a special instrument;
(3) combining the concentration measurement result of the tracer and the corresponding time, calculating the concentration distribution condition of the tracer in the landfill column under different parameter combinations according to the two-dimensional axisymmetric model and the boundary conditions of the migration of the tracer in the landfill column, and selecting a proper parameter set according to a least square method and an experimental result, namely the axial and radial diffusion coefficient, the permeability coefficient and the fracture-to-void ratio of the pores and the fractures of the saturated refuse landfill body, namely Dfr，Dfz，Dmr，Dmz，Kfr，Kfz，Kmr，KmzAnd wf。
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