Evaluation method for vertical difference of pollutant blocking performance of clay separating wall

Abstract

The invention relates to the technical field of underground environmental pollution prevention and control of refuse landfills, in particular to a method for evaluating vertical difference of performance of blocking pollutants by a clay separating wall, which comprises the following steps: S1, measuring physical characteristic parameters of a separating wall material and migration parameters of the pollutants in the separating wall material; s2, performing stress distribution analysis on the isolation wall, and establishing a relational expression based on the physical characteristic parameters of the isolation wall material, the effective stress of the isolation wall and the depth of the wall body; s3, on the basis of the relational expression and the pollutant transport law, constructing a pollutant transport model in the depth direction of the wall body of the isolation wall; and S4, predicting the migration time of the pollutants in the clay separating wall through the pollutant migration model, and evaluating the vertical difference of the blocking performance of the separating wall to the pollutants based on the migration time difference at different depths. According to the evaluation method, accurate evaluation of the vertical difference of the antifouling performance of the isolation wall is realized, and the reliability of the evaluation result and the engineering guidance value are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of underground environmental pollution prevention and control in landfills, and in particular to a method for evaluating vertical differences in the pollutant blocking performance of a clay-based isolation wall. Background Art

[0002] Landfills inevitably generate large amounts of leachate over their long-term operation, often containing a variety of complex organic and inorganic pollutants. If these pollutants are not effectively controlled and treated, they will gradually seep into the surrounding soil and ultimately contaminate the groundwater system, posing a serious threat to the regional ecological environment and human health. Clay-based isolation walls, with their unique physical and chemical properties such as low permeability, strong adsorption, and excellent chemical stability, can effectively block the migration and diffusion of these pollutants. Therefore, clay-based isolation walls have become an indispensable and critical protective feature in landfill anti-seepage systems, playing a vital role in protecting the groundwater environment.

[0003] Currently, the engineering community relies primarily on two methods to evaluate the pollutant retention performance of isolation walls: small-scale laboratory tests and numerical simulations. However, existing research methods have significant limitations: most researchers use uniform physical parameters for performance evaluation, or conduct tests and simulations only at a fixed depth (fixed effective stress). This oversimplified approach overlooks a crucial engineering reality: key engineering parameters such as permeability, porosity, and dry density vary significantly at different depths due to differences in overburden pressure. This simplification can lead to overly optimistic evaluation results, overestimating the actual anti-pollution performance of the isolation wall, thus creating potential safety hazards in engineering designs. Particularly in the long-term operation of landfills, the cumulative effect of such evaluation biases can cause pollutants to penetrate the isolation wall's protection at vulnerable locations, ultimately causing serious environmental safety issues and irreversible ecological damage. Summary of the Invention

[0004] In view of this, the present invention proposes a method for evaluating the vertical differences in the pollutant retention performance of clay-based isolation walls to solve the problem that the existing isolation wall pollutant retention performance evaluation method ignores the vertical differences in the isolation wall's ability to retain pollutants, thereby affecting the accuracy of the evaluation of the isolation wall's anti-pollution performance.

[0005] The technical solution of the present invention is achieved as follows: The present invention provides a method for evaluating the vertical variability of the pollutant retention performance of a clay-based isolation wall, comprising the following steps:

[0006] S1. Measure the physical property parameters of the isolation wall material and the migration parameters of pollutants in the isolation wall material;

[0007] S2. Conduct a stress distribution analysis on the isolation wall, and establish a physical property parameter-effective stress-wall depth relationship based on the physical property parameters of the isolation wall material, the effective stress of the isolation wall, and the wall depth;

[0008] S3. Based on the physical property parameter-effective stress-wall depth relationship and the pollutant migration law, construct a pollutant migration model in the depth direction of the isolation wall;

[0009] S4. Through the pollutant migration model in the depth direction of the isolation wall considering the vertical difference of the physical property parameters and effective stress of the isolation wall, predict the migration time of pollutants in the depth direction of the clay-based isolation wall, and evaluate the vertical difference of the pollutant retardation performance of the isolation wall based on the migration time difference at different depths.

[0010] On the basis of the above technical solutions, preferably, in step S1, the physical property parameters include permeability coefficient, porosity, void ratio, and dry density, and the migration parameters include distribution coefficient, effective diffusion coefficient, and mechanical dispersion coefficient of pollutants in the isolation wall material.

[0011] On the basis of the above technical solutions, preferably, in step S2, the physical property parameter-effective stress-wall depth relationship includes a first relationship and a second relationship. The first relationship is constructed based on the physical property parameters of the isolation wall material and the effective stress of the isolation wall; the second relationship is the relationship of the effective stress of the isolation wall varying with the wall depth constructed according to soil mechanics theory.

[0012] On the basis of the above technical solutions, preferably, the expression of the first relationship is as follows:

[0013]

[0014] Among them, k is the permeability coefficient, τ is the void ratio, n is the porosity, σ′ is the effective stress, ρ d is the dry density, and a1, a2, a3, a4, b1, b2, b3, and b4 are all undetermined coefficients determined by experiments;

[0015] The expression of the second relationship is as follows:

[0016]

[0017] Among them, σ' is the effective stress of the wall; σ v ' o is the vertical effective stress of the in-situ foundation soil outside the trench; γ' wis the effective weight of the isolation wall material; z is the calculated depth of the isolation wall; Δ is the unilateral lateral deformation of the trench sidewall; K am is the in-situ foundation soil lateral pressure coefficient, which is a function of Δ; B is the wall width; C1 is the strain value under unit stress; C cε is the modified compression index; C1 and C cε These are constants that can be obtained through indoor one-dimensional consolidation tests.

[0018] Based on the above technical solution, preferably, step S3 specifically includes:

[0019] S31. Based on the modified lateral extrusion theory, a set of governing equations describing the correlation between the physical properties of the isolation wall material, effective stress, and wall depth is established according to the first and second equations.

[0020] S32. Based on the law of conservation of mass and combining the convection, mechanical dispersion, molecular diffusion, and adsorption processes of pollutants in the isolation wall material, establish the governing equation for pollutant migration;

[0021] S33. Combine the control equations of the relationship between the physical parameters of the isolation wall material, effective stress, and wall depth with the control equations of the pollutant migration law to construct a pollutant migration model in the depth direction of the isolation wall, and set the initial conditions and boundary conditions of the pollutant migration model.

[0022] On the basis of the above technical solution, preferably, the control equations include:

[0023]

[0024] Where K'0 is the modified static earth pressure coefficient, which can be calculated based on in-situ tests or the stress distribution model of the isolation wall; γ' w A is the effective weight of the isolation wall material; d , B d , C d and D d n, ρ respectively d The coefficients of variation of , τ and k along the depth of the isolation wall can be determined through experiments.

[0025] On the basis of the above technical solution, preferably, the expression of the control equation of the pollutant migration law is:

[0026]

[0027] Where k is the permeability coefficient, n is the porosity, and ρ d is the dry density, c is the solute concentration in the solution; t is the time; x is the distance in the seepage direction; is the hydraulic gradient; is the pollutant flux caused by convection; The pollutant flux caused by mechanical dispersion; The pollutant flux caused by free diffusion.

[0028] Based on the above technical solutions, preferably, the derivation process of the control equation for the pollutant migration law is as follows:

[0029] According to the law of conservation of mass, the expression for the migration law of solute in the saturated isolation wall is:

[0030]

[0031] In the formula, c is the solute concentration in the solution; t is the time; x is the distance in the seepage direction; S is the adsorption amount of the soil to the solute; J s is the solute flux;

[0032] Among them, the expression for the solute flux is:

[0033] J s = J V + J M + J D

[0034] J V = v x nc

[0035]

[0036]

[0037] J V 、J M and J D respectively represent the solute fluxes caused by convection, mechanical dispersion, and free diffusion, v x is the Darcy velocity along the x direction, is the hydraulic gradient;

[0038] Assuming that the adsorption of the soil to the solute is linear adsorption, the expression for the adsorption amount is:

[0039] S = cK d ,

[0040] Substitute the solute flux expression and the adsorption amount expression into the solute migration law expression to obtain the control equation for the pollutant migration law.

[0041] Based on the above technical solutions, preferably, the initial conditions and boundary conditions of the pollutant migration model include:

[0042]

[0043] In the formula, x is the distance in the seepage direction, t is the time, Co For the pollutant concentration at the inflow boundary of the isolation wall.

[0044] Based on the above technical solutions, preferably, step S4 specifically includes:

[0045] S41. Use the numerical simulation method to solve the pollutant transport model, and obtain the distribution of the concentration of the target pollutant at each depth of the isolation wall changing with time;

[0046] S42. Based on the numerical simulation results, determine the transport time required for the target pollutant to reach a specific concentration threshold at each depth of the isolation wall;

[0047] S43. Compare and analyze the transport times at different depths, evaluate the differences in the pollutant retardation performance of the isolation wall in the vertical direction, and form an evaluation report.

[0048] The evaluation method for the vertical difference in the pollutant retardation performance of the clay-based isolation wall of the present invention has the following beneficial effects compared with the prior art:

[0049] (1) By systematically considering the variation laws of physical characteristic parameters such as the permeability coefficient, porosity, and dry density of the isolation wall material with the wall depth, the present invention establishes a pollutant transport model considering the vertical differences in the physical property parameters and effective stress of the isolation wall, realizes the accurate evaluation of the vertical difference in the anti-pollution performance of the isolation wall, overcomes the evaluation deviation caused by using unified parameters in the traditional evaluation method, and improves the reliability of the evaluation results and the engineering guiding value;

[0050] (2) By obtaining the relationship between the physical characteristic parameters of the isolation wall and the effective stress through an indoor consolidation test, and combining the in-situ stress distribution calculation model to establish the relationship between the effective stress of the isolation wall changing with depth, the quantitative characterization of the variation law of the physical characteristic parameters of the isolation wall with depth is realized, providing a reliable parameter basis for the construction of the pollutant transport model;

[0051] (3) Based on the modified lateral extrusion theory and the law of mass conservation, a pollutant transport model including the control equations of the correlation between the physical property parameters-effective stress-wall depth of the isolation wall and the control equation of the solute transport law is constructed. This model can accurately describe the transport characteristics of pollutants at different depths of the isolation wall and improve the accuracy of simulation prediction;

[0052] (4) The numerical simulation method is adopted to solve the pollutant transport model considering the physical properties of the isolation wall and the vertical difference of effective stress. By determining the transport time required for pollutants to reach a specific concentration threshold at different depths, the quantitative evaluation of the vertical difference in the anti-pollution performance of the isolation wall is realized, providing a scientific basis for engineering design and construction optimization. In addition, the evaluation method of the present invention has strong generality, can be applied to the transport evaluation of different types of clay-based isolation wall materials and various pollutants, and has clear operation steps and standardized evaluation processes, which are convenient for popularization and application in engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0054] Figure 1 It is a flowchart of the evaluation method for the vertical difference in the performance of the clay-based isolation wall of the present invention in blocking pollutants;

[0055] Figure 2 It is a comparison between the simulation results of the pollutant transport model of the present invention and the test data;

[0056] Figure 3 It is the effective diffusion coefficient D of the inert ion soil column test in Embodiment 1 of the present invention * Fitting result diagram;

[0057] Figure 4 It is the mechanical dispersion coefficient D of the inert ion soil column test in Embodiment 1 of the present invention m Fitting result diagram;

[0058] Figure 5 It is the breakthrough time of heavy metal pollutant Cd at different depths z of the isolation wall in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0059] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0060] As Figure 1 shown, the present invention provides an evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants, including the following steps:

[0061] S1. Measure the physical property parameters of the isolation wall material and the migration parameters of pollutants in the isolation wall material;

[0062] S2. Conduct stress distribution analysis on the isolation wall, and establish a relationship between physical property parameters-effective stress-wall depth based on the physical property parameters of the isolation wall material, the effective stress of the isolation wall, and the wall depth;

[0063] S3. Based on the relationship between physical property parameters-effective stress-wall depth and the pollutant migration law, construct a pollutant migration model in the depth direction of the isolation wall;

[0064] S4. Through the pollutant migration model in the depth direction of the isolation wall considering the physical property parameters of the isolation wall and the vertical difference of effective stress, predict the migration time of pollutants in the depth direction of the clay-based isolation wall, and evaluate the vertical difference of the pollutant retardation performance of the isolation wall based on the migration time difference at different depths.

[0065] The evaluation method provided by the present invention first obtains the basic physical property parameters and pollutant migration parameters of the isolation wall material through experimental measurement, and then establishes a relationship between physical property parameters changing with depth by combining stress distribution analysis. On this basis, a pollutant migration model considering the physical property parameters of the isolation wall and the vertical difference of effective stress is constructed. Through this model, the migration time of pollutants at different depths of the isolation wall can be accurately predicted, and the vertical difference of the anti-pollution performance of the isolation wall can be effectively evaluated. The technical solution of the present invention not only breaks through the limitation of using unified parameters in traditional evaluation methods, but also makes the evaluation results more in line with engineering practice by considering the influencing factors of the isolation wall depth, and can timely discover the weak links of the anti-pollution performance of the isolation wall. At the same time, the operation steps of this method are clear, the evaluation process is scientific and standardized, it is applicable to the migration evaluation of different types of clay-based isolation wall materials and various pollutants, and provides reliable technical support and theoretical guidance for the environmental safety protection of landfills.

[0066] Specifically, in step S1, the physical property parameters include permeability coefficient, porosity, void ratio, and dry density, and the migration parameters include distribution coefficient, effective diffusion coefficient, and mechanical dispersion coefficient of pollutants in the isolation wall material.

[0067] In step S1, the permeability coefficient, porosity, void ratio, and dry density of the isolation wall material are obtained through indoor consolidation tests and Terzaghi consolidation theory. The specific test steps can refer to the "Standard for Geotechnical Test Methods (GB / T 50123-2019)". The permeability coefficient (k, m / s) of the isolation wall material under each stage of consolidation pressure can be calculated according to the following formula:

[0068] k = C v m v γ w

[0069] Where: C v is the consolidation coefficient (m 2 / s) calculated by the time square root (Taylor) method, m v is the volume compressibility coefficient (kPa -1 ), γ w is the unit weight of water (kN / m 3 ).

[0070] In step S1, the migration parameters of pollutants in the isolation wall material are obtained through the Batch adsorption test and the inert ion soil column test.

[0071] Specifically, according to the inert ion soil column test and combined with the normalized expression of pollutant migration, the effective diffusion coefficient D * and the mechanical dispersion coefficient D m of pollutants in the isolation wall material can be calculated. The normalized expression of pollutant migration is as follows:

[0072]

[0073] The distribution coefficient is an important parameter describing solute migration, which reflects the ability of the adsorbent to retain the solute in the solution phase. The distribution coefficient (K d , L / kg) can be expressed by the following formula:

[0074]

[0075] where q e is the adsorption amount at equilibrium, mg / kg; C e is the solute concentration at equilibrium, mg / L. When the adsorption is non-linear, the distribution coefficient (K d ) is a function of concentration.

[0076] In step S2, the physical property parameter-effective stress-wall depth relationship includes a first relationship and a second relationship. The first relationship is constructed based on the physical property parameters of the isolation wall material and the effective stress of the isolation wall; the second relationship is the relationship between the effective stress of the isolation wall varying with the wall depth constructed according to soil mechanics theory.

[0077] Specifically, an indoor one-dimensional consolidation test is carried out on the isolation wall material, and the first relationship is obtained through the consolidation test. The expression of the first relationship is as follows:

[0078]

[0079] where k is the permeability coefficient, τ is the void ratio, n is the porosity, σ′ is the effective stress, ρ dis the dry density, and a1, a2, a3, a4, b1, b2, b3, and b4 are all unknown coefficients determined through indoor consolidation tests. Specifically, the second relationship is a relationship between the effective stress of the isolation wall and the depth of the wall, which is constructed based on soil mechanics theory, wherein soil mechanics theory includes any one of the "arch theory" model, the "lateral compression theory" model, and the "modified lateral compression theory" model. More preferably, the second relationship is obtained based on the "modified lateral compression theory" model,

[0080]

[0081] Among them, σ' is the effective stress of the wall; σ' vo is the vertical effective stress of the in-situ foundation soil outside the trench; γ' w is the effective weight of the isolation wall material; z is the calculated depth of the isolation wall; Δ is the unilateral lateral deformation of the groove sidewall; K am is the in-situ foundation soil lateral pressure coefficient, which is a function of Δ; B is the wall width; C1 is the strain value under unit stress; Ccε is the modified compression index; C1 and Ccε are constants that can be obtained through indoor one-dimensional consolidation tests.

[0082] Step S3 specifically includes:

[0083] S31. Based on the modified lateral extrusion theory, a set of governing equations describing the correlation between the physical properties of the isolation wall material, effective stress, and wall depth is established according to the first and second equations.

[0084] S32. Based on the law of conservation of mass and combining the convection, mechanical dispersion, molecular diffusion, and adsorption processes of pollutants in the isolation wall material, establish the governing equation for pollutant migration;

[0085] S33. Combine the control equations of the relationship between the physical parameters of the isolation wall material, effective stress, and wall depth with the control equations of the pollutant migration law, construct a pollutant migration model that takes into account the vertical differences in the physical parameters and effective stress of the isolation wall, and set the initial conditions and boundary conditions of the pollutant migration model.

[0086] Specifically, assuming that the stress on the clay-based isolation wall S-Awall is the modified static earth pressure, according to the change of effective stress σ′ along the wall depth z, and the effect of effective stress σ′ on porosity n and dry density ρ d , the influence of porosity τ and the influence of porosity τ on permeability coefficient k, the following control equations are established:

[0087]

[0088] Where K'0 is the modified static earth pressure coefficient, which can be calculated based on in-situ tests or the stress distribution model of the isolation wall; γ'w is the effective unit weight of the cutoff wall material; A d , B d , C d and D d are the coefficients of the variation of n, ρ d , τ and k with depth, respectively, and can be obtained from consolidation tests.

[0089] According to the law of conservation of mass, the expression for the solute transport law in a saturated cutoff wall is:

[0090]

[0091] In the formula, c is the solute concentration in the solution; t is the time; x is the distance in the seepage direction; S is the adsorption amount of the solute by the soil; J s is the solute flux;

[0092] Among them, the expression for the solute flux is:

[0093] J s = J V + J M + J D (4-7)

[0094] J V = v x nc (4-8)

[0095]

[0096] J V , J M and J D represent the solute fluxes caused by convection, mechanical dispersion, and free diffusion, respectively. v x is the Darcy velocity in the x direction, is the hydraulic gradient; thus, Equation (4-8) can be written as:

[0097]

[0098] Assume that the adsorption of the solute by the soil is linear adsorption, then the expression for the adsorption amount is:

[0099] S = cK d (4-12),

[0100] Substitute the solute flux expressions (4-7)–(4-11) and the adsorption amount expression (4-12) into the solute transport law expression (4-6), and the pollutant transport law equation is obtained as follows:

[0101]

[0102] Combining formulas (4-1) to (4-5) and (4-13) constitutes a numerical model for pollutant transport that takes into account the vertical differences in pollutant transport parameters such as the permeability coefficient, porosity, and dry density of the cutoff wall. This model can comprehensively evaluate the vertical differences in the performance of the cutoff wall in blocking pollutants.

[0103] Assume that the pollutant concentration in the cutoff wall at the initial moment is 0, the inflow boundary of the cutoff wall is a fixed-concentration pollutant boundary, the outflow boundary of the cutoff wall is a zero-concentration pollutant boundary, and the upper and lower parts of the cutoff wall are set as zero-flux boundaries, that is: the initial conditions and boundary conditions of the pollutant transport model include:

[0104]

[0105] In the formula, x is the distance in the seepage direction, t is the time, and C o is the pollutant concentration at the inflow boundary of the cutoff wall.

[0106] Specifically, in one embodiment, step S3 further includes model verification. Hong and Shackelford (2017) studied the transport laws of K + and Zn 2+ in zeolite-modified soil-bentonite backfill through long-term soil column tests, and gave the variation curves of the concentrations of K + and Zn 2+ with time at the outflow of the soil column. Fan Ridong (2017) studied the transport laws of heavy metals Pb out and Zn 2+ and Zn 2+ in sand-bentonite cutoff wall materials based on flexible wall permeability tests, and obtained the relevant parameters for the transport of heavy metals Pb 2+ and Zn 2+ in sand-bentonite cutoff wall materials. Yang Yuling (2017) studied the transport laws of Ca 2+ in sodium hexametaphosphate-modified calcium-based bentonite cutoff wall materials through soil column tests, and obtained the relevant transport parameters. In this section, the experimental results of ① Hong and Shackelford on the transport of K + in 5% clinoptilolite-modified soil-bentonite backfill and Zn 2+ in 5% UB chabazite-modified soil-bentonite backfill in the above literature; ② Fan Ridong on the experimental results of the transport of Pb 2+ with a concentration of 5175 mg / L and Zn 2+ with a concentration of 1625 mg / L in sand-bentonite cutoff wall materials containing 9.6% bentonite; ③ Yang Yuling on the experimental results of the transport of Ca 2+ with a concentration of 37300 mg / L in sodium hexametaphosphate-modified calcium-based bentonite cutoff wall materials are compared with the simulation results of the established pollutant transport numerical model to evaluate the reliability of the established model.

[0107] Since the soil column used by the above scholars for the test is small in size (no more than 0.075m in height and no more than 0.071m in diameter), it can be considered that the soil column is homogeneous. Therefore, when using the model for calculation, it is equivalent to a small-sized wall with the same width and depth as the height of the test soil column, and the porosity n and dry density ρ are considered to be d The permeability coefficient k does not change with depth. Therefore, A in formulas (4-2) to (4-5) d , B d , C d , D d The value of is 0, and the pollutant transport model can be simplified to formula (4-13). The specific parameter values used for model calculation are shown in Table 1, where i is the hydraulic gradient, D h is the hydrodynamic dispersion coefficient (D h =D * +D m ).

[0108] Table 1 Summary of solute transport parameters

[0109]

[0110]

[0111] Figure 2 The comparison between the pollutant transport model simulation results and the experimental data is shown. Figure 2 (a) is Hong and Shackelford's K + The simulation results of pollutant transport are compared with the experimental data. Figure 2 (b) Zn of Hong and Shackelford 2+ Comparison of simulation results of pollutant transport with experimental data; Figure 2 (c) is Fan Ridong's Pb 2+ Comparison of simulation results of pollutant transport with experimental data; Figure 2 (d) is Fan Ridong's Zn 2+ Comparison of simulation results of pollutant transport with experimental data; Figure 2 (e) is Yang Yuling's Ca 2+ Comparison of simulation results of pollutant transport with experimental data. Figure 2 It can be seen that the simulation results of the outflow ion concentration at the center depth of the wall (z = 0.032m or 0.035m) are generally well matched with the experimental data. Figure 2 There are some differences between some test data points and simulation results, but this is normal because errors are inevitable in the sampling and testing process of test data.

[0112] Therefore, the numerical model of pollutant migration proposed by the present invention, which takes into account the vertical differences in pollutant migration parameters such as the permeability coefficient, porosity, and dry density of the isolation wall, can be used to simulate the migration law of pollutants at different depths of the isolation wall.

[0113] Step S4 specifically includes:

[0114] S41. Use the numerical simulation method to solve the pollutant migration model, and obtain the distribution of the concentration of the target pollutant at different depths of the isolation wall over time;

[0115] S42. Based on the numerical simulation results, determine the migration time required for the target pollutant to reach a specific concentration threshold at different depths of the isolation wall;

[0116] S43. Compare and analyze the migration times at different depths, evaluate the differences in the pollutant retardation performance of the isolation wall in the vertical direction, and form an evaluation report.

[0117] The technical solution of the present invention will be described below through a specific embodiment:

[0118] Embodiment 1

[0119] In this embodiment, the selected clay is attapulgite with a particle size of less than 200 mesh. A set of clay-based isolation wall material ratios is selected, that is, 30 parts of attapulgite and 70 parts of Fujian standard sand are evenly mixed (denoted as S-A wall ). Other types of clays and other material ratios are also applicable. In addition, the components of pollutants in landfill leachate are complex. In this embodiment, the heavy metal pollutant Cd is selected as a typical pollutant for studying the vertical differences in the pollutant retardation performance of the clay-based isolation wall. Relevant researchers can also select typical pollutants according to their own needs and the types and characteristics of pollutants in the landfill leachate involved.

[0120] (1) Obtain the relationship between the permeability coefficient, porosity, dry density of the isolation wall material and the effective stress of the isolation wall: Conduct an indoor one-dimensional consolidation test on the isolation wall material, and obtain the relationship between the physical property parameters of the isolation wall material and the effective stress of the isolation wall through the consolidation test, that is, the relationship between the permeability coefficient (k) of the isolation wall material and the void ratio (τ), and the relationship between the void ratio (τ) and the effective stress (σ′), as follows:

[0121] k = 1×10 -10 ×e 2.4002τ (R 2 = 0.999);

[0122] τ = -0.1644log(σ′) + 0.9933 (R 2 = 0.999);

[0123] By combining the above two formulas, we can obtain the relationship between the permeability coefficient of the isolation wall material and the effective stress of the isolation wall.

[0124] In addition, the porosity n and dry density ρ of the isolation wall material can also be obtained through the consolidation test. d The relationship between it and the effective stress σ′ is:

[0125] n=-0.0698log(σ′)+0.5307(R 2 =0.981);

[0126] ρ d =0.1638log(σ′)+1.102(R 2 =0.981).

[0127] (2) Obtain the relationship between the effective stress of the isolation wall and the wall depth: According to the indoor test, the effective weight γ′ of the isolation wall material is obtained w , C1 and C cε 8.4kN / m respectively 3 , -0.08 and 0.10, in this embodiment, the calculation of the clay-based isolation wall is carried out when the wall width B is 0.8m, the wall depth H is 30m, and the in-situ foundation soil is medium-density sand (the effective internal friction angle of the foundation soil is 0.10). K am =25200(Δ / H) 2 –127(Δ / H)+0.426), when the groundwater level is at the surface, the relationship between the effective stress of the isolation wall and the wall depth is expressed as:

[0128]

[0129] (3) Obtaining the migration parameters of heavy metal pollutants Cd in the isolation wall material: wall Batch adsorption test and inert ion soil column test were carried out to obtain the migration parameters of pollutant Cd in the isolation wall material. According to formulas (2-1) and (2-2), the effective diffusion coefficient D was fitted. * and mechanical diffusion coefficient D m They are 5.32×10 -10 m 2 / s and 9.07×10 -11 m 2 / s. Figure 3 and Figure 4 As shown. The pollutant Cd was adsorbed on the isolation wall material SA by Batch adsorption test. wall The distribution coefficient is 15.74 L / kg.

[0130] (4) Calculate the migration time of pollutants at different depths in the clay-based isolation wall, and evaluate the vertical difference in the pollutant retardation performance of the isolation wall:

[0131] According to the above steps, the model parameters for calculating the migration time of pollutants at different depths in the isolation wall are shown in Table 2. Substitute the following parameters into the pollutant migration model to obtain the distribution of the concentration of pollutant Cd with time at each depth of the isolation wall.

[0132] Table 2 Summary of model parameters

[0133]

[0134] The breakthrough time refers to the time required for pollutants to migrate through the anti-pollution and anti-seepage barrier. The "Technical Code for Geotechnical Engineering of Domestic Waste Sanitary Landfill" (CJJ 176-2012) takes the time t corresponding to the ratio C of the concentration C of the target pollutant flowing out of the wall to the original concentration C0 of the pollutant out,10 / C0 = 0.1 as the breakthrough time. out,10 / C0 = 0.1 10 as the breakthrough time.

[0135] In this embodiment, the isolation wall S-A is simulated wall with a width B of 0.8 m, a depth of the isolation wall of 30 m, a water head difference h of 0.3 m between both sides of the isolation wall (the water head inside the wall is higher than that outside the wall, the left side is inside the isolation wall, and the right side is outside the isolation wall), and the pollutant migrates from left to right. The concentration distribution and breakthrough time of the heavy metal pollutant Cd at different depths z in the isolation wall S-A wall are obtained.

[0136] It can be seen from Figure 5 that the migration speed of the heavy metal pollutant Cd in the isolation wall S-A wall decreases with the increase of the depth of the isolation wall. This is because the stress in the shallow part of the isolation wall is small and the permeability coefficient is large, resulting in a low performance of the shallow part of the isolation wall in retarding pollutants. At different depths of the isolation wall, the breakthrough time of Cd is greater than 60 years. When the depth of the isolation wall increases from 1 m to 30 m, the breakthrough time t of Cd 10 increases from 71 years to 88 years, an increase of 23.9%.

[0137] Therefore, it should be noted that the performance of the shallow part of the isolation wall in retarding pollutants is weak, and the anti-pollution performance of the shallow part of the wall should be strengthened during actual construction.

[0138] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. An evaluation method for vertical differences in the performance of a clay-based isolation wall in blocking pollutants, characterized in that The following steps are involved: S1. Determine the physical property parameters of the isolation wall material and the migration parameters of pollutants in the isolation wall material; S2. Analyze the stress distribution of the isolation wall and establish a physical parameter-effective stress-wall depth relationship based on the physical property parameters of the isolation wall material, the effective stress of the isolation wall, and the wall depth; S3. Based on the relationship between physical parameters, effective stress and wall depth and the law of pollutant migration, a pollutant migration model in the depth direction of the isolation wall is constructed; S4. By considering the physical parameters of the isolation wall and the vertical difference of effective stress in the depth direction of the isolation wall, the migration time of pollutants in the depth direction of the clay-based isolation wall is predicted, and the vertical difference of the isolation wall's blocking performance for pollutants is evaluated based on the difference in migration time at different depths.

2. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 1, characterized in that, In step S1, the physical property parameters include permeability coefficient, porosity, void ratio and dry density, and the migration parameters include the distribution coefficient, effective diffusion coefficient and mechanical diffusion coefficient of pollutants in the isolation wall material.

3. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 1, characterized in that In step S2, the physical parameter-effective stress-wall depth relationship includes a first relationship and a second relationship. The first relationship is constructed based on the physical property parameters of the isolation wall material and the effective stress of the isolation wall; the second relationship is a relationship between the effective stress of the isolation wall and the wall depth constructed according to soil mechanics theory.

4. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 3, characterized in that, The first relation is expressed as follows: where k is the permeability coefficient, τ is the void ratio, n is the porosity, σ′ is the effective stress, ρ d is the dry density, and a1, a2, a3, a4, b1, b2, b3, and b4 are all undetermined coefficients to be determined through tests; The expression of the second relation is as follows: Among them, σ' is the effective stress of the wall; σ v ' o is the vertical effective stress of the in-situ foundation soil outside the trench; γ' w is the effective unit weight of the cut-off wall material; z is the calculated depth of the cut-off wall; Δ is the unilateral lateral deformation of the trench side wall; K am is the coefficient of lateral earth pressure of the in-situ foundation soil and is a function of Δ; B is the width of the wall; C1 is the strain value under unit stress; C cε is the modified compression index; both C1 and C cε are constants that can be obtained through one-dimensional consolidation tests in the laboratory.

5. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 4, characterized in that Step S3 specifically includes: S31. Based on the modified lateral extrusion theory, a set of governing equations describing the correlation between the physical properties of the isolation wall material, effective stress, and wall depth is established according to the first and second equations. S32. Based on the law of conservation of mass and combining the convection, mechanical dispersion, molecular diffusion, and adsorption processes of pollutants in the isolation wall material, establish the governing equation for pollutant migration; S33. Combine the control equations of the relationship between the physical parameters of the isolation wall material, effective stress, and wall depth with the control equations of the pollutant migration law to obtain a pollutant migration model in the depth direction of the isolation wall, and set the initial conditions and boundary conditions of the pollutant migration model.

6. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 5, characterized in that The control equations include: In the formula, K'0 is the modified coefficient of earth pressure at rest, which can be calculated based on in-situ tests or the stress distribution model of the isolation wall; γ' w is the effective unit weight of the isolation wall material; A d , B d , C d and D d are the coefficients of the variations of n, ρ d , τ and k along the depth of the isolation wall respectively, which can be determined through tests.

7. The evaluation method for vertical difference in the performance of a clay-based isolation wall in blocking pollutants as described in claim 5, characterized in that, The expression of the control equation of the pollutant migration law is: where k is the permeability coefficient, n is the porosity, ρ d is the dry density, c is the solute concentration in the solution; t is the time; x is the distance in the seepage direction; is the hydraulic gradient; is the pollutant flux caused by convection; is the pollutant flux caused by mechanical dispersion; is the pollutant flux caused by free diffusion.

8. The evaluation method for vertical difference in the performance of a clay-based isolation wall in retarding pollutants according to claim 7, characterized in that The derivation process of the pollutant migration law control equation is as follows: According to the law of conservation of mass, the migration law of solute in the saturated isolation wall is expressed as: Where c is the concentration of the solute in the solution; t is the time; x is the distance in the seepage direction; S is the adsorption amount of the solute by the soil mass; J s is the solute flux; The expression of solute flux is: J S = J V + J M + J D J V = v x nc J V 、J M and J D represent the solute fluxes caused by convection, mechanical dispersion, and free diffusion, respectively, where v x is the Darcy velocity in the x - direction, is the hydraulic gradient; Assuming that the adsorption of solute by soil is linear adsorption, the expression of adsorption amount is: S = cK d , Substituting the solute flux expression and the adsorption amount expression into the solute migration law expression, the pollutant migration law control equation is obtained.

9. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 5, characterized in that The initial conditions and boundary conditions of the pollutant transport model include: where x is the distance in the seepage direction, t is the time, and C o is the pollutant concentration at the inflow boundary of the isolation wall.

10. The evaluation method for the vertical difference in the performance of a clay-based isolation wall in blocking pollutants according to claim 1, characterized in that Step S4 specifically includes: S41. Solve the pollutant migration model using a numerical simulation method to obtain the distribution of the concentration of the target pollutant at each depth of the isolation wall over time; S42. Based on the numerical simulation results, determine the migration time required for the target pollutant to reach a specific concentration threshold at each depth of the isolation wall; S43. Compare and analyze the migration times at different depths, evaluate the differences in the pollutant retardation performance of the isolation wall in the vertical direction, and form an evaluation report.

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