Fractional-Fractal Modeling of Filtration-Consolidation Processes in Saline Saturated Soils

: To study the peculiarities of anomalous consolidation processes in saturated porous (soil) media in the conditions of salt transfer, we present a new mathematical model developed on the base of the fractional-fractal approach that allows considering temporal non-locality of transfer processes in media of fractal structure. For the case of the ﬁnite thickness domain with permeable boundaries, a ﬁnite-difference technique for numerical solution of the corresponding one-dimensional non-linear boundary value problem is developed. The paper also presents a fractional-fractal model of a ﬁltration-consolidation process in clay soils of fractal structure saturated with salt solutions. An analytical solution is found for the corresponding one-dimensional boundary value problem in the domain of ﬁnite thickness with permeable upper and impermeable lower boundaries. these using the fractional-differential which indicates the presence of sub-diffusion properties in the presented fractional-fractal consolidation model. Let us note that the above-described results are in good agreement with the results of numerical modeling on the base of the fractal advection-dispersion equation that were obtained in Reference [13] and show that the fractal formulation of the advection-dispersion equation describes both super-diffusion and sub-diffusion processes. This conclusion also holds in the case of the considered fractional-fractal mathematical model of ﬁltration consolidation in porous media saturated with salt solutions.


Introduction
The determination of the conditions for the safe functioning of industrial and domestic wastewater storage facilities, as well as numerous other engineering facilities that pollute soils and groundwater, are among the most important and relevant, primarily in the connection with environment protection issues. This makes urgent the development of effective and reliable methods for mathematical modeling of deformation and compaction (consolidation) dynamics in saturated soils, particularly, in the foundations of hydraulic structures. Theoretical studies of the peculiarities of filtration-consolidation dynamics in porous media are often reduced to the solution of boundary value problems for the corresponding systems of partial differential or integro-differential equations [1][2][3][4][5][6][7][8].
In recent decades, a number of mathematical models in fractional-differential formulation have been developed to study the features of anomalous consolidation processes taking into account memory effects and spatial correlations [9][10][11][12].
In this paper, to simulate anomalous dynamics of filtration-consolidation processes in saturated porous (soil) media in the conditions of salt transfer we use the fractional-fractal approach [13][14][15] that allows taking into account temporal non-locality of processes in soils of fractal structure in the corresponding mathematical models. We combine the space-fractal advection-dispersion equation introduced in Reference [13] with time-fractional filtration-consolidation model studied in References [10,12] obtaining a new fractional-fractal model of an anomalous process of filtration-consolidation in a compacting soil of fractal structure. Comparing to the model studied in Reference [13], the presented model is time-fractional and contains an equation for determining a velocity field taking chemical osmosis [16,17] into account. For this new model we pose an where u x is the filtration rate, H(x, t) = p/γ is the water head, p is the pore pressure, γ is the liquid density, C(x, t) is the concentration of salts in the liquid phase, k is the filtration coefficient, ν is the coefficient of chemical osmosis [2], q c is the diffusion flow, d * is the coefficient of convective diffusion [18], t is the operator of Riemann-Liouville fractional differentiation of the same order with respect to the variable t [19][20][21], ∂ ∂x α f (x) = lim is the operator of the fractal derivative [13][14][15], α > 0 is the fractal dimension. The Equations (1) and (2) are obtained combining fractal-fractional generalizations of the corresponding laws presented in Reference [14] and an approach for taking chemical osmosis into account described in Reference [16].
Following the classical soil consolidation theory of V.A. Florin [5,8] we consider an approximation of porosity change in the form ∂n ∂t ≈ 1 1+ê ∂e ∂t where n is the porosity of the medium, e is the coefficient of porosity,ê is its average value. Further we use a generalized equation of filtration flow continuity [14] for the case of fractal-structured media in the form ∂n ∂t = ∂u x ∂x α . Assuming [5,8] that changes in porosity coefficient depend only on the sum of principal stresses and the strain-stress state of soil depends only on hydraulic pressure, we obtain the following form of a linear law of compaction: Substituting Equation (1) into Equation (3) we get the equation for water head in the form where C υ is the consolidation coefficient [5][6][7][8], is the operator of the regularized fractional Caputo-Gerasimov derivative of the order β with respect to the variable t [19][20][21]. The usage of the regularized derivative is here motivated by the known restrictions on initial conditions imposed in the case then the non-regularized derivative is used [22,23] From the generalized balance equation for salts in the liquid phase in a soil of fractal structure, taking Equation (2) into account we obtain an equation for determination of salts concentration in groundwater flow in the form where σ is the porosity of the medium [18]. From Equations (4) and (5) when α → 1 we obtain a system of equations [24] of the fractional-differential model of filtration-consolidation in a soil saturated with a salt solution without considering its fractal properties. When α, β → 1 the system (4) and (5) becomes reduced to the following well-known system of equations in the classical formulation [1,2]: Using the representation of fractal derivative operator through integer-order derivative in the [13][14][15] in Equations (4) and (5) and reducing similar terms, we obtain the model's system of equations in the following form: . Within the framework of such non-classical mathematical model, the fractional-differential dynamics of a non-local in time filtration-consolidation process in a soil of fractal structure saturated with a salt solution in the case of the domain of finite thickness l with permeable boundaries is described in the domain Ω = {(x, t) : 0 < x < l, t > 0} by the system of Equations (6) and (7) with the following boundary conditions: where H 0 is the initial value of water head, C 0 is the value of salts concentration at the inlet of the filtration flow.

Numerical Modeling of Fractional-Differential Consolidation Dynamics of a Saline Saturated Soil Massif of Finite Thickness and Fractal Structure
Below we present a brief summary of a finite-difference technique for constructing an approximate solution of the non-linear boundary value problem (6)- (9).
We define the grid domain where h, τ are the grid steps with respect to the geometric variable and time, and discretize the considered problem at the time step t j+1 and in the point Fractal Fract. 2020, 4, 59 and the same notations are used for H. The operator ∆ (β) t u denotes a discrete analogue of the Caputo-Gerasimov fractional derivative D (β) t u and is defined as is the Euler's gamma function [26,27]. Let us note that in the class of sufficiently smooth functions we have D (β) [19][20][21]. Taking Equation (12) into account in Equations (10) and (11) we reduce the solution of the considered problem at the (j + 1)-th time step to the solution of the following systems of linear algebraic equations: where , The sums in F j i ,F j i are here considered to be equal to zero when j = 0. Difference Equations (13) and (14) are three-point and can be effectively solved by the Thomas algorithm [25] as follows: To determine the starting values of the coefficients, we use finite-difference analogues of the boundary conditions (15) and (16) obtaining Recalling that the Thomas algorithm is stable when linear system's matrix is diagonally dominant [25], we can state that it is stable for the systems (13) and (14) for such existing values

Results of Numerical Experiments on Modeling the Dynamics of the Consolidation Process
Numerical modeling of the dynamics of water head and concentration fields according to the presented mathematical model was performed for input data from Reference [2]. Some results obtained with respect to the dimensionless variables Figures 1-3.
The analysis of numerical experiments' results allows us to draw the following conclusions: 1. The general tendencies in the distribution of concentration and water head fields in the consolidating soil massif modeled within the framework of the presented fractional-fractal model is generally in concordance with the tendencies in the distribution of similar fields obtained using the fractional-differential model [9,24] that takes into account memory effects, but not fractal properties of the medium, as well as with the classical consolidation model [2]. 2. A decrease of the fractal dimension α for 0 < α < 1 results in both an acceleration of salinization processes in the compacting massif (Figure 2a), and an acceleration of water head dispersion in it (Figure 2b), that is, to a reduction of the compaction time compared to the case when the process is described by the fractional-differential model that takes only memory effects into account [24]. 3. With an increase of the fractal dimension α for α > 1, the processes of salinization (Figure 3a) and water heads dispersion (Figure 3b) significantly slow down compared to the case when these processes are modeled using the fractional-differential mathematical model [9,24], which indicates the presence of sub-diffusion properties in the presented fractional-fractal consolidation model.
Let us note that the above-described results are in good agreement with the results of numerical modeling on the base of the fractal advection-dispersion equation that were obtained in Reference [13] and show that the fractal formulation of the advection-dispersion equation describes both super-diffusion and sub-diffusion processes. This conclusion also holds in the case of the considered fractional-fractal mathematical model of filtration consolidation in porous media saturated with salt solutions. To determine the starting values of the coefficients, we use finite-difference analogues of the boundary conditions (15) and (16) obtaining Recalling that the Thomas algorithm is stable when linear system's matrix is diagonally dominant [25], we can state that it is stable for the systems (13) and (14)

Results of Numerical Experiments on Modeling the Dynamics of the Consolidation Process
Numerical modeling of the dynamics of water head and concentration fields according to the presented mathematical model was performed for input data from Reference [2]. Some results obtained with respect to the dimensionless variables Figures 1-3. Here C 0 = 200 g/l, H 0 = 10 m, x 0 = 25 m, t 0 = 60 days.
The analysis of numerical experiments' results allows us to draw the following conclusions: 1. The general tendencies in the distribution of concentration and water head fields in the consolidating soil massif modeled within the framework of the presented fractional-fractal model is generally in concordance with the tendencies in the distribution of similar fields obtained using the fractional-differential model [9,24] that takes into account memory effects, but not fractal properties of the medium, as well as with the classical consolidation model [2]. 2. A decrease of the fractal dimension α for 0 < α < 1 results in both an acceleration of salinization processes in the compacting massif (Figure 2a), and an acceleration of water head dispersion in it (Figure 2b), that is, to a reduction of the compaction time compared to the case when the process is described by the fractional-differential model that takes only memory effects into account [24]. 3. With an increase of the fractal dimension α for α > 1, the processes of salinization (Figure 3a) and water heads dispersion (Figure 3b) significantly slow down compared to the case when these processes are modeled using the fractional-differential mathematical model [9,24], which indicates the presence of sub-diffusion properties in the presented fractional-fractal consolidation model.
Let us note that the above-described results are in good agreement with the results of numerical modeling on the base of the fractal advection-dispersion equation that were obtained in Reference [13] and show that the fractal formulation of the advection-dispersion equation describes both super-diffusion and sub-diffusion processes. This conclusion also holds in the case of the considered fractional-fractal mathematical model of filtration consolidation in porous media saturated with salt solutions.

Fractional-Fractal Model of Filtration-Consolidation Process in Clay Soils Saturated with Salt Solutions: An Exact Solution of the Boundary-Value Problem
This section presents an exact analytical solution of a specific problem of anomalous consolidation dynamics posed within the framework of the fractional-fractal approach taking into account temporal non-locality of the process of excess pressures dissipation in a soil of fractal structure. The solution is obtained for a mathematical model of consolidation dynamics of a saline-saturated porous medium, specifically for a clay soil under the conditions of temporal non-locality of the process taking into account fractal properties of a medium.
In the case of a non-local in time isothermal filtration-consolidation process in a soil of fractal structure saturated with a salt solution, taking into account the phenomena of chemical osmosis and ultrafiltration, we use the constitutive equation for diffusion flow in the form of the following Fick's law's generalization obtained combining fractal-fractional generalization presented in Reference [14], an approach for chemical osmosis description from Reference [16] and ultrafiltration description from Reference [17]: where d u is the ultrafiltration coefficient, H is the water head, C is the concentration. Then, from the corresponding generalized equation of salts balance in the liquid phase, taking Equation (17) into account, we have an equation for the determination of salts concentration in the form

Fractional-Fractal Model of Filtration-Consolidation Process in Clay Soils Saturated with Salt Solutions: An Exact Solution of the Boundary-Value Problem
This section presents an exact analytical solution of a specific problem of anomalous consolidation dynamics posed within the framework of the fractional-fractal approach taking into account temporal non-locality of the process of excess pressures dissipation in a soil of fractal structure. The solution is obtained for a mathematical model of consolidation dynamics of a saline-saturated porous medium, specifically for a clay soil under the conditions of temporal non-locality of the process taking into account fractal properties of a medium.
In the case of a non-local in time isothermal filtration-consolidation process in a soil of fractal structure saturated with a salt solution, taking into account the phenomena of chemical osmosis and ultrafiltration, we use the constitutive equation for diffusion flow in the form of the following Fick's law's generalization obtained combining fractal-fractional generalization presented in Reference [14], an approach for chemical osmosis description from Reference [16] and ultrafiltration description from Reference [17]: where d u is the ultrafiltration coefficient, H is the water head, C is the concentration. Then, from the corresponding generalized equation of salts balance in the liquid phase, taking Equation (17) into account, we have an equation for the determination of salts concentration in the form

Fractional-Fractal Model of Filtration-Consolidation Process in Clay Soils Saturated with Salt Solutions: An Exact Solution of the Boundary-Value Problem
This section presents an exact analytical solution of a specific problem of anomalous consolidation dynamics posed within the framework of the fractional-fractal approach taking into account temporal non-locality of the process of excess pressures dissipation in a soil of fractal structure. The solution is obtained for a mathematical model of consolidation dynamics of a saline-saturated porous medium, specifically for a clay soil under the conditions of temporal non-locality of the process taking into account fractal properties of a medium.
In the case of a non-local in time isothermal filtration-consolidation process in a soil of fractal structure saturated with a salt solution, taking into account the phenomena of chemical osmosis and ultrafiltration, we use the constitutive equation for diffusion flow in the form of the following Fick's law's generalization obtained combining fractal-fractional generalization presented in Reference [14], an approach for chemical osmosis description from Reference [16] and ultrafiltration description from Reference [17]: