Soil Mechanics for Unsaturated Soils phần 9 doc

15.6 DIMENSIONLESS CONSOLIDATION PARAMETERS 439 01 , I Ill, 1 1 I I 1111, 1 I I I I II,, I I I,1111~ 0.1 0.2 z 13 0.3 0.4 ' 0.5 % 0.6 .g 3 5 0.7 0.8 0.9 1 .o 0.01 0.1 I .o Time factor (water phase), T, 10 Figure 15.18 Dimensionless time factor versus degree of consolidation curves for the water phase. Dm 0.3 i 0.4 1 0.5 8 5 0.6 0.8 0.9 . - .u .u . . . . - . -~ ranging from 0.0 to 5.0 1.0 L I , , I I IIII 0.001 0.01 0.1 1 .o Time factor (air phase), Ta Figure 15.19 Dimensionless time factor versus degree of consolidation curves for the air phase. ud = initial pore-air pressure u, = pore-air pressure at any time. Figure 15.18 shows the water phase degree of consolidation versus time factor curves for various air-water interaction constaNs. Similar curves for the air phase are shown in Figure 15.19. The interactive constant in the air phase partial differential equation was assumed to be constant for the calculation of pore-air pressure dissipation. The curves show a smooth transition towards the case of a completely saturated soil (Tenaghi, 1943) and the case of a completely dry soil (Blight, 1971). The relationships between these dimensionless numbers (Figs. 15.18 and 15.19) can be used as a general solution for one-dimensional consolidation. Appropriate curves corresponding to the air and water phases can be used for different values of consolidation coefficients and soil con- stants. Two- and Three-Dimensional Unsteady-State How and Nonisothermal Analyses The types of analyses which can be performed by a geotechnical engineer have been strongly influenced by the available computing power. During the past few years, dramatic changes have been made in geotechnical engineering practice as a result of the computer becoming a part of the office equipment. A wide range of questions can be asked by clients, and the engineer can now perform para- metric-type analyses that would not have been possible a few years ago. One of the more recent changes in the practice of geotechnical engineering is related to types of unsteady-state or transient analyses which can be performed. This chapter sets forth the theoretical basis for two- and three-dimensional unsteady-state flow analysis in unsaturated soils. Also presented is the role of nonisothermal effects as it re- lates to the ground surface boundary and internal to the soil mass. In each case, the partial differential equations are presented for the analysis. Space does not permit the de- tailing of the finite element, numerical method formulations corresponding to each of the equations. The intent is rather to initiate the reader to the fundamentals of the the- ories involved. Many situations encountered in practice have the ground surface as a flux boundary. In other words, the climatic conditions at a site give rise to thermal and moisture flux conditions at the ground surface. In particular, the conver- sion of the thermal boundary conditions into an actual evaporative flux becomes of primary interest in solving many geotechnical problems. This is an area of much needed research, and only recently have significant strides been made in providing the theoretical formulations for solving this problem. This book provides an introduction to the assessment of the moisture flux conditions at the ground surface. 16.1 UNCOUPLED TWO-DIMENSIONAL FORMULATIONS Water flow through an earth dam during the filling of its reservoir is an example of two-dimensional, unsteady-state flow. Eventually, the flow of water through the dam will reach a steady-state condition, as illustrated in Chapter 7. Subsequent fluctuations of water level in the reservoir will again initiate unsteady-state water flow conditions. Fur- thermore, infiltration and evaporation cause almost a con- tinuously changing flow condition. The transient analysis of seepage is strongly influenced by conditions in the unsaturated zone (Freeze, 1971). The following uncoupled formulation independently sat- isfies the continuity equation for the water and air phases. The interaction between the fluid flows and the soil structure equilibrium condition is not considered in the uncoupled derivation. The two-dimensional derivation considers the plane strain case where fluid flows are assumed to occur ,only in two directions (Le., the x- and y-directions). Fluid flow in the third direction (Le., the t-direction) is assumed to be negligible. 16.1.1 Unsteady-State Seepage in Isotropic Soil The term isotropy is used to refer to the coefficient of permeability (or any soil property), of the soil which does not vary with respect to direction. In other words, the coefficients of permeability in the x- and y-directions are equal at a point in the soil mass. The coefficient of permeability of an unsaturated soil, however, can vary with respect to space (Le., heterogeneity), depending upon the magnitude of the matric suction at the point under consideration in the soil mass. Consider a referential element of unsaturated soil sub- jected to unsteady-state air and water flow, as shown in Fig. 16.1. The flow equations for the air phase and the water phase can be derived by equating He time derivative of the relevant constitutive equation to the divergence of the flow rate as dcscribed by the flow law. The derivation for the uncoupled, two-dimensional case is essentially an extension of the one-dimensional consolidation equation derived in Chapter 15. The extension is associated with flow in the second dimtion. These terms are derived in a similar manner to the derivation for the one-dimensional case presented in Chapter 15. Therefore, the uncoupled 440 16.1 UNCOUPLED TWO-DIMENSIONAL FORMULATIONS 441 Water flow Air flow Water flow Air flow vw Jay V I IX Figure 16.1 Two-dimensional unsteady-state air and water flows. two-dimensional flow equations are presented without re- peating the details of their derivation. Water Phase Partial Diflerential Equation The partial differential equation for unsteady-state water flow can be written as follows: 8UW aua a2u, C; ak, au, - = -cw- + c,"- + at at a2 k, ax ax akw (16.1) c," ak, au, + CV"7 + +c - aY kw aY aY aY where uw = u, = t= e, = my = a= m; = c; = k, = Pw = g= cg = pore-water pressure pore-air pressure time (1 - my/m;3/(mF/m;3; interaction constant associated with the water phase partial differential equation. coefficient of water volume change with respect to a change in net normal stress, (a - u,) total stress coefficient of water volume change with respect to a change in matric suction, (u, - u,) k,/(p, g ma; coefficient of consolidation with respect to the water phase for the x- and y-directions coefficient of permeability with respect to the water phase for the x- and y-directions (i.e., isotropic soil); the permeability is a function of the matric suction at any point in the soil mass density of water gravitational acceleration 1 /m;; called the coefficient associated with the gravity term. terms of the right hand side of Eq. (16.1) are additional terms associated with the flow of water in the second direction (Le., the xdirection). The gravity term, cs [Le., the last term in Eq. (16.1)] is only applicable to the flow of water in the y-dimtion since this term is derived from the elevation head gradient (see Chapter 15). Air Phase Partial Dt#emntiul Equation The partial differential equation for unsteady-state air flow is as follows: au, a2u, C; ao: au, at ax2 of ax ax a2u, C; a&' au, +c;-+ c"ar + c;- + aua -=- a? Of aY aY where (16.2) constant associated with the air phase partial differential equation my = coefficient of air volume change with respect to a change in net normal smss, (a - u,) rnf = coefficient of air volume change with respect to a change in matric suction (u, - u,) S = degree of saturation n = porosity u, = absolute pore-air pressure (i.e., ii, + Gat,,,) uat, = atmospheric pressure (Le., 101 kPa or 1 atm) ; coef- w/(RT) ii,m~(l - mf/rnD - (1 - S)n ficient of consolidation with respect to the air phase for the n- and y-directions D,* = coefficient of transmission with respect to the air phasr: for the x- and ydirections (Le., isotropic soil); the Coefficient is a function of the matric suction at a point in the soil mass w, = molecular mass of air R = universal (molar) gas constant [Le., 8.31432 J/(mol * K)] T = absolute temperature (Le., T = to + 273.16) (K) to = temperature ("C). The second and third terms in Eq. (16.2) are the terms - - Df 1 c; = - associated with the air flow in the second direction. 16.1.2 Unsteady-State Seepage in an Anisotropic Soil The term anisotropy is used to refer to the soil condition where the coefficient of permeability varies with respect to direction. Therefore, at any point in the soil mass, the coefficients of permeability in the x- and ydirections are assumed to be different. The conditions associated with anisotropy are discussed first, followed by the analysis of A comparison between Eq. (16.1) and the one-dimensional water phase partial differential equation for an unsaturated soil (Chapter 15) shows that the second and third seepage. In addition, the coefficient of permeability varies with respect to space (i.e., heterogeneity) due to the variation in matric suction in the soil mass. 442 16 TWO- AND THREE-DIMENSIONAL UNSTEADY-STATE FLOW AND NONISOTHERMAL ANALYSES Unsteady-state water flow through an anisotropic soil is analyzed by considering the continuity of the water phase. The pore-air pressure is assumed to remain constant with time (Le., au,/at = 0). “The single-phase approach to unsaturated flow leads to techniques of analysis that are accurate enough for almost all practical puqmses, but there are some unsaturated flow problems where the multiphase flow of air and water must be considered’’ (Freeze and Cherry, 1979). The water phase pattial differential equation can be obtained in a similar manner to the formulation of the water flow equation for an isotropic soil. Anisotmpy in Penneabili@ Let us consider the general case of a variation in the coefficient of permeability with respect to space (heterogeneity) and direction (anisotropy) in an unsaturated soil, as illustrated in Fig. 16.2. At a particular point, the largest or major coefficient of permeability, k,,, occurs in the direction sI which is inclined at an angle, CY, to the x-axis (i.e., horizontal). The smallest coefficient of permeability is in a pepndicular direction to the largest permeability (Le., in the direction s2), and is called the minor coefficient of permeability, kw2. The ratio of the major to the minor coefficients of permeability is a constant not equal to unity at any point within the soil mass. The magnitudes of the major and minor coefficients of permeability, kWl and kw2, can vary with matric suction from one location to another (i.e., heterogeneity), but their ratio is assumed to remain constant at every point. The unsteady-state seepage analysis is generally derived with reference to the x- and y-directions. Therefore, it is necessary to write the coefficients of permeability in the x- and y-directions in terms of the major and minor coefficients of permeability. This relationship can be derived by first writing the water flow rates in the major and minor permeability directions (i.e., directions sI and s2, respectively): Heterogeneity: L, at A = k,, at E i = 1. 2, x or y Anistropy: A Direction of minor L2 GI permeability S2t v (16.3) L1 (G)~= kwi constant # 1 SI Direction of major - ’ permeability VW” Figure 16.2 Permeability variations in an unsarumted soil (het- erogeneous and anisotropic). where Vwl = 9w2 = kwl = kw2 = h, = Y= s, = sp = ahw/as, = ahw/as2 = (16.4) water flow rate across a unit am of the soil element in the sl-direction water flow rate across a unit area of the soil element in the s2-direction major coefficient of permeability with respect to water as a function of matric suction which varies with location in the sl-direction minor coefficient of permeability with respect to water as a function of matric suction which varies with location in the s2-direction hydraulic head (i.e., gravitational plus pore- water pressure head or y + u,/p,,,g) elevation direction of major coefficient of permeability, k,~ direction of minor coefficient of permeability, kW2 hydraulic head gradient in the s,-direction hydraulic head gradient in the s2-direction. &e., kwl (u, - u,)l ke., kd (u, - u,)l The chain rule can be used to express the hydraulic head gradients in the sI- and s2-directions (i.e., ah,/as, and ah,/as2, respectively) in terms of the gradients in the x- and y-directions (Le., ahw/ax and ah,/ay, respectively): ah, ax ah, ay as, ax as, ay as, ah,= + (16.5) ah, - ah, ax ah, ay + (16.6) -_ as2 ax a~,, ay as2 where ah,/ax = hydraulic head gradient in the x-direction ah,/ay = hydraulic head gradient in the y-direction. From trigonometric relations, the following relationships can be obtained (see Fig. 16.2): d~ - sina ds I - -sina dr a32 -= dy cosa. a32 (16.7) (16.8) (16.9) (16.10) Substituting Eqs. (16.5)-(16.10) into Eqs. (16.3) and 16.1 UNCOUPLED TWO-DIMENSIONAL FORMULATIONS 443 (16.4) gives Water Phase Partial Di@renl&al Equation The water phase partial differential equation can be obtained by considering the continuity for the water phase. The net flux of water through an element of unsaturated soil (Fig. 16.1) can be computed from the volume rates of water entering and leaving the element within a period of time: ah, ax aY V,I = - k,l (cosa - + sina ") (16.11) (16'12) The water flow rates in the x- and y-directions can be written by projecting the flow rates in the major and minor directions to the x- and y-directions (see Fig, 16.2): v, = vW1 cosa - vw2 sina (16.13) v,,,,, = vWl sina + vw2 cosa (16.14) where vwx = vwy = a= water flow rate across a unit area of the soil element in the x-direction water flow rate across a unit area of the soil element in the y-direction angle between the direction of the major coefficient of permeability and the x-direction. Substituting Eqs. (16.11) and (16.12) for vW1 and vw2, respectively, into Eqs. (16.13) and (16.14) results in the following relations: ah, ah, ax aY v, = - k,, cos2a - - k,, sina cosa - - kw2 sin'a - ah, + kw2 sina cosa - (16.15) ax aY ah, ah, ax aY vH3) = - kWl sina cosa - - kWl sin'a - ah, ah, ax aY + kw2 sina cosa - - kwz cos's (16.16) Rearranging Eqs. (16.15) and (16.16) yields the expressions for the rate of water flow in the x- and y-directions: = - (k," + k ") VWY (16.18) ax vy ay where k, = kWl COS'Q + kw2 sin'a k, = (k,, - kw2) sina cosa k, = (kwl - kwz) sina cosa k, = kWl sin'a + ka cos'a. Equations (16.17) and (16.18) provide the flow rates in the x- and y-directions in terms of the major and minor coefficients of permeability. These flow rate expressions can then be used in the formulation for unsteady-state seepage analyses. where V, = volume of water in the element Vo = initial overall volume of the element (Le., dx, dy, dz) dx, dy, dz = dimensions in the x-, y-, and z-directions, respectively a(Vw/Vo)/at = rate of change in the volume of water in the soil element with respect to the initial volume of the element. A summary of the constitutive equations required for the formulation of flow equations is given in Chapter 15. The net flux of water can be computed from the time derivative of the water phase constitutive equation. In this case, the time derivatives of the total stress and the pore-air pressure are equal to zero since both pressures are assumed to remain constant with time. In addition, the m; coefficient can be assumed to be constant for a particular time step during the transient process: The continuity condition can be satisfied by equating the divergence of the water flow rates [Le., Eq. (16.19)] and the time derivative of the constitutive equation for the water phase [Le., Eq. (16.20)l: (16.21) av, avw w auw at ax ay +-= -m2 Substituting Eqs. (16.17) and (16.18) for u, and vwy, respectively, into Eq. (16.21) yields (16.22) Equation (16.22) is the governing partial differential equation for unsteady-state water seepage in an anisotropic soil when the pore-air pressure is assumed to remain constant with time. In many cases, the major and minor coef Jicient of permeability directions coincide with the x- and y-directions, respectively. In this case, the a angle is equal to zero, and the governing equation [Eq. (16.22)] can be 444 16 TWO- AND THREE-DIMENSIONAL UNSTEADY-STATE FLOW AND NONISOTHERMAL ANALYSES simplified by setting the OL angle to zero [Le., Eqs. (16.17) and (16.18)]: (16.23) The k,, and kw2 terms in Eq. (16.23) are the major and minor coefficients of permeability in the x- and y-directions, respectively. These permeability coefficients are a function of matric suction that can vary with location in the x- and y-directions [Le., kWl (u, - u,) and kW2 (u, - u31. For isotropic soil conditions, the k,, and kIYZ terms are equal [i.e., kWl = kd = k, (u, - u,)] and Eq. (16.23) can be further simplified as Rearranging Eq. (16.24) gives azh, ak, ah, a2h, ak ah, ah, + + k, 7 + -, - = m,Wpwg at. kw~ ax ax aY aY aY (16.25) Substituting ( y + u,/p,g) for the hydraulic head, h,, in Eq. (16.25) and rearranging the equation results in the following form: where c: = kw/@wgmzW) c8 = l/mr. Equation (16.26) is essentially equal to Eq. (16.1) without the interaction term, C, since the pore-air pressure is assumed to remain constant with time. Therefore, it has been shown that the general governing equation for an anisotropic soil [Le., Eq. (16.22)] can be simplified for the anisotropic case where the a angle is equal to zero [i.e., Eq. (16.23)] and for the isotropic soil condition [Le., Eq. (16.24) or (16.26)]. The general governing equation, Eq. (16.22), can therefore be used to solve water seepage problems through a saturated-unsaturated flow system. For the saturated por- tion, the water coefficient of permeability becomes equal to the saturated coefficient of permeability, k, . The saturated coefficient of permeability may vary with respect to direction (i.e., anisotropy) or with respect to location (i.e., heterogeneity). In this case, both anisotropy and heterogeneity with respect to permeability are accounted for in Eq. (16.22). The coefficient of water volume change, m,W, in Eq. (16.22) approaches the value of the coefficient of volume change m, , as the soil becomes saturated. Seepage Anakysk Using the Finite Element Method Unsteady-state water seepage through a saturated-unsaturated soil system can be analyzed by solving the general governing flow equation [Le., Eq. (16.22)]. The analysis can be performed using the finite element method as de- scribed in Chapter 7 for steady-state seepage. A similar approach can be used for unsteady-state seepage, with the exception of some differences in the finite element formulation. The finite element formulation for unsteady-state seepage in two dimensions can be derived using the Galerkin principle of weighted residual (Lam et al. (1988)). The Galerkin solution to the governing equation, Eq. (16.22), is given by the following integrals over the area and the boundary surface of a triangular element (Fig. 16.3): where [B] = matrix of the derivatives of the area coordinates (Fig. 16.3), which can be written as 1 (Yz - Y3)(Y3 - YI)(YI - Y2) (x3 - x2)(xl - X3)(xZ - XI) xi, yi(i = 1, 2, 3) = Cartesian coordinates of the three nodal points of an element A = area of the element [k,] = tensor of the water coefficients of permeability for the element, which =Nodal point Yf I i=l (XI, yl) - Cartesian coordinates (1. 0, 0)- Area coordinates Figure 16.3 Area coordinates in relation to the Cartesian coordinates for a triangular element. 16.1 UNCOUPLED TWO-DIMENSIONAL FORMULATIONS 445 can be written as Either the hydraulic head or the flow rate must be specified at the boundary nodal points. Specified hydraulic heads at the boundary nodes are called Dirichlet boundary conditions. A specified flow rate across the boundary is referred to as a Neuman boundary condition. The third term in Eiq. (16.27) accounts for the specified flow rates across the boundary. The specified flow rates at the boundary must {h,} = matrix of hydraulic heads at the nodal points, that is, [L] = matrix of the element area coordinates (i.e., (L,L&}) L1&L3 = area coordinates of points in the element that are related to the Cartesian coordinates of nodal points as follows (Fig. 16.3): LI = (1/2A) {(x2Y3 - x3Y2) + (Y2 - Y3)x + (x3 - X2)Y 1 + (x1 - X3)Y) L3 = (1/W{(X,Y* - X2YI) + (Yl - Y2)x + (x2 - Xl)Y 1 k = (1/2A){(x3Yl -xlY3) + (Y3 - Yl)x x, y = Cartesian coordinates of a point be projected to a direction normal to the boundary. As an example, a specified flow rate, ow, in the vertical direction must be converted to a normal flow rate, E,, as illustrated in Fig. 16.4. The normal flow rate is in turn converted to a nodal flow, Q, (Segerlind, 1976). Figure 16.4 shows the computation of the nodal flows, Qwi and Qwj , at the boundary nodes (i) and ( j ), respectively, A positive nodal flow signifies that there is infiltration at the node or that the node acts as a “source.” A negative nodal flow indicates evaporation, evapotranspiration at the node, or that the node acts a “sink.” When the flow rate across a boundary is zero (e.g., impervious boundary), the third term in Eq. (16.27) disappears. The numerical integration of Eq. (16.27) results in a sim- pler expression of the equation: within the element tr, = external water flow rate in a direction perpendicular to the boundary of the element = PwgmzW where - [D] = stiffness matrix, that is, S = perimeter of the element. [W[k,l tBlA vw3 flow Specifiedl rate I 1 1‘ 1 1 I 1 1 vr 1 1 I I I I I I I k-!1 12-13+ Figure 16.4 Applied flow rate across the boundary expressed as nodal flows. TWO- AND THREE-DIMENSIONAL UNSTEADY-STATE FLOW AND NONISOTHERMAL ANALYSES capacitance matrix, that is, r2 1 11 11 112 matrix of the time derivatives of the hydraulic heads at the nodal points, i.e., a {hwn) at flux vector reflecting the boundary conditions, i.e., The time derivative in Fq. (16.28) can be approximated using a finite difference technique. The relationship between the nodal heads of an element at two successive time steps can be expressed using the central difference approximation: = (y - [D]) {h,)l + 2[F] (16.29) or the backward difference approximation: The above time derivative approximations are considered to be unconditionally stable. The central difference approximation generally gives a more accurate solution than that obtained from the backward difference approximation. However, the backward difference approximation is found to be more effective in reducing numerical oscillations commonly encountered in highly nonlinear systems of flow equations (Neuman and Witherspoon (1971), Neuman, 1973). The finite element flow equation [i.e., Eq. (16.28)] can be written for each element and assembled to form a set of global flow equations. This is performed while satisfying nodal compatibility (Desai and Abel, 1972). Nodal compatibility requires that a particular node shared by the surrounding elements have the same hydraulic head in all the elements (Zienkiewicz, 1971, Desai, 1975b). The set of global flow equations for the whole system is solved for the hydraulic heads at the nodal points, {Itwn}. However, Eq. (16.28) is nonlinear because the coefficients of permeability are a function of matric suction, which is related to the hydraulic head at the nodal points. The hydraulic heads are the unknown variables in Eq. (16.28). Therefore, Fq. (16.28) must be solved using an iterative procedure that involves a series of successive approximations. In the first approximation, the coefficients of permeability are estimated in order to calculate the first set of hydraulic heads at the nodal points. The computed hydraulic heads are used to calculate the average matric suction within an element. In the subsequent approximations, the coefficient of permeability is adjusted to a value depending upon the average matric suction in the element. The adjusted permeability value is then used to calculate a new set of nodal hydraulic heads. The above procedure is repeated until both the hydraulic head and the permeability differences within each element at two successive iterations are smaller than a specified tolerance. The above iterative procedure causes the global flow equations to be linearized and solved simultaneously using a Gaussian elimination technique. The convergency rate is highly dependent on the degree of nonlinearity of the permeability function and the spatial discretization of the problem. A steep permeability function requires more iterations and a larger convergency tolerance. A finer discretization in both element size and time step will assist in obtaining convergence faster with a smaller tolerance. Generally, the solution will converge to a tolerance of less than 1% in ten iterations. The unsteady-state seepage equation, Eq. (16.28), is considered solved for one time step once the converged nodal hydraulic heads of the system have been obtained. Having reached convergence at a particular time step, other secondary quantities, such as pore-water pressures, hydraulic head gradients, and water flow rates, can then be calculated using the converged nodal hydraulic heads. The equation for nodal pore-water pressures is (uwn) = ({hwn) - {yn))~wg (16.31) where {u,} = matrix of pore-water pressures at the nodal points, Le., { yn) = matrix of elevation heads at the nodal points, i.e., The hydraulic head gradients in the x- and y-directions can be computed for an element by taking the derivative of the element hydraulic heads with respect to x and y, respectively: (16.32) 16.1 UNCOUPLED TWO-DIMENSIONAL FORMULATIONS 447 where i,, iy = hydraulic head gradient within an element in the The element flow rates, v,, can be calculated from the hydraulic head gradients and the coefficients of permeability in accordance with Darcy’s law: [;,I = [kwl[Bl{hwn) (16.33) x- and y-directions, respectively. where v,, v,,, = water flow rates within an element in the x- and y-directions, respectively. The hydraulic head gradient and the flow rate at nodal points are computed by averaging the comsponding quantities from all elements surrounding the node. The weighted average is computed in proportion to the element areas. Examples of Two-LXmenswnal Problems and Their Solutions Three example problems are presented in this section to illustrate the unsteady-steady state seepage analysis using the finite element method. Lam (1984) has solved several classical problems of seepage through saturated-unsaturated soil systems. The three problems which will be discussed in this section deal with water flows through an earth dam, flow below a lagoon, and flow through a layered hill slope. In all cases, the flow regime of the problem must first be defined. This includes the spatial dimension of the soil boundaries, the boundary conditions of the flow system, and the determination of soil properties. The boundaries of different soil layers can be determined through a subsurface soil investigation. The boundary conditions of the flow system can be obtained from piezo- metric records and hydrological data. Soil properties, such as coefficients of permeability and coefficients of volume change (Le., k, and ma, can be measured using in situ or laboratory tests (Chapters 6 and 13). Example of Water How Through an Earth Dam The example problem involving water flow through an earth dam is discussed first. The cross section and discretization of the earth dam are shown in Fig. 16.5. The soil is assumed to be isotropic with respect. to its coefficient of permeability, and the permeability function used in the analysis is shown in Fig. 16.6. The saturated coefficient of permeability, k,, is 1.0 X lo’-’ m/s. The pore-air pressure is assumed to be atmospheric. Therefore, the matric suction values in Fig. 16.6 are numerically equal to the pore- water pressures, and can be expressed as a pore-water pressure head, Itp. The base of the dam is selected as the datum. In addition, a coefficient of water volume change, my, of 1.0 x kPa-’ is used in the analysis. The dam is initially at a steady-state condition, with the reservoir water level 4 m above the datum. At a time assumed to be equal to zero, the water level in the reservoir is instantaneously raised to a level of 10 m above the datum. The water level remains constant at 10 m during the transient process. The rising of the phreatic line from the initial steady-state condition (Le., at time, t, equal to zero) to the final steady-state condition (i.e., at time, t, equal to 19 656 h or 819 days) is illustrated in Fig. 16.7. The development of equipotential lines, phreatic surface, and water flow rates across the dam are illustrated in Figs. 16.8-16.11 for four different times during the transient process, The increase in the reservoir level results in an increase in pore-water pressures with time. This is dem- onstrated by the advancement of equipotential lines from the upstream to the downstream of the dam with increasing times. It should also be noted that the equipotential lines extend from the saturated to the unsaturated zones, as shown in Figs. 16.8(a), 16.9(a), 16.10(a), and 16.11(a). In other words, water flows in both the saturated and the unsaturated zones as a result of the hydraulic head differences between the equipotential lines. The flow of water in both zones can be observed directly from the flow rate vec- tors that exist in both the saturated and the unsaturated zones, as shown in Figs. 16.8(b), 16.9(b), 16.10(b), and 16.1 l(b). The amount of water flowing in the unsaturated zone depends on the rate of change in the coefficient of permeability with respect to the matric suction changes. Example of Groundwater Seepage Below a Lagoon The second example problem illustrates unsteady-state groundwater seepage below a lagoon. The lagoon is placed on top of a 1 m thick soil linear, as shown in Fig. 16.12. The geometry of the problem is symmetrical, and the liner and the surrounding soil are assumed to be isotropic with respect to their permeability. Therefore, the problem can be analyzed by only considering half of the geometry. The discretized cross section of the soil liner and its surrounding soil are depicted in Fig. 16.12. The permeability func- tions for the soil liner and the Surrounding soil are shown in Fig. 16.13. The saturated coefficients of permeability are equal to 5.0 X and 1 .O x m/s for the liner and the surrounding soil, respectively. A coefficient of water volume change, my, of 2.0 x Wa-’ is used in the analysis for both the liner and the surrounding soil. An initial steady-state condition with a groundwater table located 5 m below the ground surface is assumed (Fig. 16.14). “No flow” boundary conditions are assumed along the ground surface, the bottom boundary, and the axis of symmetry. On the right-hand boundary, a hydrostatic and “no-flow” condition is assumed to exist below and above the groundwater table, respectively. At a time assumed to be equal to zero, the lagoon is filled with water to the 1 m height, which gives rise to a 1 m pore-water pressure head. As a result, water seepage occurs from the lagoon, causing Scale for 1:160 123 geometry: F’igure 16.5 Discretized cross section of a dam for a finite element analysis involving unsteady- state seepage. [...]... p8 3 -0 .91 -0.U3 v .95 0.Y7 -0.U8 -0.51 k0.52 pI38 -0 .91 -0.W ?US -0.47 O.U9 -3 :8 1 -.3 01 P 5 -0.53 2 -0- UO O\U5 +O 97 > 99 -0.50 -0.51 -.1 06 -0.55 -0.56 -0.58 ' 0. 59 -0.W -.1 06 O.SS -0.56 p.57 -0.5s -0. 59 -0.60 O.f8 -.1 06 ,0. 59 0.60 -0.61 -.1 06 -0.50 52 Q -0.53 9- 55 -0.56 J.57 \ \ -0.62 -.8 0p 3.31 0.32 Od / p.32 -0 .90 -0 YI) -O!fS -0.W -0.US -0.50 -2 ; 5 -0. 59 -0.55 -0.58 -0.57 O.* -0. 59 -0.60... Scale for geometry 1:160 P P 8 .92 VI f -9. 87 91 -9. W -9. 59 -9. 24 -8.86 -8.52 8.3U 9- 26 9. 76 8.63 e 8m 9m 7m 6m 5m 4m.3m 2m l m Numbers are hydraulic heads in (m) (a) - = Nodal flow rate vector v (m/s) with the scale , = 2.5 x IO-* m/s - Reservoir level , , \ \ - - - - - - - - - - - Figure 16.11 Steady-state seepage through an isotropic dam at an elapsed time equal to 19 656 h (Le., 8 19 days)... -0.)8 -0.YO -0.42 -0 .95 -0-U6 -0.W -0.50 -012 -0.53 -0.55 -0.56 -0.57 0.5\ -0. 59 -0.60 -0.61 -0.33 -.2 06 -0 Y 2 p.\U -0.46 p .98 -0.50 O;U -0. 59 -0.32 - .9 03 -0.36 p9 0 -0.62 -0.36 p.?8 -0.61 -0.b8 -0 .90 -0 I 2 0.Q -0.U6 -0 .98 0.50 - -0. 59 0c -0.62 0.88 0-YO 0.U2 09 6 -0.62 0.W 0.50 0.32 03 4 03 6 0h 0.52 0.53 p- 51 -0.56 -0.57 p 5 4 p. 59 -0 BO O 6 1 -0.51 -0.56 -0.57 -0.581 -0. 59 0.80 0.60 -0.61 0.511... required for formulating the equilibrium and continuity equations Therefore, these constitutive relations are first summarized in their elasticity forms prior to the for1 mulation of the consolidation equations 16.2.1 Constitutive Relations The stress state and deformation state variables can be linked by suitable constitutive relations which incorporate soil properties in the form of coefficients For an unsaturated. .. calculated as follows (Philip and de Vries, 195 7; de Vries, 197 5; Dakshanamurthy and Fredlund, 198 1; Wilson, 199 0): (16.61) where = tortuosity factor for the soil (i.e., c = p 2 I 3 ) = cross-section area of the soil available for water vapor flow (i.e., (1 - S ) n ) Dum molecular diffisivity of water vapor in air (Le., = CY where { = volumetric specific heat of the soil as a function of water content (J/m'/"C)... volumetric solid content (Le., V , / V ) V, = volume of soils solids in the soil V = total volume of the soil I, = volumetric specific heat capacity for the water phase (Le., 4.154 X lo6 J/m3/"C for water at 35°C (Wilson, 199 0)) B, = volumetric water content (i.e., V , / V ) V, = volume of water in the soil la = volumetric specific heat capacity for the air phase 0, = volumetric air content (i.e., V... bubbles of air existed in the soil during the consolidation process A summary of the coupled formulation for three-dimensional consolidation of an unsaturated soil with a continuous air phase is presented in this section Reference is made to Dakshanamurthy et al ( 198 4) for detailed explanations on the coupled consolidation equations The constitutive relations for the soil structure, the water phase,... permeability values for the different soils involved in the system 16.2 COUPLED FORMULATIONS OF THREEDIMENSIONAL CONSOLIDATION A rigorous formulation of two- and three-dimensionalconsolidation requires that the continuity equation be coupled with the equilibrium equations This method was proposed by Biot ( 194 1) to analyze the consolidation process for a special case of an unsaturated soil The derivations... constitutive relations which incorporate soil properties in the form of coefficients For an unsaturated soil, there are three available constitutiverelations for an unsaturated soil, namely, one for the soil structure, one for the water phase, and one for the air phase In each constitutive equation, the deformation state variable can be the total, water, or air volume change, while the stress state variables... = shear modulus e, = ey = Soil Structure The constitutive equation for the soil structure is derived by assuming that the soil behaves as an isotropic, linear elastic material This assumption is acceptable in an incremental sense The constitutive relations can be developed in a semi-empirical manner as an extension of the elasticity formulation used for saturated soils The soil structure constitutive . dam. Equipotential line (m) Scale for geometry 1 :160 P VI P f 8 .92 8.63 8.3U -9. 87 -9. W 9- 26 .91 .9. 76 -9. 59 -9. 24 -8.86 -8.52 .e. 9m 8m 7m 6m 5m 4m.3m 2m lm Numbers are. incorporate soil properties in the form of coefficients. For an unsaturated soil, there are three available constitutive relations for an unsaturated soil, namely, one for the soil structure,. formulation for three-dimensional consolidation of an unsaturated soil with a continuous air phase is presented in this section. Reference is made to Dakshanamurthy et al. ( 198 4) for