The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Martinus Th Van Genuchten - One of the best experts on this subject based on the ideXlab platform.
-
Estimating Unsaturated Soil hydraulic properties from laboratory tension disc infiltrometer experiments
Water Resources Research, 1999Co-Authors: Jiři Simůnek, Ole Wendroth, Martinus Th Van GenuchtenAbstract:Four tension disc infiltration experiments were carried out on a loamy Soil in the laboratory for the purpose of estimating the Unsaturated Soil hydraulic properties. Sixteen tensiometers were installed in pairs at the following coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15, 10), (15, 15), (15, 20), and (15, 30), where r represents the distance from the axis of symmetry and z is the location below the Soil surface. A time domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of -20, -10, -5, and -1 cm, with the experiment lasting for -5 hours. The same supply pressure heads were also used for the fourth experiment, which lasted 6.25 days so as to reach steady state at each applied tension. The measured data were analyzed using Wooding's [1968] analytical solution and by numerical inversion. The parameter estimation method combined a quasi three-dimensional numerical solution of the Richards equation with the Marquardt-Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined using different combinations of the cumulative infiltrated volume, TDR readings, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyzed with Wind's [1968] method. Water contents in the retention curves were underestimated when both transient and quasi steady state experiments were analyzed by parameter estimation. Unsaturated hydraulic conductivities obtained by parameter estimation and using Wooding's [1968] analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the evaporation method.
-
estimating Unsaturated Soil hydraulic properties from tension disc infiltrometer data by numerical inversion
Water Resources Research, 1996Co-Authors: Jiři Simůnek, Martinus Th Van GenuchtenAbstract:Tension disc infiltrometers are becoming increasingly popular devices for in situ measurement of the Unsaturated hydraulic properties of Soil. Tension infiltration data are generally used to evaluate the parametersKs and ain Gardner's exponential model of the Unsaturated hydraulic conductivity. Either two measurements using different disc diameters or measurements with a single disc but using multiple pressure heads are then used. In this paper we describe a parameter estimation procedure which combines the Levenberg-Marquardt nonlinear parameter optimization method involving weighted least squares, with a quasi-three-dimensional numerical model which solves the variably saturatedflow equation. By numerical inversion of Richards' equation the unknown parameters in van Genuchten's model of the Unsaturated Soil-hydraulic properties are estimated from observed cumulative infiltration data during transient waterflow. Additional measurements of the pressure head or water content, as well as a penalty function for constraining the unknown parameters to remain in some feasible region (Bayesian estimation), can be optionally included into the parameter estimation procedure. The problem of optimal sampling design, that is, selecting the best points in space and time for making measurements, is addressed by studying the sensitivity of the objective function to changes in the optimized hydraulic parameters. We calculate objective functions based on available cumulative infiltration, pressure head, and water content measurements and also on several combinations of these data. The behavior of the objective function in three-dimensional parameter space is evaluated by means of a series of two-dimensional response surfaces. The utility of the parameter estimation procedure is demonstrated using numerically generated data. The sensitivity of the procedure to different initial estimates of the model parameters is also discussed.
Jiři Simůnek - One of the best experts on this subject based on the ideXlab platform.
-
Estimating Unsaturated Soil hydraulic properties from laboratory tension disc infiltrometer experiments
Water Resources Research, 1999Co-Authors: Jiři Simůnek, Ole Wendroth, Martinus Th Van GenuchtenAbstract:Four tension disc infiltration experiments were carried out on a loamy Soil in the laboratory for the purpose of estimating the Unsaturated Soil hydraulic properties. Sixteen tensiometers were installed in pairs at the following coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15, 10), (15, 15), (15, 20), and (15, 30), where r represents the distance from the axis of symmetry and z is the location below the Soil surface. A time domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of -20, -10, -5, and -1 cm, with the experiment lasting for -5 hours. The same supply pressure heads were also used for the fourth experiment, which lasted 6.25 days so as to reach steady state at each applied tension. The measured data were analyzed using Wooding's [1968] analytical solution and by numerical inversion. The parameter estimation method combined a quasi three-dimensional numerical solution of the Richards equation with the Marquardt-Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined using different combinations of the cumulative infiltrated volume, TDR readings, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyzed with Wind's [1968] method. Water contents in the retention curves were underestimated when both transient and quasi steady state experiments were analyzed by parameter estimation. Unsaturated hydraulic conductivities obtained by parameter estimation and using Wooding's [1968] analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the evaporation method.
-
estimating Unsaturated Soil hydraulic properties from tension disc infiltrometer data by numerical inversion
Water Resources Research, 1996Co-Authors: Jiři Simůnek, Martinus Th Van GenuchtenAbstract:Tension disc infiltrometers are becoming increasingly popular devices for in situ measurement of the Unsaturated hydraulic properties of Soil. Tension infiltration data are generally used to evaluate the parametersKs and ain Gardner's exponential model of the Unsaturated hydraulic conductivity. Either two measurements using different disc diameters or measurements with a single disc but using multiple pressure heads are then used. In this paper we describe a parameter estimation procedure which combines the Levenberg-Marquardt nonlinear parameter optimization method involving weighted least squares, with a quasi-three-dimensional numerical model which solves the variably saturatedflow equation. By numerical inversion of Richards' equation the unknown parameters in van Genuchten's model of the Unsaturated Soil-hydraulic properties are estimated from observed cumulative infiltration data during transient waterflow. Additional measurements of the pressure head or water content, as well as a penalty function for constraining the unknown parameters to remain in some feasible region (Bayesian estimation), can be optionally included into the parameter estimation procedure. The problem of optimal sampling design, that is, selecting the best points in space and time for making measurements, is addressed by studying the sensitivity of the objective function to changes in the optimized hydraulic parameters. We calculate objective functions based on available cumulative infiltration, pressure head, and water content measurements and also on several combinations of these data. The behavior of the objective function in three-dimensional parameter space is evaluated by means of a series of two-dimensional response surfaces. The utility of the parameter estimation procedure is demonstrated using numerically generated data. The sensitivity of the procedure to different initial estimates of the model parameters is also discussed.
Ole Wendroth - One of the best experts on this subject based on the ideXlab platform.
-
Estimating Unsaturated Soil hydraulic properties from laboratory tension disc infiltrometer experiments
Water Resources Research, 1999Co-Authors: Jiři Simůnek, Ole Wendroth, Martinus Th Van GenuchtenAbstract:Four tension disc infiltration experiments were carried out on a loamy Soil in the laboratory for the purpose of estimating the Unsaturated Soil hydraulic properties. Sixteen tensiometers were installed in pairs at the following coordinate (r, z) positions: (10, 2.5), (10, 5), (10, 10), (15, 5), (15, 10), (15, 15), (15, 20), and (15, 30), where r represents the distance from the axis of symmetry and z is the location below the Soil surface. A time domain reflectometry (TDR) probe was used to measure water contents at a depth of 2 cm directly below the tension disc. The first three experiments involved supply pressure heads at the disc of -20, -10, -5, and -1 cm, with the experiment lasting for -5 hours. The same supply pressure heads were also used for the fourth experiment, which lasted 6.25 days so as to reach steady state at each applied tension. The measured data were analyzed using Wooding's [1968] analytical solution and by numerical inversion. The parameter estimation method combined a quasi three-dimensional numerical solution of the Richards equation with the Marquardt-Levenberg optimization scheme. The objective function for the parameter estimation analysis was defined using different combinations of the cumulative infiltrated volume, TDR readings, and tensiometer measurements. The estimated hydraulic properties were compared against results obtained with an evaporation experiment as analyzed with Wind's [1968] method. Water contents in the retention curves were underestimated when both transient and quasi steady state experiments were analyzed by parameter estimation. Unsaturated hydraulic conductivities obtained by parameter estimation and using Wooding's [1968] analysis corresponded well. Drying branches of the hydraulic conductivity function determined by parameter estimation also corresponded well with those obtained with the evaporation method.
Wataru Ichihara - One of the best experts on this subject based on the ideXlab platform.
-
stress strain behaviour of Unsaturated Soil in true triaxial tests
Canadian Geotechnical Journal, 2002Co-Authors: Hajime Matsuoka, Dean Sun, Akiteru Kogane, Nobuhiko Fukuzawa, Wataru IchiharaAbstract:A suction-controlled true triaxial apparatus for Unsaturated Soil was developed from the existing true triaxial apparatus for sand by attaching a device to supply matric suction to specimens. Using...
-
stress strain behaviour of Unsaturated Soil in true triaxial tests
Canadian Geotechnical Journal, 2002Co-Authors: Hajime Matsuoka, Akiteru Kogane, Nobuhiko Fukuzawa, Wataru IchiharaAbstract:A suction-controlled true triaxial apparatus for Unsaturated Soil was developed from the existing true triaxial apparatus for sand by attaching a device to supply matric suction to specimens. Using the developed apparatus, true triaxial tests (σ1 σ2 σ3; where σ1, σ2, and σ3 are the three different principal stresses) on an Unsaturated silty Soil were carried out under constant suction using the negative pore-water pressure method (s = uw > 0; ua = 0) for applying the matric suction, s (s = ua uw; where ua is the pore-air pressure and uw is the pore-water pressure). It was found that the true triaxial test results under three different principal stresses are uniquely arranged on the "extended spatially mobilized plane (extended SMP)" for frictional and cohesive materials that is modified from the original SMP for frictional materials by introducing "a bonding stress, σ0 (= c·cotϕ, where c is cohesion and ϕ is the internal friction angle)." It was also found that the shear strengths of the Unsaturated si...
Delwyn G. Fredlund - One of the best experts on this subject based on the ideXlab platform.
-
Unsaturated Soil mechanics in engineering practice
2012Co-Authors: Delwyn G. FredlundAbstract:Unsaturated Soil mechanics has rapidly become a part of geotechnical engineering practice as a result of solutions that have emerged to a number of key problems (or challenges). The solutions have emerged from numerous research studies focusing on issues that have a hindrance to the usage of Unsaturated Soil mechanics. The primary challenges to the implementation of Unsaturated Soil mechanics can be stated as follows: (1) The need to understand the fundamental, theoretical behavior of an Unsaturated Soil; (2) the formulation of suitable constitutive equations and the testing for uniqueness of proposed constitutive relationships; (3) the ability to formulate and solve one or more nonlinear partial differential equations using numerical methods; (4) the determination of indirect techniques for the estimation of Unsaturated Soil property functions, and (5) in situ and laboratory devices for the measurement of a wide range of Soil suctions. This paper explains the nature of each of the previous challenges and...
-
comparison of different procedures to predict Unsaturated Soil shear strength
Geo-Denver 2000, 2000Co-Authors: Sai K Vanapalli, Delwyn G. FredlundAbstract:Several procedures have been proposed in the recent years to predict the shear strength of an Unsaturated Soil. The Soil-water characteristic curve has been used as a tool either directly or indirectly in the prediction of the shear strength along with the saturated shear strength parameters in these procedures. This paper provides comparisons between the measured and predicted values of Unsaturated shear strength using these procedures for three Soils both for limited and large suction ranges. The three Soils used in the study for comparisons have different gradation properties, percentages of clay and plasticity index, Ip values. The advantages and limitations associated with predicting the shear strength of Unsaturated Soils using the procedures is discussed in the paper. INTRODUCTION A theoretical framework for Unsaturated Soil mechanics that parallels saturated Soil mechanics is available in terms of stress state variables, namely; net normal stress, (σn ua), and matric suction, (ua uw) where σn is the normal stress, ua is the pore-air pressure an uw is the pore-water pressure. (Fredlund and Rahardjo, 1993). The framework is based on experimental studies that are costly and time consuming. Several advancements have been made in the prediction of the engineering behavior of Unsaturated Soils in recent years. The Soil-water characteristic curve has been found to be a useful tool in the estimation of engineering properties for Unsaturated Soils. Examples are the coefficient of permeability and the shear strength functions. Shear strength forms an important engineering property in the design of numerous geotechnical and geo-environmental structures such as earth dams, retaining walls, pavements, liners, covers, etc. Several procedures have been proposed in the literature during the past five years to predict the shear strength of an Unsaturated Soil. __________________________________________________________________________ Department of Civil Engineering, University of Saskatchewan, SK, Canada, S7N 5A9 These procedures use the Soil-water characteristic curve as a tool either directly or indirectly along with the saturated shear strength parameters, c’ and φ’, to predict the shear strength function for an Unsaturated Soil (Vanapalli et al. 1996, Fredlund et al. 1996, Oberg and Sallfors 1997, Khallili and Khabbaz 1998 and Bao et al. 1998). The philosophy used in each of the prediction procedures proposed by these investigators is different. Comparisons have been provided between predicted and measured values of shear strength for a limited suction range for various Soils (i.e., between 0 to 500 kPa). Escario and Juca (1989) measured the Soil-water characteristic curves and the shear strength of three Soils prior to the time when any proposals had been made for the shear strength functions. The three Soils have different gradation properties, percentages of clay and plasticity indices, Ip. These results are used in this paper to provide comparisons between the predicted and measured shear strength values both for a limited suction range and a large suction range. The study presented in the paper highlights the advantages and limitations associated with the various procedures for predicting the shear strength of Unsaturated Soils. The simple procedures discussed in this paper are of value in bringing the shear strength theories for Unsaturated Soils into engineering practice. EQUATIONS FOR INTERPRETING THE SHEAR STRENGTH OF Unsaturated SoilS Bishop (1959) proposed shear strength equation for Unsaturated Soils by extending Terzaghi’s principle of effective stress for saturated Soils. Bishop’s original equation can be arranged as shown below. ( ) ( ) ( )( ) [ ] ' tan ' tan ' φ χ φ σ τ w a a n u u u c − + − + = [1] where: τ = shear strength of Unsaturated Soil, c’ = effective cohesion, φ’ = angle of frictional resistance, (σn ua ) = net normal stress, (ua uw ) = matric suction, and χ = a parameter dependent on the degree of saturation The value of χ was assumed to vary from 1 to 0, which represents the variation from a fully saturated condition to a total dry condition. Several investigators found limitations with respect to the quantification of the parameter χ both theoretically and experimentally. Fredlund et al. (1978) have proposed a relationship to explain the shear strength of Unsaturated Soils in terms two independent stress state variables as shown below: ( ) ( ) b w a a n u u u c φ φ σ τ tan ' tan ' − + − + = [2] The shear strength contribution due to matric suction, φ , was initially assumed to be linear based on the analysis of limited results published in the literature. Later experimental studies performed over a large range of suction values have shown that the variation of shear strength with respect to Soil suction is nonlinear (Gan et al. 1988 and Escario and Juca 1989). Equation [1] can be applied for both the linear and non-linear variation of shear strength with respect to suction. Figure 1. Typical Soil-water characteristic curve showing zones of desaturation. The Relationship between the Soil-Water Characteristic Curve and the Shear Strength of Unsaturated Soils The Soil-water characteristic curve defines the relationship between the Soil suction and either the degree of saturation, S, or gravimetric water content, w, or the volumetric water content, θ (Figure 1). The Soil-water characteristic curve provides a conceptual and interpretative tool by which the behavior of Unsaturated Soils can be understood. As the Soil moves from a saturated state to drier conditions, the distribution of the Soil, water, and air phases change as the stress state changes. A typical Soil-water characteristic curve with various zones of desaturation are shown in Figure 1. The wetted area of contact between the Soil particles decreases with an increase in the Soil suction. There is a relationship between the rate at which shear strength changes in Unsaturated conditions to the wetted area of water contact between the Soil particles or aggregates. In other words, a relations hip exists between the Soil-water characteristic curve and the shear strength of Unsaturated Soils. Different Procedures for Predicting the Shear Strength of an Unsaturated Soil Vanapalli et al. (1996) and Fredlund et al. (1996) have proposed a more general, nonlinear function for predicting the shear strength of an Unsaturated Soil using the entire Soilwater characteristic curve (i.e., 0 to 1,000,000 kPa) and the saturated shear strength parameters as shown below: ( ) [ ] ( ) ( )( ) { } [ ] ' tan ' tan ' φ φ σ τ κ Θ − + − + = w a a n u u u c [3] where: κ = fitting parameter used for obtaining a bestfit between the measured and predicted values, and Θ = normalized water content, θw/θs. The shear strength contribution due to suction constitutes the second part of [Eq. 3], which is: ( ) ( )( ) { } [ ] ' tan φ τ κ Θ − = w a us u u [4] Equation [3] can also be written in terms of degree of saturation, S, or gravimetric water content, w, to predict the shear strength yielding similar results. The entire Soil-water characteristic curve data (i.e., 0 to 1,000,000 kPa) is required along with the saturated shear strength parameters in the use of Equation [3]. A bestfit Soilwater characteristic curve can be obtained in terms of a, n, and m parameters using the equation proposed by Fredlund and Xing (1994) which is shown below:
-
measurement of the coefficient of permeability for a deformable Unsaturated Soil using a triaxial permeameter
Canadian Geotechnical Journal, 1998Co-Authors: Shangyan Huang, Delwyn G. Fredlund, S. L. BarbourAbstract:The development of a specially designed triaxial permeameter for the measurement of the coefficient of permeability of deformable Unsaturated Soils is presented in this paper. The triaxial permeameter makes it possible to directly measure the coefficient of permeability at various combinations of net normal stresses and matric suction values. The volume change of the Soil specimen during the permeability measurement can also be monitored. The performance of the permeameter is demonstrated for a group of tests conducted on slurried specimens of a silty sand.Key words: Unsaturated Soil, coefficient of permeability, triaxial permeameter, volume change.
-
the relationship of the Unsaturated Soil shear strength to the Soil water characteristic curve
Canadian Geotechnical Journal, 1996Co-Authors: Delwyn G. Fredlund, Anqing Xing, M D Fredlund, S. L. BarbourAbstract:The measurement of Soil parameters, such as the permeability and shear strength functions, used to describe unsaturate Soil behaviour can be expensive, difficult, and often impractical to obtain. This paper proposes a model for predicting the shear strength (versus matric suction) function of Unsaturated Soils. The prediction model uses the Soil-water characteristic curve and the shear strength parameters of the saturated Soil (i.e., effective cohesion and effective angle of internal friction). Once a reasonable estimate of the Soil-water characteristic curve is obtained, satisfactory predictions of the shear strength function can be made for the Unsaturated Soil. Closed-form solutions for the shear strength function of Unsaturated Soils are obtained for cases where a simple Soil-water characteristic equation is used in the prediction model. Key words: Soil suction, Soil-water characteristic curve, shear strength function, Unsaturated Soil.
