On volume change dependency of the soil water characteristic curve in numerical modeling of unsaturated soils

Measurement of the soil-water characteristic curve (SWCC) in the laboratory is commonly conducted under zero confining pressure. However, in the field, the soil is under a confining stress. Therefore, it is important to study the effects of the confining stress on SWCC. In addition, the consolidation curve is normally generated under saturated conditions. However, the soil above the water table is usually unsaturated. Hence, it is also necessary to investigate the effects of matric suction on the characteristics of the consolidation curves. This paper presents the SWCCs under different net confining stresses and the isotropic consolidation curves under different matric suctions that describe the volume change characteristics of unsaturated soils with respect to stress state variables, net normal stress, and matric suction. A series of SWCCs was determined for statically compacted silt specimens in a triaxial cell apparatus under different net confining stresses. Isotropic consolidation tests under different matric suctions were also carried out. The results of the SWCC tests show that the air-entry value increased with increasing net confining stress. The yield points (i.e., yield suction, s0) obtained from the SWCC tests also increased with increasing net confining stress. The results of isotropic consolidation tests indicate the strong influence of matric suction on compressibility and stiffness of the compacted silt specimens.Key words: soil-water characteristic curve, isotropic consolidation, pore-water pressure, volume change, NTU mini suction probe, matric suction.

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A relationship describing the shear strength of unsaturated soils

Canadian Geotechnical Journal ◽

10.1139/t98-102 ◽

1999 ◽

Vol 36 (2) ◽

pp. 363-368 ◽

Cited By ~ 33

Author(s):

Daud W Rassam ◽

David J Williams

Keyword(s):

Shear Strength ◽

Soil Water ◽

Nonlinear Regression ◽

Unsaturated Soils ◽

Characteristic Curve ◽

Three Dimensional ◽

Matric Suction ◽

Linear Increase ◽

Soil Water Characteristic Curve ◽

Stress Dependent

A relationship describing the shear-strength profile of a desiccating soil deposit is essential for the purpose of analysis, especially when a numerical method is adopted where each zone in a discretised grid is assigned an elevation-dependent shear-strength value. The matric-suction profile of a desiccating soil deposit is nonlinear. Up to the air-entry value, an increase in matric suction is associated with a linear increase in shear strength. Beyond air entry, as the soil starts to desaturate, a nonlinear increase in shear strength occurs. The soil-water characteristic curve is stress dependent, as is the shear-strength gain as matric suction increases. In this paper, a three-dimensional, nonlinear regression analysis showed that a power-additive function is suitable to describe the variation of the shear strength of unsaturated soils with matric suction. The proposed function incorporates the effect of normal stress on the contribution of matric suction to the shear strength.Key words: air-entry value, matric suction, nonlinear regression, soil-water characteristic curve, tailings, unsaturated shear strength.

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Model for the prediction of shear strength with respect to soil suction

Canadian Geotechnical Journal ◽

10.1139/t96-060 ◽

1996 ◽

Vol 33 (3) ◽

pp. 379-392 ◽

Cited By ~ 733

Author(s):

S K Vanapalli ◽

D G Fredlund ◽

D E Pufahl ◽

A W Clifton

Keyword(s):

Shear Strength ◽

Soil Water ◽

Unsaturated Soil ◽

Unsaturated Soils ◽

Characteristic Curve ◽

Experimental Studies ◽

Matric Suction ◽

Glacial Till ◽

Soil Suction ◽

Soil Water Characteristic Curve

Experimental studies on unsaturated soils are generally costly, time-consuming, and difficult to conduct. Shear strength data from the research literature suggests that there is a nonlinear increase in strength as the soil desaturates as a result of an increase in matric suction. Since the shear strength of an unsaturated soil is strongly related to the amount of water in the voids of the soil, and therefore to matric suction, it is postulated that the shear strength of an unsaturated soil should also bear a relationship to the soil-water characteristic curve. This paper describes the relationship between the soil-water characteristic curve and the shear strength of an unsaturated soil with respect to matric suction. Am empirical, analytical model is developed to predict the shear strength in terms of soil suction. The formulation makes use of the soil-water characteristic curve and the saturated shear strength parameters. The results of the model developed for predicting the shear strength are compared with experimental results for a glacial till. The shear strength of statically compacted glacial till specimens was measured using a modified direct shear apparatus. Specimens were prepared at three different water contents and densities (i.e., corresponding to dry of optimum, and wet of optimum conditions). Various net normal stresses and matric suctions were applied to the specimens. There is a good correlation between the predicted and measured values of shear strength for the unsaturated soil. Key words: soil-water characteristic curve, shear strength, unsaturated soil, soil suction, matric suction.

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Predicting the permeability function for unsaturated soils using the soil-water characteristic curve

Canadian Geotechnical Journal ◽

10.1139/t94-062 ◽

1994 ◽

Vol 31 (4) ◽

pp. 533-546 ◽

Cited By ~ 445

Author(s):

D.G. Fredlund ◽

Anqing Xing ◽

Shangyan Huang

Keyword(s):

Water Content ◽

Soil Water ◽

Unsaturated Soil ◽

Unsaturated Soils ◽

Characteristic Curve ◽

Degree Of Saturation ◽

Residual Water ◽

Soil Water Characteristic Curve ◽

Coefficient Of Permeability ◽

Permeability Function

The coefficient of permeability for an unsaturated soil is primarily determined by the pore-size distribution of the soil and can be predicted from the soil-water characteristic curve. A general equation, which describes the soil-water characteristic curve over the entire suction range (i.e., from 0 to 106 kPa), was proposed by the first two authors in another paper. This equation is used to predict the coefficient of permeability for unsaturated soils. By using this equation, an evaluation of the residual water content is no longer required in the prediction of the coefficient of permeability. The proposed permeability function is an integration form of the suction versus water content relationship. The proposed equation has been best fit with example data from the literature where both the soil-water characteristic curve and the coefficient of permeability were measured. The fit between the data and the theory was excellent. It was found that the integration can be done from zero water content to the saturated water content. Therefore, it is possible to use the normalized water content (volumetric or gravimetric) or the degree of saturation data versus suction in the prediction of the permeability function. Key words : coefficient of permeability, soil-water characteristic curve, unsaturated soil, water content, soil suction.

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Equations for the entire soil-water characteristic curve of a volume change soil

Canadian Geotechnical Journal ◽

10.1139/t07-117 ◽

2008 ◽

Vol 45 (4) ◽

pp. 443-453 ◽

Cited By ~ 37

Author(s):

Hung Q. Pham ◽

Delwyn G. Fredlund

Keyword(s):

Soil Water ◽

Volume Change ◽

Curve Fitting ◽

Characteristic Curve ◽

Engineering Practice ◽

Soil Suction ◽

Residual Soil ◽

Soil Water Characteristic Curve ◽

Geoenvironmental Engineering ◽

Type Studies

Numerous curve-fitting equations have been proposed for soil-water characteristic curves. While these equations have been of considerable value in geotechnical and geoenvironmental engineering, the equations are not able to adequately fit gravimetric soil-water characteristic curve data over the entire range of soil suction for a soil that changes volume when suction is changed. Two new equations for the soil-water characteristic curve are presented in this paper. One equation has curve-fitting parameters that bear a meaningful relationship to conventional physical soil properties (e.g., air-entry value and residual soil suction), but the equation is somewhat complex. The equation is particularly useful for sensitivity type studies when undertaking computer modeling. The other equation is relatively simple to use and is developed as a conventional curve-fitting equation. The two equations are used to best-fit several soil datasets. Both equations perform well and can be used in research and engineering practice to define the gravimetric water content versus soil suction relationship for a soil exhibiting volume change.

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Effect of stress state on the unsaturated shear strength of a weathered granite

Canadian Geotechnical Journal ◽

10.1139/t04-091 ◽

2005 ◽

Vol 42 (2) ◽

pp. 624-631 ◽

Cited By ~ 75

Author(s):

In-Mo Lee ◽

Sang-Gyu Sung ◽

Gye-Chun Cho

Keyword(s):

Shear Strength ◽

Stress State ◽

Soil Water ◽

Unsaturated Soils ◽

Characteristic Curve ◽

Triaxial Tests ◽

Residual Soil ◽

Normal Stresses ◽

Soil Water Characteristic Curve ◽

Model Soil

The effect of stress state on the unsaturated shear strength of a Korean residual soil was studied using modified triaxial tests. Experimental results show that the soil-water characteristic curve and shear strength of this soil are significantly affected by the change of net normal stresses. This effect should be taken into consideration in the model to precisely describe the shear strength envelope of unsaturated soils. Thus, a new model for estimation of unsaturated shear strength is proposed using the soil-water characteristic curve and the saturated shear strength parameters.Key words: prediction model, soil-water characteristic curve, matric suction, triaxial test, unsaturated shear strength.

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Experimental Study on Soil Water Characteristic Curve of Compacted Unsaturated Soil

Advanced Materials Research ◽

10.4028/www.scientific.net/amr.168-170.1285 ◽

2010 ◽

Vol 168-170 ◽

pp. 1285-1288

Author(s):

Dong Lin Wang

Keyword(s):

Soil Water ◽

Influencing Factors ◽

Unsaturated Soils ◽

Stress Condition ◽

Characteristic Curve ◽

External Stress ◽

Method Of Least Squares ◽

Soil Water Characteristic Curve ◽

Triaxial Apparatus ◽

Soil Water Characteristic Curves

Soil water characteristic curve is one of important topics of unsaturated soils. Pressure plate extractor and GDS unsaturated triaxial apparatus are used to study influencing factors including types of soils and net mean stress. Through method of least-squares, Fredlund five-parameter model were employed to fit soil-water characteristic curves. The results show that model provided a satisfactory fit to the experimental data. Through an analysis of influencing factors, we find that not only physical condition of samples but also external stress condition can affect the shape of soil water characteristic curve.

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