The soil-water characteristic curve can be used to estimate various parameters used to describe unsaturated soil behaviour. A general equation for the soil-water characteristic curve is proposed. A nonlinear, least-squares computer program is used to determine the best-fit parameters for experimental data presented in the literature. The equation is based on the assumption that the shape of the soil-water characteristic curve is dependent upon the pore-size distribution of the soil (i.e., the desaturation is a function of the pore-size distribution). The equation has the form of an integrated frequency distribution curve. The equation provides a good fit for sand, silt, and clay soils over the entire suction range from 0 to 106 kPa. Key words : soil-water characteristic curve, pore-size distribution, nonlinear curve fitting, soil suction, water content.

Equations for the soil-water characteristic curve

The Paper presents a constitutive model for describing the stress-strain behaviour of partially saturated soils. The model is formulated within the framework of hardening plasticity using two iodependent sets of stress variables: the excess of total stress over air pressure and the suction. The mode1 is able to represent, in a consistent and unified manner, many of the fundamental features of the behaviour of partially saturated soils which had been treated separately by previously proposed models. On reaching saturation, the mode1 becomes a conventional critical state model. Because experimental evidence is still limited, the model has been kept as simple as possible in order to provide a basic framework from which extensions are possible. Tbe mode1 is intended for partially saturated soils which are slightly or moderately expansive. After formulating the model for isotropic and biaxial stress states, typical predictions are described and compared, in a qualitative way, with characteristic trends of the behaviour of partially saturated soils. Afterwards, the results of a number of suction-controlled laboratory tests on compacted kaolin and a sandy clay are used to evaluate the ability of the model to reproduce, quantitatively, observed behaviour. The agreement between observed and computed results is considered satisfactory and confirms the possibilities of reproducing the most important features of partially saturated soil behaviour using a simple general framework.

A constitutive model for partially saturated soils

The Paper presents a constitutive model for describing the stress-strain behaviour of partially saturated soils. The model is formulated within the framework of hardening plasticity using two independent sets of stress variables: the excess of total stress over air pressure and the suction. The model is able to represent, in a consistent and unified manner, many of the fundamental features of the behaviour of partially saturated soils which had been treated separately by previously proposed models. On reaching saturation, the model becomes a conventional critical state model. Because experimental evidence is still limited, the model has been kept as simple as possible in order to provide a basic framework from which extensions are possible. The model is intended for partially saturated soils which are slightly or moderately expansive. After formulating the model for isotropic and triaxial stress states, typical predictions are described and compared, in a qualitative way, with characteristic trends of the...

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A Constitutive Model for Partially Saturated Soils

Soils can rarely be described as ideally elastic or perfectly plastic and yet simple elastic and plastic models form the basis for the most traditional geotechnical engineering calculations. With the advent of cheap powerful computers the possibility of performing analyses based on more realistic models has become widely available. One of the aims of this book is to describe the basic ingredients of a family of simple elastic-plastic models of soil behaviour and to demonstrate how such models can be used in numerical analyses. Such numerical analyses are often regarded as mysterious black boxes but a proper appreciation of their worth requires an understanding of the numerical models on which they are based. Though the models on which this book concentrates are simple, understanding of these will indicate the ways in which more sophisticated models will perform.

Soil Behaviour and Critical State Soil Mechanics

The validity of the cubic law for laminar flow of fluids through open fractures consisting of parallel planar plates has been established by others over a wide range of conditions with apertures ranging down to a minimum of 0.2 µm. The law may be given in simplified form by Q/Δh = C(2b)3, where Q is the flow rate, Δh is the difference in hydraulic head, C is a constant that depends on the flow geometry and fluid properties, and 2b is the fracture aperture. The validity of this law for flow in a closed fracture where the surfaces are in contact and the aperture is being decreased under stress has been investigated at room temperature by using homogeneous samples of granite, basalt, and marble. Tension fractures were artificially induced, and the laboratory setup used radial as well as straight flow geometries. Apertures ranged from 250 down to 4µm, which was the minimum size that could be attained under a normal stress of 20 MPa. The cubic law was found to be valid whether the fracture surfaces were held open or were being closed under stress, and the results are not dependent on rock type. Permeability was uniquely defined by fracture aperture and was independent of the stress history used in these investigations. The effects of deviations from the ideal parallel plate concept only cause an apparent reduction in flow and may be incorporated into the cubic law by replacing C by C/ƒ. The factor ƒ varied from 1.04 to 1.65 in these investigations. The model of a fracture that is being closed under normal stress is visualized as being controlled by the strength of the asperities that are in contact. These contact areas are able to withstand significant stresses while maintaining space for fluids to continue to flow as the fracture aperture decreases. The controlling factor is the magnitude of the aperture, and since flow depends on (2b)3, a slight change in aperture evidently can easily dominate any other change in the geometry of the flow field. Thus one does not see any noticeable shift in the correlations of our experimental results in passing from a condition where the fracture surfaces were held open to one where the surfaces were being closed under stress.

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VALIDITY OF CUBIC LAW FOR FLUID FLOW IN A DEFORMABLE ROCK FRACTURE - eScholarship

Unsaturated soils are generally near the ground surface and are commonly overconsolidated due to environmental effects. The stress state variables for an unsaturated, in situ profile consist of the...

Influence of stress history on the strength parameters of an unsaturated statically compacted soil

The discussers would like to congratulate the authors for a well-presented paper on an important subject area that has received relatively little attention in the past. The authors should also be commended for applying the proposed model to the analysis of carefully conducted field tests (Ayad et al. 1997). Recently, the discussers have studied several aspects of desiccation and crack formation in clays (Kodikara et al. 1998; Lau et al. 1991; Lau 1987). The cracking problem is complex and the model proposed by Konrad and Ayad (1997) provides an idealized framework for modeling desiccation cracking for a special class of problems. In order to stimulate further work on this area, the discussers would like to offer the following comments. The authors have assumed that the clay remains saturated during cracking and that K0 conditions are applicable up to the onset of cracking. The authors also indicate that the model is applicable to slurries, consolidated natural soils, and compacted clays. The discussers agree that the shrinkage behavior of unconsolidated or lightly consolidated heavy clays is generally dominated by normal shrinkage, during which the soil remains saturated. For example, slurried Regina clay does not desaturate until the soil is subjected to more than 1 MPa of suction. Hence, the proposed model provides a satisfactory approach utilizing the stress paths based on the effective and total stresses. However, there is evidence that some light clays deviate from this behavior and primarily show residual shrinkage as the soil desaturates (e.g., Bronswick 1988). Similarly, heavily overconsolidated clays as well as compacted clays predominantly behave in an unsaturated manner during desiccation. Since the tensile strength of these clays is higher than that for unconsolidated clays, suctions exceeding 100 kPa can develop at the point of initial cracking. Under these suctions, these soils are most likely to be unsaturated. For example, soil-water characteristics measured for compacted soil specimens of Indian Head till have shown that the air entry value is less than 100 kPa even for specimens compacted about 6% wet of optimum (Vanapalli et al. 1996). Also, compacted clays are placed in an unsaturated state (i.e., as low as 77% saturation) in engineering practice. In view of the above comments, it would appear that the proposed approach may be applicable for some clays, but it may be necessary to more thoroughly examine the deformation behavior of soil before applying the model to all clay soils. The authors refer to the paper by Morris et al. (1992) and indicate that their approach is not suitable for crack initiation primarily because it uses an unsaturated soils framework. Lau (1987) also used the same approach for the prediction of crack depths in soils. In this study, the stresses (i.e., σi – ua, where σ is the total normal stress, ua is the air pressure, and the subscript i denotes x, y, and z cartesian axes) degenerate to the traditional effective stresses at saturation. Using unsaturated soils framework and the K0 loading condition (Lau 1987; Morris et al. 1992), the net horizontal stress (σx – ua) can be expressed as

An idealized framework for the analysis of cohesive soils undergoing desiccation: Discussion

A rigorous volume–mass constitutive model is proposed for the representation of drying–wetting under isotropic loading–unloading conditions for unsaturated soils. The proposed model utilizes concep...

Volume–mass unsaturated soil constitutive model for drying–wetting under isotropic loading–unloading conditions

A Volume-Mass Constitutive Model for Unsaturated Soils in Terms of Two Independent Stress State Variables

Routine geotechnical engineering practice has witnessed a significant increase in the usage of unsaturated soil mechanics principles. Laboratory measurement of the soil-water characteristic curve (...

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Delwyn G. Fredlund

Papers

Influence of stress history on the strength parameters of an unsaturated statically compacted soil

An idealized framework for the analysis of cohesive soils undergoing desiccation: Discussion

Volume–mass unsaturated soil constitutive model for drying–wetting under isotropic loading–unloading conditions

A Volume-Mass Constitutive Model for Unsaturated Soils in Terms of Two Independent Stress State Variables

State of practice for use of the soil-water characteristic curve (SWCC) in geotechnical engineering