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

Theories and formulations developed for the behavior of unsaturated soils are shown to include the behavior of saturated soils as a special case, leading to a generalization of the mechanics for soils. The scope of a generalized soil mechanics is illustrated through a series of visualization aids and diagrams. Man lives in direct contact with the vadose zone (ie the unsaturated zone). The upper boundary of the vadose zone (ie the ground surface) is subjected to a flux type boundary condition for many of the problems faced by geotechnical engineers. The atmosphere, down through the vadose zone to the soil below the groundwater table should be viewed as a continuum. A generalized mechanics for soils which is applicable for both saturated soils and unsaturated soils is necessary for dealing with the soil of the subterranean zone in a consistent manner (ie the saturated zone and the unsaturated zone should not be treated in isolation). There is also a need to treat the atmosphere and the soils as a unit. The soil-water characteristic curve has been shown to be a key soil function which can be used to approximate the behavior of unsaturated soils. (A) For the covering abstract see IRRD 881647.

The scope of unsaturated soil mechanics: an overview

To understand the complex soil-water interaction in an unsaturated expansive soil slope subjected to rainfall infiltration, a comprehensive instrumentation and monitoring program was carried out on an 11-m high cut slope in Hubei of China. The instrumentation included jet-filled tensiometers, thermal conductivity suction sensors, moisture probes, earth pressure cells, inclinometers, vertical movement points, artificial rainfall simulator, a tipping bucket rain gage, a vee-notch flow meter, and an evaporimeter. The technique and experience associated with each of the instrumentations are presented in this paper with an emphasis on the difference from the instrumentation in nonexpansive soils. In particular, a deliberate sealing scheme was adopted to prevent the potential bypass water flow through the shallow cracks into the installation holes for the suction sensors and the moisture probes. All the instruments worked well throughout the two month monitoring period, during which two artificial rainfall events were created. The recorded responses in pore-water pressure, water content, horizontal stress, and soil deformation were reasonably consistent with one another.

Instrumentation of an Unsaturated Expansive Soil Slope

This paper presents a numerical model to predict the volume change in an expansive soil as a result of vegetation and environmental changes. The model formulation is based on the general theory of unsaturated soil behavior. Three typical volume change situations associated with water uptake by tree roots and water infiltration into soils due to watering at surface are presented.

Prediction of Volume Change in an Expansive Soil as a Result of Vegetation and Environmental Changes

One of the challenges associated with the testing of unsaturated soils is the measurement of air volume change induced by stress changes. This paper reports the results of an evaluation of a digital pressure-volume controller as an air volume change indicator. Effects of temperature, confined volume, and precompression of the measurement medium on the p-V characteristics and the device response were determined. The device was also used to measure air volume changes of unsaturated soil specimens undergoing isotropic consolidation and axial loading in constant water content tests in a triaxial cell. The results show that the controller performs satisfactorily as an air volume change measurement device. It also permits measurement of air volume change simultaneously with the implementation of the axis-translation technique, making it a versatile device in the testing of unsaturated soils.

Air Volume Change Measurement in Unsaturated Soil Testing Using a Digital Pressure-Volume Controller

Three different chilled-mirror hygrometer test procedures were developed to investigate the time-dependent unsaturated behaviour of powdered and granular bentonite based needle-punched geosynthetic...

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Delwyn G. Fredlund

Papers

The scope of unsaturated soil mechanics: an overview

Instrumentation of an Unsaturated Expansive Soil Slope

Prediction of Volume Change in an Expansive Soil as a Result of Vegetation and Environmental Changes

Air Volume Change Measurement in Unsaturated Soil Testing Using a Digital Pressure-Volume Controller

Time-dependent unsaturated behaviour of geosynthetic clay liners