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

Weather-related geo-hazards are a major concern for both natural slopes and man-made slopes and embankments. Government agencies and private companies are increasingly required to ensure that there is adequate protection of sloping sur- faces in order that interaction with the climate does not produce instability. Superior theoretical formulations and computer tools are now available to address engineering design issues related to the near ground surface soil-atmospheric interactions. An ex- ample is given in this paper that illustrates the consequences of not paying adequate attention to the hazards of slope stability prior to the construction of a highway in South America. On the other hand, examples are given from Hong Kong and Mainland China where significant benefits are derived from putting in place a hazard slope stability management system. Some results from a hazard management slope stability study related to the railway system in Canada are also reported. The study took advantage of recent research on unsaturated soil behaviour and applied this information to real-time modelling of climatic conditions. The quantification of the water balance at the ground surface, and subsequent infiltration, is used as the primary tool for hazard level assessment. The suggested hazard model can be applied at either specific high risk locations or in a more general, broad-based manner over large areas. A more thorough understanding of unsaturated soil behaviour as it applies to near ground surface soils, along with the numerical computational power of the computer has made it possible for new approaches to be used in slope hazard management engineering.

Slope stability hazard management systems

An apparatus for performing consolidation tests under K(sub 0) conditions was designed and constructed for testing unsaturated soils. three types of consolidation tests are of interest in unsaturated soils. These are: (1) constant water content loading tests, (2) consolidation under a net total stress, and (3) consolidation under a matric suction. The apparatus had been used for simultaneous and separate measurements of pore-air and pore-water pressures for all three types of consolidation tests. The experimental results demonstrate the pore pressure and volume change behavior of an unsaturated soil during these tests.

Consolidation Apparatus for Testing Unsaturated Soils

The saturated/unsaturated coefficient of permeability is required when modeling engineering problems involving transient seepage and contaminant transport. Inaccuracies in the estimation of the coefficient of permeability function can cause errors in numerical modeling results and as a result, affect subsequent engineering decisions. Void ratio changes and degree of saturation changes influence the coefficient of permeability function of a soil. Existing methodologies for estimating the unsaturated permeability function give consideration to degree of saturation changes and may produce reasonable results for soils such as sands and silts which exhibit little volume change with suction change. However, inaccurate results are generated when the soils involved change volume as soil suction changes (e.g., initially slurry Regina clay). Revisions to previously proposed methodologies are needed for the estimation of a saturated/unsaturated coefficient of permeability function that accommodates both void ratio change and degree of saturation change when simulating the drying process. A modification of the Fredlund–Xing–Huang (Can Geotech J 31(4):533–546, 1994) estimation procedure for the permeability function is presented in this paper. Consideration is given to both volume change and desaturation of the soil as soil suction is increased. A laboratory data set for Regina clay is presented and interpreted using the revised methodology. The results illustrate a procedure that can be used to estimate the saturated/unsaturated coefficient of permeability function for a soil that undergoes volume change and desaturation as soil suction increases.

Water Permeability Function for Soils that Undergo Volume Change as Suction Changes

Soil-water characteristic curves (SWCCs) play a key role in the determination of unsaturated soil property functions for use with predictive numerical models. Most studies on the soil-water characteristic curve have been limited to the desorption branch in the low suction range (i.e., less than 1500 kPa). There is limited measured data on desorption and adsorption branches (i.e., hysteresis effects) in the high suction range. The dew-point Water PotentiaMeter (i.e., WP4-T) is a device that has been introduced into the commercial market for measuring total suction in the high suction range. The WP4-T device can reduce the time and costs associated with suction measurements in the high suction range. The performance of the WP4-T was evaluated using experimental data measured on Regina clay and Botkin silt soils. Measurements of total suction using the WP4-T device were compared with vacuum desiccator results using a range of saturated salt solutions. The results showed that the WP4-T device provides excellent results in the high suction range branches of the SWCC.

Assessment of the WP4-T Device for Measuring Total Suction

Proceedings of the Fourth International Conference on Unsaturated Soils, held in Carefree, Arizona, April 2-6, 2006. Sponsored by the Geo-Institute of ASCE; International Society of Soil Mechanics and Geotechnical Engineering, Committee TC6 on Unsaturated Soils; Canadian Geotechnical Society. This Geotechnical Special Publication contains 219 papers documenting the experience of researchers and practitioners from around the world concerning a vast array of unsaturated soil problems. Theoretical and methodological advances in laboratory testing of shear strength and volume change behavior, suction measurement techniques, soil water characteristic behavior, constitutive and numerical modeling, microscale modeling, foundation behavior, heave of slabs and pavements, evapotraspirative covers, geophysical applications, liquefaction and soil dynamics, pavements and slopes, soil-atmospheric interaction, desiccation and shrinkage, in situ testing, seepage, flow of water and gas, and design of waste depositories with emphasis on temperature effects are among the topics dealt with in the framework of unsaturated soil behavior. Materials addressed include natural soils, both expansive and nonexpansive, chemically stabilized soil, geosynthetics, geocomposite soils, and bentonite products.

Delwyn G. Fredlund

Papers

Slope stability hazard management systems

Consolidation Apparatus for Testing Unsaturated Soils

Water Permeability Function for Soils that Undergo Volume Change as Suction Changes

Assessment of the WP4-T Device for Measuring Total Suction

Unsaturated Soils 2006