This Geotechnical Special Publication contains over 215 papers presented during the Fourth International Conference on Unsaturated Soils held in Carefree, Arizona on April 2-5, 2006. These papers document 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 non-expansive, chemically stabilized soil, geosynthetics, geocomposite soils, and bentonite products.

This proceedings contains 38 papers that advance understanding of measuring, characterizing, and predicting the properties, behavior, and performance of unsaturated materials typically used in geotechnical engineering practice. The materials addressed include coarse-grained soils, compacted soils, geotextiles, loess, residual soils, shales, spoil fill, and swelling and expansive soils. These unsaturated materials are characterized with respect to soil-water characteristic curves, hydraulic conductivity, shear strength, and/or stress-strain properties. Both static and dynamic loading conditions are considered. Advances in the state of the art with respect to both laboratory and field measurements of unsaturated soil properties are included. Finally, predictions of unsaturated soil properties and their behavior with respect to specific applications, such as landfill liners and covers, heave of slabs on grade, and the bearing capacity of foundations on or in unsaturated soils are addressed.

This collection of papers presented at the sessions on ""Unsaturated Soil Engineering Practice"" held during the First National Conference of the ASCE Geo-Institute at the Utah State University in Logan, Utah from July 16-19, 1997. The significance of unsaturated soils in geotechnical engineering practice is great. The vast majority of the infrastructure is founded on and in unsaturated soil deposits. Unsaturated soil foundation problems are commonly associated with moisture-sensitive soils, wherein soils exhibit significant volume change and/or strength reduction upon wetting and represent a common geotechnical concern in both arid and humid climates. These papers address recent practice, experience, and advances in unsaturated soils engineering. The magnitude of unsaturated soil concerns is clearly identified through the diversity of topics presented. Rational, unified approaches for dealing with these unsaturated soil problems have recently emerged, providing the geotechnical engineer with tools of analysis for these problems.