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3 CHAPTER 1 STATE OF UNSATURATED SOIL 1.1 UNSATURATED SOIL PHENOMENA 1.1.1 Definition of Unsaturated Soil Mechanics To provide and agree upon a precise definition of unsaturated soil mechanics is an academic challenge in itself. Perhaps one can draw some areas and boundaries by revisiting the classical definition of soil mechanics posed by Karl Terzaghi some 60 years ago. In his seminal book of 1943, Theoretical Soil Mechanics, Terzaghi defined soil mechanics as ‘‘the application of the laws of mechanics and hydraulics to engineering problems dealing with sed- iments and other unconsolidated accumulations of solid particles produced by the mechanical and chemical disintegration of rocks, regardless of whether or not they contain an admixture of organic constituents.’’ In drawing this silhouette of soil mechanics, Terzaghi refers to three basic requirements: (1) earthen materials, (2) the principles of mechanics and hydraulics, and (3) engineering problems. The emerging appreciation of unsaturated soil in geotechnical engineering practice and education requires refinement of Terzaghi’s basic definition. The earthen materials dealt with in problems of unsaturated soil mechanics are arguably the same as in Terzaghi’s soil mechanics, referred to as ‘‘soils,’’ but under a very specific ‘‘unsaturated’’ condition. The qualifier ‘‘unsaturated’’ bears the same meaning as its alternative ‘‘partially saturated’’ and simply indicates that the degree of pore water saturation is any value less than unity or, more specifically, that a third phase of matter is introduced into the two- phase, saturated soil system. In the modern educational and professional geo- technical engineering environment, where the emphasis has historically been
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4 STATE OF UNSATURATED SOIL limited to the arena of saturated cohesive materials and completely dry or completely saturated cohesionless materials, the ‘‘unsaturated’’ qualifier is indeed significant. In dealing with unsaturated soil, one requires not only the principles of mechanics and hydraulics but also of fundamental interfacial physics. Physics in this regard refers primarily to the thermodynamic principles describing equilibrium among gas, solid, and liquid phases, the transition of matter from one phase to another, and the adsorption or desorption of one phase of matter onto or from an adjacent phase of different matter. The forces and energies associated with these multiphase interactions by their very nature separate unsaturated soil behavior from saturated soil behavior. In many practical prob- lems, where the hydrologic and stress-strain behavior of natural or engineered systems comprised of soil is strongly influenced by the presence, absence, or changes in these interfacial interactions, the traditional saturated soil mechan- ics framework often fails to satisfactorily describe or predict the behavior of the system.
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This note was uploaded on 12/30/2011 for the course CIV 7004 taught by Professor Cetin during the Spring '11 term at Auckland.

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