Soil Mechanics for Unsaturated Soils

Abstract

Numerous textbooks have been written on the subject of soil mechanics. The subject matter covered and the order of presentation vary somewhat from text to text, but the main emphasis is always on the application of the principles of soil mechanics to problems involving saturated soils. A significant portion of the earth’s surface is subjected to arid and semi-arid climatic conditions, and as a result, many of the soils encountered in engineering practice are unsaturated. This textbook addresses the subject of soil mechanics as it relates to the behavior of unsaturated soils. More specifically, the text addresses that class of problems where the soils have a matric suction or where the pore-water pressure is negative. Whether the soil is unsaturated or saturated, it is the negative pore-water pressure that gives rise to this unique class of soil mechanics problems. When the pore-water pressure is negative, it is advantageous, and generally necessary, to use two independent stress state variables to describe the behavior of the soil. This is in constrast to saturated soil mechanics problems where it is possible to relate the behavior of the soil to one stress state variable, namely, the effective stress variable. The terms saturated soil mechanics and unsaturated soil mechanics are primarily used to designate conditions where the pore-water pressures are positive and negative, respectively. Soils situated above the groundwater table have negative pore-water pressures. The engineering problems involved may range from the expansion of a swelling clay to the loss of shear strength in a slope. Microclimatic conditions in an area produce a surface flux boundary condition which produces flow through the upper portion of the soil profile. It would appear that most problems addressed in Saturated soil mechanics have a counter problem of interest in unsaturated soils. In addition, the remolding and compacting of soils is an important part of many engineering projects. Compacted soils have negative pore-water pressures. The range of subjects of interest involving negative pore-water pressures are vast, and the problems are becoming of increasing relevance, particularly in arid regions. An attempt has been made to write this textbook in an introductory manner. However, the subject matter is inherently complex. The need for such a book is clearly demonstrated by engineering needs associated with various projects around the world. The frustrations are expressed primarily by engineers who have received advanced training in conventional soil mechanics, only to discover difficult problems in practice involving unsaturated soils for which their knowledge is limited. The textbook makes no attempt to redevelop concepts well known to saturated soil mechanics. Rather, the book is designed to be an extension of classical saturated soil mechanics. As far as is possible, the principles and concepts for unsaturated soils are developed as extensions of the principles and concepts for saturated soils. In this way, the reader should be able to readily grasp the formulations required for unsaturated soil mechanics. The general format for the textbook is similar to that used in most classical soil mechanics text