The principal aim of this book is to provide a thorough grounding in unsat- urated soil mechanics principles from three fundamental pers...
The principal aim of this book is to provide a thorough grounding in unsat-
urated soil mechanics principles from three fundamental perspectives: ther-
modynamics, mechanics, and hydrology. The book is written to guide a first
course on the subject and is primarily intended for undergraduate seniors,
graduate students, and researchers with backgrounds in the more general fields
of geotechnical engineering, soil science, environmental engineering, and
groundwater hydrology.
In formulating this book,
we have maintained the opinion that a first course
in any branch of mechanics should emphasize the fundamental principles that
govern the phenomena of interest. A principles-based approach to learning is
most beneficial to the general reader and is particularly appropriate for the
subject of unsaturated soil mechanics as it remains a young, dynamic, and
rapidly emerging field of research and practice. Our general viewpoint to-
wards the pursuit of understanding is reflected by Thomas Henry Huxley’s
(1825–1895) statement: ‘‘The known is finite, the unknown infinite; intellec-
tually we stand on an islet in the midst of an illimitable ocean of inexplica-
bility. Our business in every generation is to reclaim a little more land.’’ We
hope that this book will provide the necessary background and motivation for
those who desire to explore and reclaim the ocean of unsaturated soil me-
chanics problems that nature and society continue to present.
Stress Phenomena Problems
requiring consideration of both mechanical
and chemical equilibrium are classified as stress phenomena. These include
traditional geotechnical engineering problems such as lateral earth pressure,
bearing capacity, and slope stability analysis. For each of these problems, the
strength of the soil at its limit state is the primary concern. Analysis of the
stress distribution within the soil mass and the corresponding bulk strength
becomes critically important. Limit analysis developed extensively since the
1930s for saturated soil applications formed the basis for solving most of
these types of problems. Developing elastoplastic theories for soil became the
focus of much of the geomechanics research activity during the 1970s and
1980s. Powerful numerical methods to solve the governing partial differential
equations for stress equilibrium under static or dynamic conditions have been
developed and applied to many difficult foundation problems in the past 20
years or so.
It has become clear in recent years that improved solutions of many stress-
related geotechnical engineering problems require not only sustained activities
along the continuum-based solid mechanics approach but also new theories
along a microscopic discontinuous approach for describing effective stress
under multiphase conditions.
Terzaghi’s effective stress, which is the corner-
stone of soil mechanics under saturated conditions, becomes either ineffective
or inappropriate for fully describing the stress distributions or failure condi-
tions in unsaturated soil. It has been recognized that theories for describing
the states of stress and failure in unsaturated soil require consideration of the
thermodynamic properties of the pore water in terms of soil suction, material
variables such as grain size and grain size distribution, state variables such
as the degree of saturation, and the consequent interparticle forces such as
suction-induced effective stress or suction stress.
Deformation Phenomena Physical processes characterized
by large de-
formations or strains are classified as deformation phenomena. In unsaturated
soils, these deformations are very often caused or governed by changes in the
moisture condition of the soil. Important deformation phenomena include
compaction, multiphase consolidation and compressibility, and collapsing soil
