Unsaturated Soil Mechanics

Unsaturated Soil Mechanics

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.

The history of unsaturated soil mechanics is embedded in the history of hydrology, 
soil mechanics, and soil physics. Engineering problems involving

unsaturated soil span numerous subdisciplines and practices within the general
field of civil engineering. Hydrologists, for example, have long recognized
that modeling of regional or local surface water and groundwater systems and
curring in the near-surface unsaturated soil zone. Quantitative evaluation of

moisture flux at the atmosphere-subsurface boundary requires not only knowl-
edge of the relevant soil and pore water properties but also the predominant

environmental conditions at the soil-atmosphere interface. Unsaturated soil
often comprises cover or barrier materials for landfills and hazardous waste

storage facilities of interest to the geo-environmental community. Contami-
nant transport and leaching processes are often strictly unsaturated fluid trans-
port phenomena, occurring in many cases as multiphase transport problems.

As national and international policy with regard to the health of the natural
environment is becoming increasingly more regulated, recognition of these

types of geo-enviromental issues and development of solutions from an un-
saturated soil mechanics framework is becoming more and more common.

Many of the more traditional geotechnical engineering problems also fall
wholly or partly into the category of unsaturated soil mechanics problems.
Compaction, for example, a classical application involving unsaturated soil,

has been routine practice for improving the mechanical and hydraulic prop-
erties of soil since far before the formation of civil engineering as a formal

discipline in the mid-nineteenth century. Compacted soil comprising the many
earthworks constructed all over the world is most appropriately considered

from an unsaturated soils framework. It has long been recognized that ex-
pansive soils pose a severe threat to civil engineering infrastructure such as

roads, housing, and transportation facilities nationally and internationally. Ex-
pansive soil formations in the United States alone are responsible for billions

of dollars in damage costs each year, an amount exceeding that of all other
natural hazards combined, including earthquakes, floods, fires, and tornados
(Jones and Holtz, 1973). 


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