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Geotechnical News • December 2013
www.geotechnicalnews.com
Unsaturated Soil Mechanics in
Engineering Practice
Book Reviewed by G. Ward Wilson
The recently released book titled,
“Unsaturated Soil Mechanics in Engi-
neering Practice”, constitutes a new
and exhaustive synthesis of the present
state of knowledge for unsaturated
soil mechanics. The book identifies a
range of physical processes common
to all soils near to the ground surface.
In addition to water flow through
unsaturated soils, shear strength and
volume change, the book addresses the
physical processes of heat flow, water
vapour flow and air flow. The theory
associated with each of the physical
processes is presented in a step by step
manner using common engineering
notation. The theory is followed by
descriptions of laboratory tests that
can be used to measure unsaturated
soil property functions relevant to
each physical process. An extensive
explanation is then given on how the
unsaturated soil property functions
can be estimated with the assistance
of the soil-water characteristic curve,
SWCC. Finally, protocols for prudent
geotechnical engineering practice are
presented.
An abundantly instructive and com-
prehensive chapter is solely dedicated
to the measurement and the estimation
of the soil-water characteristic curve.
This chapter shows how the SWCC
is used to estimate the unsaturated
soil property functions required for
numerical and analytical simulations
of processes common to unsaturated
soils. There is also a chapter devoted
to the quantification of the ground
surface moisture flux. For example,
it is shown how weather station data
is used to compute actual evaporation
from ground surface. Actual evapora-
tion is combined with other ground
surface water balance components
to calculate the net moisture flux at
ground surface. Engineering protocols
based on the practical experience of
the authors are described for a range
of geotechnical engineering applica-
tions.
In general, the application of unsatu-
rated soil mechanics in engineering
practice has been subdivided into
two broad categories: namely, the
Preliminary Stage and the Design
Stage. In this, it is possible to provide
estimations of the likely behaviour of
unsaturated soils based on grain-size
distribution curves or other estima-
tion techniques for the SWCC. The
preliminary type of exercises are later
followed by more detailed design
procedures.
The emphasis of the new book is
clearly centered on the ability to
estimate unsaturated soil property
functions based on the saturated soil
properties and the SWCC. An estima-
tion of the SWCC can be obtained
from the grain-size distribution curve
or correlations with soil classification
properties. There are complexities
associated with the use of the SWCC.
For example, there is a drying and
wetting SWCC. The issues related
to hysteretic and volume change are
addressed in the book.
The chapter on the use of weather
station data for the determination of
moisture and temperature flux condi-
tions at ground surface is presented
in a manner that is particularly useful
for geotechnical engineers. Details are
presented for the calculation of poten-
tial evaporation and actual evapora-
tion. Solutions for both “coupled” and
“uncoupled” procedures for satisfying
heat and moisture flow are described.
Procedures for handling the interac-
tion between precipitation and runoff
are also suggested for geotechnical
engineering problems.
Numerical modeling has become an
integral part of solving engineering
problems involving saturated-unsatu-
rated soil systems. The authors of this
new book have shown how it is pos-
sible to make use of numerical model-
ing parametric type studies to embrace
conditions that are likely to occur. The
theory and soil parameter estimation
are closely linked to software that is
able to accommodate the solution of
nonlinear partial differential equations.
All problems are formulated within a
consistent “boundary-value problem”
framework.
The need for comprehensive engineer-
ing software becomes immediately
apparent when solving problems
involving unsaturated soils. The equa-
tions that need to be solved take the
form of nonlinear partial differential
equations. Their solution must not
only ensure convergence, but also
ensure convergence to the correct
solution.
It is immediately clear from an exami-
nation of the contents that this book
is not simply a second edition of their
1993 book titled, “Soil Mechanics for
Unsaturated Soils”. The new book is
almost 1000 pages in length – almost
twice that of the 1993 book. The new
book reveals much about the progress
that has been made in unsaturated soil
mechanics over the past two decades.
Most important, the book presents
vital information regarding many of
the gaps in our original understand-
ing. Those gaps were a hindrance to
the implementation of unsaturated soil
mechanics into routine engineering
practice.
BOOK REVIEW