Environmental Engineering Reference
In-Depth Information
CHAPTER 1
Theory to Practice of Unsaturated Soil Mechanics
1.1
INTRODUCTION
or expansive soils. The shrinkage of these soils may pose an
equally severe situation. Loose silt soils often undergo col-
lapse when subjected to wetting and possibly a change in
the loading environment. The pore-water pressures in both
of the above-mentioned cases are initially negative, and vol-
ume changes occur as a result of increases in the pore-water
pressure. Residual soils have also been of particular con-
cern since their engineering behavior appears to deviate from
classical soil mechanics principles. Once again, the primary
factor contributing to the unusual behavior of residual soils
is negative pore-water pressures.
Unsaturated soil mechanics is herein presented in the
context of having a limited number of physical areas of
application, namely, water flow (and storage), air flow (stor-
age and compressibility), heat flow (and storage), shear
strength, and volume-mass change (including swelling and
collapse). The unsaturated soil theories are applied to real-
world problems and solutions are illustrated in the con-
text of a “boundary value problem.” The physical behavior
of unsaturated soil is formulated as a partial differential
equation(s) that must be solved using a numerical technique.
The partial differential equations are generally slightly too
highly nonlinear in character and as a result computer anal-
yses play an important role in solving practical engineering
problems.
Soil mechanics involves a combination of engineering
mechanics, soil behavior, and the properties of soils. This
description is broad and can encompass a wide range of soil
types. These soils could either be saturated with water or
have other fluids in the voids (e.g., air). The development of
classical soil mechanics has led to an emphasis on particular
types of soils. The common soil types are saturated sands,
silts and clays, and dry sands. These materials have formed
the primary emphasis in numerous soil mechanics textbooks.
More and more, it is realized that attention must be given to
a broader spectrum of soil materials.
There are numerous soil materials encountered in engi-
neering practice whose behavior is not consistent with the
principles and concepts of classical, saturated soil mechan-
ics. The presence of more than one fluid phase, for example,
results in material behavior that is challenging to engineer-
ing practice. Soils that are unsaturated (i.e., water and air in
the voids) form the largest category of soils which do not
adhere in behavior to classical saturated soil mechanics.
The general field of soil mechanics can be subdivided
into the portion dealing with saturated soils and the portion
dealing with unsaturated soils. The differentiation between
saturated soils and unsaturated soils becomes necessary due
to basic differences in the material nature and engineering
response. An unsaturated soil has more than two phases,
and the pore-water pressure is negative relative to pore-air
pressure. Any soil near the ground surface, present in an
environment where the water table is below the ground sur-
face, will be subjected to negative pore-water pressures and
possible reduction in degree of saturation.
The process of excavating, remolding, and compacting a
soil requires that the material be unsaturated. It has been
difficult to predict the behavior of compacted soils within
the framework of classical soil mechanics.
Natural surficial deposits of soil are found to have rela-
tively low water contents over a large portion of the earth.
Highly plastic clays subjected to a changing environment
have produced the category of materials known as swelling
1.1.1 Application of Unsaturated Soil Mechanics
in Engineering Practice
The content of this topic takes into consideration the history
of classical soil mechanics and the significant impact that
the computer has had on the practice of geotechnical engi-
neering. It is fair to say that the computer has resulted in a
paradigm shift in how geotechnical engineering problems in
general and specifically unsaturated soils problems are ana-
lyzed. The significant role that the computer has played has
also been taken into consideration in assembling the con-
tent for this topic. The nature of unsaturated soil problems
makes it essentially imperative to use numerical methods
when solving geotechnical engineering problems.
