Shear Strength Applications in Plastic and Limit Equilibrium - Unsaturated Soil Mechanics in Engineering Practice - page 612

dire consequences. Even when the water table is quite high
in the slope, it is possible to study the effect that changes
in negative and positive pore-water pressures will have on
the computed factor of safety. It is useful from an engi-
neering perspective to have a quantitative assessment of the
influence that the unsaturated soil portion has on computed
factors of safety.
The unsaturated soil shear strength properties of a soil can
readily be estimated. It is important to quantify the effect
of matric suction (and changes in matric suction) on the
analysis of the problem at hand. The influence of matric
suction at shallow depths is often much more significant than
anticipated. The engineer should carefully consider whether
there are decisions that can be made to better ensure the
maintenance of negative pore-water pressures over the life
of the structure.

actual soil behavior that form a useful categorization of the
types of analyses that can be performed in soil mechanics.
Theories of elasticity and plasticity provide categories with
theoretical limits within which the behavior of a soil mass can
be studied.
There are threemain types of soil mechanics analyses where
plastic equilibrium forms the basis of analysis: (1) lateral earth
pressure analyses, (2) bearing capacity analyses, and (3) slope
stability analyses. Plastic equilibrium satisfies equations of
equilibrium and a condition of failure (i.e., the failure cri-
terion). Typical soil mechanics analyses are derived in this
chapter based on plastic equilibrium. In each case, the soil
is assumed to have negative pore-water pressures (or matric
suctions).
Plasticity analyses can be subdivided into situations where
the pore-water pressure effects are simulated in some manner
during the testing of the soil (i.e., total stress approach)
and situations where pore pressure designations become
a part of the analysis (i.e., stress state variable approach).
Most of the consideration in this chapter is given to the
latter situation where actual or predicted pore pressures are
designated.

12.3 APPLICATION TO PRACTICAL SHEAR
STRENGTH PROBLEMS IN GEOTECHNICAL
ENGINEERING

Deformation or ground movement problems encountered in
soil mechanics can be divided into one of two categories
dependent upon the stress level involved. When deviator
stress levels are relatively low, the problems are considered
to be in the elastic range and problems can generally be ana-
lyzed using theories of elasticity. When the deviator stress
levels are relatively high, the problems are considered to
be in the plastic range and problems are analyzed using the
theories of plasticity or limit equilibrium. The elastoplastic
categories can be visualized on an idealized representation
of a stress-strain curve (Fig. 12.36).
There is a class of problems in geotechnical engineering
where the analysis is based on the assumption that the soil
behaves in a perfectly plastic manner. When the state of plas-
tic equilibrium is limited to a specific, thin zone, the problems
are referred to as limit equilibrium analyses. The thin plas-
tic zone is called a slip surface zone or a slip plane. The
assumptions of plasticity and elasticity are a simplification of

12.3.1 Prediction of Earth Pressures
Some of the earliest work in soil mechanics dealt with earth
pressures on retaining walls. However, there is little informa-
tion on the earth pressures exerted on engineering structures
backfilled and founded upon unsaturated soils. Engineers are
well aware of pressures that can be exerted by expansive soils,
but little attention has been given to developing a general earth
pressure theory for these soils. It is not adequate to request
that cohesive backfill not be used behind earth-retaining struc-
tures. Experience indicates that problems associated with the
performance of earth structures can often involve compacted,
clayey soils. Ireland (1964) stated that 68% of unsatisfactory
retaining wall performance involved cases where clay was
used as a backfill or where the retaining wall was founded
on clay.
Some of the problems encountered with earth-retaining
structures result from the tendency of clayey expansive soils to
undergo substantial changes in volume as a result of changing
environmental conditions. There are situations where unsat-
urated soils are used as backfill or where structural members
are cast in place against the soil. It is apparent that only limited
consideration has been given to the behavior of the retaining
structure under these circumstances.
A limited but theoretical analysis of earth pressures is pre-
sented for soils with negative pore-water pressures. The active
and passive earth pressures in a soil mass are discussed along
with several example problems that illustrate the concepts
involved. The meaning of the at-rest earth pressure condition
was discussed in Chapter 3 in the section on the stress state in
an earth mass. The at-rest pressure state is more appropriately
related to elastic stress theories (i.e., with zero lateral strain).

Figure 12.36 Idealized elastic-plastic behavior giving rise to two
categories of deformation analysis.