Shear Strength Applications in Plastic and Limit Equilibrium - Unsaturated Soil Mechanics in Engineering Practice - page 588

CHAPTER 12

Shear Strength Applications in Plastic and Limit Equilibrium

12.1 INTRODUCTION as virtually every physical property of unsaturated soils. It is

costly and time consuming to measure the unsaturated soil
property functions in the laboratory because of the number
of data points that are required. There has consequently been
an emphasis on the estimation of unsaturated shear strength
functions.
Many engineering projects may not be able to justify the
measurement of the unsaturated shear strength relationship
for an unsaturated soil. However, an estimation of the unsat-
urated soil shear strength envelope may provide the engineer
with a significantly improved ability to analyze practical
problems where a portion (or all) of the soil profile involves
unsaturated soils. The contribution of soil suction to shear
strength generally results in a significant increase in bearing
capacity and an increase in the factor of safety of slopes. At
the same time, the active earth pressure for an unsaturated
soil is decreased by soil suction while the passive pressure
in increased as soil suction is increased.
Most estimation shear strength equations proposed for an
unsaturated soil have been related to the SWCC or the clas-
sification properties of the soil. Consideration has mainly
been given to the use of shear strength estimation proce-
dures that use two independent stress state variables and the
SWCC. Empirical estimation equations associated with crit-
ical state constitutive models are not discussed in this topic.
Much of the published shear strength data are associated
with the peak shear strength of the soil.
The method of preparation of soil specimens should be
noted along with shear strength laboratory results. It is
important to know the preparation procedures for soil speci-
mens tested in the laboratory. Soil specimens can be prepared
as (i) slurry soil specimens consolidated to specified applied
pressures and then desaturated by increased soil suction,
(ii) compacted specimens prepared by premixing at a spec-
ified water content and density and subsequently varying the
soil suction that is applied, and (iii) undisturbed samples
from the field that are then conditioned with various applied
soil suctions.
A number of general observations can be made based on
various shear strength studies that have been undertaken

The analysis of lateral earth pressure problems (i.e., active
and passive earth pressures), bearing capacity problems (i.e.,
plasticity theory), and slope stability problems (i.e., limit
equilibrium analyses) all require the shear strength prop-
erties of the soil (i.e., failure criterion). The equations of
static equilibrium must also be satisfied when solving the
above-mentioned geotechnical engineering problems.
The shear strength envelope becomes nonlinear when soils
are unsaturated and soil suction varies over a wide range.
The nonlinearity of the shear strength envelope is related
to the changes in degree of saturation of the soil as soil
suction changes. The previous chapter provided informa-
tion on the measurement of unsaturated soil shear strength
parameters. There are numerous situations, however, where
it is sufficient and satisfactory to estimate the unsaturated
shear strength parameters. This is particularly true at the
preliminary design stage of a project. Numerous empirical
equations have been proposed for the estimation of the shear
strength of unsaturated soils. In most cases the estimated
shear strength equations are based on the saturated shear
strength parameters and the SWCC of the soil.
The first portion of this chapter is devoted to the “estima-
tion” of the shear strength envelope for an unsaturated soil.
The estimation procedures are of significant importance to
geotechnical engineering practice. Mention is also made of
another set of shear strength equations that fall under the
category of fitting equations. In this case, it is assumed that
shear strength data with respect to soil suction is available
and one of the proposed fitting equations is then best fit
through the data points. Figure 12.1 illustrates the difference
between estimation procedures and fitting techniques.

12.2 ESTIMATION OF SHEAR STRENGTH
FUNCTIONS FOR UNSATURATED SOILS

Unsaturated soil properties take the form of nonlinear math-
ematical functions with respect to soil suction. This is true
for the shear strength properties of unsaturated soils as well