Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 520

CHAPTER 11

Shear Strength of Unsaturated Soils

11.1 INTRODUCTION the same classification properties and similar stress histories

can generally be considered to be the same soil and their
shear strength properties can be quantified by testing several
soil specimens.
Several soil specimens that qualify as the same soil (i.e.,
according to their classification properties, volume-mass
properties, and stress history) are required for the measure-
ment of shear strength parameters. The specimens should be
prepared at the same initial water content and with the same
compaction effort if the strength parameters for a compacted
soil are being measured. The compacted soil specimens can
then be allowed to equalize under a range of applied stress
conditions and then tested for shear strength. Conversely,
soils compacted at different water contents result in differ-
ent densities and are generally considered as different soils
(Fig. 11.1). Soils compacted to similar densities at water
contents above and below optimum water content conditions
can have quite different soil fabrics. It is anticipated that
the compacted soils from below and above optimum water
content would likely have different “peak” and possibly
“critical state” shear strength parameters. Soils compacted
above and below optimum water content conditions should
be viewed as different soils.
Two apparatuses commonly used to measure shear
strength are triaxial equipment and direct shear equipment.
The theory associated with various types of triaxial tests
and direct shear tests on unsaturated soil is presented along
with the methodology for interpreting the test results. The
laboratory test should reasonably represent the in situ and
loading conditions that are likely to occur in the field.
Various stress paths can be simulated when using triaxial
test equipment.

The safety of many engineered structures is dependent upon
the strength of the underlying soil. Bearing capacity, lateral
earth pressures, and slope stability are examples of common
geotechnical applications that depend on the shear strength
of the soil. The soils used for the construction of engineered
structures are often unsaturated soils. It is important to quan-
tify the shear strength of the unsaturated soil and to also be
able to quantify changes in shear strength that might occur
as a result of water infiltration into the soil.
The general principles associated with the shear strength
of a saturated soil also apply to unsaturated soils. For
example, the strength of a soil changes when the stress state
of a soil is changed. Also, concepts of dilation and changes
in strength with strain are similar for both saturated and
unsaturated soils. Shear strength can be related to the stress
state of the soil. In the case of an unsaturated soil the stress
state can be described in terms of the net normal stresses

σ

u w .
The term matric suction is generally used when describing
the shear strength of an unsaturated soil; however, the term
soil suction could also be applied since the shear strength
concepts are of importance over the entire range of suction
values. The influence of soil suction on shear strength in the
high-suction range may be significantly reduced; however,
general concepts pertaining to the influence of soil suction
are important to understand.
The shear strength of an unsaturated soil is presented both
in an analytical and a graphical manner. The presentations of
laboratory test procedures assist in visualizing the changes
that occur when going from unsaturated to saturated con-
ditions and vice versa. The possibility of nonlinearity and
hysteresis in the description of shear strength is also taken
into consideration.
Each soil sampled from in situ conditions has an inherent
stress history. The stress history has an influence on the
soil fabric or soil structure. The geotechnical engineer must
discern when soils can be assumed to be the “same soil” and
when they must be considered as “different” soils. Soils with

u a and the matric suction u a −

−

11.2 THEORY OF SHEAR STRENGTH

Theories of shear strength for an unsaturated soil have been
proposed as extensions to the concepts and mathematical
equations that have been used for shear strength theories for
saturated soils. The shear strength section commences with