Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 554

The porous disks are usually replaced by metal or plastic
disks on the top and bottom of the soil specimen. The spec-
imen is enclosed in a rubber membrane during the test. The
undrained test results on unsaturated soils can be interpreted
within the context of unsaturated soil shear strength theory.

term consolidation is used to describe the process whereby
excess pore pressures due to the applied stress are allowed
to dissipate, resulting in volume change. The pore fluids are
allowed to drain following the application of the confining
pressure. On the other hand, the consolidation process will
not occur if the pore fluids are maintained in an undrained
mode. The terms “consolidated” and “unconsolidated” are
used as the first designation for categorizing triaxial tests.
The application of the axial stress is considered as the
second stage or the shearing stage in the triaxial test. Shear
stresses are generally applied to the soil specimen through
the application of a vertical compressive load. The total con-
fining pressure generally remains constant during shear. The
axial stress is continuously increased until a failure condi-
tion is reached or a limiting strain condition is achieved.
The axial stress generally becomes the total major principal
stress σ 1 in the axial direction while the isotropic confining
pressure becomes the total minor principal stress σ 3 in the
lateral direction. The total intermediate principal stress σ 2 is
equal to the total minor principal stress σ 3 (i.e., σ 2 =

11.5.7 Unconfined Compression Test
The unconfined compression test procedure is similar to the
undrained test procedure with the exception that no confin-
ing pressure is applied to the soil specimen (i.e., σ 3 =0).
The test is commonly performed in a simple loading frame
by applying an axial load to the soil specimen.

11.6 INTERPRETATION OF TRIAXIAL TEST
RESULTS

Various test procedures can be used for triaxial shear
strength testing of unsaturated soils. It is important to
interpret the meaning of the test results within the context
of unsaturated soil shear strength theory. There are basic
principles of interpretation that can be used for each of
the triaxial test procedures. The triaxial test is usually
performed on a cylindrical soil specimen that is enclosed
in a rubber membrane and placed in a triaxial cell. The
cell is filled with water and pressurized in order to apply
a constant all-around pressure or confining pressure. The
soil specimen can be subjected to an axial stress through a
loading ram in contact with the top of the soil specimen.
The application of the confining pressure is the first
stage of a triaxial test. Either the soil specimen can be
allowed to drain (i.e., consolidate) during the application of
the confining pressure or drainage can be prevented. The

σ 3 ).
Figure 11.43 illustrates the stress conditions associated
with a consolidated drained triaxial test. The pore fluid
drainage conditions during the shearing process are used as
the second designation in categorizing triaxial tests. The test
is referred to as a drained test when the pore fluid is allowed
to flow in or out of the soil specimen during shear. On the
other hand, a test is called an undrained test if the flow of
pore fluid is prevented. The pore-air and pore-water phases
can also have differing drainage conditions during shear.
Triaxial test procedures for unsaturated soils are de-
signated based upon the drainage conditions adhered to
during the first and second stages of the triaxial test. The

Figure 11.43 Stress conditions at various stages of consolidated drained triaxial compression
test.