Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 552

Fredlund (1982b). The specimen was first trimmed to the
desired diameter and height and then mounted on the
presaturated high-air-entry disk. Appropriate measurements
of the volume-mass properties of the specimen were taken
during the setup of the specimen. A coarse porous disk
and loading cap were placed on top of the specimen. The
specimen was then enclosed in two rubber membranes. The
specimen had a composite membrane consisting of two
slotted aluminum foil sheets between rubber membranes.
The purpose of the aluminum foil is to greatly minimize
air diffusion from the specimen. O-rings were placed over
the membranes on the bottom pedestal.
Spacers (i.e., consisting of pieces of 3.2-mm plastic tub-
ing) were inserted between the membranes and the loading
cap to allow air within the specimen to escape while water
was added to the sides of the specimen. The cylinder of the
triaxial cell was installed and the cell was partially filled with
water (i.e., about one-half way up the specimen). Water was
allowed access to the soil specimen either by being added
manually or by being applied through the air pressure line
connected to the loading cap.
The specimen was left for several hours to allow the dis-
tribution of water throughout the specimen. The relaxation
process was continued until air could no longer be seen
escaping from around the top of the specimen. The matric
suction in the soil was assumed to be essentially zero at the
end of the wetting process. The desired matric suction value
was then applied to the specimen. Time was allowed for
equalization to the new applied stress conditions.

the expulsion of air bubbles. The volume change measuring
devices, including the diffused air volume indicator, should
be initialized (Fredlund, 1973a).
The soil specimen should then be prepared for testing.
The initial confining air and water pressures to be applied to
the soil specimen can be set on the pressure regulators. The
confining pressure along with the air and water pressures can
be applied to the soil specimen. An initial water pressure of
30 kPa or greater is desirable in order to provide sufficient
differential pressure for flushing air from the base plate. The
diffused air volume that accumulates while performing the
test can be measured on the diffused air volume indicator.

11.5.3 Consolidated Drained Test
The initial stress state for the soil specimen is established by
applying a prescribed confining pressure σ 3 , pore-air pres-
sure u a , and pore-water pressure u w . The confining, pore-air,
and pore-water pressures are applied by opening the appro-
priate valves on the triaxial control panel in the mentioned
respective order. Other valves on the base pedestal remain
closed during the test except during the flushing of diffused
air from below the high-air-entry disk.
The vertical deflection and the radial deformation are peri-
odically monitored to measure the overall volume change of
the specimen. The volume of water flowing in or out of the
specimen is recorded on the water volume change indica-
tor. The water volume change indicator valves remain open
during a consolidated drained test, except when flushing dif-
fused air from the base plate. Air volume change is generally
not measured. Consolidation is assumed to have reached an
equilibrium condition when there is no longer a tendency
for the overall volume to change or the flow of water from
the specimen.
Upon attaining an equilibrium condition under the applied
pressures (i.e., σ 3 , u a , and u w ), the specimen is sheared by
compression at an appropriate strain rate. The magnitude
of the axial load applied to the specimen can be recorded
using a load cell. The axial load measurements are con-
verted to a deviator stress σ 1 −

11.5.1.2 Matric Suction Relaxation for Direct Shear
Te s t
A similar relaxation procedure has been used to prepare
soil specimens for direct shear testing (Gan, 1986; Escario
and Saez, 1986). The two halves of the direct shear box
are lightly sealed together using vacuum grease. The vac-
uum grease ensures that water applied to the top of the
soil specimen will flow toward the high-air-entry disk. It is
important to not smear vacuum grease onto the surface of
the high-air-entry disk.
The soil specimen is mounted into the shear box. Then
the coarse porous stone and loading cap are installed. The
initial matric suction in the soil specimen can be relaxed by
adding water to the top of the soil specimen.

σ 3 . The shearing process
is conducted under drained (or open-valve) conditions for
the applied pore-air and pore-water pressures. The overall
and water volume changes are monitored throughout the
shear process. The shearing process is terminated when the
selected failure criterion (e.g., maximum deviator stress) has
been achieved.
Diffused air is generally flushed from the base plate once
a day during both consolidation and shearing. The frequency
of the diffused air measurement depends on the applied air
pressure and the air-entry value of the ceramic disk. The
diffused air volume can be measured less frequently when
the applied air pressure is low. The water volume change
correction associated with diffused air becomes necessary
whenever tests extend over a period of several days.
The diffused air in the base plate can be flushed into the
DAVI by applying a pressure differential of 7 kPa or higher

11.5.2 Triaxial Test Procedures
Triaxial testing equipment allows flexibility in that a variety
of test procedures can be used. The triaxial cell must be
modified in accordance with using the special design con-
siderations previously explained. Several procedural checks
should be conducted prior to performing each test. The high-
air-entry disk should be saturated. Attempts should be made
to thoroughly flush water through the compartment below
the high-air-entry disk and all the connecting lines to ensure