Shear Strength of Unsaturated Soils - Unsaturated Soil Mechanics in Engineering Practice - page 553

between the base plate and the diffused air volume indicator.
Each apparatus needs to be tested to assess the differential
pressure at which diffused air can readily be removed from
the base compartment. If the applied differential pressure is
too high, the water from below the compartment may tend
to cavitate and form small air bubbles. It is also desirable to
not significantly alter the water pressure in the base plate.
It may be necessary to elevate the air backpressure in the
diffused air volume indicator during the flushing process.
The valves on the water volume change indicator are
closed during the flushing process so that the water volume
change indicator is bypassed. Diffused air can be flushed
from the base plate using a series of water surges through
the compartment below the high-air-entry disk. Diffused
air moves into the diffused air volume indicator and dis-
places the water in the burette. A few seconds may be
required between each surge. The water pressure in the base
plate only deviates momentarily from its set value when
using this procedure. The valves to the water volume change
indicator are opened while the valves associated with the
flushing process are closed after the diffused air volume has
been measured.
The procedure involving consolidation and shearing is
repeated during each stage of a multistage test. The consol-
idation for each stage should preferably be performed with
the deviator stress reduced to zero. The deviator stress can
be brought to zero by release of the axial load. The shearing
process for each stage of a multistage test should be stopped
when the maximum deviator stress is imminent, except for
the last stage, where the specimen can be sheared to a larger
strain.

line to the base plate is opened, there will be a quick equal-
ization of the water pressure in the compartment below the
high-air-entry disk.
Diffused air is removed from the base plate by momen-
tarily opening the water pressure valve, which produces a
pressure difference across the base plate. The valves to the
diffused air volume indicator are closed at the end of the
diffused air volume measurement. The undrained pore-water
pressure is then returned to the value existing prior to the
flushing process. Disturbance to the undrained condition of
the soil specimen should be minimal if the flushing time
is short.

11.5.5 Consolidated Undrained Test with Pore
Pressure Measurements
The soil specimen is consolidated using the procedure
described for the consolidated drained test. After equilib-
rium conditions have been established under the applied
pressures (i.e., σ 3 , u a , and u w ), the soil specimen is sheared
under undrained conditions with respect to the air and water
phases. Undrained conditions during shear are achieved by
closing valves on the pore-air line and the pore-water line.
The pore-water pressure developed during shear can be
measured on the pressure transducer mounted on the base
plate. A pore-air pressure transducer should be mounted
on the loading cap, if possible, when measuring pore-air
pressure changes. It may be somewhat difficult to maintain
undrained air conditions due to air diffusion.
The diffused air volume can be measured during the
constant-water-content test; however, there is no way of cor-
recting the measured pore-water pressures for air diffusion.
Difficulties associated with air diffusion may be the main
reason why few consolidated undrained tests with pore-air
and pore-water pressure measurements have been per-
formed. Thicker high-air-entry disks help to reduce the effect
of air diffusion.

11.5.4 Constant-Water-Content Test
The initial consolidation process is carried out in the same
manner for the constant-water-content test as for the con-
solidated drain test. When equilibrium conditions have been
achieved under the applied pressures (i.e., σ 3 , u a , and u w ),
the soil specimen is sheared under drained conditions for
the pore-air phase and undrained conditions for the pore-
water phase. The pore-air pressure is maintained at the value
applied during consolidation. The pore-air pressure valve
remains open during consolidation and shear. On the other
hand, the pore-water pressure valve remains closed during
shear in order to produce undrained pore-water conditions.
The pore-water pressure is measured using a pressure trans-
ducer mounted near the base plate of the triaxial cell.
Diffused air volume should also be measured during shear
under undrained water phase conditions. The water pressure
in the base plate should be recorded prior to the flushing pro-
cess and reset to the same value after flushing. The water in
the pore-water pressure control line should first be subjected
to the same pressure as recorded in the base plate. The air
backpressure in the diffused air volume indicator should be
adjusted to a pressure slightly lower than the water pressure
recorded in the base plate. When the valve on the water

11.5.6 Undrained Test
The procedure for performing an undrained test on an unsat-
urated soil specimen is similar to the procedure used when
performing an undrained test on a saturated soil specimen.
The unsaturated soil specimen is tested at its initial water
content or matric suction. In other words, the initial matric
suction in the specimen is not relaxed or changed prior to
performing the shear strength test.
There is no consolidation process allowed since the con-
fining pressure σ 3 is applied under undrained conditions for
both the pore-air and pore-water phases. The specimen is
axially compressed under undrained conditions with respect
to both the air and water phases. The undrained test is
usually run at a strain rate between 0.017 and 0.03% per
second, and no attempt is made to measure the pore-air and
pore-water pressures. Conventional triaxial equipment can
be used to perform the undrained test on unsaturated soils.