Environmental Engineering Reference
In-Depth Information
Figure 11.59 Modified direct shear apparatus for testing unsaturated soils (from Gan and Fred-
lund, 1988).
relative to the upper portion of the box. The test procedure is
similar to that used for a conventional direct shear apparatus.
A motor that provides a constant horizontal shear displace-
ment rate is connected to the shear box base. The shear box
is seated on a pair of rollers that can move along grooved
tracks on the chamber base. The top box is connected to a
load cell which measures the shear load resistance. Vacuum
grease is placed between the two halves of the shear box
prior to mounting the soil specimen in the shear box.
The plumbing layout for the control board of the modified
direct shear apparatus is similar to that used for a triaxial
test; however, it is difficult to measure water volume changes
because of the small water volumes involved. The satura-
tion of the high-air-entry disk and the flushing of entrapped
air from the base plate and its connecting lines should be
performed prior to commencing the test. The initial air and
water pressures to be applied to the soil specimen can be set
on the pressure regulators while the soil specimen is being
assembled.
The tests are generally performed as consolidated drained
direct shear tests. The soil specimen is usually given access
to water after being placed into the direct shear box. The
chamber cap is applied following saturation of the soil. The
predetermined vertical normal load, air pressure, and water
pressure are then applied to the specimen in this sequence.
The vertical normal load is applied through the loading ram
at the same time as the air and water pressures are applied.
It is important to ensure there are no leaks in the system. For
example, the leakage of air from the chamber surrounding
the soil specimen will cause a continuous water vapor loss
from the specimen. The water pressure applied at the base
plate is measured using a pressure transducer. Measurements
of vertical deflection and water movement from the speci-
men can be taken at various time increments. Consolidation
takes place under the applied vertical normal stress, the air
pressure, and the water pressure. Equilibrium is assumed to
be complete when there is no further tendency for overall
volume change and water volume change.
The soil specimen is sheared at an appropriate horizontal
shear displacement rate after equilibration has been reached.
The horizontal shear load resistance is measured using a load
cell. Readings are also taken of vertical deflection, horizontal
shear displacement, and water volume change during shear.
Shearing can be terminated either when the horizontal shear
stress resistance has reached its peak value or when the hor-
izontal shear displacement has reached a designated limiting
value. In the case of a multistage test, the shearing process
for each stage should be stopped when the peak horizontal
shear stress appears to be imminent.
11.7.2 Interpretation of Direct Shear Test Results
A soil specimen is placed in the direct shear box and con-
solidated under a vertical normal stress σ. The pore-air and
pore-water pressures must be controlled at selected pres-
sures during consolidation. The axis translation technique
can be used to impose matric suctions greater than 1 atm.
The direct shear test can be conducted in an air-pressurized
chamber in order to elevate the pore-air pressure to a mag-
nitude above atmospheric pressure. The pore-water pres-
sure can be controlled below the soil specimen using a
 
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