Triaxial Compression Test - Soil Properties: Testing, Measurement, and Evaluation - page 343

brought into contact the final time, record the reading on the
deformation indicator to three significant digits. During this proce-
dure, take care not to apply an axial stress to the specimen exceed-
ing approximately 0.5% of the estimated compressive strength. If
the weight of the piston is sufficient to apply an axial stress
exceeding approximately 0.5% of the estimated compressive
strength, lock the piston in place above the specimen cap after
checking the seating and alignment and keep locked until applica-
tion of the chamber pressure.
(3) Place the chamber in position in the axial loading device. Be care-
ful to align the axial loading device, the axial load-measuring device,
and the triaxial chamber to prevent the application of a lateral force
to the piston during testing. Attach the pressure-maintaining and
measurement device and fill the chamber with the confining liquid.
Adjust the pressure-maintaining and measurement device to the de-
sired chamber pressure and apply the pressure to the chamber fluid.
Wait approximately 10 min after the application of chamber pres-
sure to allow the specimen to stabilize under the chamber pressure
prior to application of the axial load.
Note 1 —In some cases the chamber will be filled and the
chamber pressure applied before placement in the axial
loading device.
Note 2 —Make sure the piston is locked or held in place
by the axial loading device before applying the chamber
pressure.
Note 3 —The waiting period may need to be increased for soft
or partially saturated soils.
(4) If the axial load-measuring device is located outside of the tri-
axial chamber, the chamber pressure will produce an upward
force on the piston that will react against the axial loading device.
In this case, start the test with the piston slightly above the spec-
imen cap, and before the piston comes in contact with the speci-
men cap, either (1) measure and record the initial piston friction
to three significant digits and upward thrust of the piston pro-
duced by the chamber pressure and later correct the measured
axial load, or (2) adjust the axial load-measuring device to com-
pensate for the friction and thrust. If the axial load-measuring
device is located inside the chamber, it will not be necessary to
correct or compensate for the uplift force acting on the axial load-
ing device or for piston friction. In both cases record the initial
reading on the deformation indicator when the piston contacts the
specimen cap.
(5) Apply the axial load to produce axial strain at a rate of approx-
imately 1%/min for plastic materials and 0.3%/min for brittle
materials that achieve maximum deviator stress at approximately
3 to 6% strain. At these rates, the elapsed time to reach maximum