Position the specimen in the chamber and assemble the triaxial chamber
6.2 Bring the axial load piston into contact with the specimen cap several times to permit
proper seating and alignment of the piston with the cap.
6.2.1 During this procedure, take care not to apply a deviator stress to the specimen exceeding
0.5% of the estimated compressive strength.
6.2.2 If the weight of the piston is sufficient to apply a deviator stress to the specimen
exceeding 0.5% of the estimated compressive strength: the piston should be locked in
place above the specimen cap after checking the seating and alignment; and left locked
until application of the chamber pressure.
6.3 Place the chamber in position in the axial loading device.
6.4 Carefully 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.
6.5 Attach the pressure-maintaining and measurement device.
6.6 Fill the chamber with the confining fluid to a predetermined level.
6.7 Adjust the pressure-maintaining and measurement device to the desired chamber pressure
and apply pressure to the chamber fluid.
6.7.1 If the axial load-measuring device is located outside the triaxial chamber, the chamber
will produce an upward force on the piston that will react against the axial loading
device. In this case, start the test with piston slightly above the specimen cap, and before
the piston comes in contact with the specimen cap, measure and record the initial piston
friction and upward thrust of the piston produced by the chamber pressure. Later correct
the measured axial load, or adjust the axial load-measuring device to compensate for the
friction and thrust.
6.7.2 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 loading device or for
piston friction.
6.7.3 In either case, record the initial reading on the deformation indicator when the piston
contacts the specimen cap.
6.8 Approximately 10 minutes after the application of chamber pressure (see Note 1) begin to
apply the axial load to produce axial strain at a rate of approximately:
1%/minute for plastic materials
0.3%/minute for brittle materials that achieve maximum deviator stress at
approximately 3–6% strain.
6.8.1 At these rates, the elapsed time to reach maximum deviator stress will be approximately
15–20 minutes.
Note 1—The purpose of this time interval is to allow the specimen to stabilize under the
chamber pressure prior to application of the load.
6.9 Record load and deformation values at approximately 0.1, 0.2, 0.3, 0.4, and 0.5% strain,
and at increments of about 0.5% strain, then to 3%; and thereafter at every 1%, except
that the load and deformation may be recorded at 2% increments of strain for strains
greater than 10%. Take sufficient readings to define the stress-strain curve; hence, more
frequent readings may be required in the early stages of the test and as failure is
approached.
6.10 Continue the loading to 15% strain except:
Loading may be stopped when the deviator stress has peaked and dropped 20% or
When 5% axial strain occurs after a peak in deviator stress.