Triaxial Compression Test - Soil Properties: Testing, Measurement, and Evaluation - page 352

PART II. CONSOLIDATED UNDRAINED (CU) TEST

The consolidated undrained (CU) triaxial compression test is performed
by placing a saturated specimen in the triaxial chamber, introducing
lateral (confining) pressure, and allowing the specimen to consolidate
under the lateral pressure by leaving the drain lines open. The drain
lines are then closed, and axial load is applied at a fairly rapid rate
without allowing further drainage. With no drainage during axial load ap-
plication, a buildup of excess pore pressure will result. The pore pressure

PROCEDURE

during the test must be measured to obta in the effective stress needed
to plot the Mohr circle. (The effectiv e stress

σ equals the total pressure σ

minus the pore pressure —that is, .) The pore pressure can be

determined using a pressure-measuring device connected to the drain
lines at each end of the specimen.
A step-by-step procedure is as follows:

σ σ μ

(1) With the saturated specimen encased in a rubber membrane
and inside the triaxial chamber, assemble the triaxial chamber in
the axial loading device. Carefully adjust the piston of the cell
so that it just makes contact with the top platen of the specimen.
Fill the chamber with fluid and apply lateral pressure (s3).

(2) With the connecting drain lines attached to the graduated
burette, open both top and bottom drainage valves. This allows
the specimen to consolidate under the applied lateral pressure.
Completion of consolidation is indicated when the water level in
the burette stabilizes. Determine the volume of water extruded
from the specimen during consolidation ( V ) by measuring the

volume of water accrued in the burette at the completion of
consolidation.

Δ

(3) Connect the drain lines to the pore pressure measurement de-
vice. Bring the piston into contact with the upper platen, and set
both the proving ring dial and the deformation dial to zero. Apply
axial load to produce axial strain at the rate of approximately
1%/min. As axial load is applied, the specimen's pore pressure will
increase. Record proving ring dial readings, deformation dial read-
ings, and pore water pressure measurements at about 0.1, 0.2, 0.3,
0.4, and 0.5% strain. After 0.5% strain, record at every increment
of 0.5% strain up to 3% strain and thereafter at every 1% strain.
Continue loading to 15% axial strain, except that loading may be
stopped when the deviator stress has peaked and then dropped
20%, or when the axial strain has reached 5% beyond the strain at
which the peak in deviator stress occurred.

(4) Upon completion of the test, stop the compression and release
the axial load. Remove the test specimen from the chamber, and
make a sketch or take a photograph of the test specimen at failure.
Record the angle of failure surface. Use the entire specimen to
determine its water content (see Chapter 4).