Triaxial Compression Test - Soil Properties: Testing, Measurement, and Evaluation - page 342

scarified prior to the addition of material for the next layer. The
tamper used to compact the material shall have diameter equal to
or less than one-half the diameter of the mold. After a specimen is
formed, with the ends perpendicular to the longitudinal axis,
remove the mold and determine the mass and dimensions of the
specimen. Perform one or more water content determinations on
excess material used to prepare the specimen in accordance with
ASTM Test Method D 2216.

Note 2 —It is common for the unit weight of the specimen after
removal from the mold to be less than the value based on the
volume of the mold. This occurs as a result of the specimen
swelling after removal of the lateral confinement due to the
mold.

PART I. UNCONSOLIDATED UNDRAINED (UU) TEST

PROCEDURE To carry out an unconsolidated undrained (UU) triaxial compression

test, a cylindrical soil specimen is encased in a rubber membrane and
placed in the triaxial chamber, and a specific (and constant) lateral
(all-around) pressure is applied by means of water or compressed air
within the chamber without allowing the specimen to consolidate
(drain) under the lateral pressure. A vertical (axial) load is then
applied externally and fairly rapidly without allowing drainage of the
specimen and is steadily increased until the specimen fails. The ex-
ternally applied axial load that causes specimen failure, and the lat-
eral pressure are recorded. The soil specimen is then removed and
discarded, and another specimen of the same soil sample is placed in
the triaxial chamber. The procedure is repeated for the new specimen
for a different (either higher or lower) lateral pressure. The axial load
at failure and the lateral pressure are recorded for the second test.
The entire procedure is usually repeated for two or more other speci-
mens of the same soil, each with a different lateral pressure.
The actual step-by-step procedure is as follows (ASTM D 2850-03 [3]):

(1) Place the membrane on the membrane expander or if it is to
be rolled onto the specimen, place the membrane onto the cap or
base. Place the specimen on the base. Place the rubber membrane
around the specimen and seal it at the cap and base with O-rings
or other positive seals at each end. A thin coating of silicon grease
on the vertical surfaces of the cap or base will aid in sealing the
membrane.

(2) With the specimen encased in the rubber membrane, which is
sealed to the specimen cap and base and positioned in the chamber,
assemble the triaxial chamber. Bring the axial load piston into con-
tact with the specimen cap several times to permit proper seating
and alignment of the piston with the cap. When the piston is