Triaxial Compression Test - Soil Properties: Testing, Measurement, and Evaluation - page 339

The unconfined compression test (Chapter 21) is a special case of
the triaxial compression test in which the confining pressure is zero.
Note that triaxial tests can be applied to both cohesive and cohesionless
soils, whereas unconfined compression tests are limited more or less to
cohesive soils.
There are three basic types of triaxial compression test procedures
as determined by sample drainage conditions: unconsolidated undrained,
consolidated undrained, and consolidated drained. These can be defined
as follows [2].
Unconsolidated undrained (UU) tests are carried out by placing
a specimen in the chamber and introducing lateral pressure without
allowing the specimen to consolidate (drain) under the confining pres-
sure. Axial load is then applied fairly rapidly without permitting
drainage of the specimen. The UU test can be run rather quickly because
the specimen is not required to consolidate under the confining pressure
or drain during application of the axial load. Because of the short time
required to run this test, it is often referred to as the quick, or Q, test.
Consolidated undrained (CU) tests are performed by placing a
specimen in the chamber and introducing lateral pressure. The speci-
men is then allowed to consolidate under the all-around confining pres-
sure by leaving the drain lines open (see Figures 22-1 and 22-2). The
drain lines are then closed, and axial stress is induced without allowing
further drainage.
Consolidated drained (CD) tests are similar to CU tests, except that
the specimen is allowed to drain as axial load is applied, so that high
excess pore pressures do not develop. Because the permeability of clayey
soils is low, axial load must be added very slowly during CD tests so that
pore pressure can be dissipated. CD tests may take a considerable
period of time to run because of the time required for both consolidation
under the confining pressure and drainage during application of axial
load. Inasmuch as the time requirement is long for low-permeability
soils, this test is often referred to as the slow, or S, test.
The specific type of triaxial compression test (i.e., UU, CU, or CD)
that should be used in any particular case depends on the field condi-
tions to be simulated. The UU and CU tests are covered in this chapter.

Axial loading device (see Figure 22-3)
Axial load-measuring device
Triaxial chamber (Figures 22-1 and 22-2)
Specimen mold, cap, and base
Rubber membranes, membrane stretcher, rubber binding strips
(Figure 22-4)
Deformation indicator
Calipers
Vacuum pump
Sample ejector (for undisturbed, thin-wall-tube sample)

APPARATUS
AND SUPPLIES