Civil and Building Engineering.

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Figure 3.36 Photograph of a triaxial cell.

covering the sides of the specimen because it is flexible; thus, the exterior vertical surface of the specimen is a principal surface. If the vertical surfaces of the specimen are principal surfaces, then any horizontal plane through the specimen is also a principal surface. When a compressive load is applied through the loading piston, the vertical stress acting on horizontal planes is the maximum principal stress ( 1) and the horizontal stress acting on vertical planes is the minimum principal stress ( 3). In the triaxial test, the intermediate principal stress ( 2) is equal to the minor principal stress. The axial stress applied to the soil specimen by the loading piston is ( 1 3), and this quantity is called the principal stress difference.

If the loading piston is attached to the top cap, it is possible to apply a tensile load to the specimen and make the horizontal stress the major principal stress. This makes all vertical planes the major principal planes, with the intermediate principal stress equal to the major principal stress. Tests per- formed in this manner are called extension tests. In extension tests, the stress on horizontal planes is still compressive; an extension test is not a tensile test.

Triaxial tests are performed in two or more stages. In the first stage, the specimen is subjected to an initial state of stress. This stress may be hydrostatic (also called isotropic) or may be made to simulate the in situ state of stress by using different values for the vertical and radial stresses (this state is called anisotropic). For simplicity in the following discussion, the assumption is made that the initial state of stress is hydrostatic.

The triaxial test specimen may be allowed to consolidate after the confining stress is applied. If consolidation is permitted, multiple stages of consolidation pressures may be used. The specimen is sheared in the final stage and may or may not be allowed to drain during shearing. If the specimen is allowed to drain, it is possible to measure any volume change in the test specimen during the shearing stage.

There are three possible types of triaxial tests, depending on the combination of drainage conditions during the application of confining stresses and during shear. The three types of tests are consolidated-drained, consolidatedundrained, and unconsolidated-undrained. Short descriptions of these types of triaxial tests follow.

Consolidated-Drained Triaxial Tests In this type of test, the soil specimen is allowed to consolidate completely under the initial state of stress prior to initiating shear. During shearing, either the axial deformation is applied so slowly that the porewater pressures have time to dissipate (the drains are left open) or else the axial stress is increased in small increments and each increment of pressure is maintained constant until the porewater pressure has dissipated. The amount of time required to shear the soil while allowing for
dissipation of porewater pressures is determined from the consolidation properties of the test specimen. Common names for this type of test include consolidated-drained (CD-test), drained (D-test), and slow (S-test).
The amount of time required to complete a consolidated-drained test is long compared to that of the other two types of triaxial tests.

Consolidated-Undrained Triaxial Tests In this type of test, the soil specimen is again allowed to consolidate fully under the initial state of stress in manner similar to that used in the consolidated-drained test. During shear, however, the drainage lines are closed and the specimen is sheared to failure under undrained conditions. This test is commonly referred to as a consolidated-undrained test (CU-test), a consolidated-quick test (CQ-test), or an R-test.
There are two varieties of consolidated-undrained tests. In one type, the specimen is sheared quickly in about 10 to 15 minutes. In the second type, the porewater pressures developed in the soil specimen are measured using porewater pressure transducers connected to drainage lines from the specimen.
When the porewater pressures are measured, it is necessary to shear the specimen slowly enough for the porewater pressures to equilibrate throughout the test specimen. The additional time required for shearing is determined from the time needed to complete consolidation of the test specimen prior to shearing. While the time required for porewater pressure equalization in the specimen is often on the order of several hours, the testing time is usually much shorter than the time required to complete shearing in the consolidated-
drained test.

Consolidated-Undrained Triaxial Tests In this type of test, the soil specimen is again allowed to consolidate fully under the initial state of stress in manner similar to that used in the consolidated-drained test. During shear, however, the drainage lines are closed and the specimen is sheared to failure under undrained conditions. This test is commonly referred to as a consolidated-undrained test (CU-test), a consolidated-quick test (CQ-test), or an R-test.
There are two varieties of consolidated-undrained tests. In one type, the specimen is sheared quickly in about 10 to 15 minutes. In the second type, the porewater pressures developed in the soil specimen are measured using porewater pressure transducers connected to drainage lines from the specimen.
When the porewater pressures are measured, it is necessary to shear the specimen slowly enough for the porewater pressures to equilibrate throughout the test specimen. The additional time required for shearing is determined from the time needed to complete consolidation of the test specimen prior to shearing. While the time required for porewater pressure equalization in the specimen is often on the order of several hours, the testing time is usually much shorter than the time required to complete shearing in the consolidateddrained test.
Unconsolidated-Undrained Triaxial Tests In this type of test, the soil specimen is not allowed to consolidate under the initial state of stress or to drain during shear. This test is commonly called an unconsolidated-undrained test (UU-test)ora quick test (Q-test).
Unconsolidated-drained tests cannot be performed because consolidation would occur whenever the drained specimens were opened during the shearing stage.

Test Nomenclature. The above notations were developed at various institutions at various times. The quick versus slow type designation was developed at Harvard University during the 1930s, when quick tests meant that the specimen was sheared too quickly for the porewater to get out even though the drainage lines were open throughout the test. Drained test were performed slowly, so they were called slow tests. Actually, drained tests can be performed quite rapidly if the soil has high permeability (hydraulic conductivity), and undrained tests can be performed very slowly—for example, to study the creep-deformation properties at constant water content—so the quick versus slow designation soon became meaningless, though still widely used. Between World War II and the early 1960s, much of the important research on the shearing properties of soils was performed at the Imperial College of Science and Technology and at the Norwegian Geotechnical Institute, where the more descriptive terms—drained versus undrained and consolidated versus unconsolidated—were developed. These latter terms have a disadvantage in conversation since the prefix un- may be either slurred, and thus missed, or else emphasized so much that the sentence is disrupted. Confusion in spoken discussion can be avoided by using the terms proposed by Casagrande (1960):