There are three fundamental triaxial tests that can be conducted in the laboratory, each allowing the soil response for differing engineering applications to be observed:
- Unconsolidated Undrained test (UU), also called quick triaxial
- Consolidated Undrained test (CU)
- Consolidated Drained test (CD)
Unconsolidated Undrained (UU)
UU triaxial test is also called total stress test since effective stresses in the specimen are not known. With this method, the shear strength is measured in terms of total stress. This triaxial test has a big advantage: there is no saturation stage and stress is applied quickly (without pore water drainage), so it can be completed in less than half an hour.
Tests are often carried out on three specimens obtained from the same sample, each subjected to a different confining pressure. Provided that the soil is fully saturated, the shear strength will be the same for each test and is known as “undrained shear strength”.
Consolidated Undrained (CU)
CU/CD triaxial tests are also called effective stress tests. Here, the shear strength is measured in terms of effective stress. The specimen is saturated and allowed to consolidate (soil consolidation is the process that changes its structure and water content) at the required confining pressure. The aim here is to bring the specimen as close as possible to conditions at its natural state in the ground. Once consolidated, the specimen is subjected to a controlled application of load, during which no drainage is allowed so that pore pressure can be measured.
Since the shear strength is affected by the effective stresses, by testing a set of three specimens at different confining pressures, it is possible to define the failure envelope according to Coulomb’s model and define the parameters c’ and φ’, as well as evaluating the undrained shear strength.
Consolidated Drained (CD)
With this method, the shear strength is measured in terms of effective stress. The specimen is saturated and allowed to consolidate at the required confining pressure. Once consolidated, the specimen is subjected to a controlled application of load, during which moisture is allowed to drain without the possibility of pore pressure building up.
Since the shear strength is affected by the effective stresses, by testing a set of three specimens at different confining pressures, it is possible to define the failure envelope according to Coulomb’s model and define the parameters c’ and φ’.
It is common for UU triaxial tests to be used for short-term engineering problems and CU and CD triaxial to assess long-term engineering problems.
