Triaxial Test
The triaxial compression test is a laboratory test used to evaluate the undrained strength of soils under different drainage and consolidation conditions. In the test, the sample is loaded axially while confined by a rubber membrane, which allows for control of drainage conditions. The test provides positive control of drainage conditions and the capability to assess the effect of consolidation pressure on the undrained strength.
The test is conducted on an undisturbed cylindrical sample, which is confined by a rubber membrane in a triaxial cell. The sample may be subjected to a UU (unconsolidated-undrained) test, where the drain valves remain closed throughout the test, or a CU (consolidated-undrained) test, where the sample is allowed to consolidate under the chamber pressure before the undrained axial loading is started.
The undrained shear strength (su) is defined as one-half of the peak value of the principal stress difference (deviator stress) (P1 - P3). For UU tests, the axial loading is applied at a strain rate of approximately 1 percent/min. For CU tests, the slower strain rates are suggested to more closely approximate field rates of loading.
The UU test provides a measure of Su at the in situ water content of the sample, but the value of Su often underestimates the in situ su due to disturbance and stress relief effects associated with sampling and testing. Conversely, CU tests conducted at the in situ effective overburden pressure may overestimate the in situ undrained strength of normally and lightly overconsolidated clays.
CU triaxial tests can also be used to evaluate the relation of undrained strength to consolidation pressure by conducting a series of tests at several consolidation pressures in excess of the in situ effective overburden pressure of the sample. These tests estimate the increase of Su with increasing consolidation pressure and decreasing water content. These tests are also used in conjunction with the normalized analysis of Bur to evaluate soil strength parameters.