The triaxial compression test is a more sophisticated test procedure for determining the shear strength of soil because we can measure both drained as well as undrained shear strength.

Generally 1.4 inch diameter (3 inch tall) or 2.8 inch diameter (6 inch tall) specimen is used. Specimen is encased by a thin rubber membrane and set inside a plastic cylindrical chamber. Cell pressure is applied in the chamber (which represents σ₃) by pressurizing the cell fluid (generally water). Vertical stress is increased by loading the specimen (by raising the platen in strain controlled test and by adding loads directly in stress controlled test, but strain controlled test is more common) until shear failure occurs. Total vertical stress (σ₁) is equal to the sum of σ₃ and deviator stress (σ_d). Measurement of σ_d, axial deformation, pore pressure and sample volume change are recorded.

Depending on the nature of loading and drainage condition, triaxial tests are conducted in three different ways.

1. UU Triaxial test
   

   For the UU test, the specimens (assumed to be saturated prior to test) are subjected to a confining fluid pressure in a triaxial chamber. In this test, the loads are applied quickly, and the sample is not allowed to consolidate during the test. The sample is compressed at a constant rate (strain-controlled).

2. CU Triaxial test
   

   In a consolidated undrained test the sample is not allowed to drain. The shear characteristics are measured under undrained conditions and the sample is assumed to be fully saturated. Measuring the pore pressures in the sample (sometimes called CUpp) allows approximating the consolidated-drained strength.

3. CD Triaxial test
   

   In a consolidated drained test, the sample is consolidated and sheared in compression slowly to allow the pore pressures built up by the shearing to dissipate. The rate of axial deformation is kept constant, i.e, strain is controlled. The idea is that the test allows the sample and the pore pressures to fully consolidate (i.e, adjust) to the surrounding stresses. The test may take a long time to allow the sample to adjust, in particular low permeability samples need a long time to drain and adjust strain to stress levels.

The triaxial test gives shear strength of soil at different confining stresses. Shear strength is important in all types of geotechnical designs and analyses.