Triaxialtests

Triaxial tests are standard laboratory experiments in geotechnical engineering used to determine the mechanical behavior of soils by applying confining pressure around a cylindrical specimen and imposing axial loading. They provide measurements of shear strength, stiffness, and deformation under controlled drainage and pressure conditions and can be performed on sands, clays, and other geomaterials.

The tests are conducted in several drainage configurations. In unconsolidated-undrained (UU) tests the specimen is not pre-consolidated and drainage is prevented, yielding undrained shear strength su. In consolidated-undrained (CU) tests the sample is isotropically consolidated to a confining pressure before shearing under undrained conditions, allowing measurement of pore pressure and the undrained response. In consolidated-drained (CD) tests the specimen is consolidated and then sheared with drainage, yielding drained strength parameters c′ and φ′.

Procedure: A saturated cylindrical soil specimen is placed in a triaxial cell, confining pressure is applied around the specimen, and consolidation is performed or not depending on the test type. Axial loading is then applied while monitoring axial load, cell pressure, and pore-water pressure. In CD tests drainage is allowed so pore pressure remains low; in CU and UU tests drainage is blocked and pore pressure rises.

Interpretation: The results are plotted on Mohr circles or Mohr-Coulomb envelopes. In drained tests the shear strength parameters c′ and φ′ are obtained from the envelope of shear strength versus effective normal stress; in undrained tests the undrained shear strength su is determined from the failure state on the Mohr circle. Pore pressure response and Skempton’s B may also be evaluated.

Applications and limitations: Triaxial tests inform foundation design, slope stability, and earth-dam analysis by providing strength and deformation data. Limitations include sample disturbance, scale effects, drainage control, and rate sensitivity. Advanced variants study cyclic loading, anisotropic consolidation, or strain-rate effects to simulate field conditions.