Triaxial shear test

During the test, the surrounding fluid is pressurized, and the stress on the platens is increased until the material in the cylinder fails and forms sliding regions within itself, known as shear bands.

The stress on the platen is then reduced and the water pressure pushes the sides back in, causing the sample to grow taller again.

For soil samples, the specimen is contained in a cylindrical latex sleeve with a flat, circular metal plate or platen closing off the top and bottom ends.

The distance that the upper platen travels is measured as a function of the force required to move it, as the pressure of the surrounding water is carefully controlled.

This enables the investigation of stress paths not capable of being generated in axisymmetric triaxial test machines, which can be useful in studies of cemented sands and anisotropic soils.

Triaxial tests of classical construction had been criticized for their nonuniform stress and strain field imposed within the specimen during larger deformation amplitudes.

[7] The highly localized discontinuity within a shear zone is caused by the combination of rough end plates and specimen height.

In upgraded apparatuses the tough end plates are replaced with smooth, polished glass, with a small filter at the center.

Thus, the contact zone between sample and the end plates does not buildup unnecessary shear friction, and a linear / isotropic stress field within the specimen is sustained.

This is in contrast with classical setup, where the specimen forms a bugle in the center, while keeping a constant radius at the contact with the end plates.

Isotropic volume change is especially important for CU testing, as cavitation of pore water sets the limit of undrained sand strength.

Control of the apparatus is highly automated, thus cyclic loading can be applied with great efficiency and precision.

The combination of high automation, improved sample durability and large deformation compatibility expands the scope of triaxial testing.

The Danish triaxial can yield CD and CU sand specimens into plasticity without forming a shear rupture or bulging.

[12] This allows to control the specimens to a very high degree, and observe sand response patterns which are not accessible using classical triaxial testing methods.

Triaxial apparatus with sample attached ready for testing.
The Danish triaxial in action
Post-liquefaction testing. The fine sand specimen was liquefied during consolidates undrained (CU) cycles and recovered with consolidated drained (CD) cycles many times. The wrinkles formed due to the volume change imposed by iterating between CU liquefaction and draining. In a liquefied state the sample becomes soft enough to imprint thin latex. During CD cycles - stiff enough to preserve the imprinted pattern.