Triaxial test (CU, CD, UU)
Introduction
A triaxial shear test is a method of measuring the mechanical properties of many deformable solids, especially soils (e.g., sand, clay and rock as well as other granular materials). There are many variants of the essay.[1][2][3][4].
In a triaxial shear test, stress is applied to the material sample being tested in a manner that produces stresses along an axis other than stresses in a perpendicular direction. Typically, this is achieved by placing a specimen between two parallel plates, which apply stresses in one direction (commonly vertical) and apply fluid pressure to the specimen to apply stress in the perpendicular direction. The following discusses devices that allow the application of different levels of stress in each of the three orthogonal directions, referred to as "True Triaxial Tests."
The application of different compressive stresses in the test apparatus causes shear stress to develop in the sample. Loads can be increased and deflections monitored until specimen failure. During the test, the surrounding fluid is pressurized and the stress in the plates increases until the material in the cylinder fails and forms sliding regions within itself, known as shear bands. The geometry of the cut in a triaxial test generally causes the specimen to shorten and bulge along the sides. The tension on the roller is then reduced and the water pressure pushes the sides inward, causing the sample to increase in height again. This cycle is usually repeated several times, while stress and strain data are collected on the sample. During the test, the pore pressures of the fluids (e.g., water, oil, or gases) in the sample can be measured using the Bishop pore pressure apparatus.
From the triaxial test data, it is possible to determine the fundamental material parameters on the sample, including shear strength angles, expansion angles, and apparent cohesion. These parameters are then used in computer models to predict how the material will behave in a larger scale engineering application. An example would be predicting the stability of the soil on a slope: whether it will collapse or whether it will withstand shear stresses and remain in place. Triaxial tests are used in conjunction with other tests to determine such engineering predictions.
During cutting, a granular material will typically have a net gain or loss in volume. If it was originally dense, it usually gains volume, a characteristic known as Reynolds dilatancy. Or if it was very loose, contraction may occur prior to the start of the cut or simultaneous with the cut.
Sometimes testing of cohesive specimens is performed without confining pressure (unconfined compression test). This requires a much simpler and less expensive device. Sample preparation, although applicability is limited to samples whose sides will not crumble when exposed, and the confining stress is less than the in situ stress provides results that may be overly conservative. The compression test performed for concrete strength testing is essentially the same as that performed with apparatus designed for larger specimens and higher loads typical of concrete testing.