Guidelines to Follow Before Triaxial Test

Triaxial testing or triaxial shear testing is a method of measuring the mechanical properties of different types of deformable solids, particularly, soil, rock, and granular materials. Several variations of the test exist and proper method must be determined according to the type of material being tested and its application.

In a triaxial test, stress is applied to a material sample. Unlike a typical stress test, however, stress is applied from different perpendicular directions. This is achieved by placing the material sample between parallel platens, effectively applying stress in one direction, and then applying fluid pressure in perpendicular directions. When the test apparatus applies compressive stresses on the specimen, it causes shear stress to develop within the sample. Loads can also be increased, while deflections are being monitored until the sample begins to fail.

During a triaxial test, the surrounding fluid is pressurized. Stress on the platens is also increased until the material fails and forms sliding regions referred to as shear bands. The shape and geometry of the shearing causes the sample to shorten while bulging out on the sides. When the stress on the platen is reduced and the water pressure compresses the sides back in, the sample grows taller again. The same cycle is repeated several times while the tester collects strain and stress data on the sample.

During test execution, specimens, particularly soil samples are contained in a cylindrical latex sleeve that has a flat, circular metal plate on both the top and bottom ends. The cylinder is then placed into a hydraulic fluid bath, which provides pressure around the cylinder. The top platen is mechanically driven up or down long the axis to squeeze the material in. The distance the top platen travels is recorded as a function of the force needed to move it, as the water pressure surrounding the cylinder is carefully controlled.

When testing high-strength rock, the sleeve used is often a thin metal sheeting instead of latex. Triaxial testing to failure on hard rock is seldom done because of the extremely high forces and pressures needed to break the sample. Measuring effective stress on a material sample can be done by using one porous platen surface and then measuring fluid pressure. Effective stress can be calculated from the total pore and stress pressure.

There are many different types of triaxial test, depending on the type of information needed. The most common variations include Consolidated Drained or CD test, Consolidated Undrained or CU test, and Unconsolidated Undrained or UU test.

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