Tri-axial test

A tri-axial test is one of the common methods to measure the mechanical properties of soil and rocks under different stress conditions. It is one of the most commonly used tests for assessing the shear strength and stress-strain behavior of soil. It determines the two important shear parameters of any soil: cohesion (c) and friction angle (φ). These parameters are used for analyzing the stability of slopes, designing retaining walls, assessing the soil’s resistance to sliding and its ability to support loads.
In terms of sample preparation and drainage condition, the test can be carried out for Unconsolidated-Undrained (UU) and Consolidated-Undrained (CU) soil samples. In UU test, the soil sample is prepared and tested without any prior consolidation i.e. in its unconsolidated state. During testing, the confining pressure represents the lateral stress acting on the sample but remains constant throughout the test. Drainage is not allowed, meaning excess pore water pressure generated during the test is not allowed to dissipate, simulating undrained condition. As a result, UU test helps in determining shear parameters of soil under undrained loading condition.
Whereas, in CU test, the soil sample is confined under pressure to simulate the stress history experienced by the soil in the field. The additional axial stress is applied after than which is increased gradually to achieve the desired consolidation state. Here, drainage is not provided as well, meaning excess pore water pressure doesn’t dissipate. However, prior consolidation allows the little dissipation. This test helps to assess the undrained shear strength and understand the soil’s response to additional loading after a consolidation process.
Tri-axial test can also be performed in soil samples under drainage condition where the excess pore water pressure is allowed to dissipate during the application of axial stress. This test carries similar significance as mentioned before and it is typically performed on granular soils or soils where drainage can occur.