What is Triaxial Testing and is it the Best Method for Testing Soil?

Those familiar with soil testing probably already know that there are a number of ways to test the soil. One of the most common methods is the Standard Penetration Test, which is best known for its simplicity and versatility but is held back by its lack of accuracy compared to more advanced options.

More advanced methods include, of course, Cone Penetration Testing and Mud Rotary Drilling, both of which are common. Another common method is the Triaxial Testing method. 

What is Triaxial Testing?

In order to conduct Triaxial Testing, you need a Triaxial Apparatus, which is made up of a Triaxial cell, universal testing machine and pressure control panel.

For testing soil and other loose granular materials like sand and gravel, the material is placed in a cylindrical latex sleeve and submerged into a bath of water, or another liquid, which puts pressure on the sides of the cylinder.

A circular metal plate at the top of the cylinder called a platen, then squeezes the material. The distance the platen travels is measured, along with the net change in volume of the material.

Like Cone Penetration Testing, Triaxial Testing is used to measure the properties of soils, but can also be used on more solid materials like rock. Typically, Triaxial Testing is used to solve problems of stability by:

• Determining the shear strength and stiffness of soil when retaining reservoirs of water
• Measuring stress/strain behavior
• Monitoring the internal response of the particulate medium

It is also used for pore water pressure measurement and determining contractive behavior, which is common in sandy soil. As such, this soil testing method is well-suited to helping engineers improve their building designs while limiting structural/build failures by imparting a proper understanding of material behavior and an assessment of the characteristics of a build site.

Primary benefits of Triaxial Testing include its versatility over other tests, like Direct Shear Testing, as well as its ability to study the effects of fluids on mechanical properties of solids.

Additionally, Triaxial Testing is useful for determining deformations where Poisson’s ratio of the material is important.

Is Triaxial Testing the best method for soil testing?

As is usually the case, there’s no clear cut answer to whether Triaxial Testing is the best soil testing method. The fact is, it’s more basic than Cone Penetration Testing, which also offers the significant benefit of testing on-site.

However, Triaxial Testing’s ability to study the effects of fluids in a controlled environment and determine deformations makes it a solid choice in certain scenarios.

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