What Is a Triaxial Test? | Different Types of Triaxial Test

Triaxial (shear) tests are an excellent way of measuring the mechanical properties of soil, rock and granular materials, with results used to solve a wide range of geotechnical engineering problems.

In this guide, we’ll first explore what the triaxial shear test is and how it’s carried out, then look at the different types of triaxial test and finally how the results can be used in geotechnical engineering.

Use the links below to jump to the section you’re most interested in:

• What is a triaxial test?
• Triaxial shear test procedure
• The types of triaxial test
• How triaxial test results are used
• Triaxial testing of geogrid-stabilised materials

What is a triaxial test?

A triaxial test is a type of shear test that measures the mechanical properties of deformable solids, including cylindrical core soil, rock or granular material. The aim of a triaxial shear test is to establish the shear strength of a sample by replicating the in-situ stresses it would experience in the ground where it was taken from.

In geotechnical engineering, triaxial tests provide design engineers with the data they need to calculate the effects of certain loads on their projects. By determining the shear strength of a soil or rock sample, we can work out how the ground beneath a foundation or embankment will respond to different loads – and the conditions under which it will experience shear failure.

For this reason, triaxial shear tests have a wide range of applications in the design of:

• Walls and slopes
• Embankments
• Foundations
• Earth dams
• Subsoil structures
• Tunnel linings.

Triaxial tests are carried out on high-quality specimens with a height-to-diameter ratio of about 2:1. Specimen diameters typically range from 38mm to 100mm, although much larger specimens, up to 0.5m in diameter and 1m long, can be tested in large-scale equipment.

Ground Coffee - Episode eleven (Bishop's triaxial test ). Andrew Lees visits the first triaxial testing machine in the UK.

Triaxial shear test procedure

In a typical triaxial shear test procedure, the soil or rock specimen is enclosed in a rubber membrane and then placed in a water-filled cell, which is pressurised to recreate in situ stress conditions that result in the diagnosis of the specimen’s shear strength properties.

The vertical stress to the specimen is then decreased or, more usually, increased by means of the loading ram to cause shear stress to develop in the specimen. The difference between the cell pressure and the vertical stress is called the deviatoric stress and it can be increased all the way to shear failure of the specimen.

The types of triaxial test

There are three main types of test:

• Consolidated Undrained triaxial test (CU)
• Consolidated Drained triaxial test (CD)
• Unconsolidated Undrained triaxial test (UU)

The decision to use one of these three triaxial test types is based on the type of material that is being tested. The different tests vary in terms of whether they permit water flow in or out of the specimen during the consolidation and shear stages of the test (more on this below).

Consolidated Undrained (CU) triaxial test

In Consolidated Undrained (CU) and Consolidated Drained (CD) triaxial shear tests, the sample is saturated before testing begins and excess pore pressure dissipation during consolidation to reach equilibrium conditions is allowed. The aim here is to bring the specimen as close as possible to conditions at its natural state in the ground. 

In the subsequent shear stage when the deviatoric stress is imposed, excess pore pressure dissipation is permitted in CD triaxial tests (usually performed on sands) but not permitted in CU triaxial tests (usually performed on clays). Excess pore pressure is commonly measured in CU triaxial tests so that the effective stress in the specimen can be determined.

Consolidated Drained (CD) triaxial test

As with the CU test, the Consolidated Drained triaxial test begins with a saturation stage, followed by a consolidation stage, and ending with a very slow shear stage.

It is the longest type of triaxial test due to the slow consolidation and shear stages. Pore pressure in the sample must not be allowed to build up in this type of test and any pore pressures must be allowed to disperse. CD triaxial tests are best for determining long-term geotechnical engineering problems.

Unconsolidated Undrained (UU) triaxial test

An Unconsolidated Undrained (UU) triaxial test is a ‘total stress’ test since effective stresses in the specimen are not known. This makes it a rather approximate method to determine a soil’s mechanical properties. 

The Unconsolidated Undrained (UU) triaxial test has a big advantage: there is no saturation stage and stress is applied quickly (without pore water drainage), so it can be completed in less than half an hour. As a result, it is sometimes called a ‘Quick Undrained’ test but should be regarded as a characterisation test rather than an accurate parameter measurement method.

Large scale triaxial tests can also be carried out on dry materials, such as aggregate, with pressure applied using a vacuum instead of cell water. Additionally, unconfined compressive tests, without any confining pressure, can be carried out on cohesive soils but tend to give overly conservative results.

How triaxial test results are used

Triaxial test data can be used to derive fundamental material properties, including internal friction angle, cohesion, dilatancy angle and stiffness. Results from triaxial tests are used in almost every aspect of geotechnical engineering, from slope stability analysis, to pavement design, tunnelling and bearing capacity calculations in temporary works and foundation design.

Triaxial testing of geogrid-stabilised materials

Large scale triaxial testing is critical in predicting the performance of granular layers mechanically stabilised with a geogrid mesh and was key to the development of Tensar’s T-Value method for designing working platforms. 

Find out more on our geogrids and geosynthetic solutions pages.

Prof. Alan Bishop

For more information on the life and works of the triaxial test pioneer Alan Bishop, have a read of The Bishop Method by Laurie Wesley with contributions from our very own Mike Dobie. Available from all good booksellers!