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, how it’s carried out, the different types of triaxial tests 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?
• What is a triaxial test used for?
• Triaxial shear test procedure
• 3 types of triaxial test
• What is the difference between a drained and an undrained Triaxial test?
• The application of the triaxial test results
• Triaxial testing of geogrid-stabilised materials

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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. A triaxial shear test aims to establish the shear strength of a sample by replicating the in-situ stresses it would experience in the ground where it was taken.

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 retaining system
• 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.

What is a triaxial test used for? 

The triaxial test is a cornerstone laboratory method in geotechnical engineering, providing invaluable insights into the mechanical behaviour of soils, rocks, and other granular materials. Its primary purpose is to determine the fundamental strength and deformation characteristics of these materials under various stress conditions, mimicking what they would experience in the ground.

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 stabilisation analysis to pavement design, tunnelling and bearing capacity calculations in temporary works and foundation design.

Engineers rely on triaxial test results for a wide range of applications, including:

• Determining shear strength parameters: This is arguably the most critical application. The triaxial test enables engineers to determine the key shear strength parameters of a soil, which are crucial for predicting its stability and load-carrying capacity.
• Assessing soil stiffness and deformation: Beyond strength, the test provides data on how a soil deforms under stress. 
• Analysing the stability of geotechnical structures: The shear strength parameters obtained are crucial for the design and analysis of various civil engineering structures, including construction foundations, retaining walls and slopes, embankments, railway trackbed improvements, and underground structures.
• Evaluating soil behaviour under different drainage conditions: The ability to control drainage in a triaxial test allows engineers to simulate both short-term (undrained) and long-term (drained) loading conditions, which is vital for accurate design.
• Research and development: Triaxial tests are fundamental for academic research and the development of new soil models and constitutive relationships.

3 types of triaxial test

There are three main types of tests:

• 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). To learn more, take a look at our article on the permeability of soil.

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 to determine the effective stress in the specimen. 

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 compression 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.

What is the difference between a drained and an undrained triaxial test?

The fundamental difference between drained and undrained triaxial tests lies in the drainage conditions allowed during the shear stage. This difference dictates how pore water pressure behaves and, consequently, how the soil's strength and deformation characteristics are interpreted.

Here is a comparison table of these two tests:

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 using 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.

3 Stages of Triaxial Test Procedure

Stage 1: Saturation

The primary goal of this stage is to ensure that all the voids within the soil specimen are completely filled with water, and any trapped air is removed. This is crucial for accurate pore water pressure measurements and for simulating realistic in-situ conditions where the soil is often saturated below the water table.

A common technique for saturation is to apply a "back pressure" to the pore water within the specimen while simultaneously increasing the confining pressure (cell pressure) in the cell. 

The degree of saturation is typically monitored by measuring Skempton's B-value, with a value of 0.95 or greater generally indicating sufficient saturation.

Stage 2: Consolidation

This stage aims to bring the soil specimen to an effective stress state that is representative of its actual stress conditions in the ground. During consolidation, the specimen is subjected to a confining pressure, and drainage is allowed, permitting the expulsion of pore water and volumetric deformation.

After saturation, the drainage valve opens, allowing excess pore water pressure to dissipate as confining pressure is applied to the specimen, leading to consolidation. This process is monitored by measuring expelled water volume until negligible changes indicate that primary consolidation is complete and equilibrium under effective stress is reached. 

Stage 3: Shear

In this final stage, the consolidated soil specimen is subjected to an axial load until it fails, allowing for the determination of its shear strength and stress-strain behaviour.

In confined pressure tests, an axial load is applied to a specimen at a constant strain rate while controlling drainage. Axial load, deformation, and pore water pressure are recorded until failure criteria like peak deviator stress are met, allowing for the determination of shear strength parameters.

Triaxial testing of geogrid-stabilised materials

Large-scale triaxial testing of geogrid-stabilised materials is critical in predicting the performance of granular layers mechanically stabilised with a geogrid mesh, including Triax geogrids, which provide enhanced reinforcement and stability under load. This testing 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.

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!

How Tensar can help

At Tensar, we can offer you design support for your geotechnical engineering projects, including expert guidance on triaxial testing and the use of geogrids for optimal soil stabilisation. You may also want to explore our free geotechnical design software, Tensar+. For any specific questions or assistance, please reach out and contact us.