Triaxial Testing

What is Triaxial testing?

Triaxial testing is a specialty method used by geo-technicians to study soil under varying stress conditions.

The process involves applying axial and lateral stresses to a soil sample in a controlled laboratory setup. This is to help determine shear strength, deformation characteristics, and failure modes of soils, which is important for designing structures and engineering projects.

What is it used for?

By understanding how soils react to stress when implementing the Triaxial testing method, it allows us to be able to design structures, assess stability, plan foundations, and look at ways in which we can improve various projects that involve buildings, roads, tunnels, and more.

How to measure pore water pressure in Triaxial test?

Pore pressure is generated inside the specimen during testing, and can be measured by means of pressure transducers.

Different types of Triaxial tests with definitions of each

There are three main types of triaxial tests: Unconsolidated Undrained (UU) Test, Consolidated Undrained (CU) Test, and Consolidated Drained (CD) Test.

Here’s a brief definition of each:

Unconsolidated Undrained (UU) Test (Quick Test):

This is a triaxial test in which a saturated soil specimen is rapidly loaded without giving it time to consolidate under the confining pressure. In this process, the pore water pressure is not measured.

The purpose of utilising the UU Test is to determine the undrained shear strength of cohesive soils under rapid loading conditions. This process is appropriate for when it comes to assessing the stability of quick construction processes or natural slopes.

Consolidated Undrained (CU) Test:

The CU triaxial test is where soil specimens are first subjected to a saturation and will consolidate under pressure. Then, they are loaded without allowing drainage of pore water during axial loading.

The purpose of the CU test gives Geo-technicians the ability to measure both the cohesion and frictional resistance of soils whilst also considering the effects of consolidation, which is relevant for understanding the behaviour of soils under relatively slow loading conditions.

Consolidated Drained (CD) Test:

The CD triaxial test is where soil specimens are first consolidated under a confining pressure and then subjected to axial loading while allowing pore water to dissipate.

Through undertaking the CD testing, our geo-techs can determine the drained shear strength and deformation behaviour of the soil under slow or sustained loading, which can provide more insight into long-term stability and settlement characteristics.

What is a repeated load Triaxial test?

This is a geotechnical laboratory test that is used to assess how soil responds to repeated or cyclic loads. It involves subjecting a specimen to both vertical and horizontal stresses while maintaining a confining pressure.

The repeated load triaxial test allows engineers to understand how these materials behave under repetitive loading conditions, which is crucial for designing stable and durable structures like pavements and foundations.

What is the difference between drained and undrained Triaxial test?

The main difference between drained and undrained triaxial tests is in the drainage conditions:

Drained Triaxial Test: In this test, water can flow out of the specimen, simulating well-drained conditions. It measures long-term soil behaviour.

Undrained Triaxial Test: Water is sealed in the specimen, representing poorly drained or rapid loading conditions. It measures immediate soil behaviour.

Our Projects

Examples of some of our projects that have facilitated Triaxial testing include the following:

Torrens to Darlington Ground Investigations