The triaxial test

by Andrew Lees, on 13-Mar-2020 05:26:33

Triaxial 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.

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 triaxal test ). Andrew Lees visits the first triaxal testing machine in the UK.

Test procedure

The specimen is enclosed in a rubber membrane and then placed in a water-filled cell, which is pressurised to recreate in situ stress conditions.

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 based on whether water flow in or out of the specimen is permitted during the consolidation and shear stages of the test: Consolidated Drained (CD), Consolidated Undrained (CU) and Unconsolidated Undrained (UU).

In CD and CU 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 tests (usually performed on sands) but not permitted in CU tests (usually performed on clays). Excess pore pressure is commonly measured in CU tests so that the effective stress in the specimen can be determined..

A UU 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. However, the test has a big advantage: there is no saturation stage and stress is applied quickly (and no pore water drainage), so it can be completed in less than half an hour, hence 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 results are used

Triaxial test data can be used to derive fundamental material properties, including internal friction angle, cohesion, dilatancy angle and stiffness. Results 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 geogrid and was key to the development of Tensar’s T-Value method for designing working platforms. Find out more next time…

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!

Topics:Temporary Works