Triaxial testing of geogrid-stabilised materials

Following on from our last blog, this time we look at how large scale triaxial tests are critical in predicting the performance of granular materials mechanically-stabilised with geogrid.

Ground Coffee - Episode twelve (triaxal testing of gegrids ). Andrew Lees visits Tensar's very own triaxal testing machine

Typically, finite element analysis (FEA) models used in geotechnical design characterise geogrid in terms of its in-air tensile stiffness or strength properties, together with the mechanical characteristics of the soil without geogrid, often measured using triaxial tests.

However, this approach can lead to significant under-prediction of geogrid performance, particularly for stabilisation geogrids. This is because their performance depends critically on the interaction between the aggregate particles and the geogrid apertures (‘mechanical interlock’).

Triaxial testing of the aggregate and geogrid together overcomes this, as it enables the performance of a composite material to be measured. Testing has to be carried out at a much larger scale to overcome particle size effects, typically on 0.5m diameter, 1m long samples, with a single disc (or more) of geogrid placed at the midpoint of the dry, compacted aggregate.

Placing such a large sample in a water-filled cell (as in conventional triaxial testing) would be complex and expensive, so instead, the sample is enclosed in a rubber membrane and vacuum applied to create the confining stress of up to 80kPa.

Tensar used large scale triaxial testing to develop our T-Value Method for working platform design. Five hundred large scale triaxial tests, using different combinations of aggregate and geogrid, verified that the stabilisation effect could be measured.

The results showed that peak shear strength in geogrid-stabilised aggregate is enhanced compared with non-stabilised aggregate. In other words, more stress is needed to shear and dilate the sample and larger strains are needed to cause significant softening of the stabilised granular material, compared with the non-stabilised material.

Verification through field trials

The method was then subjected to full scale field trials on a UK site and by the University of Saskatchewan in Canada where full scale plate load tests were carried out – You can see this in action here. 

Want to learn more about Tensar's T-Value Methodology?

You can access our T-Value Guide to temporary works design here.