T-Value: A new direction for working platforms

The T-value design method enables a more accurate assessment of the positive effect of stabilising geogrids on the bearing capacity of a granular working platform.

Ground Coffee Episode 6 - T-Value: a new direction for working platforms

Granular layers are often placed over weaker clay soils to improve the bearing capacity of working platforms and spread foundations. Incorporating geogrids within the granular material can improve bearing capacity significantly, so thinner layers can be installed, saving time and money through reduced excavation and imported materials.

Assessing the benefits of geogrid in design

The common approach to working platform design, BR470 Working platforms for tracked plant, considers geosynthetics but, because their effects are based upon individual manufacturers’ guidance, this can lead to inconsistency and uncertainty for designers and engineers carrying out design checks. This can mean that geosynthetics are not specified, so potential benefits and savings are missed.

Current design guidance also considers the bearing capacity of the granular material and the tensile strength of the geosynthetic separately, rather than how the two can work together to improve stability and safety.

This approach is not appropriate to stabilising geogrids, like Tensar TriAx ,because, rather than acting in tension to reinforce granular material, an aggregate layer stabilised with TriAx performs as a composite, due to the interlocking mechanism and particle confinement developing between the aggregate and the geogrid. This prevents lateral movement of the granular material, creating a mechanically stabilised layer that increases bearing capacity and controls differential settlement.

The T-Value Method

Tensar’s new design approach, the T-Value Method, is based on the relationship between bearing capacity and the load transfer efficiency of a granular layer, expressed as a T-value. This is dependent on the shear strengths of the granular layer and the subgrade beneath.

Beverley Sewage Treatment Works - Using Tensar’s T-Value Method, meant a large working platform could be 350mm thinner. See case study here.

For the first time, the full benefits of stabilising geogrids can be incorporated consistently in designs for a range of platform materials, in different ground conditions, allowing engineers to compare designs with, or without, geogrid. It can also be applied to both surface and shallow embedded foundations, circular and rectangular in shape, with dry or saturated granular layers.

Verification through laboratory testing and field trials

The T-Value method is based on finite element analysis and verification that the stabilisation effect could be measured was demonstrated using 500 large scale triaxial tests of different combinations of aggregate and geogrid.

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. 

You can access the published papers mentioned in Andrew's Ground Coffee below:

ICE - The bearing capacity of a granular layer on clay

Ground Engineering - Bearing capacity of a geogrid-stabilised granular layer on clay

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

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