Tensar’s T-value method enables a more accurate assessment of the positive effect of stabilising geogrids on the bearing capacity of granular working platforms and has been shown to cut construction costs by up to 30%, says Tensar senior application technology manager Andrew Lees.

Granular layers are often placed over weaker clay soils to improve the bearing capacity of working platforms and spread foundations. Incorporating geogrids within granular material can improve bearing capacity significantly, so thinner layers can be installed, saving time and money by reducing 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 manufacturers’ guidance, this can lead to inconsistency and uncertainty.

Furthermore, the granular material’s bearing capacity and the geosynthetic’s tensile strength are considered separately, rather than the two working together to improve stability and safety.

This is inappropriate to stabilising geogrids because, rather than acting in tension to reinforce granular material, an aggregate layer stabilised with geogrid performs as a composite, due to interlocking and particle confinement between the aggregate and the geogrid.

This prevents lateral movement of the particles, creating a mechanically stabilised layer that increases bearing capacity and controls differential settlement.

The T-Value Method

Tensar’s 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 depends on the shear strengths of the granular layer and the subgrade beneath.

So, 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.

The method, based on finite element analysis and laboratory testing of stabilised granular materials, can also be applied to both shallow foundations and to dry or saturated granular layers.

The T-Value Method in action

A 6,000m2 working platform was needed to enable piling for three, 20m diameter final settlement tanks at a wastewater treatment works in north east England. Ground conditions were challenging, with thick deposits of weak organic clays across the site.

The working platform was designed to BR470 and, based on the worst case scenario of the heaviest plant and subgrade undrained shear strength of 4kPa, the platform was to be up to 1.25m thick, comprising 6F2 aggregate with multiple layers of reinforcement geogrid.

This would have meant importing large quantities of aggregate and significant excavation and removal of soils, replacing them with large volumes of imported aggregate.

Tensar proposed using its T-Value design method to assess whether a thinner platform stabilised with geogrid could be used, without impacting on bearing capacity and load transfer efficiency.

This showed platform thickness could be cut to 900mm, without impacting on bearing capacity and load transfer efficiency. Excavation depth could also be reduced by 350mm, saving about 30% on platform costs, and cutting carbon emissions by 40%.

To learn in more detail about Tensar’s T-Value methodology, join Tensar at Croke Park, Dublin where they will be hosting a free technical workshop on the March 12, 2020. Register to attend here: http://info.tensar.co.uk/dublin2020