When civil contractors required a rapid solution to transform 3,500m² of almost lagoon-like land into a transport park for articulated lorries, the unique properties of Tensar’s new triangular structured geogrid – TriAx™ – offered particular performance benefits.
By using this latest geotechnical development, the contractors, Leiths (Scotland) Ltd, were able to stabilise the soft and variable ground at the Cumbernauld depot of ARR Craib Transport Ltd in just ten days; achieving significant cost and time savings over conventional methods.
“The ground was marshy with some very soft spots, but as soon as the TriAx geogrid went down with a single layer of fill over it, you could see it stiffening up into a firm surface under the plant wheels,” says Brian Lund, Leiths’ Project Manager. “Normally it takes several layers of compacted fill over a bi-axial geogrid to stop poor ground like this buckling under load, but with TriAx™ it was almost immediate, meaning much less time on-site for us.”
The project, completed in May 2007, was the creation of a vehicle park for ARR Craib within the Blairlinn industrial estate, south of Cumbernauld, Lanarkshire. The objective was to provide off-road parking for articulated lorries on the industrial estate. However, some of the available ground was very poor, with wet soft clays; meaning a nominal CBR of 1% had to be assumed for the sub-grade for design calculation purposes.
Alistair Gilchrist, Area Civil Engineer for Tensar International, stated: “Consulting engineers WA Fairhurst contacted me for a rapid solution as the contractors were already on site. We calculated a design which used red blaes as the main fill between two layers of TriAx with a thin topping of Type 1 sub-base. Normally, a site like this would have to be excavated to a depth of up to a metre, and backfilled with expensive high grade aggregate to be sure of sufficient stability. This was a highly cost effective and quick solution as TriAx facilitated a shallower excavation using less expensive and lesser quantities of fill.
“The contractors had to raise the surface to the existing level, and the actual depth of fill varied between 400mm and 1000mm across the site, with an average of around 600mm. Furthermore, the reduced quantities of fill also meant many less transport journeys and thus the solution had a smaller environmental impact.”
The unique, multi-directional stiffness properties of TriAx™ were also an important performance benefit in countering the impact of heavyweight vehicles making complex point-turn manoeuvres on the un-surfaced yard. Its innovative triangular configuration creates a mechanically stabilised structure better able to withstand wheel loading from any direction. Where traditional square or rectangular bi-axial geogrids offer stiffness in two primary directions, TriAx is proven to have high in-plane stiffness through a complete 360 degrees.
“The loading from slow-moving and turning articulated vehicles is considerable,” says Alistair Gilchrist. “A vehicle park, in particular, needs to be able to accommodate load every which way, unlike a road where traffic tends to be carried in a linear direction.”
Tensar – the original inventors of geogrid technology – has spent six years perfecting TriAxs innovative triangular structure to achieve significantly improved aggregate confinement and soil stabilisation properties compared to conventional bi-axial geogrids. Independent trials show that TriAx geogrids create an even more efficient stabilised layer over a weak foundation soil to increase the load bearing capacity of that layer by 2.3 to 3 times, in comparison with an unreinforced layer of the same thickness.
Its superior performance in multi-directional stiffness and aggregate confinement also enables a reduction in granular layer thickness and the potential to achieve cost-in-use savings. For example, in trial sites where the use of biaxial georgrids saved 13% over conventional solutions, new TriAx was shown to save 20%, representing an additional seven percent saving in overall construction costs. This equates to a saving of 98p per sq metre.
