Tensar GeosyntheticsThe A2 is part of the Northern Key Transport Corridor, connecting Belfast to Derry, via Coleraine. The scheme involved upgrading 6.6km of the A2, between Maydown and the City of Derry Airport, to dual carriageway standard.

A section of road widening required the road to be extended into a lagoon and for band drains to be installed to help speed up consolidation settlement. A Tensar Mechanically Stabilised Layer, using TriAx geogrids, was used to cap the weak deposit and allow initial, safe site access over several metres of a very weak organic clay material, having an undrained shear strength of 5kN/m2.

In the first instance a winch and pulley system was used to pull the geogrid across the weak material, aggregate was then carefully placed and the mechanical interlock between the stiff monolithic geogrid and the lagoon backfill created a working platform that could be used for initial access for light construction plant. Further aggregate was then added and an upper layer of TriAx geogrid was used near to the finished surface to complete the mechanically stabilised layer installation.

Basetex, a high strength-geotextile, was used to stabilise the 3.5m high embankment against deep seated shear surfaces and was placed on top of the lagoon backfill, at the base of the embankment fill.

Tensar provided an Application Suggestion to Hyder Consulting Engineers that indicated the use of geogrids and geotextiles to achieve the lagoon capping and long term stability of the embankment.

Left: Installation of TriAx geogrid as part of the lagoon capping.
Right: Example of Tensar software used in the stability analysis.
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Contractor: Lagan Construction
Consulting Engineer: Hyder Consulting Engineers
Client: DRD Road Services


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