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Geogrid makes the grade

GEOSYNTHETICS

A Highways Agency road construction specification trial is using geogrids to reduce aggregate requirement.

Improvement works on the A1 near Morpeth in Northumberland are providing the opportunity to trial a number of geogrid techniques.

As part of its search for a best value road construction solution, the Highways Agency (HA) has approved the use of Tensar International's SSLA30 biaxial geogrid by (NCC).

The council's highways department and its geotechnical consultant Scott Doherty Associates are using the geogrid for a 'grade separated junction' to replace five 'at grade crossings' on a three-mile stretch of the dual carriageway.

Many accidents have occurred on this stretch of the A1 near Stannington when vehicles attempt to turn right across the lanes of oncoming traffic. NCC designed a junction to lead traffic on and off the A1 with four 500m slip roads and minor roads connecting to the original junctions.

The slip roads will also provide a temporary bypass for several months, carrying 16,000 vehicles a day while the main A1 is closed for construction of a bridge, roundabout and underpass required for the junction.

Tensar says that while its geogrids have been used to provide high performance ground stabilisation with reduced aggregate thickness for temporary access roads, until now its use in a permanent roadway has largely been confined to private roads in the UK.

NCC asked the HA to depart from the standard specification on cost and environmental grounds, supported by documentation from Tensar.

The HA said: 'The proposal has been discussed with other members of the geotechnical team who have agreed that this is an ideal opportunity to carry out a trial using geogrids to reduce capping layer quantities used on the trunk road network.'

Ground conditions consist of glacial till deposits with soft areas and occasional hard erratics. After examining site investigation data, Scott Dohert chose a CBR of 2% for design purposes.

The typical pavement construction specification (HA 25/94 Part 2) was 150mm of Type 1 sub-base and a 600mm capping material (Class 6F1 or 6F2), overlain by the black top layers.

A class 6F2 material was selected which meant that the normal specification of Tensar SS30 biaxial geogrid was changed to SSLA30. This has a larger aperture size to accommodate the increase in maximum aggregate particle size, so full interlock of the granular particles can be maintained. This also benefited the contractor, as the finer grade C6F1 is more expensive.

Using its in-house software, Tensar proposed that with SSLA30 geogrid laid over the subgrade, the capping layer could be reduced to 300mm. This lessened the amount of aggregate required for the capping layer by 50%.

NCC estimated cost savings in aggregate alone to be between £50,000 and £100,000. The solution also meant less spoil was removed.

NCC is incorporating a short length of pavement with standard 600mm capping thickness on one of the on/off slip roads. Its performance will be monitored using a falling weight deflectometer and compared to the adjacent Tensar reinforced section, so that an assessment can be made of the benefits of using Tensar geogrids to reduce capping layer thickness over the lifetime of the pavement.

Portuguese prevention

Tensar geogrids are being used to prevent erosion on a new motorway near Lisbon. The A10 will wind through mountains between Bucelas and Arruda do Vinhos to provide better access to the Portuguese capital's northern suburbs.

To prevent erosion from the heterogeneous soil structure of embankments up to 60º, a Tensar GM4 geogrid was specified. It is being fixed to the face and combined with grass seeded topsoil.

The motorway is the 7km first phase of a 40km connection to the new Lisbon airport at Ota. The project involves a 100m tunnel and a 1100m viaduct. Ground conditions are alternate layers of limestone bench and marl. To prevent the erosion of the marl, 40,000m2 of Tensar GM4 was installed. This was anchored at the top and bottom of slopes with aggregatefilled trenches and fixed to the face with pins. Adjacent rolls of geogrid are overlapped and stitched together with HDPE braid.

The GM4 was laid over 100mm of topsoil seeded to create a grass layer to prevent erosion and help the embankment blend in with the surroundings.

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