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Tyne trades up to new model

GEOSYNTHETICS CONTRACTS :

Geogrid reinforced soil walls are supporting a new access bridge to an industrial estate in Newcastle upon Tyne. Dan Simpson reports.

TENSAR International has designed and supplied geogrid reinforcement and its modular block wall system in a £170,000 contract for a road to a new light industrial estate in Newcastle upon Tyne.

The geogrid-reinforced soil supports access to the development from Scotswood Road, in the western part of the city, and includes a bridge over an inlet of the River Tyne which replaces an old concrete bridge. Main contractor for the project is Edmund Nuttall.

'One of the difficulties was that the old bridge had to be kept open while the new access road was being built, ' said Tensar area civil engineer Jim Penman.

The new structure was built around the old bridge, allowing one carriageway of the new road to be opened and the old bridge to be removed, he explained.

The new bridge comprises two concrete arches supported on three piled abutments, with the Tensar wall built up around the arches. The reinforced soil component of the structure is made up of Tensar's British Board of Agrement approved modular block system.

The block facing units are founded by cast insitu concrete footings, with treated fill underlying the whole structure. The base blocks for the modular block wall are mortared on to the concrete footings but the remaining blocks are dry laid.

'This means the speed of construction in very fast, ' said Penman. 'The whole idea of the system is that it is cheap, quick and easy to construct. It does not need specialist teams; normal labourers can construct a wall after only a few hours training. '

Because the site was next to a tidal section of the Tyne, the lower five layers of blockwork had to be constructed between tides to get the correct compaction in the fill between the geogrid layers, Penman said.

After placement of the mass concrete footing and base units, a layer of Class 6I/6J fill was compacted up to the required levels, allowing a minimum of 150mm Type A drainage material immediately behind the face.

Geogrids were then cut to appropriate lengths and attached to the modular blocks using Tensar's connectors which achieve high strength connection between the grid and the blocks.

Grid lengths varied from 8. 4m at the base of the highest walls to 3m at the top of the wall and the grid spacing varied from 300mm in the lower part of the retaining walls to 450mm at the top.

Similarly, stronger 120RE grids were used at the base of the higher structures reducing to 80RE just over half way up and to 55RE towards the top.

To attach a non-structural facing to the reinforced soil wall, ties were placed in specially designed recesses within the modular blocks. These allow the steel ties to move vertically to accommodate different course thicknesses.

To prevent the new structure running into the old bridge, a temporary stop wall, comprising Tensar's steel mesh panel wall system, was constructed part way through the building of the geogrid structure.

A set of 16mm diameter mild steel bars were bent to the appropriate face angle and then fixed to steel mesh panels to form the main framework. The geogrid was connected to this panel by wrapping it around the bars and looping it back to itself. A geotextile liner prevents soil squeezing through apertures in the panels during compaction, so this method does not require any additional internal or external support.

Design of the walls was based on a two-part wedge failure criterion using Tensar's in-house design software, Winwall.

Wall construction began at the end of January 2001 and was complete by the end of June.

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