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Shore Thing

Contractors are strengthening a hillside cutting and working around a Victorian retaining wall to build a Tesco superstore in Sheffield. NCE reports.

Anew 7,500m2 Tesco superstore in Sheffield, is being built on a steep hillside.

The store will sit on a site cut into the Spital Hill in Victorian times. The site is mostly flat and the steel framed store will now cover much of it. The ground floor car park under the main retail area will be accessed via entrances from a road running along the Don river valley at the front of the site. But it will also have a second entrance at the first floor level, entering from the “back” of the site and down a ramp.

The store proper will have an at grade entrance uphill from the ground floor car park.

Uphill entrance

To create the uphill car park entrance a supporting platform of reinforced soil is being built up along the Spital Hill to extend the ground area at the top. Part of the store will also sit on piles inserted through this platform, which will rest on a reinforced soil wall.

The reinforced soil wall is being constructed in front of an existing Victorian stone retaining wall with a large masonry retaining wall of buttresses and arches supporting it. The parapet wall at the top of the retaining wall will be removed to allow the pavement area to be extended beyond it as public space. The pavement area will also carry a link from the road running along Spital Hill to a ramp leading down to the lower level car park.

The old 500mm thick stone block retaining wall cannot be touched structurally.

It includes a series of half arches, and Sheffield City Council is concerned that there are voids behind it.

Design and build

Contractor Bowmer & Kirkland (East Midlands) is carrying out the work under a design and build contract.

While the wall is not in any danger, the council stipulated that any new structure should not impose extra loads.

The design team turned to a reinforced soil solution behind the wall. Working with supplier and contractor Tensar it is building a 6m to 10m wide, 160m long platform in front of the wall, with a concrete block facing wall rising to a maximum of 10m. It butts up against the old wall, but transfers no loads to it.

The ground beneath the soil layers had to be strengthened with stone trenches. “They work pretty much like vibro stone columns,” says Tensar northern area civil engineer Craig Andrews.

A load distribution mattress was laid along the trenches. It comprised three layers of Tensar’s TriAx grid material sandwiched into a 500m thick layer of crushed stone.

Reinforced soil wall

On top of this foundation the reinforced soil wall itself has been rising, using a Tensartech wall system. It comprises pressed hollow concrete blocks which are lightweight and easily dry-assembled with a plastic pin system to link the blocks.

Blocks 200mm deep and 450mm long on the front face are made for Tensar by Anderton Concrete.

“We built a standard concrete foundation 250mm high for the wall along the outside,” says Andrews. “That has to be fairly accurate and clean for dry walling, which is more intolerant of perturbations or perhaps trapped pebbles, than a mortar laid wall.”

Behind the blocks goes a granular fill, which is being compacted in layers. Every two blocks rise in the wall sees a geogrid laid, in this case a Uniax Geogrid. These lock into a groove between the blocks using plastic end pieces “which are very positive connections”, Andrews points out.

Further up as loads diminish the grid is reduced in frequency to every three block layers in
the wall.

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