A reinforced soil wall on strip stone trench foundations has proved an unusual solution for part of a new Tesco store in Sheffield. GE reports.
The city of Sheffield, close to the Pennines, has its fair share of hillsides and steep slopes.
And it is on one of these, near the main line railway station, that a new Tesco Extra, selling clothes and household goods, is being built.
The store will sit on a site that was cut into the Spital Hill in Victorian times.
It is mostly flat having been the site of Wicker Station on a small branch line.
More recently the site has been occupied by a used car retailer.
A steel frame store will now cover much of this area, providing 7,500m2 of retail space at first floor level, leaving the ground floor area underneath for car parking.
The car park 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.
In fact, this entrance will be part of a main frontage for the site.
The store proper will all be at this top level, and will open onto the Spital road where it levels off along the hillside.
New pavements will be created to make a pleasant open space.
To do this 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.
“They are also unsure from the records they have, just what fill lies behind it,”
The parapet wall at the top of this old Victorian 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 retaining wall cannot be touched structurally.
The impressive stone block wall is half a metre thick with a series of half arches, and appears initially to be solid enough, like much 19th century engineering.
But the local authority, Sheffield City Council, is not so sure. What information it has suggests there could be voids behind it.
“They are also unsure from the records they have, just what fill lies behind it,” says Andrew Robertson, design manager for main contractor Bowmer & Kirkland (East Midlands), which is carrying out a design and build job for the client.
While the wall is not in any danger, the council stipulated that any new structure should not impose additional loads on the wall.
Therefore all extra risk had to be with the new structure.
To build up the site in front of the original retaining wall meant creating some kind of independent platform in front, which normally would have been a reinforced concrete wall.
But, says Robertson, that would have meant major formwork, concrete deliveries and a big increase in the overall carbon impact of the project.
So instead the design team turned to a reinforced soil solution.
Working with supplier and contractor Tensar it is building a 6-10m wide platform in front of the wall, with a concrete block facing wall rising to a maximum of 10m.
The 160m length of this platform butts up against the old wall, but transfers no loads to it.
Before building up this reinforced soil wall layer by layer, there had to be some ground improvement for the foundation, says Tensar northern area civil engineer Craig Andrews. “That is achieved using stone trenches. They work pretty much like vibro stone columns.”
Ground at the site is more or less alluvium everywhere, from the Don flood plain, with a depth up to a maximum of 18m.
At the back edge of the site with the new wall, it is about 4m thick. More than 60 short parallel trenches were excavated here at 2.5m centres, each with a 1m width and built up with stone compacted in layers 150mm thick.
Fill was sourced from the site using crushed concrete slabs which had previously overlain the whole area.
“In fact the alluvium was quite soft at this point, quivering like jelly when it came up and some of the trenches were done at 1.2m width to be more certain,” says Robertson.
To avoid potential differential settlement in between the trenches, 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.
“You get a very good interlock between the grid and the stone and the combination spreads the load well,” says Andrews.
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.
A tapered shape makes the blocks narrower at the back, which allows them to be built up on a curve, down to a 3m radius if necessary says Andrews.
“We built a standard concrete foundation 250mm high for the wall along the outside. That has to be fairly accurate and clean for dry walling which is more intolerant of perturbations.”
Blocks 200mm deep and 450mm long on the front face are made for Tensar by Anderton Concrete.
They are done with a split face design so that when two blocks are broken out of the mould they have a rough textured surface which is more pleasant aesthetically.
“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.
The grids embed tightly into the fill in the embankment.
Further up as loads diminish the grid is reduced in frequency to every three block layers in the wall.
Once the embankment reaches full height it will be used to partly support pavement and road at the top and to partly support the new store frontage.
“The store is a steel frame” says Robertson “and is supported on piles all over the site.
Local subcontractor Green Piling is installing some 900 driven steel tubular piles into bedrock with diameters between 177mm and 340mm.
“Some of these will be driven down through the new reinforced soil wall” says Robertson.
An advantage of the grid is that it punches through cleanly for piles unlike geofabrics which would tend to “feather” and get dragged into the pile hole.
Following that will be structural steel frame - some is already rising on site - and fitting out before Sheffield gets its new superstore in a little under a year.