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Piling: Game of two halves

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Ground engineering work being carried out for a new housing development in Luton is straightforward in one part but demanding in another.

Keller is delivering a mixture of complex and simple solutions for Redrow Homes’ redevelopment the site of the former Vauxhall car factory in Luton. The site has lain empty for 15 years as developers scratched their heads over how to find a commercially viable way to deal with the steep slopes.

Located next to Luton Airport Parkway station and near the end of the airport runway, it would be hard to miss Redrow’s solution for the site – one of the largest soil reinforced earth walls undertaken by Keller. The complex bit is the use of Chalk won from the site as the fill for the wall, while the 1,000 or so piles needed for foundations on the scheme, known as Napier Park, are relatively straightforward.

The redevelopment of the 22ha site will include retail outlets, leisure facilities, hotels and new homes. The site is divided between several developers, but Redrow is the first to start work and is planning around 500 homes, new woodland, a wildflower meadow and new public space on its 8ha part of the plot.

Redrow senior project manager Andrew Fuller said: “The topography of the site is challenging. The scale of groundworks needed here is unusual even on a major project.”

Redrow has said that its scheme design responds to the existing landscape, with a curved access road winding through the site, which leads to different character areas of development.

The development is split between seven apartment blocks close to the front of the plot and 180 houses on the road which winds up the site.

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The Textomur wall is 175m long, has a 65° face angle and reaches up to 9m in height.

The boundary between these two elements is clear to see at the moment with Keller’s Phi Group working on the huge Textomur soil retaining wall dividing the two.

The Textomur wall is 175m long, has a 65° face angle and reaches up to 9m in height.

Work on it started in early July and – weather permitting – is expected to be completed later this month.

Phi Group supervisor Ricky Bishop says it is not so much the size that is a challenge, but the fill material being used at Napier Park.

Chalk may not seem the ideal material but Bishop says that there was a £1M cost difference between using the Chalk or removing it from site and importing more conventional granular fill material.

“We’re laying it in 150mm layers and compacting it in four passes with no vibration to avoid destroying the structure of the material,” explains Bishop. “We’re trying to keep the moisture content down to keep handling issues to a minimum but we’ve had challenges with material being stockpiled.

“Handling releases moisture within the material and increases the pore pressure. This does dissipate and the material will dry out but not if it is handled too much and the structure is lost.”

Phi only had access to the central 80m section of the wall initially so the structure is being built in three sections, adding to the challenge.

The face is formed by preformed rebar cages and the fill material is faced by Paragrid geotextile that extends up to 9m back into the slope.

Void formers are being placed within the wall to allow piling for the houses above to be completed without compromising the integrity of the wall.

“The Paragrid is preloaded at the front and then pre-tensioned before the fill material is placed,” says Bishop. Given the challenging material, it is no surprise that there is a rigorous testing regime in place. This is being undertaken by RSK.

Nuclear density tests are being carried out at every 600mm rise in the wall and plate tests are being done at every 1,200mm. “We are getting good density results with only 3% to 4% air voids,” says Bishop. “Plate load tests fail if done too soon after the fill is placed but pass if the material is allowed to ‘cure’ for a few days.”

The weather has not helped and Bishop is concerned about the forecast as the project extends into the autumn.

Tarpaulins are being used to cover the fill when it rains in a bid to keep moisture levels down, but frequent showers mean that Bishop’s team may soon be qualified for jobs working with the rain covers at Wimbledon or Lords.

Bishop says the wall is one of the most challenging he has worked on.

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King post walls are being used to retain smaller heights in the housing development

By contrast, Keller site supervisor Karl Chaplow who is overseeing the piling work for the apartment blocks describes his part of the work as relatively straightforward.

Keller has already constructed three sections of contiguous piled wall for the basements on the four apartment blocks. These were formed by 58, 900mm diameter CFA piles at 1050mm centres and 86, 600mm diameter CFA piles constructed to depths of up to 13m.

Work in the bearing piles for the first four apartment blocks started in late August. In total over 900 piles are planned for this part of the development and Chaplow says that there are only 180 of the 400mm diameter, 1025kN capacity piles left to construct out of 742. Keller is also constructing 165, 350mm diameter, 980kN capacity piles for the development. The bearing piles are going to depths of 12m to 15m.

Installation of the 350mm diameter, 11m deep piles for the houses will follow later this autumn, but first Chaplow’s team will move onto constructing the king post walls around the perimeter of the site. Here is where the piling work will become more challenging as there are six sections of king post wall with curves and right angles planned. The king post walls will be supported by

750mm diameter CFA piles with I-beam posts to support the wall above ground.

“King post walls are being used where vertical retaining walls are needed and will retain heights up to 4.4m,” says Chaplow. “We are using a CFA rig for the 750mm diameter king post piles because the rig is on site for the rest of the foundations work.”

While the site does not yet look like somewhere you would want to call home, Redrow has said that the first plots will be ready next February and it expects to complete work on the scheme in 2021.

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