Going over old ground

Peterborough is the target of ambitious growth plans that means new housing must be built on brownfield sites. Damon Schünmann went to the Lincolnshire town to check on progress.

The Potters Way site in Peterborough was once home to a sewage treatment works that closed when it became obvious it could not cope with the Lincolnshire city's expansion. But after laying dormant for about 16 years Kier Residential subsidiary, Allison Homes, bought it for a housing development.

Housing work in the area is being driven by the government's sustainable communities plan. More specifically, the East of England Regional Spatial Strategy aims to grow the city's 160,000 population by up to 50,000 by 2021, meaning about 1,000 new houses are rising up each year to meet the estimated 25,000 target needed.

By the time the developer came on site the sewage remediation had completed but this still left a number of concrete tanks lying underground. Although tanks lie up to 6m deep, once site workers break out those that would conflict with the development's foundations, they are infilling the voids and compacting the ground to an additional 3m height, in line with the rest of the site, as a precaution against flood defence failure. This is because the River Nene is less than 1km away to the south and a marshy flood plain extends towards the site.

"The ground was compacted in 300mm layers and capped with clay as part of the remediation strategy but in the end it was not really needed as there wasn't anything horrific left in the ground to warrant it," says Allison Homes project manager, Mark Tipping.

Although the remediation - done by sister company Kier Construction - finished last month, it did not target the refuse tip at one end of the site. This will possibly get a gas membrane seal the development from leachate with any gas then vented off. Ground conditions included Cornbrash Limestone at the site's edges that was taken out when the sewage tanks were installed. But this has been put to use, after crushing, to form a piling mat meaning nothing has gone offsite.

The developer has taken something of a gamble with this, its biggest project to date. "There are 550 apartments and 56 town houses," says Tipping, "If we can sell 150 a year it will take four years but the job's finish date depends on how they sell." Tipping says the company, a traditional Lincolnshire house builder, has already invested about £10m in the project.

"We won't get anything back until we sell some and even then it's only a few hundred thousand at a time." He also mentions that although Allison Homes has never built a lift or an underground car park before, this site will feature 29 lifts and undercroft parking as well.

May Gurney won a £153,000 contract to initially install 250, 450mm diameter 1240kN CFA piles for 116 apartments in two blocks. "That's two piles per apartment - which doesn't sound many when you put it like that," laughs Tipping. Main piling began at the beginning of last month and was due to finish four weeks later.

May Gurney southern area manager, Mike Cowan says: "This job is unusual because of the ground profile. In other parts of Peterborough we have been able to found piles in the Cornbrash Limestone layer, but here this strata is thinner and has been taken out in areas for the old sewage works so we must found in the underlying Blisworth Limestone."

Although the S.I. covered the site quite extensively, it could not cover areas under the tanks. Even though there was good information, the subcontractor did additional boreholes below the tanks to verify the stratum there and decided to found in the lower band of limestone. It may however, still be possible to found in the Cornbrash in other areas as it varies in thickness across the site.

With satisfactory data coming from the extra boreholes and two preliminary static load tested piles, May Gurney was able to fine tune the original design.

"The design 12 months ago had 600mm piles and we knew what ground we had here. But with the extra boreholes we then knew 450mm piles could be used," says Tipping.

Cowan adds: "There were significant savings as there is about half the concrete needed with the smaller piles."

It was important to verify that a 450mm CFA pile would go through the good quality Cornbrash Limestone layer but the rig crews have not reported problems as even where it was not taken out, while building the sewage works, it is generally only 0.5m thick.

The piling contractor used a powerful 80t Llamada P-150-TT rig with a rock auger to ensure penetration.

The river's marshy flood plain at the east of the site must also be remediated to accommodate a crescent of town houses, now that Allison Homes has completed a flood risk assessment. But it cannot begin building until a sluice gate at the Dog in the Doublet (a pub that has given its name to an area) becomes automated.

Peterborough City Council spokesman Mike Lennox says: "The area does not normally flood but it's at a point where the Nene becomes tidal. After heavy rain in 1998, the locks upriver to the west were opened to allow water through from Northampton and because of this there was flooding here."

Future work just south of the site may include a marina for the River Nene that the developer may get involved in if parent company Kier Group makes a successful application to the council.

However, one legacy of the old site works still exists and cannot be removed. "There are two dirty grey about 5m high pipes at the entrance that we wanted to do away with," says Tipping. "But we were told nearby Hampton would flood with sewage so we will make them into a feature of some kind instead, perhaps encasing them in bright shiny stainless steel instead of them being covered in bitumen. There has been talk of making them into some modern art or a clock tower but Anglian Water doesn't want anything fixed to them so any suggestions are welcome," he laughs.