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Fundamental ground revival

Foundations in peaty ground are always a challenge. But when the site is also contaminated and in a floodplain over an aquifer then it calls for every trick in the book. Richard Bennett reports from Bury St Edmunds.

In the wake of last month's flooding the Government promised to tighten up the Environment Agency's power to prevent housebuilding on floodplains. This sounds like a good idea and for greenfield sites most people would say it generally is. But the Government is also committed to developing new housing on urban brownfield sites, which are often in floodplains. Resolving these potentially conflicting requirements can generate interesting work for civil engineers, including some tricky foundation problems.

This was the case for a site in Bury St Edmunds, Suffolk, where developer Bloor Homes is redeveloping 4ha of brownfield land on a floodplain. Having been occupied by a steel stockholder, caravan site and greyhound stadium, the site at Taffyn Meadows had become derelict.

The local authority, St Edmundsbury Borough Council, was keen to regenerate the site, but it suffered from a number of problems including multiple ownership, poor drainage and contamination from sugarbeet washings, pesticides, herbicides and flytipping.

Bounded by a school, gasworks and housing, the site forms a gently sloping valley around a water channel which flows into the River Lark.

Ground conditions are dominated by a 6m thick layer of soft peat which flanks the channel.

The area, once a section of East Anglian fenland, was regularly flooded and the name 'Taffyn' is actually a corruption of 'Tay Fen'. The area is subject to zone one groundwater protection by the EA, as a chalk aquifer underlies the peat and ther is a drinking water abstraction point nearby.

Although the EA initially objected to development on the site, the local authority was keen to see the residential development go ahead and relaxed some of the planning constraints to make the site economic to develop. 'We were very very keen to see it brought through, ' says Peter Fuller, senior planning officer with the council, 'and we have done what we can to support the developers.'

Bloor Homes eventually bought the site to build 160 houses. The £1M groundworks and foundations contract was let to Keller, which soon realised it was dealing with a particularly difficult site. 'It's one of the most unusual sites we have had for years, ' says Keller director Martyn Singleton.

This is echoed by Andrew Marns, Bloor Homes' land manager. 'It's the most complex site I've seen in 15 years, ' he says.

Housing developments are usually built from the road frontage backwards into the site, so the developer can start selling properties. Here, it meant foundation work started on the old clay fill material flanking the peat. Keller installed concrete strip footings for the houses on top of vibro stone columns which improve bearing capacity by compacting a 500mm annulus of the surrounding ground.

This allowed bricklaying to start, but as foundation work moved further into the site the peat made things more difficult. Piling was the obvious answer but the EA was concerned that by founding into the chalk, driven piles with smooth sides could provide a vertical pathway for contaminants to flow into the aquifer. So Keller chose driven cast in-situ piles which bind into the soil, blocking contaminant migration.

In all 800 piles driven 18m through the peat and into underlying chalk will be needed. The peat is consolidated by using a 3m thick surcharge of imported fill, with band drains to relieve pore pressure. Overall ground level over the whole site will be raised by 2m, with up to 300mm of predicted settlement.

Where the programme did not allow enough time for consolidation under surcharge, Keller used dynamic compaction, with an 8t weight dropped up to 15m to condense the surface down to 6m. Dynamic compaction has also been used in conjunction with band drains. 'It's the first time we have used DC over band drains, ' says Singleton.

Estate roads on the site are founded on 7m long vibroconcrete piles into the chalk. Load transfer structures will be placed under the road at potential sites of differential settlement, such as the transition from dynamically compacted to piled ground.

Now a third of the way through a four month period of groundworks, Keller is relatively pleased with progress, which has included the wettest October on record. All the first phase driven piles are now in and attention is moving towards placing the 90,000m 3of fill required to surcharge the peat.

Difficult brownfield sites such as Taffyn Meadows are likely to be increasingly common as the Government strives to meet its ambitious target of 60% of development on brownfield sites. Further restrictions on development in floodplains will make this target even harder to achieve, and require increasing flexibility from planners, the Environment Agency and developers. But it should make lots of interesting work for engineers.

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