According to the government, around four million new homes are required by 2018 and if we are to protect green belt areas and the finite land space available in the UK, this means building on increasingly difficult ground conditions. It has been calculated that up to half the sites involved in the UK have had some previous development and according to a Construction Industry Research and Information Association (CIRIA) report, over 850,000 acres has been contaminated. As if this weren't enough, many brownfield sites lie within flood plains, a number of sites assigned as growth areas susceptible to highly shrinkable clays and increasing tree protection means sites often cannot be cleared, leaving deep and difficult roots to be negotiated.
One company with innovative solutions to this and other site risk problems is Abbey Pynford, a specialist ground engineering solutions company that has developed a proprietary piled raft house foundation system. Currently suitable for up to and including five storeys in height, it can accommodate a wide range of variations to the foundation footprint, allows for all piling methods -- bored, driven or augured -- as well as ground improvement techniques. There is also little or no spoil to be removed from site either.
The system is known as ‘Housedeck’ and four versions are available -- all of which meet NHBC and LANTAC technical requirements. There is the standard ground bearing slab; an anti-heave option incorporating a void; a gas membrane option; and vibro replacement stone columns. The voided system is ideally suited to complex brownfield sites that incorporate hazardous soil gases such as methane, carbon dioxide, or radon -- formed when uranium found in many soils and rocks undergoes radioactive decay. These gases can also be present in greenfield sites because of naturally occurring organic deposits such as peat. The void also allows for ground heave in shrinkable or dessicated clay areas.
The system has a remarkably low ‘construction depth’, using a structural reinforced concrete slab only 225mm deep that covers the entire footprint of the building. This is supported directly on piles -- between 140mm and 350mm in diameter without the intervention of ground beams or pile caps -- and steel reinforcement ensures that loads on the slab from the building superstructure are transferred to the ground via the piles, without the risk of the piles ‘punching’ through the slab.
Housedeck has been chosen for a challenging site on shrinkable clay at Chattenden, near Rochester in Kent, where developer Ward Homes has just launched into the final two phases of its Toad Hall development. Around 90 dwellings are due to be completed by spring 2008. Piles transmit building loads down to levels of the clay that will never dry out and the steel reinforced structural slab spans from pile to pile without the need for ground support, allowing it to be positioned well clear of any potential heave problems.
Construction is straightforward, the topsoil is stripped and subsoil levelled, then 50mm of blinding concrete goes down. Using lightweight piling rigs, 200mm diameter augured piles at a maximum of 3.2m centres were installed to an average depth of 12m.
Quite apart from the obvious advantages offered by Housedeck where contaminated land is concerned, it offers real benefits in dealing with hidden underground obstructions on site too. Site investigations rarely locate all underground obstructions and all too often problems are discovered after the work has started. With traditional pile and beam foundations spaced in straight lines beneath the ground, there is very little scope for changing the pile locations if an unexpected obstruction is found.
Disposing of the excess soil generated when laying traditional foundations on contaminated ground can also cause a real problem for developers in some parts of the country. Because of the dramatic reduction in the removal of waste generated when installing Housedeck, compared to typical foundation systems, Housedeck can help to make considerable site savings.
This was the case on the Isle of Wight recently, where difficult ground conditions were exacerbated by the lack of available landfill sites. Stoneham Construction chose Abbey Pynford's Housedeck system for the construction of 22 houses and flats at Binstead on the Isle of Wight.
Mike Munns, Managing Director of Stoneham Construction Ltd. explains why Housedeck was so valuable to the development. "Few sites are entirely obstruction-free, so designers need foundation solutions that are flexible enough to accommodate different layouts, depending on what they find in the ground below. Also, the system does not require excavations for ground beams so we didn't have a problem with excess foundation soil. This particular job involved a sloping site as well, which can be difficult for traditional foundations -- and there were two large trees in the middle of the site, which had tree protection orders.
So we couldn't in any way damage them or their roots". |
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