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Going underground

Geotechnics Foundation reuse

Reusing old foundations when urban sites are enlarged or redeveloped can save costs. David Hayward reports.

Foundation costs and carbon emissions both slashed by over 50% - quite an impressive offer to today's sustainability aware clients. Add to this faster, less complicated underground construction works, with signi-cantly simpli-ed superstructure design, and the package looks decidedly utopian.

But to the increasing number of clients planning to enlarge existing town centre of-ces or rebuild on brown-eld sites, this can be a wish list turned into reality. So claim the authors of a just published new guide on the Reuse of Foundations for Urban Sites (RuFUS).

Britain's major cities are rapidly becoming as crowded below ground as they are above.

Redevelopment of urban sites, or the existing buildings on them, is increasingly expensive, especially in foundation costs.

Adding a few stories on top to increase an old building's capacity usually means additional beefed-up foundations. The alternative of demolishing the old to build new can trigger an equally large bill to remove or avoid original foundation slabs and piling.

Enter then the enlightened developer and his design consultant who -rst investigate whether their enlargement or rebuild can economically reuse existing foundations. And often, say proponents of the RuFUS concept, they can -- so long as client teams are prepared to take on board the inherently increased risks.

fiYou must have the back-up of an analysis team of like-minded professionals - contractors, designers, client, even insurance specialists - all prepared to think out of the box and work interactively in parallel rather than linearly, fl says Rab Fernie, technical director of geotechnical contractor Cementation Foundations Skanska, and a leading contributor to the RuFUS guide.

fiYou must also get involved early, at a project's inquiry stage, yet still have the vision to cancel the whole idea if it fails to look cost-effective. fl Such fiforward thinkingfl construction teams must risk the time and additional expense of research and testing needed to offer such an innovative design, Fernie claims. Plus they must accept the -nancial risk of pre-tender delay, insurance challenges and, ultimately, technical incompatibility of the old piling, leading to a rejection of the whole potentially clever idea.

But the possible gains are considerable, Fernie argues.

Reusing the existing support structure can offer 60% cheaper foundations, completed a third quicker compared with new ones. It can simplify superstructure reconstruction, avoiding complex belowground piling and new column installation, operations often needing large holes cut into existing basement slabs.

And removing the need for hundreds of new piles also offers clients enviable sustainability and recycling brownie points, plus substantial overall cost savings.

Fernie's company now considers the RuFUS concept for all its 1,000 major project inquiries received annually but, he estimates, less than 10% could win through to be costeffective. fiWe have used it only a few times so far, although I see the market taking off and doubling within -ve years as the new guide helps demystify the idea to engineers and clients, fl he claims.

Cementation's ggest success to date is an enlarged car park in Coventry, where the developer's design involved straddling extra oors above on new foundations. Cementation priced the conforming design needing several hundred new piles but also suggested an alternative, reusing existing piles that, by fortunate coincidence, the foundation specialist had itself installed years earlier.

The alternative won through.

Foundation costs were slashed 60% and the work completed a month quicker, saving the client £1M overall (see box).

It is London that could offer the main UK playground for this emerging technique, Fernie predicts. The capital's buildings are designed for 100 years but are often demolished or remodelled in less than 30.

Removing old foundations is expensive and risks an invasion of archaeologists. Building new ones within, or just outside, the building's footprint is often technically or geometrically challenging; if not totally impractical.

fiThe key is to establish quickly if research needed on existing foundations could result in their cost-effective reuse, fl says Fernie. Rarely are original piling and test results available years later, and Fernie cites two recent examples to illustrate a potential dilemma.

Reusing quay wall piles to support housing at Chatham docks looked a cert for RuFUS.

But a back-of-the-envelope analysis of the £50,000 investigation programme needed quickly established that with barely a third of the piles reusable the idea was not costeffective.

No time or money wasted here; unlike the similar sum spent on a two-year real investigation into the reuse of foundations beneath a power station site.

The result - this time expensively - was the same: finot -t for purposefl.

Even a favourable technical analysis, proving suf-cient spare-load capacity in existing piles, does not automatically spell success. fiRisk remains the biggest challenge, fl warns Fernie.

The possibility of differential movement between old and any new piling, plus structural and legal guarantees needed for the insurance market, are all potential problem areas.

Unsurprisingly, it is price that heads that risk list. Who pays for the initial, often lengthy, bespoke research and testing of old foundations, which may - or crucially may not - offer the client substantial savings?

fiSometimes the savings gained can turn a marginally cost-effective project into a safe prospect. But why should a specialist subcontractor like us, or any of the professional team, fund the risk that offers only the client a cost saving? fl Fernie argues.

fiUltimately it must be the developer who pays. And that demands a positive, trusting client, plus total commitment from everyone involved. fl

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