Foundation engineers are increasingly looking at reusing piled foundations.
Paul Wheeler finds out why.
If you are the sort of person who lunches with geotechnical engineers, chances are you may already know something about re-using foundations. Among the geotechnical chattering classes - especially those who practise in London - it is everyone's favourite topic of conversation.
Tim Chapman, associate director at Arup Geotechnics in London, says that at any one time the company typically has three or four jobs on the go in which it is actively investigating the reuse of foundations. Before 1999, such jobs were decades apart.
Chapman refutes the suggestion that he is something of an industry champion of the approach. He says: 'I'm not trying to sell it, I'm just saying it's an option that should be considered.'
But because the unknowns are greater than with conventional jobs there have got to be major advantages to make it viable. 'So to be effective, it's got to be cheaper or quicker or easier or safer, ' he says. 'If it isn't, we would recommend a new foundation every time.'
The fact is that reusing an existing foundation is, increasingly, the only way forward for many projects. This is because the available space for new foundations is rapidly diminishing, as successive generations of piles are installed for each new building.
'Once a building reaches the end of its life, its piles tend to be abandoned, adding to the accumulation of obstructions in the ground and leaving little space for new piles to support the developments of the future, ' explains Arup senior geotechnical engineer Jan Windle.
'Over time, the progressive abandonment of successive sets of piles, without steps to promote foundation reuse, will leave entire central city areas 'sterilised', with no space for economical new foundations, ' says Chapman.
When other obstructions such as tunnels or valuable archaeological deposits are considered, many sites find space for new foundations is scarce and diminishes with each new wave of development.
For geotechnical engineers, finding locations for additional piles has become an increasingly difficult task. Often this results in the need for large and expensive transfer structures to span areas where piles cannot be installed.
It is on sites such as these, Chapman believes, that reuse of old foundations may be the only way to allow more versatile space planning for new developments, as well as allowing more rapid, economical and ultimately sustainable construction.
Chapman says four factors govern the cities in which reusing foundations is likely to become a major issue: First a city has to support high value activities and land prices have to be high. 'Many buildings in central London now have an economic life of little more than 20 years, which, of course, is much shorter than their structural design life, ' he says.
'With the introduction of large bored piles in the 1950s, we've just got to the stage where prime sites are now coming up for their third piled redevelopment - and things are getting tricky.'
This issue is very much a London one at the moment.
Elsewhere there is less economic pressure to have the latest, most efficient buildings; and as a result redevelopment takes place over a longer cycle.
A second factor is historical.
Due to the Blitz, London needed large-scale redevelopment after the Second World War. Other European capitals, such as Paris did not. As a result Paris has a better preserved, and protected, historical centre, with new business areas like La Defence generally sited outside the city centre.
This ties in closely with factor number three - the UK's tight planning controls. These not only restrict what land is
Geology also exerts a significant control on the viability of re-using foundations.
Big buildings in London need deep, large diameter piles. Contrastingly in New York a spine of granite runs at shallow depth down the centre of Manhattan.
Here, installing foundations for a new skyscraper is more or less a matter of stripping things back to bedrock and starting again.
While foundation re-use is pretty much restricted to London for the time being, many other major cities - particularly other European and older north American cities like Chicago - are not far behind.
For this reason the UK's Building Research Establishment (BRE) has established, and is project managing, a Europeanfunded research project on this very topic (see box). Related research is starting at North Carolina State University in the USA, led by Dr Debra Laefer.
Chapman believes one of the most important distinctions when reusing existing foundations is that risk comes at a later stage in a project than it does with a conventional build - and this has to be anticipated and accommodated from the outset.
In a project where reuse of foundations is being looked at, a secondary site investigation is needed after site demolition.
'At this stage you don't know which way you are going to jump, ' says Chapman. 'It's a major decision point that you don't have in a normal project.'
That said, he maintains that there is nothing 'weird or clever' about making use of old foundations. 'It just comes down to the application of good engineering principles.'
Rebuilding the great pile
Reuse of foundations is not new - it's just that it's fallen out of favour over the last century or so.
According to Peter Ackroyd's book, London: the Biography, in Elizabethan times (1558 to 1603), in an attempt to curb London's urban sprawl, new building was only allowed if it were raised 'on old foundations'.
Ackroyd goes on to comment: 'It had been said that no stone ever leaves London but is reused and redeployed, adding to that great pile upon which the city rests'.
In fact, London was obliterated at least 17 times between the Roman occupation and the Great Fire of 1666 and of course was most recently razed in the Blitz of 1940. Each time it has rebuilt itself on the remains of the previous city.
It was only with the construction of large structures in the 19th century that engineers recognised that purpose-designed foundations were needed to prevent excessive differential settlement.
This has developed over the past two centuries to the point that nearly all structures are now built on dedicated foundations, with the reuse of old foundations occurring very rarely in the developed World.
RuFUS drills into the issues
The Re-use of Foundations for Urban Sites (RuFUS) is a European Union funded research project to develop guidelines that will allow foundations to be reused more often.
The project is co-ordinated by BRE, and project partners are Arup and Cementation Foundations Skanska from the UK, Soletanche Bachy from France, the Technical University of Darmstadt and Federal Institute for Materials Testing and Research (BAM) from Germany, consultant Stamatapoulos Associates of Greece and the Swedish Geotechnical Institute.
The $3.5M match-funded project kicked off in early 2003 and is due to last three years.
It will culminate in an international conference to be held in early 2006 at which the final research and guidance on foundation reuse will be published, as well as a suite of papers demonstrating the successful reuse of foundations for a wide range of projects.
BRE's Tony Butcher says: 'A major part of the RuFUS initiative is demonstrating to construction project teams that foundation reuse is a viable alternative. All the participants in a new development need to believe that the technique is not completely novel.' This includes the structural and geotechnical design engineers, checking authorities such as Building Control departments in the UK, the project promoter, funders and prospective insurers.
One of the big problems foundation engineers face when looking to reuse foundations is that as-built construction records are typically very poor. RuFUS will set out the need to make and preserve decent records during construction - and establish a common storage medium.
Chapman says: 'You often find that on any site about 5% of the piles have 'personality' because significant issues have arisen during their installation and testing.' And although everybody on site at the time of construction will know about these anomalous areas, come back in 20 years and all that knowledge will be lost and forgotten.'
He maintains that at the end of a project it would only take a few days for a junior engineer to produce a report that detailed all the site problems and how they were closed out.
While this information could prove extremely valuable during the life of the new building - if for example structural alterations were needed - it becomes priceless when the site next undergoes full-scale redevelopment. 'People should already be thinking along these lines - it's a real no-brainer, ' says Chapman.
He also feels it is likely that new foundations going in today will be reused in the future, which increases the need for independent verification - a view which is at odds with recent moves to self certification.
Another aim of RuFUS is to develop new instrumentation for 'smart' foundations, which will monitor their actual behaviour for direct comparison with the design behaviour. 'This will show the in-service performance of the foundation, the distribution of loads and the change in distribution with time. These data - in conjunction with better preservation of piling records - will facilitate any future reuse during redevelopment of the site, ' says Butcher.
The project website can be accessed on: www. webforum. com/rufus The RuFUS project team welcomes feedback from anyone with an interest in the process or with relevant experience that they wish to share. Email Tony Butcher at BRE: butchert@bre. co. uk