Picture: Flickr/Kingsimmy
Last week developer Lipton Rogers and fund manager Axa announced that they had purchased the halted Pinnacle tower in central London, with the intention of building a redesigned tower in its place.
Brookfield Multiplex, which stopped construction of the planned 288 metre tower in 2012 after building the foundations and the first seven storeys of the core, has now been signed on as pre-construction adviser by Lipton Rogers and Axa.
So how exactly do you deal with the challenge of a four-year old concrete stub that belongs to an abandoned architectural design? Construction Manager spoke to Nick Ling, technical director at global engineering consultancy Mott MacDonald to find out what potential issues the developers and Brookfield Multiplex will face.
He pointed out that it’s not that unusual to have a site that is encumbered by existing foundations – there are plenty of examples in London where a new building has been built on the substructure of a previous building and the industry has well-documented procedures to follow.
“London is a very congested city and sites may have several generations of foundations to deal with. In general our experience is that there are three solutions when faced with existing foundations,” says Ling.
It is not unusual to build on existing foundations in London> Flickr/Matt Brown
“First, you can pull out the existing foundations. This can be costly and is rarely done, especially when the piles are deep, which they would be for a tall building.
“Alternatively, you can build new foundations around the existing piles. This gives the client comfort about the design, but means you have to design the new building around what is already there.
“Or third, you can reuse the existing foundations, perhaps in combination with new ones. This can be problematic when records are poor as you will have to undertake a lot of investigation, which also comes with a cost and risk.
“The positive for the Pinnacle is that the records should be very good, with a high level of knowledge about what was built, by a credible design and construction team.”
“The new contractor would have to carry out testing and due diligence to provide assurance that the foundations will work for the new scheme.”
Nick Ling, Mott MacDonald
With Brookfield Multiplex now back on board, some of the potential legal and insurance complications have been removed, he says.
“In many cases if you use a building’s original foundations there is the legal issue of who is responsible for guaranteeing the work as fit for purpose.
“[There have been cases where] the original contractor may be unwilling to provide a warranty as their work is not being used as intended. The new contractor would have to carry out testing and due diligence to provide assurance that the foundations will work for the new scheme. There may be a residual risk in this, for which a new contractor may charge a premium. The developer will need to know who is responsible if there is a fault further down the line so the lawyers will be all over this.
“Employing the original contractor is one solution to help get around this, which is what appears to be happening at the Pinnacle site.”
But one near certainty is that we won’t be seeing the stub for much longer, Ling predicts. “I doubt that the stub will survive. I think that the building will be brought back to foundations if the design changes are substantial. Then the engineer will have the normal challenge of building a substructure to transfer the loads of the redesigned building to the foundations. This might require the sinking of additional piles and dealing with the resultant differential settlement issues.
“I have every confidence that a good engineer will approach this in the right way based on a wealth of well documented research to ensure the new building is structurally sound.”
