Foundations on a London development proved unsuitable for reuse, but their replacements will come with a complete performance history for designers of the future.
London's South Bank is changing.
The area around the Tate Modern art gallery and the Thames Path is the focus of extensive regeneration which extends into Southwark, the borough nestling between the transport hubs of Waterloo and London Bridge.
One development in particular sets the tone. Bovis Lend Lease's Bankside 123 scheme will transform a site on Southwark Street into a £200M office-led development for Land Securities.
Following completion of the piling and ground engineering contract for Bankside 1, Cementation Foundations Skanska (CFS) has returned to site for the £2.2M foundations contract for Bankside 2 and 3. This phase follows a similar blueprint to the first, using large diameter rotary bored piles, with about 75% drilled under bentonite.
The previous building on the site was supported by 450mm dia meter driven Frankipiles, up to 15m deep and founded in the river terrace gravel deposits. Structural engineer Whitbybird considered reusing them, but decided they could not be relied on to meet current quality standards. So for phase two CFS will install 147 piles, ranging in diameter from 750mm to 1500mm, to depths between 27m and 50m.
The deeper piles will be founded in Thanet Sands.
Removing more than 300 of the original Frankipiles was the 'rst step. This was largely done during the enabling works as part of the £8M first phase and is the largest single pile removal contract CFS has carried out to date.
Project manager Morph Kassir says: 'Part of the ' rst phase of the contract was the construction of a 400m long secant pile retaining wall to allow development of the basement structure for buildings 2 and 3 under a separate contract.
'The good thing about CFS being awarded this second phase is we can hit the ground running - we're forecasting quite a fast programme, with completion within three months.' In this latest Bankside contract CFS is continuing its collaboration with Cambridge, Cranfield and City Universities in the research and development of an international programme highlighting the potential for the reuse of foundation piles.
The Reuse of Urban Foundations (Rufus) initiative was piloted at Bankside last year, but this time the team will install prototype fibreoptic instrumentation within the bearing piles, the first time such systems have been used on a live site.
Fibre-optic systems were chosen because of their durability and low sensor cost, the intention being to monitor strain in the piles during the structure's entire lifecycle.
The intention is to 'future-proof' the foundations, enabling their reuse, by providing pile performance data for designers of any later structures on the site.
In a number of secant piles surrounding the basement, inclino meters were installed to measure wall de'ection during excavation. Numerous 3D surveying targets were also located on the secant wall, capping beams and surrounding site to accurately measure movements in three dimensions.
'The project is interesting as, given the ground conditions, the design solution we found was one that included relatively few, but very high capacity piles, ' Kassir adds.
'It makes the monitoring fascinating from a technical point of view, but also arguably more important than in other schemes, so that we know and understand how they are behaving in relation to the ground.' The CFS team concedes that the technologies are not altogether new, but says the challenging ground conditions, combined with the opportunity to monitor the behaviour of the foundations, makes for an excellent project on which to develop the Rufus concept further.
CFS's contract looks set 'nish this month and aims to match the ' rst phase for on-time delivery - completion was a week ahead of schedule.