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Bankside team designs out the risk

PILING - The combination of piling techniques used at Bankside 123 in Southwark make it one of Cementation Foundation Skanska's most challenging projects to date.Paul Wheeler reports

NOT SO long ago Southwark was a forgotten and largely overlooked pocket of inner London.Despite its riverside location and proximity to the City, the area remained run down. But that was before the Jubilee Line extension, Tate Modern and Millennium Bridge provided the catalyst to transform Bankside, as Southwark's river frontage is known, into one of London's most chi-chi locations.

Now the regeneration is spreading back from Thames, most notably with the redevelopment of Bankside 123, a major office-led development of three buildings immediately south of the formidable Tate Modern.

Developer Land Securities is behind the scheme to replace the rundown, early 1960s-built PSA offices on Southwark Street. Early focus has been on demolition and enabling works for Building 1, which will become the new headquarters for publisher IPC.

Unusually, foundation engineering has been a major component to the enabling works.This, explains Mohsen Vaziri, director with structural engineer Whitbybird, is because the biggest constraint on site was the forest of existing piles, installed to support the 13-storey PSA building and its two-level diaphragm wall-supported basement.

The previous structures were founded on 450 mm diameter driven Frankipiles, up to 15 m long and based in the river gravel deposits.These were evidently adequate for purpose through the 40-year life of the previous structures but, although Whitbybird looked into making use of these existing foundations, they were simply not up to modern design standards.

The only real option was to design the new scheme with relatively few, very high capacity piles and to remove the existing foundations where they clashed with the new.

Land Securities reasoned that the best way to mitigate the considerable programme risks associated with the existing foundations was to let construction of the new perimeter retaining wall and removal of foundation obstructions as a separate enabling works package.

This would give much greater certainty over the production of the new piles and remove the main element of risk ahead of the main building programme.

To do so, Cementation Foundations Skanska bored through and crushed more than 300 piles using powerful rotary rigs.

The new 400m-long retaining wall runs around the site perimeter and includes a section that bisects the site isolating Building 1 from Buildings 2 and 3, which will be developed under a separate contract.The wall has been designed to support the three-level basement excavation without any temporary works in the basement area.As a result the wall piles are 'working very hard'says Cementation project director Martin Kenwright, but the benefits of unrestricted working space in the top-downconstructed basement more than offset the cost of making the secant piles bigger.

Cementation completed its enabling works, including the substantial new secant-piled perimeter retaining wall on June 18.Groundworks subcontractor McGee then moved onto site, excavating it down to site formation level, enabling the main works for Building 1 to start on schedule in early July.

Land Securities novel packaging of the enabling works effectively resolved the issue of risk in the ground and isolated it from the main build.This was critical because the basement is being built topdown.Once the load-bearing foundations are in place, work on the three-level basement excavation and superstructure will be simultaneous, saving, says Ian Ronchetti, project director with Bovis Lend Lease, seven weeks on the 20-month programme.

Foundation work since July has focused on load-bearing piles for Building 1, which was drawing to a close when Construction News visited the site.

Whitbybird's design, which aims to minimise the number of foundation openings because of the obstruction issues, relies on 67 very high capacity piles installed on a 9 m by 9 m grid. Piles are between 1.2 m and 2.4 m in diameter and are founded in the Thanet Sand at depths of up to 53 m.

Mr Vaziri says: 'We looked at shorter, underream piles based in the London Clay but, with such high loads, differential settlement was always going to be a concern, so we went deeper into the Thanet Sand.'

Even so, there was still a potential settlement issue, which was resolved through a number of workshops involving Whitbybird, Cementation, Bovis Lend Lease and Land Securities.

Mr Kenwright says: 'We designed out concerns over settlement by slightly increasing the pile diameter.This moved the design thinking away from base grouting and ultimately offered considerable cost and programme benefits.'

Cementation's construction technique using bored piling under bentonite, adds Mr Kenwright, is similar to the method it pioneered across the Thames at Moorhouse in 2002. Each pile took three days to construct.

Piles are reinforced to their full depth to resist heave within the London Clay following unloading of the ground; this is not only from the removal of the original buildings but also from the new 9.5 m-deep basement excavation. Pile reinforcement cages incorporate 50 mm diameter steel tubes, which will be used for sonic logging to check the integrity of each pile.

All but three of the piles incorporate plunge columns, which will form the main load-bearing columns within the basement.Tolerances on verticality and position are to standard steel fabrication limits.

This Cementation achieves using its Cemloc adjustable frame system, which is set up at the top of the pile and ensures pinpoint accuracy for the plunge columns.

Faced with a tight schedule, Cementation developed a programme to complete one pile a day, which meant there was a lot of equipment on a congested site.This made site logistics and co-ordination between the three main subcontractors active on site - McGee, Cementation and column prefabrication supplier Burns - absolutely critical.

Bovis Lend Lease's Mr Ronchetti believes the detailed planning on site was very successful and meant the 'different trades dove-tailed very effectively'He adds that the site benefited from a genuine team approach, which, for example, involved daily meetings to co-ordinate who was doing what and where during the next shift.This process, says Mr Ronchetti, was aided by the simple graphical trick of marking the proposed working positions of all plant to scale on the site plan.

The proof of the pudding, adds Mr Kenwright, is that the foundation installation was completed a week ahead of programme. Cementation is temporarily off-site, but will return to construct the load bearing foundations for Buildings 2 and 3 next year.

Mr Kenwright rates the project as one of the most significant in Cementation's recent history, in that it involves many aspects that are right at the leading edge of piling technology.

Although, as he concedes, most of these elements, such as installing large-diameter piles under bentonite into Thanet Sand, coring through heavily reinforced concrete obstructions and plunging columns are not new, these factors combined at Bankside to make for a very testing contract.