The UK’s tallest building requires some spectacular foundations. Gemma Goldfingle finds out how the award-winning Arup/Bachy team dealt with challenging groundworks.
The Pinnacle is the latest giant to join the cluster of skyscrapers in the City of London. At 62 storeys and 290m-high, the twisting, turning tower - dubbed the helter skelter - will be the tallest in the UK when completed in 2012.
Such monuments rarely come without huge engineering challenges and so it was here.
One of the first obstacles the Arup/Bachy team had to overcome was the nature of the ground itself.
The Pinnacle will exert loads reaching up to 70MN and its piles drive through through London clay and into more unstable Thanet sands.
To make matters worse, the Bishopsgate site of the Pinnacle covers the footprint of three previous buildings and is cluttered with piles, including 1.5m diameter bored piles and 4.5m underream bells of the former Standard Chartered Bank building.
Finding records of this previous work was no easy task.
On a site so cluttered with existing piles, minimising the amount of concrete to be cored was essential.
At some points 20m of reinforced concrete had to be bored through, which ruled out minipiles and caissons.
“It sounds outrageous: bell coring existing piles and then base grouting into Thanet sands - who would do that?”
As there was a question mark over the performance of underreeam bells in Thanet sands, base grouted piles were the preferred option.
“It sounds outrageous: coring existing [underream] piles and then base grouting into Thanet sands - who would do that?” says Arup geotechnical director Dinesh Patel.
To make the “outrageous” solution possible, Arup sought early contractor involvement from Bachy Soletanche to ensure the solution was buildable.
Their work in overcoming the many complex issues was recognised at the recent GE Awards, where the project took the honours in the projects over £1M category (see page 35).
The solution involved supporting the megaframe with huge 63m deep piles and reusing a large number of existing piles from demolished buildings where lighter loads would be exerted.
Base grouted piles in Thanet sand have only been used twice before in the UK - at Canary Wharf, where Thanets are found at depths of only 25m, and more recently at the Moorhouse development in the City of London.
Knowledge of pile performance in these strata is limited at best.
“This is a huge step change in load from Moorhouse. Column loads are 35MN at maximum at Moorhouse, at the Pinnacle we are supporting 45MN,” says Patel.
The Pinnacle site is underlain by up to 6m of made ground, overlying 3m of Brickearth and Terrace gravels before a 35m depth of London clay begins.
Underlying this layer is 18m of Lambeth group then 11m of Thanet sands.
Early predictions suggested it would take four days to construct each pile, longer if shaft or base coring was needed.
As a result there was concern that low shaft frictions might occur in overlying London and Lambeth clays.
The use of pairs of 1.8m diameter piles per column was considered, which would prevent the Thanet sands being infiltrated, but the level of shaft and base coring would mean huge delays in the programme.
Using a single pile per column was seen as the optimum solution, but each pile would need to reach 63m and would require a diameter of up to 2.4m.
Pre-planning was essential while working in the unknown. Data was pooled from the two previous projects in Thanet sands.
To supplement this, the team convinced client Arab Investors to spend £250,000 on a test pile.
“Although this was a lot of money, it was essential to make sure the piles could carry the loads in Thanet sands. These are not only the deepest piles in the UK, but the largest diameter ever to be base grouted,” claims Patel.
The developer was also keen to reuse the existing foundations on the site.
“With existing underream piles reaching down to about 30m, it didn’t make sense to core through to construct new piles,” says Arup project director Paula Walsh.
“There has been a huge change in mindset in the past five to 10 years. Clients just wouldn’t have felt comfortable reusing piles. That is changing, as sustainability and cost become increasingly important,” she adds.
But to reuse the old foundations, Arup had to first establish what was there and if it would configure with the loads in the new design.
Unfortunately, digging out the records from the original construction was no easy task.
“The challenge of building in the City is often what has gone on in the past,” says Patel. “It requires a great deal of detective work to find out the history of each site and what is hidden underground.”
Record keeping was not such a high priority 25 years ago when the previous buildings on the Pinnacle site were constructed.
Patel and his team did a lot of research seeking information from parties who worked on the original site.
“A lot of door tapping was involved. Luckily the contractor who worked on the Standard Chartered Bank still operated in the UK so we gained some information about what was there, but no definitive pile records,” says Patel.
Conversations with the previous contractor revealed that not all of the piles on site were fully reinforced, thwarting the original plan of reusing all of the existing piles.
The northern half of site had only short cages, and cracking was experienced below the cage during the original basement excavation.
All these piles had to be cored and grouted before the structure could be created.
There was a strong risk that cracking could occur if the piles were reused, so new piles had to be used for the northern half of the site.
“It was a cluttered site. We had more concrete than soil,” says Walsh.
“The client had wanted to extend an existing three-storey basement, but breaking through that amount of concrete would be a nightmare, so we decided to stick with the existing basement profile.”
“There has been a huge change in mindset in the past five to 10 years. Clients just wouldn’t have felt comfortable reusing piles. That is changing, as sustainability and cost become increasingly important,”
The little information known about this much-developed site was also going to bring problems.
This was made clear when desk research uncovered photos of the temporary works showing sacrificial temporary king posts supporting the perimeter wall of the 1980s buildings.
Groundworks contractor Keltbray also got involved in the project early.
To cut the risk of hitting obstructions it was decided to pile from the ground floor slab with pre-drilled holes using specialist plant. An 18m temporary casing was installed after the hole was cut, ready for piling.
“Piling contractors are good at building piles, groundwork contractors are skilled at removing obstructions. There was always a risk of hitting an underream or temporary post, but by getting Keltbray to open up holes we minimised the risk of delaying the project further down the line,” says Patel.
Each pile took two days to construct, working 17 hours a day.
The pile was bored down to 57m - using Bachy’s BG40 rig - where the Lambeth group begins to become sandy.
Bentonite was then flooded into the casing to toe level and the next 4m were drilled using the auger.
A digging bucket with teeth was used to remove soil until about 150mm above the final toe level and then a cleaning bucket with blades was used to clean the pile and cut the final length.
As a load of 20MN is transferred to the base, it is essential that it is stable.
Using the cleaning bucket prevented suction being generated at the base and stopped the sand being disturbed.
Patel calls this technique a “Bachy special”, and says the early involvement with the contractor was vital in getting it right.
Following this operation, a steel plate attached to a fabric tape measure was lowered to the base to check hardness.
This was done immediately after the base cleaning before the installation of the reinforcement cage.
A special ranking system was devised for the scheme using these measurements.
Once the pile quality was deemed acceptable, 250m3 of concrete grout was then poured through the full length of the tremmie while bentonite was pumped out.
The Bachy and Arup team worked together to engineer an innovative way of delivering the grout without using a costly double cage system.
Using a traditional perimeter tube - a manchette - would not have given a uniform covering, so a pipe was toggled through the centre of the pile to deliver the grout.
“Despite the complex nature of this project everything went like clockwork. Working together as a team was essential,” says Patel.
Piling began in July 2008 and was completed in September last year.
The project remains on track to meet its 2012 completion date when it will take its place both on the London skyline and in the history books.