Keltbray brings Drapers' Gardens back down to earth
00:00 28 Mar 2007
|
Drapers’ Gardens is – or rather, was – a building of contradictions. It was until very recently one of London’s most visible buildings, disliked by many because of its proximity to St Paul’s Cathedral when viewed from Waterloo Bridge.
Trying to find the building in the tangle of City streets that surround it is surprisingly difficult, however. The tallest building ever to be demolished in the UK, the tower is now half its original 100m height as Keltbray’s demolition work drops it by more than a floor per week. It is tucked away in a secluded square on pedestrianised Throgmorton Avenue, just a few metres from the Bank of England.
Designed by Richard Seifert, the architect behind the Natwest Tower, and completed in 1967 on land owned by the Drapers’ Company, the building has divided opinion for 40 years. Cited by some as one of the few successful 1960s towers, many more considered the building an eyesore. Seifert himself described it as his proudest achievement.
The demolition process is making the building’s unique structure much more apparent. The base podium section, which originally housed a car park and plant rooms and which served no structural purpose, has been stripped away to free up space for waste crushing and machinery, including a mechanical muncher constantly processing material.
This has exposed the narrow central stem on which the building appears to perch precariously, and the huge concrete cantilever above, shaped to correspond with the eight solid steel internal columns either side of a concrete core. This existing structure proved ideal for cantilevering the huge amount of scaffolding required to surround this massive structure – 500t in total erected over 12 weeks.
Scaffolding
"If we had had to scaffold down to the basement level, there wouldn’t have been enough room at the base to process the waste," says Keltbray project manager Nick Smith.
The 67-week, £5.3m demolition process began in May 2006 with an asbestos strip, at the same time as the scaffolding and Monarflex covering were being put up. Seven weeks were then spent removing the plant room from the top of the building before work began removing the 30 floors. Work was halted several times during January and February due to very high winds, but the project is on schedule.
"We are beating our original target of a floor a week," says Smith. "It’s a learning curve, and as more efficient ways are found of tackling the situation it becomes a factory process."
Each floor generates 700t of waste, which can be removed by only one road, the rest being private. Smaller rubble is disposed of via the building’s lift shafts, while larger pieces are lifted down by the huge tower crane, the mast of which was erected in July to a height of 160m. The crane is lowered by means of a collar, each 6m section of the mast removed as the building drops.
The logistics of working in such a cramped environment are what makes this job special, Smith says. "It’s been fantastic, the most interesting and technically challenging job I’ve ever worked on," he enthuses. "It’s also one of the most striking buildings, largely because of where it is. If it wasn’t in central London, it wouldn’t be the same.
"People’s reaction as they walk past is amazing, there’s real wow factor seeing this huge structure coming down here so close to neighbouring buildings. I think the café across the road has seen a massive upturn in business as people sit and gaze at the work going on!"
The project also shows the public that demolition is a highly skilled process, Smith believes. "The most difficult element of this job is that it’s in the centre of London’s financial district, with all the logistical and time constraints that that brings," he says.
"Demolition happens a lot quicker than the build did, so we have to manage that and react to changes. Jobs like this prove demolition is not just grunt work, but is in fact very technical."
Maintaining good relations with the neighbours, which include major financial institutions as well as smaller businesses, was vital, says Smith. Every one was visited and given a presentation about the project, and all are kept up-to-date with regular newsletters.
Local businesses
The staff of JP Morgan, whose building directly abuts the excavation, expressed such a keen interest in the project that Keltbray ran a competition to offer a small group a full site tour, asking them to guess how many piles underpinned the original building (answer: 195).
As well as a City of London Considerate Construction Scheme Gold Award for the overall project, this open approach also brought an individual gold award for Joe Allison, the project manager in charge of demolition.
"Joe has worked on jobs in the City for 20 years, so his experience is invaluable," says Smith. "We’ve had very few complaints from the neighbours. By getting everyone on board, they know what’s coming and when."
The rich archaeology yielded by the site also attracted interest. Keltbray piled the entire perimeter of the site, and is excavating down to 7m below ground level. The silent hydraulic zero sheet piles have been butted up against party walls, and offer water cutoff and ground support. Ground anchors have been used to free up as much space as possible for the dig, and because of the proximity of the adjacent buildings.
The site will be handed over to the building contractor in October as a tidy sheet-piled box. Drapers’ Gardens will be replaced by a glazed 14-storey Foggo-designed building.
In the meantime, the specialists from Pre-Contract Archaeology have made a surprising number of discoveries, and have increased their team on-site from 20 to 50 individuals. Keltbray has lent a hand with the heavy digging, leaving the experts to concentrate on the more detailed work.
With London Wall just a few hundred metres away, the site falls within the Roman city limits. As well as Roman coins and keys, plus a figurine and some ceramics, the archaeologists found drainage ditches that would have once run into the ancient River Walbrook, a tributary that joined the Thames at Cannon Street up until the late second century. This suggests that existing records of the position of the Walbrook are slightly wrong, a significant find. They also found a Roman road and the remains of houses.
"To us, the site looks unremarkable, but an archaeologist can see it all as it was in their mind’s eye," says Smith. "It’s been fascinating to watch them work."
Concrete cantilever
Keltbray’s last two jobs will be removing the concrete cantilever, which will be hammered from above by machines balanced on the remaining central core, and taking out the substantial concrete basement raft. Measuring 32m x 27m x 3m and heavily reinforced using twisted square bars, a typically 1960s’ design, this has been predrilled with more than 100 holes to weaken the base before the time comes to remove it.
As much work as possible, including the asbestos strip, was done in-house. "We are able to undertake asbestos-only jobs, as well as civil engineering work, which many people don’t realise," says Smith.
The complex temporary works on the site were designed by Keltbray’s own Wentworth House Partnerships. The company also has a specialist works division, which recently worked on the collapsed building in Whitechapel.
"Being able to offer so much in-house eases the interface between different parts of the build. We can reassure the client that there will be no phasing problems. We pride ourselves on being a one-stop shop."