Designed to withstand the impact of a 500 lb bomb, these sacrificial slabs are 1.4-3.2 m thick and reinforced with a mesh of steel tramlines that were ripped up from the surrounding roads. If that was not enough, contractors used high strength, 70-90 kN concrete to increase the density.
Now Bovis Lend Lease, with the aid of demolition contractors Robore Cuts and McGee, is undoing that work. Under normal circumstances it might be a case of just using heavy excavating equipment to break up the slabs, but with Gordon Brown's helpers still occupying part of the site, battering and blasting is out of the question during the day. Indeed civil servants have banned noisy work from 8am to 8pm.
Quiet nights
What about breaking up the slabs at night? Unfortunately, the Treasury on King Charles Street is just a couple of streets away from Downing Street. And local planners have banned noisy work from 8pm to 6am so that local residents – including baby Leo Blair – are not disturbed.
Julian Daniel, Bovis Lend Lease's project director for the £112m redevelopment of the Treasury, explains that this would leave only two hours a day for the heavy demolition – not nearly enough for Bovis to finish the PFI contract in the two years allowed.
So Daniel called in specialist demolition contractor Robore Cuts. More engineer than contractor, Robore is cutting up the giant slabs using a diamond saw, which costs £200/m of saw. Once the blocks have been cut up into segments light enough for the tower crane to carry – about 20 tonnes – Robore uses hydraulic bursters to pop the blocks out of the slab ready for craning out. The bursters provide up to 13,000 psi of pressure – that is about 300 tonnes of lifting power delivered through the head of the burster.
The bursters
The bursters, explains managing director Robert Harverson, are also inserted within the blocks to create cracks along the fault lines in the slabs. The cracks are wedged open and holes are drilled in for the lifting gear to be attached. The detonation slabs cover about 65% of the floor area of the Treasury, explains Harverson, and will take 21 weeks to remove.
While Robore is undertaking the £1m demolition of the detonation slabs, McGee is carrying out a £7m general demolition contract within the main Treasury building. Bovis brought McGee in on negotiated terms and the contractor started on site on 17 July, just 10 days after Bovis took control of the site and only two months after the PFI contract was finally signed.
Foster & Partners
The task for McGee is not straightforward either. The Foster & Partners design entails converting the building from one dominated by cellular offices to an open plan environment. This means taking out all the floor slabs of the building so that they can be replaced by raised access floors and also stripping back 60% of the internal walls.
Because the office has a Grade II* facade, which must remain intact, the demolition is being carried out top-down. This means the process is complex. First, the rubble must be taken away using skips that are craned in. McGee explains that there are six 4.5 yd3 skips in constant use taking out the rubble from the clinker concrete slab. It is expected that the skips will be in use nine hours a day for 10 weeks.
And to make sure that this vast demolition job is not hampered by the weather, temporary roofs have been constructed above the building. This is just as well, considering the amount of rain that has fallen over the past couple of months.
A lot of the internal walls are also being demolished to make way for the open plan design, so sophisticated shoring has to be used to stop the facade collapsing in. And huge transfer beams, some nearly 1 m deep and weighing 2.5 tonnes, criss-cross the already exposed upper floors – making the site look more like a dry-dock for a battleship.
Costs
Lower down the six-storey building, as yet untouched by McGee's 160 site workers, a dense web of props and structural scaffolding holds up stair cores and other vital structural members. The contractor estimates that the temporary works will cost £600,000 and the scaffolding £1.5m.
Even the soft stripping of the internal walls is not simple, says Daniel, who worked for Tarmac on the Gravesend PFI hospital before working for Bovis at Saddlers Wells. "Some of the walls are 6 m high and are solid masonry. It's one of the hardest soft strips I've ever seen," he says.
McGee reckons that by the end of the three-month demolition contract, the firm will have stripped 12.5 km of internal walls, which is about 30,000 m3 of rubble. cm
Source
Construction Manager
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