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Double act's firm Friends

Henry Demolition is working hand-in-glove with piling contractor Fondedile to push ahead on a new London office building. Kate Sleep reports

HENRY Demolition has saved both time and money for client Friends Provident by proposing a novel working method for a new development on St James' Street in London.

The demolition company is knocking down the old office block at the same time as specialist contractor Fondedile is sinking piles for the new building, Metro House.

Andrew Routledge, Henry joint managing director, says: 'We suggested the method and an enlightened client and professional team took it on board. We have a close relationship with Watermans [the client's engineer] which helps.'

Piling and demolition were the first two contracts awarded on the job; Henry and Fondedile worked together for the client before the main contractor Kier was appointed.

Demolishing a building with another specialist on site throws up obvious safety issues. But during the dual work period, a safe working environment is created by using a two-layer buffer zone. No demolition work is carried out less than two floors above the piling team, creating a natural crash mat.

There are benefits to be gained by working this way, according to Tom Henry, Henry Demolition's contracts manager.

'Carrying out piling and demolition simultaneously has advantages other than just time - it cuts down on noise and dust pollution, because piling is kept within an existing structure,' he says.

Like most demolition jobs, Henry has reduced dust even further by cladding the entire structure in a heavy duty monoflex hoarding.

It has also laid salvaged carpet on the scaffolding boards to collect glass shards and stop debris falling between the gaps.

The building is being dismantled from top to bottom with one side disappearing faster than the other. The uneven work pattern is required to maintain support to neighbouring buildings before a temporary steel structure is erected.

At the same time as Henry is dismantling the building, Fond-edile is in the basement working on its 10-week programme to install the 256 piles needed for the new building.

Henry slaves for the piling team; moving debris out of the way of the six piling rigs and making sure the high water table does not slow things up.

One problem caused by demolishing and piling a building at the same time is that Fondedile cannot get access to all its piles, so it will have to return to site for two weeks once Henry is finished.

The existing building has two party walls, which both need support during demolition and construction, one to a far greater extent.

It was the need for design and erection of the temporary support towers which drove the client to employ the demolition and piling contractors ahead of the main contractor; the former designs the system and the latter sinks temporary piles for it to sit on.

The bank next to Metro House is a tall narrow structure with a party wall which requires two major structural steelwork towers to support it. The towers require party wall agreement, which can be a lengthy procedure.

Henry's engineer Dewhurst McFarlan design-ed the towers and then client engineer Watermans and party wall engineer Watts had to approve the design before the party wall agreement got the green light.

Design and installation of the towers has created a number of problems for Henry Demolition.

The towers will eventually sit on 26 temporary piles, with a pile cap below the ground floor level of the new building. This method allows the new build works to carry on around the towers, which will be removed once the new structure is up to full height.

The designers did not know what was under the existing building, so it was a case of making an educated guess, explains Mr Henry.

'Because the old building was a steel structure we thought there'd be a ground beam,' he says.

As it turned out, it was a mass concrete foundation. Ordinarily this would be broken out and the pile cap cast in place. But because of the programme constraints there is no time to break out the concrete, so the two towers will be supported by a steel grid on top of the piles, which is tied into the existing structure.

Demolition of the building will continue and the concrete will be taken out as part of that sequence.

These unexpected ground conditions and the very tight programme forced Henry to fabricate the towers before design approval; a risk which the firm was confident enough to take, says Mr Routledge. 'It is a comfortable risk because we are loading onto the existing founds, which could be stronger than the pile cap,' he says.

And time is of the essence, since Henry needs road closures (which are only granted at weekends) to lift the towers in, and December 6 is the last date the firm will be allowed one before Christmas.

Once these towers are in place, the whole of the old building can be demolished to make way for the main contractor.

Kier will maintain the tight time schedule, starting to build the new office on one side of the site before demolition work is complete on the other.