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Home is where the art is

Structures Site preparation

Three 'ugly sisters' and a massive underground bunker complex were a major challenge on one high profile PFI project in London. Andrew Mylius reports on a game of two halves.

There was never any doubt that site preparation for the new £330M Home Office headquarters in Marsham Street, Westminster, would be tough. During the second world war, a complex of subterranean bunkers had been constructed as an emergency operations HQ for Churchill and his cabinet:

Consisting of two massively reinforced, converted gasholder tanks, or 'rotundas', and a purpose built 'citadel', the three storey deep bomb proof hiding place had been built colossally over strength, with heavily reinforced, 3m thick walls, roof and floor slabs.

Three 21 storey towers had been erected above the shelter in the 1960s, using the existing structures for foundations.

While pulling down the 'three ugly sisters' was a conventional demolition challenge, hacking the bunkers' top storey off would require serious muscle.

Even the largest hydraulic breakers and munchers available struggled to make an impression, says Roger Hewitt, technical director at consultant Pell Frischmann. The three tower blocks came down like skittles when demolition subcontractor Brown & Mason got to work in April 2002. But time soon started running away when the firm hit the bunkers.

The new Home Office building is being delivered under a £330M private finance initiative (PFI) deal - construction is worth £180M - by a consortium of Pell Frischmann, French contractor Bouygues and architect Terry Farrell, with HSBC bank. It consists of three office buildings with three residential blocks to the rear.

Under the PFI deal, the consortium will rack up penalties of over £1M a month for delays beyond the set hand-over date of January next year.

So finding ways to accelerate demolition and speed construction of the new buildings became a major concern.

'Demolition was going so slowly that Brown & Mason went to the Westminster planning authority and asked for permission to complete the work with explosives, ' Hewitt recounts. 'This was exactly when the war in Iraq started.'

To the contractor's surprise its application was approved.

Charges were laid in 2.5m deep holes at 1m centres to crack the structure, helping the breakers take more manageable bites.

Even so, demolition work was only completed late August last year. The ability to claw back time against schedule has been thanks to the scheme's design and Bouygues' construction technique.

The rotundas were big and strong enough to commandeer for foundations providing movement problems could be overcome - there were worries that, following demolition of the 21 storey towers, the new five storey Home Office buildings would not provide enough stabilising mass to prevent uplift. Accordingly the rotundas were infilled with demolition waste.

Compensation grouting was used to stabilise areas of gravel where the new buildings' foundations fell outside the footprint of the rotundas and citadel, preventing differential settlement.

'We managed to make major time and resource savings by reusing the existing structures and demolition waste, ' Hewitt sums up.

Meanwhile, Bouygues had started construction of the buildings at the southern end of the site long before demolition had ceased at the northern end. A speedy, modular method that is common in France but little used in the UK was adopted, says Bouygues production manager Denis Motard.

This consists of insitu reinforced concrete columns at 7.5m centres, spanned by 520mm wide precast concrete beams.

These were corbelled to support precast floor slabs. Insitu concrete stitches and floor topping were cast integrally.

Each level of the buildings is a repeat of the one below.

Bouygues brought in its own design steel shuttering for the columns - it came in two halves, complete with working platforms and guard rails, making for rapid erection and stripping. Precast elements were shipped in from Belgium using just in time delivery to ensure that the tightly constrained site was not clogged up with surplus materials.

Motard admits that his 'biggest challenge has been the programme', but explains that Bouygues has dealt with it 'by selecting very good suppliers and subcontractors'.

This gave reassurance that there would be no delays due to delivery hold ups or quality issues.

'We have had a standard tender process but for each bidder we wanted to visit other sites and their production yards to see what kind of product they were able to deliver.'

Meanwhile, Bouygues' main construction operation has been manned by 15 managers drafted in from France and by workers from Portugal. 'We didn't want to mix people who think in different ways. The French and Portuguese work a lot together - we work to the same rules, ' Motard says - though British and Irish workers generally dominate follow on trades.

Fit out of the first building started while foundations for the third were still being installed. Now furniture installation is under way in the first building, while the third is shortly to be handed over for fit out, ready for occupation by 3,400 civil servants in five months time.

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