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Top of the waiting list

Geotechnics Hospital building programme

Some building projects demand a whole host of intricate foundation remedies. Alan Sparks checked out the prescription for one London site.

Anew £225M state of the art training hospital is springing up opposite Euston station. But, although progress is fast, there is a 170m long headache that runs along the site boundary - the oldest Tube tunnel on the London Underground network.

Protecting the listed brick tunnel that lies just 1m below the busy street level is fundamental if the 32 year PFI contract is going to make the joint venture of Balfour Beatty and Amec (BCJV) a profitable one. Contractor McGee has erected the steelwork and O'Rourke will construct all retaining walls and concrete frames - a total concrete volume of more than 37,000m 3. With a floor area of over 72,500m 2, the aluminium/glass clad project is due for completion in April 2005.

Restraining surrounding buildings, Tube lines and highways are 787 piles, some up to 40m deep. The extensive temporary works accounts for 520 of these. Ground conditions were not straightforward either, as half of the site is liable to settlement and the other half subject to heave, and only a 10mm differential settlement is acceptable.

But the consequences of failing to hold back the Tube tunnel below Euston Road carried the greatest threat, so a belt and braces solution was prescribed.

Steel I beams, over 15m long and 914mm deep, were plunged up to 6m into 1,500mm diameter C50 grade concrete bored piles.

Once formed, these king posts then prop the basement walls of the office block that previously occupied the site. Inclinometers within the piles are linked up to a 24 hour monitoring log that allows engineers to keep a watchful eye on settlement.

The piles used a hollow steel sleeve to prevent the gravel layer overlying the clay from falling into the bore, and a polymer based medium was chosen to ensure no water ingress. Environmental concerns are the usual justification for this choice over more common bentonite.

But not this time.

'Primarily, the polymer solution was selected as it bought us space. The kit is only half the size of that needed for the bentonite de-sanding plant, ' said BCJV site director Andy Clarke.

Although the site covers a large area, space is tight and has meant that every square metre was garnered wherever possible. 'With four piling rigs and three tower cranes on the go at the same time, you find that once you've allowed a 40m radius exclusion zone around these, remaining space is in pretty short supply, ' explains BCJV project director Stuart Fraser.

'We have needed to reorganise the programme constantly to maximise the efficiency of the interface between different trades. But managing the risk facing the 170 workers on site was the main driver in our philosophy, ' says Clarke. 'The size of the temporary king posts looks big, but in our opinion, any extra cost outweighs the added risk that a less conservative design would have carried.'

Pinching space wherever possible was seen as a key target for the programme team. 'One way we have increased our working area has been by avoiding raking piles wherever possible, ' adds Clarke.

Flying shores 30m long constructed from 550mm diameter circular hollow sections perform this task through the main rib cage of the site - each carrying a 1,000kN load. These were flatjacked into position to exert a permanent outward force on to the walls. Day and night, the stress in these members is also remotely monitored.

In one corner of the site this was not possible, and complex steel frames have been used to restrain the top of the contiguous piled retaining wall. Where there was little distance behind these walls and no horizontal propping was realistic, raking piles had to be used.

'However, in one area of the site, there was a large enough shelf behind the contiguous piled retaining wall to install 1,200mm diameter concrete piles, ' says Clarke. 'A reinforced concrete temporary waling beam spanning behind these was cast to tie a 60mm diameter rod back to the wall, therefore avoiding raking props and buying us some more room.'

Piling specialist Amec Stent completed the 11 month piling works in January, effectively leaving a blinded surface.

O'Rourke then entered the fray to form the structural frame and the permanent solution retaining walls.

So precise is the jumpform system used by O'Rourke that the walls on the central core of the 17 storey tower are only 200mm thick. Additives, such as pulverised fuel ash (PFA), are needed to attain the necessary workability of 200mm slump to counter any voids within the heavily reinforced frame, and yet still find a high enough strength gain to build up from it the next day. Current progress stands at the rate of one floor per week.

'Concrete was the only option here as limiting vibration and noise from the nearby traffic was extremely important for this particular building, ' explains Fraser.

At its peak, there were 150 lorry movements every day, with nowhere to wait around the site as it lies in the heart of a red route. 'The only way we were able to cope with the amount of traffic was to build a bridge right through the centre, ' shrugs Clarke nonchalantly.

'We used the bridge for loading and unloading - we just couldn't have done it without it.'

Steel trussed columns were prefabricated off site and brought in single pieces to be sunk into concrete piles that are 3m in diameter. The guide frames were specially made and a mighty job in themselves.

When the building climbs, a 2m thick concrete slab will replace the bridge. Originally, this slab was only to be designed for pedestrian loading, as it will eventually support the main entrance to the hospital. But the design was stepped up to carry full lorry loading - allowing use for site traffic throughout construction.

The basement will house six radiotherapy bunkers. These require a 125mm thick steel shield to be embedded in the 2m thick walls. The floor of the basement is constructed from an 1,800mm thick raft slab with heavy reinforcement mats top and bottom.

In every way this is a large job and could be the tip of the hospital building programme iceberg that will rely on public private partnership agreements. 'Normally, this job would cost up to 20% more under a traditional contract, ' says Fraser. 'It just would not have been built without a PFI contract.'

Should a leak spring from one of the retaining walls in 20 years time, and radiotherapy treatment have to be suspended, the responsibility would fall squarely on the shoulders of BCJV. 'The penalty would be unthinkable.' But, for Fraser, this responsibility does not kill the thrill from working on such a flagship project.

'Without doubt, this is the most interesting job I've been involved with. We have been consulting with the client on everything from heavy ground engineering through to clinical design. This is a once in a lifetime job.'

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