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Multiple bypass

PILING & FOUNDATIONS

Close co-ordination of foundation work is helping the smooth construction of Derby's new 'superhospital'.Max Soudain reports.

Constructing a new hospital is always a huge undertaking, let alone trying to build one on the site of the active hospital it is replacing. But this is what is happening at Derby City General Hospital, where redevelopment work is well under to create a new 'superhospital'.

Southern Derbyshire Acute Hospitals NHS Trust says the City General and Derbyshire Royal Infirmary rely on each other to provide a full range of services, but because they are 4km apart it is 'not the most efficient and effective way to organise acute care in Derby'.

The £336M redevelopment will unite the city's main medical disciplines on a single site, as well as providing another 600 beds and more operating theatres to cope with increased demand.

Derbyshire Royal Infirmary will be redeveloped as a community facility providing rehabilitation and therapy services.

Work was procured under the government's Private Finance Initiative. The design, build, finance and operate contract was awarded to Skanska/Innisfree in June 2002.

Skanska Derby, a joint venture of Skanska Integrated Projects and M&E firm Skanska Rashliegh Wetherfoil, began the first, two-and-a-half year phase of the six-year project in spring 2003.

The team is responsible for construction of the new hospital as well as 30 years of hard and soft maintenance - everything except clinical functions.

'The contract was won on a straight tender, ' says Skanska Derby project director Cliff Bryant. 'The contract is almost a 50/50 split between M&E and building - it is an integrated approach.'

Two main buildings are being constructed at the General Hospital site: the acute hospital, which has six storeys, and a fourstorey day care and outpatients facility. Only a quarter of the original buildings will remain.

During construction, 40% of departments will be housed in £30M of temporary structures, before moving into some of the new buildings at the end of phase one. Even this will be temporary, with further shuffling of departments after demolition and completion of the two hospital buildings, explains Skanska Derby package manager Matt Barnes.

Demolition of the General Hospital buildings and excavation of the pad foundations began last August and was finished in November. Although some material had to be disposed of offsite - 'there was some asbestos, which was removed by a specialist contractor', says Barnes - most is being recycled on a site across the road. Brick and inert material is dried, crushed and used, among other things, for the piling mat.

Skanska Cementation Foundations came on site in September to carry out vertical load tests on three CFA piles.

Contracts manager Martin Cooper says: 'The original tender was based on 750mm, 950mm and 1,050mm rotary bored piles, but after the rotary anchor piles for the test were installed, it was decided to switch to CFA.'

Driven piling had already been ruled out because of the sensitivity of operating theatres to vibration. 'In places we are piling just 2m away from hospital buildings, ' Cooper says.

The main reason for the switch was groundwater, which is at various levels - particularly between 6m and 7m at the base of the fill and clay underlying the site.

Beneath is Grade IV to Grade I Mercia Mudstone, where the piles are founded.

'We would have had to use long casing or tremie in concrete if we had used rotary bored piles, ' Cooper explains. 'Piles were tested to 250% working loads and design changed accordingly.'

Cooper points out that just because Cementation's parent firm was building the hospital, it did not mean winning the piling contract was automatic. 'The contract was a competitive tender with other piling firms - everything had to be transparent because it is a PFI project, ' he says.

Columns for the six-storey acute hospital will be supported on single piles installed on a 6m by 6m grid (although there is some variation).

There are three loading ranges:

l750mm diameter, 11.5m long piles carry up to 2,900kN - 'but in places carry as little as 200kN';

l750mm diameter between 17m and 18m long piles carrying 4,200kN;

land 900mm diameter, 18m to 19m long piles carrying 5,500kN.

'Rather than short and fat piles we went for longer and thinner ones, ' Cooper says.

The heaviest loads are under the helipad but heavy loads are found 'all over the place'. Piles also had to have sufficient capacity to allow up to three more storeys to be added in the future.

Pile construction was straightforward. There are few lateral loads and the piles are founded in the competent Grade I and II mudstone. 'There are no special settlement criteria. Pile tolerance is 75mm in plan and 1:75 vertically.'

Standard Class II 40N concrete is being used for the piles, which are reinforced with cages of eight T20 bars, starting 3m to 4m below cut-off level, which varies across the site - 'So cages are up to 9m long but are mostly 6m.'

Cementation's contract originally included construction of a 50m long contiguous pile wall for the service trench through the middle of the building.

'This was made up of 600mm diameter, 11m long piles for a 5m retained height, which would have been very expensive, ' Cooper says. 'Instead the trench was made shallower, removing the need for the wall.'

The outpatients building is only four storeys and there are just two load cases. Loads up to 2,000kN will be supported by 750mm diameter, 8m long CFA piles; for 3,500kN loads, 750mm diameter, 13m long piles will be used.

Piling proper began at the end of October. Three rigs were used at the peak of work, one on the acute hospital, one on the outpatients building and one on the contiguous wall.

Up to 10, 900mm diameter and 16, 750mm diameter piles were installed each day in this first phase of piling, which saw 800 piles completed by the end of January. Contractor Laing O'Rourke moved in at the beginning of January to start erecting the reinforced concrete superstructures.

Cementation will return to install a similar amount of piles for the second phase, starting in autumn 2006. The new hospital is due to open in 2008.

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