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Chalk challenge Piling for the new Norfolk & Norwich Hospital has to cope with variable chalk bedrock that has been eroded into a complex system of subterranean valleys.

Construction of one of the first of the Government's Private Finance Initiative hospitals is well under way. Principal contractor John Laing started work on the new £200M Norfolk & Norwich Hospital in Colney, on the western outskirts of Norwich, in January this year, and piling contractor May Gurney Technical Services is now forming the extensive foundations for the 11 buildings that will make up the new medical facility.

Designed to replace the current Norfolk & Norwich Hospital in the centre of the city, the new unit will comprise a total of nearly 90,000m2 of facilities, as well as parking provision for around 2,350 cars with associated roadways and infrastructure.

The greenfield site covers an area of some 255ha on land previously used for farming. Ground conditions are difficult however, as the underlying chalk bedrock has been eroded into a complex system of subterranean valleys, infilled with glacial material consisting of clay, sand and gravel of varying depth and strength. This, combined with problems inherent with founding in the undulating chalk, created significant challenges for companies tendering for the foundation work.

The piling contract awarded to May Gurney originally spec- ified the design, supply and instal- lation of bored concrete piles. But as part of the tendering process, the design and construct team were open to proposals that specified other economic and buildable sol- utions that could produce cost savings. Stewart Sam- ple, John Laing manager responsible for the piling work, explains. 'We knew that the underlying strata was at variable depths which would require the piling contractor to come up with the solution for best pile performance. Our require- ment was for a total package providing both design and cost certainty.'

So before submitting a tender, May Gurney carried out considerable background research using local knowledge and aerial photographs of the site, including one taken 30 years ago which showed - to some extent, at least - the contour of the chalk. With this information on board, the company was then able to put together a total costed package for the whole job.

May Gurney's solution was to install some 3,000 bored piles of 300mm and 350mm diameter. Piling division director Chris Heath says that the company wanted to come up with a solution that was both cost effective and the most technically sound for the ground. 'We decided that we would have to guarantee that every pile would found into chalk if we were to achieve best pile performance with the minimum number of piles.'

The com-pany rejected the alternative solution of shorter driven piles because of the variation in the ground and the lack of confidence in the founding strata, with driving resist- ance not giving a reliable indication of pile capacity. 'We could have ended up with driven piles going to some considerable depth in the chalk and, worse still, a pile refusing in a dense layer above weaker strata could have a very low capacity without anybody knowing it,' says Heath.

Using information obtained from the initial site study, May Gurney persuaded Laing that further investigation was needed on selected areas of the site. This allowed a reasonable picture of the contour of the chalk to be produced and, on the assumption that this survey was sufficiently accurate, the company was able to come up with a total fixed cost for the contract. But it is now responsible if piles have to be extended. 'We're having to do a mini site investigation for each pile group,' says Heath.

Work started at the beginning of March, with test piles being installed. It is due to finish towards the end of July. Among the rigs on site is the Llamada P-100-TT, which is capable of forming 30m deep Continuous flight auger piles up to 1,200mm diameter. This, says Heath, means that May Gurney has the capability to overcome most problems that the difficult ground may throw up.

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