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Loaded question

Foundations - Chalk with an identity crisis has been causing concern in Cambridgeshire.Damon Schunmann investigates.

Addenbrooke's hospital in Cambridge claims its two new blocks housing wards, a theatre, university research facilities and five floors for the Medical Research Council will make it the first in the country to combine patient care with academic research.

Care and research has also gone into the piling designs at the site due to uncertainty in the ground. This is not because the stratum is unclear as much as because the local chalk does not seem to know how it should be reacting to load.

The ground is 1m fill overlying Lower Chalk and Gault Clay at about 17m down. But a problem for pile designers was that the chalk masquerades as clay.

'Previous designs developed over the years were not appropriate because they simply wouldn't have generated sufficient load capacity, ' explains subcontractor May Gurney general manager Steve Longdon.

The difficulty for subcontractors was whether or not to use existing CIRIA reports (PG6 Piling in chalk and PR11 Foundations in chalk) for guidance.

Longdon says 50% thought the ground would behave as a chalk and 50% thought it would behave as a clay. 'This raised questions as to what was right so we were called in.'

May Gurney has previous experience from in and around Addenbrooke's, and believed clay piles to be the right approach.'

To prove it, May Gurney proposed a suite of preliminary test piles. It installed four CFA piles, three 450mm diameter and one 600mm diameter, as well as a research and development pile for its own work in advancing designs. The results showed an unacceptably high variance in load settlement performance.

Longdon says: 'One of the strange things was that the 600mm pile performed worse than the two at 450mm that were founded at the same depth.' This demonstrated the differing load bearing capacities in the chalk.

As a result May Gurney's design assumed the chalk was clay, and increasing pile lengths meant that most will now, ironically, be founded in the underlying clay anyway.

A bonus of the test piles was that back analysis of the results to gauge chalk settlement allowed refinement of the design. The main works piles could be shortened by reducing the factor of safety to two.

'We will still do three working pile tests to verify the design although we're confident in it, ' Longdon reassures.

The main CFA piling work will see 178, 450mm, 1,250kN load piles and 318, 600mm diameter, 2,120kN load piles, all of which will be installed to between 6m and 21m depth.

A bridge to the existing hospital will link the new buildings and restricted access for piling here means a Hutte 202 mini-rig will be used.

An Alfred McAlpine Capital Projects-Haden Young joint venture is the main contractor on the £73M, 32 year private finance initiative scheme from NHS Addenbrooke's Trust.

Alfred McAlpine is responsible for the civil and structural work while Haden Young is handling the mechanical and electrical side.

The 150m by 45m site is constrained and the JV is looking to offsite prefabrication to keep trades and activities to a minimum. The main site access is also a blue route for ambulances and has to be kept clear at all times.

Piling work started in October and is due to finish before Christmas. Main works are scheduled for completion in just over two years time.

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