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Underground economy

THE OBSERVATIONAL METHOD has produced significant design savings on underground construction for the new Nuffield Hospital in Leeds.

Foundation contractor May Gurney Piling used the approach to design a 7m high basement piled retaining wall, reducing unnecessary excavation of the underlying hard mudstone and sandstone.

The wall had to act as a free-standing cantilever, as the design of the superstructure would not cater for additional horizontal loads from the ground retaining systems.It also had to be watertight, because there is a risk of contamination if a large sewer nearby ruptures.

May Gurney was working for contractor Shepherd Construction on its design and build project. It originally proposed a hard/soft CFA secant pile wall faced with a drained cavity blockwork skin, but suggested that the soft piles could be removed during construction, as there was little risk of water ingress.

Instead, the wall was built as a contiguous piled structure, lined with a waterproofed reinforced concrete wall spanning between the capping beam and the ground floor slab.Geotechnical design was by consultant Babtie.

As part of the observational method, May Gurney installed inclinometers at the centre of two of the piles to provide information on the forces acting on the wall. All piles were installed using the firm's Llamada 73T rig.

Readings were taken before, during and after excavation. The observed head deflection was 18mm compared with a theoretical deflection of 20mm. Observed maximum bending moments were 93% of calculated figures.

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