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Box digging

UNDERGROUND CONSTRUCTION

Because space is tight at the Moor House site, excavation was a complex process. The shaft was advanced in 1.5m sections and spoil was lifted out of the shaft in a skip by a mechanical hoist mounted on a frame above the ground floor.

The skip then ran along the underside of the first floor before being lowered through an opening in the ground floor to the basement level 'muck bin'. An excavator emptied the bin daily into a lorry for disposal offsite.

The hoist also carried precast tunnel segments (manufactured by Charcon) back to the base of the shaft. A ring of segments was placed and bolted to the ring above before the annulus behind was grouted.

'The biggest challenge was ensuring that we could work 24 hours a day during the shaft dig, ' says Skanska's Matt Cova. This prevented collapse, particularly through the Lambeth Group.

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