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UNDERGROUND MOVEMENT: Construction of the North Downs tunnel on the Channel Tunnel Rail Link is advancing as fast as 6m a day. Contracting joint venture Eurolink is building the tunnel as part of Contract 410. The consortium's members comprise Miller Civil Engineering, Dumez GTM and Beton & Monierbau. They are using a combination of 25mm diameter rock bolts and lattice girder supported shotcrete to form the 13.5m wide, 250mm thick primary lining through solid but faulted Kent chalk. Excavation of the 3.2km tunnel uses road header and back acting machines. Wet process shotcrete is applied using a manually controlled robotic nozzle. Start of work at the northern portal was delayed by the discovery of swallow holes. The tunnel heading will be excavated full length by next May, before the bench and then invert are removed. Construction of the 500mm cast in-situ reinforced concrete lining is expected to begin six months later. The consortium is also working on the adjacent £30M, 1.2km Medway Crossing contract. This will be built 70m south of the existing M2 viaduct and will comprise two incrementally launched reinforced concrete box viaducts and a three span 350m balanced cantilever central section across the river. (Full report next week)

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