Deputy Prime Minister John Prescott was this week scheduled to witness the breakthrough of the North Downs Tunnel on the Channel Tunnel Rail Link, two months ahead of schedule. The moment will mark a milestone in construction of the 3.2km tunnel - the UK's first high-speed rail tunnel - over 100m below Bluebell Hill at the central point in the tunnel. The 13m wide, 10m high single bore tunnel which starts from Boxley Valley north of Maidstone and emerges alongside the M2 south of Chatham is being constructed with a sprayed concrete lining similar to that used on the troubled Heathrow Express project.
However this tunnel passes almost entirely through chalk rather than soft London clay. The breakthrough will see only the tunnel crown complete from end to end. Rockbolting and roadheader machines will be taken back to the portals to begin excavation of the tunnel down to invert. A permanent reinforced concrete lining will then be poured against the shotcrete lining. The tunnelling contract 410 is being carried out by the Eurolink consortium of Miller, Dumez GTM and Beton & Monierbau in combination with contract 350, the Medway Bridge project, at a total cost of £110M.