Breakthrough of the North Downs Tunnel on the Channel Tunnel Rail Link was achieved last month- two months ahead of schedule.
The 3.2km structure is the UK's first high-speed rail tunnel, passing up to 100m below Bluebell Hill at its centre.
The 13m wide, 10m high single bore tunnel starts from Boxley Valley north of Maidstone and emerges alongside the M2 motorway south of Chatham. The breakthrough saw only the tunnel crown complete from end to end.
Roadheader machines were taken back to the portals to begin excavation of the tunnel down to invert.
Casting of the 350mm thick permanent reinforced concrete lining begins this month.
Tunnelling contract 410 is being carried out by Eurolink consortium of Miller, Dumez GTM and Beton & Monierbau in combination with contract 350, the Medway Bridge project at a total cost of £110M.