PROGRESS ON CTRL'S NORTH DOWNS TUNNEL

This article reports on the project for constructing the 3.2km long North Downs Tunnel in Kent, using the New Austrian Tunnelling Method (NATM). The tunnel will be an essential part of the Channel Tunnel Rail Link (CTRL) Ashford, and London St Pancras. The article describes the tunnel contract, the tunnel design, the tunnel construction and lining, the monitoring regime, and progress to date. Several possible types of design were considered, but the required tunnel dimensions and the predicted ground conditions led to choosing a single-tube, large-span NATM tunnel design. Safety provisions are vital, and include high-level evacuation walkways on either side of the tunnel. Between January 1994 and October 1998, pre-construction site investigations were conducted, and 30 boreholes and 17 trial pits were excavated to determine the geology and ground conditions. Most of the tunnel will be through various types of chalk, but marl is also present. A strict environmental standard for design and construction was imposed by the legislation for the CTRL. Tunnelling began on 16 April 1999, and is being conducted from two portals, using top heading, bench, and invert excavation, using a Paurat E242 roadheader and a Liebherr 932T excavator. Monitoring allows real time analysis of ground conditions during construction.