North Downs Tunnel technical overview

摘要

The North Downs Tunnel forms part of Section 1 of the Channel Tunnel Rail Link (CTRL, route shown in Figure 1), to be opened later this year, and is the only CTRL tunnel constructed by Eurolink using the principals of NATM. The tunnel lies to the south of Chatham with it's north-west portal (London Portal) situated at the head of the Nashenden Valley and it's south-eastern portal (Country Portal) located as the tunnel emerges from beneath the escarpment face of Bluebell Hill. At 3.2km in length and an excavated cross section of up to 174m~2 (13m diameter), it is also one of the largest rail tunnels to be constructed in Europe. The tunnel traverses through a varied sequence of chalk beds at a depth of up to 100m below the ground surface. Constructed five months ahead of programme and substantially under target cost, the background to its design and construction is therefore of considerable interest with much of its success being attributed to the technical innovation, value engineering and integrated observational approach adopted throughout its construction. A summary of some of the more important aspects is outlined below.