CHALKING UP A VICTORY

Travellers are now commuting regularly through the Channel Tunnel for both business and pleasure, and construction of the high-speed Channel Tunnel Rail Link (CTRL) in South East England is now progressing. This article reports on one of the key CTRL contracts, for the construction of the huge 3.2km long North Downs Tunnel through Bluebell Hill. Seven days a week, 24h a day, tunnellers are drilling another 8-10m of the tunnel, whose construction is ahead of schedule. The task of opening the bore to its full 166m2 cross section is currently proceeding twice as fast as planned. A few large tunnel boring machines (TBMs) and very few staff are involved in the excavating and shotcreting work. By the end of 2003, the tunnel will carry both tracks of the CTRL, with trains passing each other at 290kph. The tunnel cross section is large, because plenty of space is needed to allow for aerodynamic effects around the dynamic envelopes of the trains. Value engineering, based on experience of excavation through chalk and a revised structural analysis, led to the decision to reduce the tunnel lining from its originally planned 500mm thick reinforced concrete to a 350mm thick unreinforced in situ concrete shell, saving #5M cost. The face is being excavated by a Paurat 242 roadheader, and Liebherr 932 Litronic excavators are also being used.