North Downs Tunnel
The tunnel was constructed by mechanical excavation of the weak chalk, followed by application of a sprayed concrete lining (SCL), prior to construction of an in-situ reinforced concrete secondary lining. Due to the environmental sensitivity of the area, no pressure relief shafts were provided.
It was completed in December 2001 as part of the first phase of the CTRL, £5 million under budget and five months ahead of schedule. Over 100 workers working in shifts 24 hours a day made this possible, despite the site being perceived as a major risk before the project.
The tunnel is 3.2 kilometres (2 mi) long, with an internal diameter of 12 metres (40 ft) and a cross-sectional area (CSA) of 150 square metres, and descends to a depth of 100 m (325 ft) below the chalk hills. This made it one of the largest (in terms of CSA) and deepest twin-track railway tunnels ever constructed in the UK.
Trains using High Speed 1 can reach speeds of 300 kilometres per hour (186 mph) but, for safety reasons, they will travel at a lower speed through the tunnel and across the Medway Viaduct over the River Medway.
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