North Downs Tunnel

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The High Speed 1 tunnel country portal, under the North Downs at Blue Bell Hill

The North Downs Tunnel is a railway tunnel that carries High Speed 1 through the North Downs, at Blue Bell Hill near Maidstone in Kent, south-east England.

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 km (two miles) long, with an internal diameter of 12 m (40 ft) and a cross-sectional area (CSA) of 150 m², and descends to a depth of 100 m 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 km/h (186mph) but, for safety reasons,^{[citation needed]} they will travel at a lower speed through the tunnel and across the Medway Viaduct over the River Medway.