Geology and geotechnics of the HS1 North Downs tunnel

The 3.2 km long North Downs Tunnel is located to the east of Maidstone and is part of HS1, the Channel Tunnel Rail Link. Commissioned in 2003, the tunnel is the largest and deepest twin track railway tunnel ever constructed in the United Kingdom conveying high speed trains at speeds of up to 300km/h, at depths reaching 100m beneath the main chalk escarpment comprising Upper, Middle and Lower Chalks. This paper presents both geological and geotechnical aspects associated with the design and construction of the tunnel together with the results of hydrofracture testing of the various chalk units. Details are also provided into the geological conditions, ground deformations and surface settlement experienced during tunnelling and measures and monitoring procedures put in place to control the construction process.