 North Downs Tunnel

The North Downs Tunnel, also known as the Blue Bell Hill 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.