C103 - THE TALE OF TWO TUNNELS

This article describes the complex job of boring a 6 m i.d. tunnel beneath an operational, brick-lined rail tunnel that is 140 years old and is just 4.5 m away. The job was part of the TL2K's new Section 2 Contract C 103 (C103). The tunnels run northeast to southwest at minimum depth of 5.5 meters down to a maximum of 20 meters at the nadir. Both start from an enlarged cut and cover box. They sit above the groundwater in stiff to very stiff London Clay. TBM erection started in May 2003 and was completed in late July. The new tunnels connect into the existing Thameslink rail lines and approach their reception eyes at about the same level. The existing tunnel was built using cut and cover between 1865 and 1868 and has been found to be robust and capable of accommodating the movements resulting from the new tunneling works, reducing costs by cutting mitigations needed.