Jacked installation of underbridges

This paper describes the development and use of jacked tunnel methods for installing underbridges below rail tracks and highways avoiding disruption to traffic movements. A range of techniques based on jacking precast units have been developed in the UK since 1967, when the abutments for a small underbridge below the main rail line near Exeter were jacked into place, the bridge deck being installed in a short track possession. This was the forerunner to a number of developments and projects, including modular methods to create bridge piers and abutments, box jacking and advanced support structures. The developments of the jacked deck and arch methods based on preformed lining units being jacked in on a slide path, a hybrid of deck slide and tunnelling, have greatly increased the possible clear spans, installation lengths and structure configurations. The key development issues are illustrated by reference to completed projects.