Farm Road Bridge, UK: a new ‘top-down’ integral bridge using an existing arch as falsework

Abstract

Electrification of the Great Western Railway Route being undertaken requires the alteration of existing infrastructure. Farm Road Bridge is a railway overbridge located adjacent to the Severn tunnel in the UK. The original masonry arch arrangement was foul of the electrification gauge. The bridge was reconstructed in 2015 with the innovative use of an integral form. The bridge was constructed in an unconventional manner with the deck and piled walls built on top and beyond the existing arch span. Once the new bridge was completed, the arch structure was demolished from below. The bridge was designed to act as an ‘aqueduct’ in extreme flood events. The monolithic nature of the integral structure ensures the bridge is watertight and prevents flooding of the Severn tunnel. As the bridge was designed to fit over the existing arch, the deck has varying depths, slimming down from 1 m at the abutments to 0·5 m thick at the centre of the 20 m span. The increase in road level is thus minimised. The change in relative stiffness of the deck means internal forces are redistributed towards the ends of the span. This paper outlines the design and construction of the permanent and temporary works for this structure.