An unconventional viaduct design for the A82 road widening

The A82 is a primary route in Scotland that follows the shoreline of Loch Lomond. Following a landslide in the late 1970s, traffic lights were implemented, limiting vehicles to one-way traffic. A widening scheme was required to restore the two-way carriageway over a 400 m length. This scheme included a six-span 168 m viaduct curved in plan to follow the existing road alignment. The form of the structure was highly influenced by the site topography, the adjacent soil/rock slope profile and the loch shoreline. It aimed to provide an economical, buildable solution while minimising intrusion into the sensitive environment and minimising traffic disruption during construction. To meet these criteria, the design was developed to suit a phased construction. A full-length contiguous piled retaining wall supported the existing carriageway and facilitated installation of a trapezoidal steel–concrete composite box-girder viaduct. Partial construction of the deck slab allowed a temporary switch of traffic, enabling the remainder of the bridge works to be completed on the original road alignment. Once constructed, the 3000 t deck was jacked-up to its permanent position using 82 synchronised hydraulic cylinders. This paper summarises how design and construction of the viaduct was achieved in its challenging environment.