DEVELOPMENT OF THE SECOND SEVERN CROSSING

This paper describes the design and construction of the Second Severn Crossing, which is three miles downstream of the existing Severn Bridge between England and Wales. In October 1990, the Government signed a formal Concession Agreement with Severn River Crossing Plc to lease and maintain the existing bridge, design and construct the new bridge, and operate and maintain both bridges for up to 30 years. The narrow shipping channel and difficult geographical conditions there led to a bridge design, which includes a cable-stayed bridge with 456m central span, linked to the banks of the Severn by two long multi-span approach viaducts. Before preparing the design, it was decided to: (1) minimise the amount of work to be done in the river; (2) maximise the prefabrication or precasting and modularisation of as many bridge components as practicable; and (3) base construction methods on the extensive use of jack-up barges or self-elevating platforms. The concrete caisson shells for the foundations are up to 350m by 13m by 13m and weigh up to 2000t; they were filled with concrete after placing. The central bridge pylons and viaduct piers are made of concrete. The steel deck sections are pre-assembled on-shore.