The Second Severn Crossing provides the main road link between England and Wales, and carries more than 60,000 vehicles a day.
Halcrow, in partnership with French consultant SEEE, designed the new crossing on behalf of the Laing-GTM that won a £330 million contract to design and build the bridge.
The new 5.2km long crossing was built to relieve congestion on the original Severn Bridge. It crosses the Severn estuary 4.8km downstream of the existing Severn Bridge, between English Stones and Gwent, and supports a six-lane motorway. The crossing is made up of a central cable-stayed bridge, with a 456m main span over the navigation channel, and approach viaducts built of post-tensioned, glued segmental, precast concrete box girders.
Halcrow-SEEE were appointed designers to the consortium, initially to prepare the bid and subsequently to provide detailed design and construction supervision, including site investigations and hydrogeological studies.
With high winds and a 14.5m tidal range, the Severn estuary is a demanding environment for construction, so the underlying philosophy for both design and construction was to minimise offshore work by using precasting as much as possible.
The cable stayed section of the crossing has two, 137m high, hollow reinforced concrete pylons, built using “jump-form" climbing formwork system and in situ reinforced concrete. The bridge deck units are structural steel with a reinforced concrete roadway, and each unit weighed 180t and measured 34.6m in width and 7m in length.
The 4.2km of approach viaducts were constructed in 100m spans using the balanced-cantilever method, with 27 glued segmental precast concrete sections needed to form each carriageway of every span. Over 2,200 viaduct units, each weighing up to 200t, were manufactured in a purpose built precasting yard.
The majority of the piers were built using glued segmental, concrete box sections, post-tensioned using unbonded tendons. Most of the pier foundations were formed on rock within 37 precast concrete caissons, each 53m long and weighing up to 2,000 tonnes. They were placed using purpose designed marine equipment, and then filled with concrete, taking between 12 and 20 weeks to cast.
Halcrow-SEEE designed the bridge to withstand earthquake loading, and used extensive computer modelling to design the foundations to resist ship impact. We used both physical and mathematical modelling to analyse the effect of the bridge on the regime and environment of the estuary.
Special hybrid caisson/piled foundations were designed to support the bridge where it crosses a tunnel that carries the main rail line between England and Wales to ensure that load is not transferred onto the tunnel, and a monitoring system was developed to detect deformations in the tunnel lining.
Construction of the crossing took four years, and required 320,000 cu m of concrete, 30,000t of reinforcing steel and 150,000m of pre-stressing steel. It was officially opened in 1996.