Second Severn Crossing graphic

Classic Projects: Second Severn Crossing

Three decades after the opening of the original Severn Bridge a second crossing was opened to alleviate congestion caused the huge growth in traffic streaming into Wales.

The Second Severn Crossing (SSC) is the most recently constructed bridge over the estuary separating England and Wales. Five miles downstream from the original motorway link, which was constructed in the 1960s, the SSC closely follows the line of the Severn rail tunnel that has been in place for more than a century. It is also close to a ferry route that dates from the days of the Roman Empire, which illustrates how few crossing points there are at this point in the river.

The need for a second crossing arose from the increase in motorway traffic across the Severn Bridge in the 1980s. A rise of 63 per cent in traffic flow, along with repeated closures due to high winds and congestion, meant that the M4 and M5 network could no longer cope. After a period of consultation and tendering, the Secretary of State for Transport selected a bid from John Laing and GTM-Entrepose that led to the formation of Severn River Crossing PLC, which was to design, build, finance, maintain and operate the new bridge.

The second crossing was not to be simply an enlarged version of the original, which is a combination of a suspension bridge, two viaducts and a cable-stayed bridge. The SSC, although bigger in scope, was less complex architecturally, being essentially an enormous viaduct with a central cable-stayed span over the Shoots channel at its centre. Because of its location, local geology and other environmental factors associated with the point where the lower reaches of the River Severn meet the Severn Estuary, the task of construction was complex.

The SSC connects Sudbrook, Monmouthshire, with Severn Beach in South Gloucestershire. At this point on the river there is a tidal range of 14.5m and notorious currents. This meant that the bridge needed to be prefabricated wherever possible and put in place using a fleet of 10 floating craft and seven back-up barges, often working within a two-hour tidal window. The first job was to get the foundations in place, requiring the placing of 37 precast caissons weighing 2,000 tonnes apiece. These were then filled with concrete that took up to 20 weeks to cast.

The caissons were positioned using a computer-controlled dynamic positioning system that could hold the modified barge on station to within a tolerance of half a metre. Once the tide had gone out the construction area was left dry and the caissons could be sealed around their 300m perimeter.

Pier construction and movement of viaduct and pylon segments relied on floating precast sections into place. Much of the bridge deck was moved into place from the construction yard by road, having been 'match bolted' prior to dissassembly and transportation to ensure a perfect fit.

The construction phase took four years and came in at a cost of £330m, compared with the £8m of the original motorway crossing three decades earlier.

But improvements were significant, especially in the way in which the new bridge coped with lateral winds. Its sides are fitted with baffles that improve stability, especially for high-sided vehicles. This means that speed restrictions in windy conditions that were a common feature of the Severn Bridge are now less frequent, while the overall design of the bridge makes it more resistant.

The SSC carries three lanes in both directions as well as a hard shoulder (compared with the Severn Bridge's two lanes) and so handles volume well. It is able to accommodate 75 per cent of traffic across the river with comparative ease.

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