Prince of Wales Bridge

Prince of Wales Bridge
Gloucestershire, Monmouthshire
The Second Severn Crossing, viewed from Severn Beach
Location
Type:	Central span: Cable-stayed bridge Approach viaducts: Segmental bridge
Carrying:	Six-lane M4 motorway
Crossing:	River Severn
Location
Location:	51°34’28"N, 2°42’6"W
Structure
Length:	3 miles 528 yards Main span: 1,496 feet
Type:	Central span: Cable-stayed bridge Approach viaducts: Segmental bridge
History
	Built 5 June 1996
Information

The Second Severn Crossing or Prince of Wales Bridge is the M4 motorway bridge over the River Severn between Gloucestershire and Monmouthshire, inaugurated on 5 June 1996 by HRH The Prince of Wales to augment the traffic capacity of the original Severn Bridge built in 1966. The bridge marks the lower limit of the River Severn and the start of the Severn Estuary.

Its location is farther to the south than the original bridge and, being more in line with the landward sides of the M4, is a shorter journey than the original crossing. The junctions at each end are designed for most traffic to use this crossing, and in order to use the original Severn Bridge crossing, one has to leave the M4 and join the M48 either at Aust or near Magor. The new crossing carries more traffic than the Severn Bridge, which is still in use. It is much wider than the Severn Bridge, having three lanes and a hard shoulder each way, compared with the two lanes, cycle path and narrow footpath of the original crossing. The path taken by the bridge is close to that of the Severn Tunnel, which has carried the railway line beneath the river bed since 1886.

Much of the estuary is mudflats at low tide, but at high tide can be covered by as much as 16 feet of water. This has presented the engineers with a unique set of constraints, with packets of work being scheduled at low tide and needing to be completed, with the short windows allowed by the tides.

The concession given to the consortium which financed, built and operate the bridge required them to take over the outstanding debt on the original Severn Bridge and to operate the two bridges as a single entity. Tolls were set annually by the government based on the change in the previous year's Retail Price Index. On the expiry of the concession the consortium was required to hand the bridge over into public ownership. In July 2017 it was announced that the toll would be abolished, and as announced on 2 October 2018, tolls were scrapped on 17 December 2018.

Background

The Severn Estuary presented a barrier between the Bristol area and South Wales. The estuary has a maximum tidal range of 48 feet, the second highest in the world and during a rising or falling tide, strong currents of up to 8 knots. Much of the estuary is mud flats that are exposed at low tide that have been designated a Special Protection Area. The central part of the estuary is a navigable channel which, at the site of the bridge, is known as "The Shoots". The bridge is upstream from Avonmouth and the Port of Bristol, but downstream from the Port of Sharpness. The Gloucester Harbour Trustees have responsibility for controlling navigation in the estuary's tidal waters upstream from the bridge.

Until 1966, road travellers had the option of either using the Aust Ferry which had operated since mediæval times (and as a car ferry since 1926) or by making the 57-mile detour via Gloucester. In 1966 the first Severn road bridge, a four-lane suspension bridge was opened carrying the M4 motorway across the Severn. By 1984 traffic across the first Severn Bridge had tripled and it was projected that by the mid-1990s the old bridge would be running at capacity. A study was commissioned into the building of a second crossing – either a tunnel or a bridge. The consultants reported back in 1986 recommending that a new bridge be built downstream from the existing bridge.

In 1988 it was announced that tenders would be invited from private consortia to fund, build and operate the bridge for as specified period. The consortium would also take over the management and the £100 million debt of the old bridge. Tenders were invited in 1989 and in 1990 the concession to build the bridge was awarded to Severn River Crossing plc. Construction work started on 26 April 1992 and the bridge was opened by the Prince of Wales on 5 June 1996.

Design

The Shoots Bridge; the shipping channel lies between the two towers

The architect and aesthetic consultant for the bridge was Ronald Weeks of Percy Thomas Partnership. The bridge which has portals close to Sudbrook in Monmouthshire and Severn Beach in Gloucestershire has three principal sections – a 25-span viaduct on the Gloucestershire side of length 2,300 yards, a 24-span viaduct of length 2,270 yards on the Monmouthshire side and the bridge itself, a 1,036-yard structure with a 120 feet of navigational clearance, giving a total of 3 miles 528 yards. The central section, called the Shoots Bridge, is of cable-stayed design and the central span (between the bridge pylons) is nearly 1,500 feet in length. The approach viaducts are of a segmental bridge design. The crossing forms a very slight "S" curve – the roadway having an approximate east-west alignment at each of the portals, while the central bridge follows an alignment which is approximately WNW to ESE.

The Severn Railway Tunnel passes under the estuary bed on a line which is generally about 500 yards upstream of the bridge, but which passes under the line of the bridge close to the Gloucestershire shore.

The deck, which carries three lanes of traffic in each direction is 113 feet 6¼ inches wide. The sides of the bridge are fitted with 10-foot baffle plates to reduce lateral wind loads coming from the Severn Estuary onto the traffic and this has reduced the number of times that speed restrictions have been needed. The overall design of the new crossing makes it more resistant to high winds than the old Severn Bridge.

Construction

Construction of the Second Severn Crossing

The crossing was built by a business consortium under a public-private partnership. A company called Severn River Crossing plc, led by John Laing plc and GTM-Entrepose, was formed to build the new crossing. This company also took over the responsibility of managing and maintaining the old Severn Bridge crossing, as well as managing and maintaining the new crossing. The cost of constructing the new crossing was expected to be paid for by tolls collected from motorists using the two crossings. Work on the new crossing began in 1992. Completion was in 1996.

Sub-assemblies for the bridge were constructed onshore and then shifted by a large tracked vehicle onto a barge (the SAR3), prior to being floated out on the high tide to the appropriate site. The 37 bridge pier foundations on the approach viaducts are 321 feet 10½ inches apart, and consist of open concrete caissons weighing up to 2,000 tons, which were founded on the rock of the estuary bed. The decking consists of 12-foot post-stressed match cast sections, weighing 200 tons each.

The cable stayed section of the crossing is over 3,427 feet long, consisting of a 113 feet 6¼ inch-wide deck made from steel plate girders with a composite reinforced concrete slab. These were prefabricated on shore and put in place using balanced cantilever methods. There are two high twin leg, reinforced and pre-stressed concrete pylons carrying 240 cables which support the bridge deck rising to a height of 490 feet above the river bed or 330 feet above the bridge deck. Cable vibrations were experienced during construction and secondary cables were added to eliminate this. To avoid detracting from the aesthetics of the primary cables, the secondary cables are very slender and are not very noticeable.

Outside links

("Wikimedia Commons" has material about Prince of Wales Bridge)

• Severn River Crossing PLC
• Timelapse video of the crossing on YouTube

Bridges and crossings on the River Severn
Severn Estuary	Prince of Wales Bridge	Severn Tunnel	Aust Severn Powerline Crossing	Aust Ferry	Severn Bridge	Severn-Wye Cable Tunnel