Second Severn Crossing

Infobox_Bridge
bridge_name=Second Severn Crossing


caption=Second Severn crossing seen from the English side looking West, January 2006
official_name=
carries=6 lane M4 motorway
crosses=River Severn
locale=South West England/South East Wales
maint=
id=
design=central span: Cable-stayed bridge
approach viaducts: Segmental bridge
mainspan= convert|456|m|ft|0
length= convert|5128|m|mi|2
width=
height=
clearance=
below=
traffic=
open=5 June 1996
closed=
toll=
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coordinates= coord|51.5745|N|2.7016|W|region:GB_scale:30000|format=dms|display=inline,title
lat=
long=The Second Severn Crossing (Welsh: "Ail Groesfan Hafren") is a motorway crossing over the River Severn between England and Wales, inaugurated on 5 June 1996 by Charles, Prince of Wales to augment the traffic capacity of the original Severn Bridge crossing built in 1966. The bridge marks the lower limit of the River Severn and the start of the Severn Estuary.

Its location is further to the south than the old bridge and being more in line with the landward sides of the M4 motorway, is a shorter journey when travelling from England to South Wales. The junctions at each end are designed for most traffic to use this crossing – to use the old Severn Bridge crossing one has to leave the M4 and join the M48 motorway either at Aust or near Magor. The new crossing carries more traffic than the Severn Bridge, which is still in use. It is wider than the Severn Bridge, having three lanes and hard shoulder each way, compared to the two lanes, cycle path and footpath of the original crossing.

Tolls

In contrast to the original Severn Bridge crossing, the tolls are collected on the Welsh side near Rogiet (but in the same westbound direction as on the first crossing). Tolls can be only be paid for by cash or cheque, or by use of the Severn TAG system, which is a wireless system by which payments are paid electronically as vehicles drive through the toll lane. Cash can be paid at the manned booths or coins can be used at the automated coin gates.

Tolls charges are based on a three tier pricing system:

Motorcycles and other vehicles driven by a UK disabled badge holder are exempted from paying a toll, although these vehicles must stop at the toll booth to confirm their eligibility.

The toll prices are updated every year on 1 January in accordance with the Severn Bridges Act of 1992.cite web | url= http://www.severnbridge.co.uk/Tolls2008.pdf | format= PDF | work= Severn River Crossing Plc | title= Severn Bridge Tolls | accessdate= 2007-12-26]

Design

The crossing is convert|3.186|mi|km miles long, consisting of a single central navigation span over the "Shoots" channel and approach viaducts on either side. The Shoots channel is the deepest section along the width of the Severn estuary, and a longer span was required to ease the passage of shipping under this section. The central bridge section, called the Shoots Bridge, is of cable-stayed design and the central span (between the bridge pylons) is convert|456|m|ft|abbr=on in length. The approach viaducts are of a segmental bridge design. Its Welsh end is in Monmouthshire; its English end at Severn Beach in South Gloucestershire. The crossing forms a gentle S shape and near the English side crosses over the top of the Severn railway tunnel.

The sides of the bridge are fitted with special railings 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

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. [cite web | url= http://www.severnbridge.co.uk/legislation.shtml | title= Bridge legislation | work= Severn River Crossing Plc | accessdate= 2007-12-26] The 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 with completion in 1996.

Sub-assemblies for the bridge were constructed onshore and then shifted by a large tracked vehicle (similar to that used to move the Apollo and Space Shuttle at Cape Kennedy) onto a barge, prior to being floated out on the high tide to the appropriate site. The 37 bridge pier foundations on the approach viaducts are convert|98.11|m|ft|1 apart, and consist of concrete pneumatic caissons weighing 37 tonnes, which were sunk into the mud of the estuary. The decking consists of convert|3.5|m|ft|1 post stressed match cast sections, weighing 200 tonnes each.

The cable stayed section of the crossing is over convert|900|m|ft|0 long, consisting of a convert|35|m|ft|0 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 convert|149|m|ft|0 high twin leg, reinforced and pre-stressed concrete pylons carrying 240 cables which support 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.

Environmental criticisms

Prior to the bridge's construction environmentalists raised numerous concerns and criticisms, chiefly about the immediate damage from construction work and the effects of long-term pollution from a projected increase in car traffic. The approach viaducts from the east sit on English Stones, a rocky outcrop uncovered at low tide. And the crossing is built near extensive mudflats in the Severn Estuary used by various migrating birds.Fact|date=September 2007

Panorama simple



fullwidth = 9800
fullheight = 1175
caption = View of the Second Severn Crossing from Severn Beach on the English side.
height = 200

References

External links

* [http://www.severnbridge.co.uk Severn River Crossing PLC]
* [http://www.iht.org/motorway/m4sscscheme.htm The Motorway Archive: M4 Second Severn Crossing]
* [http://www.youtube.com/watch?v=99i0-NgHeiU Timelapse video of the crossing] on Youtube
* [http://www.severntales.co.uk/page66.html River Severn Bridges]