vol 8, issue 2

Classic Projects: Second Severn Crossing

11 February 2013
By Nick Smith
Share |
Second Severn Crossing graphic

Find out about the River Severn’s most southerly bridge

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.

NEXT MONTH: The manual typewriter

Share |

1,000 construction workers employed

Construction period 1992-1996

Opened on 5 June 1996 by HRH Prince of Wales

Toll booths on Welsh side

Pay to get into Wales – no exit charge

60,000 vehicles use crossing per day

Carries six-lane M4 motorway

The SSC is a cable-stayed bridge with approaching viaducts

Seen from above the bridge is a slight 'S' shape

Volume of concrete 320,000m3

Weight of reinforcing steel 30,000 tonnes

Total length of pre-stressing steel 150,000m

Number of concrete deck units: 2,434

Related forum discussions
forum comment To start a discussion topic about this article, please log in or register.    

Latest Issue

E&T cover image 1607

"As the dust settles after the referendum result, we consider what happens next. We also look forward to an international summer of sport."

E&T jobs

  • Control System Engineer

    United Utilities
    • Lancaster, Lancashire
    • Up to £33415 + Comprehensive Benefits

    Provide ICA maintenance and engineering support to the Water & Wastewater Production

    • Recruiter: United Utilities

    Apply for this job

  • Signal Processing Engineer

    B&W Group
    • Steyning, West Sussex
    • Competitive Salary

    We are looking for a Signal Processing Engineer to support the R&D process on active loudspeaker products.

    • Recruiter: B&W Group

    Apply for this job

  • Principal Mechanical & Electrical Engineer

    De Montfort University
    • Leicestershire
    • Grade G: £36,672 - £46,414 per annum

    Join the Projects Team to develop and manage medium to large projects on the university estate.

    • Recruiter: De Montfort University

    Apply for this job

  • Advanced Commissioning Engineer

    National Grid
    • Nottinghamshire, Nottingham, England
    • £46000 - £57000 per year

    National Grid is at the heart of energy in the UK. The electricity we provide gets the nation to work, powers schools and lights everyone's way home. Our energy network connects the nation, so it's essential that it's continually evolving, advancing and i

    • Recruiter: National Grid

    Apply for this job

  • Electrical Design Engineer

    Oxford Instruments
    • Yatton, Bristol
    • Competitive salary plus excellent benefits

    We are looking for an electrical designer to join our engineering design team.

    • Recruiter: Oxford Instruments

    Apply for this job

  • Skilled Electrical Fitter

    MBDA
    • Bolton
    • Competitive Salary & Benefits

    What?s the opportunity?   The Electrical Fitter will carry out manufacturing and test tasks within the electrical department in accordance with product certification procedures, defined workmanship  ...

    • Recruiter: MBDA

    Apply for this job

  • Electrical Manufacturing Technician

    MBDA
    • Stevenage
    • Competitive Salary & Benefits

    What?s the opportunity?   As a qualified craftsman with experience in electrical manufacturing, the Manufacturing Technician will report to a Team Leader, receiving day to day ...

    • Recruiter: MBDA

    Apply for this job

  • Consultant Engineer (Electrical Power)

    BAE Systems
    • Cumbria, Barrow-In-Furness, England
    • Negotiable

    Consultant Engineer (Electrical Power) Would you like to play a key role in providing technical direction to the design of power systems on the Successor class submarines, which will replace the current Trident-equipped Vanguard class, currently in servic

    • Recruiter: BAE Systems

    Apply for this job

  • Electrician

    The Bristol Port Company
    • City of Bristol
    • C. £31,729 per annum plus supplements, benefits and overtime

    You’re a good team worker with a strong technical capacity – so bring your talents to a new role with one of the area’s leading employers.

    • Recruiter: The Bristol Port Company

    Apply for this job

  • Supply Restoration Team Manager (HV/SAP)

    SSE
    • Oxford, Oxfordshire
    • Salary: £37,588 to £49,645 + Car (SSE8) Depending on skills and experience

    SSE is looking to recruit a Supply Restoration Team Manager to join our existing team in Oxford.

    • Recruiter: SSE

    Apply for this job

More jobs ▶

Subscribe

Choose the way you would like to access the latest news and developments in your field.

Subscribe to E&T