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 1606

"Where would Frankenstein and his creative mind fit into today's workplace? Should we fear technological developments or embrace them?"

E&T jobs

  • Maritime Engineering Opportunities

    Defence Equipment & Support (DE&S)
    • Bristol
    • £30,424 - £35,285

    You will be working alongside a team of people who are immensely proud of what they do in providing the best possible service to our Armed Forces

    • Recruiter: Defence Equipment & Support (DE&S)

    Apply for this job

  • Engineering Manager

    BAE Systems
    • Hampshire, England, Portsmouth
    • Competitive package

    Would you like to play a vital role in managing and implementing the correct governance in order to enable BAE Systems to provide assurance and integrity of supply chain data? We currently have a vacancy for an Engineering Manager - Product Integrity

    • Recruiter: BAE Systems

    Apply for this job

  • Consultant Engineer - Test

    BAE Systems
    • Farnborough, Hampshire, England
    • Negotiable

    Consultant Engineer - Test Would you like to be a lead within an exciting team working on one of the UK's largest defence projects? We currently have a vacancy for a Consultant Engineer - Test at our site in Ash Vale. As a Consultant Engineer - Test, you

    • Recruiter: BAE Systems

    Apply for this job

  • Structural Designer

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

    Structural Designer BAE Systems is looking to recruit multiple Structural Designers to join our Maritime Submarines unit to be based in our site in Barrow-in-Furness, as the Trident Replacement Programme progresses towards the start of the build stage in

    • Recruiter: BAE Systems

    Apply for this job

  • Mechanical Design Engineer

    BAE Systems
    • England, Hampshire, Portsmouth
    • Negotiable

    Mechanical Design Engineer Would you like to work in an interesting and challenging role with the chance to gain exposure to a number of maritime projects? We currently have a vacancy for a Mechanical Design Engineer at our site in Portsmouth. As a Design

    • Recruiter: BAE Systems

    Apply for this job

  • Operations Manager

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

    Operations Manager We currently have an opportunity for an Operations Manager to join our Maritime - Submarines business area at our Barrow-In-Furness site. As the Operations Manager you will work within a Construction or Manufacturing Facility and be res

    • Recruiter: BAE Systems

    Apply for this job

  • Principal Chemist

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

    Principal Chemist Would you like to play a key role in the safety and assurance of submarines for the Royal Navy? We currently have a vacancy for a Principal Chemist at our site in Barrow-in-Furness. As a Principal Chemist, you will be carrying out a rang

    • Recruiter: BAE Systems

    Apply for this job

  • Software Engineer

    BAE Systems
    • England, Hampshire, Portsmouth
    • Competitive package

    As a Software Engineer, you will be investigating how technology and data can be used to optimise the services we provide to our clients, including the Royal Navy, and will include unique pieces of equipment at the forefront of innovation.

    • Recruiter: BAE Systems

    Apply for this job

  • Principal Control Systems Engineer

    BAE Systems
    • England, Cumbria, Barrow-In-Furness
    • Competitive package

    As a Principal Engineer you will be responsible for the design and integration of control systems at a safety integrity level (SIL) 3. This will include requirements management, system design, and integration into the wider platform.

    • Recruiter: BAE Systems

    Apply for this job

  • Ship Refit Operations Manager

    BAE Systems
    • Jubail, Saudi Arabia
    • Negotiable

    Ship Refit Operations Manager Would you like to work with some of the largest defence projects in the world, with the chance to deploy on a contract basis to Jubail, Saudi Arabia with increased allowances? An exciting opportunity has arisen to join BAE Sy

    • Recruiter: BAE Systems

    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