Second Severn Crossing

Crossing the Severn to connect London and Cardiff meant overcoming some of the highest tides in the world. By resorting extensively to prefabrication, we were able to manage the significant risk factors inherent in this project that called for the design, construction, and operation (by concession agreement) of a 5,126-metre structure encompassing a cable-stayed bridge 948 metres long and 2 access viaducts, respectively, 2,103 and 2,077 metres long. The 5,126-metre structure, providing 37 of overhead clearance, spans the Severn estuary, 5 kilometres downstream from an existing suspended bridge. It provides 3 lanes for traffic and an emergency lane in each direction, along with a central reservation. It is equipped with safety barriers and wind-barriers on each side.

BACKGROUND

In 1988, the government of the United Kingdom launched a call for tenders for the building of the Second Severn Crossing. The bridge was designed to relieve traffic congestion in the region and provide a further link between England and Wales by extending the M4 motorway connecting Cardiff and London. The structure includes 2 viaducts providing access to a cable-stayed bridge from either side. The decks on the viaducts are 33 metres wide and consist of 2 single-cell precast reinforced concrete box girders. The viaduct connecting the main structure to the Welsh side of the estuary is 2,077 metres long and includes a 65-metre end span, 20 conventional 98-metre spans, and a 49-metre console that supports the end of the cable-stayed bridge. The viaduct connecting the main structure to the English side of the estuary is 2,103 metres long and includes a 32-metre end span, a 58-metre transition span, 21 conventional 98-metre spans, and a 49-metre console that supports the other end of the cable-stayed bridge.

TECHNICAL OVERVIEW

As a result of design constraints due to project specifications, these 2 large-scale viaducts represent true pioneering achievements in the external use of prestressed concrete. The box girders, 15 metres wide at the top, were built separately with the balanced cantilevered method using precast segments assembled with bonding joints and reinforced with concrete. The segments were built in two plants located on either side of the estuary. The use of precast segments (2,302 in all) enabled us to build a concrete deck. The 946-metre-long cable-stayed bridge includes 2 pylons 137 metres high located on either side of the navigation channel. They are connected by a low tie-beam supporting the deck at 40 metres above the highest water level and a high tie-beam forming a gantry at mid-height in the stay-cable anchoring zone. The deck rests partly on the pylons’ base rails (mounted crosswise on simple bearings), partly on 4 side piers located on either side of the pylons (mounted on uplift-restraint bearings), and partly on the ends of the access viaducts (mounted on simple bearings). The stay-cable system is made up of 4 planes of 60 guy-wires consisting of nineteen to sevety-five 15-millimetre segments and ranging in total length from 35 to 243 metres, deployed on either side of the deck and the pylon shafts. The hollow pylon legs, embedded in massive precast foundations, were built on site thanks to 39 lifting operations using 3.8-metre lift increments on average. Generally speaking, the deck was built using the balanced cantilevered method from the pylons. The segments were lifted into place by 2 mobile hoisting systems deployed symmetrically at the ends of the balanced cantilevers and moved successively forward as construction progressed.

The construction methods used in this project took into account the area’s extreme weather conditions, including strong tides (up to 14 m high), strong current, and strong winds.

IMPACT

The bridge provides another connection between England and Wales, linking Severn Beach and Newport, by extending motorway M4, which now runs from London to Cardiff. The bridge provides motorists with dual 3-lane carriageway. This new crossing over the Severn estuary, located downstream from the existing suspended bridge, is most assuredly one of the achievements of late 20th-century engineering in which we deployed all of the means at our disposal to overcome challenging natural elements.
On this cable-stayed bridge-construction project, we pioneered the use of precast components assembled with bonded and bolted joints to build a concrete deck. This bold move helps achieve progress in the construction industry and prepares us for future challenges. The extensive use of precast components in all parts of the structure and the use of heavy machinery and powerful hoisting and other handling equipment proved very effective in this site that is exposed harsh weather conditions. We had to overcome extreme weather conditions, at times, in this region of Great Britain where the tides are the second-highest in the world (after the Bay of Fundy in Canada).
In addition, we used environmentally friendly methods on this project in efforts to comply with a sustainable development approach designed to preserve this special scientific interest site where many migrating birds nest. We took into account flooding and potential flooding situations, reduced noise and dust emissions, preserved water quality, deployed an environmental protection program, implemented protective measures in conjunction with work procedures, and appointed a person on site to monitor closely all environmental issues and impacts.

Project participants

Client
Severn River Crossing Plc

Project management
Maunsell & Partners

Key figures

Implementation dates
April 1992 to April 1996    

Excavation
12 million m3

Concrete
4.8 million m3

Dam
3.930 million m3

Upper Forth Crossing at Kincardine (Clackmannanshire Bridge)

Kincardine

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