Second Severn Crossing
River Severn, Redwick to Sudbrook, UK
G Maunsell & Partners
Halcrow Group Ltd
Gifford & Partners
date 26th April 1992 - 5th June 1998
era Modern |
category Bridge |
The Second Severn Crossing is a cable stayed bridge with long approach viaducts at either end. It was built to relieve the congestion on the first crossing, the Severn Road Bridge (1966). The newer bridge is the most southerly crossing of the River Severn and at the time of construction the longest river crossing in the UK. It carries the M4 between England and south Wales, while the original carries the M48.
By the 1980s, traffic volumes on the Severn Road Bridge had tripled since it opened. Studies on the feasibility of a second crossing were commissioned in February 1984. W.S. Atkins and Maunsell & Partners (jointly the Second Severn Crossing Group) were appointed to examine potential bridge and tunnel solutions. Public consultation led to tenders being invited in April 1989.
The following April, the Laing-GTM consortium won a design, build, finance and operate (DFBO) tender and formed Severn River Crossing plc (SRC) with Bank of America and Barclays Private Equity. SRC appointed Halcrow in partnership with French consultant SEEE to undertake the detailed design, and Gifford & Partners for checking.
The Severn Bridges Act, passed on 13th February 1992, enabled the crossing and all the approach motorways to be built, and the official start of works was the 26th April, with a concession period of up to 30 years. Maunsell & Partners was responsible for ensuring that the project complied with the relevant standards in their role as agent for the government, and with W.S. Atkins designed the motorway link roads at either end of the new crossing.
The River Severn has the second highest tidal range in the world at 14.5m. Its tricky currents and winds of up to 129km per hour combine to produce waves up to 2.5m high. It was therefore more efficient to do as much work as possible in locations unaffected by the river, so precasting and prefabrication was done at a 28.3 hectare site nearby. A 2km haul road was laid on a causeway over the river bed to be used for access to the English side, and may still be seen at low tide.
The crossing is 5.128km long overall and has a 37m navigational clearance to the water level. The 25-span approach viaduct on the English side is 2.103km long and the Welsh one measures 2.077km and has 24 spans. Between them is the 948m long cable-stayed bridge, known as Shoots Bridge. It has a central span of 456m over the main river channel.
The precast double decks of the structure each carry three lanes of motorway with a narrow hard shoulder and are joined by the central reserve, which was cast in situ.
The crossing is founded on 37 precast concrete caissons 53m wide with parallel sides and rounded ends. The 35 viaduct caissons weigh 1,600 tonnes each, while the bridge has two caissons of 2,000 tonnes each. They were transported from the casting yard to the river by the barge SAR3 (specially converted from part of an oil tanker) and lifted by the jack-up crane barge Lisa A, which waited until SAR3 had returned to her berth and the tide had receded before lowering the caisson into position on the river bed. The joint between caisson and riverbed was sealed and then the hollow caisson — now acting as its own cofferdam — was filled with mass concrete and capped with reinforced concrete.
The viaduct piers atop the caissons were transported to site by SAR3 and lifted into place by the crane barge Jay Robertson. They have a maximum height of 48m and are formed from precast units that were ‘match cast’ (cast adjacent to sequential units) for a good fit and prestressed together vertically. The tendon ducts were then filled with wax.
Purpose-built gantries lifted the 2,300 precast reinforced concrete viaduct deck segments, each 200 tonnes, working outwards from a pier. Segments were positioned at either end of a balanced cantilever then glued and post-tensioned, and each span has 27 segments.
Shoots Bridge is not far from Sir John Hawkshaw's Severn Tunnel (1886, ST500860) and required careful siting of its 2m diameter bored concrete pile foundations beneath its caissons that reach into the bedrock below the tunnel. The tops of the two concrete pylons that support Shoots Bridge are 137m above water level (100m above deck). Each has a pair of hollow rectangular shafts, tapering from 10.2m x 4m to 5.4m x 4m, slipformed in situ with vertical post-tensioning to the upper parts. There are lifts and steel access ladders inside the shafts, which are connected by upper and lower precast hollow reinforced concrete crossbeams.
The 7m long x 34.6m wide bridge deck units comprise 2.15m deep structural steel girders topped by a reinforced concrete roadway slab 200-470mm deep, with transverse truss girders every 3.6m. Each 180 tonne unit was test match bolted before being taken to site. The Lisa A lifted the first five units on either side of the pylons into place, but subsequent units were placed by double shear legs standing on the road deck. High strength friction grip bolts were used to connect the units, avoiding on-site welding.
There are 240 cable stays connecting the deck to the tops of the pylons, at precast anchorage tie beams. The cables have between 19 and 75 strands depending on location and are spaced about 7.3m apart. Secondary vertical two-strand cables link the main cables, keeping them in plane and reducing oscillations. Individual strands are made from seven galvanised steel wires. The bridge is designed to remain usable with any one cable missing under full load and any two missing with reduced load. There are also 3m high baffles on each side of the bridge to deflect the wind.
A monorail with rapid access train (RAT) was installed hanging beneath the crossing deck to facilitate inspection and maintenance, but it closed shortly after opening because cracks were found in the welds between steel rails.
The Second Severn Crossing now carries more than 60,000 vehicles every day and is designed to withstand earthquake loading and ship impact. It cost £330m to construct and was completed on time, to budget and — most importantly — without fatalities. It was opened by HRH Prince Charles on 5th June 1996 and won that year's British Construction Industry Supreme Award and Concrete Society Overall Winner Award. Pedestrians and cyclists are not permitted on the Second Severn Crossing. Both crossings are tolled, westbound only and have toll plazas on the Welsh side of the river.
Baffle plates were installed under Shoots Bridge in 1997 to damp the occasional vortex-induced vertical oscillations and vibrations that were observed in the winter of 1996-7. Maintenance works totalling £4.7m were carried out between March 2003 and June 2004. They included strengthening the monorail and refurbishing the RAT.
Both Severn crossings were closed in February 2009 and January 2010 because ice falling from the suspension cables and cable stays was posing a hazard to vehicles. The second crossing was closed in December 2009 for the same reason.
Contractor (1992-6): Laing-GTM
Contractor (2003-4): Laing O'Rouke
Wind tunnel testing: Rowan Williams Davies & Irwin Inc
Steelwork: Cimolai Costruzioni Metalliche
Cable geometry software: Formule Informatique
"Critical Analysis of the Shoots Bridge, Central Span of the Second Severn Crossing" by T. Lai, Proceedings of Bridge Engineering 2 Conference, Bath, April 2009, available at www.bath.ac.uk
"Severn River Crossing PLC", Severn Visitor Centre, undated brochure,
available at www.severnbridgesvisitorcentre.org.uk