European cooperation may, In some instances, seem fragile, but cross-border ventures between Europe's contractors and engineers are fast becoming the norm. A prime example is the Second Severn Bridge Crossing in the UK, where British and French contractors and engineers are working side by side to complete what will become Britain’s largest project for decades.
The $430 million (£300 million) crossing will be 5.2 km long and comprise two multi-span approach viaducts running across the Severn estuary from the Avon (England) and Gwent (Wales) shores to the main bridge spanning the navigation channel. The central spans will be carried on cables from two H-frame pylons rising 148 m above water level.
On either side of the estuary, construction yards arc rumbling with activity as the concrete caissons and viaduct and bridge spans are formed. It is with the Intention of minimising work off-shore in the face of the extreme tides racing up and cfown the estuary on each side of the. river, that the huge concrete precasting yards are operating.
Relieve Pressure
The project was conceived to boost transport links between England and Wales and relieve pressure on the existing Severn Bridge. Building will take four-years and is being carried out to a design, build and operate basis by Laing-GTM Entrepose'u as contractors, and Halcrow-SEEE (Soclete de'Etudes et d'Equip-ments d'Entreprlses)121, who have responsibility for the overall design.
Both French companies have considerable bridge experience. SEEE is the design company within GTM. Its experience in cable-stayed multispan designs ranges from the Evipos bridge and Rion Antlron fixed link crossing, in Greece, to the Honfleur bridge (main span 856 m — a world record), over the River Seine in France.
Halcrow, which traces its roots back to 1868, has de signed hundreds of bridges. Including the Orwell bridge near Ipswich, In eastern England, which, at 190 m, has the longest concrete span in Britain. The company was also Involved as design consultant for the 2.7-km Dartford Bridge crossing (SW England), and for Hong Kong's Lantau crossing.
Joint Initiative
The joint Halcrow-SEEE Initiative has assembled some 480 engineers and other specialists to deal with the complexities of the structure, . while the on-site staff crisis is of equal numbers of French and British, plus other secondments and exchanges. According to project director, John Haste, the scheme "is the most exciting challenge for all Involved", while David Mizon, Halcrow's man on site, has praise for both, with British practicality and French theo-ritical expertise complementing one another.
Initial attention in early 1992 . was focused underground. Site investigations, conducted during 1989-90, provided data for the detailed design. The crossing's two approach viaducts comprise concrete box sections with slcn der cantilevered decks. They will rise, on a series of twin piers, to the main 912m-long cable-stayed bridge, the centrepiece of the crossing.
The statistics are Impres slve : (here are over 27 approach spans on the Gwent side, and 25 on the Avon side; 42 foundations in the caissons. The total volume of concrete required will be 320,000 m3; plus over 30,000 tonnes of reinforcing steel and over 150,000 m of prestressing steel. The number of cable stays in the bridge will be 184. Over 1000 men will be employed, many from local areas of unemployment, before completion by 1996.
Bridge Pylons
The placing of the first offshore caissons started towards the end of 1992. By the spring of 1993, work will start on the piers and viaduct deck, to l>c followed by the bridge pylon-,. Actual bridge deck work wll start in the summer of ]'• '4 anc will be followed by the installation of mcchan* al and electrical services.
Up-to-date design methods for the bridge deck have led to advanced technical solutions for the prestressing of the viaducts and for the design of the composite cable-stayed deck. Powerful computer programs using three dimensional modelling techniques devel oped by the joint venture, have been used Many of the ap proach piers arc to be con structed on foundation caissons specially designed to withstand the high horizontal loads that could be imposed should vessels of 6500 dwt collide with them. Otherwise, piled foundations will be used, with special foundations being designed for the viaduct piers on either side of British Rall's Severn railway tunnel, which carries the main London-Cardiff line.
Two groups of seven-end-bearing piles, 2 m in diameter and 30 m long, will straddle . the tunnel, spanning it diagonally to its axis, and coming to within 10 m of the tunnel wall.
Rail Tunnel
Cased for the top 23 m with a 2-m-1nterii.il-dlamelcr 16-mm thick walled tube, the latter is treated with a bitumastic slip to prevent any fu lure load transfer to the ground In th<- region of the rail tunnel, The pier loads will be transferred to the sandstone by the piles which toe tn some 2 m.
Operations are supervised in the dry. behind bunds, and from large jack up or lifting barge On both sides, direct founding on rock is not possible for all bridge piers, so the piles cany loads down through the alluvium into lire underlying rock — LPS