England - Wales Bridge
Nears Completion

by Liz Clarke
Photography by Motordrive Photographic

Engineers carrying out Europe's biggest bridge-building project are fitting the final details to the span that crosses the treacherous waters of a British river estuary. The river's fast-flowing currents and the world's second-highest tidal range have made for a rather unique and difficult working environment for the project.

The new road bridge linking England with Wales across the River Severn comprises a cable-stayed main span of 456 metres across the deepest stretch of river. The span is linked to land on each side by two viaducts, each more than 2000 metres long supported by 44 concrete piers, 37 of which are based on precast reinforced-concrete caissons sunk into the riverbed. The caissons were positioned to extremely high tolerances with the help of a multi-role work barge, the SAR-3, powered by four 2000hp computer-controlled thrusters. The barge relied on an advanced system of dynamic positioning to keep the vessel stationary in the fierce tidal flow with its 14-metre range, enabling the 2000-ton caissons to be lifted clear and lowered into place to an accuracy of within 20 mm.

This dynamic positioning system, developed by bridge construction team Laing-GTM, employs laser beams directed between the SAR-3's control room and targets on shore and in the river estuary. The system has enabled work to stay on schedule in spite of occasional interruptions caused by ferocious winds and freezing winter conditions.

Two 137-metre-high pylon towers support the main cable-stayed bridge deck, the final section of which was lifted into place earlier this year. The entire length of roadway along the bridge is being fitted with three-metre windshields to protect vehicles from the notorious Severn winds that can reach 160 kph (l00 mph).

Laing-GTM, the Anglo-French contracting team responsible for building the bridge, is confident the project will be finished on time in mid-1996, adding a vital link to Britain's roads network.

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