LAST weekend saw the biggest sideways bridge slide in the world part of the 12.25 million A38 Marsh Mills viaduct replacement contract in Plymouth.
Contractor Hochtief inched the 5,500 tonne, 410 m dual carriageway slip road into position using six 100-tonne jacks.
The operation went to plan, beginning at 6.30 on Friday night and finishing 48 hours later.
In order to minimise road closures and avoid lane closure charges of up to 30,000 per day, Hochtief built the two slip road bridge decks on temporary supports next to the existing decks.
It then diverted the traffic onto the new deck while the old structure was demolished and the new substructure was completed.
Replacing the two busy slip roads running off the main viaduct in the normal manner by demolishing them and building new structures would have cause severe traffic disruption for 18 months.
Hochtiefs site manager Ray Bradbury said that the Exter bound road section was a near perfect fit.
But there was a hairy moment at the beginning of the slide. The merge slide was downhill on a 2.85 per cent slope which meant that four braking points were used. And the non-stick PTFE coated sliding surfaces had less friction than expected. This meant that the bridge set off at a greater rate than anticipated and one end skewed out of line.
Mr Bradbury said: We had to stop and take precautions we tightened up the brakes and bolted heavy sections of steel across the bearing track at several points to act as buffers.
This meant the slide took 12 hours rather than the programmed eight. But Sundays good weather brought the operation back on schedule.