COSTAIN WILL open a fourth workface on the troubled Avonmouth bridge later this month, after the Highways Agency authorised the expenditure of £800,000 on an extra set of temporary props.
The props are central to the strengthening process as they take the weight of the steel box girder while extra plates are welded on. The new team of welders from steelwork subcontractor Kvaerner Cleveland Bridge will start work in the northern approach viaduct within the next few weeks.
Costain was recently granted a 149 week extension on the original three year £50M contract to strengthen the 23 year old bridge and widen the M5 between junctions 18 and 19. However, the introduction of a new workface and other value engineering measures has meant the target date is now the end of 2000 rather than 2001.
One possible time saving measure is to use concrete instead of steel to stiffen the box girder near the approach by the piers. Dozens of new steel stiffening plates would be replaced by 450mm deep reinforced concrete poured between existing lower flange stiffeners over a distance of 2.5m each side of the pier.
However, the bridge strengthening will not now be completed before the introduction of 40t lorries in 1999. Banning 40t lorries from the crossing would be impossible as all realistic alternative routes face similar restrictions. The Agency believes the bridge will be safe, although there may be an impact on
the strengthening operations. 'There is a weigh-in-motion system which would sound an alarm if there was any danger,' said Highways Agency project manager John Bourne. 'Records show that around 15% of the lorries crossing are seriously overloaded and the bridge is taking them without any problem'.
The HA is sticking to its claim that the cost of additional work on the contract is unlikely to be more than £50M out of a total contract price of £100M. 'Our original pre-tender estimate was £81M, so the final cost will not be vastly in excess,' Bourne said.