In January 1968 the most westerly pier, number 23 collapsed leaving the deck unsupported over two spans. A temporary steel structure was installed across the collapsed section to allow for reopening of the bridge to traffic, while the permanent works were designed.
A large portion of pier 23 collapsed damaging the adjacent pier 22. the collapse of pier 23 was caused by a large scour pit on the north east side of the pier.
A temporary walkway which also carried diverted statutory undertakers services was constructed
After the collapse the road was closed and the damaged sections of the bridge demolished
The foundations of the piers are constructed on a layer of boulders placed on the river bed sand, which is susceptible to scour.
Cut off walls were sunk into the river bed around the base of the piers by a curtain of bored piles. The piles were formed by sinking lining tubes with a cutting edge through the existing foundations using drilling rigs. Once the holes were drilled to the correct depth and after insertion of the reinforcement, high strength grout was pumped in while the tube was being removed.
For the new foundations (below piers 22 and 23) the contractors elected to sink in-situ reinforced concrete piles bored down into the underlying "shillet".
Each pile group is capped by a reinforced concrete raft flush with the river bed carrying the pier proper. The piers consist of reinforced concrete cores clad by a skin of masonry 0.3m (1'-0") thick.
The reconstruction work comprised the following:
In order to prevent large scale cracking similar to that in pier 23 it was necessary, firstly, to consolidate the interior and, secondly, to reinforce the masonry to increase its strength.
The Fondedile system was adopted which consists of drilling a number of 63mm holes in criss-cross fashion through the pier, inserting 25mm diameter high tensile steel rods into the holes, after which each hole is grouted up under pressure. Each pier is thereby strengthened by a system of reinforced concrete ties which turn the whole mass of masonry into a single unit. The ends of each rod are not anchored, but the aperture is made good with special cement grout to blend in with the masonry.
Typical layout of Fondedile System installed in Pier
The carriageway widening had to be done at the expense of the footpaths for reasons of economy. The inner lines of pre-cast concrete beams installed in 1924/25, which carried the footpaths could not be adapted to allow the roadway to be extended over them. They had therefore to be removed and replaced by shallower stronger beams.
Section which shows the inner reinforced concrete beam has been replaced by a concrete encased steel beam.
The space between the building line and the quay wall south of the bridge was to small to accommodate a 9.15m (30'-0") road. the additional width was therefore gained by cantilevering out over the river. Seven reinforced concrete beams cast under and across the road reach out over the river and carry edge beams similar to the outer beams on the bridge. The gap between the new edge beams and the quay wall is filled with a reinforced concrete slab.
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