Fears of a “catastrophic failure” of a bridge in Bristol have caused the council to bring forward a £50M plan to repair a road network “deteriorating at a significant rate”.
The Cumberland Basin road system acts as the entrance to Bristol City Docks and is currently experiencing traffic levels two and a half times higher than it was meant to when it was built.
A report submitted to Bristol City Council’s cabinet stated this is causing an accelerated depreciation of the network, which opened to traffic in 1965 after construction began two years earlier.
One structure the report highlighted was the Avon Bridge, which links Cumberland Basin with Ashton Gate on the A3029 Brunel Way over the River Avon. The report stated its design was “no longer approved in the UK for any new proposed highway structures as the potential risk profile of potential catastrophic failure is considered to be unacceptably high”.
It further stated that the bridge consists of a considerable number of inherent high-risk construction design features which include post-tensioning, or half-joints, and tendons in box-beam bottoms. These features coupled with ongoing road-water and gritting salt ingress leakage the bridge has suffered from and its increasing age has given the council no option but to intervene.
To help combat extend the lifetime of the bridge, the council has been recommend to approve a £4M plan for inspections and structural maintenance over the next five years.
Major works for the structural maintenance of the bridge outlined in the plan include concrete repairs to half-joints, soffit, bridge piers and main cantilevered bridge spans and the installation of drip rails on each side of the half-joints to stop water tracking onto cantilevered sections.
Due to apparent concerns regarding the structural integrity of “half jointed”/post tensioned fixed bridges and to determine the current overall structural condition, the council conducted a post-tensioned special inspection of Avon Bridge in early 2021.
Following the completion of this report, urgent works were undertaken in 2022 to re-waterproof and re-surface the bridge deck and replace all four expansion joints to mitigate the issue of water ingress into the structure.
The whole Cumberland Basin complex contains a broad range of reinforced concrete structures in which a failed Challenge Fund Bid in 2015 by the council identified a number of concerns.
These concerns included issues with elastomeric and roller structural bearings and expansion joints between all elevated structural deck areas, ineffective waterproof membranes throughout all bridge deck areas and a sub-standard parapet system throughout.
Works under the £50M plan for the whole of the complex would include structural concrete repairs, expansion joint replacement, resurfacing and re-waterproofing of bridge decks, bearing condition assessment and potential full bearing replacement, parapet containment barrier railing refurbishment and wholesale railing replacement.
Currently over a third of Bristol City Council’s £1M annual structures maintenance budget is spent on the Cumberland Basin network alone. Since the failed Challenge Fund bid, the council’s structures team has been able to use this budget to conduct a small amount of repairs listed above.
Bristol City Council was contacted for comment.
Its report stated: “Significant elements of the Cumberland Basin network are nearing the end of its intended working design life and are now deteriorating at a significant rate.
“The overall Cumberland Basin structure complex has deteriorated much faster than originally designed for, given that the current traffic figures are currently over two and a half times the projected traffic figures in 1963-1965 anticipated, with the consequential increase in structural component deterioration.
“Given the condition of the Cumberland Basin network, additional capital investment is now required to manage its ultimate depreciation and decline, and to facilitate the emerging Western Harbour Development.”
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