Chiswick Bridge has received some long overdue TLC in the wake of previous half measures and attempts to restore it.
Project: Chiswick Bridge refurbishment
Client: Transport for London
Contract value: £9.1m
Main contractor: Bam Nuttall
Concrete subcontractor: Topbond
Parapet strengthening: Stone West
Design engineers: Ramboll Parsons Brinckerhoff JV
Start date: April 2014
Completion date: May 2015
Opened in 1933 on the same day as the Twickenham and Hampton Court bridges further upstream, the Grade II-listed Chiswick Bridge carries the A316 Great Chertsey Road across the River Thames between Chiswick and Mortlake in west London.
The bridge is a reinforced concrete multi-arch structure clad in Portland Stone. It comprises three arches that span the river with two intermediate piers founded in the riverbed.
Further arches support the road above the Thames Path, which runs on both sides of the river. A staircase is located in each corner of the bridge to connect the road with the footpaths below.
The existing parapets were constructed from Portland Stone with limited positive connection. Over time, the stone balusters have weathered – some significantly.
“On University Boat Race days, barriers had to be erected to prevent crowd loading on the parapets due to safety concerns”
A previous interim measure saw cages positioned on both sides of the balusters across the entire structure to prevent stone falling into the river below.
However, on University Boat Race days, barriers had to be erected to prevent crowd loading on the parapets due to safety concerns over loading capacity.
Limited and poor quality concrete repairs had been undertaken in the past to the arch soffits.
To address this deterioration both above and within the structure, last April Transport for London began a major refurbishment of the bridge as part of its wider structures and tunnels investment portfolio.
This scheme would strengthen the parapets to modern standards, undertake concrete repairs both externally and internally and also undertake a general refurbishment to the road surface and stairways.
Being Grade II-listed, the heritage status of the structure was a major constraint that involved close working with both the local council and English Heritage.
Innovative parapet design
An innovative design for the new parapet was used to achieve the required strength within the stonework through the use of stainless steel sections connected by stainless steel bars.
The balusters, replaced with stone from the same quarry as the original, as well as a small number that were reusable, were drilled on site so that the new vertical stainless steel bars could fit within.
The coping stone was then shaped and attached to the top stainless steel section, while lower stones were cut to fit over the lower stainless steel section.
“The result is a parapet of N1 standard, with none of the stainless steel able to be seen and the visual appearance of the structure unaffected”
This was then mechanically attached and dowelled to a new reinforced concrete beam constructed below the footway.
The result is a parapet of N1 standard, with none of the stainless steel able to be seen and the visual appearance of the structure unaffected.
In addition to the parapet reconstruction, concrete repairs have taken place to the internal cellular structure and, in particular, the internal columns that transfer the load from the deck to the arch structure.
Cathodic protection has been installed to enhance the life of the repairs along with the addition of a corrosion inhibitor.
Repairs have also taken place to the arch soffits, which have required the use of a barge located beneath each arch in turn, equipped with MEWPs and welfare facilities.
Close liaison with the Port of London Authority has enabled the co-ordination of the works around various river events, including the annual University Boat Race.
The opportunity has also been taken to restore the architectural bronze trim which lines the river arches and which in places was missing or supported by failing concrete. To achieve this, new bronze trim has been cast and installed.
Expansion joints across the deck, located at each abutment and at each side of the intermediate river piers, have been replaced with new buried joints. Concrete repairs took place to the road deck either side of the joint.
“New lamps contain LED fittings to minimise the whole-life cost of the system”
The footways have been re-waterproofed and resurfaced, and a new surface course has been laid over the bridge.
Heritage-style lamps have also been installed on the footways to match the original lamps as closely as possible, which had previously been removed and replaced by 10 m conventional columns.
These new lamps contain LED fittings to minimise the whole-life cost of the system.
The lighting on the staircases is also being replaced with heritage-style lighting.
Significant engagement with the heritage teams has seen the London boroughs, the Greater London Authority and English Heritage give their input for the lighting solutions, parapet design and concrete repair appearance.
To cap the project off, the Portland Stone façade of the bridge has been cleaned, restoring the bridge’s original glory.
The majority of the work was completed in early April, in time for hundreds of spectators to stand against the parapet and watch the climax of the University Boat Race (and the first women’s University Boat Race) which finishes just short of the bridge.
Geoff Pearce is project manager for the Chiswick Bridge refurbishment at Transport for London