TfL working round the clock to repair Hammersmith Flyover
Transport for London made the unprecedented decision to close the Hammersmith Flyover, one of the busiest roads out of London, following the discovery of a serious structural defect.
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A detailed investigation into the extent of corrosion on the steel pre-stressing cables running through the bridge had revealed far more significant deterioration than had previously been seen or expected.
The test results left TfL’s director of roads, Dana Skelley, with a difficult decision to make.
The corroded cables help to hold the spans of the concrete structure in place and are therefore integral to its design, but the A4, the route supported by the flyover, is a major arterial route connecting traffic from the capital with Heathrow Airport and beyond.
“We have been aware for a while that there is corrosion in these cables, and we have been monitoring them using acoustic methods inside the flyover for a couple of years,” says Ms Skelley.
“On the basis of the monitoring we thought that we’d have major work to do on the bridge in about 10 years.”
But in summer 2011 the team started to pick up an increase in the rate of deterioration in certain locations. Across the 16 spans of the flyover, one location was of particular concern, so TfL introduced a prop to offer some additional support to the structure.
As the cables are encased in concrete or grouting, inspecting the level of damage is difficult. A cable that looks intact in one area can be severely damaged slightly further down the line.
“The prop allowed us to do some more intrusive exploration around the location. It’s not particularly easy to drill out concrete around damaged cables unless you’ve got something like a prop to offer some back-up,” says Ms Skelley.
“It was through this greater exploratory work that we discovered some of the cables were in a worse condition than we anticipated and in areas we hadn’t expected.”
The main concern for TfL was the possible sudden failure of the bridge, as it was unlikely to display any warning signs.
“The bridge was originally constructed on top of scaffolding, because it can’t take its own weight without the introduction of the cables,” explains
The alarming aspects of the corrosion were its extent and location; the team had expected that the most serious damage would be located at the cable ends, where the steel is more likely to get exposed to water.
But they found worse levels of corrosion further along the cables and it was this discovery that prompted TfL to close the bridge until the level of damage could be fully assessed.
“The closure allowed us to do investigations and calculations that told us we were able to reopen the bridge to light traffic, which we did on 13 January,” says Ms Skelley.
The traffic restrictions meant the work could continue while the bridge kept one lane open in each direction.
Of the 16 spans, five warranted serious attention and these are the five TfL will repair before the London Games this summer.
The initial work saw the team from Amey, which is co-ordinating the repair, remove about 200 m of the central reservation in preparation for new tensioning cables to be installed to support the structure.
“The first operation was to clear the deck of the existing central reserve,” explains Amey site representative Trevor Cherryholme. “The chunk of concrete was anchored to the deck, so we had to use several techniques to remove it.
“One was air pressure hydrodemolition, which uses very high pressure water jets to break the concrete clear but leaves the steel behind. We also used diamond rope cutting, which cuts the concrete into 2 m-long sections that can then be lifted off and taken away.”
Following the removal of the central reservation, the team was able to set the strengthening slab. Over four consecutive nights the contractor laid about 170 cu m of concrete.
The next phase, which the team is working on now, involves installing 22 km of new tensioning cable above and below the bridge deck inside a specially made duct, which will supplement the load capacity of the remaining cables.
The new ducts, which are made of heavy-duty plastic, will then be filled with wax oil to prevent deterioration due to water ingress, as well as allowing the cables to be easily inspected and replaced when necessary.
“The current cables are grouted in, so it’s impossible to take them out,” says Mr Cherryholme. “With the new system it is possible to de-energise the cables and release the tension so they can be inspected. We can also re-thread the new cables at any time.”
One of the greatest obstacles faced by the team working on the bridge is the confined space inside the structure and on the surface, as it remains partly in use.
“Keeping the bridge open and working in the public domain on a very busy traffic route is quite challenging,” says Mr Cherryholme.
“Inside the structure you have between 1.8 m head height going down to 1.5 m. It’s a very confined space, the site itself is confined and there are restrictions about how many people can work in any one area.”
There are currently more than 100 operatives working round the clock on a three-shift pattern to complete the works, along with a specialist team of more than 50 designers and engineers.
But because much of the work is going on inside the structure, the intensity of work is often not visible to the public.
“The problem we have is that people assume if they can’t see anyone working, nothing is going on, when actually there are lots of people working inside the structure,” says Ms Skelley.
Done to scale
To increase the speed of the operation and ensure nothing goes wrong on site, the team runs a scale trial of the processes on a site close to the flyover. “We do a trial which runs about two days ahead of the work on site,” says Mr Cherryholme.
“We have created an area down the road which is set up to the same dimensions as inside the flyover and acts like an instruction kit.”
“We can’t risk something going wrong on deck, so it was used as a trial and a training ground,” adds Amey associate director (consulting) Alex Gilbert.
“When the crews come up on deck they know what needs to be done and we can progress. We start on Pier K, do an activity there which we learn from and then we’re able to get efficiencies as we move down the deck,” he says.
The repetitive nature of the activities means work can be completed slightly more quickly on each pier as work progresses.
“The repeat activities provide a great learning curve. So, as we do these activities, they get faster and faster,” says Ms Skelley.
“This has happened on the activities we’ve done so far; at the beginning you’re a couple of days behind programme, then by the time you’ve finished the whole operation you end up ahead of programme because of what you have learnt along the way.”
The work to strengthen the five weakest spans of the bridge will be completed before the 2012 Olympics, then TfL will return to strengthen the remaining spans in the summer of 2013, although it might look at alternative solutions to the one being used on the five worst-affected spans.
“When we have had more time to consider what to do we might use a different solution using more modern materials, but at the moment we are going with the method we know works,” says Ms Skelley.
The first phase of repairs will cost approximately £12 million and is expected to be completed in June ahead of the beginning of the Olympics in July.