Technology recently developed on the Queen Elizabeth II Bridge has virtually eliminated traffic disruption during expansion joint replacement work on a key M25 motorway viaduct.
Since 1986 the Gade Valley Viaduct near Junction 20 has carried the M25 over the West Coast Main Line (WCML), a busy local authority road, the Grand Union Canal and the Mill Stream. After almost 30 years and despite regular maintenance, two of the structure’s four expansion joints had reached the end of their life. Replacing them, however, promised to be a major exercise, with the potential for massive traffic disruption.
“We knew we would have to do much of the work from underneath, below temporary bridging ramps,” says M25 design, build, finance and operate consortium Connect Plus technical director Derek Hughes.
ajc5740 photo credit anthony cullen
Source: Anthony Cullen
“And until recently the only commercially available options for the ramps were hinged systems that could be raised during night time closures to give access from above.
“But there were several drawbacks, not least the speed restrictions that would be necessary.”
Maximum traffic speed with the ramps in place would be only 30mph, although tests showed HGVs tended to slow down to as little as 15mph. This would have hit Connect Plus with serious financial penalties
And there were logistical drawbacks as well. Once raised, the hinged ramps would form a “steel wall” across the viaduct, effectively blocking access along the deck. There were also worries about how secure the raised ramps would be in high winds.
“The breakthrough idea was to form the approach ramps from asphalt, laid to conform to standard highway comfort criteria”
Robert Percy, Flint & Neill
These were exactly the same challenges that had faced project manager Connect Plus Services when confronted with the urgent need to replace the QEII Bridge’s six expansion joints (see box). At the end of that award winning project, Connect Plus had the new hardware to hand to use at Gade Valley – and it was the responsibility of contractor Jackson Civil Engineering and designer Flint & Neill to adapt the design they had developed for the QEII Bridge to suit Gade Valley.
Flint & Neill associate Robert Percy says: “In principle, the concept was simple. The joints would be bridged by steel plates 100mm thick, with finger joints at one end to accommodate thermal movements of the deck.
“ The plates had to be 5.1m long to span across the double expansion joints on the QEII Bridge. The breakthrough idea was to form the approach ramps from asphalt, laid to conform to standard highway comfort criteria. That way, instead of the ramps acting as speed bumps, they would allow traffic to flow at normal speeds.”
Maintaining the 70mph limit across the viaduct needed ramps 48m long, laid to a sinusoidal profile. The bridging plates inherited from the QEII Bridge project were 1.2m wide and weighed in at 5.3t. On that project, truck-mounted cranes were used to handle the plates: at Gade Valley the presence of the WCML meant 25t rubber tracked excavators took over the role, to meet Network Rail’s restrictions on slewing.
There was another significant difference as well. At Gade Valley the joints are skewed by more than 30°.The bridging plates had to be staggered: this made the interface with the asphalt ramps quite challenging, says Jackson Civil Engineering framework director Paul Watson.
Maintaining the 70mph limit across the viaduct needed ramps 48m long, laid to a sinusoidal profile
“Aggregate Industries supplied a very special mix for both the QEII Bridge and here at Gade Valley. It contained very little bitumen and could be laid in one pass. Long term durability might have been a problem but that wasn’t important here.”
Simple the solution might be in principle: in practice some fine tuning was necessary. Total thermal movement is significantly less than on the QEII Bridge, but the skew required the longer length of bridging plate. A cover plate with
well-tapered edges protects the steel finger joints at one end of each plate. At the other a knuckle bearing allows some limited rotation.
To avoid what Hughes describes as the “skateboard effect” – the tendency for the plate to bounce up as a vehicle rolls off it – cover plates and knuckle bearings are bolted down to prefixed base plates anchored to the deck. Individual bridging plates are fitted with quick attachment lifting points for ease of handling.
Watson says the use of hydrodemolition to break out the concrete around the existing bearings, which was so successful on the QEII Bridge, was ruled out at Gade Valley “because of the proximity of the railway.”
He adds: “We used remote control track guided concrete saws instead. It worked well.”
At 21m long, Gade Valley’s expansion joints were longer than the 19m joints on the QEII Bridge, although there were no double installations. Reinforcement congestion around the joints was much less, so a high workability concrete mix was used to replace the old concrete and secure the joints in place instead of the advanced self-compacting mix used on the QEII Bridge.
Fabricating the bridging plate components to the essential standards of accuracy was a challenge in itself. “We could only find one firm in the UK that could laser cut 100mm plates on CNC machines,” Hughes reports.
“That was Sheffield-based Mayflower Engineering, who did a great job.”
Precision components simplified and speeded assembly insitu. Installing the bridging plates and the ramps could be accomplished during a single 10 hour night time closure.
Once the new expansion joints were installed and the new concrete had achieved sufficient strength, another night time closure sufficed for the removal of all the steel components and the asphalt ramps and the resurfacing of the immediate area.
Hughes attributes the success of the project to Early Contractor Involvement (ECI) and the collaborative nature of the design and development phase. The main objective, to minimise traffic delays and disruption, was achieved, and the prospect of many more expansion joint replacement projects around the M25 no longer seems so daunting.
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Replacing the six expansion joints on the QEII Bridge at Dartford by conventional means would have required lengthy lane closures, caused traffic delays, and led to multi-million pound financial penalties for Connect Plus, the DBFO company responsible for the 30-year M25 contract with Highways England.
The inside lane of the crossing is reputed to carry heavier traffic than any in Europe, and the crossing itself has been described as the most stressful section of the M25. Nothing that would add to the stress and frustration of drivers was acceptable.
Daytime lane restrictions were out, as were any reductions in the crossing’s normal 50mph speed limit. Existing commercially available temporary bridging systems would slow traffic speeds, and would struggle with the structure’s two double joints. A completely new bespoke approach was needed.
Connect Plus technical director Derek Hughes came up with the idea of massive 100mm thick steel bridging plates – simple, practical and fatigue resistant. Connect Plus Services managed the project from the outset, with Jackson and Flint & Neill joining the team to develop and trial the concept.
Joint replacement, QEII bridge
The choice of asphalt for the approach ramps was particularly attractive. Sinusoidal ramps just 35m long allowed traffic to maintain normal speeds in comfort. The design was proven in full-scale trials alongside the M11, where it was confirmed that even the 100mm thick plates would deflect up to 15mm as a fully loaded HGV passed over.
Hydrodemolition was the preferred method for removing the original expansion joints. Work could be carried out safely below the bridging plates as traffic rumbled past above.
Cable stayed bridges are notoriously “lively’, and a range of movements had to be accommodated during the joint replacement project. One piece expansion joints were specified again, but were deeper than the original and featured extensive use of stainless steel.
These were more expensive than the standard option, but came with a 45 year design life. The 19m long, 26t replacements arrived direct from Munich and were installed during well-rehearsed night time closures.
Overall, there was virtually no disruption to traffic, an outcome that saved at least 500t of carbon emissions while Connect Plus avoided more than £23M in road closure charges.
The work came in more than 15% below the target price and was completed on time. And the new bridging ramp system is now an asset that can be exploited on other M25 crossings, of which there are at least 30 that could benefit from this innovative technology.