The hugely complex refurbishment of London’s troubled Hogarth Flyover finished ahead of schedule this week after six weeks of frenetic 24/7 working.
Main contractor Conway-Aecom reopened it to traffic at 2pm on Sunday, rather than on Monday as originally planned.
The 17 span, 252m long structure has undergone a complex £3M+ deck and parapet refurbishment. It was originally erected in 1968 as a temporary measure to relieve pressure on the busy Hogarth Roundabout below.
The flyover is used by 10,500 vehicles a day and carries one lane of eastbound A316 traffic above the roundabout where the A4 Great West Road and the A316 meet. It was originally planned to be in position for only five years, pending a major remodelling of the junction. One of three competing proprietary temporary flyover systems was chosen (see box, below).
The hybrid design, known as the Bridgeway system, features a heavy steel substructure topped with precast concrete deck panels and a bituminous wearing course, and has performed relatively well, according to Transport for London (TfL) structures engineer Sharan Gill.
“The substructure was in good condition, but the concrete deck was suffering badly from de-icing salt attack,” she said. “Worst of all was the state of the parapets. Not only were the anchorages heavily corroded, but they were simply bolted through the edges of the deck panels, which were deteriorating badly.”
TfL’s concerns about the parapet’s vehicle containment capacity were first raised in 2012, and a series of inspections and assessments followed. A crash deck was hastily installed in December 2012, allowing TfL to consider its options. Major remedial works became imperative in October last year when a 500mm by 500mm chunk of concrete fell from the flyover.
“We looked at everything from complete demolition and replacement to eliminating the need for a flyover by remodelling the roundabout itself,” Gill said.
“But it could take up to 30 years for major changes to the roundabout to be finalised, so the flyover has to be kept open. In the end, a deck and parapet replacement was the best value for money option.”
Refurbishment was included in TfL’s current £4bn roads modernisation programme, and had to be coordinated with other road works in west London.
Traffic disruption at the Hogarth Roundabout had to be kept to an absolute minimum. TfL imposed a six week time limit on the actual construction work, although trees and vegetation on and close to the roundabout had to cleared in February in advance of the nesting season. Round the clock working had to be adopted.
To speed the replacement, much of the original design concept was retained, minimising the need to modify the substructure. C50 grade concrete was specified for the replacement panels, which are 175mm thick rather than the original 150mm and feature an upstand beam to carry the new parapets.
The flyover was closed on 14 July and demolition began.
TfL project manager Dominic Green said it was considered too dangerous to lift out the original panels by crane, given their deteriorated condition.
“We decided to break them up insitu, with the debris falling onto the crash deck,” he said.
“In the event, the concrete simply crumbled away and we were able to complete the demolition a week early.”
Precaster Hanson produced the 86, 6.5t replacement panel units at its factory in Derbyshire, and the first was installed on 6 August.
A complex traffic management programme allowed access for the two 180t and two 230t cranes required.
“The southbound A316 had to be closed for three nights to allow the 180t cranes in to install 25 panels,” said Green.
“During the day, delays at the roundabout never exceeded 25 minutes. And we managed to cope with the RideLondon bike race coming through on 10 August.”
Early contractor involvement was key to the project’s success, Green added.
This led to several significant tweaks to the original design and construction programme that reduced time on site. More time was saved by the decision to eliminate lighting on the viaduct - tests showed that the illumination provided by the roundabout lighting below was sufficient.
A much improved panel clamping system was employed, eliminating the need for insitu concrete fill between the panels. The original flyover had a 3t weight limit and a 30mph speed limit, and these parameters will not change, said Gill. “But it will now be able to take 7.5t maintenance vehicles, and the parapets will be up to current containment requirements.”
A stiff stone mastic asphalt was specified for the wearing course, on a spray-applied waterproof membrane. Kerbs were installed by Extrudakerb.
There are also rumble strips - which posed something of a challenge for the contractor. Green said: “The rumble strip machine is heavier than 7.5t, so the rumble strips had to be installed by hand, in just two days.”
Most official records describe the Hogarth Flyover as being an example of the Bridgway system developed by contractor Marples Ridgway.
But a Ministry of Transport technical memorandum from 1969 paints a different picture.
It describes three competing temporary flyover systems, of which the first is identified as “The JGL Poulson Flyover”, also known as “The Bridgway”.
Another of the competing systems was the “Braithwaite FliWay”. A surviving example of the FliWay dating from 1970 can be found at Gallows Corner in Romford, at the junction of the A12 and A127. Its vertical profile is angular rather than smoothly arched, and it required major remedial works in 2008. Large sections of deck were replaced, and new parapets fitted.