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Boston Manor viaduct will be replaced, says minister

Ministers are seeking a long term replacement for the troubled Boston Manor viaduct following last week’s dramatic closure of a vital section of the M4 because of a worsening cracking problem, NCE can reveal.

Speaking as the bridge reopened to light traffic on Friday, roads minister Mike Penning confirmed that work had already begun to look into replacing the structure in the long-term.

“It’ll cost a lot of money - it’s a major piece of infrastructure,” said Penning, although he said it was too early to discuss specifics. “We will now have discussions over what we do in the long-term.”

Those discussions are understood to be at an early stage and represent a quick about turn over the future of the structure. Just a day before Penning spoke, a Highways Agency spokesman told NCE that it believed the structure still had a working life of “decades” left.

“There’s no replacement on the cards at the moment, nor for decades to come,” said the spokesman, who added that the structure’s capability should not be understated considering that it provided an “uninterrupted service” for decades prior to the recent problems.

The spokesman stressed that the current repair work was being done alongside an inspection programme that was primarily designed to inform the viaduct’s future maintenance regime.
However, within hours the debate had shifted towards considering a replacement, a decision no doubt influenced by the extremely sensitive timing of the problems and increasing political attention.

Penning said that in the shortterm the emphasis had focused on finding a solution ahead of the start of the London 2012 Olympics.

The viaduct is on a crucial stretch of the M4 between London and Heathrow airport and is part of the Olympic Route Network. Engineers have been working around the clock under great pressure to get the repairs to a point where the structure could reopen at all.

Both Penning and transport secretary Justine Greening visited the site last week as the repairs continued amid uncertainty over how long they would take and whether the closed section between junctions 2 and 3 could be fully reopened ahead of Games traffic.

As of last Friday, the road was repaired to the point of being able to open to light traffic and a pre-closure weight restriction of 7.5t was reinstated. However, the Games Lane on the M4 - which came into operation on Monday - will have a greater weight allowance of 13.5t for the coaches that will carry athletes, officials and media arriving ahead of the Games.

Highways officials had hoped to reopen the road for the Games with no restrictions, but Penning confirmed that these will now remain in place for the foreseeable future and certainly during the Olympics.

The problem that forced the closure stemmed from a crack 10mm long and 3.5mm deep in one of the bridge’s electroslag welds, discovered on a “sensitive” area at the top of one of the steel truss girder sections of the bridge. Inspections and repairs to the electroslag welds have been ongoing since March (see box) and discovery of this most troublesome crack was made as repairs were carried out to a crack on an adjacent weld around 30mm away.

“We were right at the end of what was already a very rapid programme,” technical director for the Connect Plus contractor Derek Hughes told NCE. Connect Plus operates and maintains this section of the M4.

Despite a delicate operation to remove the crack via burring - a type of grinding that generates less heat - the crack worsened.

“In trying to remove the crack it was clear the crack wanted to keep growing,” said Hughes. He added that the increased difficulty in dealing with this crack was exacerbated by the presence of an inclusion in the weld - a piece of slag or carbon - as well as work to repair the nearby crack. As a result, the team decided to stop trying to remove the crack - which otherwise would have been ground down and infilled - and instead bolt four specially manufactured 15mm thick steel plates, each around 1.5m by 500mm, over the weld to bypass it.

Work still remains to be done, but according to the Highways Agency spokesman because of the likelihood the crack was caused by the existing repair work, “it is taking a more cautious approach to repairing the existing welds”.

Of the 64 original defective welds, 57 have been repaired.

Electroslag welding woes

Electroslag welding was used in the 1960s and 1970s but concerns emerged over the brittle nature of the welds leading to it being phased out by the 1980s.

The US Federal Highway Administration first banned its use on primary structural tension members such as flanges in 1977 following a discovery of a large crack on a highway bridge near Pittsburgh.

The welds are brittle due to the high heat input required to join the pieces of metal together.

The process involves placing the two steel plates to be joined in a vertical position about 30mm apart. Two copper shoes are attached to either side of the plates, with a bridging piece at the bottom creating a cavity.

Welders will then feed 2mm to 3mm thick filler wires into the cavity before passing a current through these wires to strike an arc in a process that is similar to striking a match. This creates a bubbling pool of metal. Wire is then continually fed into this bubbling pool, melting and filling up the cavity before eventually solidifying and forming the weld.

Welders repeat this process up the length of the plate until the whole section has been fused together.

But the process produces large grains within the weld and the heat effective zone of the metal sheets, making it brittle.

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