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Cracks trigger closure of second Olympic route bridge: full report

Engineers were this week scrambling to finish urgent repairs to Boston Manor viaduct in west London in a desperate effort to reopen a vital section of the M4 in time for Olympics traffic.

The urgency of the work intensified last week after the discovery of a new crack in the 1960s structure’s electroslag welds on Friday forced the closure of the elevated section of motorway between London and Heathrow airport.

The pressure is on the Highways Agency as Olympic bosses want to introduce M4 Games Lanes from Monday. Games lanes are reserved for the vast numbers of athletes, media and officials who are expected to begin arriving from Heathrow. An Agency spokesman said the looming Monday deadline was “closer than we’d comfortably like”.

Inspections and repairs to cracks in viaduct welds have been ongoing since March (NCE 5 April). The new crack is no longer than others already found, but the Highways Agency said its “sensitive location” triggered the dramatic action.

The special programme of work was initially triggered by reported concerns about the electroslag welding technique elsewhere in the world. The 965m long, 17 span viaduct is a mix of plate girder and truss girder construction and has 416 welds, 312 on the 14 span plate girder section and 104 on the three span truss section.

Since March, investigations have identified 64 areas needing repair with cracks up to a maximum of 15mm long. Repairs began in mid-May and 52 areas have now been fixed. A 7.5t weight restriction has been in place since investigations revealed the extent of the damage.

Inspections are ongoing, although work was supposed to be prioritised depending on the level of risk at each location. A Highways Agency spokesman said he was “not entirely sure”
why the latest crack in such a vital position had been discovered at such a late stage.

The closure of the viaduct was prompted by discovery of a hairline crack on the truss section of the bridge in a highly stressed zone over the bridge supports. The discovery was made during overnight works last Friday just west of junction 2 on the M4, prompting closure of the road between junctions 1 and 3 while repairs are carried out.

Repairs involve grinding down the crack, infilling it and then fixing a steel compensation plate over the area.

Inspections were continuing as NCE went to press on Monday and the Agency spokesman said it was still unknown whether more cracks would need repairing.

The affected section of the M4 is part of the Agency’s Area 5 network covering the M25 and radial routes into London. Area 5 is operated and maintained by the Connect Plus consortium of Balfour Beatty, Skanska, Atkins and Egis Road Operation UK under a £6.2bn DBFO deal.

Atkins is working as technical adviser on the job with Arup. Osborne is specialist contractor and the Welding Institute and the University of Manchester are providing experts.

Liability for fixing the defects is shared between Connect Plus and the Agency. The spokesman said that the priority was to get the work done as soon as possible but then there would be a period of “number crunching” to work out the full cost of the repairs as well as any wider economic losses.

The spokesman added that while the Agency was always “looking to strengthen” its structural maintenance regime there as otherwise limited scope for learning lessons because the
structure is unique.

Second closure of key route in seven months

Closure of the M4 this week is the second time in seven months that the key Olympic route from Heathrow to central London has been closed due to a structurally deficient viaduct.

In December Transport for London (TfL) was forced to temporarily shut the A4 Hammersmith flyover after it discovered serious, widespread corrosion in the precast concrete viaduct’s
post-tensioning cables (NCE 12 January).

It was able to reopen one carriageway in each direction in January but did not fully reopen it until May.

TfL is in charge of the Olympic Route Network around London and is now closely monitoring the Highways Agency’s work on Boston Manor.

“We are staying in regular close contact with them about progress,” said a spokesman.

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