Urgent inspections were taking place this week to work out how to repair cracks on a crucial M4 motorway structure in London in time for the Olympic Games.
The 15mm long hairline cracks were discovered by chance in welds on the Boston Manor Viaduct just west of junction 2 near Hounslow last week.
The Highways Agency has not confirmed the exact location of the weld cracks, but their discovery has forced it to impose weight restrictions on the damaged stretch, diverting coaches and lorries onto busy local roads.
The Highways Agency said the 1.03km long, 1960s, steel structure comprises 14 spans of plate girder construction and three spans of lattice girder construction with box girder chords. The elements are attached using a combination of bolts and welds.
The problems were first discovered on 24 March during a special investigation of the structure’s electroslag welds.
The plate girder sections of the structure have 312 welds on the 14 spans and there are 104 on the three span lattice girder deck.
Inspecting the welds involved the removal of the bridge’s protective paint coating, and it was only then that the cracks were noticed.
Until then, the problems had gone unnoticed, even during the bridge’s last six-yearly principal inspection check that was carried out in August last year.
The bridge is maintained for the Agency by the Connect Plus consortium which comprises Balfour Beatty, Atkins, Skanska and Egis Road Operation.
On discovering the first of the cracks the Agency imposed a 17t weight restriction on the viaduct while investigations took place as a “precautionary measure”.
These investigations uncovered more cracks, prompting the Agency to increase the restriction to 7.5t last Friday.
The Agency hopes to complete its inspections by the middle of this month and will then decide on a repair methodology.
It said that 2% of the structure is under examination, although it refused to reveal specific crack locations.
The solution could involve grinding out the cracks or putting steel compensation plates over them to reduce load on the welds.
The Agency expects that the repairs could be made before the Olympics but was not prepared to offer a guarantee.
The affected section is on the Olympic Route Network (ORN) and will be used to carry coaches travelling to and from Heathrow airport and the Olympic Park in East London.
It would be a major embarrassment for Games organisers if the restrictions cannot be lifted.
Transport for London (TfL) is in charge of the ORN. Following the discovery of structural problems on the nearby Hammersmith viaduct last February, TfL roads director Dana Skelley told NCE that “no other structures” were at risk.
She said Hammersmith Flyover was a unique case because of its mode of failure (NCE 9 February).
Skelley also insisted that all of TfL’s structures had been fully checked and it had a “robust regime in place” for assessing structures.
The Highways Agency, TfL and the London 2012 Organising Committee are looking into contingency plans but they are likely to rely on the current diversions for the time being.