Report concludes subcontractor error in positioning Teflon plate caused bridge accident that led to rail rush-hour chaos

Human error was to blame for the partial collapse of a rail bridge on 28 May, according to a report published today by Transport for London (TfL).

Despite the presence of TfL inspectors on site, Bridge GE19, which forms part of the East London line extension, dropped 200mm after subcontractors, using temporary supports, made final adjustments before placing it on its permanent fixtures.

A Teflon plate was wrongly positioned between the base of the bridge and the temporary supports. It is believed that, as the bridge cooled, the change in temperature resulted in its contraction by two or three millimetres. This led to a movement that caused the temporary supports to be ejected from their position and resulted in the east end of the bridge dropping onto its permanent fixtures.

Five pieces of concrete decking fell onto rail lines, causing services into Liverpool Street station to be suspended.

TfL London Rail's director of safety, Martin Brown, said: “It was a unique event that we want to ensure cannot be repeated on any other bridge installation.”

The working methods of the subcontractors were not cleared with the TfL project engineers or the site's principal contractor, a Balfour Beatty/Carillion joint venture.

“The positioning of the Teflon pad was a human error,” said Mr Brown. He admitted: “the procedures we have should have stopped this error occurring. There is a collective responsibility for ourselves and our contractors to ensure these procedures work.”

Two other East London line bridges put in place around the same time have been checked and no problems have been found.

The East London line extension is due for completion in June 2010. The bridge will carry London overground trains over the six tracks leading out of Liverpool Street.