Transport for London (TfL) has published a report into an incident on 28 May, 2008 involving a rail bridge being built outside Liverpool Street station as part of the East London Line Project.

The report found that at 19:15, the bridge dropped 200mm after sub-contractors, using temporary supports, made final adjustments before placing it on its permanent fixtures.

They wrongly positioned a Teflon plate between the base of the bridge and the temporary supports.

Their working methods were not cleared, either with the site Principal Contractor Balfour Beatty - Carillion Joint Venture (BB-CJV) or TfL project engineers.

It is believed a change in temperature as the bridge cooled resulted in it contracting two or three millimetres.

This led to a movement, which caused the temporary supports to be ejected from their position, resulting in the east end of the bridge dropping onto its permanent fixtures.

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The incident caused five pieces of concrete decking to fall onto rail lines below, and services into Liverpool Street station were suspended overnight.

TfL has tightened up supervision over its contractors as a result of the incident.

Tfl London Rail's Director of Safety, Martin Brown said: "We have investigated this incident thoroughly, identified the causes and put in place procedures so that it cannot happen again.

"It was a unique event that we want to ensure cannot be repeated on any other bridge installation.

"The temporary work, which involved jacking the bridge horizontally about 38mm to a position over the permanent fitting, was undertaken without engineers from TfL or our main site contractor, BB-CJV, being made aware of what was happening.

"The positioning of the Teflon pad was a human error, but 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.

"When planning the bridge installation it was not considered that this final repositioning work would be required.

"When it was found to be necessary, no method statement was prepared for checking. It should have been.

"Balfour Beatty-Carillion has upgraded its system of checking its sub-contractors and TfL has also stepped up its overall supervisory role."

The bridge has now been bolted and welded into its permanent position without further problems.

(CD/NS)