The slip caused material to fall onto rail lines outside Liverpool Street station during the evening rush hour on 28 May (NCE 5 June).
Speaking to NCE following its report into the incident published last week, TfL director of heath, safety and environment for its London Rail division Martin Brown said: "We successfully launched the bridge to within 1mm of its intended position. It was resting on packing material, and was to be jacked down – a systematic process."
However, pitched at a 3.3% to 3.4% angle (1:30), the bridge was resting on a tapered plate during jacking. Thermal expansion caused the bridge to move 38mm from its initial resting position after its launch (NCE 21 February). This movement had to be corrected with additional horizontal jacking.
Joint venture contractors Balfour Beatty and Carillion along with jacking specialist Fairfield Mabey had added pads coated with polytetrafluoroethylene (PTFE), also known as Teflon, to the bridge, to ease horizontal jacking.
“PTFE reduces friction when loaded. When there is movement, friction is further reduced,” said Brown.
He added that the final mechanism for the slip remains obscure, and would be difficult to model. But as work ended just before the accident, the PTFE pads were left on the bridge. As the bridge bearing and PTFE-coated surface allow movement, it is likely the bearing and bridge moved in opposite directions, precipitating the slip.
“People on-site did not understand the forces involved and the error they were making,” said Brown. “The attention to detail we gave to the successful launch was not carried through to the lowering which, with some human error, allowed the slip. This could have been prevented with a bit more work”
Brown said the slip exposed valuable lessons for what is a routine procedure and TfL shared its findings with Network Rail, the Highways Agency and others. Network Rail has allowed work on the bridge to finish. Final concrete pours are due in October.
TfL London Rail has made six recommendations from its inquiry. These include London Rail disseminating the lessons learned; reviewing the way it assures itself of contractor performance, including looking at record keeping; refining its emergency plans and reviewing escalation and on-call procedures and formal inquiry procedures.
Finally, it proposes that the Balfour Beatty-Carillion joint venture review its risk assessment procedure, and ensure that its subcontractors use the same risk assessment techniques.
Client - London Rail (part of Transport for London)
Principal contractor - Balfour Beatty-Carillion joint venture
Bridge design - Scott Wilson and Benaim
Bridge construction and temporary works - Fairfield Mabey
Erection - MJ Hughes
Detailed jacking down design - Chris Booth Associates
Temporary works design checking - Chris Adams