A DECISION to replace steel support girders with rock bolts could have contributed to the Lane Cove motorway tunnel collapse in Sydney, health and safety investigators said last month.
The report by WorkCover New South Wales, Australia's equivalent of the Health and Safety Executive, reveals that the tunnel was originally to have been supported by steel girders.
It says design and build contractor Thiess John Holland (TJH) asked designer Parsons Brinckerhoff to replace these with rock bolts.
'The ground support option of steel sets or girders for poor ground conditions in shale, was superseded and replaced with a less conservative rock bolt and shotcrete support option, ' the report says.
The Lane Cove collapse occurred on 2 November 2005 at the junction of the Pacific Highway exit ramp tunnel and the Marden Street ventilation tunnel in north Sydney (GE February 06).
The tunnel was being built with road headers excavating, and rock bolts and shotcrete used to stabilise and strengthen the walls and roof.
Parsons Brinckerhoff originally speci ed steel girders to permanently support the 9m diameter excavation where ground conditions were particularly poor, before two passes of shotcrete were sprayed 17mm and 200mm thick.
The latest report follows TJH's own investigation in January, which found that the rock bolts were too short.
However the new report states that not only were the bolts a metre shorter than they should have been - at only 4m - but the applied shotcrete was not always thick enough.
The report says the contractor failed to make the tunnel wide enough and then applied a thinner layer of shotcrete to preserve the design cross-section.