FAILURE OF under-designed rock bolts triggered the collapse of the Lane Cove motorway tunnel in Sydney, Australia, according to an independent report published last week.
The area where the tunnel roof caved in was known to have poor ground conditions, it says.
Rock bolts at least 2m longer than the 5m bolts used should have been incorporated to support the tunnel roof, said report author and Imperial College professor of soil mechanics Ted Brown.
The 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 (NCE 10 November). At this point the spans were larger than those for a single tunnel section.
After the collapse, a 6m diameter, 25m deep hole opened at street level down to the tunnel, undermining a block of flats.
Road headers were used during tunnel construction to chip away at the rock using the New Austrian Tunnelling Method. This involved excavating up to 2.5m of tunnel at a time, using road headers to form a 7m high, 9m wide tunnel.
After excavation of each section, a 100mm thick layer of shotcrete was applied, followed by 4m long rock bolts in a 1m by 1m grid.
A secondary support system of 100mm of shotcrete and 5m long rock bolts in a 1.5m by 1.5m grid was then applied, before a final shotcrete layer to cover the bolt heads.
Ground conditions included highly jointed Ashfi eld Shale, a vertical igneous rock intrusion or dyke, and fault lines.
The dyke and fault lines acted as vertical shear planes where rock masses could readily dislodge, says the report.
The many horizontal and vertical planes in the shale also reduced the 'already low compressive and tensile strengths of the rock mass, increasing the likelihood of stress-induced failures around the periphery of the excavations', says the report.
Brown's investigation also discovered that the dyke, which had been encountered throughout the course of tunnelling, had not been included in design documents for the junction between the two tunnels.
'Under these circumstances, the junction is regarded as an excavation meriting special design attention, particularly in terms of the provision of support, ' says the report.
'But at the design stage no special analysis of the - junction was carried out.' The report expresses the view that the contractor underestimated the effective span of the tunnel at the junction point.
It says that although calculating the effective span was a matter of opinion, Brown believed it to be 21m while the contractor had calculated it at 17m.
The report continues: 'Spans of 17m to 22m are very large effective spans for a material as weak as the weathered Ashfield Shale, and could well be outside the limits of precedent practice in this material.' Brown added that rock bolts at least 7m long should have been installed at the tunnel intersection.
Design and build contractor Thiess John Holland has continued tunnelling the remainder of the twin 3.6km long motorway tunnels using the same method.
But a company spokesperson said that the tunnel would be diverted away from the collapse site.