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 month.
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. 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.
Tunnelling was carried out using observational 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 Ashfield 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 stressinduced 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 under-estimated 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.
Steel arches to beef up Sydney's Lane Cove tunnel
Contractors on the partially collapsed Lane Cove motorway tunnel in Sydney, Australia have changed tunnelling methods, NCEI has learned.
The new method involves extensive probing ahead of tunnelling in the weak Ashfield Shale stratum.
Contractor heiss John Holland is also expected to use steel arch frames instead of rock bolts in weak ground when the tunnel is close to the surface.
The collapse occurred in November last year during excavation of a junction 17m below ground (NCEI last month).
Construction used road headers to chip away at the rock using the New Austrian Tunnelling Method. This involved applying two passes of shotcrete and rock bolts.
But after an independent report was published last month by Imperial College rock mechanics professor Ted Brown, NCEI understands that the construction methodology has been changed.
Future tunnelling of the Lane Cove tunnel through areas of Ashfield Shale will involve drilling 20m long holes upwards at a 10° angle near the top of the excavated face. An endoscope will then be inserted in the holes to monitor the ground.
The source said that if the Ashfield Shale was encountered where there was less than 17m of cover, steel framed arches would replace rock bolts to provide greater stability.
It is expected that the tunnel junction will be relocated away from the collapse site, although the method for supporting this part of excavation has still to be decided.
1.38 am, 2 November: Small blocks of rock and shotcrete fall from the crown of a recently excavated tunnel section.
Fractured rock against the dyke surface falls into the tunnel, destroying the structure's arching action. Shotcrete then begins to peel away as the void caused by falling rock gets bigger. Rock, including rock bolts, fall along the vertical plane of a fault line and the dyke, and horizontally along joints.
1.40am: Workers evacuate as the void propagates upwards towards the retaining wall under Epping Road and the foundation of the block of flats within 20 minutes.
2.30am: A hole up to 6m in diameter opens up above the tunnel.
6.50am: Longueville road is closed to traffic 8.25am-12.00pm, 3 November: Concrete pumped in to stabilise the hole.