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Steel arches replace rock bolts in Lane Cove motorway drive

CONTRACTORS ON the partially collapsed Lane Cove motorway tunnel in Sydney, Australia have changed tunnelling methods, a source close to the project has said.

The new method involves extensive probing ahead of tunnelling in the weak Ashfield Shale stratum.

Contractor Theiss 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 last November during excavation of a junction 17m below ground at the connection of two tunnels (GE February 06).

Roadheaders chipped away at the rock for the NATM construction which called for two passes of shotcrete and rocks bolts.

Geological features including horizontal and vertical joints, vertical igneous rock intrusions and fault lines caused the crown of the tunnel to collapse.

An independent report by Imperial College rock mechanics professor Ted Brown, published in January, said the contractor had under-designed the junction.

It is understood future tunnelling through areas of Ashfield Shale at Lane Cove 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.

A source said if the Ashfield shale was encountered where there was less than 17m of cover, the steel framed arches would replace rock bolts to provide greater stability. It is expected 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.

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