Investigations are continuing as to why one segment of a tunnel being built in Sydney, Australia, collapsed on Nov. 2. Residents above the Lane Cove tunnel were evacuated after a section being excavated by roadheaders collapsed near an exit ramp. No injuries were reported.

Rescue teams from the New South Wales Fire Brigade immediately evacuated 60 people. They reported that a growing hole 10 meters wide and 7 m deep was undermining and damaging low-rise apartment houses. The rest of the 3.6—km twin tunnel, 95% excavated, was unaffected, according to the contractor.

The joint venture of locally based Thiess Pty, Ltd. and John Holland Group Pty, Ltd. blames "unforeseen" ground conditions at the collapse area. The earth there consists of shale with intrusions, while the rest of the tunnel is mainly in good sandstone, says project spokesman David Saxelby.

Former rock mechanics professor Ted Brown will begin reviewing the cause of the collapse on behalf of the joint venture. The New South Wales state government also announced that it was conducting an investigation. Meanwhile, the contractor filled the affected area of excavation with concrete to prevent further subsidence.

Under an approximately $800-million turnkey contract that began in mid-2004, the joint venture is building three-lane twin tunnels. Running east-west, they form part of Sydney’s new beltway. Altogether, the joint venture is using seven roadheaders for excavation, employing a version of the New Austrian Tunneling Method.

The accident occurred at the tunnel’s eastern end, where a side exit ramp to Pacific Highway crosses a ventilation tunnel, according to joint venture officials. "We had a thorough geotechnical and engineering plan," says Saxelby. "That plan was followed and this particular event was absolutely unforeseen."

Excavation has taken place in about 100 m of similar ground conditions, he adds. He predicts that the project completion will remain on schedule for mid-2007.