COLLAPSE OF a section of Australian motorway tunnel during construction last week has cast further doubts on the use of the New Austrian Tunnelling Method (NATM).
The collapse occurred at the intersection of a ventilation adit and a running tunnel during construction of the privately fi nced Lane Cove tunnel, 75km north of Sydney.
NATM work on the project involved excavating up to 2.5m of tunnel at a time using road headers to form a 7m high, 8.1m wide bore.
Contractors then installed a row of rockbolts pre-tensioned to 50kN, and applied a 75mm thick layer of C28/35 shotcrete.
Rockbolts were then grouted.
Final waterproofing and another layer of shotcrete will be applied later.
The cave-in caused the partial collapse of a three-storey block of flats above the intersection.
Nobody was injured.
Ground conditions at the intersection include 'highly fractured' low strength weathered sandstone, high strength shale and laminated layers.
It is understood that engineers were rock bolting or excavating the ventilation adit at 2am last Wednesday when the collapse created a 25m deep hole.
The flats above the collapse were immediately evacuated and contractor Thiess John Holland pumped over 1,400m 3 of C40/50 concrete to stabilise the hole. This was completed by 7am the same morning.
By Sunday a structural assessment revealed that 12 units were safe to inhabit, but nine required further strengthening.
The whole structure has been temporarily shored.
A report published by the local Lane Cove council reveals that the intersection was originally located under a park, 65m east of the block of flats.
The council said that a redesign of the ventilation scheme in December 2002 led to the decision to shift the junction between the ventilation shaft and a running tunnel under the fl ats. This is believed to have worsened the collapse.
'As the location of the air tunnel seems to be a crucial element in the tunnel collapse, all the circumstances relating to the changes to the ventilation system, including what level of assessment and consultation was undertaken, need to be investigated, ' says the report.
The council also claimed that design and build contractor Thiess John Holland Joint Venture had not consulted residents adequately over the redesign.
Thiess John Holland claimed that the revised ventilation scheme had been reviewed by an 'independent verifi r' and that the community liaison group had been briefed in November 2004 over the changes. It added that the tunnel had also been built in the correct location.
Thiess John Holland has appointed rock mechanics expert Professor Ted Brown to carry out an independent review of the incident.