ROGUE SHALE conditions encountered during the construction of an Australian motorway tunnel could have led to its collapse, it was revealed last month.
A section of the Lane Cove tunnel, 75km north of Sydney, was being built using the New Austrian Tunnelling Method (NATM) when it collapsed in November at the intersection of a ventilation adit and a running tunnel (GE December 05).
Contractor Thiess John Holland said the collapse occurred in predominantly Mittagong and Ashfield shale.
It has appointed rock mechanics expert professor Ted Brown to carry out an independent review and tunnelling will only continue through the shale if he finds it safe to do so. Brown's report was due at the beginning of January.
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.
The contractor then installed a row of rockbolts pre-tensioned to 50kN and applied a 75mm thick layer of C28/35 shotcrete.
Imperial College rock mechanics senior lecturer John Harrison said shale degraded quickly when exposed. 'Shale absorbs moisture, swells and then material breaks down. If you can recognise the material you make sure you instantly cover it with shotcrete.'