ROGUE SHALE ground conditions encountered during the construction of an Australian motorway tunnel last month could have led to its collapse, it was revealed this week.
The Lane Cove tunnel near Sydney was being constructed using the New Austrian Tunnelling Method (NATM) (NCE 10 November).
Contractor Thiess John Holland said this week that the collapse occurred in predominantly Mittagong and Ashfi eld shale ground conditions.
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 fi nds it safe to do so. Brown's report is due before the end of the year.
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 that shale degrades quickly when exposed.
Shale absorbs moisture, swells and then the material breaks down. If you can recognise the material you make sure you instantly cover it with shotcrete, ' he said.