As a parliamentary inquiry assesses rehabilitation of open cut brown coal mines to reduce fire hazard, concerns remain for mine stability.
Events including batters of the Yallourn mine collapsing in 2007, the Princes Highway crumpling in 2011, and even the diagonal crack that appeared on Latrobe Road last year were cited in a lecture about mining induced ground movement.
Federation University Geotechnical and Hydrogeological Engineering Research Group director Rae Mackay presented ‘Mining Induced Ground Movements in the Latrobe Valley: what, why, when and where?’
A large water source or a period of significant rainfall, water access through the overburden via sinkholes and cracks were considered general features of such major movements.
But Dr Mackay said GHERG was also considering the tensile strength of brown coal, its residual shear strength and what is called “creep behaviour” – where a material sags or deforms under mechanical stresses.
“Basically if you leave anything for a long time it sags, there’s an opportunity to move even though you’re not changing the forces that are applied to them,” Dr Mackay said.
He explained GHERG was tackling a number of issues that were not well understood since research from the former State Electricity Commission.
“Mining in the Valley has been (happening) for quite a few years so now there are opportunities for slower processes to begin to have an impact,” Dr Mackay said.
“We need to understand movements that are occurring over the longer term, changes to materials, to material properties that have an impact on mines that have been open for a long period of time. This will have an impact on rehabilitation of the mines going forward,” Dr Mackay said.
The professor is not involved in the reopening of the Hazelwood Mine Fire Inquiry as it assesses mine rehabilitation, but said mine stability could overlap work to reduce fire hazard.
“If you can cover over a coal surface to reduce fire risk and do it well enough to reduce the opportunity for water to flow into the cracks and reduce risk of movement, there could be some cross thinking there,” Dr Mackay said.
“Simply covering over coal doesn’t stop it from moving.”