Geotechnical engineer James Doherty is helping solve one of the costliest challenges facing underground miners: how best to fill the voids left under the earth's surface when mining is complete.
Associate Professor Doherty is a geotechnical engineer who specialises in numerical modelling at the School of Civil, Environmental and Mining Engineering (CEME) at The University of Western Australia.
Filling up the stopes, or underground spaces, left after extracting ore is an important part of an underground mine's operations.
It's a big, and expensive job, especially when you consider the stopes might be as high as 100 metres in some cases.
"There's a limit to how big these voids can get, so what they do is mix the mine waste with cement and pump it underground into the stope," Associate Professor Doherty says.
"At some underground mines, the biggest cost item is the cement they add to the stuff they pump underground."
Complicating the issue is that when the cement mix combines with water to become a gel, it shrinks.
"This volume reduction has significant implications in terms of stresses and so on inside the material," he says.
"So we're trying to couple all that into a model."
The numerical modelling that Associate Professor Doherty is working on involves taking measurements within the stopes using piezometers and total stress cells.
"We're working with two mines in Kalgoorlie, one's a nickel mine and the other's a gold mine; and we're also working with a large silver mine in Queensland," he says.
"We've collected a lot of samples and done a lot of lab testing.
"I'm now in the process of building models that replicate the material behaviour - what we've observed in small samples in the lab - then
putting that into numerical models that replicate complex boundary conditions that this material might see as you're filling up a stope."
The modelling is designed to help answer two practical questions.
"That is, how fast can you fill these voids without busting the barricade that you build at the bottom?" he says.
"And ultimately, as you fill it up, how much cement is needed so that when you blast the stope next to it, the original stope can free stand and is strong enough to support its own weight without collapsing into the void you create."
(You can find out more about CEME researchers and their projects at the School's website.)
Associate Professor James Doherty (Civil, Environmental and Mining Engineering) (+61 8) 6488 3991