Navigating changing risk profiles in geotechnical environments
Despite the major progress made in rock mechanics over the last 50 years, there are still outstanding problems. Globally, the minerals industry worldwide is encountering increasing costs and demanding conditions. These include lower ore grade, unconventional deposits, skill shortages, as well as a range of social and environmental challenges.
Three rock mechanics experts from industry and academia explain the major unresolved rock mechanics problems that the industry faces and examine the potential impact of new technologies.
"One of the problems we encounter is the natural variability in the rock masses and ground conditions we are operating in,” says Dr. Alison McQuillan, director of Rocscience Australia. McQuillan is a Chartered Professional in Geotech who has provided specialist consulting services in copper, gold, iron ore, coal and diamond operations in Australia, PNG, SE Asia and Africa.
McQuillan highlights that while modelling can be used to try and better understand the potential scenarios, it can be endless. “The question is, when is enough enough?” Professionals need parameters to navigate a slew of scenarios. “With the uncertainty, we should have some rules or guidelines in place. For example, at least these volume or sensitivity analyses or probabilistic analyses need to be completed to ensure the end user has an understanding of: One, the uncertainty; two, the variability and three, the changing risk profile that then presents.”
Changing risk profile is a concern for many geotechnical engineers. McQuillan suggests that to mitigate the uncertainty, we need to leverage new technology. “If there’s anything that helps us predict the ground conditions ahead of time, that will also obviously reduce the risk,” she says.
Another area of uncertainty in rock mechanics is the representation at scale of the responses of the rock mass in different geotechnical environments. “We know very little”, says Fernando Vieira, Senior Principal Geotechnical Engineer at Cartledge Mining and Geotechnics. “In fact, as everyone knows in these fields, we have great modelling and schematics and ways of representing by artificial means what we perceive to be the response but in reality those responses are never mimicked by what actually happens.”
Over a period of over 30 years, Fernando has worked on surface and underground mines sites, on operations technical branches and in corporate. He has addressed gold, copper, coal, platinum, iron-ore and base-metal mining environments around the world. “In my career, I’ve done a lot of modelling on all sorts of platforms and applications and never ever did I see the response modelled in reality. This raises the problem of the credibility, sometimes, and reliability of modelling.”
There is also the problem of the time-dependent behaviour of rock materials. “Often, we are required in civil constructions, in underground construction type projects, to really estimate the standing times of excavations to remain stable over a certain period of time. And that is never equated into our designs, which is very problematic, indeed,” he says.
Time-dependent responses can occur during both the construction and the maintenance of underground openings depending on the environment. According to Dr Chrysothemis Paraskevopoulou, lecturer in Tunnelling and Rock Engineering at the University of Leeds, this can include creep, squeezing, swelling, stress relaxation, and strength degradation of the rock mass. Accurately anticipating the true behaviour of rock material is a challenge and one that, Vieira contends, will linger not only here “but also in Space.”
Professor Jamal Rostami, from the Colorado School of Mines, echoes Vieira’s comment on the limitations of geotechnical investigations. “Of course, we can complement that with instrumentation and sensory systems and try to get more information in the whole process,” he adds, referring to some of the technological advances in the space. “In recent years and, of course, in the future, we’re moving towards more stochastic design.”
“We really understand the boundaries of equity in the rock properties and parameters. Then we can use the concept of probability of failure, which is a much better approach in design in that it takes variability into account and essentially allows us to choose a proper risk factor instead of just fortifying the design by a lot of over design and by a factor of safety.”
For Rostami, though, perhaps the greatest challenge to the sector is ensuring knowledge is passed on to future generations, “It requires a lot of training and a lot of sharing of knowledge so that we can be successful in the future in rock mechanics.”
This article was an extracted transcript from AusIMM's AusRock Webinar: Future of Rock Mechanics. To discover some of the cutting-edge research outcomes set to shape the next evolution of mining activity, register for the upcoming Advances in Rock Mechanics specialist masterclass, powered by UNSW Sydney.