Dec '13/Jan '14

Advances in ground control

Ground support solutions and the evolution of micro-seismic monitoring technology have the potential to help operations run with fewer interruptions and greater safety

By Alexandra Lopez-Pacheco

Miners are reaching deeper and deeper into the earth, and the need to monitor and mitigate dynamic ground conditions continues to drive the development of ground control technology.

Geologists and engineers gather as much structural information as possible on rock mass when considering a mine’s design long before any drilling or blasting ever begins, but there will always be unknowns inherent to how any given rock mass will behave. Excavation, blasting and drilling can result in rock mass shifting to redistribute stress, triggering instability. Innovation in ground support solutions and the evolution of micro-seismic monitoring technology offer the potential to help operations run with fewer interruptions and improve the protection of both personnel and equipment.

Flexible roof support

For years the primary ground support tools used underground to protect against rockfall have been shotcrete – essentially cement sprayed on the rock – and screen made from steel mesh. For all their advantages, screens can be difficult to fasten to the rock and shotcrete is brittle.

“Dry shotcrete can’t move, can’t flex,” says James Bradley, mining technical leader with 3M Canada. ”What happens in underground mining is the rock tends to want to move and, when that happens, the rock is always going to be stronger than the shotcrete. When it does [move], even three millimetres, the shotcrete cracks and loses its strength.”

In 2009, Xstrata Nickel approached 3M Canada and asked the company to help develop a substitute for shotcrete. Working with a mining consortium and Xstrata, 3M Canada developed its polymeric composite membrane (PCM). Shotcrete can take four to eight hours to dry and up to 20 days to cure after spraying, but PCM takes seconds to dry and three days to cure after it is sprayed on.

PCM can be used on its own or in combination with shotcrete for added stability. “Concrete is good at compression but bad in tension; PCM has excellent tension,” says Bradley. “When shotcrete starts to move and crack, the liner takes the load – it’s almost like a skin overtop of shotcrete that keeps it from moving and cracking further, taking the load away from the rock. If a mine has pre-existing issues, operators can spray PCM over shotcrete, giving it second life. Or if a mine wants to be proactive, the mine can spray before the shotcrete cracks. In our testing, we’ve found that spraying PCM over concrete increases the peak load – the point where the shotcrete cracks for the first time – by 50 per cent.” 3M suggests PCM can increase the overall toughness of shotcrete by 400 percent.

PCM can also be used on its own. “The point of this invention was to decrease the cycle time and to increase productivity of the mine reaching ore bodies,” says Alex Isings, 3M Canada’s mining and oil and gas leader. “Since PCM cures quickly, cycle times on mine sites can be decreased. PCM also uses much less material than shotcrete, requiring fewer drums to be transported underground. It’s also orange in colour, which makes it easier to identify which areas have been reinforced and helps to reflect light in an otherwise dark environment.”

The biggest hurdle to implementing the new technology, says Isings, will be changing mindsets. “The mining industry knows concrete and steel, but it doesn’t know polymers yet,” he adds.

Ears to the ground

Over time, micro-seismic monitoring has matured as a critical risk-mitigating tool. In simple terms, micro-seismic monitoring consists of sensors installed in the mine, a junction box where the key communications equipment is stored and seismic data from the sensors collected, and computers to store and process the data and create 3D images from it. “The technology emerged with computing power, so as computers got bigger, better and faster, so did and does the technology,” says Brad Simser, a Sudbury-based ground control engineer with Glencore Canada.

“If you go back to some of the older mines, typically you had copper wire going from surface down to underground and it would branch out like a tree root system to connect with the sensors,” says Simser. “To accomplish the copper wire infrastructure was an enormous task.”

With the advent of wireless technology, the logistics of getting the system’s backbone into a mine for a new operation has become much easier. Today, the technology can far more effectively differentiate between real seismic events in the mine and vibrations caused by drilling, haulage trucks or rock rumbling down the ore pass, Simser notes. Best practices are also constantly being improved upon, and this includes not skimping on the sensors so as to provide accurate information that zeroes in on the exact area producing the seismic activity. As well, more experts are seeing the important role of micro-seismic monitoring in the mine design stage, rather than simply using it to monitor risk in ongoing operations. It provides the opportunity to collect background-level data on the normal seismicity in the mine, making it easier by comparison to recognize new issues.

The micro-seismic monitoring technology sector is dominated by two players: ESG Solutions, headquartered in Kingston, Ontario, and the Australia-based Institute of Mine Seismology (IMS), which will open an office in Ontario, in early 2014. In 2012, both ESG and IMS released new iterations of their monitoring technology. In addition to increased sensitivity, both systems address the issue of availability with built-in self-testing capabilities designed to free up the time and labour often tied up with system maintenance. Alexander Mataseje, project manager and business development specialist at ESG, explains: “In the past, troubleshooting instruments required travelling underground into the mine to visually inspect the equipment, to look for cut wires or damage and take a physical reading to determine the cause of any failures. The new Paladin IV 32-bit seismic recorder has built-in diagnostic tests that allow users to diagnose any issues directly from the seismic terminal on the surface.”

To make the IMS system more flexible, managing director Richard Lynch says the company’s new system is modular in design. “The seismic station traditionally consists of a digitizer and a CPU, which were packaged together. We’ve split these into two separate units now, making it cheaper for some applications. For example, if a customer wants to monitor four sensors in the same borehole, they used to have to buy two complete stations. Now they buy two digitizer units and just one CPU unit.”

New filtering technologies and more powerful CPUs have also improved the quantity and quality of seismic events being detected. Mataseje reports one mine using the latest system in Ontario experienced a 36 per cent increase in event detection. “In distinguishing normal seismic events from abnormal behaviour, even the most subtle trends provide valuable feedback for ground control. By detecting even more of the smallest seismic events, over time we are able to generate a comprehensive record of seismic activity, and use that for more meaningful interpretations,” he says.

For all the advances made in the technology, the ultimate goal of being able to forecast major seismic events based on data collected and identified trends remains a challenge.

“This technology provides a set of eyes to see how the rock responds to putting in tunnels and making big holes underground but it is still not a great forecasting tool to say what will happen,” says Simser. “It’s a good risk identifier: you can see a higher risk area or a lower risk area. But to know tomorrow at noon we can expect the big one, no one has been able to do that yet.” In the past, Simser has analyzed relatively large seismic events at a mine and found that less than half showed a clear connection between mining activities and subsequent seismic events and, for nearly a third, “there was perfect silence, nothing, and then out of the blue, a big event.” In theory, the potential exists to predict a significant number of major seismic events by analyzing the enormous amount of seismic monitoring data collected. “Very small seismic events won’t do observable damage but the truth is there is some damage to the rock so the cumulative effect is significant,” says Simser. “From an individual events basis, since there’s no observed damage, mines often tend to ignore it and only look at the bigger ones. Most operations I’ve seen do the barebones basics with the information: maybe dig a tiny bit into it. Not that many people have enough training and experience to dig deeper into the data, so there’s a gap there.”

Lynch notes that, “In some mines – the deep gold mines in South Africa being good examples – seismic hazard forecasts are made three times each day for every working area. It’s a difficult task, but the success rates are good enough to allow the mine to manage its seismic risk.”

A software solution to more complex seismogram analysis is still some way off, says Lynch. “Automatic algorithms, unfortunately, are still not as reliable as experienced human processors, despite two decades of research. Progress is being made, but we measure that humans are still better about 65 per cent of the time.” In response, IMS provides outsourced data processing and filtering services.

ESG has addressed the gap in analysis and training by recently launching a mining and geotechnical consulting services group. “Many mines prefer to have direct access to their seismic data. By offering additional training to clients, giving them more guidance on what to look for and how to reconcile current seismic activity with historical data, they are better able to make sense of what they’re recording,” says Mataseje. “In terms of newer advanced analysis, our consulting group offers a number of methods that go beyond just locating seismic events, but that help clients understand how and why rocks are failing,” says Mataseje. “Where I see the industry moving is in the analysis and interpretation of the data collected using advanced techniques, to help mines get the most value out of their seismic data.”

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