Use of hi-tech monitoring is helping to safeguard the operation of an essential mine rail tunnel in Canada but the techniques could benefit other geotechnical schemes too.
Advancements in geotechnical monitoring methods are leading to more audacious subterranean projects being carried out each year. These techniques are not just benefitting new developments - use of the latest approach has helped one Canadian mine maintain an historic tunnelled rail link.
Key to Iron Ore Canada’s operation in Quebec is a 1950s-built rail tunnel on the Quebec North Shore and Labrador Railway that links its mining operations in Labrador City with its port at Sept-Îles. The tunnel was drill and blasted through granite to create a more direct route for exports but high ground water levels mean there are ground movement risks that have called for careful monitoring.
“High levels of ground water in the area have left the tunnel prone to seepage, with large slabs of granite falling from the roof and shoulders of the tunnel onto the track, risking both injury to rail staff, tunnel workers and the costly delay of millions of tonnes of iron ore,” explains 3D Laser Mapping pre-sales monitoring engineer Daniel Scully, who has been working with Iron Ore Canada on monitoring the issue.
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The tunnel at Sept-Îles is approximately 600m long, 10m high, and bores through very foliated granite with multiple dykes and fault systems cross cutting the tunnel. This leaves large, unsupported wedges that can become unstable.
According to Scully, Iron Ore Canada has embarked on a geotechnical stability programme in the tunnel that utilises rock bolts to transfer the load to deeper, more stable granite and mesh to limit the impact of rockfall.
“As the tunnel cannot be closed without severe financial implications, any work carried out has to be done in line with the train schedule,” he explains. “Despite dewatering measures temperatures often drop as low as -45°C, resulting in ground water quickly freezing on the tunnel walls and displacing more layers of granite. This adds to the complex set of challenges faced by ground engineers.”
To address the issue of ice forming, engineers are using a series of industrial scale heaters. However, highly susceptible areas still become frozen and these have become part of a strategic monitoring programme to target the areas which need to be heated to melt the ice before it can lubricate any joints or fractures.
There are currently around six areas of the tunnel – around 50m in length – which are less susceptible to failure, but do not have any type of remediation in place. It’s these areas of the tunnel that are currently the subject of a light detection and ranging-based (Lidar) monitoring system, which 3D Laser Mapping has worked with Iron Ore Canada to develop.
“Iron Ore Canada requested a system that would allow them to continually monitor the stability of these unprotected areas,” says Scully. “Using a combination of short-range laser scanners and our Sitemonitor Live software, it is now possible to match time-series scans of the tunnel to warn of any changes. This unique scanner and software set-up also issues alerts in the event that change is identified outside of pre-determined parameters, allowing for a more proactive approach to be taken before an incident occurs.
“Lidar is perfectly suited to the inhospitable conditions of an underground mine, or dusty and exposed nature of an open-pit site. By using pulsed light from a laser, Lidar systems allow an operator to calculate distance and create a record of measurements remotely.
“Where Lidar has become most useful lies in its ability to create 3D representations of almost any environment. With some systems capable of firing around 1,000,000 laser pulses per second, each with a corresponding georeferenced record, it is possible to build a millimetre accurate virtual model of buildings, structures and excavations in a matter of minutes. The data collected from a laser scanner is known as a ‘point cloud’, and this data can be extracted to reveal details or patterns which are difficult or sometimes impossible to see with the naked eye.
“One of the reasons that point cloud data has become so valuable in the mining sector is thanks to the ability to compare one dataset to another. Because of the great level of detail collected by a laser scanner, it is possible to match multiple datasets collected over a period of time, making it an invaluable monitoring tool in geologically complex areas.”
Scully says that Lidar based scanners and software packages, such as Sitemonitor Live, don’t only serve to warn of impending issues, they can also help to manage engineering processes more effectively.
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Scully believes that the benefits of Lidar-based systems could have a much wider impact on the ground engineering industry. “Processmonitor Live works in a similar way to Sitemonitor Live and uses close to real-time Lidar data to calculate surface change,” he explains. “This type of technology is starting to see wider use for applications such as shotcrete measurements, tunnel profiling and other tasks that are laborious or dangerous to carry out manually.
“The challenges of an underground mine almost perfectly mimic those faced by civil engineers and construction companies working on transportation or other tunnel projects. Developing technology that is effective and easy to use in these challenging conditions has led to the development of laser scanning solutions that can be applied to improve safety and provide critical intelligence in other heavy industrial environments.
“Whether we are talking about new boring projects or the maintenance of existing constructions, Lidar can be used as a powerful tool to assess the structural integrity of almost any underground environment.”
While other parts of the industry may yet to benefit from this technology, the laser scanners in Iron Ore Canada will continue to keep a watchful eye over the rock bolting and mesh installation work until the work is completed in three years’ time.