Slope stability is critical when it comes to open pit mines and factors affecting or indicating slope stability issues need to be monitored to allow safe and continuous mine operations. Depending on the slope monitoring method used, data collection can involve manual measurements requiring field personnel to approach pit slopes or crests, where instability could occur.

With today’s advances in technology, there are alternatives to working near pit slopes and crests when collecting slope monitoring data. Using photogrammetry and 3D point cloud capabilities, teams can now record, analyze, and monitor mine slope stability more efficiently and, most importantly, from a safe distance.

WSP has been working closely with Crown-Indigenous Relations and Northern Affairs Canada (CIRNAC), at the Faro Mine Complex (FMC) located in Yukon, Canada, to successfully implement this technology.

Project Background: Faro Mine, Canada

The FMC was once the largest open-pit lead-zinc mine in the world. Today, according to the Government of Canada, it is the site of one of the most complex abandoned mine remediation projects in Canada. The Complex includes three open pits (Faro, Grum and Vangorda), now inactive, and several waste and water containment facilities. The mine closed in 1998 and is currently undergoing care and maintenance.

WSP has been involved in the environmental assessment of the remediation project and has also been mandated to work on the slope stability review of the pits since 2002, which is an important component of the care and maintenance program.

The monitoring program for the pits has historically included:

• Regular visual inspections for slope stability indicators;
• Manual measurements of the distance to the pit crests from crest regression pins; and
• Survey monitoring data from slope displacement points located near the pit crest.

Monitoring crest regression by taking manual measurements required field personnel to work close to the pit crest and over the years, safety concerns from site personnel led to the discontinuation of this method. Alternative pit slope stability monitoring options were evaluated in 2015.

Photogrammetry Monitoring Method Overview

Photogrammetry monitoring was the technology chosen over more conventional approaches such as RADAR and LiDAR surveys, as a cost-effective method of remotely and accurately monitoring pit slope stability and mitigating the safety hazard related to personnel accessing pit crests and slopes. In 2016, a photogrammetry network was set up at the FMC, which consists of aluminum survey tripods installed across from each pit’s area of interest (i.e. area of slope being monitored), and a Digital Single Lens Reflex (DSLR) camera which is placed on each tripod during data collection. Photographs of the pit slopes are collected at regular intervals and 3D models are generated using photogrammetry software. Slope displacements occurring over an interval can be detected by comparing the 3D point clouds generated from each instance of data collection.

The workflow to analyze a pit slope using this method looks like this:

• Overlapping photos are taken of the areas of interest;
• 3D point clouds are generated from the photos using photogrammetry software;
• Point clouds are compared to detect displacements occurring on the slope between the dates of photogrammetry data collection for the selected point clouds; and
• Results are analyzed and interpreted to report on slope stability.

Example Output: Faro Pit

An example of photogrammetry analysis results for the Faro pit is shown. This comparison between point clouds generated from photos collected of the Faro pit east wall in August 2020 and May 2020 identified several areas of localized erosion, sloughing, and rockfall on the pit slope. The results output also indicated areas of apparent displacement due to snowmelt and vegetation differences. A review of photographs was conducted to confirm which displacements indicated by the modelling results were due to actual slope displacements and which were due to changes in vegetation and snow cover.

The magnitudes of displacement measured from the model results indicate, in general, negative displacement (displacement into the slope due to erosion, sloughing and rockfall) of up to 1.0 m and positive displacement (displacement out of the slope due to material accumulation) of up to 0.70 m. One area of rockfall on the upper part of the slope experienced greater displacements of up to 2.5 m of negative displacement and 1.3 m of positive displacement.

The results of the August 2020 to May 2020 comparison indicate localized displacements which do not impact the overall stability of the Faro pit east wall. A “Trigger Action Response Plan” or TARP is in place for the open pits at the FMC. Under the open pit TARP, displacements indicated by photogrammetry monitoring and other pit slope monitoring components can trigger an alert level (yellow, orange, or red) depending on the magnitude, extent, and rate of displacement observed in the data. In the event that one of these alert levels is triggered, the TARP outlines the corresponding response, which can include an increase in pit monitoring frequency, as well as pit access restrictions.

Conclusion

The photogrammetry network at Faro Mine has been successfully operating since 2016 with an average accuracy of approximately 20 cm, which is considered within the acceptable range for detection of large-scale slope instability at this project site. The photogrammetry monitoring is used in conjunction with regular field personnel visual inspections and annual survey monitoring data of slope displacement points to provide information on slope stability.

The key benefits of the photogrammetry monitoring are that data can be collected quickly and remotely to improve worker safety, and at a relatively low cost. While more processing is involved to detect slope displacements compared to other survey methodologies (e.g. prisms or radar), the process can be streamlined following implementation and by engaging dedicated personnel.

Using the photogrammetry network, WSP is providing CIRNAC with a safe and reliable source of slope monitoring data, complementing the other monitoring system components. This method has proven to be a simple, accurate, cost-effective, and easily verifiable approach to successfully monitor slope displacements for the Faro, Vangorda, and Grum pits.

* This work was performed by Golder professionals who joined WSP in an acquisition completed in 2021.