This is the third in a series of articles that share some of the critical control measures learned from the largest highwall failure in mining history – the massive Manefay Slide at the Bingham Canyon Mine in 2013. This article discusses types of geotechnical monitoring systems including those that were used at Bingham Canyon to predict the failure nearly two months before the event. The early warning provided by this monitoring was a key critical control that prevented injuries and fatalities that could have occurred in this immense event.
There are several methods to monitor geotechnical movement or changes in open pit mines. Some of these methods are low tech and based on human observations, such as having all employees go through geotechnical hazard training so they can detect and report geotechnical problems as soon as they are observed. Others are extremely high tech – involving sophisticated measurement systems. In the case of the Bingham Canyon landslide, some of the earliest indications of a problem were first observed in the field by engineers and operators. However, to determine the size, scale, and timing required high precision radar systems. This article describes and compares 16 methods of geotechnical monitoring. A robust geotechnical monitoring program normally requires multiple systems – depending on the size of the operation, complexity of the geology, and amount of risk for geotechnical failures.
A summary of the various monitoring methods can be seen in the table “Geotechnical Monitoring Method Comparison”. This table compares the various monitoring methods based on the following seven qualifiers:
- Measure – What does the system measure? Ranges from observation to amount of movement.
- Type – Is the measurement taken at a point, over an area or a linear distance?
- Geometry – Is the measurement made on the highwall, on top of the surface or within the subsurface?
- Continuous – Is the measurement taken continuously or intermittently as needed?
- Portable – Is the monitoring system portable and easily moved or based in a fixed location?
- Initial Cost – Is the initial cost relatively low (thousands) or high (hundreds of thousands of dollars)?
- Alarmed – Can the monitoring be alarmed to initiate an evacuation of the mine in case of exceedance?
The field of Geotechnical monitoring is a critical control measure designed to keep miners safe. This is a field that continues to rapidly expand and improve. New technologies, such as radars and laser scanners, continue to be introduced on a regular basis. If you are aware of new geotechnical monitoring technology, please share with the author so that this table can be updated.
Geotechnical Monitoring Method Descriptions
Following is a short description of each monitoring method along with the key advantages and disadvantages of the method.
Geotechnical Hazard Training – This method trains employees that work in the mine how to identify geotechnical hazards and what to do if a hazard is observed. This method has a low upfront cost since it is usually part of the required MSHA or other required safety training. The key is to have the procedures and expectations in place for all employees to report and act upon hazards, including the authority to stop operations if a serious issue is detected. The following photo shows dust from falling rock, which is one of the geotechnical hazards that miners are trained to look for and report.
Advantages: Employees are typically in all parts of the mine and are familiar with normal conditions so they can detect changes or unique situations that other monitoring systems may not detect. Bingham Canyon had 800 pairs of trained eyes looking for geotechnical problems.
Disadvantages: There are periods when employees are not in an area and therefore changes may not be observed. This method is qualitative and small changes may not be noticed or reported.
- Documented Inspections – Like Geotechnical Hazard Training, this method relies on observations, but in this case observations of professionals such as trained geotechnical engineers, mining engineers, geologists and supervisors.
Advantages: Inspections are performed by a professional who has a higher level of knowledge and understanding of geotechnical hazards. As part of the inspection, the amount of movement or risk for failure can be further quantified with professional observations and measurements.
Disadvantage: The intermittent inspections are usually performed once per shift or per day and therefore changes would not always be detected in real time.
Synthetic Aperture Radar – Geotechnical radar systems can detect very small levels of movement or velocity changes over a large area on the surface of highwalls, in near real time. The radar systems can then be used to trend the movement over time to help predict failures or alert operations of impending highwall failures. Synthetic Aperture Radar systems have typically been semi mobile and placed in a fixed location for relatively long periods. These systems are set up to scan large areas of a pit from long distances (over 2 miles away). The following photo shows a synthetic aperture radar system in operation at the Bingham Canyon Mine.
Advantages: The fixed system acts like a sentinel, watching over a large area and reporting changes and potential hazards. The Bingham Canyon mine used Synthetic Radars to help determine the size of the Manefay and the fact that the slow-moving mass was actually accelerating and headed towards failure.Disadvantages: Fixed radars are line of site and any areas screened from the radar will not be monitored. In addition, as the monitored area is increased, either the data resolution is decreased or the time between scans is increased, resulting in less information for the entire area.
Real Aperture Radar – These systems have many of the same attributes of the stationary synthetic aperture radars, but are normally mounted on trailers or trucks. If a problem is detected, the radars are moved closer to areas of concern. The photo at the top of this article shows a real aperture radar system monitoring the Manefay scarp at the Bingham Canyon Mine.
Advantages: Since they are closer, a smaller area is monitored with a higher resolution with shorter times between scans. This provides excellent monitoring of troubled areas that are of high risk of failure.Disadvantages: Mobile systems cannot be relied upon to continuously monitor large areas. If the mobile system is moved to monitor a problem area, then larger areas of the mine would not be monitored and protected.
Robotic Theodolite Prism Network – With these systems, a robotic theodolite continuously scans the locations of a network of prisms that are placed on the high walls of a pit. The location of each prism is tracked over time and the velocity of each prism can be calculated. A single theodolite can track over one hundred prisms, however, the greater the number of prisms the longer it takes the theodolite to scan all the prisms. The following photo shows a robotic theodolite automatically scanning for prisms.
Advantages: Robotic theodolites measure the movement of a specific point so it is simple to trend the movement and acceleration over time for each point. Radars on the other hand measure broad areas instead of tracking specific points, so calculating trends and cumulative displacement is more problematic.
Disadvantages: The scattering of points may not reveal movement of smaller blocks and the movement of a few points may not be representative of an entire area. Also, there may be slight errors in the data due to diurnal or weather changes. Although these errors average out over longer time periods, velocity calculations can be influenced in the short term.
Surface Extensometers – One of the oldest methods of geotechnical monitoring is the use of extensometers to monitor changes in surface cracks. Modern surface extensometers often use a cable reel anchored to one side of the crack with the cable stretched and anchored across the crack. Any movement will pull cable from the reel, which is measured and automatically recorded. Unusual movement can then be detected and appropriate personnel alerted. The following photo shows a extensometer set up at the Bingham Canyon Mine.
Advantages: This system is a relatively simple and inexpensive way to monitor surface cracks that can be precursors to a problem. Measurements are in real time and if an unusual movement is detected an alert can be automatically sent.
Disadvantages: A crack may not be identified or accessible and thus cannot be monitored with this method. Surface extensometers also monitor only a single point and are subject to error as temperature changes impact the length of the cable.
Time Domain Reflectometry (TDR) – A TDR is a co-axial cable that is extended down a drill hole and then attached to an instrument that sends an electric pulse down the cable. The instrument can detect if there has been any distortion or changes in the cable and the location of that distortion. If there is any earth movement down the hole, the TDR can detect the location and indicate relative magnitude of that movement. However, the amount of movement is not quantified with the TDR. The following figure is an example of data from a TDR over a five month period. The small tick marks show when movement is detected on the top of Bed 1 beginning on March 17 and the movement continues to increase over time. The values in the tick marks show the change in impedance and not displacement.
Advantages: TDRs monitor subsurface movement; are relatively inexpensive if installed in holes drilled for exploration or piezometers; and the entire length of the drill hole can be monitored.
Disadvantages: TDRs do not record the actual movement in the drill hole and the readings are often taken manually (although the process can be automated).
Piezometers – Water pressure is a critical aspect of rock stability and piezometers are used to measure water levels and pore-water pressure, critical aspects to rock stability. Often water pressure is not studied until there is already a geotechnical problem, which is too late. Vibrating wire piezometers can be installed in a drill hole at either single or multiple locations to monitor changes in water levels and pressure. The data from these monitors are critical in calculating the stability of the highwall. The following photo shows a drill hole with sensors whose data is automatically sent to a central location.
Advantages, Although piezometers do not measure ground movement, they can be a key leading indicator of a potential problem by measuring changes in water levels.
Disadvantages: Piezometers only measure pressure at the hole location. The water depth and pore pressure of perched water tables or geologic irregularities may be missed if piezometers are not placed in those locations.
Microseismic Monitoring – A microseismic monitoring system uses geophones or accelerometers that detect seismic energy (the sounds of rocks moving underground).
Advantages: Multiple Microseismic monitors can be used to determine the location of underground earth movement in 3D space with triangulation.
Disadvantage: Unfortunately, the processing requirements are so extensive in these systems that they have been more useful in determining the location of movement afteran event. Additional research with this method could improve its usefulness.
GIS Data Display – The geographic information display (GIS) allows the information measured by the various monitoring systems to be stored on a network server and then displayed at any location in the network. Often this information can be combined with data from multiple sources to help geotechnical engineers detect/determine trends and understand changing situations.
Advantages: GIS Data Display systems make other measurement systems useful and effective.
Disadvantages: These systems depend on networks which may have reliability issues at some locations.
Drone Photogrammetry – One of the fastest growing monitoring methods is the use of aerial drones to take high resolution photos anywhere in the mine. With these photos, engineers can observe conditions of the entire mine. By using photogrammetry, changes in surfaces can be detected and measured from the photos.
Advantages: Drones can safely get information from inaccessible or dangerous areas.
Disadvantages: Post processing time does not allow for real-time monitoring and alerts. This disadvantage should improve as the technology matures.
GPS Prism System – similar to a robotic theodolite prism network, the GPS prism system tracks the movement of individual points on highwalls and benches using satellites instead of theodolites.
Advantages: GPS do not rely on line of sight like a theodolite, (although their signal can be blocked at times). Also, true movement vectors can be calculated to determine/improve the understanding of the mechanics of a failure mass.
Disadvantages: GPS prism system cost per point is more expensive than a theodolite prism system and results are not as accurate. These systems also rely on a base station to adjust for induced errors in GPS signals. This limits the distance that the GPS prisms can be located from the base station. There are times that GPS satellites are not available so the GPS system is not operational.
Downhole In-Place Inclinometers – Sensors are placed in drill holes to monitor ground movement as well as the direction of movement. Often multiple sensors will be placed in a single drill hole to better determine the location of movement.
Advantages: The direction and amount of underground movement can be measured continuously and an alarm can be set for when unusual movement is detected.
Disadvantages: Movement is only measured at specific points in a drill hole.
InSAR – (Interferometric Synthetic Aperture Radar) uses radar from orbiting satellites to scan the surface, which are then compared to previous scans to calculate the amount of movement in any area.
Advantages: InSAR originates from space so there should be very few areas that are not in line of sight.
Disadvantages: InSAR is only available when the right satellites are overhead and the significant amount of postprocessing time. This system is good for developing an understanding of overall movement, but not practical for real time monitoring.
Laser Scanner – laser scanners have the same benefits as radar systems. They can be configured for continuous monitoring of highwalls and set to send alerts if a problem is detected. They can be set up at either portable or fixed locations.
Advantages: Many mines already own laser scanners for planning purposes and the data can be used for multiple purposes.
Disadvantages: Lasers are impacted by dust and precipitation. During significant events, it may be difficult to gather data.
Downhole Inclinometers Probes – unlike in-place inclinometers, these probes measure the changes in the diameter of a drill hole that has been fitted with special tracks for the probe to follow. The shape of the drill hole is compared to previous scans to determine if any movement has occurred.
Advantages: This system measures movement along the entire length of the drill hole and it is relatively inexpensive for initial cost
Disadvantages: Scanning is generally performed manually. The time required for post processing of the data prevents real time monitoring and alerting capabilities.
Because of the significant risk and impact of geotechnical events in the mining industry, there is a demand for monitoring methods to continue to expand and improve. Each mine has its own unique circumstances and therefore may require a unique combination of monitoring methods that meet those circumstances. Additional information can be found in my book, Rise to the Occasion – Lessons from the Bingham Canyon Manefay Slide.
To see other articles in this series go to Sharing Critical Control Measures #1 – Knowing the Greatest Risks or Sharing Critical Control Measures #2 – Independent Experts, Black Hats, and Sharing Learnings. You can also visit my blog for these and other articles at RiseToTheOccasion.net.
If you found this article on geotechnical monitoring methods to be useful and/or interesting, please like or share with others. The genesis for this article was the result of a comment from a reader that was posted in an earlier article – so your comments make a difference.
All photos in this article come from the book Rise to the Occasion – Lessons from the Bingham Canyon Manefay Slide and are copyrighted by Rio Tinto.