What is Critical Slope Monitoring?

For over a decade, the phrase, critical slope monitoring (CSM) has been frequently used by radar companies and the mining industry. Yet no official definition has been suggested for a CSM program or what conditions constitute employing a CSM program. I am proposing eight requirements that define a CSM program that is a result of advancements in technology, daily reporting, successful interpretations of data, and personal experience using ground-based slope monitoring radar systems.

There are several reasons for employing a CSM program, but in general, it is about reducing risk and providing an adequate warning when an unstable slope presents a dangerous threat to people, buildings, and infrastructure.

Currently, ground-based radar is the only slope monitoring technology that meets the proposed requirements for use in a CSM program. The following are the requirements to implement a CSM program:

• The monitoring technology must be able to acquire, process, and present displacement, velocity, and inverse velocity data in minutes 24/7.

• The technology must include the ability to set alarm thresholds and send alarm messages via text messaging and e-mail.The technology must be able to monitor continuously, 24-hours per day in all weather conditions.Monitoring professionals need to initiate and contribute to developing a trigger action response plan (TARP), for all CSM monitoring projects.

• The CSM program should include experienced monitoring professionals from the disciplines of geology, mine engineering or geotechnical engineering.

• The monitoring professional needs to understand the implications of movement trends and how they correspond to production, working conditions, and safety.

• It is essential that the entire monitoring team, supervisors, and client understand that slope collapse is unlikely in the absence of acceleration.

• Daily active data engagement is vital since numerous sites have documented evidence that slope acceleration can begin at any time.

• Data must be reviewed throughout the day and during the night if 24/7 operations are occurring.

• Local geologic conditions need to be understood as there appears to be a correlation between rock quality, acceleration, and the time to slope failure.

It is imperative during the monitoring of unstable slopes that the requirements for a CSM program become fully embraced and include the active engagement of a slope monitoring professional to communicate and report on conditions and changes that could indicate an increased risk. Just as necessary are reports describing no changes in slope conditions, and that risk remains at a low to moderate level.

Also, monitoring technology being considered for a CSM project needs to be carefully investigated since many companies market their hardware as being able to acquire data in real-time. Technically, they could be collecting data at the speed of light, but what good is real-time data acquisition if it takes hours, days, or weeks to process and configure the data before a professional interpretation can be made? At a mine in Arizona with strong brittle rocks, acceleration typically begins without much, if any, deformation and complete failure can occur rapidly in about 3-4 hours from the time acceleration begins. Therefore, at this site, a CSM technology must acquire all data, process the data, and present the data in near-real-time, to the end-user with no additional processing requirements. Time is the most crucial factor during an acceleration event. For example, if monitoring a new location for the first time, the time to failure may be unknown due to unique geologic conditions. Using a deferred data monitoring technology primary data source for a CSM program could put people and property at a much higher level of risk.

Submitting daily reports is one technique to keep monitoring professionals engage when observing creeping slopes over long periods. The engagement also develops a more in-depth understanding of a slope's movement characteristics. Also, the radar software allows for site events such as blasting, shovel work, or other types of earthwork excavations to be documented, allowing for correlations between work and slope movement. For example, a blast event and the ensuing highwall vibration could initiate an acceleration event. If the two events can be correlated, then appropriate changes can be made to blast designs in the area. Brief daily summaries provide supervisors and management current updates on movement trends and any changes that could increase risk.

The first ground-based interferometric synthetic aperture radar (GBInSAR) system was introduced into the open-pit mining industry over a decade ago. During this period, mine engineers quickly realized that by combining its near-real-time data results with the near-real-time interpretation, they could produce the following results:

Quickly communicate changes in slope stability to mine operations, providing a warning with time to adjust. By correlating radar data to mining methods, the mining engineer can quickly communicate instructions to change mining methods to maintain and sometimes increase production. With active daily engagement, dangerous outcomes from unstable slopes risk can be reduced, improving investor confidence while reducing litigation costs from slope failures that otherwise could result in injury or loss of life. Cost analysis has shown that a proactive long-term CSM investment can reduce risk, save lives, reduce costs, and increase investor confidence.

For these reasons, ground-based radar systems have earned the trust of most large mining companies operating surface mines. Recently, a ground-based interferometric synthetic aperture radar (GBInSAR) company has created a fleet of rental units, offering a more affordable option for smaller mining companies and private sector engineering firms.

Ultimately, a CSM program is about improving safety and reducing risk. Although this post discusses the GBInSAR as the primary monitoring system for a CSM program, other technologies could qualify if they meet the CSM requirements defined here. More importantly, for a long-term proactive approach, a successful CSM program requires a group of engineering geologists or geotechnical professionals committed to safety through active monitoring engagement.

With a few exceptions, GBInSAR use and CSM programs outside of the mining sector have been limited, even with numerous examples of successful results from mine sites around the world. There are several reasons for this, including:

• The high purchase price of a slope monitoring radar system, however, rental units have recently become available.

• Poor communication and data sharing between the mining companies and private sector engineering frims.

• The private sectors limited exposure to radar capabilities, advantages, and limits.

• The seasonal and erratic nature of slope failures throughout the United States facilitates a reactive response as opposed to a proactive response.

• Federal and state agencies are slow to identify, prioritize and establish monitoring protocols for geohazard assets.

• The misconception of politicians and the general public that landslides and rockfall failures occur instantaneously with no way to predict their failure, which indirectly helps promote the reactive responses to these events.

The images below explain how quickly slope conditions can change, as illustrated in both the cumulative displacement data and the inverse velocity data for the same event. The failure data was acquired by a GBInSAR monitoring an active slope failure in northern California. The cumulative displacement data shows movement towards the radar as increasing data values in a positive direction. Cumulative displacement data shown in the negative direction or trending towards the x-axis represents movement away from the radar or, in this example, missing material from the failure that now represents a new surface farther away from the radar.

The approach to data linearity is distinct in both cumulative displacement and the inverse velocity data; however, the noise exhibited in the inverse velocity data is typical for slopes showing little to no movement. Areas of missing inverse velocity data represent no velocity data. The radar software is programmed to display the inverse of zero velocity as a dashed blue curve since the inverse of zero cannot be resolved.