Updated: Feb 3, 2020
What are the fundamental interpretations of slope movement trends shown above?
Regressive displacement indicates that movement is decreasing, and the risk to production and people is low.
Steady displacement indicates that movement is consistent at some rate resulting in a low to moderate risk for production. Even at steady rates, caution needs to be considered since high steady rates can initiate individual rockfall without overall site acceleration.
Progressive displacement indicates acceleration, which presents a higher risk to production that can become an extreme risk if acceleration continues.
There is some ambiguity with the above definitions as rock types contribute to the level of risk that displacement transmits. At some locations, the rocks can be strong, brittle, indurated, or have low cohesion. These conditions and properties can affect the level of risk described in all the movement trend definitions.
The ground-based radar systems provide numerous functions that have validated much of the current understanding of slope movement. An advantage of radar systems over nearly all slope monitoring systems is its quick scan times of an entire slope surface, data processing, and presentation of data to the end-user occurring in about 2-minutes continuously 24/7. The fast and continuous display of data can describe movement trends that are clearly defined that look nearly the same as the schematic trends shown above.
The GBInSA surface displacement maps are unique in that they display movement occurring at various rates according to values that can be assigned to multiple colors or what some describe as a heat map. The radar software allows the user to define a specific area of slope movement exhibited by the map that can vary from just a few data cells (a small area) to thousands of cells (a large area). From each defined area, cumulative displacement, velocity, and inverse velocity data can be viewed in near real-time. The continuously updated stream of near-real-time data often shows an area of movement increasing its detectable surface area, which typically corresponds to increasing displacement rates within that specific area. The growth of movement should raise concerns since acceleration could be the next data change. The opposite is often the case when an area that is showing displacement begins to decrease its detectable surface exposure could indicate a regressive trend has started. The displacement map provides logistical and visual information that needs to confirmed by analyzing cumulative displacement, velocity, and inverse velocity data.
The fast presentation of data not only provides an accurate and current display of movement trends but offers the monitoring professional a high-level of data confidence since significant time gaps in data naturally create doubt. Experience with some unstable areas has shown multiple acceleration events occurring quickly, only to regress hours later, changing to a steady or regressive trend. Short accelerating events can initiate rockfall that many observe as an indication of impending slope failure. Still, if visual observations of rockfall and deformation are the only method of monitoring, then production could suffer at the expense of field observations with no supporting data. A progressive trend is the most concerning because acceleration often leads to deadly results; however, the data exhibited during the onset of acceleration and much of its progressive trend can appear noisy. Not until the inverse velocity data begins to approach data linearity can predicting or anticipating a slope failure occur with any level of confidence.
In some cases, slow, noisy accelerating data could allow for several weeks of continuous production before evacuating an area. Monitoring a progressive trend and being able to make crucial decisions concerning safety and production requires dedicated active engagement by a site professional, or what is a critical slope monitoring (CSM) program.
The initiation of a CSM project should include site-specific details of movement trends, knowledge of site conditions from operations to geologic conditions for the development of a trigger action response plan (TARP). Employing CSM together with a TARP can avoid unnecessary or highly conservative decisions that would otherwise be costly in the absence of near-real-time data that can take advantage of accurate movement trend analyses.
The image above shows continuous cumulative displacement data (blue curve) for a 3-month interval on and unstable highwall of an open-pit mine. The solid vertical lines are manually inputted events that often correlate to changes in slope movement. At the time of the movement trends, in 2015, the interval time from acquisition to the presentation was about 4-minutes. But advancements in technology and processing have now decreased the time to about 2-minutes.
The impact of this figure is the distinct movement trends exhibited in a short interval of time. For comparison, assume we have satellite InSAR data for this area during the same 3-month interval. Cumulative displacement from satellite InSAR would show individual dots of averaged data at 11-day intervals that might be connected by a straight line to help visualize the overall trend of movement. What would probably be missing are the short interval acceleration events identified by the GBInSAR. Because of longer acquisition intervals and lengthy data processing techniques, satellite InSAR must currently be regarded as a deferred data monitoring system and not applicable as a stand-alone critical slope monitoring technology. Also, at a different open-pit mine with strong brittle rocks, the GBInSAR has documented several single and multi-bench failures that took about 4-hours from the time radar identifies acceleration to the time of failure.