Proactive Methods for Identifying the Provenance of Rockfall Using Current Monitoring Technologies

It appears that the holy grail of slope monitoring is predicting individual rockfall events. A noble endeavor, but until technology can accurately and consistently provide advanced warning, causes of rockfall and the provenance of displacement should be considered in conjunction with current monitoring technologies.  

If a slope is perfectly stable, the typical rockfall triggers include increasing pore water pressure, chemical and physical erosion, human activity, and the pull of gravity, and seismic events.  

Mining and construction can affect slope stability, often initiating slow submillimeter displacement undetectable by the human eye. Geotechnical engineers often provide a mathematical analysis based on subsurface conditions to determine the factor-of-safety (FOS) of natural and structural slopes. Building codes will usually require a minimum FOS value above 1.0 for all slopes, while a FOS of less than 1.0 defines an unstable.  

Considering the FOS, a slope experiencing submillimeter movement over a long interval of months to years that exhibits no observable surface deformation what could the eventual impact of the missed diagnosis? Could this long-term movement eventually initiate a rockfall event? Does the FOS analysis consider the submillimeter movement a problem if they cannot visually detect deformation? Is long interval submillimeter movement even a consideration?

Third-party review of planned pushbacks during active open-pit mining and resulting in slope angles with a FOS of 1.5 or higher is a typical recommendation by a third-party engineer. During an engineering analysis experiencing long interval submillimeter movement that is not recognized, are the resulting slope angles and FOS correct? Could people and equipment now be exposed to a higher risk of rockfall?

By implementing a monitoring program that includes satellite and ground-based InSAR, the potential provenance of rockfall events can be identified. Both technologies can resolve submillimeter surface displacements. 

In the case of ground-based InSAR, the analysis software allows for the under-sampling of long intervals of regular acquired data. Which means, data discarded as noise during regular day-to-day high rate scanning is analyzed using an under-sampling routine that can identify submillimeter movements that are not included during regular scanning. 

Using both InSAR technologies and creating time-series data for individual radar cells or pixels will often show the same regressive, steady, or accelerating trends exhibited in larger displacement time-series areas. However, individual radar cells often require more critical review since their data is often much nosier. 

The analysis of individual radar cells was used on the large post-failure monitoring of the Mud Creek landslide in California to provide warning of potentially imminent rockfall events. Identifying the rockfall events were correlated to field observations from spotters, which helped improve the analysis method. In addition, under-sampling ground-based InSAR data has successfully identified not only areas of potential rockfall but the eventual appearance of large unstable slope areas and failures.

Like the Doppler radar rockfall warnings and the predictive warnings issued before an earthquake in southern California, only several seconds of warning can be provided. Preventive mitigation is the logical alternative by continuing to improve seismic building codes and identifying at-risk rockfall locations on slopes. 

Using the InSAR methods of preventive analysis, mitigation efforts in the mining and private sector could reduce rockfall hazards. Rockfall protection by installing Geobrugg fences and other methods should be established ahead of potentially fatal or destructive events. We should be proactive, not reactive when it comes to safety.