Insights into Rockfall from Constant 4D Monitoring

Abstract

Current understanding of the nature of rockfall and their controls stems from the capabilities of slope monitoring. These capabilities are fundamentally limited by the frequency and resolution of data that can be captured. Various assumptions have therefore arisen, including that the mechanisms that underlie rockfall are instantaneous. Clustering of rockfall across rock faces and sequencing through time have been observed, sometimes with an increase in pre-failure deformation and pre-failure rockfall activity prior to catastrophic failure. An inherent uncertainty, however, lies in whether the behaviour of rockfall monitored over much shorter time intervals (Tint) is consistent with that previously monitored at monthly intervals, including observed failure mechanisms, their response to external drivers, and pre-failure deformation.

To address the limitations of previous studies on this topic, 8 987 terrestrial laser scans have been acquired over 10 months from continuous near-real time monitoring of an actively failing coastal rock slope (Tint = 0.5 h). A workflow has been devised that automatically resolves depth changes at the surface to 0.03 m. This workflow filters points with high positional uncertainty and detects change in 3D, with both approaches tailored to natural rock faces, which commonly feature sharp edges and partially occluded areas.

Analysis of the resulting rockfall inventory, which includes > 180 000 detachments, shows that the proportion of rockfall < 0.1 m3 increases with more frequent surveys for Tint < ca. 100 h, but this trend does not continue for surface comparison over longer time intervals. Therefore, and advantageously, less frequent surveys will derive the same rockfall magnitude-frequency distribution if captured at ca. 100 h intervals as compared to one month or even longer intervals. The shape and size of detachments shows that they are more shallow and smaller than observable rock mass structure, but appear to be limited in size and extent by jointing. Previously explored relationships between rockfall timing and environmental and marine conditions do not appear to apply to this inventory, however, significant relationships between rockfall and rainfall, temperature gradient and tides are demonstrated over short timescales.

Pre-failure deformation and rockfall activity is observed in the footprint of incipient rockfall. Rockfall activity occurs predominantly within the same ca. 100 h timescale observed in the size-distribution analysis, and accelerated deformation is common for the largest rockfall during the final 2 h before block detachment. This study provides insights into the nature and development of rockfall during the period prior to detachment, and the controls upon it. This holds considerable implications for our understanding of rockfall and the improvement of future rockfall monitoring.