Obtaining a thorough and accurate rock slope characterization data set can be very costly or impossible when the critical slope exposures are inaccessible or hazardous. To overcome these challenges a terrestrial-based LiDAR scanner was used to collect a 3-D point cloud model of one such slope near Newhalem, Washington, from which initiated the 2003 Afternoon Creek rockslide. This paper reports on the methodology and analysis techniques used to collect the 3-D point clouds, and extract the orientation, spacing, and persistence of important discontinuity sets. It includes a discussion of the benefits and limitations of these methods, as well as practical recommendations based on this case study.
Accurate rockslide characterization is difficult, in part, because of the large degree of uncertainty regarding the spatial and physical properties of the unstable rock mass, including, but not limited to, the geometry and interconnectivity of controlling discontinuities, and mechanical properties of the discontinuities and rock mass. This uncertainty can be reduced (although not eliminated) by a thorough and accurate site investigation and data collection program. The traditional data collection program for a hazardous rock slope typically involves discontinuity orientation measurements through scan-line survey and outcrop mapping. However, obtaining a thorough and accurate data set can be costly or impossible, especially when the critical slope exposure is steep, high, hazardous, or in other ways inaccessible.
To overcome these challenges, a terrestrial-based, Light Detection And Ranging (LiDAR) scanner was used to collect discontinuity and rock mass characterization data on a steep, inaccessible, hazardous rock slope, at the site of a recent rockslide near Newhalem, Washington. The November 9, 2003 Afternoon Creek rockslide involved a volume of approximately 750,000 m3, a portion of which fell more than 600 meters in elevation down a steep slope onto Washington State Route 20 (SR20), an important route through the North Cascade Mountains. The slope continues to threaten SR20 due to potentially unstable material at the top ridge formed by the earlier event. The slope is large, and exceptionally steep. Access to the unstable zone is extremely limited, requiring workers to be dropped at the top by helicopter and then use ropes to rappel down, and maneuver around the main failure escarpment.
LiDAR scans were performed, with an Optech ILRIS-3D laser scanner, from several vantage points ranging from 100-m to 1,000-m distance to the slope face. The result of each laser scan is a three-dimensional point cloud of the scanned surface from which discontinuity orientation, persistence, and spacing data were extracted. This paper reports on the methodology and analysis techniques employed to characterize the structural patterns of the Afternoon Creek rock slope, using the results of a terrestrial-based LiDAR survey. It includes a discussion of the benefits and limitations of these methods, and practical recommendations based on the case study.