Comparison of Traditional and Emerging Technologies for Assessing Slope Stability in an Urban Environment

Quarrying operations between the late 19th and early 20th century transformed the once moderately rolling, stable eastern slope of San Francisco’s Telegraph Hill to a slope with near-vertical rock faces prone to rockslides. One such failure occurred following an intense rain event in December 2019 on a portion of slope directly beneath the Filbert Steps, a popular tourist attraction leading to the iconic Coit Tower. As part of the emergency response, geological mapping using three independent methods was performed. These methods were manual measurements using a geological compass, unmanned aerial system (UAS) data capture, and terrestrial Light Detection and Ranging (LiDAR) data capture. The results of this study showed structure from motion (SfM) models derived from UAS photogrammetry to be an effective method for characterizing a rock mass, capable of obtaining similar results to more expensive pieces of equipment (i.e., LiDAR) and eliminating the safety concerns associated with mapping unstable slopes using a geological compass. The large dataset of geological structure orientations obtained from the UAS imagery-based SfM model facilitated relatively robust analyses at an early stage in the project, and provided a basis for slope stabilization recommendations both for emergency response and permanent restoration. This paper presents a comparison of the three methods used for geological mapping and provides commentary on the effectiveness of UAS mapping for provision of efficient slope stabilization recommendations.