ROCFKALLS AND SMALL PRECURSOR DEFORMATION DETECTED USING OBLIQUE AERIAL SFM PHOTOGRAMMETRY: CASE HISTORY FROM TENNESSEE PASS, CO

Large rockfalls occurred in April and December 2014, impacting U.S. Route 24 near Tennessee Pass, Colorado. Following these rockfall events, Colorado Department of Transportation (CDOT) commissioned a study to monitor and assess the rockfall source zones.

Oblique aerial photographs of the slope were collected from a moving helicopter on five occasions between April and June 2015. From each oblique aerial photogrammetry (OAP) survey, we generated 3D models and completed 3D quantitative change-detection to determine model differences as small as 5 cm. Analysis showed that a number of small rockfalls and debris slides occurred over the course of the study. Additionally, results suggest that a large ‘cap block’ composed of at least three independent and detached blocks, ranging from 10 m3 to 100 m3, underwent downslope displacements from 5 to 15 cm. OAP change-detection results were validated with a displacement signal from a crackmeter that had been previously installed on the cap block. The OAP change-detection and the crackmeter data were in good agreement at this location, with both showing close to 5 cm of movement over the course of this study. Additional analysis showed that the block accelerated during a three-day precipitation event in early May, which came at the end of a prolonged period of freeze-thaw action.