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Monitoring large-scale rock slopes for the detection of rockfalls using structure-from-motion photogrammetry at Debeque Canyon, Colorado
Elbahnasawy, Mohamed
Elbahnasawy, Mohamed
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2023
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This research investigates the frequent rockfall events in DeBeque Canyon along I-70. It uses the multi-epoch photogrammetric monitoring datasets collected by the Colorado Department of Transportation between 2014 and 2021. The study aims to assess the effectiveness of the direct geo-referencing approach in creating large-scale photogrammetric models without ground control points (GCPs). It also aims to develop a workflow for creating a regional-scale rockfall inventory and characterize the spatial variability of rockfall characteristics. Furthermore, the research seeks to evaluate the impact of pre-existing rockmass structures on rockfall frequencies, sizes, and shapes.
Comparison of the developed photogrammetric point clouds created using a direct geo-referencing approach to lidar surveys revealed a good matching precision. The precision was as good as 0.059 m in terms of root-mean-squared (RMS) difference metric. For efficient handling of large-scale, multi-epoch models, the study implemented construction of photogrammetric models for only the first and last acquisition. The corresponding image datasets for intermediate acquisitions were manually reviewed. This approach enabled rapid identification of the temporal occurrence of each rockfall. Segmenting photogrammetric models into smaller segments minimized "bowl-effect" distortion and reduced processing time.
The study revealed that rockfall activity vary along DeBeque Canyon corresponding to changes in lithologies, rockmass conditions, and the presence of oversteepened areas. Increased rockfall activity can be attributed to factors such as prevalence of weaker rockmasses, increased degree of fracturing, human interference, and presence of steeper slopes. The temporal rockfall rates increase in years with a higher number of days with snow thickness exceeding 1 inch. The study found that pre-existing rockmass structures influenced rockfall failure mechanisms, shapes, and scaling exponent of the power-law equation. The scaling exponents of the magnitude-cumulative-frequency (MCF) curves were found to be impacted mainly by variations in lithology and degree of fracturing. The expected range of block volumes obtained based on structural mapping was larger than the actual rockfall volumes. This discrepancy occurred due to model resolution limitations for structural mapping. It also resulted from the occurrence of smaller rockfalls due to intact rock failure between mapped joints and rockfalls not bounded by joint sets.
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