VIRTUAL STRUCTURAL MAPPING USING 3-D DIGITAL ROCK SLOPE MODELS, I-90 NEAR SNOQUALMIE PASS, WASHINGTON

3-D digital models have been effective tools for structural mapping and engineering design in two rock slope stabilization projects along I-90 in the Cascade Mountains about 100 km east of Seattle: Midway Curve (Milepost 66) and Snoqualmie Pass East (Hyak to Keechelus Dam). In each project, we used digital SLR cameras and commercial photogrammetry software to create several dozen models. Each model consisted of several hundreds of thousands of xyz points integrated with a digital photograph, with RMS errors typically on the order of millimeters and average xyz point spacing on the order of centimeters. Lines of sight nearly normal to outcrop faces produced the best results, and in a few cases extremely oblique lines of sight produced unusable results. We used the 3-D models for collaborative virtual mapping, allowing the project teams to safely and efficiently identify joints and faults of interest (both polygonal planar patches and linear traces). The orientations of patches and traces were calculated, displayed on equal area nets in real time, and exported for slope stability analysis and design. We measured joint orientations in the field using a compass and found them to be in excellent agreement with the digital results. Although properties such as iron staining can be identified on the photographs, fieldwork is necessary to characterize the degree of weathering, rock quality, joint filling, joint aperture, and other important properties. Other uses of the models included generation of profiles for remedial design and measurement of units exposed in the outcrops.