Yukon River bridge landslide: Preliminary geologic and geotechnical evaluation

This report presents the findings of a geologic and geotechnical evaluation of a landslide at the Yukon River bridge (the E.L. Patton Bridge). The Yukon River bridge landslide occurred in fall 2012 between approximately 375 and 575 feet west of the bridge. Although there was no damage to the bridge foundation, the landslide's close proximity to the bridge and concerns over additional failures prompted multiple evaluations, including landslide documentation, drainage assessments, and geotechnical studies. This report was prepared to convey the general characteristics of the rock mass, characteristics of rock discontinuities, and the geomorphic expression of the 2012 landslide in the vicinity of the bridge. We determined that the fractured bedrock and steep slopes in the bridge vicinity are prone to slope failure. However, this assessment should be considered preliminary and should not be used to determine areas of potential future slope instabilities. A significant amount of additional exploration and slope stability analysis is required to fully understand the landslide hazard in the area of the bridge, as well as the potential for reactivation and/or expansion of the 2012 landslide into adjacent slopes.