Geomorphology and engineering geology of a landslide in ultramafic rocks, Dawson City, Yukon

• Published in: Engineering Geology
• Main Authors: Brideau, M, Stead, D, Roots, C, Orwin, J
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Elsevier 2007
• Subjects: Dawson City; Yukon; Dawson
• Online Access: http://hdl.handle.net/2292/12616; https://doi.org/10.1016/j.enggeo.2006.10.004

The Dawson City Landslide is located at the northern edge of Dawson City, Yukon and took place prior to 1740 years B.P. The landslide occurred at the contact between the metasedimentary rocks of the Yukon–Tanana Terrane and the overlying altered ultramafic rocks of the Slide Mountain Terrane. The main part of the failure comprises altered ultramafic rocks, which have a lower Geological Strength Index (rock mass quality) than the underlying metasedimentary sequence. Division of the study area into smaller structural domains identified discontinuity sets associated with faulting along the contact of the two lithological units. A kinematic analysis of the identified discontinuity sets suggests that a toppling failure mechanism may be feasible. Due to the highly fractured nature of the ultramafic rock mass, a circular failure controlled by the low strength of the rock mass is considered a potential mechanism for the Dawson City Landslide. Limit equilibrium and finite difference numerical modelling codes support a circular failure mechanism hypothesis. However, a combination of high pore water pressure (ru=0.3), and seismic loading are required to produce a factor of safety b1.0. The dominant trends of linear geomorphic features (tension cracks, trenches, anti-slope scarps, ridges) correspond to a discontinuity set recognised in the headscarp of the failure. Tension cracks exposing roots and split trees indicate recent movement of a small section of the headscarp that is potentially unstable. The downslope risk represented by this unstable block is difficult to assess due to the uncertainties related to entrainment of talus material and potential travel distance over the rough and irregular surface of the middle coarse debris material. The movement rate of this block should be monitored and the surrounding area routinely investigated for the development of other potentially unstable sections of the landslide. Sheared trenches, stretched roots and split trees suggest that the middle to lower section of the debris is currently moving. http://www.sciencedirect.com.ezproxy.auckland.ac.nz/science/article/pii/S001379520600281X