Investigation of a landslide in discontinuous permafrost near Chitina, Alaska

Thesis (M.S.) University of Alaska Fairbanks, 2011Few studies have investigated landslides in frozen ground. As Alaska's population grows and climate warms, we can expect an increase in landslide frequency. This thesis presents mapping and subsurface analysis of a slide occurring in discontinuous permafrost near Chitina, Alaska. The active slide intersects the McCarthy Road and occurs within an underlying clayey glaciolacustrine diamicton deposit in an area previously used as a dump. The multifaceted investigation included landslide feature mapping, seismic surveys, tree-ring sampling, and a slope stability analysis to back-calculate residual soil strength properties. Mapping indicated that the overall landslide extent is larger than the active slide area. Analysis of tree-rings suggested movement occurred as at least two events. Seismic surveys produced good correlation with drilling data and effective subsurface imaging. Boring logs indicated that the permafrost table was at greater depth inside the active slide area than immediately adjacent along the roadway. These observations indicate that the failure mechanism for the active portion of the slide is different than that of the overall slide area. Back-calculation of the clay properties indicates a residual effective friction angle of 22.1 Permafrost degradation due to thermal effects of the dump may contribute to ongoing movement of the active slide area.1. Introduction -- Objectives -- Literature review of mass wasting in permafrost regions -- Flow -- Solifluction -- Skin flows -- Bimodal flows -- Multiple retrogressive flows -- Slides -- Block and multiple retrogressive slides -- Rotational slides -- Falls -- Summary -- References -- 2. Investigation of a soil slope failure near Chitina, Alaska -- Abstract -- Introduction -- Background -- Geology -- History of the slide area -- Summary of subsurface conditions -- Field methods -- Mapping -- Seismic survey -- CSL3 -- CSL4 -- CSL1 and CSL2 -- Dendrogeomorphology -- Results -- Results -- Mapping -- Seismic survey -- Dendrogeomorphology -- Slope stability analysis -- Conclusions -- Acknowledgements -- References -- 3. Conclusions -- Recommendations for future work -- Appendix: results of unconsolidated-undrained triaxial tests