FACTOR OF SAFETY IMPLICATIONS FROM A SLIDE CAUSED BY GLACIAL EROSION

Glacial erosion lengthens valley walls due to the downcutting of valley floors. Although this lengthening implies that the hill slopes should become increasingly unstable over time, the ice thickness counteracts the lengthening by effectively creating a new valley floor and decreasing slope length. This inhibits deep-seated land sliding because the basal shear stress the hill-slope experiences is dictated by ice thickness. Glacial advance and thickening lowers shear stress. A retreat and thinning of the ice means a lengthening of the valley wall, causing an increase in shear stress that triggers landslides. A landslide that may reflect this feedback is present in Taan Fjord in South Alaska where recent retreat of the Tyndall Glacier has been accompanied by the formation of a landslide on the west wall of the fjord.

For an undergraduate research program, I gathered data concerning the height, width, and surface slope of the landslide in order to develop a model that relates shear stress to different glacier thcknesses. A profile of the landslide was produced using data collected with an Electronic Distance Measuring device (EDM) and a bathymetric survey of the fjord floor using a depth finder. Ice heights from past advances of Tyndall Glacier were determined from glacial trim line elevations measured in the field. I used the factor of safety for landslides as an assessment of the shear stress. Landsliding following retreat of ice suggests that the shear stress threshold for failure was surpassed. A comparison of the factors of safety before and after ice retreat should give a range in values for hill slope failure in this particular case. The preliminary factor of safety with ice is 150 kPa - .68742 N and is 150 kPa - .68719 N without ice.