Date of Award

8-2018

Degree Type

Report

Degree Name

Master of Science (MS)

Department

Geology

First Advisor

Tammy Rittenour

Second Advisor

Joel Pederson

Third Advisor

Carol Dehler

Abstract

On August 5th, 2014, a hillside failed behind a North Salt Lake City, UT neighborhood threatening several homes. Aerial Photography, Digital Elevation Models (DEM), geochemistry, rain gage and seismic data were used to test the influence of contributing factors in this landslide failure. Aerial photographs available from 1993 to present were examined for signs of tension cracks suggesting impending ground motion, as well as documentation of human modification along the hillslope. Repeat DEM analysis of elevation and slope of the hillside before and after the slide were examined to characterize the pre-failure hillslope and subsequent landslide. Geochemical analyses were run on samples of the Tertiary Norwood Tuff within and outside of the landslide boundaries. Precipitation and seismicity data were collected and compiled to identify if they played a role in initiating the landslide.

Analyses indicate that the hillslope was previously part of a gravel pit in the 1990s, which was reclaimed in the early 2000s. Tension cracks began to appear by 2002, indicating downslope movement of unconsolidated material. DEM analysis reveals that the hillside was not at a critical angle for failure, suggesting that the linear, graded hillslope was stable prior to the landslide release. Geochemical analysis of Norwood Tuff indicated it is clay-rich, containing montmorillonite, an expandable clay mineral. Two severe rain storms occurred within six days of the slide, providing the fluid for saturation of the montmorillonite clays in the altered Norwood Tuff underlying the landslide. No notable seismic events occurred leading up to the slope failure.

After the landslide event, the scarp and toe of the slide became over-steepened. The toe is not stable and continued movement is expected and has been observed. The driving forces for this slide included human modification to the slope both during gravel pit operation and post reclamation construction that under-cut the toe of the hillslope and changed local hydrology of the area by decreasing infiltration above the slope. This was compounded by a large precipitation event that saturated the clay-rich material, which was the specific trigger for this landslide event.

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