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A Geotechnical Investigation of the October 2011 Cedar City Landslide, Utah

Tizzano, Ashley S
http://rave.ohiolink.edu/etdc/view?acc_num=kent1397555352

Abstract Details

2014, MS, Kent State University, College of Arts and Sciences / Department of Earth Sciences.
During the morning of October 8, 2011, a massive landslide caused severe damage to State Route 14 (SR 14) in Cedar Canyon, eight miles outside Cedar City, Utah. The landslide detached approximately 1.5 million cubic yards of material from the south side of the canyon, displaced parts of the road and covered the remainder of a 1200 ft (365 m) stretch of SR 14 under more than 100 ft (30 m) thick debris. The stratigraphy of the canyon where the landslide occurred includes the cliff-forming Tibbet Canyon Member of the Straight Cliffs Formation (limey sandstone) and the underlying slope-forming Tropic Formation (shale) and Dakota Formation (mudstone and sandstone with coal horizons), all Cretaceous in age. The landslide initiated in the Straight Cliffs Sandstone and propagated as a translational slide along the contact between the colluvial soil and the underlying bedrock. Utah Department of Transportation (UDOT) drilled three borings through the landslide material, placed slope inclinometers in the borings, and conducted a geophysical survey from the crest to the toe of the slide. I used detailed line survey and window mapping methods to collect orientation data for 186 discontinuities within the Straight Cliffs and Dakota Sandstones. Stereonet plots of discontinuity orientation data, generated by the DIPS software, revealed the presence of three principal joint sets that contribute to slope instability at the site. Samples of the colluvial soil and the bedrock were tested in the laboratory to determine relevant engineering properties including natural water content, density, and shear strength parameters (cohesion and friction) of soil, bedrock, and soil-bedrock contact. The SLIDE software program and data generated in the laboratory were used to perform a stability analysis which indicated a factor of safety of 0.8 to 1.2 for the dry conditions and 0.3 to 0.4 for fully saturated conditions (water table at ground level). A sensitivity analysis was performed by adjusting the water table height to create partially saturated soils and determining the friction angle needed for a safety factor of one, keeping the cohesion value as determined in the laboratory. For an average friction angle value of 29o, a water table height of approximately 3.3 ft (1 m) above the soil-bedrock contact is required to cause the failure; however, it may be slightly different than that depending on the type of bedrock lithology (shale, mudstone, sandstone) controlling the failure. The results of this study show that a combination of factors was responsible for causing the Cedar City landslide including a relatively steep colluvial soil slope (~30°), extra weight on the soil from previous rockfalls, and buildup of pore pressure due to a rainstorm prior to failure. Remedial measures used by UDOT to stabilize and repair SR14 include re-grading the slope of the failed colluvial soil to 2H:1V, placing a riprap-filled ditch along the road for drainage purposes, installing a retaining wall below the road on the east side of the landslide, re-compacting the subgrade material under the road and reconstructing the pavement.
Abdul Shakoor (Advisor)
210 p.
Geology; Geotechnology
landslide; slope stability; Cedar Canyon; Utah

Recommended Citations

Citations

  • Tizzano, A. S. (2014). A Geotechnical Investigation of the October 2011 Cedar City Landslide, Utah [Master's thesis, Kent State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=kent1397555352

    APA Style (7th edition)

  • Tizzano, Ashley. A Geotechnical Investigation of the October 2011 Cedar City Landslide, Utah. 2014. Kent State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=kent1397555352.

    MLA Style (8th edition)

  • Tizzano, Ashley. "A Geotechnical Investigation of the October 2011 Cedar City Landslide, Utah." Master's thesis, Kent State University, 2014. http://rave.ohiolink.edu/etdc/view?acc_num=kent1397555352

    Chicago Manual of Style (17th edition)

kent1397555352
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This open access ETD is published by Kent State University and OhioLINK.