Characterization of Failure Parameters and Preliminary Slope Stability Analysis of the Cedar Canyon Landslide, Iron County, Utah

Abstract

The 2011 Cedar Canyon landslide, eight miles (12.8 km) east of Cedar City, Utah, severely damaged State Route 14. Bedrock stratigraphy at the site consists of the cliff-forming Straight Cliffs Formation and the underlying slope-forming Tropic Shale and Dakota Formation. The lower part of the Straight Cliffs Formation and all of Tropic Shale and Dakota Formation are covered with a thick deposit of colluvial soil that has accumulated at the base of the steep cliff formed by the Straight Cliffs Formation. The landslide initiated in the colluvium and propagated as a translational slide along the contact between the colluvial soil and underlying bedrock units. The Utah Department of Transportation (UDOT) drilled and installed slope inclinometers in three borings and conducted a geophysical survey from the crest to the toe of the landslide. We collected orientation data for 186 discontinuities from the Straight Cliffs and Dakota Formations to determine the principal joint sets for use in kinematic analysis; tested samples of colluvial soil and bedrock to determine natural water content, density, and shear strength parameters of soil, bedrock, and the soil-bedrock contact; used SLIDE software to perform a preliminary stability analysis; and performed a sensitivity analysis with respect to varying positions of the water table. Results show that a combination of relatively steep colluvial slope (~30°), extra weight from previous rockfalls, and buildup of pore pressure due to a rainstorm prior to failure, contributed to the landslide.