Effects of atmospheric variability and remediation techniques on the stability of an interstate highway embankment

Abstract

This thesis presents the findings of Phase III of an ongoing investigation of a seasonally episodic, infiltration-induced landslide, known as the Straight Creek slide, which has caused over 2 m of roadway subsidence along a stretch of I-70 since construction of the highway embankment in 1973. While the general causes of instability were determined in previous phases, the factors causing variability in slide movement and the effects of remediation techniques were not well understood prior to this study. The purpose of Phase III is to reduce these uncertainties, and to propose an alternative remediation technique. Phases I and II (2010 to 2014 and 2014 to 2016) of this investigation, conducted as a partnership between the Colorado Department of Transportation and researchers at the Colorado School of Mines and the U.S. Geological Survey Landslide Hazards Program, identified the slide triggering mechanism as the interaction of rapid infiltration of spring snowmelt with site stratigraphy. This infiltration, in combination with a large and steeply graded watershed composed of high hydraulic conductivity material, produces high groundwater flux through the embankment; as the conductivity of the embankment material and native material beneath it are substantially lower than in the watershed above, this high flux creates high pore pressures in saturated zones and low suction in unsaturated zones. This produces low effective stress along the slide failure surface, which runs underneath the embankment, resulting in seasonal instability and movement. Phase III incorporates borehole logs, atmospheric data, streamgage data, inclinometer data, and piezometer data with coupled hydro-mechanical stability analyses to evaluate the effect on the stability of the Straight Creek slide of variability in infiltration characteristics and of remediation projects undertaken in 2011 and 2012. Of those studied, the most important single-year atmospheric factor causing instability is found to be the amount of snowmelt, followed by the rate at which it infiltrates; other single-year factors, including early partial snowmelt and summer rainfall, are found to have a relatively negligible effect. Consecutive years of high or low infiltration are shown to have a compounding effect on site hydrology, so that stability in a given year is influenced by the preceding years. The weight-reducing caissons installed in 2011 and 2012 are found to have no substantial effect on slide movement, as they reduce normal forces and frictional resistance along the failure surface as well as gravitational driving forces. The horizontal drains installed near the slide toe are found to only reduce pore pressures and increase stability in a limited area due to the low conductivity of surrounding materials. Based on these analyses, a new drainage system is proposed and evaluated, which would be installed above the highway embankment in order to intercept groundwater before it encounters the conductivity contrast. Numerical modeling demonstrates that this design can dramatically improve slope stability.