Abstract

Intermittent movement of the Red Creek Landslide has caused settling and cracking of U.S. Highway 50 west of Gunnison, Colorado, for more than 30 years. Significant degradation of the roadway has resulted in extensive repair costs and traffic delays. Geomorphic and computer stability analyses support the theory that the active Red Creek Landslide is the partial reactivation of a larger, ancient paleolandslide, with a basal rupture surface located predominantly in the Morrison Formation. The original paleolandslide may have been triggered by an earthquake coupled with a high water table. Creation of the Blue Mesa Reservoir submerged approximately 50 percent of the paleolandslide. Some clay layers within the Morrison Formation are susceptible to severe weakening by slaking, and the resulting reduction in strength, combined with a pre-existing failure plane at depth, contributed to the recent partial reactivation of the paleolandslide. Seasonal rapid drawdown of the Blue Mesa Reservoir acts as the trigger for movement, and large drawdowns have been correlated with periods of slope movement. The effects of rapid drawdown could be minimized if reservoir drawdown can be limited to rates less than the critical calculated values. Historic excavation near the toe may explain why other paleolandslides around the reservoir have not reactivated under identical drawdown conditions.

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