Geologic Causation and Mitigation Design for the US-89 Landslide Disaster, Bitter Springs, Arizona

摘要

Large ancient rock landslides or "megaslides" exist throughout much of the Grand Canyon area in Arizona. On February 20, 2013, a 500-foot-long section of pavement failed, closing US 89 in the Bitter Springs area. The Echo Cliffs of Northern Arizona consists of steep Triassic sedimentary rock that has been tectonically uplifted into a large monocline. Vertical relief of the cliffs is 1,500 feet. Slope failures along part of the Echo Cliffs are unique because they occur in the upper 1,000 feet and do not extend to the base of the cliffs. Geomorphic mapping and extensive subsurface exploration indicates that the cliffs initially went through a period of extensive debris slide failures (Pleistocene). Successive erosion and exposure of Chinle Formation claystone, along with the wet Pleistocene climate, resulted in the formation of large translational megaslides. Individual blocks are thousands of feet wide and range from 300- to 500-feet-thick. This translational failure is part of the toe of the ancient megaslide. Failure occurs along a relatively shallow dipping slip surface. Subsurface exploration indicates that, the re-activation of the landslide occurred under "dry" conditions. Re-activation of the landslide is considered to be due to long-term and episodic erosion at the base of the megaslide as well as long-term creep and plastic deformation of the Chinle Formation. To rebuild, a new road alignment shifted just east of the active slide, an extensive rock cut and subsequent massive rock buttress fill with an estimated volume of 1,000,000 yd. is proposed to stabilize the landslide.