THE DEBEQUE CANYON LANDSLIDE: A CASE STUDY IN SITE CHARACTERIZATION OF COMPLEX LANDSLIDES

A critical part of engineering analyses of moderate- to large-size landslides is the geologic characterization. In the authors' experiences the site investigations and resulting characterization of the geologic materials and processes often lack the detail and accuracy necessary to develop a valid conceptual model. The purpose of this presentation is to describe a successful site characterization program that was part of a comprehensive analysis of a complex landslide.

The DeBeque Canyon landslide is located 21 miles east of Grand Junction, Colorado. The climate is semi-arid and the geology consists of nearly flat-lying interbedded sandstones, siltstones, and weak shales mantled by discontinuous mid-Pleistocene loess deposits. A catastrophic event in the early 1900s reportedly diverted the Colorado River and caused flooding of the railroad corridor across the river. Reactivations of the landslide rubble damaged Highway 6-24 in 1958 and Interstate 70 in 1998. One of the major concerns was whether a large portion of the canyon wall could, again, fail catastrophically. This concern prompted a multi-disciplinary study and analysis of the landslide in 1999.

Early in the study a literature review was conducted that included establishing landslide age and recurrence intervals from historical documentation, correlation between moisture and slide events, and a review of similar slide performance in that setting. Photogrammetric analysis was also conducted to discern historic landslide behavior and measure movement over time from historical air photos. Detailed mapping of the landslide and surrounding area was accomplished on high-resolution air photos at a scale of 1:1200 and was projected onto a 2 ft. contour interval base map. The mapping and field observations were used for preliminary interpretation of slide mechanisms and design of a subsurface investigation and monitoring program. The slide history, mapping, subsurface investigation and preliminary monitoring provided the details required to construct a three-dimensional conceptual model of the landslide setting and slide mechanisms. This conceptual model of geologic materials, subsurface geometry, and processes involved were used as a basis for engineering analysis to predict future movements and design mitigative solutions.