IMPLICATIONS OF THE SAGE VISTA LANE LANDSLIDE FOR HILLSIDE DEVELOPMENT IN LANDSLIDE AREAS IN NORTHERN UTAH

Movement of the Sage Vista Lane landslide initiated on April 28, 2005, and impacted four recently constructed, attached residential townhouses along the east and upslope edge of a partly completed residential hillside development in Cedar Hills, Utah. By May 13, continued movement of the landslide had severely damaged the upslope side of the townhouses and forced their abandonment. The Sage Vista Lane landslide was caused by the nearly complete reactivation of a historical slide that had reportedly last moved in 1983, suggesting inadequate characterization of the potential landslide hazard in a predevelopment geotechnical investigation. Development-related hillside modifications included excavation of the building pad on or near the toe of the historical landslide and a cut slope in the lower part of the slide, construction of a cobble-lined drainage ditch atop the cut slope, and temporary irrigation of the cut slope for seeding purposes. Landslide movement initiated only a short time following these modifications during the second wettest water year on record in Cedar Hills, but less than a year after the end of a multi-year drought. The relatively small time interval between hillside modifications and movement of this and several other recent damaging landslides suggests a failure of consultants to recognize the inherent marginal stability of pre-existing landslides in northern Utah.

The reactivated landslide is part of a larger prehistoric landslide complex that consists of clay-rich debris derived from the Mississippian Manning Canyon Shale, a landslide-prone unit that underlies much of the east bench area of several Utah Valley communities including Cedar Hills. Reactivation of the 1983 landslide in 2005 included all but its southwestern and lowermost part. Observations by residents suggest that the initial toe of the Sage Vista Lane landslide emerged about midway up the cut slope. Landsliding consisted of two phases of rapidly accelerating movement separated and followed by movement at a very slow rate. The landslide reactivated again in March 2006 and minor movement lasted through late April. Despite land drains installed in the lower part of the slide to improve stability, the landslide continues to pose some hazard to downslope residential units and infrastructure.