SLOPE-STABILITY IMPLICATIONS OF GROUND-WATER-LEVEL FLUCTUATIONS IN WASATCH FRONT LANDSLIDES AND ADJACENT SLOPES, NORTHERN UTAH

Accurate slope-stability analysis of pre-existing landslides and adjacent, potentially landslide-prone slopes requires a realistic estimation of maximum ground-water levels. Previous researchers have documented a long-term rise in ground-water levels since the 1960s in unconsolidated deposits near recently active landslides. Ground-water levels rose mostly during and immediately prior to a wet cycle that started in 1967. Between 1967 and 1998, cumulative excess precipitation totaled about 26 inches in Salt Lake City, coincident with a long-term rise in ground-water levels by as much as 25 feet to historically high levels and active landsliding by the late 1990s. Ground-water-level monitoring since early 1999 in and near five Wasatch Front landslides shows that seasonal ground-water levels rise in response to snowmelt, and locally to summertime lawn watering and extreme precipitation events. Seasonal groundwater levels during the measurement period fluctuated up to a maximum of 14.6 feet. Our data indicate more variability in seasonal peak ground-water levels than in seasonal low levels in a single well. Ground-water levels rise more rapidly in wet years than they decline in dry years, suggesting that infiltration of water into a slide is a more efficient process than dewatering of a slide. Extensional ground deformation features including fissures, grabens, upslope-facing scarps, and back-tilted surfaces facilitate rapid infiltration of local precipitation and snowmelt. In the Sherwood Hills landslide in Provo, rising ground-water levels in the lower part of the landslide, accompanied by first declining and then rising levels in the upper part of the slide, preceded the onset of damaging movement in 2005, suggesting a dynamic ground-water-level-fluctuation model for landslide reactivation.