| Description |
Red Butte Canyon is a federally protected research area located outside Salt Lake City, Utah east of the University of Utah. The Wasatch Fault Zone, an active fault capable of producing magnitude 7.0 earthquakes, is also located less than one mile west of Red Butte Canyon. A series of geomorphic features exist in Red Butte Canyon including landslides, rock falls, creep, and steep slopes within both the primary Red Butte Creek channel and the secondary Parleys Fork channel. Possible paleochannels exist adjacent to a mapped landslide at Red Butte Creek and Parleys Fork. A detailed geologic and engineering investigation was executed to address these unusual geomorphic features. Research objectives included detailed geologic mapping in the study area to better evaluate geologic characteristics and determine the characteristics of subsurface sediments. Additional objectives included performing a groundwater and slope stability evaluation within selected areas of the study area. Methods included drilling and sampling, seismic refraction, groundwater measurements, geotechnical engineering testing, slope stability modeling, and radiocarbon dating to evaluate the geological and geotechnical characteristics of geomorphic features. A combination of sediment type, grain size distributions of soils and radiocarbon dating suggest that subsurface sediments closest to the landslide area formed in an alluvial / lacustrine depositional environment suggesting a lake existed in Red Butte Canyon behind a landslide dam at some point in the past. Charcoal samples discovered in the subsurface yielded radiocarbon ages of 4,370 ± 30 years, in close agreement with independent paleoseismology studies for on the Wasatch Fault dated 4,000 ± 500 years ago, suggesting the lacustrine sediments formed in response to the earthquake and landslide. Slope stability analysis investigated undrained and drained conditions and utilized the Pseudostatic Method. The results show that selected slopes within the field site were stable under drained conditions except when a high groundwater table exists. Slopes were also less stable under drained conditions, particularly when a peak horizontal ground acceleration exceeded 0.1 g - 0.15 g. The reduced stability of slopes under seismic loading conditions also suggests that the landslide was triggered by an earthquake in the aforementioned time frame. |
