Landslide Hazard Studies at GEO-HAZ

(Updated Jan. 16, 2008)

Slope Stability Studies—Ski Areas

Slope Stability and Geology at Snodgrass Mountain, Crested Butte Ski Area

GEO-HAZ is directing comprehensive geologic and slope stability studies for a proposed new ski area on Snodgrass Mountain, for the Crested Butte Mountain Resort. Past landsliding is extensive on the SE slope of the mountain, where the Tertiary laccolith of Snodgrass Mountain is underlain by Mancos Shale (the same geology as at Mt. Crested Butte). Over the past 12 months we have mapped the entire mountain in GIS, drilled 16 boreholes, installed 18 recording piezometers including 4 nests, 8 inclinometers, 2 stream gages (flumes), and 20 landslide monitoring monuments. Drilling was supplemented by nearly 10,000 linear ft of seismic tomography surveys, both P-wave and S-wave. As soon as geotechnical shear strength data are available, we will be analyzing the pre- and post-development stability of various mapped landslides in support of ski area design.

Slumps on Funnel Trail at Snowmass Ski Area, Pitkin County, Colorado

Recent slumping on the Funnel Trail at Snowmass was interfering with ski area operations, so GEO-HAZ was employed by SE Group for the Aspen Ski Company to analyze the cause of failure and suggest mitigation. Failure was caused by a combination of: (1) weak Mancos Shale beds dipping parallel to the slope, (2) a thin blanket of till upslope of the alump soaking up snowmelt that got perched on the shale, and (3) flay slopes and insufficient surface drainage in the till area. Mitigation consisted of rerouting surface drainage in the till area above the slump, to tie in with the roadside ditch network that already existed.

Geologic Hazards Associated with Front Side Improvements, Keystone Ski Area, Colorado

GEO-HAZ performed GPS-based geologic field mapping of potential geologic hazards in areas proposed for improvements at Keystone Ski Area.

Powderhorn Ski Area, Mesa County, Colorado:

geologic component of the Environmental Assessment for a major ski area expansion and facility upgrade. Main concerns are shallow landsliding affecting lift towers, and the effect of runoff from artificial snowmaking on slopes that are near threshold of slope stability.

Keystone Ski Area, Summit County, Colorado:

geologic component of the Environmental Assessment for the Jones Gulch Expansion. Main concerns are shallow landsliding and the possible reactivation of landslides by snowmaking and trail- and road-building activities.

Buttermilk Ski Area, Aspen, Colorado:

geologic component for the Buttermilk Master Plan, with special reference to the movement of historic and prehistoric landslides on this large dip-slope of Mancos and Dakota Formations. The analysis also describes rockfall hazards associated with the proposed gondola between the Aspen Highlands base area and the top of Buttermilk.

Older Studies at Other Ski Areas (descriptions on request): Aspen Highlands, Aspen Mountain, Breckenridge

Slope Stability Studies—Residential Developments

Landslides at Suncrest Development, Draper, Utah

GEO-HAZ produced a Landslide Inventory Map at 1:12,000 scale of the 3700-acre Suncrest Development, digitized and georeferenced to the developer’s site survey system. Following that, we embarked on a 3-year slope stability analysis of various build-out stages within the development, under contract to Professional Services Industries (PSI), Salt Lake City, UT office. GEO-HAZ was responsible for geologic mapping, structural mapping and interpretation, landslide mapping; trench siting, logging, interpretation, and sampling; collection of shear strength samples; design and interpretation of geophysical lines; location and interpretation of geotechnical borehole results; construction of geologic cross-sections for computer stability analysis. This project was the most expensive landslide study ever performed in Utah, including a total of more than 80 trenches, 30 boreholes, and miles of seismic refraction tomography surveys.

Debris Flow Hazards at Top of Mill Subdivision, Aspen, Colorado

GEO-HAZ reviewed prior reports and consulted on the design of a debris flow deflection wall for a future residence in the Top of Mill subdivision, which lies at the base of the Aspen Mountain ski area.

Slope Stability Studies—Commercial Developments

The Timbers Lodge, Keystone, Colorado:

geologic mapping and age-dating of landslides along the access road to the Timbers Lodge. Performed slope stability calculations of various road alignments and cross-sections, along with slope reinforcement and drainage strategies, to identify a design that could attain a factor of safety against failure of 1.35, as specified by the U.S. Forest Service.

Regional Slope Stability Mapping

Ridgetop Spreading, Splitting, and Shattering Related to Earthquakes in Southern California

Ridgetop Spreading, Splitting, and Shattering Related to Earthquakes in Southern California—mapping phase, 1998

Ridgetop Spreading, Splitting, and Shattering Related to Earthquakes in Southern California—trenching phase, 1999

Annual Project Summary (PDF file)

GEO-HAZ performed a 2-year assessment for the U.S. Geological Survey of gravitational spreading hazards in the San Gabriel and Santa Susana Mountains of southern California. During strong earthquake shaking (frequent in southern California mountains) mountain ridges undergo a loss of strength that results in ridge spreading, with faulting, splitting, and shattering affecting the ground surface. The results of the study were later published in 2003 as California Geological Survey CD-ROM 2003-05, which won the Holdredge Award from the Association of Engineering Geologists (http://devaegweb.i4a.com/i4a/pages/Index.cfm?pageID=3755).

For details on ordering the CD, go to http://www.conservation.ca.gov/cgs/information/publications/release_statements/Documents/CD2003_05.pdf

Kingston, Jamaica Landslide Susceptibility Maps

Kingston, Jamaica, Landslide Susceptibility Maps

The fringes of the fast-growing Kingston metropolitan area are comprised of steep slopes developed on highly-weathered rocks. During occasional intense precipitation from hurricanes and tropical storms, these slopes landslide, with attendant losses in human life and property damage. In cooperation with the University of the West Indies, Mona (Dr. Rafi Ahmad) and the Organization of American States (OAS), GEO-HAZ produced two landslide-susceptibility maps of the greater Kingston region. The maps were produced by applying an enhancement of the standard DeGraff Landslide Susceptibility Matrix approach to a 15 m GIS model; all map algebra was performed in IDRISI. In addition to the standard susceptibility variables of geologic unit, slope angle, and slope aspect, we additionally analyzed landslide susceptibility to distance to mapped faults (which turned out to be a very strong control for deep landslides) and slope plan curvature (which turned out to be important for shallow landslides). Because shallow (colluvial) landslides respond to different controls than deep landslides, we produced a susceptibility map for each type, based on splitting the initial landslide inventory map into shallow and deep landslides.

For details on the study, go to: http://www.oas.org/cdmp/document/kma/udspub5.htm

Seattle Earthquake-Induced Landslides

Seattle Earthquake-Induced Landslides

This pioneering study, performed in 1986, was the precursor to all succeeding studies in the Seattle area. It was the first to use GIS, the first to base it’s landslide inventory on the records kept by the City of Seattle, and the first (and only) to assign annual probability estimates to different landslide scenario maps, based on shallow groundwater levels. The computational flow charts for calculating infinite-slope factor of safety, as performed in raster GIS, are generic and can be applied to any area.

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