United States Department of Transportation
i
Effects of Permafrost and Seasonally Frozen Ground on the Seismic Responses of Transportation Infrastructure Sites
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2010-02-01
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[PDF-1.84 MB]
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TRIS Online Accession Number:01164850
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Edition:Final Report
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NTL Classification:NTL-ENERGY AND ENVIRONMENT-Environment Impacts;NTL-HIGHWAY/ROAD TRANSPORTATION-Soils and Geology;
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Abstract:The state of Alaska is located in one of the most seismically active zones in the world. Several large magnitude earthquakes (the Prince William Sound Earthquake, March 1964 and the Denali Earthquake, November 2002) have occurred in the state and caused considerable damage to its transportation system, including several highway bridges and other infrastructures. Some of the damages could be related to frozen soil effects. However, only limited research has been carried out to investigate the effects of frozen soils on the seismic site response. A comprehensive analytical investigation of the effects of frozen soils including seasonally frozen soils and permafrost on the seismic site response has been conducted. Two sites, i.e. the C St-O’Malley Bridge site in Anchorage and the Goldstream Creek Bridge site in Fairbanks, were selected to represent typical sites with seasonally frozen soils and discontinuous permafrost, respectively. Two generic soil profiles were constructed based on the geologic and geotechnical data available for these two sites. A set of input ground motions has been selected from available strong-motion databases and scaled to generate an ensemble of seismic hazard-consistent bedrock motions. One-dimensional equivalent linear analyses were applied to analyze the seismic site response for three levels of seismic hazard, i.e. MCE (2% probability of exceedance in 50 years), AASHTO Design Earthquake (7.5% probability of exceedance in 75 years) and IBC Design Earthquake (two-thirds of MCE). A series of parametric studies were conducted for assessing the sensitivity of the results to the uncertainties associated with shear wave velocity of frozen soils, thickness of seasonally frozen soil, permafrost table/thickness, and bedrock table. Our results show that the presence of frozen soils, particularly permafrost, can significantly change the ground motion characteristics. It is concluded that it is generally conservative to ignore the effects of seasonally frozen ground on site response. It is, however, not always conservative to classify permafrost soil sites by using average shear wave velocity of the upper 30 m frozen or unfrozen soils and use code-defined site coefficients for seismic design. For permafrost sites similar to the GC Site-Worst Case Scenario, that is when the permafrost table is at -50 to -130 ft, and the bedrock table is at -230 ft or shallower, and the soil types are similar to the GC Site - Worst Case Scenario, the average response spectra obtained from this study could be used in seismic design.
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