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TitleEvaluating the frequency dependence of dynamic soil properties in Leda Clays
AuthorCrow, H L; Hunter, J; Motazedian, D; Cascante, G
SourceProceedings of the 5th Canadian conference on geotechnique and natural hazards; 2011, 8 pages
Alt SeriesEarth Sciences Sector, Contribution Series 20110018
Meeting5th Canadian Conference on Geotechnique and Natural Hazards; Kelowna; CA; May 15-17, 2011
Lat/Long WENS -76.0000 -75.5000 45.5000 45.2500
Subjectsengineering geology; clay analyses; clay mineralogy; clay minerals; clays; sensitive clays; marine clays; clay properties; sediment stability; seismic risk; seismic interpretations; Leda Clays; Champlain Sea
Illustrationsgraphs; plots
ProgramEastern Canada Geohazards Assessment Project, Public Safety Geoscience
AbstractIn Ottawa, Ontario, unusually high amplification ratios have been measured in clayey silts ('Leda Clays') at low levels of earthquake-induced ground shaking. However, the contribution of material damping to the overall ground motion at soft soil sites across the city is not well understood. Therefore, low-strain dynamic properties (shear wave velocity and damping characteristics) of Leda Clays were tested in the field and through lab analyses using new and traditional approaches. Downhole seismic testing was carried out in two deep Leda Clay boreholes in the Ottawa area (96m, 54m) using monofrequency seismic signals between 10-100Hz, and two identical downhole triaxial geophones. Interval damping levels ranged from 0.2% to 0.4% between 30m and 90m depth. This indicates that in relatively homogenous Champlain Sea deposits, very little elastic energy is lost at low strains, which contributes significantly to the observed elevated amplifications experienced during small strain earthquakes in the Ottawa area. These results were compared with low-strain laboratory testing carried out on Leda Clay samples using the newly developed non-resonant (NR) approach at the University of Waterloo. The NR method evaluates the frequency dependence of damping and shear wave velocity (Vs) at various strain levels. Ongoing geotechnical research is now evaluating the frequency dependence of dynamic soil properties in Leda Clays at higher strains using the NR technique. These combined results will provide important soil parameters for ground response analyses at soft soil sites in Eastern Canada.