Monofrequency in situ damping measurements in Ottawa area soft soils

In Ottawa, Canada, unusually high amplification ratios have recently been measured in clayey silts (called ‘Leda Clays’) at low levels of earthquake-induced ground shaking. However, the contribution of seismic Q, or material damping (ξ=1/2Q), to the overall ground motion at soft soil sites across the city is not well understood. This research investigates attenuation measurements in soft soils (Vs<250 m/s) for ongoing seismic hazard evaluation in the Ottawa area. The work focuses on in situ measurements of damping in two deep boreholes drilled into Leda Clay. To investigate the possibility of frequency-dependent dynamic properties of these materials at low strains, a new approach to the spectral ratio technique has been developed for the measurement of Qs in the field using a mono-frequency vibratory source (generating signals between 10 and 100 Hz), and two identical downhole 3-component geophones. Monofrequency signals also allowed for the measurement of dispersion (variation of velocity with frequency). Analysis of the data show that dynamic properties are, for the most part, independent of frequency in the homogenous silty soils, yielding negligible variation in shear wave velocity (<2 m/s) across the frequency test band, and small strain Qs's ranging from 170 to 200 (damping of 0.25–0.30%) over soil thickness intervals ranging from 10 to 60 m. At intervals within 20 m of the ground surface, laminated silt and clay beds of elevated porosity are found to have slight influence on the frequency dependence of damping for frequencies greater than 70 Hz (damping increase to 0.6%).

► Monofrequency spectral ratio method developed to measure damping ratio (ξ) in situ.
► Method tested in glaciomarine silts where amplification from M2–3 earthquakes is high.
► Monofrequency spectral ratios result in much less error than broadband spectral ratios.
► Damping ranged between 0.2% and 0.4% at ∼10−6% strain; little frequency dependence between 10 and 100 Hz.
► Soil properties influence frequency dependence of damping (observed increase to 0.6% at 70 Hz).