Doctor of Philosophy
The Metro Vancouver region lies in a seismically active region with numerous areas exhibiting thick liquefiable layers and shallow groundwater levels. The liquefaction susceptibility classification is verified by considering the thickness of liquefiable layers from 808 cone penetration tests (CPT). Probabilistic seismic-induced liquefaction assessment based on the national seismic hazard model of the 2020 National Building Code of Canada is accomplished to account for the contribution of all seismic sources to the liquefaction evaluation. The liquefaction hazard curves from 900 CPT and shear wave velocity profiles are generated by incorporating the liquefaction potential index (LPI) and Ishihara-inspired liquefaction potential index (LPIISH) into the performance-based liquefaction assessment. The first probabilistic liquefaction hazard maps of Metro Vancouver are presented using the targeted return periods of 475 and 2,475 years. These liquefaction hazard maps reveal that liquefaction manifestations are expected in Richmond and Delta regions. In addition to considering seismic loading uncertainty, variability of soil resistance and the liquefaction triggering model are also accounted for through Monte Carlo simulation to embed the factor of safety against liquefaction (FSL) and the required soil improvement to prevent liquefaction (ΔqL) into the performance-based assessment. It yields the FSL and ΔqL hazard curves to determine the targeted design level. The probabilistic FSL, ΔqL, liquefaction return period and Hcr maps built on a large number of in situ data and semi-sophisticated analyses should be used together to mitigate against liquefaction hazard in the region. Useful regression models to predict ΔqL for return periods of 475 and 2,475 years from the soil resistance are developed. Deterministic analyses are performed for Cascadia subduction zone earthquakes using ground motion prediction equations of the 6th national seismic hazard model to determine the range of Mw-amax combinations that result in the observed paleo-liquefaction features in the region. A probabilistic framework is developed to account for the uncertainties of soil resistance, groundwater table depth, maximum ground acceleration, and the liquefaction model. Our probabilistic results also reveal that Cascadia interface earthquakes with Mw > 8.9 lead to a 31-57% probability of liquefaction triggering in the region. Magnitude-bound curves are also developed for Cascadia interface earthquakes.
Summary for Lay Audience
Liquefaction is a phenomenon in which the soil loses its strength due to an earthquake’s seismic loading and may act like a liquid. Most areas in the Metro Vancouver region are prone to liquefaction triggering because of loose and thick liquefiable sandy ground that is saturated and exposed to a relatively high rate of earthquakes. Future earthquake-induced liquefaction may result in ground damage such as sand boiling or ground cracking or as much as lateral ground spreading. Regional liquefaction hazard mapping for Metro Vancouver will be beneficial to identify the areas with a likelihood of none to extreme liquefaction manifestations.
This thesis utilizes the most recent simplified procedures to evaluate earthquake-induced liquefaction hazard that are based on existing databases of earthquake-induced liquefaction evidence. In addition, this thesis utilizes the most comprehensive database of soil resistance measurements developed for the region to date. Since there are lots of uncertainties inherent in a conventional liquefaction evaluation, a probabilistic liquefaction analysis is employed to account for the variabilities of input parameters and the simplified liquefaction model. This thesis’ development of a performance-based liquefaction evaluation presents a comprehensive picture of liquefaction hazard considering local measures of subsurface soil resistance and return periods of regional seismic sources. The derived liquefaction hazard estimates are spatially predicted (mapped) for two targeted return periods of 475 and 2475 years to generate regional seismic-induced probabilistic liquefaction hazard maps for the Metro Vancouver region. The maps show that most areas in Richmond and Delta cities will exhibit severe surficial liquefaction manifestations at the ground surface given either return period.
Evidence of old (paleo) liquefaction features in Metro Vancouver dates back to less than 3500 years ago and confirms our predictions of high earthquake-induced liquefaction potential for the region. Considering the Cascadia subduction zone as the earthquake source to trigger liquefaction, we use the latest national seismic source model and ground motion prediction models to estimate the likely the peak ground acceleration shaking and associated earthquake magnitude that would have initiated the observed paleo-liquefaction in the region with both deterministic (not considering the uncertainty) and probabilistic (accounting for the uncertainty) frameworks.
Javanbakht Samani, Alireza, "Probabilistic seismic liquefaction hazard mapping of western Metro Vancouver, British Columbia, Canada" (2023). Electronic Thesis and Dissertation Repository. 9881.
Available for download on Tuesday, December 31, 2024