by Sheri Molnar, Thursday, November 8, 2018
Passive seismic techniques that record Earth’s background seismic noise wavefield have gained significant popularity in the last few decades. Passive seismic methods are non-invasive and non-disruptive to the site and therefore provide environmentally-sensitive methodology to infer subsurface geology. The use of passive seismic methods in current geotechnical engineering practice is rare but will increase in future. This presentation will include the basic theory of passive seismic methodology, why these passive methods are best combined with active-source seismic methods, and present case-study examples from nearly 20 years of experience in developing and applying passive seismic methods across Canada, Chile and in Nepal.
Both active- and passive-source seismic methods are utilized to map subsurface ground conditions across western communities of Metro Vancouver related to a multi-year seismic microzonation mapping project, including shaking, liquefaction and slope stability hazards. The project is supported by Emergency Management British Columbia and the Institute for Catastrophic Loss Reduction. Key tasks have involved developing a 3D geodatabase from previously collected geological, geophysical and geotechnical datasets, and performing seismic testing for subsurface site characterization which began this past summer 2018. In addition, all available recordings from 7 moderate earthquakes of magnitude > 4.3 between 1976 and 2015 are utilized to provide a comprehensive assessment of observed site amplification in Greater Vancouver. This presentation will include a status update of the Metro Vancouver microzonation mapping including preliminary non-invasive seismic testing results and comment on challenges in development of the underlying datasets.
Lessons Learned from Geotechnical Failures - 2018 Fall Cross Canada Lecture and VGS Annual General Meeting
by Alex Sy, Thursday, October 18, 2018
Despite advances in geotechnical engineering, failures do occasionally occur because of unknowns, uncertainties, inexperience, miscommunications, etc. However, failures do provide valuable lessons for the profession that can be learned to minimize future failures. This lecture will present three examples of geotechnical failures in British Columbia, in which the author was engaged to carry out forensic engineering. Pertinent details of the geotechnical failures and their causes are described for the following three case histories: (1) the dyke breach at the Stanley Street Pump Station located on the North Arm of the Fraser River in New Westminster; (2) the excessive foundation settlement at the Queensborough Middle School using stone column foundations in very soft soils at the east end of Lulu Island, and (3) the damaging ground movements at the Panorama/Ridgeview Subdivision located on an old landslide or “earthflow” in Chilliwack. Subsequent remedial solutions and lessons learned are also discussed.