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Presented by Dr. Jonathan Bray, UC Berkeley Faculty Chair

Liquefaction triggering procedures provided results consistent with observations of liquefaction across much of Christchurch, New Zealand during the 2010-2011 Canterbury earthquakes. However, there are cases where liquefaction assessment methods indicate that there should have been severe liquefaction-induced ground failure, yet none was observed. The overestimation of liquefaction-induced ground failure by current procedures is troublesome because engineers often require expensive ground improvement or building foundations where these procedures indicate that liquefaction is likely. The over-estimation of liquefaction triggering by current procedures is due to their inability to capture the seismic performance of stratified silty soil deposits. The empirical database used to develop these procedures consists primarily of liquefaction triggering data from sand sites.  
Stratified silty soil swamp deposits in Christchurch have mitigating effects on the manifestation of liquefaction beyond what can be captured by current liquefaction triggering procedures. Differing surficial geology and depositional environments explain in part the limitations of current liquefaction evaluation procedures in the silty soil sites in Christchurch. Consideration of depositional environment distinguishes between liquefaction performances that could not be differentiated through the CPT-based assessment alone. CPT resolution cannot capture the thin layering at these stratified sites, and current liquefaction triggering procedures do not consider the effects of the stratification on pore water pressure movement in the soil deposit. Careful logging of continuous high-quality sampling provides important insights on in-situ stratification at these silty soil swamp sites, discerning differences in stratigraphy resulting from differences in the depositional environment.

Speaker: Jonathan Bray, Ph.D., P.E., NAE is the Faculty Chair in Earthquake Engineering Excellence at UC Berkeley. Dr. Bray is a registered professional civil engineer and has served as a consultant on important engineering projects and peer review panels. He has authored more than 350 research publications on topics that include liquefaction and its effects, seismic performance of earth structures, earthquake ground motions, and surface fault rupture. He created and led the Geotechnical Extreme Events Reconnaissance (GEER) Association. Dr. Bray is a member of the US National Academy of Engineering and has received several honors, including the Terzaghi Award, Ishihara Lecture, Peck Award, Joyner Lecture, Middlebrooks Award, and Huber Research Prize.