Advancing the understanding of liquefaction ejecta through case histories

Abstract

Case histories have been critically important in advancing the field of geotechnical earthquake engineering. The sequence of Canterbury, New Zealand, earthquakes in 2010-2011 produced an unparalleled quantity of high-quality liquefaction manifestation data. The data include airborne LiDAR-based measurements of ground surface elevation, aerial and ground photographs, property inspection reports, cone penetration tests, borehole logs, groundwater depth measurements, and peak ground acceleration (PGA) estimates. These data were utilized to develop the first liquefaction ejecta database. It contains 235 well-documented case histories with estimates of liquefaction ejecta-induced settlement and parameters describing ground conditions and ground motions. The settlement was estimated using the LiDAR-based and photographic-based approaches. Most of the sites in the database have thick, clean sand deposits. About one-half of these sites settled at least 50 mm due to ejecta when shaken by a M_w 6.1-equivalent PGA > 0.40 g. The severity of liquefaction ejecta at these clean sand sites tended to be underestimated by the state-of-practice liquefaction induced-damage indices. The underestimation was greater by indices that did not consider post-shaking hydraulic mechanisms. Strong shaking and liquefaction induced by the M_w 6.2 Feb 2011 earthquake formed cracks in the crust, which likely exacerbated the severity of liquefaction ejecta in the following M_w 6.2 June 2011 earthquake. The liquefaction ejecta case histories provide important insights, some of which are shared in this paper.