NZSEE Conference 2026 Wellington
Liquefaction assessment is a key aspect of earthquake geotechnical design in New Zealand particularly in high seismic areas such as Wellington. Simplified methods, such as Idriss & Boulanger (2014), are widely used for their efficiency and strong empirical foundation. However, these approaches assess soil layers independently based on peak ground acceleration (demand) and relative density (resistance), without accounting for interactions within the full soil profile. Dynamic effective stress analyses (ESA) overcome these limitations by simulating earthquake time histories, pore pressure generation and dissipation, and the coupled soil response throughout the deposit. This paper compares simplified methods with dynamic effective stress analyses (ESA) for three alluvial profiles across Wellington, where liquefaction strongly influences foundation and structural design. The following challenges are investigated: (1) do thin weak layers embedded in dense soils liquefy? (2) do dense layers within weaker strata liquefy? The possible implications for foundation design are discussed. Applying ESA alongside simplified methods improves understanding of liquefaction risks, particularly in layered alluvial deposits where simplified methods may predict isolated “pockets” of liquefaction or overlook mechanisms that can affect strength loss. This approach can result in a more targeted and costeffective risk mitigation and foundation design by distinguishing conditions where liquefaction effects are either more severe or less critical than simplified assessments suggest.
