NZ Geotechnical Society (NZGS) 22nd Symposium 2025 Auckland
Liquefaction assessment in pumiceous deposits is an ongoing hot topic for researchers and geotechnical engineers in New Zealand. The fragile nature of the pumice particles makes pumiceous deposits vulnerable to particle crushing during conventional penetration testing procedures (e.g. cone penetration test, CPT) as well as dynamic loading. Therefore, conventional liquefaction triggering procedures that are based on correlations developed on case studies of hard-grained soils are not applicable to pumiceous deposits. In most cases, pumiceous sands have been found to have higher liquefaction resistance compared with hardgrained soils. Despite receiving notable research attention in the past two decades, assessment of liquefaction hazard in areas underlain by pumiceous soils is still inconsistent in everyday geotechnical practice. In our experience, pumice content determination and cyclic triaxial testing are typically undertaken for high-value and/or high-risk projects. However, for medium- to small-sized projects, geotechnical engineers may often adopt liquefaction assessment methods developed for hard-grained soils for pumiceous sites, potentially resulting in conservative solutions. This is often the case due to a myriad of reasons, such as balancing the cost of more advanced site investigations against perceived benefits, tight design programmes, and the experience of geotechnical engineers in undertaking more advanced investigations.
This discussion paper focuses on reviewing relevant research findings and current engineering guidelines on liquefaction assessment in pumiceous deposits. Ultimately, the paper aims to inspire constructive symposium debates about further guidance that can be provided to assist geotechnical engineers in characterising pumice sites even for small-to-medium scale projects. Suggestions are provided for potential geotechnical characteristics (specifically, threshold for fines content, pumice content, and specific gravity-pumice content correlations) that might provide clearer direction for scoping more advanced investigations and suitable liquefaction assessment procedures. Additionally, the paper discusses the potential for a more concise project risk framework.
