Geotechnical assessments for design of foundations in lower-seismicity liquefaction prone areas – a case study

M. C. McLean, B. O’Loughlin
ENGEO, Tauranga.

ABSTRACT

This paper presents a case study of a Ministry of Education (MoE) building project on potentially liquefiable dune sands at Mount Maunganui, Tauranga. It presents our methodology for assessing liquefaction vulnerability including triggering response and step change behaviour, how these aspects were considered during design in compliance with NZS1170, the MoE Structural and Geotechnical Requirements (SGR), and how project challenges introduced by the release of the National Seismic Hazard Model (NSHM) during design were overcome.

Liquefaction triggering just beyond Serviceability Limit State (SLS1) meant that careful consideration and guidance needed to be given to the level of foundation and structural resilience. Our analyses determined that punching failure and deformations could be addressed to avoid collapse, however significant repair and/or rebuild would be necessary at both SLS2 and Ultimate Limit State (ULS), requiring a resilient solution incorporating shallow ground improvement and a robust ground beam foundation system.

This paper presents the liquefaction and soil structure interaction analyses undertaken to develop a solution to increase seismic resilience and safeguard the building from potential building code updates. It also discusses how achieving minimum code requirements may result in low levels of seismic resilience, and why assessing triggering response and step change behaviour is necessary to inform robust geotechnical and structural considerations for foundations and ground improvements that deliver an appropriate level of seismic resilience.

Liquefaction triggering and foundation design in lower seismicity areas is a relatively common but less so talked about challenge for consultants. With increasing regulatory requirements being introduced by building consent authorities to improve resilience, increasing focus is being placed on triggering assessment to determine liquefaction vulnerability and foundation resilience. This paper introduces the approaches taken, lack of clarity in existing codes and guidelines, and presents the benefits of adopting MOE’s SGR for foundation design resilience for the project.