Paper contribution

Advancing Slope Stability Practice in New Zealand: The NZGS Guidelines and their application in Greywacke

Eleni Gkeli; Nick Peters; Alan Wightman; Doug Mason

Proceedings
EUROCK 2026
Pages
745-750
Submitted
28 January 2026
Accepted
15 June 2026
Published
12 August 2026

Summary

Abstract

The New Zealand Geotechnical Society (NZGS), in collaboration with the New Zealand Natural Hazards Commission (NHC), is developing a modular Slope Stability Guidance series to support consistent and reliable assessment of landslides and slope instability across New Zealand. This paper presents Unit 7C.2 of the guidance series, which addresses slope stability in Torlesse greywacke, New Zealand’s most widespread basement rock and a material commonly associated with landslides triggered by rainfall and seismic loading. Torlesse greywacke is characterised by intense deformation, heterogeneous weathering, and closely spaced, variably persistent discontinuities, resulting in complex slope failure mechanisms that challenge conventional rock slope assessment approaches. The paper demonstrates how the guidance was developed, by synthesising recent New Zealand earthquake case histories, published laboratory test data, and practitioner experience. This approach resulted in practical, engineering‑focused guidance for characterising greywacke rock masses, interpreting failure mechanisms and selecting appropriate strength parameters for stability analysis. By addressing the challenges associated with typical site investigation constraints especially in urban environments, Unit 7C.2 streamlines slope stability assessments in New Zealand, while it enhances their reliability and consistency. The principles and approaches adopted in the development of the guidance could be applicable to other similar rock conditions and steep landslide prone terrain worldwide.

Contributors

Authors

  1. Eleni Gkeli New Zealand Geotechnical Society, New Zealand; Stantec, New Zealand ORCID 0009-0000-0152-8331
  2. Nick Peters Tonkin and Taylor, New Zealand
  3. Alan Wightman ENGEO, New Zealand
  4. Doug Mason WSP, New Zealand

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Works cited

References

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