Unit 7A.2: Volcanic Ashes is a reference document developed) to provide guidance on the assessment and management of slope stability in volcanic ash soils, particularly in the central North Island of New Zealand.
The document outlines the unique geotechnical properties of volcanic ash deposits, which includes their ability for high water retention, sensitivity, and variable strength, which present distinctive challenges for slope stability. It covers the geological setting, characteristics, and distribution of volcanic ash soils, as well as typical failure modes, causes of instability, and recommended investigation and analysis methods. Attention is given to the behaviour of sensitive ash layers, the impact of rainfall and groundwater, and the importance of site-specific assessment.
The guidance also details best practices for analysis, design, and mitigation of slope instability in volcanic ash terrains. It provides practical recommendations for geotechnical investigations, laboratory testing, and parameter selection, and discusses suitable mitigation measures such as drainage, structural solutions, and exclusion zones. The document is supported by case studies from the Bay of Plenty and Tauranga regions, illustrating real-world failures and lessons learned.
Comments on Unit 7A.2 should be addressed to the NZGS secretary (secretary@nzgs.org) and should be forwarded no later than Friday 16 January 2026. PDF Markups are preferred where this is possible.
Thank you to the authorship team of David Sullivan, Harshad Phadnis, David Milner and Clive Anderson!