Geotechnical issues in seismic assessments: When do I need a geotechnical specialist?

The Canterbury Earthquakes Royal Commission recognised the need for greater focus on geotechnical investigations and assessment as part of engineering projects. The current revision of the New Zealand Society for Earthquake Engineering’s (NZSEE) guidelines document “Assessment and Improvement of the Structural Performance of Buildings in Earthquakes” June 2006 (Red Book) has no specific guidance for consideration of geotechnical aspects as part of a detailed seismic assessment and mitigation design for an existing building. NZSEE and the Ministry of Business, Innovation and Employment (MBIE) have addressed this by adding a chapter on geotechnical assessment. That geotechnical chapter (C4) is in draft and will be formally released in early 2017. In parallel to Chapter C4, the New Zealand Geotechnical Society (NZGS) and MBIE are developing guidelines to help summarise current practice in earthquake geotechnical engineering. These documents provide guidance on geotechnical input, but only provide limited guidance on the question of when to get a geotechnical specialist involved. This paper highlights issues in building assessment and design which can arise if geotechnical issues are not adequately considered. These include; variability of ground conditions, “brittle” behaviour of some foundation types, changes in soil- foundation performance under cyclic loading, and soil-foundation-structure interaction.