SEISMIC DESIGN WORKING GROUP

22 February 2006
NZGS Guidelines on Geotechnical Seismic Design

The NZGS Biennial Symposium “Earthquakes and Urban Development” took place in Nelson last week. It was well attended with over 200 delegates, mainly geotechnical engineers and engineering geologists from consultancies and GNS. The proceedings run to 476 pages.

During the symposium we held a panel discussion “Geotechnical Seismic Design Standards”. The main impetus for this discussion was the impending replacement of NZS 4203 by NZS 1170. While far from complete, NZS 4203 gave some useful guidance to geotechnical practitioners including a specific rule for “equivalent static method of analysis” of earth retaining structures. However, NZS 1170.5 states that seismic design of soil retaining structures and civil structures including dams and bunds is outside the scope of the standard and also states that the effects of slope instability and liquefaction are not addressed.

When 1170.5 replaces 4203 a serious vacuum will exist. Even with the very limited guidance given in NZS 4203, we are aware that there is significant and undesirable variability within geotechnical earthquake engineering in New Zealand. Ad hoc attempts are being made by individuals and organisations to interpret 1170.5 for geotechnical design in ways that were perhaps never intended by the authors of that standard.

By show of hands, the meeting unanimously endorsed the motion that the Society should form a working group to prepare a set of guidelines to assist geotechnical practitioners with geotechnical seismic design.

Some urgency was seen as being necessary with a desire to avoid a long and protracted process. To this end, a two stage approach was suggested with the first stage being an interpretation of 1170.5 with appropriate factors for geotechnical design, to be established in conjunction with the authors of 1170.5, and establishing the relevant philosophy for geotechnical seismic design. A second stage would be to develop more specific guidelines for a range of geotechnical systems.

We are well aware that standards for geotechnical seismic design are under development world wide, notably Eurocode and ISO. We have no desire to “reinvent the wheel” but there is a need to adapt such initiatives to fit the New Zealand environment and philosophy.

The meeting also strongly endorsed the view that “guidelines” are far more desirable than “codes” or “standards” in this area. Flexibility in approach is a key part of geotechnical engineering and the technology in this area is rapidly advancing.

The working group will include a representative from the 1170.5 panel, SESOC, and NZSEE as well as NZGS selected experts in seismology and earthquake geotechnical engineering.

 

 

 

 

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