The design of permanent slopes for residential building development

Abstract

The NZ Building Code requires a minimum Factor of Safety (FOS) of 1.5 for permanent slopes in building developments. No further qualification is made on how to calculate this FOS or indeed if there are other more important requirements. NZ Territorial Land Authorities vary in their interpretation of the applications of the Code. For example, many require the conservative approach of a FOS ≥ 1.5 under full saturation of slopes regardless of soil conditions, geometry and geohydrology. However, this FOS requirement is not mandatory and alternative solutions may be accepted by Territorial Land Authorities. The Building Act allows for consents to be issued under section 36(2) for land stability. There are also parts of the Earthquake Commission Act which need to be taken into account. Much time has been wasted discussing consent applications on an ad hoc basis. What is needed is a consistent approach to stability assessments as agreed by Territorial Land Authorities and the geotechnical community.

This paper draws on the results of a questionnaire to Councils and geotechnical consultants throughout New Zealand proposed to promote a consensus of interested groups. The Hong Kong system is also considered as a basis in which the quality of technical data, economic risk and risk to life will be considered. Reference is also made to the Australian position on stability assessments and their risk approach. The results of this review will be used to promote revisions to the NZ Building Code and the required factors for the stability of permanent slopes.