It is recognised that earthquake design of high cut slopes in hilly to mountainous terrain is important to ensure that the next generation of transportation routes have adequate resilience to provide access to communities following an earthquake event. There is very little guidance in New Zealand or worldwide on the seismic design of cut slopes. Research and development of guidelines for the seismic design of high cut slopes was carried out by Opus for the New Zealand Transport Agency. The guidelines propose a new performance based approach where the need for resilience underpins the design. The design loadings and approach are based on the importance of the route and the resilience expectations for that link, given the national resilience context. A new Resilience Importance Category (RIC) is proposed to guide the design approach. The guidance addresses a number of critical issues in the seismic design of cut slopes, such as selection of topographic amplification factors, the use of pseudo-static ground accelerations for design, and mechanisms of slope instability. The new principle of resilience based design has a focus on achieving resilience of access for communities, through consideration of the performance of cut slopes in earthquakes, and also the likely time taken for restoration of access. The proposed approach to account for topographical amplification effects is based on the research to date as well as specific numerical analyses, and could be refined as further research information comes available.
New Zealand has rugged and mountainous terrain and major transportation routes are often associated with high cut slopes. It has been always recognised that the resilience of transportation routes in New Zealand greatly depends on the performance of these high cut slopes in earthquakes. This has been distinctively demonstrated in the 14 November 2016 M7.8 Kaikoura earthquake event, by the significant cut failures and landslips that caused one of the primary road and railway transportation corridors in the South Island to close for many months (Figure 1).
Currently there is very little guidance available for the earthquake design of high cut slopes either in New Zealand or internationally. The potential for topographical amplification of earthquake shaking, and the observation of the large landslides that have affected transportation routes in earthquake events has raised the awareness of the need for research and development of guidelines for the seismic design of high cut slopes. The New Zealand Transport Agency engaged Opus International Consultants to carry out this research and the development of guidance.
The research objectives included review of the performance of high cut slopes in recent worldwide earthquakes, consideration of the influences of the distinctive aspects of New Zealand’s seismicity and topography, review of relevant recent research on topographical effects from New Zealand and overseas and limited numerical analyses on characteristic topographies in New Zealand, review of current design practice in New Zealand and overseas and development of guidelines for the earthquake design of high cut slopes in New Zealand. The New Zealand topography and Seismicity as well as the existing design guidance were presented by Brabhaharan et al 2015. This paper will focus on the other aspects of the research and the development of the design guidelines.