The adoption of design motions with a 2% rate of exceedance in 50 years in the National Building Code of Canada (NBCC 2005) has had a major impact on geotechnical engineering practice in Canada. The peak ground accelerations were doubled compared with the previous motions which had an exceedance rate of 10% in 50 years. The increase in accelerations has had a huge effect on assessments of liquefaction potential and slope stability, because the methods of assessment in common use depend on peak ground acceleration. This paper describes typical problems encountered in Canadian practice with use of the low probability motions and describes some measures for alleviating the impact on design, while maintaining the code objective of life safety
