Discussion on the allowable safety factor of slope stability for high rockfill dams in China

The allowable safety factors of slope stability for rockfill dams in Chinese design codes are defined irrespective of dam heights and are mainly based on the engineering experiences with respect to the rockfill dams which are lower than 200 m. It is well known that safety margins for different structures are not comparable when safety factors are taken as the measure of safety margin. Thus whether the current allowable safety factors are applicable to the rockfill dams with heights of over 200 m has been controversial in the engineering and academic field. Reliability is well known as a unified measure of safety margin for different structures. Therefore, the slope safety margins of rockfill dams with different heights can be compared in terms of slope reliability indices derived from probabilistic methods. Based on the statistical analysis of nonlinear shear strength parameters of rockfills from 14 Chinese dams, the slope reliabilities of fictitious rockfill dams of different heights designed with equal allowable safety factor are calculated and compared in this paper. It is shown that the slope failure risk of higher dams is larger than that of lower dams if the dams are designed with equal allowable safety factor rather than equal reliability. Therefore, from the perspective of maintaining consistent slope reliability levels among rockfill dams with different heights, the allowable safety factors should be prescribed in terms of dam height, i.e. the allowable safety factor for higher dams should be larger than that for lower dams.