Reliability based seismic slope failure assessment of a high concrete face rockfill dam close to an active fault zone

This study assesses the earthquake-induced failure risks of the upstream and downstream slopes of a concrete face dam (CFRD), which is under the influence of a nearby active fault, using the reliability-based analysis method. For this purpose, peak ground accelerations (PGAs) that may occur at the dam site were calculated probabilistically and their contribution to the sliding risks of dam slopes was determined in accordance with the conditional probability theorem. Upstream and downstream slopes of CFRDs are usually conventionally designed as 1.3–1.5H:1V and 1.3–1.6H:1V, respectively throughout the world. Such slope design is considered sufficient for almost every case. The findings of this study are fully applicable and very important for the structural design of all concrete-faced rockfill dams (CFRDs) worldwide. Since, the slope design of CFRDs is similar and usually traditionally (1.3–1.5H; 1V for upstream slope and 1.4–1.6H:1V for downstream slope). However, this study proves that only conventional upstream slope design (1.3–1.5H:1V) can be safe for even very high peak ground accelerations (PGAs) occurring due to a nearby active fault, but contrary to expectations, downstream slope of CFRDs cannot be sufficiently safe and maintain its slope sliding safety when PGAs reach high levels. When the critical PGA level is exceeded, the downstream slope carries the risk of losing its stability with high probability, as emphasized in this study. Therefore, this study provides a new design strategy and extremely important information for dam design engineers which is to design a rather flatter downstream slope such as 1 vertical to 1.8–2.0 horizontal or, instead, to design a downstream rockfill berm supporting conventional design if CFRDs are to be built in high seismic hazard potential areas or close to active faults. Suggested such type of designs will be economical and sufficiently safe.