ORIGINAL PAPER Judgement of rapid drawdown conditions in slope stability analysis Xiao-ping Hou 1 & Sheng-hong Chen 2 & Isam Shahrour 3 Received: 5 June 2020 /Accepted: 17 April 2021 # Springer-Verlag GmbH Germany, part of Springer Nature 2021 Abstract The rapid drawdown of reservoir may have a significant impact on the stability of adjacent slopes. In order to avoid potential risks, it is important to calculate the change of slope safety factor prior to the reservoir operation. The finite element method is used to analyze the transient seepage during drawdown, and then the pore water pressures are introduced into the stability computation based on limit equilibrium to obtain the transient safety factor. Computations show that for slopes with a specific geometry, the safety factor ratio depends on the parameters K/(S y v) (where K is the permeability coefficient, v is the drawdown speed, and S y is the specific yield) and c′/(γHtan ϕ′) (where c′ and ϕ′ are the effective cohesion and friction angle, γ is the soil unit weight, and H is the slope height). By considering a wide range of K/(S y v) and c′/(γHtan ϕ′) values and different slope geometries, the percent reduction in critical safety factor of slope during drawdown relative to that during steady-state seepage is obtained. The drawdown condition that causes a large percent reduction in safety factor is judged as a rapid drawdown, and the opposite is a slow drawdown, which does not affect the slope design. This paper presents a series of charts for engineers and designers to judge rapid and slow drawdown conditions. Before the reservoir operation, the appropriate drawdown speed is selected according to the charts to ensure a slow drawdown for adjacent slope, while in the slope stabilization design, only the rapid drawdown stability analysis needs to be performed. Keywords Slopes . Rapid drawdown . Transient seepage . Stability analysis . Safety factor Introduction Engineering practice shows that the reservoir drawdown con- ditions may have an important impact on the stability of adja- cent slopes (International Committee on Large Dams 1980; Lawrence Von Thun 1985; Paronuzzi et al. 2013; Sun et al. 2017). Indeed, during a rapid reservoir drawdown, the pore water in soil cannot drain at the speed of the reservoir draw- down. Consequently, the phreatic surface in the slope could become higher than the water level in the reservoir, resulting in transient seepage flowing out of the slope. The pore water pressure generated by transient seepage reduces the shear strength along the potential sliding surface, which probably destabilizes the slope. In practical engineering, precautions should be taken to prevent slope instability due to a rapid drawdown (Abramson et al. 2002). Previous studies on the impact of the reservoir drawdown speed on slope stability classified the drawdown conditions into rapid drawdown and slow drawdown (Liu et al. 2005; Berilgen 2007; Sun et al. 2017, 2018). Since rapid drawdown is detri- mental to the stability of the slope, it must be taken into account in the stabilization design of dam and reservoir bank slopes (US Army Corps of Engineers 2003). The ratio K/(S y v) was com- monly used in the previous studies as the indicator for judging rapid drawdown (Mao 2003), where K is the soil permeability coefficient (unit: m/d), v is the speed of water-level drawdown in the reservoir (unit: m/d), and S y is the specific yield, also known as drainage porosity, which is a ratio less than or equal to the effective porosity, indicating the ratio of the volume of water that an unconfined aquifer will release from storage by gravity to the total volume of saturated aquifer. Experiments show that when K/(S y v) ≤ 0.1, the lowering of the phreatic surface is very small relative to the water-level drawdown, so the drawdown is judged to be rapid; when K/(S y v) > 10, the * Xiao-ping Hou xiaopingyatou@163.com 1 Key Laboratory of Ministry of Education for Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Northwest A&F University, Yangling 712100, China 2 State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China 3 Laboratoire Génie Civil et géo-Environnement, Université Lille 1, 59655 Villeneuve d’Ascq, France https://doi.org/10.1007/s10064-021-02253-y / Published online: 20 April 2021 Bulletin of Engineering Geology and the Environment (2021) 80:4379–4387