Evaluation of the upstream slope stability in earth dams under rapid drawdown conditions (Case study: Shaharchay dam)

Rapid drawdown is one of the key factors that can threat the stability of earth dams and neglecting it can lead to serious damages. This factor causes the earth dam break by inducing slip potential in upstream slope. Rapid drawdown occurs when the water level in reservoir of an earth dam reduces in a short period of time with a high rate, whilst the water level in the core cannot follow the water level of the reservoir. This phenomenon could happen for any reason, such as an earthquake, sudden formation of large cracks, arching phenomenon, flood, withdrawal of large amount of water for various purposes and etc. Due to discharge and decrease of water level in the reservoir, the hydrostatic pressure on the external surface of the upstream slope fades away while the balancing hydrostatic pressure inside the dam still remains. This remained hydrostatic pressure is one of the most important factors of slipping risk on the upstream slope. Nevertheless, breakage of a dam not only eliminates the funds spent for its construction, entirely, but also wastes the collected water in the dam without deployment and above all, destroys many facilities and people property and also brings casualties. On the other hand the majority of dams in our country are among earth types which are exposed to rapid drawdown condition for various reasons. Therefore, a comprehensive study in this field is needed and a definition for the drawdown condition has to be defined, in which the stability of the dam is not endangered. In this research, stability of earth dams exposed to rapid drawdown is studied by employing finite element method (using Geo studio software) which is one of the most accurate and recent methods for the analysis of earth dams. For this purpose, the Shahrchay dam’s cross section was selected as a case study and modeled by SLOPE/W and SEEP/W from the Geo-Studio software package. Then, numerical model was calibrated by the instrument data using back analysis technique. The analysis was conducted by considering circle failure planes for the condition of water drawdown from normal water level up to the bottom level of water intake. In the performed analysis, the effects of changes in hydraulic conductivity coefficient of the core, rate of water withdrawal in the reservoir and also impact of upstream drainage on the rate of water drawdown in the core, were evaluated. Also, water surface level in the core were calculated and plotted for various states. The results showed that safety factor in the upstream slopes decreased as the water withdrew from the reservoir. However, the safety factor increased after a certain time by complete withdrawal of water through the core. When the discharge rate is higher, the safety factors variation graph is more V-shaped like and critical safety factor occurs at earlier time. Also, when the full discharge of reservoir is necessary in case of emergency, it is recommended that the Shahrchay dam be discharged up to 32 cm per day to prevent the risk of the upstream slope failure. According to analyzing different drawdown conditions with different functions (linear, step and variable-speed nonlinear function), changing of the linear drawdown function to step drawdown function leads to 5 percent increase in the safety factor of the upstream slope, while altering the linear drawdown function to non-linear drawdown function increases safety factor of the upstream slopes by 7 percent, approximately. Studying changes in the core conductivity coefficient observed that when the core conductivity coefficient is equal to 5×〖10〗^(-10) m⁄s , the water level inside the core won’t drop, even at very low discharge rate. This can be very dangerous and cause instability in the upstream slope. Also, it was observed that while locating the upstream drainage system, the drain space is enclosed inside the dam, and considerable amount of pore water pressure is dissipated as the water level is the reservoir drops. By locating the drains 1, 2 and 3, the safety factor increases, respectively, up to 10, 13 and 7 percent. Through comparing horizontal and vertical drainage systems, it was found that the horizontal drainage system is 6 percent more efficient for draining water from the dam body and therefore reducing pore water pressure in drawdown condition.