Assessment of the upstream slope failure of a dam due to repeated cyclic drawdown

Abstract Khlong Pa Bon Dam in Thailand underwent the greatest differential drawdown in impounded water in 2014. Unexpected deformation of the upstream slope of the dam was observed on June 27 in the year, after operation for 10 years. The drawdown was hypothesized as the possible cause of the slope deformation. The results from piezometers showed that the upstream slope remained partially undrained after sudden drawdowns. A rapid drawdown analysis confirmed that the movement did not occur due to the sudden drawdown of water in 2014. Back analysis revealed that the shear strength of the embankment slope was near its residual value at the time of failure, which was validated by the movement of inclinometers. Ring shear tests were used to determine the residual shear strength of the failure zone, and the results were validated by the finite element method in ABAQUS. Furthermore, the results of the numerical analysis demonstrated that strain softening was the major cause of the slope movement.