Keywords: concrete dams, hydrodynamic pressures, dynamic response, amplification, equivalent springs.

It is widely accepted that the dynamic response of any retaining system is not yet clearly understood mainly due to the dynamic interaction between the system and the retained material. Especially, the case in which the retained material is liquid (e.g., water behind dams) is of much more greater interest for engineers, as during a seismic excitation significant hydrodynamic pressures may be developed.

The dynamic response of various types of dams which retain a semi-infinite deposit of water has been examined in the past analytically, numerically and experimentally. Until now the design of dams (either rigid or flexible) is based on the method described in the pioneer work of Westergaard, in which: (a) the dam is considered rigid, (b) water is considered incompressible, and (c) water extends to infinity (theoretically) behind the dam, thus, forming a semi-infinite deposit. The aforementioned assumptions lead to increased seismic pressures acting upon the dam. Consequently, they lead to rather conservative designs, thus, their efficiency must be re-examined in order to achieve a more realistic assessment of the hydrodynamic distress of dams and to obtain more economical designs. Especially when a concrete dam is founded on soft soil, the dynamic interaction between the dam the reservoir and the soil becomes much more complex, and cannot be realistically simulated by such simplified methods.

In the present study, the dynamic interaction between a concrete dam, the retained water, and the underlying soft soil is investigated. Initially, the dynamic pressure distributions are calculated in the case of resonance, whereas the resultant water thrust and moment at the dam base are evaluated as functions of the imposed steady-state frequency. Subsequently, the water is replaced by a pair of springs to simulate the transitional and rotational response of the reservoir. The stiffness constants of these springs are estimated by approximate empirical formulas that have been developed in this work. Finally, the distress contours in the dam body are presented for various cases of the reservoir depth and the underlying soil layer stiffness. The results justify the necessity for a more elaborate consideration of the aforementioned dynamic interaction complex phenomena and their impact on the seismic design of concrete dams.

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