2023 – The Importance of Water Compressibility in Seismic Analyses of Concrete Gravity Dams

Sam Lalli, Francisco Lopez, Michael McKay, Stella Harrison

Finite Element Analysis (FEA) of concrete dams is becoming increasingly more complex in order to better capture their behaviour under both static and seismic loading. One paramount aspect of this is the modelling of the dam-foundation-reservoir interaction during earthquakes, as the generated hydrodynamic pressures against the dam wall have a significant impact on the total response of the system.

Historically, hydrodynamic pressures have been estimated assuming that the water of the reservoir is an incompressible fluid. However, several studies have shown that ignoring the compressibility of water can either underestimate or overestimate the seismic response of concrete dams. Compressibility of fluids is often neglected in FEA without an understanding of the implications of such an assumption.

This paper presents a practical 4-step procedure for assessing the influence of water compressibility on the seismic response of a concrete gravity dam. The procedure can be followed to make an initial assessment on whether compressible water should be used in the nonlinear time- history FEA or whether the more simplistic incompressible water approach could be used without significantly impacting the seismic response.

A worked example of a generic concrete gravity dam illustrated the use of the 4-step procedure, demonstrating that the influence of water compressibility was relatively minor.
Through a sensitivity analysis it was found that the influence of compressibility of water on gravity dams is most significant in dams with a low ratio of the fundamental frequency of the reservoir to that of the dam alone (Ωr), in relatively flexible dams, dams in relatively rigid foundations, dams in narrow valleys and FE models considering a massless foundation.

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