The paper provides the methods for evaluating the strength parameters of earth dams under forced oscillations. The methods of solving the problem are based on the expansion of sought for solution in terms of eigenmodes of elastic structure oscillations. Linear steady-state and unsteady forced oscillations of three different earth dams were studied with account of structural heterogeneity and viscoelastic properties of structure material under various dynamic effects. To describe the viscoelastic properties, the Boltzmann-Volterra hereditary theory of viscoelasticity is used. The results of investigations made it possible to reveal a number of effects that arise under forced oscillations in a dam in the pre-resonant, resonant, and post-resonant modes of oscillations.