  We consider the effect of an array of plates or beams over a semi-infinite
elastic ground on the propagation of elastic waves hitting the interface. The
plates/beams are slender bodies with flexural resonances at low frequencies
able to perturb significantly the propagation of waves in the ground. An
effective model is obtained using asymptotic analysis and homogenization
techniques, which can be expressed in terms of the ground alone with effective
dynamic (frequency-dependent) boundary conditions of the Robin's type. For an
incident plane wave at oblique incidence, the displacement fields and the
reflection coefficients are obtained in closed forms and their validity is
inspected by comparison with direct numerics in a two-dimensional setting.
