Herein, we analyze the damping that arises from the soil-water interac
tion within nearly-saturated soils during steady-state vibrations. Bas
ed on the two-phase theory of Biot, we analytically derive the spectra
l response of a column made of nearly saturated poroelastic soils as a
function of soil stiffness, degree of saturation, porosity, specific
gravity, and permeability. A parametric study determines the influence
of these parameters on the fundamental period, maximum amplitude, ene
rgy dissipation, damping factor, and maximum pore pressure. In departu
re from common beliefs in soil dynamics, we conclude that the soil-wat
er damping within fully-saturated sands is not negligible compared to
structural hysteretic damping.