Modelling of frictional soil damping in finite element analysis

In soil dynamics, the soil is often described as a viscous material. In a viscous
material however, the dissipated energy is assumed to be proportional to the wave frequency,
which is absolutely not applicable to soils. It is therefore better to use a concept of damping
based on dry particle friction. A non-viscous model based on this concept results in a
damping ratio that becomes constant for small deformations for both sand and clay, and is
also independent of frequency or shear strain amplitude. This behaviour corresponds with
laboratory measurements and requires only one damping parameter which can be obtained
from laboratory tests.
This model is implemented in Plaxis as a User-Defined Soil Model to analyse the problem of
a strip footing subjected to a dynamic load. The initial results are rather remarkable. . For
example, a Power Spectral Density plot of the velocities shows that not all frequencies seem
to be damped equally and at some distance the input frequency is not even present.