In Situ Determination of Soil Stiffness and Damping

Determination of in situ dynamic soil properties is fundamental to the prediction of the seismic behavior of foundations and soil embankment structures. Both elastic (stiffness) and inelastic (damping) values are required for computational analysis. To be of value to engineers, the geophysical inversion should employ the same soil model as used in the dynamic analysis software. Current engineering practice employs a Kelvin-Voigt model (spring in parallel with dashpot). The relevant wave equation is a third-order partial differential equation. This paper demonstrates how to collect in situ field data and solve for stiffness (scaled shear) and damping values by a method consistent with this constitutive model. Measurements of seismic wave amplitude decay and velocity dispersion are simultaneously inverted for the required stiffness and damping values. These in situ stiffness and damping values are directly comparable to those obtained by resonant column measurements in the laboratory. Furthermore, the results may be directly input into currently available engineering software to provide values of stiffness and viscous damping. This paper includes both synthetic (finite difference) and field data examples that illustrate the method.