Mechanical characterization of heterogeneous soils with surface waves: experimental validation on reduced-scale physical models

The characterization of heterogeneous soils using common geotechnical techniques often proves impossible when the size of the heterogeneity is larger than a few tens of centimetres. Geophysical investigation techniques based on seismic wave propagation can help engineers to characterize the mechanical properties of such materials. In this paper, both refracted and surface waves are used to estimate the mechanical properties of an equivalent homogeneous medium. A summary of the main results obtained numerically using finite-element computations and homogenization theory is presented. It is shown that, for first-mode surface-wave wavelengths larger than 7.5 times the nominal size of the heterogeneity and within certain heterogeneity concentration ranges (up to 50% for matrix dominant soils), surface waves homogenize the soil in accordance with classical homogenization theory. To validate these numerical results, a reduced-scale model was built and seismograms, generated with a falling weight, were recorded. The phase-velocity dispersion curve of the generated surface waves is inverted in order to obtain the shear-wave velocity of the heterogeneous layer. The compressional-wave velocity is calculated by means of seismic refraction analysis. Velocities obtained on the reduced-scale model correspond to those predicted by homogenization theory from individual measurements of matrix and inclusion velocities.