Pre-loading effect on site response: Site amplification and soil properties mismatch

A field experiment with a temporary embankment loading gave the opportunity to evaluate changes of both soil properties and site response, due to a low stiffness shallow soil model. The Quarter Wave-Length approximation is mainly used to quantitatively indicate the expected changes. The original Vs models derived by seismic prospecting methods (Raptakis. Soil Dynamics and Earthquake Engineering 2012;34: 69-77), similarly performed prior to and after preloading, were used for site amplification estimates as a function of frequency. The quantitative comparisons of either results derived by field techniques combined to each other or those of each technique separately before and after preloading, reveal changes in site response showing a clear de-amplification after preloading but of lower order than stiffness increase. This fact may have beneficial manpower and cost consequences on the acquisition of the most “detailed” and “reliable” Vs input model for site response studies. Among others, the fact that a significant disparity between amplifications is observed only after preloading indicates that changes in the soil properties refer to the ground hypotheses for which the exploration tools present by principle limitations and drawbacks.