SITE RESONANCE FREQUENCY ASSESSMENT THROUGH DYNAMIC LOAD TESTING (DLT): EARLY RESULTS OF A TEST CARRIED OUT IN THE EXPERIMENTAL SITE OF FIVIZZANO (MS, ITALY)

The assessment of the local seismic response is one of the fundamental phases when it comes to defining the levels of seismic hazard at a detailed scale. these are typically known as seismic zonation studies. the estimation of the site specific resonance frequency is a parameter of relevant interest within such studies as it is associated with the maximum seismic amplification which has to be related to the frequency of resonance of buildings. the ambient noise based methods and other experimental methods for measuring this parameter are not always applicable because they are strongly influenced by contrasts of impedence and by lateral subsurface heterogeneities. Furthermore, they sometimes present limits that are not always clearly known. On the other hand, the classic analytical approach that makes use of integrated and multidisciplinary Investigation methods, which estimate the local amplification parameters, mainly in terms of amplification factors and response spectrums, currently presents uncertainties and limitations. these are essentially associated with the difficulty of modelling the mechanisms of propagation of the seismic waves within a 3D system. the authors of this paper propose the experimental measurement of the site resonance frequency by means of Dynamic Load Testing (DLT). This is done by presenting the operative procedures, the data processing and the results obtained from a test undertaken at the experimental site of Fivizzano (Massa-Carrara, Italy) where a detailed study of the local seismic response was also undertaken. the test involved the delivery of sinusoidal forces with maximum amplitude of 20 kN and a range of frequencies between 0 and 15 Hz by means of an electro-mechanical generator. Simultaneous seismic monitoring was undertaken through a multichannel acquisition system. Velocimeters were placed on the ground and in a nearby strategic building. the measured frequency of resonance was compared to the predicted value obtained from empirical-analytical solutions and application of horizontal to vertical spectral ratios (HVSR) method and to the value measured by a seismic station equiped with two coupled accelerometers, one at the surface and one at depth; these sensors allowed an accurate analysis of the effects of the drift deposits overlying the bedrock, that is the transfer function of the stratigraphic column at the site.