Seismic site response analyses of ideal medium-stiff soil deposits

This paper focuses on the stratigraphic effects of medium-stiff soil deposits and describes the results of 1D non-linear seismic response analyses carried out using the computer code Deepsoil. A set of 30 soil profiles were considered varying the bedrock depth H in the range of 20-60 m and assuming a compliant bedrock. A set of horizontal acceleration time histories, recorded on almost flat rock outcropping sites, with amplitude, energy and frequency content varying in wide intervals were used as input motions. The parameters defining the backbone curve have been calibrated using shear modulus reduction curves available in the literature for soils of low to medium plasticity. The analysis results are presented in terms of stratigraphic and spectral amplification factors obtained from the motion computed at ground surface. The analysis results show that the stratigraphic amplification factor is significantly influenced by the peak outcrop acceleration and bedrock depth while it is almost unaffected by the data sorting with reference to earthquake magnitude; accordingly, original empirical predictive models have been proposed to estimate the stratigraphic amplification factor as a function of the most influencing parameters.