Seismic amplification factors for low plasticity medium-stiff soil deposits

The seismic response of low plasticity medium-stiff soil deposits has been investigated through non-linear one- dimensional analyses carried out with reference to a set of 30 soil profiles, with bedrock depths varying in the range 20–60 m. More than 300 horizontal acceleration time histories recorded on almost flat rock outcropping sites have been used as input motions with amplitude, energy and frequency content varying in wide intervals, which should lead to outcomes of general validity. In the modelling of soil hysteretic behaviour, the backbone curve was described by a modified hyperbolic model calibrated against well-established stress-dependent re- lationships describing the variation of shear modulus and damping with the strain level. 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 magnitude sorting of the data; accordingly, original empirical predictive models have been proposed to estimate the stratigraphic amplification factor as a function of the peak outcrop acceleration and bedrock depth. Also, it has been demonstrated that, regardless of the amplitude of the input motion and whatever is the sorting of data with reference to magnitude and bedrock depth, a direct proportionality exists between the stratigraphic and the spectral amplification factors, implying an almost constant value of the spectral shape ratio. Finally, the relevance of using stress-dependent shear modulus and damping curves has been pointed out to allow a reliable evaluation of the stratigraphic and spectral amplifications.