Nonlinearity and strength in 1D site response analyses: a simple constitutive approach

When performing seismic site response analyses in the design practice, one of the main issues consists in finding a compromise between the limited availability of site-specific experimental data to characterise the cyclic soil behaviour and the necessity of a proper constitutive representation of such behaviour, including nonlinearity, hysteresis and strength. This paper presents a new constitutive approach for 1D nonlinear soil models, capable of describing such features with a simple and easy to calibrate constitutive equation, requiring the definition of few parameters, all derived from conventional field and laboratory data. The main element of novelty in the proposed approach consists in recognizing that the soil shear strength is a key ingredient not only to limit the maximum shear stress that the soil can experience at large strains, but also to describe the dependence of the nonlinear soil properties on mean effective stress and plasticity index, as observed in the medium strain range. Based on a thorough comparison with laboratory data, it is shown that the proposed model provides a very good description of the soil behaviour, in the whole strain range. Its application in site response analyses is further validated on centrifuge data and verified against other nonlinear constitutive soil models. Finally, using a well-characterised site in Italy as reference, it is demonstrated that an erroneous prediction of the soil shear strength can lead to a misinterpretation of the amplification phenomena within the soil deposit, together with a possible gross underestimation of the actual surface acceleration.