Stiffness degradation, G/G0, curves of Champlain clay at St-Adelphe, Quebec, and the associated variation of its damping ratio with shear strain are constructed in this study using the new combined triaxial simple shear (TxSS) apparatus. The apparatus offers the ability to obtain the stiffness (G/G0) and damping ratio of soils over a wide strain spectrum from 0.001% to 10%. The value of the small-strain stiffness of the tested clay is further confirmed through another series of piezoelectric ring-actuator technique (P-RAT) tests. Although, the stiffness degradation curve of the tested clay follows to some extent traditional curves suggested in the literature, the examined Champlain clay exhibits a different trend with respect to hysteresis damping, especially at large strains (>1%), and available analytical models could not successfully predict the damping behavior of the Champlain clay at such a strain level. A new constitutive model is therefore presented as a modification of the original Sig4 model considering the pore-water pressure built up with shear strain. Stiffness degradation and damping ratio versus shear strain curves of Champlain clays estimated using the proposed soil model are similar to their experimentally determined counterparts even at large shear strains where other models tend to misjudge the damping behavior of the clay.

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The prediction of seismic ground response is conditioned by the knowledge of each material behavior of soil deposits. The recourse to plasticity criterion to simulate cyclic behavior of soils under seismic loading is becoming more realistic. In this study, an elasto-plastic constitutive equation is cast within the framework of one dimensional finite element (FE) soil column model to account for the spatial and material nonlinearity of the secant shear modulus. To account of the spatial non linearity, shear modulus is written in terms of rigid base shear modulus and height of the soil column, while for material nonlinearity, the shear modulus degradation is deducted by the application of the isotropic evolution of the Von Misès criterion. Obtained results proved the efficiency of the proposed methodology and the predictive capability of the elaborated elastoplastic model which captures both small- and large-strain behaviors. They likewise highlight the important roles that play the spatial and material shear modulus variation in the prediction of the seismic soil responses.