During an earthquake, excess pore water pressure generation in saturated silty sands causes a reduction in shear strength and even liquefaction of the soil. A comprehensive experimental program consisting of undrained cyclic simple-shear tests was undertaken to explore the key factors affecting the energy-based excess pore water pressure generation models for non-plastic silty sands. The examined influencing factors were non-plastic fines content (less than and greater than the threshold value congruent to 25%), packing density, vertical effective stress, applied cyclic stress ratio, and soil fabric. The relationship between excess pore water pressure ratio and dissipated energy per unit volume was found to be mainly dependent on the relative density and fines content of soil, whereas the cyclic stress ratio, initial vertical effective stress, and soil fabric (i.e. the reconstitution method) appeared to have a minor impact. A revision of the original energy-based model developed for clean sand by Berrill and Davis was proposed to improve prediction accuracy in terms of residual excess pore water pressures versus normalised cumulative dissipated energy. Nonlinear multivariable regression analyses were performed to develop correlations for the calibration parameters of the revised model. Lastly, these correlations were validated through additional cyclic simple-shear tests performed on different silty sands recovered at a site where liquefaction occurred after the 2012 Emilia Romagna (Italy) earthquake.

Fully liquefied soils behave like viscous fluids, and models developed within the framework of soil mechanics fail to catch their behaviour on the verge of liquefaction or after it. Several research works have shown that modelling the liquefied soil as a fluid is physically more convincing. Such an equivalent fluid can be characterised via an apparent viscosity (g) (sharply dropping when liquefaction is triggered) which can be modelled as a power law function of the shear strain rate (pseudo-plastic fluid), depending on two parameters: the fluid consistency coefficient (k) and the liquidity index (n). With this approach, it is possible to consider a simple correlation between the equivalent viscosity and pore pressure increments independent on the equivalent number of cycles, whose parameters can be calibrated from the results of stress-controlled laboratory tests. The paper investigates the effect of some relevant experimental factors (effective vertical stress, stress path, frequency and waveform of the applied cyclic load, soil fabric and pre-existing shear stress) on the apparent viscosity of soils during their transition from the solid to the liquefied state, and therefore also on the pore pressure increments generated by the stress path. To do that, the results of stress-controlled laboratory tests performed in a sophisticated simple shear apparatus, along with published data, have been interpreted in terms of the apparent viscosity. Simple correlations in terms of viscosity-based pore pressure generation and pseudo-plastic behaviour are proposed and confirmed from the results of 1D non-linear site response analysis for the (c) 2023 Production and hosting by Elsevier B.V. on behalf of The Japanese Geotechnical Society.