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Cyclic undrained triaxial tests were carried out on saturated and unsaturated loose specimens of three fine-grained materials. The results confirm that, as the saturation degree decreases, liquefaction resistance sharply increases. In the interpretation of results, the first step was the assessment of the reliability of the use of the 5% strain amplitude criterion to define the attainment of liquefaction. With this aim, the tests were analysed assuming that the soil behaves as an equivalent viscous material. It is shown that the characteristic values of viscosity defining the start and completion of liquefaction depend on soil grading, increasing as the coefficient of uniformity increases. The equivalent viscosity corresponding to a condition of double strain axial amplitude of 5% is fairly constant for all soils. Then, starting from the observation of the relationship between the final volumetric strain and the initial degree of saturation, a theoretical interpretation of the tests is reported. It is shown that the behaviour of the tested sands can be interpreted quantifying the thermodynamic specific energy spent to reach liquefaction, considering the contribution of all phases (soil, water and air). The position of the liquefaction resistance curve depends on the volumetric component of such a specific energy.

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Liquefaction is a phenomenon marked by a rapid loss of soil strength and stiffness, which generally occurs in loose saturated sandy deposit during earthquake because of the generation of excess pore water pressure.Several experimental researches concluded that liquefied soil behaves as a fluid during ground movement, but after the earthquake motion ceases, due to the dissipation of excess pore water pressure and soil dilatancy, the liquefied soil recovers its initial stiffness and returns to behave as a solid. Such a change of state can be analysed by considering the soil as an equivalent visco-plastic material, characterized by an apparent viscosity () that changes during the cyclic loading. Following this approach, the authors analysed the results of some cyclic undrained triaxial tests carried out on reconstituted and undisturbed (frozen) specimens of sandy and gravelly soils in terms of apparent viscosity decay law (-Ncyc), highlighting the relevance of  as physically based parameter for the correct identification of the liquefaction triggering. The experimental results confirm that the apparent viscosity decreases with the increase of the shear strain rate and highlight that the flow characteristics of liquefied soils (consistency coefficient and liquidity index) are affected by both grain size distributions and soil state condition (relative density and confining stress).

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