On the apparent viscosity of granular soils during liquefaction tests

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 (eta) 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 (eta-N-cyc), highlighting the relevance of eta 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 conditions (relative density and confining stress).