On the apparent viscosity of granular soils during liquefaction tests.

Liquefaction is a phenomenon marked by a rapid loss of soil strength and stifness, which generally occurs in loose saturated sandy deposit during earthquake because of the gen eration of excess pore water pressure. Several experimental researches concluded that liq uefed soil behaves as a fuid during ground movement, but after the earthquake motion ceases, due to the dissipation of excess pore water pressure and soil dilatancy, the liquefed soil recovers its initial stifness 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 (η) that changes during the cyclic loading. Following this approach, the authors analysed the results of some cyclic undrained triaxial tests carried out on recon stituted and undisturbed (frozen) specimens of sandy and gravelly soils in terms of appar ent viscosity decay law (η-Ncyc), highlighting the relevance of η as physically based param eter for the correct identifcation of the liquefaction triggering. The experimental results confrm that the apparent viscosity decreases with the increase of the shear strain rate and highlight that the fow characteristics of liquefed soils (consistency coefcient and liquid ity index) are afected by both grain size distributions and soil state conditions (relative density and confning stress).