An experimental study on the apparent viscosity of sandy soils: from liquefaction triggering to pseudo-plastic behaviour of liquefied sands

The complete loss of soil strength and stiffness, which occurs during soil liquefaction, marks a change of state of the soil that switches from solid to liquid. In particular, several researchers reveal that the soil behaves as an equivalent visco-plastic material, characterized by an apparent viscosity (η). The paper aims to show the large potentiality of the apparent viscosity in the study of liquefaction. To do that, a dataset of already published undrained cyclic triaxial tests on five different sands has been processed according to a viscous perspective. Firstly, the research shows the relevance of η as a physically based parameter for the correct identification of the liquefaction triggering. Additionally, the experimental data confirm that the relationship between the apparent viscosity and the shear strain rate is a power law function as that characterizes non-Newtonian fluids. Such relationship allows to study the behaviour of liquefied soils, as long as the two parameters, k and n on which the relationship depends, are known. However, the direct dependence observed between two parameters allows to simplify the calibration procedures, implying the calibration of only one parameter (k), which would seem to be linked to the soil capacity (CSR). Finally, the results of laboratory tests have been extended to those of a nonlinear dynamic response analysis of a real site, located in Pieve di Cento (Italy), affected by extensive liquefaction phenomena during the 2012 Emilia Romagna earthquake. Starting from the expected value of CSR at each depth, k and n have been easily found through the proposed correlations. The good agreement of the calibrated pseudo-plastic law with the results of the dynamic analysis confirms the reliability of the proposed approach.