Soil Liquefaction: Mechanism and Assessment of Liquefaction Susceptibility

The basic mechanisms of earthquake-induced soil liquefaction are introduced by considering the shaking of a block on a thin granular layer, which mechanical behaviour is modelled with a hypoplastic constitutive model. If the block is founded on a dry cohesionless soil or drainage of the granular layer is fully allowed, the soil densifies and the block settles step-wise. On the other hand, if drainage is impeded pore pressure develops and effective pressure decays with increasing number of shaking cycles, until, depending on the initial density, either a quasi-stationary cyclic state is reached or the effective pressure vanishes (liquefaction). The coupled nature of dynamic problems involving soils is also shown by the results of the analyses, i.e. the motion of the block causes changes of the soil state which in turn affect the block motion. Investigations of soil liquefaction under dynamic earthquake-like excitation with a 1-g laminar box confirm the predicted behaviour. The same constitutive equation is applied to the numerical simulation of the propagation of plane waves in homogeneous and layered level soil deposits induced by a wave coming from below. Welldocumented sites during strong earthquakes are used to verify the adequacy of the hypoplasticity-based numerical model for the prediction of soil response during strong earthquakes. It is concluded that liquefaction susceptibility during strong earthquakes can be reliably assessed with the proposed method. The influence of local site conditions, seismic excitation and nonlinearity of the soil behaviour on the ground response can be realistically taken into account by the model.