Seabed Seismic Motion in Presence of Seismic Induced In-Profile Liquefaction

Cyclic loading caused by earthquake in sandy soils often leads to the development of positive pore pressures. In extreme conditions, the pore pressure may increase until reaching a state of zero effective stress associated with a dramatic reduction of the soil shear stiffness and strength. Even when liquefiable soils are found at depth, and capped by a non-liquefiable crust, once liquefied they can act as a seismic isolator, significantly modifying both amplitude and frequency content of the vertically propagating shear waves. As a result, seabed seismic motions at offshore sites characterized by the presence of liquefiable layers within the soil stratigraphy may change considerably with respect to a non-liquefied scenario. Non-linear Seismic Site Response Analyses (SSRA) are often used for site-specific evaluation of seismic input for offshore projects. Severe earthquake motions induce non-linear soil response associated with significant stiffness reduction for soft soils. The hysteretic non-linear soil behavior leads to modifications in terms of magnitude and frequency content compared to the postulated stiff soil input. This is even more important where pore pressure build-up and liquefaction may occur, leading to further modification of the seismic accelerations at mudline. However, standard industry practice consists in performing total stress SSRA that are not able to model the softening response in presence of liquefiable soil layers. This paper compares the seabed seismic motion assessed by means of total and effective stress SSRA in order to evaluate the effect of the soil stiffness degradation in liquefied layers within the soil profile, building upon the findings of Ardoino et al. (2015), who observed in-profile liquefaction to have a limited effect on mudline response spectrum. In particular, it is shown how modelling methodologies able to replicate the transient nature of excess pore pressure build-up during earthquake excitation (i.e. time-domain analyses), are better suited to capture seismic motion modifications in presence of in-profile liquefaction, with respect to response spectrum analyses. The effects of deep foundations embedded across the liquefied layers, on the propagation of the seismic motion, is also investigated and discussed.