Spatiotemporal evolution of excess pore pressures in a silty seabed under progressive waves during residual liquefaction

Both transient and residual pore pressure responses are induced as ocean waves propagate over a silty seabed. In this study, the spatiotemporal evolution of excess pore pressure in a silty seabed under progressive waves was physically modelled in a large wave flume. Three typical stages were identified in the process of residual liquefaction via flume observations, including quasi-elastic, intensive build-up of residual pore pressure, and continuous liquefaction stages. During the initial quasi-elastic stage before the liquefaction of the silt bed, the transient pore pressure can be well predicted by the analytical solution based on poro-elastic theory. After that, the residual pore pressure builds up intensively to its maximum value, even in the case that the transient pore pressure is nearly negligible at deep soil layer. Once the residual liquefaction occurs, the poro-elastic theory becomes invalid for describing the pore pressure response. The residual liquefaction is not simultaneously induced within the entire bed, but gradually progresses downward from the shallow layer of the silt bed to deeper. The pore pressure amplitude is significantly amplified after the silt liquefies, while no amplitudeamplification was observed within the un-liquefied silt. An amplification ratio (zeta) is proposed to characterize the amplification effect and distinguish the onset of residual liquefaction. The value of zeta during the continuous liquefaction stage is found to be one order of magnitude larger than that in the quasi-elastic stage. Comparisons with the existing centrifuge tests further indicate that the critical cyclic stress ratio for the silt bed is much smaller than that for the sand bed, implying the silt bed is more prone to residual liquefaction.