Seismic analysis of submarine slopes: Retrogressive and three-dimensional effects

The stability analyses of submarine slopes, to this date, are mostly based on the classical methods of slope stability analysis such as the limit equilibrium method. While appropriate for static slope stability analysis, those methods have some limitations when used in seismic analysis of saturated soil slopes involving soil liquefaction. This study aims at filling some gaps in the current approach by using a state-of-the-art method for effective stress, seismic analysis of submarine slopes. The method proposed here implements a fully coupled, dynamic, finite element approach and a multi-yield surface plasticity model for simulating non-linear soil behaviour under dynamic loads.; The objectives of this research are aimed at addressing the needs of geotechnical practice: (1) to provide a procedure for analyzing seismically induced retrogressive slope failures and to use this procedure for explaining the mechanisms and identifying the main factors affecting the extent of those slope failures; (2) to assess the three-dimensional effects in seismic analysis of submarine slopes, in order to provide geotechnical practitioners with a reliable tool for extrapolating the results of manageable 2D seismic analyses to real 3D configurations; and (3) to design a procedure for constitutive model parameter calibration based on liquefaction strength analysis, using limited amount of experimental data and accounting for uncertainties in soil properties.; By modelling the retrogressive failures, the study highlights the importance of accounting for the potential of retrogression in regions that are seemingly safe but can be affected by such phenomenon. Risk assessment of infrastructures (e.g. pipelines) located on such seemingly safe zones should include estimation of retrogression distance. This is similar to accounting for the potential hazard of debris run-out for infrastructures located below the potentially unstable slopes. In this part of the study, a new method is introduced for simulating successive failures due to loss of support. For the various configurations of seabed slopes analyzed here for assessing the effects of gentle seafloor slope and presence of a layer with low permeability, it was found that the final linear extent of retrogressive failures are 5 to 20 times larger than those of the initial failure, which is usually the only stage of failure accounted for in practice.; Three-dimensional effects are assessed by comparing results of two- and three-dimensional analyses, in terms of predicted displacements, shear strains, and excess pore water pressure ratios. Limits of applicability of the 2D, plane strain analysis assumptions are quantitatively assessed. Some regression models are also presented that express ratios of 3D to 2D predictions as a function of slope width/height ratio and earthquake peak acceleration. The results of the present dynamic, fully coupled, non-linear analyses are also compared with those of static slope stability analyses. The comparison indicates that the trend of decrease in the ratio of 3D/2D response as a function of slope width/height ratio is very similar for both approaches. However, the applicability limit of the 2D assumption is found to be slightly lower in dynamic analysis (width/height ratio of about 3--5, with larger values corresponding to larger seismic accelerations) than in static analysis (width/height ratio of about 5) for the same level of tolerance (15%). Moreover, for B/H > 6--7, the differences between 3D analysis predictions on the symmetry plane of the slope and 2D analysis predictions are found to be insignificant. (Abstract shortened by UMI.)...