Numerical investigation of the instability process in underwater sedimentary slopes subjected to seismic action

The sedimentation process preconditions the strength and stress state of soils in sloping seabed, yet it is often ignored in studies of the seismic-induced instability of underwater slopes. Additionally, the conventional total stress-based analysis struggles to explicitly capture excess pore pressure variation and effectively assess sedimentary slope instability under seismic excitation. In this study, an effective stress-based two-step numerical approach is proposed to investigate the contribution of sedimentation and seismic excitation on the instability process of a practical slope case. First, the sedimentation process is replicated, with the results mapped to the initial state of the seismic analysis. Then, an explicit hydro-mechanical model considering the cyclic strength degradation is proposed for seismic analysis. A searching algorithm is presented to dynamically identify the potential sliding surface and quantify real-time stability throughout the sedimentation-seismic process. Last, the approach is applied to consecutively simulate the entire sedimentation-seismic instability process of the Goleta slide. Results indicate that weak layers formed during sedimentation become preferential zones for the development of sliding surfaces, which propagate in a planar pattern under seismic excitation. During the process, the soils experience significant strength degradation (50 % at the sliding surface) due to strain softening and pore pressure accumulation. Parametric analysis indicates lower sedimentation rates tend to result in shallow slides of under-consolidated soils, while higher sedimentation rates lead to substantial pore pressure accumulation, causing deep-seated sliding. This work highlights the preconditioning effect of rapid sedimentation, and contributes to the scientific prediction of seismic geohazards in underwater slopes.