Methodology for seismic and post-seismic stability assessing of natural clay scope based on a visco-plastic behavioural model in simplified dynamic analysis

A co-seismic viscoplastic sliding model, composed of two consequential behaviour phases, was realised in order to assess the co-seismic and post-seismic stability of natural slopes. The model takes into consideration the development and distribution of available strengths in pre-seismic conditions, as well as the viscoplastic behaviours manifested during monotonic and impulsive fast shearing tests on different clayey soils. In relation to the strength increase produced by the shear displacement rate, phase I is present during sliding on pre-existing failure surfaces at the residual state and/or in weak bands at the fully softened state. In this latter case, this is limited to small displacements. Conversely, phase II is characterised by strength decrease and occurs if and when the inertial dynamic load mobilises the "impulsive critical shear strength", which is greater than the shear strength available in the pre-seismic static field. This implies the development of a first failure or a new failure surface with high shear displacements along these surfaces. The simplified dynamic analysis for infinite slopes, integrated by the behaviour model introduced in this paper, highlights a less conservative nature in comparison to that of the classic Newmark approach with one single exception. This occurs on attainment of the "static break point" where the co-seismic displacements obtained are comparable or even greater than those attainable from the classic Newmark approach. Furthermore, in relation to the co-seismic development of shear strength, it is possible to estimate in the short term as well as in the long term the post-seismic instability after the main shock.