Influence of cyclic strength degradation on a Newmark-type analysis

During strong earthquakes soils develop significant deformations that may affect the stability condition of geotechnical systems, causing the activation of plastic mechanisms and the occurrence of permanent displacements. This paper focuses on the seismic behaviour of natural slopes for which the magnitude of earthquake-induced permanent displacements is noticeably affected by the cyclic behaviour of soils. Accordingly, the possibly occurring cyclic reduction of shear strength must be considered to avoid unsafe estimation of the slope seismic performance. To this purpose, a modified Newmark-type analysis is proposed in the paper assuming a time-dependent value of the slope yield acceleration coefficient kc computed as a function of a degradation parameter R, quantifying the relative reduction of kc with respect to its initial value, and the number of equivalent loading cycles. The results of an extensive parametric analysis were used to derive some displacement predictive models for a safe estimate of the influence of cyclic strength degradation.