Seismic slope stability of embankments: a comparative study on EC8 provisions

According to EC8 provisions, seismic stability assessment of natural slopes is currently performed based on simplified methods i.e. the pseudostatic and the Newmark’s sliding block method. The application of these methods requires the beforehand consideration of major assumptions necessary for the selection of either the seismic coefficient or the acceleration time history of the rigid block.

Although both ULS and SLS are defined according to acceptable level of deformations at the slope, the assigned level of displacements is not clarified. In the current study the seismic slope stability of embankments is analysed employing two different methodologies depending on the depth of the failure surfaces, in order to evaluate the implications of the underlying assumptions of EC8 provisions.

For intermediate depth of failure surfaces a decoupled procedure is followed, estimating thus separately their dynamic response and the permanent seismic displacements. After calculating the horizontal and vertical seismic coefficients from parametric dynamic numerical analyses the seismic permanent displacements were estimated according to Newmark’s sliding block approach.

Seismic coefficient spectra for circular failure surfaces, based on two levels of acceptable seismic displacements, are proposed. For deep failure surfaces, coupled analyses of a SDOF system with sliding potential along its base have been performed and the critical acceleration was established. The results indicate that displacements are prohibited at certain combinations of (a) the ratio of the shear strength of the interface to the maximum applied acceleration, and (b) the tuning ratio (ratio of the eigenperiod of the SDOF to the period of excitation).