Topographic Effects on the Seismic Coefficient and Earthquake-Induced Permanent Displacement of Earth Slopes

Abstract

Topographic irregularities have found to considerably affect the amplitude and frequency content of ground motions, but this effect is not included in current pseudo-static and displacement-based methods of seismic slope stability analysis. In this study, two-dimensional (2D) seismic response of step-like slopes under vertically propagating in-plane shear waves (SV waves) is assessed to investigate the topographic effects on seismic coefficient and permanent displacement of earth slopes. The analyses are performed for slopes with different heights, inclinations, soil types subject to artificial input motions with various frequencies. The decoupled method is adopted to separately calculate the seismic response of slopes and the permanent displacements. The slip surface of slopes and the characteristics of sliding mass are firstly derived through static stability analysis, and the seismic coefficient of sliding mass is then evaluated by assuming no sliding scenario occurs. The permanent seismic displacements are finally computed by the Newmark method. The one-dimensional (1D) seismic analysis is also performed for the sliding mass, and the results are compared with those from the 2D analysis to provide some insights into the topographic effects on the assessment of the seismic stability of earth slopes. It is found that the conventionally used ratio of slope height to the wavelength may not consistently perform well to describe the topographic effects for slopes over a rigid bedrock, where the soil layer amplification is dominant. The ratio of the fundamental site period to the predominant frequency of the input motion can be used to characterize the topographic effects on the seismic coefficient and earthquake-induced permanent displacement of earth slopes. In addition, the 1D analysis does not consistently provide conservative results compared with 2D analysis for the full cover sliding cases.