A 2D Seismic Stability and Deformation Analysis

Two-dimensional (2D) seismic stability and deformation calculations for a 16.5-meter high levee embankment are presented. The embankment is to be constructed on potentially liquefiable foundation soil. A liquefaction triggering analysis is conducted that includes stress calculations for the pre-earthquake condition under steady-state seepage using the finite element program PLAXIS, seismic response calculations using the finite element program TELDYN, and liquefaction resistance estimates. The results of the 2D analysis suggest that the potentially liquefiable soil is confined to approximately the uppermost 10 meters of the natural soil under a limited area of the downstream embankment toe. The 2D analysis clearly illustrates the increased liquefaction resistance and the safety factor against liquefaction in the foundation soil beneath the major portion of the embankment due to the increased confining stress of the embankment loading. The 2D liquefaction triggering analysis results are used to estimate the end-of-earthquake strength using the procedure proposed by Seed and Harder. The two-dimensional distribution of end-of-earthquake strength is used for a limit equilibrium, post-earthquake, static stability analysis to calculate the factor of safety of the embankment-foundation system. The calculated factor of safety of 1.1 indicates that no excessive deformation is expected after the duration of earthquake shaking. The limit equilibrium stability analysis using the end-of-earthquake strength is repeated to locate the critical slip surface and the corresponding yield acceleration. The acceleration time history for the mass defined by this critical slip surface is calculated by TELDYN. The permanent deformation of the embankment during the design earthquake is estimated based on the acceleration time history and yield acceleration using the procedure developed by Newmark.