Seismic Performance Evaluation of Concrete Gravity Dams Subjected to Mainshock and Aftershocks

The dams with unconventional cross-sections are vulnerable to strong earthquakes, with most of the damages occurring at the neck region where there is a change in the slope of the cross-section at the downstream side. In the event of a sequence of mainshock and aftershocks, complete failure of the neck region is anticipated. With this background in view, two such dams, Koyna and Pine Flat dams, are taken as illustrative examples and are analyzed under a sequence of mainshock and aftershocks.

A nonlinear time history analysis of a 2D cross-section of the tallest non-overflow monolith is performed for near and far field station earthquake records. The cross-section is analyzed for the mainshock and aftershock effects by joining the segments of the earthquake with an adequate time gap created by zero excitations. The time gap is decided such that the free vibration of the dam under the influence of one earthquake shock comes to rest with any possible residual displacement. The analysis considers the inelastic effect of the damage plasticity model available in ABAQUS.

The results of the numerical study indicate that some portions of the dam included in the neck region may be severely damaged if the effect of aftershocks is considered in the analysis, even at a PGA level of 0.4g. The extent of the damage depends on the type of earthquake and the geometry of the dam cross-section.