The objective of this research project is to study the effects of differential excitations along the base of a dam, caused by spatially varying earthquake ground motion. The standard procedure is to assume that all points at the base of the dam simultaneously experience the same earthquake-induced ground motion - that is, that the dam is shaken by the motion of a rigid plate on which it rests. In contrast, this study accounts for phase delay and incoherence effects of the base excitations arising from the propagation and scattering of seismic waves. Appropriate ground motion data are now available from the analysis and stochastic modeling of recent accelerograph recordings on and near earth dams. The research program involves: 1) The selection of a suitable ground motion model from these data; 2) A linear random vibration analysis of a three-dimensional earth dam model to study the effects of spatially varying ground motions; 3) The development of approximate excitation specifications that yield reasonable responses when the dam is analyzed using a two- dimensional model; 4) A preliminary assessment of topographical effects; 5) Investigation of the non-linear response using the method of equivalent linearization, with verification by Monte Carlo simulation. The results from this research are relevant to safety evaluations of existing earth dams, as well as to the design of new earth dams.
