Seismic Response of Earth Dams in Triangular Canyons

The effects of narrow and sloping canyons on the dynamic response of embankment dams is investigated by a three-dimensional finite element analysis. Analytical results for embankments with several width to height ratios are compared with the results of plane strain solutions for comparable sections and significant differences are shown to exist between the two solutions. Comparisons are presented of the natural frequencies of vibration of dams in rectangular and triangular cannons of different slopes with those of long dams for which the assumption of plane-strain response is valid. For an embankment in a triangular valley with a width to height ratio of three and subjected to transient ground motions, the computed accelerations at the mid-section were about 60 to 80% higher in a three-dimensional response analysis than in a plane-strain analysis. On the other hand the computed shear stresses for the same section in the three-dimensfonal analysis were generally about one half the values computed from plane strain analyses. It is concluded that the dynamic response of an earth embankment is significantly affected by the shape of the canyon in which it is constructed.