Vibration Characteristics of Dams in Narrow Canyons

Abstract

Results from simplified analytical and numerical models on vibrational characteristics of earth dams in narrow canyons are presented. The effect of the canyon length over height ratio on the fundamental period of the dam is illustrated for five idealized canyons shapes: increasing canyon narrowness results in a decrease of the natural period. In contrast, a deformable underlying soil layer in the middle of the valley is shown to lengthen the natural period. Useful closed-form expressions for natural frequencies, displacement shapes, participation factors, and acceleration amplification functions are presented in a table. Acceleration amplification functions at midcrest are plotted versus a dimensionless frequency for various canyon shapes. A simple model of a dam in a semicylindrical canyon is used to demonstrate that flexible canyon boundaries may significantly reduce mid-crest accelerations due to energy radiation. Moreover, soil nonlinearities that develop during strong base motion may reduce drastically the high-frequency, high-amplitude midcrest accelerations caused by the narrow canyon.