INFLUENCE OF ELASTICITY OF FOUNDATION ON SEISMIC RESPONSE OF GRAVITY DAMS

Abstract

Gravity dams form a lifeline of our economy. In order to design earthquake resistant dams and to evaluate the safety of existing dams, it is required to estimate time periods, frequencies, mode shapes, earthquake induced stresses and deformations. The study presented here makes an effort to study the influence of elasticity of foundation on seismic response of gravity dam. The effect of depth of mesh on dynamic response has also been studied. Amongst the various methods available for carrying out the stress analysis of dam, the finite element analysis is considered superior. Hence finite element method has been used in this thesis for static and earthquake analysis of dam section including foundation. The computer software COSMOS/M has been used for carrying out the analysis. The plain strain assumption is used for idealizing the dam section. Dam foundation interaction has been considered by considering a part of foundation contributing to the stiffness of the dam.,The dams, which have been considered for analysis, are Koyna dam (height = 103.2m), Pine flat dam (height = 122m) and Bhakra dam (height = 225m). Various types of loading such as dead load, hydrostatic pressure, uplift pressure, earthquakes load and hydrodynamic pressure has been considered in the analysis. The response indicates considerable variation in the behavior of the dam when dam-foundation interaction is considered particularly when elasticity of foundation is equal to or lower than that of the dam. The time period decreases as the ratio of modulus of elasticity of foundation and dam (Ef/Ed) increases. There are variations in mode shape also with the change in El/Ed ratio. The depth of mesh also has a considerable affect on dynamic response of dams.