SEISMIC RESPONSE OF HIGH CONCRETE GRAVITY DAM INCLUDING DAM-RESERVOIR-FOUNDATION INTERACTION EFFECTS

Abstract

Gravity dams form a lifeline of a country economy and its failure will create huge loss of life and properties. The dynamic analysis of a concrete gravity dam is a reasonably complex problem. The response of a dam subjected to dynamic loading is a combined effect of the interaction among dam, reservoir and foundation systems. Hence, in order to design earthquake resistant dams and to evaluate the safety of existing dams, it is necessary to study the various aspects influencing the seismic response of a high concrete gravity dam. The linear response of an idealized high concrete monolith gravity dam with horizontal harmonic and vertical ground motion is presented to show the importance of parameters such as effect of foundation flexibility and reservoir interaction in terms of crest displacements and stresses at the heel of dam. In the study presented herein, four various models have been adopted and effect of foundation and reservoir has been illustrated clearly, where all the possible ways such as dam with fixed base, dam with block-foundation, dam with block-foundation and reservoir and dam with spring instead of block foundation and reservoir are used to highlight the importance of foundation flexibility and reservoir on the basis of response of dam in both 'frequency and time domain. The modelling and analyses have been carried out in Finite Element Package ANSYS 10.0. In the case of frequency extraction analysis using IS 1893:1984 and Westergaard's technique it is found that frequency obtained is high as compared to fluid element modelling since water compressibility effect has been neglected in IS 1893: 1984 and Westergaard's techniques. In case of model analysis, the water inside and surrounding the structure alters the dynamic characteristics of the structural system, increasing the periods of the fundamental modes of vibration and modifying the mode shapes. As a result reduction in fundamental frequency is noticed with consideration of dam-reservoir-foundation system. The dam-reservoir-foundation system showed increase in response of dam when both foundation and reservoir interaction have been considered iii and there is significant decrease in frequency when reservoir comes in consideration as compared to only dam-foundation interaction. . The parametric studies showed that the response of dam structure in terms of various quantities such as displacements and stresses mainly depends on foundation modulus of elasticity and height of reservoir. In case of flexible foundation significant change in response was observed as compared to rigid foundation. Fluctuations in reservoir height also showed variations in response of dam in terms of quantities such as displacements and stresses. From the results obtained in harmonic analysis after the present study, we can certainly see that the dam-reservoir-foundation rock system shows more response at reduced frequencies as compared to dam-foundation model which is strongly influenced by water interaction in dam. In case of the results for the vertical ground motion during harmonic analysis it is found that the higher harmonics of the system sometimes play role in the behaviour of system. When water is present the resonant amplitudes seem to be increasing in some non-uniform pattern unlike the uniform increase that occurs for horizontal ground motion which is indicating towards some counter action between the dam's inertia loading and the hydrodynamic added load when motion is of vertical nature. So the impact of the vertical ground motion on dam-reservoir-foundation system is important under seismic zones. The equivalent spring-damper model for replacing the foundation can be of great benefit in case of non-linear analysis with respect to time required for ,non-linear analysis. In case of linear time history studies for various soil sites such as hard, medium and soft, it is found that different responses are obtained for the dam structure though having same PGA. It can also be observed that for an accurate dam analysis, it is necessary to include the interaction effect in the dynamic analysis, especially for dams which are built on seismically prone sites. While the result of non-linear analysis for dam with fixed base with and without reservoir shows that there is increase in response when reservoir interaction is considered.