Risk Assessment of Concrete Gravity Dams under Earthquake Loads

Abstract

Dams are important structures to supply water for irrigation or drinking, to control flood, and to generate electricity. In seismic regions, the structural safety of concrete gravity dams is important due to the high potential of life and economic loss if they fail. Therefore, the seismic analysis of existing dams in seismically active regions is very crucial to predict responses of dams to ground motions. In this thesis, earthquake response of concrete gravity dams is investigated using the finite element (FE) method. The selected dam is the Pine Flat Dam which is located in the Central Valley of Fresno County, California. The dam-water-foundation rock interaction is taken into account in developed FE model by considering compressible water, flexible foundation effects, and absorptive reservoir bottom materials. In addition, Dams are usually analyzed using deterministic analysis method; however, several uncertainties affecting the results should be considered in the analyses of dams, such as material properties, inaccuracies of modeling, the water level in the reservoir, and the aleatoric nature of earthquakes. Therefore, the uncertainties regarding structural data and external actions are considered to obtain the fragility curves of the dam-water-foundation rock system. The structural uncertainties are sampled using Latin Hypercube Sampling method as a practical and efficient way for addressing such a complex problem. The fragility curves for base sliding and tensile cracking limit states are obtained performing non-linear time history analyses. Normal, Log-Normal and Weibull distribution types are used in order to fit fragility curves. The effect of the minimum principal stress on tensile strength is considered and found to be insignificant. It is also found that the probability of failure of tensile cracking is higher than that for base sliding of the dam. In addition, the loss of reservoir control is unlikely for a moderate earthquake.