SEISMIC PERFORMANCE EVALUATION OF CONCRETE GRAVITY DAMS

Abstract

Concrete gravity dams are crucial to hydro energy, irrigation, and water storage. Their failure can result in catastrophic consequences for life, the environment, and the economy. During earthquakes and extreme loading conditions, over-stressing, cracking and subsequent sliding and overturning may lead to a catastrophic failure of the dam. The evaluation of a dam’s seismic performance is generally based on an expert’s judgment assessing the magnitude of tensile stresses and their spatial distribution within the dam’s cross-section. Many of dams were designed before the development of modern earthquake engineering using outdated methods that underestimate the seismic forces. This is compounded by a lack of historical data and codified guidelines for evaluating performance and safety of dams. While contemporary Finite Element (FE) codes are widely used to simulate non-linear behavior of concrete dams and evaluate the seismic performance, the complexity and intricacies of setting up a non-linear FE model may discourage and can be challenging for a practicing engineering to conduct in-depth seismic studies. The thesis presents a relatively simplified framework that uses available FE techniques to evaluate limit states and quantify damage, potentially addressing some of these challenges. To begin with, the seismic evaluation of an example concrete gravity dam is conducted by employing a decoupled approach, wherein the dam is divided into distinct sub-regions based on the post-earthquake stability limits outlined in existing literature. The dam’s nonlinear behavior is modeled using the concrete-damaged plasticity model considering both dam-reservoir and dam-foundation interactions. The viscous-spring absorbing boundary technique is utilized to absorb outgoing waves at the truncated foundation boundary, and the equivalent nodal force input method is used as the seismic input mechanism. Incremental dynamic analyses (IDA) is used to delineate and quantify various damage states in Pine Flat Dam. A local damage index premised on the tensile damage of the finite element is used to compute/quantify the damage. These derived damage states are then examined in iii relation to various Engineering Demand Parameters (EDP) and Intensity Measures (IM). However, all parameters display weak correlation with the resultant damaged state. In the subsequent step, an investigation of the seismic damage and performance assessment of the Pine Flat Dam under three reservoir level conditions is considered. Using IDA and adopting the modified, decoupled, zone-based damage assessment methodology stated above, the mean IDA curves based on crest displacement and different local damage indices are computed. Both horizontal and vertical ground motions are used as seismic input at the foundation base. Critical damage areas were identified using the IDA curves and it is observed that the change in reservoir levels are shown to alter damage patterns. The inter-relationship between the damage states and Engineering Demand Parameters (EDPs) show high variability and the complexity involved in the prediction of seismic damage and damage states. The development of fragility curves is presented to provide insight into exceedance probabilities of each sub-region and of overall dam corresponding to three different limit states. Variations in susceptibility to different levels of damage at different reservoir and spectral acceleration levels are observed from the fragility functions. These fragility curves serve as a quantitative measure for seismic vulnerability and can aid in assessing the risk associated with each specific sub-region of the dam. The study discusses the seismic behavior of the dam highlighting the intricate interplay between reservoir levels, structural response, and damage states. The insights emphasize the necessity of incorporating these dynamics into dam safety assessments and engineering practice.