Dynamic Response And Damage Mechanism Of Concrete Gravity Dams Under Extreme Loadings

With the development of hydropower engineering technology，the majority ofconcrete dams with the height of100m~300m are being built or to be built incountries with active seismic activities. The safety of dams can be guaranteed undernormal conditions. Because of the uncertainty of the earthquake ground motion,concrete dams are likely to experience cracking under strong earthquake groundmotions. Seismic safety evaluation of high dams remains a crucial problem in damconstruction. Due to their significant political and economic benefits, they arepossible targets for terrorist attacks or intentional explosions. The possible failure ofdams can cause the most undesirable impact on the downstream populated area alongwith a considerable amount of devastation. Therefore, study on the dynamic response,damage mechanism, seismic and antiknock performance of concrete gravity damssubjected to extreme loadings is crucial to our social and economic development inthe significant working of disaster prevention and mitigation, which is also the keytechnical problem and important strategic task in the construction of hydraulicengineering.Because of the very serious consequences of dam failure, the safety andprotection issues of high concrete dam under extreme sudden loads should be givenmore attention. The extreme loadings, which can cause disaster to concrete gravitydams, mainly includes strong earthquake loading and explosion shock loading. Inorder to reveal the dynamic response behavior and damage mechanism of concretegravity dams under strong earthquake ground motions and explosion shock loading:(1) crack propagation process, dynamic response, potential failure modes and ultimateseismic capacity of concrete gravity dams under strong earthquake ground motionshave been investigated; the effects of initial cracks, strong motion duration,mainshock-aftershock seismic sequences and near-fault ground motions on theseismic performance and dynamic response of the concrete gravity dams arediscussed;(2) shock wave propagation characteristics of underwater explosion nearboundaries are studied; damage modes and dynamic response of concrete gravitydams subjected to underwater and air explosions are compared; dynamic responsebehavior, failure modes, antiknock performance, damage mechanism and damage prediction of concrete gravity dams subjected to underwater explosion have beeninvestigated.The major creative contributions of this paper are summarized as follows:(1) Seismic fracture processes of dams are discussed, and typical potentialfailure modes are presentedDynamic response, cracking process and crack opening of concrete gravity damssubjected to strong ground motion are studied by employing the XFEM. The validityof the algorithm is verified. The mesh-sensitivity of the model is examined. Seismiccrack propagation process, failure modes and initial crack location of concrete gravitydams under different levels of earthquake ground motions are analyzed. The typicalpotential failure modes of concrete gravity dams under strong earthquake groundmotions are presented. The effects of initial cracks on the initial crack, crackpropagation process, failure modes, crack propagation depth and seismic response ofthe concrete gravity dam are investigated. The seismic weak parts of concrete gravitydams are obtained.(2) The seismic cracking model with initial cracks is established, and anevaluation method of ultimate aseismic capacity is proposedThe concrete gravity dam with initial cracks is established. The effects of initialcracks on the initial crack, crack propagation process, failure modes, crackpropagation depth, dynamic response and seismic performance of concrete gravitydams are investigated. Based on the performance-based seismic fortification criterionand theory, a comprehensive evaluation method of ultimate aseismic capacity forconcrete gravity dams is proposed. This method is conducted for analysis of nonlinearseismic response by considering demand-capacity ratios (DCR), cumulativeoverstress duration, failure mode, stability, and plastic zone.(3) Both local and global damage indices are established, and the influenceof strong earthquake on the seismic performance is discussedA damage mechanics model associated with strain softness is proposed to reflectreasonable damage dissipation of concrete fracturing. According to the characteristicsof the cracking damage development, both local and global damage indices areestablished as the response parameters. Strong motion durations of the selectedas-recorded accelerograms are obtained based on the definition of significant duration.The influence of strong motion duration on the accumulated damage of concretegravity dams is examined. The construction method of mainshock-aftershock seismic sequences is established. The single mainshock and aftershock ground motions areconnected to form the seismic sequences with a time gap. The effects of seismicsequences on the accumulated damage of concrete gravity dams are investigated. Thedynamic coupled model of dam-reservoir-foundation systems are presented based onthe Lagrangian formulation. Nonlinear seismic damage analyses of concrete gravitydams subjected to both near-fault and far-fault ground motions are performed. Theinfluence of forward directivity effects on the dynamic damage and seismicpersormance of concrete gravity dams is discussed.(4) The cavitation model for underwater explosion is established, and theshock wave propagation characteristics near boundaries are exploredThe validity of the numerical model for capturing the unsteady cavitation isexamined in the analysis. The shock wave propagation characteristics of underwaterexplosion near the free surface and the structure surface are discussed. The shockwave-free surface interface and shock wave-structure interaction are revealed. Theinfluence of the air-backed and water-backed plates on the shock wave propagationcharacteristics and cavitation effects is also compared. The unsteady cavitations andsurface cutoff effects near the free surface and the structure surface are described andcaptured. An analytical comparison of the shock wave propagation characteristics ofthe water-backed plate against the air-backed plate is brought out.(5) An effective numerical method for antiknock analysis of dams ispresented, and the damage characteristics of dams are comparedA fully coupled numerical approach with combined Lagrangian and Eulerianmethods is established for concrete gravity dams subjected to blast shock loadings. Alarge-scale and highly nonlinear calculation method is proposed. Lots of complexissues, such as the strain rate effect, shock wave-structure interaction and structuralresponse, are taken into consideration in the establishment of gravity dam fullycoupled model. The validity of the coupled model is examined by comparing resultsobtained from the proposed method with those reported in the literature. Theinfluence of the detonate medium on the peak pressure, impulse and velocity of shockwave is studied. The dynamic response behavior and damage characteristics ofconcrete gravity dams subjected to underwater explosion and air blast are compared.(6) The antiknock performance of dams is discussed, and the damageprediction model of dams subjected to underwater explosion is firstly proposed The dynamic response and damage effects of concrete gravity dams subjected tounderwater explosion is discussed. The propagation characteristics of shock wavenear the dam structure is analyzed. The influence of the standoff distance, chargeweight, detonate depth and the upstream water level on the antiknock performance ofconcrete gravity dams is investigated. The damage mechanism, failure modes andvulnerable part of concrete gravity dams subjected to underwater explosion areobtained. Lowering the water level is an effective measure to guarantee the safety ofdams. The damage conditions of concrete gravity dams subjected to explosion shockloading are divided according to the characteristics of the cracking damagedevelopment. The damage prediction model of concrete gravity dams subjected tounderwater explosion is firstly proposed. Critical curves related to different damagelevels are derived. The safe standoff distances are obtained.