Determination Of Calibrated Earthquakes For Hydropower Projects In Western China

After the2008Wenchuan Ms8.0earthquake, the seismic safety of dams inhydropower projects in western China, especially those of large scales, has attractedmuch attention from public and engineering communities..The Federal Emergency Management Administration of the United States(FEMA) has proposed the concepts of maximum credible earthquake (MCE) andmaximum design earthquake (MDE) for the purpose of analyzing and designing theseismic hazard of dams. In definition, the MCE is the largest earthquake that mayoccur on a recognized fault or within a particular seismic area under the currenttectonic framework. The MDE refers to the maximum ground motion on a damcaused by an assumed earthquake, based on which the dam and associated structuresare designed. The MDE may be set equal to the MCE or to a designed earthquake lessthan the MCE, depending on the circumstances. In general, the associatedperformance requirement for the MDE is that the project performance withoutcatastrophic failure, such as uncontrolled release of a reservoir, although significantdamage or economic losses may be tolerated.Compared with the MDE of the United States, the Interim Regulation ofAseismic Design and Special Report Compile for Hydropower (2008) proposed theconcept of calibrated earthquakes. In order to prevent the dam from uncontrollablefailure when struck by destructive earthquakes, this interim regulation suggests thatfor large hydropower projects, the seismic ground parameters on the basis of thedesigned earthquake should be increased according to the calibrated quake. Forexample, the first-level water retaining structure can take1%probability of exceedingin100years or ground motion parameters of maximum credible earthquake (MCE) torealize the calibration.For the regions with high seismic hazard, foreign researchers generally take theparameters of the control maximum credible earthquake (CMCE) as MDE forearthquake proof and disaster prevention. Compared with the value of2%probability of exceeding in50years taken for the design earthquake of water retaining structureby foreign researchers, Chinese researchers take the value of2%probability ofexceeding in100years, which is higher than the former and suggests that the currentdesign earthquake standard of China is high. Because of the differences of designearthquake standards between China and abroad, and also because of there is fewrelative literature in China, the determination of the calibrated earthquake needs to bestudied further, especially for western China with high seismic hazard. This thesispresents case studies on the dams of two hydropower stations in western China. Itfocuses on comparison and analysis of the determination of MCE and the calibratedearthquake of dams using the deterministic method in different seismotectonicenvironments. The results are summarized below.(1)By analyzed both implication and derivation processes of seismic code of thekey buildings (e.g. the backwater building of hydropower engineering), this worksuggests that the ground motion parameters of exceeding probability is2%in100years as design basis. Then, the dam failure analysis is made using calibratedearthquake from the perspective of peak acceleration for prevention.(2)This thesis discusses the main technique procedures of estimating the PGA ofcalibrated earthquake in western China using the maximum credible earthquakemethod, and proposes a new method to evaluate the magnitude of MCE and providedthe PGA of MCE of both seismogenic structures and background earthquakes.(3)By estimating the calibrated earthquakes of the two dams sites, this workobtains the typical PGA of dam site No.1as0.599g, which is derived from maximumcredible earthquake method, and the typical PGA of dam site No.2as0.211g, which isderived from exceeding probability1%in100years, respectively. These resultsdemonstrate that if the current maximum credible earthquake method or probabilisticmethod of exceeding probability1%in100years may contain unsafe factors. So thelarger value should be used as the typically PGA for calibrated earthquakes.