Study On The Ultimate Seismic Performance Of Concrete Gravity Dams Based On The Incremental Dynamic Analysis

China is one of the most abundant water resources in the world.In order to fully develop and utilize the hydropower energy,with the promulgation of the 13 th five-year plan,the state council has proposed to speed up the construction of 172 major water conservancy projects.By 2020,China will build more than 98000 reservoir dams.Under the action of sudden extreme load,the dam is affected by the seismic load,and there may be serious damage.When the seismic load exceeds the limit seismic capacity of the dam itself,even more serious dam break will occur,which will lead to relatively serious secondary disasters in the downstream cities and regions.Therefore,in extreme response of dam under earthquake load behavior,potential failure modes,the safety of the dam after a sudden earthquake load evaluation,are important contents in the major work of earthquake disasters in our country.With the supports of the National Natural Science Foundation(51309143)and Excellent Master’s Thesis Support of China Three Gorges University(2017YPY020),the seismic analysis model of the concrete dam-reservoir-foundation interaction system which is based on the viscous-spring boundary method and fluid solid coupling is firstly established.According to the incremental dynamic analysis(IDA),the ultimate seismic capacity,dam failure mechanism and failure modes of the system are studied.The main contents are listed as follows:1)The viscous-spring boundary method is employed to simulate the dam-foundation interaction system.The seismic analysis model which takes account into the radiation damping effect of infinite foundation is established.The dynamic peak stresses of the dam are reduced by 10%～30% when compared with the traditional massless foundation model.The fluid-solid coupling method is employed to simulate the dam-reservior interaction.The peak stresses of the dam body are reduced by 10%～20% when compared with the traditional added mass method.Through the implementation of above two methods on ABAQUS software,a highly accurate and efficient seismic model for the gravity dam-foundation-reservoir system is established.2)Two seismic analysis models of the dam-foundation-reservoir system,in which one is based on the traditional added mass and massless foundation method and the presented one is based on the viscous-spring boundary and the fluid-structure coupling method,are established,respectively.The stress responses in the presented model are reduced by about30%.Based on the extended finite element method(XFEM),the results of the crack propagation model for concrete gravity dam established by the viscous-spring boundary method and fluid-structure coupling method are closer to the actual cracking position by the vibrating table tests.It is shown that the nonlinear crack propagation process of the dam can be simulated more accurately by using the presented seismic analysis model.3)Based on the incremental dynamic analysis(IDA),the ground peak acceleration(PGA)is taken as the seismic intensity index IM,and the relative displacement of the crest is taken as the index DM of the structural performance parameters.The viscous-spring boundary method and fluid-solid coupling method are used to establish the seismic analysis model of the dam-foundation-reservoir interaction system,and the seismic performance of the concrete gravity dam is evaluated.Taking the Koyna gravity dam as an example,16 seismic records are selected.Using the quantile analysis method,the overall safety margin of the new model is higher 10%～20% than that of the traditional model for PGA with different guarantee rates.The calculation results of the traditional model are somewhat conservative.4)Based on the incremental dynamic analysis(IDA),three kinds of dam relative displacement angles are introduced as the index DM of structural performance parameters.The viscous-spring boundary method and fluid-solid coupling method are used to establish the seismic model for the dam-foundation-reservoir interaction system,and the seismic performance of the Jin’anqiao concrete gravity dam is evaluated.The Jin’anqiao dam has a MDE level of about 0.35 g,and the safety guarantee MCE level is approximately 0.47 g,which indicates that the dam has a security redundancy rate of 25% at the 84% guarantee rate.It is highly likely to meet the security assurance level.The performance level of the PGA corresponding to each performance level at the critical part of the crest to the slope is consistent with the overall seismic performance level of the dam,indicating that the safety of the slope of the gravity dam determines the overall seismic level of the dam.5)The failure modes of the Jin’anqiao dam can be roughly divided into the following five categories: the downstream slope area of the dam is damaged.The crack will extend diagonally upstream to form a penetrating crack.At the junction of dam heel and dam foundation,the crack will extend downstream along the dam foundation interface.The slope area in upstream is damaged and the crack extends horizontally downstream.The failure will happen at the junction of the downstream RCC zone of the dam.The crack will extend upwards at an angle of about 45° to form penetrating cracks on the upstream surface.It is damaged on the roof of the gallery due to stress concentration,and the crack in this area will merge with cracks in the rolling zone and form the main crack that penetrates through the dam body.6)The functional safety level of the Jin’anqiao Dam is about 0.42 g.The level of its functional guarantee is basically determined by the crack extension length at the downstream slope of the dam head;the level of safety assurance is about 0.6g,and the level of safety assurance is basically from the lower part of the dam body.The cracks on the upstream slope and the length of the cracked zone in the middle of the dam head are determined.