Seismic Vulnerability Analysis And Seismic Safety Assessment Of Overall Stability Of Concrete Dam-Foundation Systems

The scale and quantity of high dam projects built and being built in China are unprecedented in the world.These high dams are mainly concentrated in the strong earthquake active area of Western China with poor regional structural stability,and most of them are concrete dams whose seismic safety are of great concern.Seismic safety evaluation of dams is a key issue in engineering design.One of the main failure modes of high concrete dams under earthquake is overall sliding instability failure of the dam body along with part of the foundation.This paper focuses on the overall seismic stability safety evaluation of high concrete dam-foundation system.The finite element model of concrete dam-foundation coupled system is established under earthquake considering the uncertainties of mechanical parameters of sliding surface of the controlled sliding blocks in dam foundation rock mass.Seismic vulnerability analyses of concrete gravity dams and arch dams are carried out respectively,and a preliminary framework for probability-based seismic safety evaluation of the overall stability of concrete dam-foundation system is constructed.The main contents are as follows:(1)The dynamic contact model considering the residual cohesive is developed.Seismic sliding stability analysis of a concrete gravity dam-foundation system is performed.The influence of the residual cohesion on the residual sliding displacement,sliding crack length,maximum opening displacement and stress of dam body is studied by using the improved dynamic contact model.It is concluded that the consideration of the residual cohesion can lead to a more reasonable and economical design for the seismic sliding stability control of gravity dams.(2)The finite element model of a concrete gravity dam and arch dam-foundation system is established.For the gravity dam,the contact nonlinearity of sliding surfaces of deep sliding blocks is considered.For the arch dam,nonlinear behaviors,including the contraction joint opening and closing,the failure of the dam-foundation interface and the boundary of the probable sliding block of the dam abutment,are considered comprehensively.Based on the Incremental Dynamic Analysis(IDA),the sensitivity analysis of shear strength parameters on the contact surface to the seismic sliding stability of concrete dams is carried out.The procedure is proposed to evaluate the seismic sliding damage levels of concrete dams using residual sliding displacement and sliding area ratio as damage indexes.The relative importance of the shear strength parameters is quantitatively evaluated.The mechanism of shear strength parameters affecting the deep sliding instability of gravity dam and the seismic instability failure process of arch dam abutment is revealed.(3)Based on the above established model,the uncertainty analysis of seismic sliding stability of concrete dam-foundation system is carried out by Latin hypercube sampling method and IDA method considering the uncertainty of shear strength parameters of sliding surfaces.It is concluded that the randomness of shear strength parameters leads to the variability of seismic sliding failure process of concrete dam-foundation systems.For concrete arch dam-foundation systems,based on the proposed damage indexes containing the residual sliding displacement and sliding area ratio,the influence of shear strength parameters on the variability of seismic sliding stability of concrete dams is discussed.It is discovered that the sliding area ratio index can more comprehensively present the whole process of sliding occurrence and development,while the residual sliding displacement more intuitively presents the final sliding instability.It is suggested that both of them can be used as a convenient and effective index to evaluate the overall stability of arch dam abutment against seismic sliding.For both concrete gravity dam and arch dam-foundation systems,the residual sliding displacement or sliding area ratio-based mean IDA curve shows good consistency with the results of basic parameter model.(4)Based on above calculation and analysis,the concept of seismic vulnerability analysis is introduced,and the procedure for seismic vulnerability analysis of concrete dams is given.The threshold value corresponding to the inflection point on the average residual sliding displacement and sliding area ratio-based IDA curve is proposed as the criterion to determine the seismic sliding stability performance levels of concrete dams.The limit states with respect to the seismic sliding failure process of gravity dams and arch dams are defined respectively.According to the given procedure of seismic vulnerability analysis,the seismic vulnerability curves of gravity dams and arch dams at different performance levels are obtained.The differences between seismic vulnerability curves with or without residual cohesion are discussed.Based on the obtained seismic vulnerability curves,the seismic performance of concrete gravity dams and arch dams with or without residual cohesion is analyzed under maximum design and credible earthquakes,which provides a basis for seismic safety evaluation of concrete dam-foundation systems.(5)Based on the obtained analytic function of seismic vulnerability curve,the seismic risk analysis of concrete dams is carried out through introducing a power exponent function for seismic hazard curve and combining the probabilistic seismic demand model.Thus,the performance-based seismic safety assessment of concrete gravity dams and arch dams is carried out.The probabilities of gravity dams and arch dams reaching different limit states within the design basis period are given,which provide a reasonable scientific basis for the seismic safety assessment under extreme earthquakes.