Acceleration Distribution Research For Seismic Response Analysis Of Concrete Faced Rockfill Dams

As a serious natural disaster, severe earthquakes bring great losses to both property and life. Concrete faced rockfill dam, as a familiar type of dams, its instability is the most frequent reason between all of those who cause the break down. Nowadays, high rockfill dam construction is in the ascendance in China. Therefore, the antiseismic stability is the key factor for the engineering safety of the high rockfill dams and this research is important and exigent considering the latency risk of large reservoirs.The pseudo-static method stated in the criterion is widely used in antiseismic stability analysis of rockfill dams, and seismic coefficient is the key data in this method. But the criterion has been established in quite a early time and the data is mainly adopted form 2-D calculation, which nowadays cannot incarnate the seismic response of high rockfill dam, neither include the effect of valley shape and ground motion characteristic. With the development of computer science, large-scale 3-D finite element analysis of rockfill dams is now possible for the seismic characteristic research, especially the seismic coefficient research using 3-D finite element analysis to high rockfill dams, in providing the theoretic valuable data in theory and practice for the antiseismic analysis of the rockfill dams. A great quantity of calculation is performed to rockfill dams using the equivalent linear visco-elastic model, and statistical analysis is also realized.By the comparing analysis of the antiseismic stability methods of rockfill dams, the shear wedge method and the lumped-mass method can only be applied to limited boundary conditions; they cannot simulate dam-foundation system with different terrene situation and cannot simulate the dynamic shear stress distribution on the plane section. It is not enough if only use linear elastic model or visco-elastic model to situations that the rockfill material has obvious dynamic nonlinear character. The finite element method has the advantages of applicability to nonlinear heterogeneity problem, adaptation to complex boundary condition, applicability to static as well as dynamic calculation. Using finite element numerical method can obtain relative accurate solution for seismic response analysis of high rockfill dams.Using the above method, the effect of dam height, slope ratio and valley width to the seismic coefficient is studied. Especially, systematical analysis is performed to the acceleration distribution orderliness of the center section. Some meaningful conclusions are obtained. The amplification effect between 0-5/6H is relatively small. When the height is above 5/6H, the amplification effect will increase remarkably. When the valley is confined, the amplification effect at the top of a 300m-height dam is smaller than a 100m-height dam. When the valley is openness and the slope is cliffy, the dam height almost has no effect to the acceleration at the top of the dam. To high concrete faced rockfill dam, with the slope less cliffy, the maximum value of acceleration at the top of the dam becomes small. When the dam in a wide valley is not so high, the slope ratio almost has no effect to the amplification effect. But to high dams, when the slope is cliffy, the width of the valley has obvious effect to the amplification effect.The effect of earthquake characteristics to seismic response is also studied, and in the calculation four typical seismic waves are adopted. It is indicated that, with the increase of amplitude of acceleration inputted, the amplification effect at the top of the dam is diminished. This is because when the acceleration becoming larger, the shear stress becomes larger and the shear modulus becomes smaller. The spectrum characteristic of a seismic wave has great effect to the seismic acceleration response of the dam.The acceleration distribution along the dam axis at the top of the dam is studied. It is indicated that the acceleration distribution along the dam axis is symmetrical about the midline of the dam. With the valley width increasing, the maximum acceleration moves from the center section to the sideward section symmetrically. For confined valley, the maximum acceleration calculated using 3-D model is larger than that calculated using 2-D model. For wide valley, the maximum acceleration located at the place nearing the bank.2-D calculation cannot show the place where the maximum acceleration is happen. Therefor, the 2-D calculation is not anymore applicable for the wide valley because of the above results. It is commended that, to high rockfill dams, the seismic coefficient along the dam height as well as along the dam axis should be afforded.Statistical analysis is also made to the seismic coefficient, and an empirical equation for calculating the maximum acceleration is provided. Seismic stability analysis of rockfill dams using pseudo-static method can be modified according to the equation.Two actual projects of rockfill dam engineering are researched, they are'Quxue concrete faced rockfill dam on the Shuoqu River in Sichuan province'and'Xilongchi asphalt concrete faced rockfill dam in Shanxi province'. They are first meshes using 3-D finite element method, and then seismic response analysis is studied. The acceleration distribution of the dams is especially studied. The calculation result can prove the design scheme and offer some experiences to practical engineering construction.