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Abstract Based on accurately obtained data of Tangjiashan barrier dam on geological structure(three-layer structures from top to bottom：gravel soil，blocks and gravels，stratoid cataclastic rocks) and the relevant parameters，using the Visual Modflow，which is a three-dimensional visualization software，seepage field of the dam under four conditions of water levels(710，720，730 and 740 m) of the dammed lake are simulated. According to the results of simulated calculation，each soil layer¢s seepage velocity and seepage gradient are calculated，and the analytical results show that the first and the second layers have the characteristics of coarse particles and high permeability，so the overall performance is of a stable flow，with the characteristics of seepage and stable change of overall gradient，and it will not have turbulent conditions of low-permeability clay，inflexion of seepage gradient and the characteristics of piping. The maximum seepage gradient in gravel soil of the first layer is more than allowed gradient in the transition between the first layer and the third layer，so it will cause sporadic or partial seepage destruction on downstream of barrier dam. And simulated results show that gravel soil of the first layer will experience seepage failure when the water level of dammed lake is more than 726 m. With the uplift of water level，the dam-breaking mode of overall barrier dam is that gravel soil of the first layer will be eroded and washed away by the seepage failure and head erosion，also by the down-cutting caused by overflow. The blocks and gravels of the second layer are gradually washed away and down-cut because the first layer is washed and water velocity increased，but it will not happen overall breaking. The stratoid cataclastic rocks of the third layer will remain stable，and the depth of erosion and down-cutting is the top of the third layer.
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Received: 28 January 2010
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