Abstract: The characteristics of percolation stability and measures for seepage control of dam foundations, which are the deep alluvion of the river valleys in Sichuan are described in detail in this paper. The records of the hydroelectric plant Yingxiuwan, the Yuzixi I and Gongzui cofferdam etc. are presented.Finally, based on the data resulting from the experiments, the relationship between the critical hydraulic gradient for piping of the alluvion and the permeability coefficient are found, so are the relationship between the critical hydraulic gradient for piping of the alluvion and the grain size （<φ2mm） percent finer. It is very convenient and effective this results will be used in the design and construction of medium-sized and small structures of water conservancy and hydroelectric power.