Abstract: The sandy gravel layer at the lower part of the double foundation of river dikes is dominant, its permeability coefficient is larger than 0.02 cm/s, and it is a kind of piping material. The destructive tests on piping failure of this material are performed. Experimental results indicate that the seepage passages occur at the upper part of the sandy gravel layer, and the percentage of fine particle carried out reaches 80% to 100%. When the percentage of fine particle is smaller than 20%, it reachs 100%. The structure of soil skeleton doesn’t change, but its permeability coefficient becomes larger. When the percentage of fine particle is larger than 20%, the seepage passages will affect the stability of the upper layer.