Qiangtang alluvial layers are mainly composed of sandy silt and have “highly silty and weakly clayey” characteristics. Seepage failures can be easily induced in these layers by the hydrodynamic pressure of underground water. Silt is a kind of complex transitional soil between sand and clay; and its property is different from both sand and clay. Seepage stability tests are conducted with the testing apparatus developed by authors on samples of five typical types of silt in Qiangtang alluvial layers (including sandy silt and clayey silt) as well as samples of undisturbed soil with horizontal beddings. The results of the tests show that Qiantang alluvial silt has poor seepage stability and low impermeability. In terms of the sandy silt from Qiangtang alluvial layers. (1) The critical hydraulic gradient (icr) of sandy silt ranges from 1.01 to 1.25. (2) It turns into local flowing soil when failure occurs. (3) Its transition from the critical state to the failure state occurs instantaneously; and thus deep excavation in such sandy silt layers can very easily cause seepage failure. As with the Qiangtang clayey silt. (1) Its critical hydraulic gradient increases with the content of clay. (2) Its transition from the critical state to the failure state is relatively slow compared to the sandy silt. (3) It fails in the form of whole flowing soil. The tests also reveal that the seepage stability of Qiantang alluvial layers is anisotropic. The seepage perpendicular to the horizontal beddings has the maximum icr

which is about 2.5 times the icr of the seepage parallel with the beddings. The research provides more knowledge on the prevention of silt seepage failure.