Supposing that the layer is consisted of two ideal elastic spheres with different diameters, the permeability coefficient of the layer can be calculated. After piping taking place and small grains being carried away, the permeability coefficient will change. The piping model is made to quantificationally calculate the permeability coefficient and hydraulic head of any point. The model explains the causes of piping and the development of concentrated passage, and it provides theoretical basis for prevention of dam piping.
