Abstract: The seepage failure is a common failure mode of earth-rock dams because of the influence of dam body defects or trans-dam structures,and the piping failure constitutes a large proportion.A series of studies on piping have been conducted,and the corresponding models have been put forward.However,no deep studies on the development process and the failure rules after piping have been carried out.In this paper,the critical hydraulic gradient of moveable grain and the change of void ratio during the piping development process are given.Moreover,the calculation methods for the discharge through piping passage and the control condition of the transition from orifice flows to weir ones are also derived.Based on the above results,a mathematical model of piping development is put forward,and the gradation and the density are both analyzed by use of it.The relationship among the discharge,velocity and porosity is established.Comparison between this model and other existing models shows that the present model gives more rational consideration of the influence of break development process on dam material characteristics.The actual discharge hydrograph tests on the dam break have confirmed the rationality of the model.