The failure of dikes is mostly caused by the development of piping. Whether or not piping erosion develops is mainly determined by the critical hydraulic gradient. In this paper, a multi-particle model for the critical hydraulic gradient of dike piping is derived. The model is based on the Terzaghi formula, and considers particle size distribution characteristics and the interaction between the sand sample and the water flow. Using a global sensitivity analysis of the model, it was found that the determinants of piping occurrence are mainly the physical characteristics of the soil particles and the particle size distribution characteristics. Laboratory tests and a field case analysis (in China) of piping were carried out. The derived model was found to perform better than Terzaghi's model.