A series of flume model tests was conducted to study the process and mechanism of piping in a three-stratum dike foundation, which was composed of weak permeable clay layer, sand layer and strong permeable sand gravel layer. To observe the process clearly, fine sand layers of different colors were laid on the gravel layer according to the color sequence. The thickness of colored sand was changed to investigate the effect of thickness on the process and mechanism of piping. The experimental results indicate that the different thicknesses of color sand results in the different critical hydraulic gradients, eroded masses and developing speeds of piping channel, which are quite different from those in two-stratum dike foundation. The critical hydraulic gradient depends on a number of factors including the properties and integrity of the damaged soil. The sand layer makes the flux insensitive to the erosion at the early stage of seepage deformation. Furthermore, it is also found that several times of intermittent erosion occur at the same upstream hydraulic head. One of the reasons for this is that the particles are clogged in the process of movement, and the other reason is that the stress release of the soil near the boundary of piping channel after the erosion of soil at the frontend, resulting in a decrease in the soil resistance over time.