In order to determine the critical head values of boiling and piping, several experiments were performed for 3 cases of model dykes on 7 kinds of pervious foundations. The results obtained are as follows : 1. It appears that the coarser and the denser the foundation material, the higher the critical heads of boiling and piping, and that the lower the permeability of the foundation, the higher the critical heads of boiling and piping. 2. A difference in head between the moments of boiling and piping is greater in the case 2 or case 3 than in the case 1 because of the additional hydraulic resistance. And it is found that the coarser the foundation material, the greater the head difference. 3. The critical heads of boiling and piping is directly prortional to the seepage length. 4. The piezometric heads close to the singular point are of the same magnitude, provided that the geometry of the model dyke and foundation material are the same. 5. Variations of the weight of model dyke can not affect the critical head. According to the conclusions shown above, the critical head of piping can be more practically predicted for prototype using the results from laboratory tests on scale model.