Flow tests under high hydraulic gradients were carried out to estimate the internal stability of various granular materials with grains ranging from .08 to 16mm. Twenty grain size distributions were tested, either continuous or discontinuous (gap-graded). After the tests, the materials were separated into three layers and the grain size distribution of each layer was measured. The soils were classified as stable if the maximum difference between the passing percentages of the upper and lower layer did not exceed 5% and as unstable otherwise. The experimental data were compared with the results given by some internal stability criteria which are obased on very different approaches. The same method was applied to about 40 soils from the literature. It was found that the simple criterion of Kezdi, which consists in separating the soils into coarse' and fine' fractions at different values of the diameter and applying Terzaghi filter rule to the two fractions, led to global agreement with the experimental data, whereas the two other criteria gave unsatisfactory results. However, a few differences were noted between Kezdi classification and the results of the tests, showing the need for further research, especially in one case (over 65) where the soil was classified as stable and proved unstable.
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