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Variables that can be considered in experimental investigations of internal instability of earth dam materials are often bounded by practicality and feasibility constraints. To overcome these shortcomings, a total of 164 previous published experimental results were compiled to study statistically the relative significance of vertical effective stress, grain size ratio (D15/d85), fines content, coefficient of uniformity (Cu), and specimen length in the mechanism of internal instability. The present statistical results agreed with previous experimental findings on the combined dominant roles played by fines content and stress in the internal instability. However, the stress reduction factor was found to be inversely correlated with the vertical effective stress, which was not reported previously. The scale effect of specimen size has significant positive correlations with both the stress reduction factor and critical hydraulic gradient, whereas the influences of Cu and D15/d85 were minimal, in contrast to previous experimental findings. An improved theoretical hydromechanical envelope for predicting internal instability of cohesionless soil in the space of stress-gradient–stress reduction factor was proposed in this study. The unstable envelope moved with increasing applied hydraulic gradient until the state of material touching the envelope. [ABSTRACT FROM AUTHOR]