Criteria for assessment of internal stability of granular soil

An attempt is made to assess the internal instability of granular soils based on analysis of variation of the secant slope of the soil particle size distribution (PSD) curve according to three commonly used semi-empirical criteria: the Kezdi criterion, the Sherard criterion and the Kenney and Lau criterion. A computer code ASISGS is developed to first obtain the secant slope curves of soil PSD curves and then the potential for internal instability in 34 soil specimens is analysed. Comparison with experimental results indicates that the Kenney and Lau criterion successfully distinguishes between stable and unstable soil particle distributions for a majority of soil specimens. The Kezdi and the Sherard criteria are not always reliable, however, and are barely applicable when the finer fraction is less than a specific threshold value, especially for internally stable soil specimens which would be deemed as internally unstable. The intersections of the secant slope curves and the limited slope lines demonstrate that the threshold values can be taken as 15% and 12·95% for the Kezdi criterion and the Sherard criterion, respectively. Finally, a new synthetical chart is proposed for assessing internal instability of granular soils; the results derived are in good agreement with experimental results.