Comparison of two criteria for internal stability of granular soil

Internal instability describes the loss of soil particles as a result of seepage. Two criteria are commonly used to determine the potential for such instability within a granular soil that is subject to seepage flow. The criteria are similar, in that they both require an evaluation of the slope of the grain-size distribution curve. However, the manner in which the evaluation is made yields a subtle difference between the respective methods. Comparison of the methods reveals that the difference can have implications for evaluation of broadly graded soils, and guidance is suggested for their use with greater confidence in engineering practice.