Internal erosion: critical hydraulic gradient in one-dimensional vertical seepage and its relation to soil gradation

A hydromechanical envelope that governs the onset of soil erosion by seepage-induced internal instability is examined with reference to theoretical and laboratory stress-gradient paths for one-dimensional upward and downward seepage flow. The method is validated from permeameter test results on glass ballotini. Using the theory, analysis of a database of 22 gradations establishes a semi-empirical relation between attributes of a grain size distribution curve and a stress-reduction factor governing potential for internal erosion. The semi-empirical relation provides a screening tool for susceptibility of a soil to seepage-induced internal instability.