Observations on internal stability of cohesionless soils

INTRODUCTION Internal stability refers to the ability of the coarser fraction of a soil to prevent migration of its finer fraction as a result of seepage flow. Skempton & Brogan (1994) report findings from piping tests on four well-graded to gap-graded sandy gravels. Flow was imposed in an upward direction, without surcharge loading and without vibration. An abrupt transition was noted from stable to unstable states, defined by the ratio D9 15=d9 85 1⁄4 4 as postulated by Kezdi (1979) and attributed to work published in 1969. The latter concept was conceived independently by de Mello (1975), advocated by Sherard (1979) for a ratio 4 to 5, and recommended by Lowe (1988) for a ratio of 5. It involves splitting the gradation at arbitrary points along the curve, to yield D9 15 of the coarse fraction and d9 85 of the fine fraction, as illustrated in Fig. 1 for gradation G3-C*. In this study we further evaluate the Kezdi (1979) criterion, through laboratory permeameter testing of five soils (Fig. 1 and Table 1), and compare the results with a dataset of 34 gradations extracted from Kenney & Lau (1985, 1986) and Honjo et al. (1996).