Strength changes during internal erosion of gap-graded soils

Internal erosion from internally unstable soils can negatively affect long-term strength of embankment dams. Investigations of the packing behaviour of a gap-graded internally unstable soil identified two distinct microstructures: (a) a microstructure in which the non-erodible coarser fraction remains at its densest packing when the proportion of erodible particles is below a transition finer fraction and (b) a microstructure in which the coarser fraction becomes looser as the finer fraction increases above this transition. Laboratory tests, using water-soluble salt as the erodible particles, showed that for the first microstructure mechanical behaviour was unaffected by particle loss (i.e. suffusion). It was only for the second microstructure that particle loss affected mechanical behaviour (i.e. suffosion). When the proportion of erodible particles exceeded a higher critical finer fraction, coarser particles became dispersed, no longer forming a skeleton from which particles could erode, resulting in internal stability.