Effects of suffusion in embankment dam filters

This study investigates how varying degrees of internal instability of silt–sand–gravel soils affect the grading characteristics due to suffusion. Suffusion is an initiation mechanism of internal erosion, which is a major cause of the failure of embankment dams. A database comprising gradations representative of those used for embankment dam filters has been compiled. Three are based on downward seepage tests by the authors and the remaining 23 are based on reported results of tests published by others. These 26 gradations are cohesionless sand–gravel soils and silt–sand–gravel soils with < 15% fines, some of which have slightly plastic fines. Internal stability is evaluated using two empirical criteria for shape analysis of the particle size distribution curve. By relating the internal stability to typical grading characteristics, the effects of internal instability are determined. Gradually, more severe effects are observed as the internal instability increases. For clearly unstable soils, with a stability index (H/F)min < 0·5 and a probability of internal instability pinitial > 70%, the effective grain size D15 is increased, on average, by a factor of 9 relative to its initial value. Coarsening of a critical filter to an impervious dam core may adversely affect the filter's protective ability.