Assessment of the internal stability of a dam core

Internal erosion due to seepage forces represent one of the main causes of embankment dam safety incidents and failures. This phenomenon can initiate by internal instability where the finer particles in a dam core can erode through voids formed by the matrix of coarser particles. The assessment of internal stability of soils in existing dams or for future projects is of utmost importance for a dam owner. An application of three internal stability assessment methods based on the geometry of grain size distributions (construction control data) was realized for the silt-sand-gravel core of a 171 m-high rockfill dam. This led to contradictory results. The representativeness of conclusions drawn from such applications should not be judged only by considering laboratory testing conditions used in establishing the various methods. Results from field monitoring data must also be taken into account. The use of grain size distribution criteria is a first step to assess internal stability. A thorough assessment of the internal stability of an existing dam would require the analysis of the core and filter materials by considering the spatial variability of their geotechnical properties as well as the interactions of geometric, hydraulic and mechanical susceptibilities to internal instability. Specific tests may be required for new projects particularly if the grain size distribution of soils under consideration is gap-graded or has a flat tail of fines.