Assessing potential for internal erosion in glacial moraine core embankment dams

Internal erosion, the process of wash-out of fine-grained material by seepage flow from an embankment dam filling material, can pose a major threat to dam safety. Many embankment dams in Sweden, having the typical composition with an impervious core of widely-graded glacial moraine (till) and protective filter of glaciofluvial sand and gravel, have experienced internal erosion that surfaced in the shape of sinkholes, turbidity and sudden increased leakage. The general consensus, as far as internal erosion is concerned, is that the downstream filter to the core is the most important protection against internal erosion. This paper is based on a study comprising 45 existing Swedish embankment dams and the results reveals that: on one hand a majority of the dams have filters that do not satisfy current filter requirements, and on the other hand only a few of these dams have developed internal erosion that surfaced in the shape of sinkholes or other related signs. This indicates that the current filter criteria is somewhat limited from an analysis point of view, preferably intended for design use with built-in factors of safety. A more discriminatory predictor appears to be needed if embankment dams with widely graded glacial cores with high potential for surfacing internal erosion are to be analytically distinguished from dams with low potential, apparently internal erosion free. This paper will show that not only filter coarseness needs to be addressed when analyzing potential for internal erosion in embankment dams with glacial moraine cores, but also the internal stability of the core and filter.