Internal erosion, the process of washout of fine-grained material by seepage flow from an embankment dam filling material, can pose a major threat to dam safety. Internal erosion at a far reached stage can surface in the shape of sinkholes, sudden turbid leakages and settlements, indicating that the filling material in the dam is undergoing potentially significant property changes. It was early on found, for instance by Sherard [1979], that embankment dams comprising broadly graded material appear to develop sinkholes more frequent than dams of other material components. And considering that typical Swedish embankment dams are composed of material with broad and wide gradations of glacial origin, in particular the impervious core of broadly graded moraine (till), it should come as no surprise that many embankment dams in Sweden have shown signs of deterioration that can be related to internal erosion. The susceptibility for internal erosion in Swedish embankment dams was perhaps first noticed in a survey carried out by Nilsson et al [1999], where 20 % of the incorporated dams were estimated being affected by internal erosion. To reduce the risks connected to internal erosion it can be necessary to implement dam safety measures preferably at an early stage before the internal erosion progressed to surfacing, but distinguishing which dams that hold a real potential future risk for internal erosion can prove difficult. This paper is based on an evaluation comprising 45 existing Swedish embankment dams and the results show that a more discriminatory predictor, than today’s filter criteria, appears to be needed if embankment dams with widely graded glacial cores with high potential for surfacing internal erosion are to be singled out from dams apparently internal erosion free. The results indicate that not only filter coarseness needs to be addressed when assessing potential for internal erosion in embankment dams with glacial moraine cores, but also the internal stability of the core and filter.

Embankment dams comprising broadly graded materials of glacial origin have in the past been identified as being prone to develop sinkholes more frequent than dams composed of materials of other origin. Sinkholes on the crest of a dam are many times an indicator of internal erosion. Internal erosion can initiate and continue to develop for many reasons, but mainly due to root causes coming from core/filter properties and interaction, possibly affected by the dam design and/or construction related reasons. In this paper a review is made on moraine core dams in the literature in relation to performance history of internal erosion. Dam data comprising 90 existing moraine core embankment dams has been compiled and assessed with objective to investigate the occurrence of indicative properties of dams that have developed internal erosion. The data set of dams consists predominately of Swedish dams, but it also includes moraine core dams located in the rest of Scandinavia, in North America, Russia and Australia. The investigation shows that a coarsely graded filter, grading instability of the core and filter, and high susceptibility for filter segregation are properties that are over-represented and possibly indicative for moraine (till) core dams with performance history of internal erosion.

Internal erosion occurs when fine-grained particles are washed out from a filling material by seepage. Sinkholes and settlements on the dam crest, sudden increased leakage, or muddy seepage may be manifestations of progressed internal erosion. Dams with cores of glacially formed broadly graded soils have been found to be more likely to experience internal erosion than dams composed of other materials. This paper describes and reviews case histories of 30 existing dams constructed from broadly graded soils with performance history of internal erosion. Understanding i) typical signs and observations of internal erosion, ii) the timing of internal erosion incidents, iii) the possible early-warning signs, iv) the possible warning-time, and v) the location of the internal erosion incident on the dam body – may provide tools for early-warning detection of internal erosion prone dams and improve the assessment of internal erosion in existing dams.