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 thirty existing dams constructed from broadly graded soils, with a performance history of internal erosion. Understanding i) the 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.
