ICOLD defined a rockfill dam as, "an embankment type of dam, dependent for its stability primarily on rock. As rockfill dams must contain an impervious zone - now usually selected earth with filter zones, comprising a substantial volume of the dam - the term Rockfill dam usually represents a dam that contains more than 50% of compacted or dumped pervious fill. The dam is dependent for watertightness on an impervious upstream blanket or an impervious core."
Like an earth dam it is composed of fragmental materials, with each particle independent of the others. The mass stability is developed by the friction and inter-reaction of one particle on another rather than by any cementing agent that binds the particles together.