A hydraulic fill dam is one in which the material is transported in suspension in water to the embankment where it gets placed by sedimentation. The sorting effect of flowing water is utilised in creating a fine-grained core at the centre of the embankment with coarse shells on the sides. In a semi-hydraulic fill dam the material is transported by hauling units and dumped at the edge of the embankment. It is then washed to its final position by water jets. The use of this type of dam is rare, because;

(C) Sowers, George Earth and Rockfill Dam Engineering

Drainage of the core takes place in two ways, some of the water percolates horizontally into the more pervious shell. The remainder moves upward to the surface, allowing the centre of the dam to subside. The downward movement eventually develops arching in the core and prevents its full consolidation.