Upon Review of Engineering Design Documents, it is not clear what level of compaction was achieved on the stilling basin wall backfill. In reference, "Retaining Walls Stilling Basin, Foundation Report, 1 May 1955," it states that "the gravel was bulldozed over the slope into the void behind the training wall. Caterpillar tractors, equipped with bulldozer blades, then worked the gravel into all the corners and leveled uneven areas preparatory to placement of more gravel."

Values estimated in the original design documents vary slightly, i.e., a phi-angle of 37 degrees is used in a 1951 design document of stilling basin gravity walls and a phi-angle of 40 degrees is used in a 1953 design document of stilling basin walls. For reference, the phi-angle for the well-compacted embankment shells of the wing dams was determined to be 43 degrees (1989).

It is assumed that the backfill was not compacted beyond the ordinary movement of placing and spreading equipment. Using standard values from reference tables for medium dense coarse grained cohesionless materials (sandy gravel or gravelly sand), the following values were estimated;