The hydrostatic pressure per square foot at 90 ft. depth is,
62.5 × 90 = 5,625 lbs.
The dam, having a factor of safety of 10, must present a resistance of,
5,625 × 10 = 56,250 lbs.,
or 28 tons per square foot.
The theoretical width of bank corresponding to 90 ft. head and a factor of 10 is shown by the dotted triangle (A-B-B) to be 450 ft., ([B, Fig. 19]) with slopes 2½ on 1.
To this must be added the width due to the height of crest above the water surface in the reservoir and the width of crest.
The former would be,
2 (2½ × 10) = 50 ft.,
and the latter by Trautwine’s rule,