LII. 



C. W. DARLEY. 



B = Radius in feet. 



D = Depth of water to be impounded which should be calculated 

 from the maximum estimated highest overflow or flood level. 

 P = Water pressure in tons per square foot. 

 ^ Z>x62-5 



= Dx -027902 



T = 



.Flood Level. 



2240 

 RP 

 s 

 . • .T = RD x -0014 when s = 20 tons 

 T=RD x -0023 when s = 12 tons 



The thickness of any curved dam at 

 any depth may thus be graphically 

 determined by a simple diagram the 

 top thickness being increased to 3 ft. 

 or 3 ft. 6 in. up to 5 ft., or more, 

 where floating timber is expected to 

 be carried over during floods. The 

 area of the triangle forming the theor- 

 etically safe cross section x s = the 

 total thrust on each abutment. 



There would be no saving of material in constructing a concrete 

 dam, having a limit of resistance of 20 tons per square foot to a 

 greater radius than 500 feet or with a limit of resistance of 12 

 tons per square foot to a greater radius than 300 feet, because the 

 required thickness would, in both cases, be about equal to that 

 necessary for a dam of gravity section having a line of resistance 

 within the middle third. A slight curve in all long dams is, 

 however, advisable, to allow of more freedom of movement under 

 changes of temperature, and to obviate as far as possible, trans- 

 verse cracks due to the contraction of the concrete in setting. 



The average crushing strength of a large number of specimens 

 of concrete made with the usual proportions of dry materials, viz. 

 1 cement, 2j sand, 2 J shivers, and 3 hard metal 1| in. gauge, six 

 months old, has been ascertained by testing to vary from about 

 70 tons to 145 tons per square foot — 80 tons may be taken as a 



