DAMS 



79 



Js superior to the plain earth-filled type, in that less damage would 

 be caused by an overflow. 



If only poor material can be obtained for the earth-fill, it is 

 necessary to provide a puddle or concrete core, the same as with 

 the previous construction, and the wetted surface should also be 

 protected by a rip-rap or concrete facing. 



Masonry Dams. Masonry dams may, according to their 

 design, be divided in two general classes, gravity dams and arched 

 dams, and these further into solid or buttressed structures. 



Gravity Dams. Gravity dams must resist any tendency 

 toward sliding or overturning. 



Assume a dam structure of a trapezoidal cross-section and 

 with the water surface level with the crest, as in Fig. 33. Then 

 the pressure in pounds 

 acting on the up-stream 

 side of the dam per foot 

 length is equal to 



62.4 X //* 

 P = ^ X sec 6. 



Where 



H = Head in feet; 

 = angle of dam sur- 

 face with the 

 vertical; 



62 . 4 = weight of 1 cubic 

 foot of water. 



Water Level 



FIG. 33. Cross-section of Gravity Dam 

 (Water same level as crest). 



This pressure acts perpendicularly to the surface be at a point 

 two-thirds the height of the dam, figured from the top. The 

 leverage with which this force tends to overturn the structure 

 about point d is equal to the perpendicular distance between this 

 point d and the continuation of the pressure line P, i.e., dk. The 

 overturning force is, therefore, equal to PXdk foot-pounds. 



The overturning force must be counterbalanced by the weight 

 of the structure. This is equal to W and it acts perpendicularly 

 from the center of gravity. Its leverage about the point d is equal 

 to di and the resisting force is, therefore, equal to WXdi foot- 

 pounds. 



The center of gravity of a trapezoid may graphically be found 



