f DAMS 85 



undercutting and erosion at the lower end of the toe when passing 

 severe floods, and permitting a quiet discharge without subjecting 

 the masonry structure to dangers from vibrations. 



The illustrations in Fig. 36 and 37 show a typical design of a 

 masonry dam of the straight gravity type. 



Buttressed Dams. This type of gravity dam has been devised 

 with a view of utilizing the material more economically than is 

 possible in a gravity structure, a typical design being illustrated in 

 Fig. 38. As seen, it is a hollow structure consisting of a concrete 

 deck supported at stated intervals by buttresses or piers per- 

 pendicular to the axis of the dam. As the downward pressure 

 of the water is relied on to a great extent to give the structure 

 stability, the upstream face should have an incline of not more 

 than 45 with the horizontal. The thickness of the deck should 

 be proportioned in accordance with the hydrostatic pressure, and 

 it should vary uniformly from the base to the top, being some- 

 times reinforced with steel to increase its strength. Careful 

 precautions should be taken to make the structure water-tight, 

 and drains should be provided as well as passageways for interior 

 inspection. 



This type of dam requires very good foundations. As the 

 entire pressure must be withstood by the buttresses alone, it is 

 evident that the base width of these at right angles to the axis 

 will have to be considerably greater than for a gravity type struc- 

 ture. 



Arched Dams. These may be either of solid or buttressed 

 design, curved in a horizontal arch with the abutments braced in 

 the rock on the sides of the gorge or canyon, thus giving greatly 

 increased stability. It is not considered good practice, however, 

 to rely entirely on the arch action and dams of this class are, 

 therefore, as a rule designed as a combined arch and gravity type. 

 In fact, the dam is often designed purely as a gravity structure, 

 and the added strength given by its curved form is simply assumed 

 to increase its safety to that extent. 



It has been the general practice to build these dams in one con- 

 tinuous arch, and Jorgensen in " Journal of Electricity" states that 

 for spans less than 600 feet a curved dam of this type requires less 

 material for the same factor of safety than a straight gravity dam. 

 If the gap to be closed is over 600 feet, the cross-sectional area of 

 the arch becomes nearly as great as the cross-sectional area of a 



