88 DAMS AND HEADWORKS 



gravity dam for equal stresses, and when it is considered that the 

 arch is always longer than the chord, it is evident that the limit 

 of economy for a single arch span has been reached, if special 

 conditions are not present. 



Recently the multiple-arch type of dam has come into exist- 

 ence and gives promise of allowing big reductions in the quan- 

 tity of material required for structures safely spanning gaps of 

 any width. 



Figs. 39 and 40 illustrate an arched dam and Figs. 41 and 42 a 

 dam of the multiple-arched type. 



General Rules Governing Design of Dams. The following 

 regulations governing the design and construction of dams were 

 recently issued by the New York State Conservation Commission. 



" Complete plans, elevations and sections of all proposed 

 dams must be submitted and approved of by this commission 

 before any work on the dam can be commenced, and the site must 

 be examined and approved of by this commission, both before 

 and after it has been prepared. 



" Foundation Bed: Dams must be built upon a firm, compact, 

 impervious and natural foundation bed, from which all perish- 

 able material has been removed. Earth foundation beds must be 

 ploughed or trenched. Masonry must be carried into solid rock 

 at the base and sides, wherever practicable, and also have channels 

 cut into the rock bed sufficient to afford a firm hold for the dam. 

 Rock foundations must have all loose material removed; the 

 crevices for 200 feet above and for 100 feet below the dam, must 

 be thoroughly filled with concrete or grout, and the whole surface 

 under the dam thoroughly washed. Masonry dams over 35 feet 

 in height must have the rock bed drilled for hidden fissures and 

 tested by compressed air; these holes must be filled with grout 

 under a pressure equal to the maximum ultimate pressure. 



" Calculations: Dams must be stable at any section and under 

 all conditions. The compression upon masonry on the upstream 

 face shall be 10, 14 and 18 tons per square foot and for the down- 

 stream face 8, 10 and 14 tons per square foot, depending on the 

 mass; the first for walls less than 12 feet thick and buttressed 

 dams, and the last for solid masonry dams over 150 feet in height 

 with the best of work done under the inspection of a competent 

 engineer approved by this commission. 



" All cement must be Portland and up to the standard of the 



