78 DAMS AND HEADWORKS 



of gravel, sand and clay, and if this is readily obtained, the struc- 

 ture is generally built homogeneous, as in Fig. 31. 



There are many different methods of placing the material, 

 such as providing trestles and dump-cars, cable ways, etc. If the 

 material is taken from a higher elevation than the dam, and water 

 is plentiful, the hydraulic method of rilling may be used and is 

 generally found very economical. 



If good material is not to be found near the site, puddle or 

 concrete cores must be built to insure an impervious structure, as 

 shown in Fig. 31. 



Such a puddle core is preferably made of a mixture of clay and 

 gravel, this being considered superior to clay alone. It is placed 

 in the center, with the finer material next and the coarser outside. 

 It should be protected from becoming dry, in which case it would 

 crack and permit the water to seep through. Enough water is, 

 however, generally percolating through the structure to keep it 

 moist. The fill towards the outside surface should, however, be 

 kept as dry as possible to keep it from disintegrating, and it is, 

 therefore, advisable to install an efficient drainage system on this 

 side. 



To protect the wetted side from the effect of the water it is 

 usually constructed with a rip-rap, and sometimes a concrete 

 facing may be advisable to prevent seepage. The other side 

 should also have a covering of rip-rap or gravel, or it should, at 

 least, be sodded. 



Rock-fill Dams. A typical construction of this type of dam 

 is shown in Fig. 32, the essential difference between the same and 



FIG. 32. Rock-Fill Dam. 



an earth-filled dam being the rock-filled part which forms the 

 down-stream section, while the other side is filled with earth and 

 gravel. 



The rock-fill serves as a support for the earth-fill, which makes 

 the dam impervious, and it is, therefore, evident that this type 



