DAMS 



77 



than the high-water level and the width of the crest varies any- 

 where from 8 to 10 feet for low dams, to 20 or more for the highest 

 one. 



One of the most important things in its construction is to secure 

 a water-tight foundation. Hardpan and clay are good founda- 

 tions while soft soil and rocks with fissures are very bad. The 

 site must be cleared of tree stumps, roots, etc., and it is always 

 necessary to remove the soil for a depth of 1 to 2 feet. One or 



FIG. 30. Earth-Fill Dam with Puddle Core. 



El. 231 



OnUide Surface to be DrWMd 

 with Boil aad Seeded 



Original Ground Surface 



FIG. 31. Earth-Fill Dam with Impervious Puddle Core. 



more trenches are dug parallel to the axis of the structure, to hold 

 the material, and if the soil is pervious it may be necessary to 

 provide a puddle core, as shown in Fig. 30, in order to prevent the 

 water from seeping under the dam, or piling may have to be 

 driven down to bedrock. 



The material which goes into the structure must be found near 

 the dam site, and its character, therefore, determines the method 

 of construction to a great extent. The best material is a mixture 



