76 DAMS AND HEADWORKS 



Soft foundations usually require a dam with wider base, and 

 it may be necessary to first fill in with rock or gravel, while if the 

 soil is pervious piling may also be required. Undermining should 

 also be guarded against by extending the facing at the toe. 



Figs. 28 and 29 show a rock-filled crib dam of modern design. 

 The up-stream side has been given such a slope that the stability 

 of the dam is assured even under the greatest floods, the weight 



FIG. 29. Cross-section of Timber Crib Dam Shown in Fig. 28. 



of the water acting to hold it down, so that the higher the flood 

 the greater the stability. The down-stream side is also sloping 

 and tapers off into a long apron so designed as to take care of the 

 overflow without shock or commotion. A sluiceway is provided 

 at one end of the dam, and at the other there is a concrete cham- 

 ber or forebay serving as intake to the pipe lines supplying the 

 plant. The openings to this forebay are controlled by gates and 

 are provided with the usual screens for the exclusion of trash. 



Earth-fill Dams. This type of dam generally has a trapezoidal 

 cross-section and consists, as the name implies, of an earth-fill 

 faced with some harder material. It cannot be overturned and its 

 stability depends on the imperviousness of the material used in its 

 construction. It is not intended to be used as a weir, and in case 

 of overflow is liable to be disintegrated and washed away. For 

 this reason, earth-fill dams must be provided with spillways if 

 there is danger of flood-waters passing over the crest. They are 

 not intended for very high structures, and while dams of this type 

 have been built for heights above 100 feet, about 50 and 75 feet 

 is more common. There are no definite rules laid down for cal- 

 culating the dimensions, but it is considered good practice not to 

 let the slope of the wetted side exceed 1 in 3, while the outside 

 slope may be 1 in 2. The height should be at least 10 feel higher 



