124 



Earth dams will not ordinarily be feasible in the Knobstone region, 

 owing to the general lack of clay of good imddling qnalities for the 

 core or the dam. In the edge of the limestone region snch clay may be 

 available, bnt would in most cases have to be moved down very steep 

 slopes at considerable expense. Except on large contracts, where the 

 construction of a cable-way would be worth while, the use of clay for 

 short dams would probably be more expensive than concrete\ In some 

 instances it might be advisable to use earth embaukments with a rein- 

 forced concrete core, mortised well into the rock, and extending to the 

 top of the structure. With this type of construction any sort of material, 

 having the re(iuisite stability, could be used for the embankments, since 

 the waterproof qualities of the dam would depend entirely on the core. 

 Great care is necessary in this typo of dam to prevent settling of the 

 embankments in such a way as to warp or crack the core. It is best that 

 no water should be allowed to come against the dam until thorough 

 settling has taken place. For long dams in the Knobstone region some 

 such construction as that just described is almost a necessity.^ It may ba 

 said finally that timber dams arc '>nly makcsliifts, and should not bo 

 tolerated by any conununity. 



As an example of a successful concrete arch dam in the Knobstone 

 region, a brief descrijition may be given at this point of the dam recently 

 built by Indiana T'niversity. The cross-section of this dam is shown in 

 the accompanying figure (Fig. 1), and photographs of the dam and 

 pond in Fics. 2, :\ and 4. The length of the dam on the rock substratum 

 is IIGJ feet, and on the crest 2(H) feet. The thickness at the base is 

 2S4 feet, and the total height subject to water pressure is 34 feet. The 

 maxinunii height above the valley alluvium is 2S feet. The dam is 

 stepped up in ledges on both the up-stream and down-stream faces, and 

 the cross-section is such that ample stability is provided, even without any 

 arching. The arching (to a radius of .'540 feet) gives very greatly in- 

 creased stability mider water i)ressnre, and vastly decreases the liability 



■ As a niattor of fact, nearly all of tho bids on eartli dams for the University 

 were liigher than on tlie type of concrete dam constructed, and to be described 

 later. 



2 A type of dam, consist ini;- of a thin plate of reinforced concrete, supported 

 by buttresses of concrete is described hi I'.uel and Hill's treatise on reinforced con- 

 crete, and has actually been constructed, in a few cases. This type of dam uses a 

 minimum of structural material, but demands a considerable outlay for forms. 

 It would probably cost about the same as a good rubble concrete dam. 



