122 



FORESTRY AND IRRIGATION. 



March, 



other drainage Ime, where banks sud- 

 denly approach each other, and where 

 an artificial embankment of modern di- 

 mensions will complete the enclosure of 

 a reservoir of considerable capacity, the 

 water supply to be furnished by its own 

 drainage basin or to be led into it b}' a 

 ditch. 



Where it is practicable to build the 

 structure of earth it is usuall}^ far 

 cheaper than masonry, wood, stone, or 

 any other construction possible. This 

 possibility is dependent upon the exist- 

 ence near at hand of suitable material 

 for an earth dam, and in a less degree 

 of an impervious earth foundation upon 

 which it may rest. If the dam is not 

 to be high, it is possible to safeh' con- 

 struct it of earth upon a rock foun- 

 dation by taking particular pains to 

 make a tight bond between the earth 

 and rock ; but it is a difficult matter, 

 and where its height will bring a heavy 

 water pressure to bear it is usually not 

 practicable to make a sufficiently tight 

 bond between the earth and the rock. 

 In such a case an earth dam may be 

 built with a masonry core carried a 

 short distance down into the rock and 

 built up through the center of the dam 

 as high, or higher, than the water sur- 

 face is to be. 



Where the foundation is of earth this 

 should be stripped of all organic matter 

 down to a reasonably compact loam or 

 clay, and the earth placed in the em- 

 bankment should be freed from all or- 

 ganic matter. The ideal mixure for the 

 bank is clay and sand and gravel in ap- 

 proximately equal parts, or in any pro- 

 portions such that no one of the three 

 shall constitute less than 20 per cent, 

 nor more than 60 per cent, of the bulk. 

 In general, the finer materials and the 

 larger proportion of clay should be 

 placed in the center of the dam and the 

 side next the water, while the larger 

 proportion of gravel should be in the 

 down-stream half of the dam. These 

 materials may be placed by scrapers or 

 carts, and shovild not be dumped in piles, 

 but should be scattered in layers from 

 4 to 6 inches in thickness, and sprinkled 

 with a hose or sprinkling cart, so as to 

 make them thoroughly moist, but not 

 so wet as to constitute what would be 



called mud. While in a moist condition 

 the material should be well trampled or 

 compacted b}' rollers. An ideal method 

 of construction is to bring the material 

 into place by means of scrapers, and to 

 have a heav}^ sprinkling wagon, drawn 

 by four horses, the wheels having wide 

 tires and so arrangecf as not to track. 

 The sprinkling cart then becomes a 

 roller, and, together with the trampling 

 of the men and horses handling the 

 scrapers and the team drawing the cart, 

 performs the office of compacting the 

 embankment. The wagon should be 

 kept moving and performing the func- 

 tion of a roller, even though continuous 

 sprinkling is not required. 



Where the material, and especially 

 the clay, is sufficiently abundant, such 

 a structure may be made virtuall}' im- 

 pervious. In case the clay is scarce or 

 must he hauled a long distance, it may 

 be found advisable to build a core of 

 carefully selected materials, mixed with 

 a proper amount of clay and carefully 

 "puddled " that is, moistened, mixed, 

 and compacted. The rest of the dam 

 may then be made of coarser materials, 

 reliance for impermeability being placed 

 upon the puddled core. The founda- 

 tion of the dam should be rendered 

 rough, so that there will be no definite 

 horizontal joint between the dam and 

 its foundation. This may be done by 

 plowing deep furrows across the ravine, 

 or, still better, by digging deep trenches 

 parallel to the axis of the dam. The 

 top width of the dam should be not less 

 than five feet, and wider for higher 

 dams. A good formula is five feet plus 

 the square root of the maximum height 

 of the dam. 



The slopes of the dam should be suffi- 

 cientl}^ flat so that there will be no 

 danger of the earth rolling down the 

 slopes that is, they must be flatter than 

 the ' ' angle of repose ' ' of the material 

 of which it is constructed. Conserva- 

 tive slopes are 3 horizontal to i vertical 

 on the water slope, and 2 horizontal to 

 I vertical on the down-stream slope. 

 Steeper sides may be given where the 

 material is especially favorable and 

 where important wave action is impos- 

 sible; but it may be said that the water 

 slope should not be steeper than i on 2 , 



