Earth dams are made of carefully selected material, preferably gravel containing enough clay to make a water-tight puddle. Too much clay is to be avoided: it may range from 5 to 30 per cent of the remainder, according to the kind of material. The water slope being more or less saturated should be much flatter than the dry slope, the approved practice being a slope of three horizontal to one vertical. The earth is built up in shallow layers which are thoroughly compacted by rolling, trampling by horses or solidified with water.
Earth dams fall usually within four classes: (1) The homogeneous bank of earth; (2) the bank of earth with puddle core; (3) the bank of earth with a core of masonry or sheet pil ing; (4) the bank of earth puddled on the water slope.
The object of the masonry core is chiefly to ing from a height, the finer material up-stream, and having very flat slopes. Such a dam is in reality a mound, and can be made of a wide variety of material, the stability depending upon a moderate ground-water plane through the body of the fill rather than on impermeability. Such plane should not be sufficient to produce springs that will disturb the material, and the fill can be tightened or ((sealed" by means of roily water or mud against the up-stream face. Such dams are sometimes built with wood core walls and sluiced only on the up-stream side.
Besides the Tomhannock Dam here illus trated may be mentioned the following notable earth dams: The Ekruk Dam (India) is 7,300 feet long and 72 feet high, the reservoir holding 24,870, 000,000 gallons.
The Belle Fourche Dam (South Dakota) is 6,493 feet long, and has a maximum height of 122 feet. It is 19 feet thick at the top and 620 feet at the base. It is built of heavy clay, in cut off water seeping through, not to resist the hydrostatic pressure. The thickness of the core walls at the water level is usually one-seventh to one-sixth of the head in the reservoir.
Earth-dams have failed by over-topping from insufficient spillways or deficient free board, from saturation and sloughing of the rear slopes, and from cutting out around pipes carried through the base. Embankments sub ject to water action should have slopes much in excess of the so-called ((natural(( or dry slope. Present practice tends to an impermeable up stream face and upper toe wall or intercept to a safe depth with good drainage behind, thus developing the full stability of the material. Pipe lines through •made banks are to be avoided, and in any location great care is needed to prevent seepage along the pipe line. The minimum thickness at the top is 10 feet, and 30 feet is regarded as a safe maximum.
The hydraulic-filled dam is probably a de parture in the right direction. Dams of large
section are filled by water carriage or by dump 6-inch layers, packed by steam rollers. The up-stream face is covered with heavy concrete slabs. The reservoir holds 66,500,000,000 gal lons.
The Gatun Dam (Canal Zone) is 7,700 feet long, 115 feet high, 100 feet wide at the top and 2,019 feet at the base. It contains 21,146,000 cubic yards of earth. The hydraulic fill has a base 1,200 feet wide.
The Lahontan Dam (Nevada) is an earth fill of gravel and silt built up in 4-inch layers. It is 1,300 feet long and 124 feet high. The thickness at the bottom is 620 feet, and at the top 20 feet. On the water side it has a facing of rips p 24 inches deep.
The San Leandro Dam (California) is 500 feet long and 125 feet high. It is 28 feet thick at the top and 1,700 feet at the base. About one-third of the material was sluiced in by the hydraulic method.
The Tabeaud Dam (California) has a length of 636 feet and a height of 120 feet. It is 20 feet thick at the top and 620 feet at the base.
It ifalltlilfl lafi r IP clay; iiit ,&-rinelt :layers;! wttiell Werti;s killed, trampled` by' het Hes and) odifett I lengthwise ' by • cartivheela: The:Itip4treariv )slope' is ptiddied 'arid voyeruct ' with lees & rock' fill. • 1 • - - • • • ' ' - • I • ' Tke• -Arrowhead Darn !! (California) is the seeptut higheit 'earth dint 'in • the world —222 feet! Iltisl850 'feet king, is 20 feet Wide at tell)! 950 1 eet 'at • the base. It 'has it vire wall . ofYconeifete feet high, 26 feet :thick at dife) batteand ; at the top. Thon earth , irase dumped elty and' hydranlicked'irtto'plade•.: -!!•: A The Calaveras! Dam, an adjiinctiof the Sin Francisco water supply, built '19131/6, is• the' highest earth dam. on. record. It is' 1,260 Vein% long, with pi axitutim1itightr led feet above bed rock. , - It is' 25 feet thick at 'the to and" 1,312 feet at the base. The material was sluiced in,' the'eent•o'being the 'finest day and 'silt de posited iwrstill water to simulate lactistrineclay.' It was washed out down an °pea I 'the clay content oaring from 20 to SW per cent, the remainder being sand and gravel. The was raised to the dam by mud- Timhps.- - - - _ ,-_,- - --- - -_---- - - - - • -•; - ' timber-dam across flow ing streams for Power- sea uni*rsal in the pioneer eve erit . of the cou try. Timber was plentiful and Use.s wid ly familiar. Wooden wheels with horizon tales utilized moderate heads, and these wars also limited by the height of banks so as to avoiexcessivelfoVage that the flow of the dry season could be us the daylight hours.