EARTH DAMS are formed by depositing the natural soil from the vicinity of the site in thin layers to form the structure and carefully rolling or otherwise rendering conlpaet each layer be fore another is added. Water is sometimes ap plied to the earth to help compact it. Soil that will compact readily and be as little porous as possible should be selected, but it is difficult and generally impracticable to make earth damns im pervious to water. Since continuous percolation through an earth dam would lead to its ruin, it is customary, where an attempt is not made to secure imperviousness through the whole struc ture, to place a water-tight barrier either on the upper face of the dam or at its centre. The former is known as a lining and the latter as a heart or core wall. Both a lining and a heart wall may be used. Heart-walls may be com posed of a carefully selected mixture of clay and loam or sand, called puddle: of concrete; or of stone masonry plastered with cement. One of the advantages of the heart-wall is that it can be carried well into the Iced and banks of the val ley, beneath and beyond the main part of the dam, which is a great safeguard against leaks between the natural ground and the artificial structure. Whether or not the upper face of the dam is lined to prevent leakage, it must be paved with stone. concrete. or brick to prevent dunnage lc the earth slope by the action of the waves at the water-level, which latter varies with the fullness or depletion of the reservoir. \\ here a water lining is desired, puddle is generally em ployed, often with concrete or masonry above it. Asphalt has recently been used to supple ment concrete, brick, or stone lining. Occasion ally the lower slope of earth dams is paved; in high dams it is frequently built with a level place or berm part way up its height. Water is rarely allowed to come within five feet of the top of earth dams or reservoir embankments and it may be kept et en lower. The minimum advisable thickness of the base will increase with the height of the dam and the gentleness of the slopes. The angle of repose, or natural slope, of ordinary earth, dumped in banks. gives a base of one and one-half feet to one foot of height, but. wet earth has a less angle of repose. It is common, therefore, to give dams of ordinary earth a slope of 2 to 1 on the lower or dry face, and or 3 to 1 on the wet face, and even these figures may be exceeded. Some earth dams are backed with loose stone or rock, to give greater stability. Occasionally the material composing earth dams is brought into place by means of flowing water, instead of by carts, scrapers, or buckets, running on and dumped that there were then ten earth dams GO feet or over in height. as shown by the accompanying
table. Later information shows that the fourth darn in the table, now known as the San Leandro, has a total height of 158 feet from the lowest point of the foundation to its crest. and has no heart-wall; also that it extends 125 feet above the original surface. The Tabraud dam, near Jack ',on. Cal., completed in December, 1901, has its crest 120 feet above rock foundation, and 110 feet above the natural surface of the ground. It has no heart-wall.
See Engineering News, July 10, and September 11, 1902. for illus trated descriptions of the Ta brand and San Leandro dams, respectively.
Earth dams or earth embank ments for reservoirs are among the oldest of engineering struc tures. having been built for irri gation thousands of years ago, in Egypt, India, and other Oriental countries.
AlASONEY DAMS, particularly of notable size, are of comparative recent origin, their construc tion having awaited the development of modern engineering. :Moreover. while masonry clams of great height date from the sixteenth century (see table), it was only during the last half of the nineteenth century that their design accorded with the great principles of engineering—maxi mum strength with a minimum of material and coat. The accompanying table, taken from \Veg mann. The Design and Construction of Dams (New York, 1S99), will serve as a basis for tracing the development of the most notable masonry darns of the world during the last three centuries, terminating with the new Croton Dam, under cointruction in 1901. In 1900, or since the table was compiled, the contract was let from a cableway (q.v.) Hydraulic fill dams is the name applied to this rather novel class of structures. This process was used to build a part of the San Leandro and Temeseal dams of the water-works supplying Oakland, Cal., and also in building earth dams at La Slesa and San Joaquin (Lake Christine), Cal., and Tyler, Tex. Earth dams vary in height from a few feet to 100 feet or more, and in length from a few score to thousands of feet. or even to miles. although most of the structures miming into miles are more properly called reservoir embankments. A summary of the heights of earth dams in the United States. for •ater-works purposes alone, compiled from figures included in The Manual of American Water-Works for 1889-90, showed for the Waehusett Dam, which is a worthy rival of the new Croton Dam. Some figures concerning the Wachusett Dam and the Assuan Dam on the Nile (see Reservoirs, below) hare been added to the table.