Page:The New International Encyclopædia 1st ed. v. 05.djvu/874

DAMS AND RESERVOIRS. 758 DAMS AND RESERVOIRS. the water-level, which latter varies with the lullness or depletion of the reservoir. V here a water lining is de.sired, 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 U'o'r: mw^^^^^^'S^i^^^^'^^^'-^w^^^^fWfW^'^ Fig. 1.— CR08P-SECTI0N OF EARTH DAM, with concrete heart^wall and upper slope paved : Metropolitan Water- Woriis, Southborough. Mass. place or herm part waj' 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 even 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 2% 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 fiowing water, instead of by carts, scrapers, or buckets, running on and dumped that there were then ten earth dams 00 feet or over in height, as shown by the accompanying table. Later information sliows that the fourth dam 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- son. C'al.. completed in December, 1<»01, 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 Neus, July 10, and September 11, 1902, for illus- trated descriptions of the Ta- braud and San Leandro dams, respectively. Eartli dams or earth embank- ments for reservoirs are among the oldest of engineering struc- tures, having been built for irri- gation thousands of jears ago, in Egypt, India, and other Oriental countries. M.soxKY Dams, particularly of notable size, are of comparative recent origin, their construc- tion having aAvaited the development of modern engineering, iloreover. while masonry dams of great height date from the sixteentli century (see table), it was only during the last half of the nineteenth century that their design accorded M'ith the great principles of engineering — maxi- mum strength with a minimum of material and cost. The accompanying table, taken from Weg- mann, Tlic Design and Construction of Dams (Xew York, 1899), will serve as a basis for tracing the development of the most notable masonry dams of the world during the last three centuries, terminating with the new Croton Dam, under construction in 1901. In 1900, or since the table was compiled, the contract was let PABTICULAH9 OF TeS HiGH EaRTH DAMS NAME AND LOCATION Druid Lake, Baltimore. Md Pilarcitos, San Francisco. Cal San Andreas, San Francisco, Cal Lake Chabot, Oakland, Cal Middle Branch, New York, N. Y... , isterdain, N. Y' Kauffman, Pottsville, Pa No. 2, Oneonta, N. Y Owego, N. Y Waverly, N. Y Height. It. 119 95 93 SO 73 65 SI 60 CO (10 Length, ft. Xot given. 640 640 300 740 410 600 250 322 Heart-wall Puddle. None.* None.* Not given. Rubble masonry. Not given. Masonry. •These two dams, while without heart- walls in the damproper, have puddle-fllled trenches, 26 and 47 feet deep, re- spectively, from the natural surface to solid rock. 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 Temescal dams of the water-works supplying Oakland, Cal.. and also in building earth dams at La Mesa 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 running into miles are more properly called reservoir embankments. A summary of the heights of earth dams in the United States, for water-works purposes alone, compiled from figures included in The Manual of American Water-Works for 1889-90, showed for the Wachusett Dam, which is a worthy rival of the new Croton Dam. Some figures concerning the Wachusett Dam and the Assuan Dam on the >>ile (see Reservoirs, below) have been added to the table. -Masonry dams are designed as though they were monolithic structures, and for this reason, as well as because of the fact that the pressure against the face of the dam tends to rupture it both vertically and horizontally, the blocks of stone are not laid in regular courses. Portland cement mortar is used to bind the stones in one homogeneous mass, or the dam may be composed of irregularly shaped masses of stone with the intervening spaces filled with concrete, or it may be made of concrete alone. (See Cement.)