DAMS AND BESERVOIKS. 760 DAMS AND KESERVOIRS. Overturning is guarded against by giving the structure sueli a cross-section that the lines of pressure will be thrown within the centre third of the dam. To provide against crushing, a material with high resistance to such action is selected and the structure is so proportioned that Original Sijrfi3te_ ■^cayarflo ---^^ ^ "Cut Stone ^T^-^l}t>:Bei/H -^ UnEarfh^ -s^i I'ti^iLUAI^'.H. Fig. 2. — CROSS-SECTION op a masonry dam (New Croton Dam for New York City Water- Works). its own weight will not crush the material in its lower part. As a general rule the pressure should never exceed fifteen tons per square foot, and with some materials it may need to be as low as six tons. Obviously the only way to pre- vent excessive or crusliing pressures at the bot- tom of ver' higli dams is to diminish the thick- ness as the height increases. The action of ice and of the actual or possible current of over- flowing water renders it necessary to make the top of the dam thicker than would be required to resist water pressure alone ; otherwise the dam might be tapered to a knife edge at the top. The common type of cross-section for high masonry dams approaches a right-angled tri- angle, with the perpendicular side up-stream, but it varies from a real triangle in having both sides curved somewhat, particularlj- so as to give a broader base, and in having the extreme upper part built with nearly parallel sides, while the top is fiat, or perhaps more or less rounded. While some of the early dams were quite bold in cross-section, most of them were far otherwise. The French engineers were the first to apply the results of theoretical study of the subject to the design of the cross-section of dams. M. Sazilly discussed the questions involved in a memoir published in Amialcs des Fonts et Chatisses (Paris, 1853). M. Delocre carried the work on to more rational conclusions, which were made public in 1S5S, and on which the design of the Furens Dam, near Saint Etienne, France, was constructed (1862-G6). A memoir on these stud- ies was published in the journal just named in 1866. Although this dam was only 6 feet higher (170.6 against 164.24 feet) than the Puentes Dam, completed in Spain in 1791, the French dam was 0.91 feet thick at the top and 161.02 at the base, while the Spanish dam was .3.5.73 feet thick at the top and 144.29 feet at the base. The French dam ^^■as curved in plan and the Spanish Avas polygonal. An Englishman, Prof. W. .J. Rankine ( q.v. ) , made the next notable study of the subject. (See his Miscellaneous Scientific Papers. London, 1880.) A number of eminent American engineers connected with the new Croton Aqueduct and its adjuncts, for Xew York City, made studies which resulted in the design of the highest masonry dam yet attempted, which was put >inder construction in 1892. Very extensive investigations were made in this con- nection by Mr. Edward Wegmann, who published an important treatise, entitled The Design and Construction of Masonry Dams (New York, 1888), to which new matter was added and a new title given in 1899, .so as to cover all classes of dams. Arched or Curved Dams have given rise to a great amount of discussion as to the possibility of utilizing the arch principle to resist a part of the thrust of the water on the dam, instead of relying wholly on gravity dams, or those with a section which gives sufficient weiglit to resist overturning and sliding. The most notable dams designed on this principle are the Bear Valley, Zola, and Sweetwater dams, details of which are given in the table. The first two are declared by Jlr. James D. Schuyler, in his work on reservoirs (see below), to be "so slender in profile as to be absolutely unstable were they built straight." The Bear Valley Dam is only"3.17 feet thick at the top, 20 feet thick at the bottom, and is 60 feet in height. Co.xcRETE ilASONRY Dams are not essentially difTerent from other masonry structures, except in their composition. (See" Cement and Con- crete.) Perhaps the most notable concrete dam in the world is that near San Mateo, Cal., built by the Spring Valley Water-Works Company of San Francisco. This dam, which is of the arched type, had attained a height of 13.4 feet in 1888 or 1889, but is designed to i-each ultimately 170 feet, with a top width, when completed, of 25 feet and a width at the base of 176 feet. Rock-Fill Dams are built of large stones, or rock, loosely put in place, but with liand-laid face or slope walls. To make such dams water- tight, or sufficiently so for the objects to be attained, the up-stream or wet slope may be faced with plank, concrete, concrete and asphalt, or steel. It is also possible to use earth to form either the upper or lower section : or riveted steel plates may be built in the centre of the structure. A masonry wall, with earth above and rock fill below, faced on the lower slope with stone laid in mortar, is another variation. The adoption of this form of construction is generally in the interests of economy, in localities where the transportation of cement would be very costly, where e^irth dams are out of the question, and where stone is abundant and easily thrown into place. The Escondido Dam, built by a Cali- FlG. 3.— ROCK-FILI, DAM WITH STEEL HEAET-WALL OR DIA- PHRAG-M. Southern California Wat^r Company, San Diego, Cal. fornia irrigation district of that name, is one of the most notable of the rock-fill structures. It is 76 feet high, 10 feet thick at the top. and 140 feet thick at the base, has top and bottom lengths of 380 feet and 100 feet respectively. The hand-laid dry wall on the upper or wet slope is 15 feet thick at the base and 5 feet at the top. It is covered with redwood plank and the space between the plank and the stone was rammed full of concrete. The joints in the planking were calked with oakum and daubed
