CONCRETE MASONRY DAMS are not essentially different from other masonry structures, except in their composition. (See CEMENT and Cos CRETE.) Perhaps the most notable concrete dam in the world is that near San :Mateo, Cal., built by the Spring Valley Water-Wo•ks 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 reach ultimately 170 feet, with a top width, when completed, of 25 feet and a width at the base of 176 feet.
RocK-Fm. DAMS are built of large stones, or rock, loosely put in place, but with hand-laid face or slope %vans. 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 lie 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 earth dams are out of the qUestion, and where stone is abundant and easily thrown into place. The Escondido Dam, built by a Cali 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 with asphalt. The Lower Otay Dam, near San Diego, Cal., completed in 1897, is a rock-fill dam with a steel core. The dam was started in masonry, but being carried to a height of 40 feet above its lowest point. when its top length was only S5 feet, it was decided to change the design. An inverted T-iron (thus, 1 ) was bolted to the masonry and steel plates one-third of an inch thick, 17 ?0 feet long, and 5 feet high were riveted first to the T-iron, then to each other until three courses had been placed. The plates were dimin ished in thickness as they neared the top. The dam is 161 feet high above its lowest point, 130 feet high above the natural earth, and is of rock fill for 121 feet. The steel plates were protected by a coat of hot Alcatraz asphalt, then a layer of burlap, then harder asphalt, and finally one foot of Portland cement concrete, on each side. A part of the rock fill was deposited in place by the force of a very heavy blast and the rest was transported from the quarry by a cableway (q.v.) 94S feet long. Nearly 180,000 cubic yards of stone were used. The stream flow must be passed around and not over rock-fill dams.