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make the wall of the dimensions shown in the table. Wet them thoroughly, then mix and place the concrete.
“Use proportions one part Portland cement to two parts clean, coarse sand to four parts screened gravel or broken stone.
“Take special care to make the concrete water-tight by using a wet mix. If possible, lay the entire dam in one day, not allowing one layer to set before the next one is placed. If it is necessary to lay the concrete on two different days, scrape off the top surface of the old concrete in the morning, thoroughly soak it with water, and spread on a layer about 1/4 inch thick of pure cement of the consistency of thick cream, then place the fresh concrete before this cement has begun to stiffen.
“If the forms on the lower side of the dam are well braced, the forms on the upstream side may be removed in three or four days, and the pond allowed to fill. The forms on the down-stream face should be left in place well braced for two or three weeks. No finish need be given to the surface.”
Reinforced-Concrete Dams.t-Reinforced concrete is particularly adapted to the construction of dams. When so used there is a
TABLE XXXI.-DIMENSIONS FOR SMALL DAMS AND QUANTITY
OF MATERIALS FOR DIFFERENT HEIGHTS OF DAMS. Proportions: 1 Part Portland Cement to 2 Parts Sand to 4 Parts Gravel or Stone
* Make deeper if necessary to get a good foundation.
A large double load of sand or gravel is about 40 cubic feet. † This discussion is arranged from “Concrete, Plain and Reinforced,” by Homer A. Reid.
great saving in material, and on this account a reduction in cost of, in some cases, as much as 20 per cent. Again, the space under the apron may be utilized for storage or power-house purposes, as for the location of turbines, clectric generators, etc. Another advantage is that of securing a practically impervious curtain face wall, without any of the dangerous leaks so troublesome to locate in some masonry structures. If sufficient number of reinforcing rods are used and run in every direction there will be little or no danger of cracking in the deck concrete.
The design of steel dams is that of a triangle with the upstream face so flatly inclined that the water pressure is made to give increased stability by its weight, and this basic principle has been the leading feature in the development of dams of reinforced concrete,
which were first introduced in the Eastern States about the year 1902 by the Ambursen Hydraulic Construction Company, of Boston.
About 30 dams varying in height from 10 to 80 feet, some over 1,000 feet long, have been erected during the last 8 years, many of them attracting marked attention by the engineering profession.
The design of these dams illustrates very strikingly the adaptability of reinforced concrete to new conditions. The principle followed in the design is that the vertical pressure of the water is , utilized to firmly hold the dam down on its foundation.
With the usual type of gravity dams, the up-stream face is vertical or nearly so. The pressure of the water is thus exerted horizontally, tending to overturn the dam, which must therefore be made heavy enough to prevent same from occurring.
In the reinforced-concrete dam, the slope of the water face may be so fixed that the pressure on the foundation is controlled by the designer, and the safety factor is made at least five.
The usual type of reinforced-concrete dam consists of an inclined slab of reinforced concrete extending from the heel to the crest, and spanning between and supported by transverse buttresses of concrete, resting upon the foundation. Another inclined slab may or may not be used to form an apron or spill-way. The deck
is usually increased in thickness from the crest to the heel on account of the increase in pressure as the water deepens.
The principles governing the design of reinforced-concrete dams are the same as those used for the design of masonry dams as far as the external pressures are concerned. However, as reinforced-concrete dams are usually of triangular cross-section, they have a much wider base than masonry structures, which greatly increases their resistance to overturning. This resistance is further increased by the weight of the water above the face or deck, which usually has an inclination of from 30° to 45° with the horizontal.
An increase in the height of the water flowing over a masonry or solid dam increases the pressure thereon and causes the line of pressure to rise, thereby greatly increasing the overturning moment on the dam without in any way increasing the resisting moment to the same.
FIG. 102.—Ashokan Dam of the New Water Supply System for the City of New York.
One of the Largest Dams in the World.
In a triangular dam, however, with a broad base, as in the hollow reinforced-concrete dam, when the head of water flowing over the dam is increased, the lines of pressure become more nearly vertical, the overturning moment is actually reduced, and the stability is in no way endangered. Owing to the reduction in weight it may be necessary sometimes to fill hollow dams with sand, earth, or gravel to increase its resistance to sliding.
Reinforced-concrete dams are particularly fitted to poor tion conditions on account of the broad base and consequent low unit pressures. This will often enable a large saving in cost.
Concrete Reservoirs.-The construction of reservoirs of concrete present but few features not already discussed in the sections on walls and dams. The principal difficulty encountered is in obtaining a watertight bottom, as extensive areas of shallow concrete are subject to cracking on account of settlement, shrinkage, and expansion. The best means to avoid this cracking is by having a double lining. The under lining is laid in a continuous sheet and covered with a sheet of a good asphaltic material, and over this is placed concrete, in sections ten feet square, the joints between the sections being filled with an asphaltic material.