Dam, an obstruction or barrage employed for raising the level of water in a stream, or for excluding it from an enclosure. Structures of the latter kind are called coffer dams, and are used to afford facilities for excavating. Dams for raising the level of water in streams have a variety of purposes, such as furnishing water power to machinery, supplying aqueducts for conveying water to towns or for irrigation, and for effecting slack-water navigation of rivers. The material and construction of a dam will depend upon its object and upon the cross section of the body of water it is intended to restrain.' For still and shallow water of not more than 5 ft. depth, where the foundation is firm and comparatively impervious, an embankment of stiff clay, 8 or 10 ft. in breadth, and well rammed, will be sufficient, if a gate is provided to keep the level of the water below the top of the embankment and thus prevent its wearing away. Where the confined water has much depth and breadth at the dam, the construction requires great strength, and therefore must be of materials capable of being firmly joined together and also to the banks of the stream.

Stone masonry well laid in hydraulic cement and framework of timber, or the two in combination, are the materials usually employed in the construction of dams of great strength. When dams for slack-water navigation are built upon streams which are subject to heavy freshets, the selection of the site is very important. It is generally advisable to place them where the stream is pretty wide, for the purpose of allowing a ready flow over the dam during high water. If built at a narrow place, the restraint to the outlet would so increase the hydraulic as well as the hydrostatic pressure that great expense would necessarily be incurred in making a structure sufficiently strong and securely joining it to the banks, and in many cases the object could not be accomplished. Sometimes the dam is built in a straight line transverse or diagonal to the current. The diagonal line allows the readiest flow, but the transverse is generally preferable on account of making less disturbance in the bed of the stream below. It may also, for greater strength, be built in the form of an arc with the convex surface toward the current, or it may have two or more straight lines, the angles pointing up stream.

In constructing a dam, it must be borne in mind that the pressure of water is in proportion to its depth, but the circumstances not only vary with difference of location, but in the same location in consequence of changes which constantly take place in the current of the stream. Often during a freshet the surface becomes exceedingly rapid, so as to exert great force against the upper part of the dam, and calculations based upon hydrostatic pressure alone would prove erroneous. The rule therefore is to supplement mathematical calculations by a judgment as to the requirements necessary in each particular case, and to make the structure strong enough at every point to withstand whatever force may be brought against it under any possible circumstances. Dams are often built of a framework of timber, a plate resting upon posts which in turn are supported by a sill, reaching across the stream. Upon the plate rafters are laid, one end resting upon the plate and the other reaching up the stream and resting upon the rocky bed or upon another sill. Across the rafters planking is placed of sufficient strength to withstand the hydrostatic pressure, as well as that of the running water. The dam across the Connecticut river at Holyoke, Mass., completed in 1849, is 1,017 ft. long and 30 ft. high.

It is formed of square timbers inclined 22° to the horizon, having one end bolted to the rock and the other resting upon a timber framework. From the crest of the dam descends an apron 4 ft. long, which slopes also at an angle of 22°. This dam has withstood the heaviest freshets in the Connecticut river without having given way in any part. The water is delivered by 13 gates to a main canal faced with masonry, 140 ft. wide at bottom, 144 ft. at top, and 22 ft. deep. The motive power afforded by this dam is said to be the best in the United States. The barrage across the river Furens in France, for the primary purpose of forming a reservoir to hold the waters of the river during freshets from inundating the town of St. Etienne, and the secondary one of supplying it with water, is one of the remarkable pieces of civil engineering of the day. The dam is 164 ft. high, and between the extreme ends at the top, where it is anchored in the rock, the distance is 100 metres or 328 ft. The adjoining diagram, fig. 1, shows a horizontal ground section of the foundation, and also that of the curved upper edge, which is an arc whose chord is 100 metres, and its versed sine 5 metres.

The facings of both sides are curved surfaces, calculated as near as possible to allow that breadth of structure which at every point would bear a constant proportion to the strain given by the forces exerted against it. The soil upon which the dam is built is mica schist, and the structure is sunk into the rock at the bottom and sides. In both the excavation was continued until perfectly solid rock was reached. The wall is built of solid masonry, the stone being carefully selected, and not placed in tiers, but so joined as to produce the effect of a monolith, and, with the exception of a small portion at each end, was laid in hydraulic cement. The work was begun in 1858, but it was not till 1865 that it could be tested by a great depth of water. In December of that year the Furens was greatly swollen, and the reservoir was filled to the height of 46 metres, and in the following March to the height of 47 metres. The pressure produced no movement of mass, or escape of water, except a dampness owing to its being forced through the pores of the material by the enormous pressure, which at the depth of 47 metres or 154 ft. was more than 67 lbs. to the square inch, or about 4 1/2 tons to the square foot. - A Coffer Dam is a barrier erected to exclude water from an enclosure, usually for the purpose of allowing excavations to be made.

It may be constructed m a variety of ways, and of different materials, depending on the size of the work, the depth of the water, and the material of which the bottom is composed. If it is of clay or similar substance, piles may be driven about the enclosure, in two, three, or four rows, and the intervening spaces filled with gravel and clay. Between the piles of the inner row clay, or if necessary hydraulic cement, may be used, after which the water may be pumped out. (See fig. 2.) If the bottom is of quicksand, the ' construction of a coffer dam which will allow of much excavation is attended with great difficulty. Such was the case in building the dry dock for the Brooklyn navy yard; the exposure to water, however, was only on one side. For more than 60 ft. below the superstratum of black mud the bottom consisted of an impalpable micaceous sand, which, saturated with water under pressure, flowed like a mobile dense liquid. In this material a pit was to be excavated having an area of over two acres at the top and one at bottom, and a depth of 37 ft. below mean high water. Piles of yellow pine 40 ft. long and 15 in. square were driven in close contact around the outer end of the proposed excavation, the intervals between the rows being 10 or 12 ft.

These were filled in with the sand excavated. The piles were held together with oak wales and two-inch tie bolts. This barrier was forced inward by a pressure of less than 10 ft. of water, and when the excavation reached 30 ft. the water came in under the piles and filled the pit. A new and stronger dam was then commenced, and piles from 10 to 15 ft. longer than the first were driven close together in two rows 30 ft. apart, outside of the old work, and the space between filled with gravel and coarse stone. A third row was driven within the old work from 12 to 15 ft. below the proposed level of the foundation, and the space behind filled in with gravel. This dam was retained in place during the progress of the excavation only by the closest attention, although the walls were over 60 ft. thick and contained six rows of piles of timber from a foot to a foot and a half in diameter. When the bottom will not admit of pile driving, a frame of cribwork, lined with planking, made water-tight and loaded with stone, is sunk in position, and any crevices at the bottom are stopped with clay or hydraulic cement or concrete.

If a shaft of only a few feet in diameter is required to be sunk into a rock for the purpose of excavating, a coffer dam may be constructed of an iron cylinder, which, having a strong flap of India-rubber sheeting around its bottom, may be made tight by means of weights. As the excavation proceeds, an inner cylinder may be slid down below the surface of the rock, like the slide of a telescope. It is, however, difficult to get and maintain such a piece of apparatus in position in a harbor where moderate waves are liable to be raised. Cribwork which may be weighted to any required degree and floated into position, and sunk by additional weight of stone, will generally be found more practicable. A combination of cribwork and iron cylinder in a coffer dam is represented in the article Blasting.

Fig. 1. - Dam across the Furens.

Fig. 2. - Coffer Dam.