Page:The American Cyclopædia (1879) Volume V.djvu/654

 650 DAM FIG. 1. Dam across the Furens. secondary one of supplying it with water, is one of the remarkable pieces of civil engineer- ing 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 con- stant 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 perfect- ly 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 Ibs. to the square inch, or about 4% tons to the square foot. A COFFER DAM is a barrier erected to exclude water from an en- closure, usually for the purpose of allowing excavations to be made. It may be construct- FIG. 2. Coffer Dam. . ed in a variety of ways, and of different ma- terials, 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 diffi- culty. 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 impal- pable 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. b'elow 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 ex- cavation 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 con- crete. If a shaft of only a few feet in diame- ter 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 in- ner cylinder may be slid down below the sur- face 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 re- quired degree and floated into position, and sunk by additional weight of stone, will gen- erally be found more practicable. A combina- tion of cribwork and iron cylinder in a coffer dam is represented in the article BLASTING.