large as that required to protect cribs sunk upon the natural bed; they would not however, spread to such a distance from the channel-side of the pier as to be an obstruction to navigation. A clearer understanding of the subject will perhaps be derived through the following illustrations, the first of which exhibits the end view of a crib set horizontally upon a natural sand-bed in a depth of 14 feet of water. The width of the crib is 20 feet; the height is represented as being 15 feet. It is shown in a proper position before having been subjected to the action of the water. Fig. 2 is not an exaggerated exhibition of the same crib after having been subjected to the action of the water. Many instances are known of cribs having settled much more seriously. In this representation it is shown that a portion of the sand-bed has been washed from under the crib, and that the stone filling has settled down with it, and run through the grillage; but in the discussion of the relative cost of cribs, sunk with and without artificial foundations, the quantity of stone which may have actually run into the sand below the crib will not be estimated for a greater depth than is shown by the sketch, nor will the detrimental effect produced by the distorted position of the crib, nor the imperfect construction involved through the necessary insertion of wedgeshaped timbers to provide a proper bearing for the superstructure, be taken into the account of dollars and cents. Fig. 2 represents the stone as having settled into the sand below the natural bed, on the outer or seaward side, to a depth of 4 feet, and on the inner or channel side, to a depth of 2 feet. It also shows the crib itself as having settled 3 feet on the outer, and 1 foot on the inner sides. The quantity, therefore, contained in the prism of stone which has subsided below the natural bed, for a crib of dimensions 32 by 20 feet, approximates 32 by 20 by 3 feet = 1,920 cubic feet, or 15 cords, and the mean subsidence of the crib equals 2 feet, or two courses of timbers 12 by 12 inches square. Two middle courses of a crib of the above dimensions contain, as per bill of materials used, 276 linear feet of framed timber, and 346 pounds of iron bolts. The average cost of these materials, with the framing, would be approximately as follows, the prices being taken from the contracts awarded by the Government in 1871, viz: For 15 cords of stone ballast, at $8.25... For 276 linear feet of framed timber, at 31 cents.. Total cost involved by subsidence of crib..... $123 75 85 56 13 84 223 15 Fig. 3 represents the same crib as being set upon a foundation of loose stone, in the same natural depth of water. The stone are shown as having been pressed downward to a distance of 3 feet below the natural bed of the lake by the superincumbent weight of the stone in the crib, and after its having been subjected to the action of the water. For the convenience of computation, it is shown in a more symmetrical shape than it would actually assume. The quantity of stone contained in this artificial foundation, having a mean dimension of 28 by 4 by 32 feet, equals 3,584 cubic feet, or 28 cords, of 128 cubic feet, which, at $25 per cord, would cost $231, showing an excess of $7.85 over the cost involved by the subsidence of crib placed upon the natural lake-bed. But it is assumed that the crib which is set upon a foundation of stone rests in a horizontal position, with its two upper courses above water-surface, while, in the first case, it has settled and tilted so that the seaward side is 2 feet below the surface, and the inner side at water-level, a mean horizontal of which would show it to be 1 foot below the surface of the water. It is clear, then, that a gain is effected by adopting the system of stone foundation, which is equivalent to three courses of crib-construction in value at the prices above specified, as follows, viz: 414 linear feet of framed timber, at 31 cents.... 520 pounds of iron drift-bolts, at 4 cents... 10 cords of stone ballast, at $8.25... Amount gained......... Minus excess of... Total amount gained by using an artificial foundation of stone....... Or a saving of about $7 per running foot of pier. $128 34 20 80 82 50 231 64 7 85 223 79 Besides which, the cost of furnishing and driving the necessary piles for carrying the superstructure, the loss of timber occasioned by cutting wedge-pieces for leveling up the work, and the extra framing of the wedge-timbers, have not been taken into the account. |