Handbook of Railroad Construction; For the use of American engineers. / Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States.

by George L. Vose

FOUNDATION BY CAISSON.

282. In deep water the coffer-dam becomes very expensive, on account of the size and length of the piling, and the quantity of bracing required. In such cases recourse is had to the caisson; which is simply a box in which the masonry is built, and afterwards sunk to the proposed site. The manner of putting down a piece of masonry by caisson will best be shown by an example.

Suppose we wish to sink a pier thirty feet long, twenty feet high, and six feet wide, in twenty feet of water.

Let the caisson bottom be of two courses of square 12 × 12 timbers, fastened strongly at right angles to each other. Let the courses of masonry be two feet thick. Assume the weight of a cubic foot of stone as one hundred and sixty lbs., a cubic foot of wood at thirty, and of water sixty-two lbs. per foot.

Every floating body will sink until it has displaced a quantity of water equal to its own weight.

If the bottom is ten feet wide and thirty-five feet long, it will weigh

35 × 10 × 2 × 30 = 21,000 lbs.

one course of masonry weighs

30 × 6 × 2 × 100 = 57,600 lbs.

one course of side timbers, 12 × 12, which are laid upon the sides of the raft,

(2 × 35 + 2 × 8) × 30 = 2,580 lbs.

Now load the bottom with one course of masonry and three courses of side timbers, and we have

Stone	57,600	lbs.
Bottom of caisson	21,000	lbs.
Three side courses	7,740	lbs.
In all:	86,340	lbs.

which divided by 62, gives 1,392; which divided by the area of the caisson bottom, gives

1392
350 = 3.98

or nearly four feet, for the depth at which the caisson will float. This leaves the sides one foot above the water surface.

Putting on a second course of masonry and three more side courses of timber, we have

Floor	21,000	lbs.
Two courses masonry	115,200	lbs.
Six side courses	15,480	lbs.
In all	151,680	lbs.

which divided by 62, and by 350, gives seven feet very nearly; leaving the top one foot above the surface.

In the same manner we proceed until the caisson grounds upon the bed, which has been previously prepared, either by pile-driving or by dredging. The bottom being reached, the sides are taken off, and the masonry remains upon the floor. The caisson may at any time be grounded by filling with water, and may be raised again by pumping out. The masonry may be laid either from barges or rafts at the site, or at the shore. Guide piles are necessary to insure the descent in the proper manner, and to prevent overturns.

In laying stone under water, it is to be remembered that masonry submerged loses 62
100 nearly of its weight, and is consequently more liable to be injured by shocks than when above the surface.

CHAPTER XIII.
SUPERSTRUCTURE.

283. Nothing aids more the proper accomplishment of any object than a correct idea of what is wanted. The following definition is given by Mr. W. B. Adams, of what good superstructure should be:—

“The principal requirements of permanent way are: That it shall be well drained, especially in contiguity to the substructure; that the weight and damaging power of the locomotives and rolling stock should be considered the data for calculation; that the strength, hardness, and tenacity of rails, and the immobility of the substructure should be adapted to the hardest work to which the railway is to be subjected; that the substructure should have an amount of bearing surface proportioned to the load to be borne, and the nature of the rail and ballast; and a sufficiently fair hold in the ground to prevent looseness or lateral motion, from the side lurches of the engines and trains; that the rails should possess so much vertical and lateral stiffness, either in themselves or in their fastenings, as to prevent all deflection; and have sufficient hardness of surface not to laminate or to disintegrate beneath the rolling loads; also, to have sufficient breadth or tread surface to diminish the crushing effect of the wheels.

“They should be as smooth as possible, to prevent concussion, and be laid at the proper angle, and the curves regularly bent, so as to insure the accurate tread of the wheels. The joints should be so made that the rails may practically become continuous bars, yet with freedom to contract and expand without being too loose. And with all this there should be interposed between the rails and the solid ground, some medium sufficiently elastic to absorb the effect of the blows of the wheels, without being crushed or forced down into the ballast, and yet stiff enough to keep the upper surface of the rails in a uniform plane.”