Circular caissons of metal and reinforced concrete have come into use more recently and are frequently employed where foundations of small area are feasible, and in a few instances for foundations of larger area where circular piers acre to be constructed. They are frequently used for the foundations of highway bridges where con siderable depths must be reached, a pair of cylinders braced together being employed for each pier. Circular caissons of small diameter are constructed with single walls and a cutting edge at the bottom, those of larger diameters having double walls with space between the walls for loading with concrete.
Steel walls are commonly used for circular caissons in this country, although small sections are frequently of cast-iron pipe resting upon a steel cutting edge. In the foundations of the California City Point Coal Pier, 4-foot cast-iron pipe was used in lengths of 12 feet bolted together.' A conical steel section S feet in diameter was used at the bottom to give large bearing area, and the concrete filling in the pipe was reinforced with vertical steel.
In constructing foundations for torpedo boat berths at Charleston, S. C., steel cylinders 8 feet in diameter and -12 to 52 feet long were used as The cylinders were sunk through a bed of sand and about 4 feet into a bed of blue clay, which sealed the bottom, the soil inside being then excavated to near the bottom. Some wooden piles 4.5 feet long were driven inside the cylinder and the bottom section 5 feet deep filled with concrete, inclosing the tops of the piles. A form was then set up inside the cylinder and 4-foot reinforced con crete columns constructed to the top, the forms and cylinder above the bottom section being then removed.
Cylinders S feet in diameter were used for the foundations of the bridge over the Atchafalaya at Morgan City, La. (see Baker's Masonry Construction). These were sunk to a depth of 120 feet below high water and from 70 to 115 feet below the mud line. Below the river bottom, the cylinders were of cast iron 11 inches thick and above of wrought iron inch thick.
A double-wall caisson of steel was used for the pivot pier of the Omaha Bridge and Terminal Company's bridge across the Missouri River at Council Bluffs, Iowa The caisson was of steel, 40 feet outside and 20 feet inside diameter and was sunk through sand and clay and coarse sand to the rock 120 feet below low water. The spaces between the walls were filled with concrete to furnish weight for sinking. In sinking, the material was dredged from inside the caisson, and water jets were used upon the outside to reduce the friction. Twenty 3-inch vertical pipes were carried down inside the outer cylinder to the cutting edge to provide.for operation of water
jets.
Reinforced concrete walls are gradually coming into use for cylinder caissons and seem to offer advantages for the purpose. The weight of concrete is of help in sinking and obviates the necessity of so much temporary loading, which is an item of considerable expense, while the greater durability of the concrete as compared with steel is also of value. Gravel filling may sometimes be employed in a reinforced concrete cylinder, while the steel cylinder should be filled-with con crete.
Concrete cylinders are cast in place by using adjustable forms for building up the upper end as the cylinder is sunk. In some instances, however, they are cast in sections off the work and placed in position after hardening.
Reinforced concrete cylinder caissons were used in the foundations of the lumber docks at Balboa, Canal Zone.' The caissons were made 8 feet in outside and 6 feet in inside diameter and were pre-cast in sections 6 feet long. The bottom sections had conical exterior sur faces, widening to 10 feet in diameter and fitted into a cutting edge made of steel plates as shown in Fig. 121. The sections were reinforced with vertical bars and horizontal rings of steel, and were fastened together by means of six 1-inch anchor bars 12 feet long, which pass through cores molded in the shell. The rods were fastened together by the use of sleeve nuts which were adjusted in niches molded in the shell for the purpose.
The caissons were sunk 60 to 70 feet to rock, by laborers excavating inside of them, the water being kept down by pumping. The cutting edge was embedded about a foot in the rock, and a conical depression was blasted out of the rock in the center to give the concrete filling a strong bond.
Caissons having shells 6; feet in outside and 4 feet in inside diameter were used in the foundations for the Penhorn Creek Viaduct of the Erie They were reinforced with 4-inch horizontal rings spaced 6 inches apart. The caissons were built in place in sheeted pits, 12 feet square and 15 feet deep, collapsible steel forms 5 feet long being used and 29 feet of caisson built at one operation, which after being allowed to set was sunk and another section added, depths of about 70 feet being reached in this way. The concrete was allowed to harden six days before sinking, which was accomplished by dredging with an orange peel bucket, and sometimes using a water jet. The jets were usually necessary below depths of about 40 feet. Four 1='_-,-inch pipes suspended from the derricks and guided by hand were employed. The jets were used around the upper part of the exterior faces of the caissons to within about 20 feet of the bottom.