FILLING THE AIR-OHAMBER. When the caisson has reached the required depth, the bottom is leveled off—by blasting, if neces sary,—and the working chamber and shafts are filled with concrete. Sometimes only enough concrete is placed in the bottom to seal the chamber water-tight, and the remaining space is filled with sand. This was done at the east abutment of the Eads Bridge, St. Louis, Mo., the sand being pumped in from the river with the pump previously used for excavating the material from under the caisson.
Eads Bridge. The foundations of the steel-arch bridge over the Mississippi at St. Louis are the deepest ever sunk by the pneumatic process, and at the time of construction (1870) they were also very much the largest. The caisson of the east abutment was an irregular hexagon in plan, 83 by 70 feet at the base, and 64 by 48 feet at the top-14 feet above the cutting edge. The working chamber was 9 feet high. The cutting edge finally rested on the solid rock 94 feet below low water. The maximum emersion was 109 feet 81 inches, the record depth for pneumatic work until 1911 (see § 884). The other caissons were almost as large as the one mentioned above, but were not sunk so deep.
The caissons were constructed mainly of wood; but the side walls and the roof were covered with plate iron to prevent leakage, and strengthened by iron girders on the inside. This was the first pneumatic caisson constructed in America; and the use of large quantities of timber was an important innovation, and has become one of the distinguishing characteristics of American practice. In all subsequent experience in this country (except as mentioned in 890), the iron lining for the working chamber has been dispensed with.. The masonry rested directly upon the roof of the caisson, i.e., no crib-work was employed. In sinking the first pneumatic founda tion an iron coffer-dam was built upon the top of the caisson; but the last—the largest and deepest—was sunk without a coffer-dam, a departure from ordinary European practice, which is occasionally followed in this country (see § 868-69).
Brooklyn Bridge. The foundations of the towers of the first suspension bridge over the East River, between New York City and Brooklyn, sunk in 1869-72, are the largest ever sunk by the pneumatic process. The foundation of the New York tower, which
was a little larger and deeper than the other, was rectangular, 172 by 102 feet at the bottom of the foundation, and 157 by 77 feet at the bottom of the masonry. The caisson proper was 311 feet high, the roof being a solid mass of timber 22 feet thick. The working chamber was 91 feet high. The bottom of the foundation is 78 feet below mean high tide, and the bottom of the masonry is 461 feet below the same. From the bottom of the foundation to the top cf the balus trade on the tower is 354 feet, the top of the tower being 276 feet above mean high tide.
To make the working chamber air-tight, the timbers were laid in pitch and all seams calked; and in addition, the sides and the roof were covered with plate iron. As a still further precaution, the inside of the air chamber was coated with varnish made of rosin, menhaden oil, and Spanish brown.