RATE OF SINKING. The work in the caisson usually con tinues day and night, winter and summer. The rate of progress varies, of course, with the size of the caisson, the rapidity with which the masonry can be placed, the kind of soil, and particularly with the number of bowlders encountered. At the Havre de Grace Bridge, the average rate of progress was 1.37 ft. per day; at the Plattsmouth bridge, 2.22 ft.; and at the Blair Bridge 1.75 ft. per day. Speeds of 6 and 8 feet per 24 hours have been maintained for a few consecutive days with large caissons.
The above rates of sinking were greatly exceeded in the case of the small caissons for column foundations of tall buildings. In constructing the Broad Exchange Building, New York City, 88 caissons were sunk an average of 30 feet in 47 days, one being sunk 27 feet in 20 hours and 2 feet in 1 hour.
The maximum depth to which the pneumatic process has been applied is 113 feet, in 1911, at the Municipal Bridge across the Mississippi River at St. Louis; and the next deepest was at the Eads Bridge in St. Louis (§ 889). At the first Memphis bridge (§ 891) the depth was 106.4 feet. In the last two cases the pressure was continued at or near the maximum for several days. At the Williamsburg Bridge over East River, in
New York City, the maximum depth was 107.5 feet, but this depth extended over only a very small area and the maximum pressure was for only a few minutes. Except in these instances, the com pressed-air process has never been applied at a greater depth than about 90 feet. The pneumatic process is limited to depths not much greater than 100 feet owing to the deleterious effect of the com pressed air upon the workmen (see § 895-98).
Theoretically, the depth, in feet, of the lower edge of the caisson below the surface divided by 33 is equal to the number of atmospheres of pressure (15 lb. per sq. in.); but the depth does not always indicate the pressure. In sand or silt the pressure may be either a little more or a little less than that corresponding to the depth, and in clay the pressure may be considerably less than the theoretical amount. At the Earls Bridge the actual pressure varied between 45 and 50 pounds for 53 working days, and at Memphis from 40 to 47 pounds for 75 days.