Canadian engineer . overed ; and the cofferdam kept well above the water level. When the shoe of thecaisson was bedded in the river bottom, the air pipes wereconnected with the compressors and the caisson filled withcompressed air. On starting operations in No. i caisson,the shoe was 20 feet below water level and nine pounds pres- and closed as required. A ;4-inch plug-cock tapped into thepipe above this valve and left open while blowing out ma-terial, helped considerably. A 4-inch hose, 15 feet long witha 3K-inch nipple on the end, was kept with its nozzle in apool of water formed by the jet;
Canadian engineer . overed ; and the cofferdam kept well above the water level. When the shoe of thecaisson was bedded in the river bottom, the air pipes wereconnected with the compressors and the caisson filled withcompressed air. On starting operations in No. i caisson,the shoe was 20 feet below water level and nine pounds pres- and closed as required. A ;4-inch plug-cock tapped into thepipe above this valve and left open while blowing out ma-terial, helped considerably. A 4-inch hose, 15 feet long witha 3K-inch nipple on the end, was kept with its nozzle in apool of water formed by the jet; and the material to be ex-cavated shoved by hand to the nozzle by the sand hogoperating the pipe. The pressure of the air forced it up andout without the loss of very much air from the chamber. In the hard-pan excavation, the material was taken outfor from three to four feet below the shoe of the caissonbut leaving a bank of material, clear around, supporting thecaisson. Excavation was then suspended, and this bank. ycrrical ^cition Sections and Plan of Pneumatic Caisson. sure was used. The water inside the caisson being drivenout under the shoe, the air men, or sand hogs as they arecalled, entered the caisson through the man-lock; and exca-vation began. Three eight-hour shifts of about twenty meneach were worked continuously. The air pressure and upper was picked down to a level bench, leaving the caisson sus-pended by the friction on its sides. When everything wasclear below the shoe, the blow-pipes were opened and theair pressure lowered three or four pounds. The weight ofconcrete on top was so regulated that the caisson would then L February 29, 1912. THE CANADIAN ENGINEEE 339 settle down on to the hard bench below the shoe. Clay wasthen used to mud-up underneath the shoe, to prevent theleakage of air; and excavation commenced again. About 900 cubic yards of material was excavated fromNo. I caisson, including about 50 cubic yards of bed rockwhich had to be blasted out in o
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