COMPRESSED-AIR Psoosss. The plenum or compressed air process consists in pumping air into the air-chamber, so as to exclude the water, and forcing the pile or caisson down by a load placed upon it. An air-lock (I 864) is so arranged that the workmen can pass into the caisson to remove the soil, logs, and bowlders, and to watch the progress of the sinking, without releasing the pressure. The vacuum process is applicable only in mud or sand; hut the compressed-air process can be applied in all kinds of soil.
Many times in sinking foundations by the vacuum process, the compressed-air process was resorted to so that men could enter the pile to remove obstructions; and finally the many advantages of the compressed-air process caused it to entirely supersede the vacuum process. At present the term pneumatic process is practically synonymous with compressed-air process.
The first foundations sunk entirely by the com pressed-air process were the pneumatic piles for the bridge at Roches ter, England, put down in 1851. The depth reached was 61 feet.
The first pneumatic caisson was employed about 1870, at Kehl on the eastern border of France, for the foundations of a railroad bridge across the Rhine.
The first three pneumatic pile foundations in America were constructed in South Carolina between 1856 and 1860. Im mediately after the civil war, a number of pneumatic piles were sunk in Western rivers for bridge piers. The first pneumatic caissons in America were those for the St. Louis Bridge ($ 889), put down in 1870. At that time these were the largest caissons ever constructed, and the depth reachedó 109 ft. 8i in.ówas not exceeded until 1911 0884).