PNEUMATIC PROCESS.
The principle involved is the utilization of the difference between the pressure of the air inside and outside of an air-tight chamber. The air-tight chamber may be either a pneumatic pile— an iron cylinder which becomes at once foundation and pier,—or a pneumatic caisson—a box, open below and air-tight elsewhere, upon the top of which the masonry pier rests. The pneumatic pile is seldom used now. There are two methods of utilizing this difference of pressure,—the vacuum process and the plenum or compressed air process.
The vacuum process consists in ex hausting the air from a cylinder, and using the pressure of the atmos phere upon the top of the cylinder to force it down. Exhausting the air allows the water to flow past the lower edge into the air chamber, thus loosening the soil and causing the cylinder to sink. By letting the air in, the water subsides, after which the exhaustion may be repeated and the pile sunk still farther. The vacuum
should be obtained suddenly, so that the pressure of the atmosphere shall have the effect of a blow; hence, the pile should be connected by a large flexible tube with a large air-chamber—usually mounted upon a boat,—from which the air is exhausted. When communication is opened between the pile and the receiver, the air rushes from the former into the latter to establish equilibrium, and the external pressure causes the pile to sink.
To increase the rapidity of the cylinders may be forced down by a lever or by an extra load applied for that purpose. In case the resistance to sinking is very great, the material may be removed from the inside by a sand-pump (§ 877), or an orange-peel or clam-shell dredge (¢ 845); but ordinarily no earth is removed from the inside. Cylinders have been sunk by this method 5 or 6 feet by a single exhaustion, and 34 feet in 6 hours.