Pneumatic Caissons

Although the sand-lift is efficient, there are some objections to it: (1) forcing the sand out by the pressure in the caisson decreases the pressure, which causes, particularly in pneumatic piles or small caissons, the formation of vapors so thick as to prevent the workmen from seeing; (2) the diminished pressure allows the water to flow in under the cutting edge; and (3) if there is much leakage, the air compressors are unable to supply the air fast enough.

During the construction of the St. Louis Bridge, Capt. James B. Eads invented a mud-pump, which is free from the above objections to the sand-lift, and which in mud or silt is more efficient than it. This device is generally called a sand-pump, but is more properly a mud-pump.

The principle involved in the Eads pump is the same as that employed in the atomizer, the inspirator, and the injector, viz.: the principle of the induced current. This principle is utilized by dis charging a stream of water with a high velocity on the outside of a small pipe, which produces a partial vacuum in the latter, when the pressure of the air on the outside forces the mud through the small pipe and into the current of water by which the mud is carried away. The current of water is the motive power.

Fig. 94, page 438, is an interior view of the caisson of the Baltimore and Ohio R. R. bridge at Havre de Grace, Md., and shows the general arrangement of the pipes and the mud-pump. The pump itself is a hollow pear-shaped casting, about 15 inches in diameter and 15 inches long, a section of which is shown in the corner of Fig. 94. The water is forced into the pump at a, impinges against the conical casing, d, flows around this lining and escapes upwards through a narrow annular space, f. The interior casing gives the water an even distribution around the end of the suction pipe. The flow of the water through the pump can be regulated by screwing the suction pipe in or out, thus closing or opening the annular space, f. To prevent the too rapid feeding or the entrance of lumps, which might choke the pipe, a strainer—simply a short piece of pipe, plugged at the end, having a series of i-inch to finch holes bored in it—was put on the bottom of the suction pipe. The discharge pipe of the mud pump terminates in a "goose-neck" through which the material is discharged horizontally.

The darkly shaded portions of the section of the pump wear away rapidly; and hence they are made of the hardest steel and constructed so as to be readily removed. Different engineers have different methods of providing for the renewal of these parts, the outline form of the pump varying with the method employed. The pump used at the St. Louis Bridge was cylindrical in outline, but otherwise essentially the same as the above.

In order to use the mud-pump, the material to be excavated is first mixed into a thin paste by playing upon it with a jet of water. This pump is used only for removing mud, silt, and soil containing small quantities of sand; pure sand or soil containing large quantities of sand is "blown out" with the sand-lift.

The water is delivered to the mud-pump under a pressure, ordi narily, of 80 or 90 pounds to the square inch. At the St. Louis Bridge it was found that a mud-pump of 3i-inch bore was capable of raising 20 cubic yards of material 120 feet per hour, the water pressure being 150 pounds per square inch.* Neitfier the sand-lift nor the mud-pump is suitable for the excavation of stiff clay; and, as at the Memphis Bridge the caissons were large and were to be sunk a considerable distance through stiff clay, Mr. George S. Morison invented a device for hoisting clay in a bucket by means of compressed air. The clay hoist consisted of a cylinder and piston placed at one side of the top of the material shaft. The piston was actuated by air pressure, and was connected to a cable to which was attached a bucket working up and down through the material shaft. At the top of the shaft were two doors operated by levers from the outside. The bucket held 6i cu. ft. The device was very effective.

Moran

Moran's air-lock consists of a lock, at the top of the material shaft, closed at both top and bottom by a pair of sliding doors so arranged as to permit of hoisting buckets of material out of the air chamber by means of a derrick and cable. The doors are moved lJr com pressed air, and are inter locked so that one can not be opened until the other is closed. On the cable is a stuffing-box which fits into a semicircular groove in the edges of the two halves of the upper door, and permits the bucket to be raised or lowered while the upper door is closed. This lock is very effective, since in ordinary operations the bucket usually passes the lock with only about 5 seconds delay, and can do it with a delay of only 2 seconds.

A combination of the pneumatic process and that of dredging in the open air through tubes has been employed extensively in Europe. It seems to have been used first at the bridge across the Rhine at Kehl. The same method was used at the Brook lyn Bridge. The principle is rudely illustrated in Fig. 95, page 439. The central shaft, which is open top and bottom, projects a little below the cutting edge, and is kept full of water, the greater height of water in the column balancing the pressure of the air in the chamber. The workmen simply push the material under the edge of a water shaft from whence it is excavated by an orange-peel or clam-shell dredge ($ 845).

Blasting. Bowlders or points of rock may be blasted in compressed air without any appreciable danger of a "blow out" or of injuring the ear-drums of the workmen. This point was settled in sinking the foundations of the Brooklyn Bridge; and since then blasting has been resorted to in many cases. Bowlders are some times " carried down," that is, are allowed to remain on the surface of the soil in the working chamber as the excavation proceeds, and subse quently imbedded in the concrete with which the air-chamber is filled.