Pneumatic Caissons

Havre de Grace Bridge.

Fig. 93, page 434, and Fig. 94, page 438, show the construction of the caisson, crib, and coffer-dam employed by Gen. William Sooy Smith in 1884 in sinking pier No. 8 of the Baltimore and Ohio R. R. bridge across the Susquehanna River at Havre de Grace, Md. This is the type formerly employed, by that engineer in a number of large bridges erected by him, and in a general way is the ordinary form used by American engineers for pneumatic caissons for bridge work.* The caisson shown in Fig. 93 has a slight flare, but subsequent experience has shown this to be undesirable, since the caisson can be guided more certainly when it has vertical sides.

Table 67 gives the dimensions and quantities of materials in the pneumatic foundations of this bridge, and Table 69 (page 449) gives the cost.

Before the construction of the Eads Bridge (§ 889) at St. Louis, Mo., in 1870-74, the air-lock had always been placed at the top of the air-shaft, and was of such con struction that to lengthen the shaft, as the caisson sunk, it was neces sary to detach the lock, add a section to the shaft, and then replace the lock on top. This was not only inconvenient and an interruption to the other work, but required the men to climb the entire distance under compressed air, which is exceedingly fatiguing (see f 895). To overcome these objections, Eads placed the air-lock at the bottom of the shaft in the air-chamber. This position is objectionable, since in case of a "blow-out," i.e., a rapid leakage of air,—not an unfre quent occurrence,—the men may not be able to get into the lock in time to escape drowning. If the lock is at the top, they can get out of the way of the water by climbing up in the shaft.

At the Havre de Grace Bridge (§ the air-shaft was con structed of wrought iron, in sections 15 feet long. The air-lock was made by placing diaphragms on the inside flanges of the opposite ends of the top section. A new section and a third diaphragm could be added without disturbing the air-lock; and when the third dia phragm was in place, the lower one was removed preparatory to using it again. Some engineers compromise between these two posi tions, and leave the air-lock permanently at some intermediate point in the pier near the bottom (see Fig. 92, page 432).

It will be shown presently (§ 897) that with deep founda tions it is very desirable to have an elevator for carrying the workmen up and down, and hence it is better to have the air-lock near the bottom of the'shaft; but for the safety of the men it should not be in the working chamber. However, an elevator for the men is a

comparatively recent invention, and is used only in the deepest work.

In the early application of the pneumatic method, the material was excavated with shovel and pick, elevated in buckets or bags by a windlass, and stored in the air-lock. When the air-lock was full, the lower door was closed, and the air in the lock was allowed to escape until the upper door could be opened, and then the material was thrown out. This method was expensive and slow.

Auxiliary Air-Lock.

In the first application of the pneumatic process in America (§ 862), Gen. Wm. Sooy Smith invented the auxiliary air-lock, F G, Fig. 91 (page 430), through which to let out the excavated material. The doors, F and G, are so connected to each other that only one of them can be opened at a time. The excavated material being thrown into the chute, the closing of the door F opens G and the material slides out. This simple device is said to have increased threefold the amount of work that could be done.

Sand-lift.

This is a device, first used by Gen. Wm. Sooy Smith, for forcing the sand and mud out of the caisson by means of the pressure in the working chamber. It consists of a pipe, reaching from the working chamber to the surface (see Fig. 91, 92, and 93), controlled by a valve in the working chamber. The sand is heaped up around the lower end of the pipe, the valve opened, and the pressure forces a continuous stream of air and sand up and out. Mud or semi-liquid soil may be removed by this means by immersing the lower end of the tube and opening the valve; but this method is most effective with sand.

The sand-lift is eight to ten times as expeditious as the auxiliary air-lock. Of course, the efficiency of the sand-lift varies with the depth, i.e., with the pressure. The "goose-neck," or elbow at the top of the discharge pipe, is worn away very rapidly by the impact of the ascending sand and pebbles. At the Havre de Grace Bridge, it was of chilled iron 4inches thick on the convex side of the curve, and even then lasted only two days. At the Brooklyn Bridge, the dis charge pipe terminated with a straight top, and the sand was dis charged against a block of granite placed in an inclined position over the upper end.