PNEUMATIC CAISSONS. A pneumatic caisson is an immense box—open below, but air-tight and water-tight elsewhere,—upon the top of which the masonry pier is built. The essential difference between the penumatic pile and the pneumatic caisson is one of degree rather than one of quality. Sometimes the caisson envelops the entire masonry of the pier; but in the usual form the masonry en velops the iron cylinder and rests upon an enlargement of the lower end of it. The' pneumatic pile is sunk to the final depth before being filled with concrete or masonry; but with the caisson the masonry is built upward while the whole pier is being sunk downward, the masonry thus forming the load which forces the caisson into the soil. A pneumatic caisson is, practically, a gigantic diving-bell upon the top of which the masonry of the pier rests.
The principles involved in the construction of pneumatic caissons can be best explained in connection with a description of a few noted examples.
The apartment in which the men are at work is known as the working chamber or the air-chamber. The mass of timber between the top of the air-chamber and the lowest course of masonry is called the roof of the caisson. The shaft (shown in black) through the roof of the caisson, and connecting with a similar shaft (shown in white) through the pier, is called the air-shaft, and is for the ascent and descent of the men. The air-lock—situated at the junction of the two cylinders which form the of a short section of a large cylinder which envelops the ends of the two sections of the air-shaft, both of which communicate with the air-lock by doors. The small cylinders shown on each side of the air-shaft are employed in supplying concrete for filling the working chamber when the sinking is completed. The pipes seen in the air-chamber and projecting above the masonry are employed in discharging the mud and sand, as will be described presently. The timbers which appear in the lower
central portion of the working chamber are parts of the trusses which support the central portions of the roof of the caisson.
_ The main portion of the caisson is built of timbers 12 inches square drift-bolted together. Some of the roof timbers are omitted and the space is filled with stone or concrete to facilitate the floating of the caisson from the place of construction to the bridge site. The side walls of the working chamber in this form of caisson are unusually strong. The timbers forming the inclined face are 17 inches square, and are continuous from one outside wall to the opposite one, being cut to a 12-inch square having vertical sides where they enter the outside wall and also where they intersect the other inclined wall. Mr. Morison thought this construction was necessary, but experi ence seems not to justify the expense of this type of construction.
The masonry is usually begun about 2 feet below low water, the space intermediate between the masonry and the roof of the working chamber being occupied by timber crib-work, either built solid or filled with concrete. In Fig. 92 the masonry rests directly upon the roof of the air-chamber, which construction was adopted for the channel piers of this bridge to reduce to a minimum the obstruction to the flow of the water.
Frequently a coffer-dam is built upon the top of the crib (see Fig. 93, page 434); but in this particular case the masonry was kept above the surface of the water, hence no coffer-dam was employed. When the coffer-dam is not used, it is necessary to regulate the rate of sinking by the speed with which the masonry can be built, which is liable to cause inconvenience and delay. When the coffer-dam is dispensed with, it is necessary to go on with the construction of the masonry whether or not the additional weight is needed in sinking the caisson.