Caissons are of two forms, the "erect" or "open" and the "inverted." The former is a strong water-tight timber box, which is floated over the site of the work, and being kept in place by guide piles, is loaded with stone until it rests firmly on the ground. In some cases the stone is merely thrown in, the regular masonry commencing with the top of the caisson; which is sunk a little below the level of low water, so that the whole of the timber is always covered, and the caisson remains as part of the structure. In others, the ma sonry is built on the bottom of the caisson, and when the work reaches the level of the water the sides of the caisson are removed. The site is prepared to receive the caisson by dredging and depositing a layer of concrete, Or by driving piles, or a combination of both.
The inverted caisson is also a strong water-tight box, open at the bottom and closed at the top, upon which the structure is built, and which sinks as the. masonry is added. This type of caisson is usually aided in sinking by excavation made in the interior. The processes employed to aid the sinking of the inverted caissons are called the "vacuum" and the "plenum." The vacuum process consists in exhausting the air from the interior of the caisson, and using the pressure of the atmosphere upon the top of it to force it down. Exhausting the air allows the water to flow past the lower edge into the interior, thus loosening the soil.
The plenum or compressed-air process consists in pumping air into the chamber of the caisson, which by its pressure excludes the water. An air lock or entrance provided with suitable doors is ar ranged in .the of the caisson, by which workmen can enter to loosen up the soil and otherwise aid in the sinking of the caisson vertically by removing and loosening the material at the sides. If the loosened material is of a suitable character it is removed with a sand pump; if not, hoisting apparatus is provided and, being loaded into buckets by the workmen, it is hoisted out through the air lock.
Freezing Process. This process is employed in sinking foundation- pits through quicksand and soils saturated with water. The Poctsch-Sooysmith process is to sink a series of .pipes 10 inches in diameter through the earth to the rock; these are sunk in a circle around the proposed shaft. Inside of the 10-inch pipes 8-inch pipes closed at the bottom are placed, and inside of these are placed smaller pipes open at the bottom. Each set of the small pipes is connected in a series. A freezing mixture is then allowed to flow downwards through one set of the smaller pipes and return upwards through the other. The freezing mixture flows from a tank placed at a suffic ient height to cause the liquid to flow with the desired velocity through the pipes. The effect of this process is to freeze the earth into a solid wall.