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Box and Open Caissons

caisson, water, bottom, top, foundation, sunk and timbers

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BOX AND OPEN CAISSONS Box Caissons.—A caisson is a watertight casing within which the work of placing a foundation may be clone. The casing forms a shell which contains and usually remains a permanent part of the foundation. Caissons are of three general types: Those closed at bottom, known as box or erect caissons; those open at both top and bottom, known as open caissons; and those closed at top, called pneu matic or inverted caissons.

Box caissons of timber are commonly employed When masonry foundations are to be placed upon piles cut off under water. These caissons are water-tight boxes, open at the top, which may be floated into position over the piles upon which they are to rest and then sunk by building the masonry inside them. The floor and lower part of the caisson is usually a permanent part. of the foundation, but the sides which extend above. the water are intended to act as coffer dams during construction of the masonry and are removed upon com pletion of the work.

The construction of box caissons varies with the depth of water in which they are to be sunk and the shape and dimensions of the foundations. For light work, timber studding with plank sides and bottom may be sufficient, while in heavier work, a bottom of two or more thicknesses of 12X12 inch timbers, with sides built up of similar timbers on top of each other, and drift-bolted together, or timber framework with vertical staves may be used. The bottom must be capable of carrying the load of masonry required for sinking and the sides must resist the water pressure or the outward pressure of material with which it may be filled. The caisson may be built on land, launched and floated to the site of the foundation, or when heavy timbers are to be used for a floor, it may more easily be built afloat.

Timber box eissons are occasionally used as a base for foundations upon fairly firm soil. The excavation must be made to the depth required before the caisson is sunk. Such caissons were used in the foundations of the south pier of the Duluth Ship canal. "They 1 were from 24 to 30 feet wide, 21 feet high, and from 50 to 100 feet long. The floor was S inches thick laid close, the channel side had

a solid wall of a double thickness of 12X12 inch timbers, while the opposite side was composed of a single thickness of 12X12 inch timbers laid close. Connecting and bracing the two walls were trans verse bulkheads of 10 X 12 inch material spaced 4 feet center to center "The caissons were built in the harbor, towed to the site, and sunk by filling with rock and gravel. After sinking, the caissons were cov ered with a layer of heavy timbers, in which was built the concrete pier, the top of the caisson being slightly below low water level." For work of this kind, a timber crib or grillage which is not water tight is sometimes used for the lower part of the foundation, the top of the crib being below low water. A box caisson is then sunk on top of the crib. The floor of the caisson carries the masonry superstruc ture, and the sides, which are intended only to exclude water during construction, are removed when no longer needed. Reinforced concrete box caissons have been used in some instances. They may he made part of the permanent structure above as well as below the low water level, and do not need the cofferdam sides.

Box caissons of small size have sometimes been sunk several feet into soft material by the use of water jets under the bottom. A number of pipes are run through the bottom to carry the water, which washes the material from underneath and allows the caisson to sink.

201. Types of Open Caissons.—An open caisson consists of a easing, with one or more openings extending through from top to bottom, intended to he sunk through soft materials which may he displaced by the weight of the caisson or removed by dredging through the openings. The caisson is always an integral part of the foundation. It may be simply a shell to contain concrete upon which the main reliance for strength is placed, or the caisson itself may be designed to bear the loads coming upon the foundation and the filling for the purpose of sinking and anchoring it.

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