The open-caisson method is extensively used and has been em ployed in placing foundations when the depth is too great for any of the other methods in common use. The caissons may be made of timber, steel, or concrete, and vary widely in design, according to the size and character of the foundation to be constructed. Three types of open caissons are in use: (1) Single-wall caissons of timber, consisting of an outer watertight wall with the bracing necessary to enable it to hold its form; (2) cylinder caissons, consisting of a single or double cylindrical shell of steel or concrete with a single opening at the center; (3) caissons having several openings or wells, with double walls between and around them. The double walls are joined at bottom into cutting edges, and the spaces between them filled with concrete or other materials to aid in sinking.
Caissons of the first type are used where the depth of sinking is small or the material through which they are to be sunk is soft. They are frequently employed for piers where the foundation is upon rock with little or no soil above it, and a shell is needed within which the concrete body of the pier may be formed. Cylinder caissons are used for foundations of small area which must be sunk to consider able depths through soft materials. The method with several open ings is used for larger foundations requiring sinking to considerable depths.
203. Single-wall Timber Caissons.—Single-wall caissons are constructed in the same manner as box caissons, without the bot toms. The walls are commonly built up with 12X 12 inch timbers or with 12 inch plank laid flat. They are sunk upon a bottom of rock or other firm material which has been prepared to receive them. It is then only necessary to provide an outer wall of the form desired for the foundation, with bracing to resist the water pressure when pumped out, and a means of carrying sufficient load to sink it.
When the site is covered with soft material, sinking is accomplished by weighting the top of the caisson with some material which may afterward be removed, by dredging the soil from inside the bottom, and sometimes by using a water-jet to wash the soil from under the walls. After sinking, the bottom is sealed with concrete deposited under water and the caisson pumped out, after which the concrete filling may be placed in the open air. In some instances the filling is all placed through the water without out the caisson, in which case, it would not be necessary that the caisson be water-tight, but it must he capable of holding the concrete filling.
The permanent portion of a timber caisson usually terminates below low water, the part extending above the water being removed after serving as a cofferdam within which the masonry has been con structed_ In constructing the Columbia River Bridge of the North Coast Railway' open caissons were used to provide forms for the con struction of concrete piers upon the hard bottom of the river. The
depth of water was about :30 feet and velocity of current seven miles per hour. The construction of the caissons is shown in Fig. 120.
The walls consisted of 12 x 12 inch timbers framed and braced to conform to the shape of the pier. The caisson was sunk by weighting with rails held in racks upon the sides in order to keep the load near the bottom and prevent capsizing. The concrete was deposited through the water to the depth of 32 feet, large buckets with movable bottoms being used for the purpose.
In the construction of a pivot pier on rock foundation, the En gineering Department of Boston used a single-wall circular caisson GO feet in diameter and :30 feet high as a form for the concrete body of the pier. The caisson was built of about 1-1u of 3 X 12 inch yellow pine planks, 5 feet long, laid flat and breaking joints. The ends were beveled to make radial joints, and each plank secured to those below it by 1-inch oak tree nails 9 inches long, two at each end of each plank. In addition, the planks were well spiked to the lower courses throughout their lengths with 0-inch spikes. The courses were also secured together by 4 X 12 inch vertical planks opposite alternate joints.
Before placing the caisson, the site was dredged to rock. " There 1 was no attempt to construct the crib so that on the bottom it should conform to the variations in the rock surface. Instead, the bottom of the crib was made level and it was sunk until it took bearing on only a portion of the lower edge at the highest rock level. Then to provide continuous bearing to all parts of the circumference, and especially to complete the inelosure of the crib and confine the con crete that was afterward deposited within it, wooden boxes of vary ing sizes, but averaging about 4 feet square and 4 feet deep, were filled with lean concrete, lowered to the bottom and placed by divers under the edge of the crib to form a continuous wall. After the concrete boxes were placed, the excavation outside the crib was back-filled with gravel until the whole crib was surrounded by filling to about 29 feet below low water or some 2 feet above the bottom courses of plank. This backfilling formed an effectual seal to retain the concrete which was deposited in water inside the crib without unwatering the crib." 203. Cylinder Caissons.—The method of sinking wells by using a curbing of brick masonry which sinks as the earth is excavated from the bottom has been in common use for many years. A wooden cut ting edge is constructed and the brickwork started on top of this and built up as the sinking progresses. This method has been used for a long time in India for bridge foundations and in a number of instances in Europe. Usually work of this character has been of small diam eter and sunk to comparatively shallow depths, but in some in instances large shafts have been sunk by this method, and depths of over 200 feet have been reached.