Concrete cylinder caissons 0 feet in outside di:mneter, 8 inches thick and from 33 to 55 feet deep were used in the for the storehouse of the Boston :1rnny Supply Base; 577 of these piers were placed in 110 working days.' Pits 12 feet deep and 10 feet 4 inches square were dug and concrete cylinders 22 feet high constructed in the pits. The caissons were sunk below the bottoms of the pits by men digging the earth from inside then and forcing them clown by the use of jacks. The forms were removed and excavation begun twenty-four after pouring the concrete. When a sufficient depth could not be reached by this method, the concrete cylinder was continued at the bottom in open cut behind poling boards. After the concrete shell reached solid clay, the hole was belled out below the end of the shell to give a larger bearing to the base of the pier and the whole filled with concrete. Ground water was kept down by constant pumping.
204. Dredging through Wells.—When foundations are to be sunk to considerable depths through soft materials, the method of dredging through wells is very commonly employed, wherein, caissons of wood, steel, or concrete are built with vertical openings, or wells, extending through them. The body of the caisson surround ing the wells is filled with concrete to provide weight for sinking, and the soil at the bottom is removed by dredging through the wells, or by men in open excavation when the water can he kept down by pumping.
When the foundation is to be sunk through deep water, the cais sons may be built on land or on barges and floated to the site. When the site for the foundation is on land or in shallow water, the caisson may be started in place, in an open cut or inside of cofferdams, and built up as the sinking proceeds. As the position of the caisson can not be accurately- controlled in sinking, it is necessary to make the horizontal area covered by it larger than that of the foundation it is to carry. In a large caisson, the descent is guided by the manner of excavation, when resistance is met upon one end which tends to tip the caisson, the excavation is confined to that end until it is righted. If the caisson is narrow and the wells in one line, the control in a transverse direction is often difficult. Obstructions, such as boulders or sunken logs under the cutting edges, offer the most serious obstacles to work of this kind. These are not met at great depths and are com monly removed by divers, or sometimes by the use of a water jet.
Timber caissons have been employed more frequently than metal ones in this country.
The first use of deep open caissons in America was in the con struction of the foundations of the Poughkeepsie bridge over the Hudson River.' The largest of these caissons was 60X 100 feet in plan for the bottom 40 feet, narrowing to 40 feet in width at the top. There were 14 wells, each 10X12 feet, separated by one longitudinal and six transverse walls. The cutting edges at the bottom were 12X 12 inch white-oak timbers, and the walls were of hemlock, solid and triangular in shape for the lower 20 feet, widening at that height to their full widths. The end walls and longitudinal walls were hollow above this height. The six transverse walls were solid and 2 feet thick for the full height.
The hollow walls were filled with gravel in sinking the crib and the soil was excavated through the wells with a clam-shell bucket.. The caisson was 10.1 feet high and was sunk until the top was 23 feet below low water, the last dredging being clone with the top submerged. The wells were then filled with concrete, and a box caisson with bottom of grillage 6 feet thick was sunk on top and the masonry of the pier built up in this as a cofferdam, the sides being removed when the masonry was above water.
Open timber caissons were used in the foundations of the bridge of the Oregon Railway and Navigation Company across the Willamette River at Portland, Oregon.`' They were 36 X 72 feet with six well holes, each 9 feet square, the cutting edges being made of steel plates inclosing the bottom timbers, with 6 X 6 X --inch angles at the bottom. The lower 11 feet of the crib was of solid timber, triangular in shape, with vertices at the cutting edges. Above that height, walls 12 inches thick were carried up and the entire spaces around the wells filled with concrete as the caissons were sunk. The caissons rest upon cemented gravel 120 and 130 feet below low water, and when in final position the wells were filled with concrete. The crib proper ends at 20 feet below low water and the upper part of the caissons were built to be used as cofferdams within which the superstructure of the pier could be constructed.
"In the construction of each of these piers a substantial dock was first constructed in the river, consisting of about 100 piles well driven down, capped, and braced together. Borings were then made around the entire perimeter of the crib at spaces about S feet apart, and the elevations of hard material at all points were determined. It was found to be on a considerable slope, showing a difference of elevation of 22 feet for opposite diagonal corners. When these elevations were determined, pipes were successively sunk at numerous points around the perimeter and in the location of the cross walls, and holes drilled in the hard material to a common level sonic 2 feet below the lowest elevation of the top of the cemented gravel. As soon as the drilling at each hole had been completed to the proper elevation, a cartridge of black powder and dynamite in a sheet iron case was lowered to the bottom of the hole and discharged by an electric battery. This process was repeated at such frequent intervals as it was deemed would produce a bottom uniform in character throughout the entire area of the crib. Thus the blasting for leveling the cemented gravel was carried on before an excavation was made through 50 feet of gravel and sand. In the meantime the steel cutting edge had been set up and riveted together on ways in a shipyard convenient, and enough timber put on to float the crib, which in this condition was some 30 feet high. It was then floated into position in the dock already pre pared and other piles driven on the open end of the dock, entirely inclosing the crib." Iron and steel caissons have been extensively used by English engineers, but in this country the use of metal has usually been restricted to the cylindrical form, as timber has generally been found cheaper and more satisfactory.