Pnetunatic Caisson In this method two water-tight box-like structures built of timber or of metal are used. They are connected together one above the other, the lower caisson being inverted, and the upper one open at the top. This structure is floated and anchored over the position selected for the pier and the masonry built inside the upper caisson until the lower edge of the lower cais son rests on the bed of the body of water. The contained water in the lower caisson is particular to the other three. The river bed consisted principally of sand, and some clay and boulders. Below this, at a depth varying from 45 to 75 feet below high water level, the formation consisted of gneiss rock with a very irregular surface. The caisson was sunk through the sand, clay and boulders until it rested on the rock, which was then blasted away and stepped so as to make a fair hearing for the edge on all sides. The caisson measured 60 feet by 76 feet on the sides, with a total depth of 19 feet, and a working chamber 71/4 feet in height. The walls were 2 feet 9 inches thick and were built of two courses of 12 by 12-inch timbers, the outer course placed longi tudinally and the inner vertically. Two layers of 3-inch planking were placed on the outside, and one layer on the inside. The bottom of then expelled by compressed air, allowing men to enter and excavate the underlying material within the limits of the caisson. As the ex cavated material is discharged by being passed out through an air-lock in the top of the cais son, and the masonry work is added to course by course, thus keeping it always above the surface of the water, the caisson sinks lower and lowered into the substrata until it reaches solid rock or other firm material. The lower caisson is then filled with concrete, and the sides of the upper caisson are removed, leav ing the masonry on the foundation formed by the lower caisson with its concrete filling. Fig. 7 shows the longitudinal section of a cais son of this type.
The pneumatic method, although one of the most effective, is limited in its application to work at a depth of about 100 feet below the surface of the water, as it is impossible for men to work conveniently and effectively under a greater air pressure than that required to sustain a column of water of that height.
A clear idea of the application of the pneumatic caisson method to works of great magnitude may be obtained from a description of the caissons used in constructing the foun dations for the piers of the Williamsburg sus pension bridge, completed between Manhattan and Brooklyn, N. Y., in 1904.
The following description is that of the north caisson of the New York tower, pictured as Fig. 8, but it also applies in almost every ---- -- the walls was provided with a cutting edge, which extended continuously around the whole caisson. This cutting edge was not provided for the purpose of cutting through the bed of the river, but to enable the worlanen to use their tools close to the outer edge of the walls of the caisson, which were nearly three feet in thickness, and also to facilitate the removal of boulders. The roof of the working chamber was 5 feet in thickness, and consisted of several layers of 12 by 12-inch timbers. The whole caisson was stiffened with massive steel-plate riveted-trusses, which extended across from wall to wall. All the timbers of the caisson
and of the roof were drift-bolted together so as to give great rigidity to the structure, and to make it water-tight. Additional strength was given to the working chamber by two solid bulkheads, which extended entirely across and divided it into three compartments, openings being provided for the passage of workmen. A massive framework or gridiron formed of 16 by 16-inch timbers was bolted together and to the side walls at the level of the lowermost timbers with 1 steel tie-rods. From each intersection of this framework vertical posts reached to the roof and were tied to gether and stiffened to resist lateral distortion, by diagonal struts and tie-rods, as shown in Fig. 9, practically forming two steel trusses 9 feet 3 inches in depth, with a weight of 10 tons each. They were a novel feature in this class of work, but were rendered necessary on account of the shallowness of the caisson. The roof was pierced with 7 shafts, each about 3 feet in diameter, for the passage of men and which was in the form of drift-bolts. Without the concrete filling it weighed 1,965 tons, and contained 6,000 yards of concrete above the roof of the working chamber. The upper cais son was 50 feet deep and contained 29,000 cubic feet of timber and 32 tons of iron.
The sinking and concreting was accom plished in three months and six days. Down to the depth of 55 feet the men worked in eight-hour shifts. Below this depth the shifts were successively shortened as greater depths were attained. Although the air pressure at the depth of 107 feet was 46 pounds per square inch, there was very little sickness among the workmen.
Other notable examples of the pneumatic caisson method are the foundations of the centre pier of the Harlem River bridge, New material, and several pipes ranging from 1 to 5 inches in diameter, for supplying air and water, blowing-out sand, and for carrying electric light wires. These shafts were of circular section, and were put in place in lengths of 8 feet as the masonry work was carried upward.
The piers on the Brooklyn side were con structed in a similar manner, with the exception that they were carried to a depth of 107 feet below high water. The last caisson to be sunk passed through 50 feet of water, 20 feet of sand. gravel and boulders, 30 feet of hard clay and hard-pan, and 12 feet of rock. The ex cavating of the rock was rendered necessary by the steepness of its slope.
The lower caisson measured 63 feet by 79 feet, and contained 74,700 cubic feet of timber, and 98 tons of iron, the greater portion of York city; the Benares bridge over the Ganges River, India; the Poughkeepsie bridge over the Hudson River, New. York; the Hawkesbury bridge, New South Wales, Australia; and the Jubilee bridge over the Hooghly River in Ben gal. The centre pier of the Harlem River bridge supports the thrust of a 510-foot steel arch on each side of it, and stands on a timber caisson 54 feet by 104 feet, with a depth of 13 feet. It is divided into three compartments by vertical partitions which not only serve to strengthen the caisson, but protected the men during the blasting operations necessitated by the greatly inclined surface of the rock.