Engineering Wonders of the World
Volume I

THE BRIDGES OF NEW YORK CITY. 263 Building the Williams- burgh Bridge. river bottom curved outline. On this account they would make the structure cheaper, since each running foot of bridge costs much more than a foot of approach viaduct. With straight backstays, of course, it is necessary to provide separate trusses to carry the shore span roadway. It will be seen, however, that the more recent Manhattan Bridge returns to the older arrange- ment of the shore spans, suspending the roadway of the shore spans directly from the cables, so that perhaps this is the better arrangement after all. But the topography of the ground surface affects the question. The first work which the constructors of the Williamsburgh Bridge had to do was to build the stone piers on which the towers stand. The piers are out in the water, and, of course, should rest on solid rock. To prepare for this, holes were bored in the with tubular diamond drills, which yield samples of the material through which they go. These showed at what depth rock would be reached. It was thus learned that one of the foundations would have to go down over 100 feet below water-level to get to rock. Then the work of sinking pneumatic caissons on which to build the masonry of the piers was begun. The caisson method is nowadays used for most piers built in water wherever driven piles are not considered a satisfactory foundation. A great box of many layers of heavy timbers is built, closed on the sides and at the top, but open at the bottom. This box or caisson is sunk to the bottom in the exact location of the pier, and then compressed air is forced into the caisson to keep out water and enable diggers to get to work inside ex- cavating the soil under it. Meanwhile the stonework of the pier is being built up on the roof of the caisson, to hold it down against the buoyancy of the water and force its edges downward into the soil as the work of the diggers progresses. When the bottom edges Fig. 8.—HANDLING GIRDER AT CENTRE OF MAIN SPAN, WILLIAMSBURGH BRIDGE. of the caisson get down to rock suitable for a foundation, the interior of the caisson is filled up with concrete, the workmen backing out as this filling progresses. The masonry above the caisson having been continued up mean- while, the pier is now complete, with a broad base of concrete resting on an assured founda- tion. Two such caissons were sunk for each tower, there being a separate pier under either half of the tower. To show that caissons are by no means trifling affairs (and those of the Brooklyn and The Piers and Towers. Manhattan Bridges are much larger), we may note that each, of the four was about 63 feet wide, nearly 80 feet long, and from 19 to 53 feet high. The depth of founda- tion below water-level differs, being 55 feet in the shallowest and 107| feet in the deepest. The stone piers resting on them rise to a height of 23 feet above water. All this foundation work took well over a year to execute. When it was finished, the steel towers could be built. Four immense steel posts rise from each pier, the eight posts being braced together very strongly to form a substantial tower. For about 100 feet they rise vertically to the level of the road- way, an®! thence converge, so as to give the