The water shafts were a very simple but ingenious means by which the mud and rock excavated inside the work chamber could be hauled out with no loss of compressed air whatsoever, and with none of the time required to move through an air lock.
The water shafts were seven feet square, open at top and bottom, and they extended like twin wells through the roof of the caisson straight down into the work chamber to a depth nearly two feet below that of the caisson’s bottom edge. These shafts were also built of boiler plate and once the caisson was in operation they would be filled with water to a level sufficient to “lock in” the compressed air below.
At the base of the shafts, at the two points where they extended deeper than the caisson itself, open pits would be dug in the river bottom, which would also fill with water. These would be the delivery ends of the shafts and the columns of water within the shafts would be kept suspended (kept from flooding in on the workers) by the pressure of the air within the chamber.
The water shafts, as one magazine of the time explained for its readers, were essentially huge barometers that measured the pressure of the air inside the caisson. The shaft itself was the barometer tube, filled with water instead of mercury, and the pool at the bottom was the cistern. Every pound of pressure in the caisson above normal atmospheric pressure (which of course was bearing down on top of the column of water) forced the water a little more than two feet higher in the shaft.
To get rid of the material they excavated, the men would shovel it into the pits, or pools, at the base of the shafts, where it would be hauled up and out by big clamshell dredge buckets dropped down from above, directly through the shafts of water. The theory was the buckets could work as fast as the men could feed them. It was a neat, efficient system, so long as the water in the shafts stayed at the proper level. But if the volume of water in one shaft became too great—too heavy, that is, for the compressed air below to support it—then the water in the pit would flood out into the work area. Or, if for some reason, the volume of water decreased to the point where its weight was no longer enough to counteract the pressure in the chamber, then there would be a terrific release of air, or blowout, from below.
The supply shafts were only twenty-one inches in diameter and simply the means by which Roebling intended to get the necessary cement, sand, and gravel into the caisson once the excavation was finished.
John Roebling’s thought had been to make the interior of the caisson one big open space, with no divisions or supports to get in the way of the excavation work. But his son had to abandon that idea for several reasons. First of all, since the caisson would have to be launched like a ship—only in this case a ship built upside down—there would have to be supports of some kind between the launchways and the roof. Washington Roebling also anticipated that the cutting edge of the caisson would be striking on boulders on its way into the riverbed and when that happened he did not want the entire weight resting solely on those few points. But it was chiefly because of the particular nature of the East River that he decided to divide up the work chamber with a number of supporting walls.
The East River connects the Upper Bay of New York with Long Island Sound, and because it has two entrances—at the tip of Manhattan and at Hell Gate, the opening to Long Island Sound—and two distinct tidal movements arriving at these points at different times of the day, its currents are quite unlike those of any ordinary river. The water is full of whirlpools and eddies caused by a bottom of jagged peaks and huge potholes, some as much as fifty feet deep. And with the tides surging in and out of the narrow openings, the currents are swift, turbulent, and something very serious to contend with. Even with a favorable wind the great sailing ships of the day could make little headway against an outgoing East River tide and would often stand in considerable numbers in the bay, like small armadas, waiting for the tide to change.
“The extreme rise and fall,” Roebling explained, “is seven and a half feet. If the inflated caisson is just barely touching the ground at high water, it will press upon the base with a force of 4,000 tons at low tide, all of which has to be met by the strength of the shoe and the frames.” Not until the caisson was permanently “righted down” under several hundred tons of tower stone would this powerful, potentially destructive up-and-down action stop.
So he had Webb & Bell build in heavy truss frames of pine posts and stringers, with three-inch sheathing on each side and side braces to the roof every six feet. There were five of these inside partitions, each running the width of the caisson and dividing the interior into six separate chambers, each 28 by 102 feet. Once the caisson was in the water and resting on the bottom, doors would be cut in the partitions so the men could go back and forth from one chamber to another.
As the mammoth timber box grew on the ways, it looked like nothing ever seen before in an East River yard. Seven launchways were required (one for each of the outer edges, five under the interior partitions), and the total weight of the structure, by the time it was ready for launching, would be six million pounds, or three thousand tons, which was, for example, a thousand tons more than the Challenge, leviathan of the clipper ships built in East River yards. The caisson would contain some 110,000 cubic feet of timber and 230 tons of iron and it was being built to go down the ways in the usual fashion of a great ship, its long side toward the water. For the time being it stood fifty feet back from the ends of the ways, and as everyone who had had any experience with shipbuilding knew, the great danger of launching so large a mass was the chance that one end might get going faster than the other and the whole gigantic affair would wedge tight on the ways. It was also necessary of course that the caisson get up enough momentum coming down the ways to overcome the immense resistance offered by the water. So just getting the thing launched was an engineering problem of very major proportions. Indeed, Roebling said later that the problems of launching the caisson and of protecting it against sea worms caused him more anxiety than the prospect of sinking it.
As might be expected, all such questions and the steady progress of construction were of enormous interest to innumerable bystanders. Day after day people came down to the yards at the foot of Noble Street to take a look for themselves, even after the first snows arrived. Newspapermen and some of Tweed’s people came over from New York, as well as a number of engineers, not the least of whom was Captain Eads from St. Louis, whose own caisson was being sunk beneath the Mississippi by this time and who happened to be in New York on some other matter.
So concerned were the Webb & Bell people over the problems involved, so different was this job from any they had ever handled in all their previous experience, that they had insisted on being paid in advance—$100,274.51.
Once the caisson was in the water, the plan was to tow it downriver. How seaworthy it would prove to be in the turbulent current was another open question. After giving the caisson a thorough inspection, James B. Eads told Roebling he could expect trouble and said it might topple over if he inflated it during the trip downstream, which was exactly Roebling’s intention.
In the meantime, however, the waterfront had to be cleared at the point where the caisson would be docked and the riverbed had to be dredged deep enough for the huge structure to be floated into place. The clearing of the site began on Monday, January 3, 1870, and because the winter turned out to be abnormally mild, the work there, like the work at the shipyard, moved along faster than expected. Any other winter it would have been impossible to do much of anything.
Clearing the site took about a month. For daily commuters on the Fulton Ferry it all provided an interesting show and the first real sign they had had that the bridge was actually under way. About half of one big ferry pier had to be dismantled, fender sheathing torn out, massive stone-filled cribbing removed, and all without disrupting ferry service. An enormous steam crane, called the “Ox,” was brought in on a barge to pull out the old piles, and as they came up one after another, there was much amazement over the toll the East River had exacted.
Each one was infested with thousands of sea worms in the area between the low-water mark and the mud line. As Roebling noted, “A pile which was sixteen inches in diameter below the mud, perfectly sound and free from worms, would be found eaten away to a thin stem of three inches just above the mud, and all timber was affected alike.” Then so that no one missed the point, he added, “This shows the necessity of going below the top of the riverbed with our timber foundation, and also proves its entire safety in that position.”
Once the old dockwork was out of the way, a large basin was to be built to contain the caisson, open toward the river but bounded on three sides with new piling. Within this area the riverbed was to be dredged to a uniform depth of eighteen feet at high tide, or deep enough to keep the caisson afloat at all stages of the tide. The dredges made great headway at first, as long as there was only surface mud to contend with, but then they hit hardpan and boulders. “The character of this material was next to solid rock,” Roebling wrote. The dredges could make but the slightest impression upon it. “Recourse was necessarily had to powder,” and the blasting commenced at night, from about seven in the evening until daylight, when traffic was light on the river and few people were about the ferry slip. Holes were driven into the river bottom with steel-headed iron piles. Then blasting charges were packed into iron canisters and dropped into the holes by professional divers. When the divers were out of the way and the pile drivers hauled back to a safe distance, the charges were set off by electricity.
Three pile drivers were kept in action, and with a little practice the men had the work down to a neat system, setting off some thirty-five blasts every ten-hour shift. During the day the dredges moved in and cleared out the results of the night’s work.
A number of the boulders encountered were too large to be picked up by a dredge and had to be dragged clear—the divers assisting underwater. The whole process was about six times as expensive as normal dredging, but still quite effective, and it provided valuable knowledge of the ground the caisson would have to penetrate. On one side, for example, near the new piling, a dozen blows of the pile driver would sink an iron pile forty feet through soft clay, but in the center area it took a hundred blows to go three feet. Toward the ferry the clay gave way to boulders of all sizes, closely packed, with coarse sand in between, and at the open end of the basin, on the river side, all soft strata had been washed away, leaving hardpan.
As time passed, Roebling decided to concentrate the dredging along the lines of the caisson’s edges and frames; the parts in between could be removed later, he said, from inside the caisson. He also had two holes blasted to an extra depth to accommodate the water shafts.
The work went slowly now, and while the blasting and dredging provided valuable knowledge of the riverbed, that knowledge itself was a most sobering reminder of the magnitude of what they were undertaking. To sink a wooden box as big as a fair-sized railroad station straight down through such material, and underwater, keeping the thing absolutely level the whole time, and bringing it to rest finally—perhaps fifty feet down—and at the exact spot it was meant to be, was a very tall order indeed. And added to that, along toward the end of January, reports began coming in from St. Louis of a strange malady among the men working inside the Eads caisson.
James Buchanan Eads was an authentic American genius and one of the looming figures of the nineteenth century. Slim, leathery, highly opinionated, disliked by many, he had survived an extraordinary life on the Mississippi that had included a lucrative underwater salvage business, a financially disastrous attempt at glass manufacturing, and the building of a fleet of ironclad gunboats during the Civil War. These slow, squat, ugly warships, built before the Monitor or the Merrimac and nicknamed “the Turtles,” had played a decisive part in defeating the Confederates on the Mississippi, along with the rams built by Charles Ellet. Eads had not designed the ships himself, nor had he gone into battle with them as Ellet had with his rams, but he had organized everything, having timber cut in Minnesota and Michigan, iron armor rolled at St. Louis and Louisville, keeping four thousand men at work on a night-and-day basis, and financing much of the operation out of his own pocket. At the time Washington Roebling was distinguishing himself on Little Round Top, Eads’s gunboats were assisting Grant in the successful siege of Vicksburg.
In early 1870 Eads was approaching fifty. He was the sort of person who liked to play chess with two or three others at a time, and in a recent weight-lifting contest among some of his blacksmiths, he had come in second.
During his years in the salvage business Eads had worked with diving bells up and down the Mississippi and was said to know more than any man alive about the river’s treacherous currents and the character of its bottom. This had been an important factor when he presented St. Louis and New York financial backers with his radical proposal for a bridge over the Mississippi. But it was his unbridled self-confidence and his reputation as a man who could get things done that mattered most in the end. He managed to convince men who had worked with the country’s foremost engineers that he, James B. Eads, was the one man fit to bridge the Mississippi at St. Louis, that the bridge he wanted to build was the only answer, and this despite the very well-known facts that he had had no formal training as an engineer and that he had never once built a bridge before. Both Charles Ellet and John A. Roebling had prepared plans for suspension bridges at St. Louis back in the 1850’s. Later, the year before he died, Roebling had done an entirely new set of plans, combining both suspension cables and parabolic arches. But Ellet’s and Roebling’s ideas had been turned down. (The St. Louis people were fools, John Roebling wrote to his son.) Now Eads and his bridge were the talk of St. Louis.
The great need was for a bridge to carry a railroad and highway over the river without interfering with steamboat traffic. The Mississippi at St. Louis is about the same width as the East River. Instead of a heavy iron truss, the customary thing then for railroad crossings, or a suspension bridge, Eads had conceived a mammoth arched bridge, with arches of steel set on stone piers. He intended to span the river with just three of his steel arches, the biggest of which, the center span, would be longer than any arch of the time by several hundred feet. To avoid interfering with river traffic during construction, his assistant, an engineer named Henry Flad, had devised a cantilever system nobody had tried before. The halves of each arch would be built out toward one another from their respective stone foundations, like great jaws slowly closing over the river, which was the conventional way, except that here the temporary supports needed (until the jaws joined) would be supplied from above. The usual practice was to prop such arches up from below with temporary timber “falsework” that could be torn out once the bridge was finished. But since this would be impossible, obviously, if the river was to be kept clear, Eads would hang the arches from overhead cables attached to temporary wooden towers built above each of his stone piers.
So the design of the bridge, the material he intended to build it with, the way he planned to build it, just about everything about the bridge, was unorthodox and untried. And when he had first proposed it, Andrew Carnegie had decided that somebody who knew about things mechanical, as he said, had better look over the plans.
Carnegie’s interest in the bridge was twofold. He had been approached by Eads’s St. Louis backers to see if he might be interested in selling some of their bridge bonds. Also, it was a few years before this that he had organized his Keystone Bridge Company, one of the first to specialize in manufacturing iron railroad bridges. Carnegie enjoyed talking about his love of bridges. Like Thomas Pope and John Roebling he saw them, he said, as testimonials to the national spirit and professed great personal satisfaction in the part he played in building them.
The Keystone company was now being invited to come in on the St. Louis job as consultants and to handle the superstructure. So Carnegie, quite sensibly, asked for an opinion on the bridge from Keystone’s chief engineer and president, J. H. Linville, whom Carnegie described wi
th customary enthusiasm as “the one man in the United States who knew the subject best.” This was an overstatement, but Linville was certainly among the finest men in engineering. He had been bridge engineer for the Pennsylvania Railroad before Carnegie hired him and the huge iron truss he had built over the Ohio at Steubenville in 1864 was considered the outstanding structure of its kind.
Linville asked that a set of Eads’s plans be sent to him. He examined them carefully, then, a little like the paleontologists who had been asked to give an opinion on the Cardiff Giant, he solemnly declared the subject preposterous. “The bridge if built upon these plans will not stand up; it will not carry its own weight,” he told Carnegie in private, and presently, in a formal statement, he called the bridge “entirely unsafe and impracticable” and said any association with it on his own part would imperil his reputation and was therefore out of the question.
Linville was quite wrong and Carnegie, who knew nothing about engineering, urged Linville to lead Eads “into the straight path.” Eads, however, was not about to be dissuaded or to have any outsider, regardless of reputation or connections, begin doctoring his bridge. In the end he would convince even Linville that he knew what he was doing. The Keystone company went to work on the bridge; Carnegie went off to London to sell a block of bonds to the American financier Junius Morgan, father of J. P. Morgan; and by the summer of 1867 Eads was confidently proceeding with the preparatory work for the first abutment beside the St. Louis waterfront. In neighboring saloons it was said that the bridge would take seven million dollars to build—and seven million years.
As things turned out the final cost would come to something near ten million, and seven years would go by before the job was completed. Once in use the bridge would be acclaimed by everyone, and by engineers especially. As one engineering historian would write, the bridge was “an achievement out of all proportion to its size,” something Washington Roebling thoroughly appreciated at the time Eads came over to visit the Webb & Bell yards.
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