The Great Bridge (27 page)
Collingwood and Paine were in charge of clearing boulders from beneath the shoe and seeing that the caisson settled properly. “Levels were taken every morning on the masonry above,” Collingwood wrote later, “and a copy furnished the general foreman…. If the caisson were level, the usual method followed in lowering was to begin at the central frame, and loosen the wedges regularly from the center towards the ends. The two frames next to these were then treated in like manner, and finally the outer two. When no obstructions occurred, the blocks would all be gone over several times in the course of a day, and the caisson would settle easily, at the rate of three or four inches in 24 hours.”
At first, however, things had not gone that way at all. Through July and on into early August, the rate of descent had been less than six inches a week, and the boulders, instead of diminishing in number, as had been expected, became more plentiful. It was a hopeless rate of progress Roebling reported to his directors. At this rate it would take nearly two years to sink just the one caisson.
Boulders within the work chambers were the lesser evil. Before they could be hauled up the water shaft, they had to be split into manageable pieces, never an easy job, but at least they could be dealt with under comparatively reasonable conditions. Boulders under the shoe, however, or those found beneath the frames, were each a major undertaking. The removal of a boulder from under the shoe, for example, went as follows.
The ground around the inner side of the boulder had to be dug away with pick and shovel, with the excavation filling with water as fast as the men worked. Then the boulder had to be drilled by hand, underwater, and a lewis inserted, a dovetailed iron eyebolt to which a hoisting rig of some kind could be attached. In the early days of the work, double sets of block and tackle were tried, with a gang of thirty or forty men hauling at the ropes, while others worked furiously with winches and crowbars. But very often the boulder refused to budge. So Roebling had hydraulic jacks lowered through the supply shafts. These were of a kind designed for pulling instead of lifting and had a capacity of two to three tons. The water chamber on such a jack was above, not below, the piston, and the piston rod had a big hook at the end instead of a lifting shoulder. This hook was attached to the iron eye in the boulder, while the opposite end of the jack was chained fast to the nearest substantial timber or, better still, to the ceiling. The jack pump was then set in motion and, as Roebling said, it would prove itself a “very effective instrument.” There would be an immense momentary strain, then the boulder would give way and come sliding into the caisson, where it would be broken up..
When a boulder appeared to extend several feet outside the caisson, no attempt was made to haul it in. Rather the part inside was slowly split up until enough had been removed for the caisson edge to clear.
But whichever way they were handled, a few good-sized boulders beneath the shoe could hold up everything for three or four days. Such delays were maddening, and there were more and more of them as time passed and equipment began to break down or the water shafts failed to function as they were supposed to. The big clamshell buckets, armed with seven-inch teeth, were formidable-looking affairs and under normal conditions one of them could dredge up more than a thousand yards a day. This, in theory at least, meant that the equipment in use should have been able to haul out the whole volume of material that had to be removed for the tower foundation in about a month’s time. But the buckets kept breaking down or getting caught under the bottom edges of the water shafts. As it was, the job would take five months, and these, as Roebling wrote, were “five months of incessant toil and worry, everlasting breaking down and repairing, and constant study to make improvements wherever possible.” Bucket teeth that worked well for scooping would not last a day at grappling with stones. For every two buckets in working order, three were being repaired. “There was, indeed, one period,” Roebling said, “when we were almost tempted to throw the buckets overboard…”
One of the greatest early disappointments was to find that the buckets were unable even to dig their own hole under the water shafts, as they were supposed to. Much of the time the buckets failed even to bite into the material dumped into the hole unless a couple of men were kept constantly stirring the pool, “to keep the stuff alive,” as Roebling said. But even then the bottom of the pool kept filling in and had to be dug out by hand repeatedly.
Stone and clay would pack solid and actually fill the hole in a few hours, such was the incredible nature of the material being excavated. So it became necessary to feed all the stones into the pool at one time, separately, then the clay by itself. The kind of bucket in use could lift any stone it could catch hold of, but such a stone, or a chunk of split boulder, had to be placed just so in the hole for the bucket to get a proper hold, and the stones could only be taken out one at a time. Whenever a badly placed stone got wedged under a shaft, which happened fairly regularly, somebody had to dive under to see what could be done. “When the lungs are filled with compressed air,” Roebling wrote, “a person can remain under water from three to four minutes.” He knew this, it seems, from personal experience.
Any material fed into the pools from other parts of the caisson could be properly prepared, as it were, for the dredges to handle, but when the trouble was inside the water-shaft pools, as often happened, or when the pool had filled in, one to two days would be lost while the shaft was sealed off on top, with an iron cap, the water forced out by compressed air, the pool pumped dry, and the pit dug out by hand to a depth of six to eight feet. And the whole time this was going on (about two days on the average), the other shaft had to handle all the work. There were, in fact, so many occasions when the pools had to be cleared in just this fashion, so many repairs needed on the buckets, that most of the time only one water shaft was in operation.
Furthermore, whenever the work was held up a day or two, and the caisson stopped settling, its movement immediately afterward could be quite erratic, coming in sudden, unpredictable, uncontrollable starts. This, Collingwood explained, was due to the earth compacting around the caisson, as it does around a pile when driven slowly. “At such times it would seem impossible to get it started, and when once movement began, it was almost sure to split a set of blocks before it was arrested.”
Once, after the caisson had been at rest for several days because of breakdowns in equipment, all the usual steps were taken to get it started again, but to no avail. The blocking was eased, the shoe was cleared of obstructions, and still the caisson just hung there, motionless, with nothing holding it. The men did not know quite what to make of this. The only real significance of the episode, however, was that it gave the engineers a chance to compute roughly how much side friction the caisson had to overcome during its descent. As Collingwood figured it and reported later to a meeting of the American Society of Civil Engineers, the average pressure in the chamber at the time was seventeen pounds per square inch, giving a lifting force from the compressed air only of 20,400 tons. The bearing surface (posts and frames) was carrying about 625 tons and estimates were that the whole outer edge was probably carrying about that much again, which gave a total upholding force of somewhere near 21,650 tons. But the total weight of the caisson then, including the stonework on top, was judged to be 27,500 tons. Therefore, when it failed to move, the weight being carried by side friction alone was 5,850 tons. So this meant that along with everything else that had to be overcome to get the caisson down even a single inch, there was about 900 pounds of friction working against every square foot of the exterior surface.
To add to the over-all physical discomfort of everyone involved, blowouts continued and with greater frequency than Roebling had figured on. After each initial rush of air out one side or other, a returning wave would follow, inflowing river water that would stand knee-deep over the work surface until the air pressure eventually forced it out. Blowouts were usually caused by changes in the tide, which in the early stages affected the balance of pressures inside and out and which apparently Roebling had anticipated. But even the wake of a passing steamer could cause enough of a change in the water level to bring on a blowout, and this came as quite a surprise.
To build up additional weight on the caisson, some of the excavated material was dumped on top, in the spaces not taken up by the masonry. The rest of the material was dumped into carts that ran on inclined tracks down to big scows tied up on the riverside. (Once the caisson was in position this side had been closed in with a cofferdam, as the others were, and docks and tracks and turnarounds for the stone carts had been built.)
Eventually, when the caisson got down about ten feet below the river bottom, water ceased to come in at all, so tightly was the ground packed about the outer sides. Now, much to their amazement, the boulder crews encountered a new phenomenon. As Collingwood wrote, “It was not an uncommon occurrence in removing a large boulder, that an opening would be made entirely outside the caisson, for three or four feet.” Sometimes when this happened a man might crawl inside, beyond the limits of the caisson, that is, to dramatize the uncanny nature of such a space, not to mention his own nerve.
To step up the pace, Roebling organized a special force of forty men who worked at boulders exclusively, from eleven at night until six in the morning, when the regular shift came on. In time everybody grew more accustomed to the work. Roebling, in the words of William Kingsley, gave “the work his unremitting attention at all times,” but especially at critical points was he “conspicuous for his presence and exertions.” Like his father, he demanded much of every man under him, and even more of himself.
As the weeks passed he found that a slight lowering of the air pressure inside the chamber could work wonders whenever added weight was advantageous. The compressors would be slowed a little and the caisson would immediately bear down harder. It was a ready, effortless way to apply an additional twelve hundred tons or so any time that was needed, and for only as long as needed.
But when the caisson had reached a depth of some twenty feet, or approximately half the distance Roebling intended to sink it, the boulders became so large and numerous that there was no choice left but to begin blasting.
The idea of using powder on the boulders had, of course, been considered from the start. It would have saved all kinds of time and effort obviously, and as things grew increasingly difficult and frustrating inside the caisson, the men were more than ready to give it a try, whatever the supposed risks involved. But Roebling had held off. In such a dense atmosphere, he reasoned, a violent concussion might rupture the eardrums of every man inside. Smoke from the explosions might make the air even more noxious and certainly more unpleasant than it already was. The doors and valves of the air locks might be damaged.
His greatest fear, however, was the possible effect on the water shafts. The two immense columns of water that stood above the work chambers and every man in the caisson were held there in a critical balance only by the pressure inside the chambers. The margin of safety was just two feet of water—the distance from the surface of each pool and the bottom edge of each shaft. An explosion inside the caisson, Roebling explained, might suddenly depress the level of the pool and allow the air to escape underneath. A water shaft might blow out, in other words. All the compressed air would escape in one sudden blast and almost certainly with the following immediate consequences: with the work chambers instantly deflated, so to speak, the full weight of the caisson would come down all at once, smashing blocks and frames and outer edges. The impact might be so great as to crush every interior support and everyone inside; and in the early stages of the work, the river would have rushed in and drowned everyone. What the effect might be on top was anybody’s guess, but it was realistic to assume that all that water bursting out of a shaft would be about the same as a major explosion.
Still, Roebling knew, such prospects, however sobering, were all hypothetical. There was no past experience to go by. So whether he was right or not remained to be seen. With luck, he might be wrong. He decided to find out.
He began by firing a revolver with successively heavier charges in various parts of the caisson. When it was clear this was perfectly safe and causing no adverse effects, he set off small charges of blasting powder, fired by a fuse, gradually working these up in magnitude until they were on the order of what was needed to get on with the work. The concussions bothered no one especially, nor did they have any noticeable effect on the air locks or water shafts. “The powder smoke was a decided nuisance,” Roebling said. “It would fill the chambers for half an hour or more with a thick cloud, obscuring all the lights.” But this he alleviated greatly by switching to fine rifle powder.
The results were spectacular. With a little practice the work moved ahead as never before. A long steel drill would be hammered into the rock to make a hole for the blasting charge and the charge would be tamped in and set off.
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“As many as twenty blasts were fired in one watch,” Roebling reported, “the men merely stepping into an adjacent chamber to escape the flying fragments.” The hard crystalline traprock split more easily than the tough gneiss or rotten quartz boulders. Invariably the traprock broke neatly into three equal-sized boulders. The caisson now began descending twelve to eighteen inches a week, instead of six.
Care was taken to guard against fires igniting in the yellow-pine roof and the men did their best not to injure the shoe with the charges they set off beneath it. But the shoe by this time was in such shape that a little more damage hardly mattered. The armor plating was bent and torn, the shoe itself cracked or badly crushed in dozens of places.
