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The Caissons

The content below is from #174 of Who’d a Thunk It? Podcast

SPECIAL THANKS TO ThePublicDomain.com for this weeks episode and providing an awesome source.

RECOMMENDATION SEGMENT

  • This week I recommend you take your time appreciating your infrastructure such as bridges.
    • That’s right, this week’s recommendation segment ties directly into the main event.
    • Pittsburgh, my home, has 446 bridges which is more than Venice Italy… the city known for being under water.
    • Today’s topic focuses on the Brooklyn Bridge, it is located in New York City and reportedly has 789 bridges in total.
    • Those bridges aren’t just some simple thing, they are marvels of engineering, hard work, and human accomplishment.

NOW FOR THE MAIN EVENT

  • Have you ever walked past a bridge or pier and thought: “how the hell did they make this?”
    • I do, frequently. Engineering is miraculous in its accomplishments. Today engineers use all sorts of designs to lay the underwater foundations for structures like bridges. These are called caissons.
  • Let’s talk about a bridge NOT made with modern engineering and technology, the Brooklyn Bridge.
    • The Brooklyn Bridge was the longest suspension bridge in the world upon its completion in 1883 and remained so for the next 20 years. It stretches just shy of 6,000 feet end to end and, at its base, 140 feet side to side. Designer John Roebling meant for the bridge’s towers to be its most conspicuous features. Soaring 278 feet above the East River and weighing 140 million pounds each, they surely are. But, in my opinion, the most interesting features are the caissons that sit below the towers and support much of the weight of the whole span. These caissons that sit as supports at the bottom of these behemoth towers are partially made of wood!
  • In the early 1870s, two massive wooden boxes called caissons were sunken to the bottom of the East River.
    • The French word “caisson” is derived from the Italian “cassone” and means a large box. The caissons were the first part of the bridge to be built. The idea was to flip the box over, pressurize it with air to force the water out, and sink it to the bottom of the river. The mammoth boxes were built on land and slid into the river like a ship being launched into the water. The one closest to Brooklyn was 168′ x 102′ and 14′ tall, and weighed six million pounds. Nothing heavier had ever been launched before.
    • They were built entirely of heavy oak and pine timbers, bolted together, sealed to be airtight, and sheathed in tin. The thickness measured 15 feet on the roof of one caisson and 22 on the other, the density required to hold the millions of pounds of limestone and granite of the towers. The wood is still there. Turns out, wood can last indefinitely when completely submerged. It becomes waterlogged, which pushes out the oxygen, in turn preventing the growth of microbes that cause decay.
This sketch, by Stanley Fox at Harper’s Weekly, shows one of two caissons during construction on land in 1870 (top) and how it was expected to look after being launched. Picture courtesy Brooklyn Museum.
  • Workers would enter inside these boxes beneath the river in order to build the Brooklyn Bridge towers.
    • With the caisson resting on the river bed, men entered the pressurized interior through an air lock and began digging. They shoveled clay, rock, and boulders into pools at the bottom of two square “muck tubes” (also called waste shafts), where it was clawed out by a top-mounted derrick dropping a clamshell bucket down the tube.
    • As men carved away the river bed from underneath and masons built the tower atop it, the caisson slowly sank.
  • The grueling work of excavating the mud and rock from inside the caissons was done by hundreds of low-paid laborers.
    • Water filled the muck tubes. You might wonder why the water doesn’t flow back into the caisson below. It stayed put because the pressure of the air in the caisson’s interior pushed back against it. At the same time, the weight of the water pushed downward, trapping the air in the caisson. It was a delicate balance that had to be maintained; if the volume of water suddenly decreased in the tubes, there would not be enough weight to restrain the air in the caisson. It would blow up and out, depressurizing the caisson and possibly causing it to implode under the crushing weight of the tower.
  • The more mud they shoveled out, the deeper the caisson would sink.
  • Life in the caissons was brutal and to most workers, it must have felt like Dante’s inferno. They endured arduous and hazardous labor in unbearable heat.
    • In an episode known as “The Great Blowout,” the delicate balance of air pressure was disrupted and catastrophe alsmost struck. Very early on a Sunday morning when no one was in the caissons, the water level in one of their muck tubes fell dangerously low. During working hours, with the clamshell buckets busily pulling soil up through the muck tubes, sediment floated in the water that substantially increased its weight. Men usually monitored the water levels and filled the muck tubes when necessary. On this calm morning, however, no one noticed that the level dropped. The clear water in the tube, its heavy silt settled to the pool below, lost its struggle to hold the air in the caisson and exploded out like a volcano. But the caisson did not crumple like its designer feared. Although it settled hard about 10 inches into the packed earth, Roebling’s wooden box held the falling weight of 35 million pounds.
“Inside views of the East River Bridge caisson, Brooklyn, N.Y. / from sketches by our special artist,” from Frank Leslie’s Illustrated Newspaper, 1870. The pool beneath the muck tube can be seen in the lower left photo. Image courtesy of Library of Congress.
The Brooklyn-side tower rising atop the sunken caisson. Photo courtesy New York Public Library.
  • Many workers experienced a mysterious illness which they called caisson disease subjected the workers to paralysis and death. The cause of caissons disease was unknown at the time, but we now know it was the now common illness known to divers all over the world as The Bends.
    • Decompression sickness, also called generalized barotrauma or the bends, refers to injuries caused by a rapid decrease in the pressure that surrounds you, of either air or water. It occurs most commonly in scuba or deep-sea divers, although it also can occur during high-altitude or unpressurized air travel.
    • Bubbles forming in or near joints are the presumed cause of joint pain (the bends). With high levels of bubbles, complex reactions can take place in the body. The spinal cord and brain are usually affected, causing numbness, paralysis, impaired coordination and disorders of higher cerebral function.
  • About 1/3 of caisson workers experienced the Bends, but they were quickly replaced with droves of migrant workers from Germany, Ireland, Italy willing to work for just $2 a day
    • Eventually, the caissons reached a depth where the soil was dense enough to support the weight of the bridge. They were filled with concrete, where they remain today inconspicuously carrying the tens of millions of pounds of masonry and steel that stretches across the East River, and the more than one-hundred thousand cars, four thousand cyclists, and ten thousand pedestrians that cross the bridge every day.

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