Not being from New York, I never considered the Brooklyn Bridge more than in passing until a few years back when I read David McCullough’s book, The Great Bridge. I was fascinated.
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. Would you believe they’re made partly of wood?
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 down a way. 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.
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. See the Harper’s Weekly sketch above. As men carved away the river bed from underneath and masons built the tower atop it, the caisson slowly sank.
Water filled the muck tubes, as you can see in the above illustration. 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.
In an episode known as “The Great Blowout,” this scenario almost unfolded. 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.
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.
“National Register of Historic Places Inventory–Nomination Form.” United States Department of the Interior. 1975.
Fisette, Paul. Wood Myths: Fact and Fiction about Wood. UMass Amherst Department of Environmental Conservation. 2005
McCullough, David. The Great Bridge: The Epic Story of the Building of the Brooklyn Bridge. New York: Simon and Schuster. 1982.
Roebling, W. A. Pneumatic Tower Foundations of the East River Suspension Bridge. New York: George W. Averell. 1872.
Rosenberg, Andrew. “Guide to the Brooklyn Bridge,” NYC The Official Guide online. 6 Nov 2019.
Trachtenberg, Alan. Brooklyn Bridge: Fact and Symbol. Chicago: The University of Chicago Press. 1965,