The tunneling crews that built New York City’s subways and sewers around the turn of the last century only spent part of their shifts digging tunnels.
Half their workday was devoted to decompression, so the urban miners working beneath the earth wouldn’t die from “the bends,” the atmospheric pressure-related illness that afflicts deep sea divers if they surface too quickly. It meant that every shift had to have two crews on at once — one digging, one decompressing.
So why don't people have to go through decompression to ride the subway?
8 comments:
I read a lot about The Bends in a book about the building of the Brooklyn Bridge, an engineering marvel in its day. I had heard about this so many times in NYC lore and was really trying to understand the concept.
My understanding (FWIW) was that the workers at risk for the bends were in depths well below where subways normally travel, and were subjected to rapid ascent, before it was known not to do that.
I never heard about the bends in relation to building the subways, tho. One set of subways were built by cut-and-cover - so tunnels were exposed, the bends wouldn't apply. The others were bored through, but I never had the sense they were deep enough to cause decompression issues.
Now I really want to know! I think we have a book about the building of the NYC subways. When I'm home I will look this up.
722 Miles, the building of the NYC subway
and
The Great Bridge, the epic story of the building of the Brooklyn Bridge, by David McCollough <--- an excellent book
I think I found an answer. From the site NYC Subway.org:
At one time the digging was being carried on under thirty-seven pounds air pressure, which was the maximum reached. The maximum allowed is only fifty pounds to the square inch. When the pressure was at its highest an average of more than a thousand men were employed, daily, owing to the short shifts permitted by law for caisson workers. At this period of the work the men were employed only one and one half hours at a stretch, entering the tunnels twice daily for an aggregate of three hours. The air pressure at the time of "holing through" had been reduced to eighteen pounds and will be continuously reduced as the tunnel is caulked and made water tight, so that normal pressure will have been reached by the time the tunnel is ready for its concrete lining.
It sounds like the dangerous pressure occurs before the tunnels while they are under construction, before they are finished. Once finished, the pressure is gone.
I don't know the science behind this, but it explains why we can use subways safely.
I'm disappointed that I had forgotten that the people who built the subway suffered from the bends, something I once knew.
* while the tunnels are under
That's interesting (those poor workers who were the first ones to discover you could get the bends from tunnelling!), but I still don't understand why the pressure existed when they were digging but doesn't any more.
To construct the Brooklyn Bridge, for example, pylons were erected to support the bridge. But those pylons needed to be built at the bottom of and in the middle of the river, which presented a problem. To allow for construction at the bottom of the river, a caisson (a very large wooden box with no bottom) would be built and sunk to the bottom. Water would be pumped out from that box. These caissons were pressurized to keep the water out - as long as the air pressure was higher than the water pressure, there would be no leaks. The men worked in these caissons under high air pressure. This provided for a dry environment while they were working on the river bed. This had nothing to do with depth, as I understand it.
Oh, that makes better sense, thanks! I didn't know what a caisson is, and it didn't occur to me to look it up because I figured "caisson worker" = "people who are digging subway tunnels".
The story of how Roebling and the other engineers devised the use of those caissons, and what the caissons enabled them to do, is fascinating. It changed bridge building forever.
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