What's Under Your Feet in New York City? — Practical Engineering

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[Note that this article is a transcript of the video embedded above.]<br>New York City is unlike any other city in the United States. It’s the most densely populated area in the country by far, absolutely packed with buildings, business, homes, and people. But for all that you can see walking the streets or flying overhead, there’s a whole other world of infrastructure that makes the city possible below the surface. What if you could peel back the paving and soil to see what one of my favorite authors, David Macaulay, called the city’s “massive root system.” Let’s take a tour underneath the Big Apple. I’m Grady and this is Practical Engineering.<br>Let’s pick a generic Manhattan intersection to start this journey. You’ve got cars, buses, bikes, buildings, sidewalks, traffic lights, fire hydrants, hotdog stands, manholes, and more. But open it up and there’s the whole other world below the surface. We’ll start with the water lines.<br>If you think about it, water pipes could run overhead like electrical wires. Digging trenches is a lot of work, after all. And fixing underground pipes is pretty disruptive to streets. Of course, New York isn’t unique in having them run underground. It’s standard practice across most of the world for a lot of simple reasons: water is heavy, so continuous support along the length of a pipe is a structural convenience. Water also freezes, so putting pipes underground below the frost line prevents them from freezing in the cold winters. It also protects them from a whole host of natural and human-caused hazards like car crashes and rogue parade balloons. That’s important because a broken water main can be a big problem.<br>When you see a huge rooster-tail of water spraying from the street, it’s easy to wonder why we need water mains to run at such high pressures. Of course, pressure helps move water through pipes to all the individual places where it’s needed across the city. But maybe more importantly, that pressure pushing out keeps contamination from getting in. You want any crack, hole, or break in a water main to be a one-way street. If anything’s moving from one side of the pipe wall to the other, it’s pretty important that it happens from in to out.<br>Like electricity, water lines typically run in somewhat of a grid pattern. This adds redundancy, providing multiple paths for water to reach a destination so that taking a line out of service doesn’t disrupt the flow to residents. It also makes sure that all the water in the pipes is constantly flowing. If you build a water distribution system like the branches of a tree, you end up with a lot of dead-ends where water can slow down or even stagnate, making it unsafe to drink.<br>New York City’s water system is famously gravity fed, with most of the source water coming from upstate at a higher elevation. It also requires no filtration because the source watersheds are fiercely protected to keep contamination out. But the City doesn’t just assume things are good. Dotted throughout the streets are more than 900 water sampling stations that let officials collect and test the quality of the water at the end of the distribution system to make sure it’s safe to consume.<br>If you could peel back the soil and look at the city’s water distribution system, you’d see water mains down nearly every street; shutoff valves used to isolate individual lines for maintenance or repairs, connections to street and wall hydrants where firefighters can hook up their engines, and service lines that tap into the mains to supply each individual building. You’ll notice that few utilities run under the buildings themselves. The main reason is that we need to be able to access them to fix them if needed. The other reason is that buildings often have their own underground structures, specifically piles, piers, or drilled shafts that serve as their foundation. I have a whole video on deep foundations if you want to learn more after this.<br>Unlike water pipes, it is pretty typical to see electrical distribution lines running overhead on utility poles everywhere across the globe. You won’t see this in most parts of New York City, though. Roughly 85 percent of the electrical lines are underground. Part of it’s about looks: lines clutter up the space and require dedicated rights of way that limits the use of that space. Another part is safety: keeping people and vehicles free and clear of distribution level voltages. And, of course, there’s reliability. When a heavy storm takes out an above-ground utility pole in a suburban neighborhood, the ensuing power outage is an inconvenience. That same outage in Manhattan could affect a lot more people.<br>There’s a lot of confusion about underground electrical service. You can kind of divide the grid into three distinct categories defined by voltage ranges: there’s transmission (where power moves over...