What's Under Your Feet in New York City?

[00:01] 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,  

[00:16] 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  

[00:32] a tour underneath the Big Apple. I’m  Grady and this is Practical Engineering. Let’s pick a generic Manhattan intersection  to start this journey.

[00:49] 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.

[01:02] 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.  

[01:19] 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.  

[01:36] 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. When you see a huge rooster-tail of water spraying  from the street, it’s easy to wonder why we need  

[01:53] 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,  

[02:10] 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. Like electricity, water lines typically run in  somewhat of a grid pattern. This adds redundancy,  

[02:27] 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  

[02:39] 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. New York City’s water system is famously  gravity fed, with most of the source water  

[02:53] 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  

[03:08] 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. If you could peel back the soil and look  at the city’s water distribution system,  

[03:22] 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  

[03:35] 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  

[03:47] 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.

[04:03] 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,  

[04:15] 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  

[04:32] 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.

[04:49] You can kind of divide the grid into three  distinct categories defined by voltage ranges:   there’s transmission (where power moves over very  long distances at hundreds of thousands of volts),  

[05:04] distribution (where it’s carried throughout  a populated area at a a few thousand volts),   and finally service (the voltage at the  plug). Putting service lines underground   is pretty straightforward. You might even  have an underground line at your house  

[05:18] running to a lamp or a detached garage. Putting  transmission lines at hundreds of thousands   of volts underground is a pretty extreme  engineering challenge because of insulation,  

[05:30] heat buildup, and capacitance. Undergrounding  distribution lines lies somewhere in the middle. One of the big upsides of running lines above  ground is the availability of air. Air is free  

[05:43] and it works pretty well as an insulator if you  keep enough space around energized conductors.   You only need actual insulators at the pole.  Putting lines at tens of thousands of volts   underground requires pretty expensive insulation  that prevents arcs to ground or other phases and  

[06:00] resists the effects of water, a hazard that  is inevitable for every underground utility. We often call an electrical interconnection  a “grid,” but that term mostly applies to the   high-voltage bulk power system covering whole  states or countries. It’s not really a good  

[06:16] description at a city scale. Most urban areas use  what’s called a radial system for distributing   electricity, which is more akin to branches of a  tree than a mesh. For a single-family residential  

[06:30] home, you might share a transformer with a few  houses. That connects to the distribution feeder,   and you can follow that line all the way back  to the substation. Each feeder is essentially   a one-way dead end for power flow. There may be a  crossover somewhere for redundancy, but it’s not  

[06:47] an inherent part of the radial architecture.  In New York City, it’s totally different. Throughout the five boroughs, New York  City operates about 70 separate so-called   “secondary networks,” each of which is served  by somewhere between 8 and 28 feeder lines  

[07:04] from an area substation. Rather than individual  transformers that serve one or two buildings,   there are network transformers dotted around the  city, usually in concrete vaults belowground,   each connected to one of the redundant feeders  from the substation. Because they’re underground,  

[07:20] these transformers have to be capable of  operating while fully submerged in water. Those transformers drop the voltage to the  service-level where a grid of conductors   spread out to all the buildings in the area. These  are true networks, actual grids of service-level  

[07:36] voltage with multiple redundant pathways for  energy to take (not like the branches of a   tree at all). And there’s another way it’s  not quite like the rest of North America. A typical service transformer in the US  gives you “split phase power”. It takes  

[07:52] one phase from the grid and provides  two energized lines we call hots. Each   hot leg has a voltage sine wave between  neutral that is 180 degrees out of phase.   So between one hot and the neutral, you get  120 volts. That’s a typical wall outlet.  

[08:07] Larger appliances and EV chargers use both  hot lines to get 240 volts. In New York City,   the service networks are different. They  use a three-phase system just like the   rest of the grid. Each phase is offset by 120  degrees. Most individual apartments or houses  

[08:24] get two of the three phases. So in the city,  you still get 120 volts from hot to neutral,   but you only get 208 volts between the hots. Most  large appliances are designed to work on both 240  

[08:37] and 208 volts in the US because of this mixing and  matching with single phase and three phase power. Larger buildings like skyscrapers usually get  their power at a higher voltage directly from   the feeder and use their own transformers  on maintenance floors to provide service  

[08:52] throughout the building. So ConEdison maintains  basically two power grids, each underground,   one for the feeders between 13,000 and 27,000  volts and one for low voltage service. They  

[09:06] both run through ducts that travel below  streets and sidewalks and are serviced in   the thousands of underground manholes and vaults  throughout the city. With everything protected   underground with lots of redundant paths,  New Yorkers enjoy one of the most reliable  

[09:20] electrical services in the country, but  obviously, that reliability comes at a   price. The “service network” architecture  is one reason why New York City has some of   the highest electricity prices in America. But  wires aren’t the only way New Yorkers get power.

[09:38] Looking back at our generic intersection,  you’ll see something that is not present in   most other cities, and certainly not at the scale  we see here. New York City has a district heating   network, offering steam as a public utility.  It’s not as extensive as the power system,  

[09:54] with pipes mainly confined to Manhattan, but  it is the largest of its kind in the world by   far. There are about 1,500 customers with steam  service delivered through the underground lines.  

[10:06] It’s mainly used for heating buildings and  for hot water, but it can also be used for   sterilization in hospitals, cleaning dishes  at restaurants, for presses in dry cleaning   facilities, and somewhat counterintuitively, for  air conditioning through steam-driven compressors.

[10:23] Of course, there are a lot of engineering  challenges with running steam pipes   underground. Thermal expansion is a big  one. If you shut one of these lines down,   it changes in temperature by roughly 300  degrees Fahrenheit or 170 degrees Celsius.  

[10:39] You need regular expansion loops or heavy-duty  slip joints that can absorb the physical movement   created by those huge swings in temperature.  You also have to deal with condensation.   Some of that steam naturally condenses into  water, and if you don’t get it out of the pipe,  

[10:55] that liquid can act like bullets inside the high  speed steam flow. I have a video on that from way   back in the day if you want to learn more. Steam  traps can discharge condensate while keeping the   steam inside the pipe. Occasionally you get a  steam leak or just a spot where groundwater is  

[11:12] coming into contact with the hot pipes, creating  a cloud of steam through a manhole to the street.   ConEd puts these orange smokestacks to  divert the vapor away from the public   until they can fix the underlying issue. Those  steam lines are insulated to keep the heat in,  

[11:27] and usually deeper than other utilities to  avoid heating up the surface or the other lines. One of those lines you definitely don’t want  to heat up is natural gas. A large percentage   of buildings and households in New York City use  natural gas for heating, cooking, and hot water.  

[11:43] Some of the gas lines are low-pressure mains  that connect directly to buildings through the   meter. The newer lines run at higher pressures and  require a building regulator. For safety reasons,   these are often installed outside, so they’re  easy to spot. New York has put limits on when  

[12:00] natural gas can be included in new construction,  so these lines might become a thing of the past,   joining other abandoned lines underground. Because  of the cost and complexity of decommissioning   utilities, it’s not uncommon to simply abandon  them in place. In fact, there are plenty of lines  

[12:17] underground that aren’t doing anything at all,  including a once-sophisticated pneumatic tube   mail delivery system, which spanned 27 miles and  delivered mail throughout the city until it was   shuttered in the 1950s when the maintenance  costs started outweighing the benefits.

[12:33] Also like other places, New York City needs  telecommunications: telephone, cable, and   fiber lines. And like all their other utilities,  these are running below the streets, weaving their   way around everything else near the surface. But  unlike a lot of the utilities, telecommunications  

[12:50] are somewhat standardized. That’s because, in  1891, the city granted a franchise to a company   called Empire City Subway. That name uses a more  generic meaning of subway, an underground path,  

[13:04] rather than the much more famous one you’re  thinking of (which we’ll get to soon). Empire   City Subway’s whole job was and still is to build  and maintain a massive network of underground  

[13:16] utility ducts. Anyone wanting to run telephone,  cable, or fiber lines through Manhattan   or The Bronx has to rent space inside those  conduits rather than digging their own paths. But even with that standardization, you can see  that things are starting to get pretty cramped.  

[13:33] In many cases, especially older parts of the  city, very little of this infrastructure was   planned out comprehensively. As each service  was installed, it had to find space among the   others. And because much of it is pretty old,  most streets are either imperfectly mapped,  

[13:50] or not mapped at all. Making repairs  is its own kind of treasure hunt.   Utilities workers and engineers often just call it  “the spaghetti” because of how big a mess it can   be. To combat their proverbial pasta problems, the  City is working on a comprehensive, 3D database of  

[14:07] underground utilities to help with construction,  repairs, and emergency response, but it’s an   enormous challenge to combine records that,  in some cases, can be more than 100 years old. Vacuum excavation has made a big difference  in being able to dig around utilities safely.  

[14:24] Water or compressed air breaks up the soil and  the truck can suck it up. This makes it a lot   quicker to expose buried utilities without  damaging them, especially compared to an   excavator or backhoe bucket. Often you have to  support utilities from above during repairs to  

[14:40] keep them from sagging down and flexing, which  could lead to breaks and service disruptions.   Older materials used for pipes and ducts like  cast iron and vitrified clay are brittle and  

[14:52] relatively weak. Taking away that continuous  underlying support to repair something below   requires a lot of care. This is just a ton  of work compared to cities where there’s   less density in the buried utilities. But  that’s still not everything under the street.

[15:09] Of course you have sewers that carry wastewater  away. They’re usually deeper than the other   utilities, and for a few reasons: Sewers rely  on gravity to maintain flow, so the slope   carries them further below the ground. You also  generally want to have them below water lines,  

[15:25] just to be absolutely sure that a sewage leak  doesn’t find its way through the soil toward   fresh water. Of course, all American cities have  sewer lines, but New York has some big ones.   They’re not quite like the cartoon-style tunnels  with handy ledges that act like walkways for crime  

[15:42] fighting reptiles, but some of the hand-laid brick  pipes are still in use. This is a big city that   produces a lot of wastewater, and when all those  lines start to converge and concentrate toward the   treatment plants, it takes big pipes to carry it  all. They may not look quite like the cartoons,  

[15:58] but some actually are cavernous enough to  walk through standing completely upright. These big old sewers are made even more  complicated by stormwater. Much of the   sewer system in New York City was built before  modern environmental rules. The goal back then  

[16:14] was to get it out of town, not to treat it,  so it didn’t really matter that the stormwater   was diverted into sewage pipes. It was a good  thing, actually, because it diluted the sewage   that was just being discharged directly  into waterways. The problem is that, now,  

[16:30] it matters a lot. On rainy days, the treatment  plants don’t have the capacity to clean up both   the sanitary sewage (the stuff that comes  from sinks, toilets, and showers) and all   the stormwater runoff from streets and buildings.  So they still have to discharge untreated sewage  

[16:46] on occasion. The city has roughly 400 outfalls  where these “combined sewer overflows” occur. Of course, dumping raw sewage is highly  restricted under modern laws. You can’t  

[16:58] pollute natural waterbodies without consequence.  But, it’s a practically impossible problem to fix,   at least all at once, since these systems were  built this way over decades and decades. Instead,  

[17:11] the city operates under a consent decree with  the state that basically says, “We won’t fine   you for the overflows, but you have to implement  a plan that puts a stop to this eventually.” Lots   of major US cities are in the same boat. And New  York City really has been working to fix it. From  

[17:28] curbside rain gardens to massive underground  retention tanks like the one at Newton Creek,   New York is slowly chipping away at the number of  sewer overflows. In newer parts of the city, the   sanitary and storm sewers are entirely separate.  Catch basins line the streets where stormwater  

[17:45] is diverted to underground pipes. That’s  just one more separate system of underground   utilities to find space below these crowded  streets. Go a little deeper and there’s more. New York City is famous for its subway, one of  the largest and busiest rapid transit systems  

[18:01] in the world. They mostly run in cut-and-cover  tunnels below the streets. This is a simple idea:   dig a trench down from the surface of the road,  construct the floors, walls, and roof of the  

[18:13] tunnel, then backfill and replace the road on top.  It’s disruptive to the street, but much simpler   and more cost-effective compared to tunneling  methods that don’t disturb the surface. There  

[18:25] are many instances of multi-level tracks where  one line crosses another that required elaborate   steel framing to support everything during  construction. Deeper sections of the subway,   like those under the East River, required  alternative methods. The most recent projects have  

[18:41] used tunnel boring machines, which are much more  expensive, but increasing the depth helps avoid   existing utilities and minimizes disruption at the  street level. And you also have the ventilation   structures and the elevators and escalators  and stairs that connect between subways and  

[18:57] the surface. The stations themselves are mostly  subterranean, since that’s where the trains are. Of course, there are tunnels for cars and trucks  below the surface in New York City too that pass   under the East River and Hudson River. Sometimes  it makes more sense than building a bridge,  

[19:12] and other times it’s necessary for  grade separation to keep traffic moving. Go even deeper below the street, and we’re back  to water. New York has three primary tunnels that   bring the fresh water into the city from upstate.  Tunnel 3 is one of the largest and most expensive  

[19:30] capital construction projects in the City’s  history. Started in 1970, it’s still under   construction and will be for at least another  decade. It’s really deep, roughly 650 feet or 200  

[19:43] meters below the surface. That’s nearly half the  Empire State Building! That extreme depth is to   avoid the underground traffic jam of all the other  utilities we’ve talked about, but also because   it keeps the tunnel in hard rock that’s better  able to withstand the monstrous pressure inside.

[20:00] The underground of New York is almost a city  within a city. We kind of get used to having   all these utilities and services that it’s easy to  forget the physical space they all take up and the  

[20:13] work that goes into installing and maintaining  them. Besides what they provide us, there are   little signs at the surface as reminders: manhole  lids, puffs of steam, metal grates where you can   barely catch a glimpse of the machinery below. And  then sometimes the reminders are a little more in  

[20:30] your face, like when an intersection shuts down  for a careful and intricate replacement project,   weaving new pipes between those that might be  a century old. These kinds of disruptions are  

[20:42] hard to love, but they do give you a chance  to appreciate everything that’s underneath. You can see in photos of excavations in  New York City that just about everything   is customized. With such a sprawling collection  of utilities built over more than a century,  

[20:58] there just isn’t that much standardization. It’s  the same thing with a lot of the demonstrations I   build for videos: you can’t pick this stuff up  off the shelf. Most of my builds have custom,   one-off parts to make them work. But you don’t  need a multi-billion dollar municipal budget to  

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