Learn how the engineered world around you actually works — why trains almost never crash, why dirt roads ripple into washboards, how a fine mist can stop a tunnel fire, how a city moves a river 500 kilometres, and what really happens inside a zip. This course collects five of the best real-world engineering explainers from Tom Scott, Practical Engineering and Veritasium, each one built on visits to the actual infrastructure and interviews with the engineers who run it.You will start with railway signalling: the block systems, interlocking and automatic protection that make rail one of the safest ways to travel, explained from the cab and the signal box. From there, the physics of road corrugation — the self-amplifying feedback loop between wheels and gravel that no amount of careful driving prevents — then the Tyne Tunnels' water-mist fire suppression, a system designed around the hardest problem in tunnel safety. The fourth section follows the Los Angeles Aqueduct from the Sierra Nevada to the city: siphons, cascades and the century-old gravity-fed design that still works today. The course closes small, with the zip — a mechanism you use ten times a day, manufactured in greater numbers than there are stars in the Milky Way, and a beautiful case study in how tiny geometry does big work.No equations or prior background needed — just curiosity about the built world. By the end you will notice the engineering hiding in plain sight on every journey you take, and understand the safety systems, feedback loops and design trade-offs that keep modern infrastructure running.
For as long as we’ve had clothes, we’ve done laundry. Long before detergent pods and spin cycles, people figured out the simple trick of rubbing textiles against rough surfaces, which squeezes dirty water out of the fibers and pulls cleaner water back in. Roughly around the 1800s, that idea got a dedicated tool that you still occasionally see today, even if only in jug bands. The washboard spread across the world as a faster, more practical way to scrub shirts and pants. Its use continued to grow until automated electric machines took over in the early 20th century. But we’re not here to talk about laundry. That era wasn’t just the heyday of the washboard. It also marked the start of the highway era (at least in the US), when road construction boomed, and traffic skyrocketed, including on the unpaved routes that connected farms, towns, and job sites. If you’ve ever driven on a dirt road, you know one of their biggest problems. In some places, the surface organizes itself into a repeating pattern of ridges and valleys that can rattle your teeth, shake your suspension to pieces, and make the vehicle feel like it’s skating. It’s obnoxious at low speeds. At higher speeds, it can become legitimately dangerous because bouncing tires can cause you to lose control of steering and braking. In 1924, this phenomenon was known as “rhythmic corrugations.” By 1929, people were calling them “washboard” roads. And nearly a century later, they’re still a persistent problem all over the world. What’s surprising is that this is still an active area of research. In the past decade or so, there’s been a burst of papers trying to explain exactly how washboarding forms and how to stop it. I was reading one of those studies when I saw the experimental setup they used. The moment I saw it, I knew I had to build one. Let’s talk about why this happens, and why there’s still more to learn about washboarding. I’m Grady, and this is Practical Engineering. Roughly 35% of the countless thousands of miles of roadways in the United States are unpaved. That’s technically true, but a little misleading since most of those roads are in rural areas that carry only a tiny fraction of the traffic volume compared to the roads paved in asphalt or concrete. This is not a measured or reported statistic, but some back-of-the-envelope math shows that only about one percent of traffic happens on unpaved roadways in the US. That’s not true in other parts of the world, though. We call that outer layer of a road the wearing surface (for obvious reasons). Of course, using a hard material like asphalt or concrete as the wearing surface has a lot of benefits: it cuts down on dust, it’s generally smoother and more comfortable to drive on, it’s easier and safer to drive at high speeds, and it’s more durable. But as good as it is, paving doesn’t make sense in every situation. Almost everything in transportation engineering is an economic decision. Roads seem kind of mundane and simple, but you have to consider the scale. Think about a basic home improvement project like installing new flooring in a kitchen. Maybe it’ll cost you a few thousand dollars. Then consider that a mile (or 1.6 kilometers) of two-lane roadways has, very roughly, 600 kitchen’s worth of surface area. For a simple commute to work, we’re talking about thousands or even tens of thousands of kitchens now. This is silly, but the point is that roads are extremely expensive, not because they’re particularly sophisticated, but because they’re huge. You don’t get a monthly “road bill,” but if you add up gas taxes, tolls, and the slice of general taxes that fund roads, the average household in the US is paying roughly on the order of an electric bill each month. So engineering decisions about roadways are largely driven by costs. Here’s a simple example: An unpaved road is pretty cheap to build. Before we consider the traffic, it sits pretty low on this graph. But as traffic increases, so do the maintenance costs associated with regrading, adding new gravel that’s lost over time, controlling dust through chemical application, etc. Its fixed cost is low, but its variable cost is steep because it’s not as durable. A paved road is expensive to build initially, but it requires less maintenance and that maintenance is less sensitive to traffic volume, so its variable cost is relatively flat. You can see there’s a breakeven point based on traffic volume, below which it makes more sense not to pave a road. And that’s just looking at traffic. When you factor in other complexities like the availability of materials, the abundance of skilled contractors who can do the work, and climate factors, there are plenty of situations where paving a road isn’t the best economic decision. In many countries, pavement is a luxury reserved for only the busiest highways. My point in all of this is to show you that washboarding isn’t just a problem of the past. It’s a real and present challenge, so it makes sense that we’re still trying to understand how and why it happens. I mentioned there’s a lot of ongoing research into this question. Those papers can be pretty hard to wrap your head around. So I figured I’d build a demonstration so we can see it happen in real time. A lot of these parts came from my friends at Send-Cut-Send. My local hardware store didn’t have a circular sand track and a motorized wheel-arm assembly on the shelves, so I had to design this myself. I could have spent days out here fabricating it, too, but I’d rather spend that time making more videos for you. Send-cut-send makes it so effortless. They give anyone with a CAD file access to laser cutting, CNC machining, bending, powder coating, and more. They don’t have minimum quantities. It’s super fast, super easy, made in the USA. They have completely changed my workflow out in the shop, and they support what I’m doing on the channel. This is the easiest recommendation I can make: If you need parts made from metal, acrylic, or any of the other materials they have available, give them a try at the link below. Here’s how I set this up: In the center I have a stepper motor connected to a gearbox. I chose a stepper because the hardware is relatively cheap these days, and with the built-in encoder, I don’t need a separate system to keep track of the speed. The power supply and driver for the motor are over here to the side. I’m pretty new to this stepper stuff. This is not related to washboards, but I just think it’s cool. This controller sends pulses to the driver, which energizes the coils of the motor in a careful order. The trick is that it also knows whether the motor actually did what it said to do, so it can adjust or throw a fault if something goes wrong. For the 90s kids, it checks itself before it wrecks itself. It took me some time to wrap my head around it, but now I just have a button and knob that do exactly what I want. This arm is attached to the gearbox and has a wheel assembly on one side and some bolts for a counterweight on the other. The wheel runs in a circular track of sand I made from polycarbonate sheets. I used polycarbonate rather than my typical acrylic because it can flex more without breaking. Turns out that not much will stick to it, though, so I ended up bolting the pieces together. Now that this is set up, I can just push go and watch what happens. And what happens isn’t really too interesting at first. This does just about what you’d expect it to: the wheel digs a rut in the sand as it goes around and around the track. Right now, it’s moving at about one meter per second or roughly 2 miles per hour. I let this run for an hour with pretty much no change in the behavior. But watch what happens when I ramp up the speed just a bit. And actually listen too, because it’s easier to hear than to see at first. After only a few laps around the track, you start to hear a rhythm. It doesn’t take long at all before the wheel is literally galloping between each bump. I have to be honest - I didn’t think it was going to work this well. Let’s take a closer look to see what’s happening. I’m going to smooth out the sand and start this again. But, critically, there’s no way for me to make this surface mathematically smooth. There are always going to be some small, random bumps and dips. And it turns out that’s important here. My wheel has some freedom to move around this clevis pin, so it can go up and down bumps like a regular vehicle would. The wheel’s not rigidly being pushed through the material; it’s in a conversation with the sand. Because there is some irregularity in the surface, the force between the wheel and the sand is irregular too. When we hit a bump, the force is briefly higher, causing the sand to be plowed forward. And when the wheel encounters a dip, the force is less so less material is shifted. That fluctuation in forces matters because the wheel starts changing the surface of the track unevenly. As the wheel rolls up off a bump, it unloads, but what goes up must come down. The wheel has inertia, so when it falls, the downward force on the surface is higher than when it’s just rolling along. If the speed is fast enough, that impact happens a little after the lowest point, so the sand is pushed and piled downstream of where the wheel hit, creating a new bump where there wasn’t one before. One bump becomes two, becomes three, and so on. Each pass makes them a little bigger and organizes them a little better until you get that rhythmic corrugation in the surface. Eventually the bumps are big enough that the wheel follows a ballistic trajectory, literally jumping off the surface with each one, and slamming back down on the other side in just the right place to push more sand up the following bump. The only thing limiting the bumps from growing indefinitely is the natural repose of the sand. It can only stand up so steeply before it flows down into the next dip, filling it somewhat. That’s why these washboards can travel like sand dunes, slowly moving down the road. Understanding all this, it makes sense why washboards often emerge in transitional areas of a road: curves, changes in slope, and at connections to paved roads. You need a fluctuation in tire force to get the process started, and these are places where, even without a bump, there’s a higher likelihood of a sudden change in force that could start to shift the material. What’s interesting is how robust the instability is. You would think that the differences in vehicle weights, wheel bases, suspensions, and speeds would tend to smooth things out. It turns out, they don’t. In fact, you don’t even need a wheel to do this. Researchers have shown that even a simple angled plate that acts like a plow will form ripples above a threshold speed as long as it has the freedom to move with the bumps. It all feels a bit counterintuitive; it’d be easy to assume that an angled plate would smooth out the road like icing on a cake. A big part of the reason things don’t smooth out is that the washboards are self-perpetuating. They impose a periodic force on vehicle tires that, in turn, amplifies the forces creating the corrugations. It’s like the Tacoma Narrows Bridge where the wind amplified the torsion and the torsion amplified the wind's effect on the deck: a positive feedback loop of instability that only gets worse over time. The washboarding of roads is just one example of what scientists call a pattern-forming instability, where structure emerges from a system that starts out more-or-less uniform. We see them everywhere in nature, from chemical reactions to the markings on animals. And washboarding has some natural analogs. In rivers, moving water transports sediments in a way that can form ripples on the bed. I caught a cool shot of this phenomenon in action for my video about reservoir sedimentation. At a larger scale, river meanders are often described in the same way. Even a straight channel will, under the right conditions, begin to drift sideways as curvature redirects the fastest flow toward the outer bank, strengthening erosion there and deposition on the inside. I have a couple of videos on that process, too. But the fluid doesn’t have to be water. In places without much vegetation, wind-driven sand doesn’t naturally smooth out. Instead, it forms dunes that can march across the landscape. In all these cases, the same basic feedback loop is at work: the driving force reshapes the surface, and the reshaped surface redirects the driving force. It’s the same thing with washboards. Small bumps change how a wheel loads and shears the gravel, and that shifted, uneven loading moves material in a way that makes the bumps grow into a repeating pattern. Characterizing the factors that play a role in the formation of washboarding helps you make engineering decisions about how to avoid it in the first place. The obvious solution is to use a more durable wearing surface, like asphalt. But we’ve already covered why it’s not always the best option. Plus, it’s not like asphalt is immune to damage. Speed is the most critical factor for washboarding. Maybe slowing down is a useful suggestion on a personal driveway, but it’s not particularly viable as a widespread solution on public roads, especially the low-traffic roads where enforcement of speed limits is…less than strict. We want to get to where we’re going, and we want roads that can safely handle the speed. So most of the focus is actually on the materials used in roadway construction. Typical roads use a base layer below the wearing course. It can get a lot more complicated than this, but in nearly every case, the base material is usually doing the heavy lifting. The material used for road base is broadly graded, meaning it has particles of lots of different sizes, from gravel size all the way down to microscopic clay-sized grains. Often it’s made from crushed rock, which naturally produces a good mix of particle sizes. All those different sizes, plus the angularity of the crushed stone, help the road base lock together into a dense, strong layer. It’s actually pretty remarkable. If you’ve ever walked on well-compacted road base, it feels almost as hard as concrete. But remember that a typical road base is meant to be confined by some kind of pavement. You don’t have to worry about particles being dislodged or shifted because they’re trapped underneath the asphalt. So, generally, the wearing surface for an unpaved road is going to perform best if it has more fines than a typical road base, locking those bigger aggregates in better and offering more resistance to shearing. This is a pretty careful balancing act. Too many fines, and the road is excessively dusty when dry and slippery when wet. Too few fines, and the surface won’t lock together, leading to washboarding and raveling. It’s a pretty fine line to walk for an engineer, and the trouble is that it’s not always a simple matter to source the right material. Road base is widely available because we use so much of it, so quarries and suppliers tailor their equipment and processes to meet those specifications. It can be much harder to get an aggregate operation to produce or blend a custom batch of material that works well for unpaved roads. In fact, except for the biggest projects with a commensurate budget, the material chosen for an unpaved road is almost always a compromise between what works best and what the local quarry can give you. Another option might be synthetic materials that can augment the properties of soil. These are used in all kinds of situations, including roadway construction, so it makes sense that they might help prolong the life of unpaved roadways and reduce the maintenance costs associated with washboards. I think it might be fun to test some of these out in the test track here, since I already have it built. Let me know if you’d like to see a video on that. For something so common, washboarding is a reminder that we’re still actively learning how the built world behaves, especially when it isn’t perfectly rigid, smooth, or controlled. Researchers are still teasing apart the details of how speed, tire pressure, suspension, moisture, and grain size conspire to pick a spacing and then lock it in. Unpaved roads aren’t a relic of the past. They’re critical infrastructure for huge parts of the world, linking farms to markets, kids to schools, and communities to clinics. And if a road corrugates, it’s not just annoying. It slows everything down, shakes vehicles apart, can lead to crashes, and makes maintenance a constant uphill battle. The more we understand the physics of washboarding, the better we can engineer roads that stay safe, smooth, and reliable, even when the only tools you have are local materials, a grader, and a limited budget.
For this stop on my road trip, I was visiting the River Tyne, and I was going to be getting very cold and wet, but not in the way you might expect. The Tyne was the boundary between the counties of Northumberland and Durham, up in the north-east of England. These days it’s more complicated, but historically the Big River marked the border. And while I started this video on the north side of the river, in Northumberland, the reason I was there is over in Durham. In 1968, the Tyne Road Tunnel opened to the public, with a second tunnel added in 2011. I parked up at the tunnel headquarters, and I was there for three things: first, so the team could show off behind-the-scenes of the tunnel. Second, to learn about a fire suppression system that I’d never heard of before. And third, to stand under that system while it was being tested. So, I put on safety gear, and the first stop was the control room. -Come in. -Oh, blimey. -This is the control room. -You have a proper mission control here. That’s David, he’s the operations manager. And his team were watching more than 200 cameras inside the tunnels, helped by an automatic system that detects if anything’s out of place. We’ve got the vehicle coming in here, in the northbound tunnel. We’ll wait for one to come through. -Okay. -Nice blue truck there. -Yep, big truck. -Right back round. -Yeah. And then it will come along to here. Oh, so it’s always… And we follow the vehicle all the way along. As soon as it starts to leave one camera, it joins the next one. Yeah. It will go all the way along to the very end and it comes back round on the second row. Right! The two tunnels are very different in construction: the first was dug by hand and by machine 15 metres between the top of the tunnel and the river bed. The second is an immersed tube, constructed on land, sunk down into a trench, sealed up and drained. Then the trench is filled in and covered in a lot of rocks. There he is. Approximately two and a half, three minutes to get through the tunnel. Most of the time, of course, it’s all quiet in the control room. But that control room is there for a reason. We approximately have between 7 to 15 breakdowns a month. -Okay. -But then at the weekend we might have a drunken person trying to walk through the tunnel. We have animals trying to get through the tunnel. It can be vast and varied. That’s more often than I’d think. So we have a target time of ten minutes to try and get that breakdown out. At rush hour, there’s a calculation that for every minute that the tunnel’s closed, that’s one mile of traffic jam outside. Right. And every time you have to hit the… I assume there is a button which I’m not allowed to point my camera at somewhere, that is the emergency stop. -Have a look here. -Oh, okay. I actually was allowed to point my cameras at the safety systems, but I did agree to let the Tunnels team look at this video before it goes out, and if you see anything blurred or marked as removed, that is for security. They don’t have any editorial control over the words I’m using or the story I’m telling, though. There were big emergency buttons on that screen, three of them all labelled “STOP”, and Jack, controller on shift, said that hopefully they never have to use them. -We tend not to press the big red ones. -No! If you do, something’s gone wrong there. -It just scares you. -(laughter) Jack also told me about the ventilation in the tunnel, which is gonna be important later. There are jet fans mounted on the roof to keep the air moving, but they’re not always needed. What you’ll see is something called the piston effect which when there’s quite a few vehicles in there, it naturally draws fumes out of the tunnel. Yeah. Okay. Lower times of traffic, such as now or through the night, the piston effect decreases... -Right. ...so, you rely more on the help of the fans. Stick a pin in that “piston effect”, we’ll come back to it. But with the sun set and light fading, we walked out to the tunnel itself, with a few other folks from the team in tow. We had to cross the road, which would normally be complicated, but they timed it well: there were two vehicles passing through with dangerous cargo onboard, which meant the tunnels would be closed to any other traffic for just a couple of minutes. Those are the dangerous vehicles that were going through. Petrol, by the looks of them. And they just asked…I’ve got to keep quiet while we’re walking across. We were well outside rush hour, it was nearly 9pm, so, that closure wasn’t gonna cause too much trouble. And it gave us a gap to get across without slowing anyone down. And there go the cars. Time to go into the tunnel, the oldest of the two, with one of the big ventilation chimneys looming in the distance. We were starting in what’s called the Escape Gallery. This is now hats, gloves. -Yeah, right. -It’s a hat and glove area. -Helmet on, helmet tightened. -Just at the back, underneath. Got it. Thank you so much! That’s very loud and very close. You’ll probably feel that wind, that positive pressure. Oh, yes. The escape gallery is kept at positive pressure, air constantly forced in, because if something happens in the tunnel and people have to get out of their cars and evacuate, that gallery needs to have clean air no matter what. This is the big chimney. Oh! That’s the big chimney! So, gloves on. As you go down, one at a time, it’s not very far, this one. Yeah. The next stop was down a ladder into an area that the public definitely aren’t allowed into: the service gallery. You’ll probably hear a few vehicles rumbling over our heads... -Yeah. -...but this is absolutely amazing. This is underneath the road deck. -Oh! -And you can see… -Oh... ...I’m six foot five, so you can kind of work the distance out. Yeah. But you can hear the rumbling, especially when you get the trucks coming through. That’s going over our heads. Supporting stanchions either side of the tunnel. -Yeah. -And this is the void. On the side were four big pipes: two for drainage, and two for firefighting. The one down below, the smaller one, is the mist system line. -Mm-hm. -The mist system… Oh right, yeah. But that’s not the same as the fire main? That’s not the same as the fire main. It’s two different systems. One for when the firefighters turn up and need to plug-in. -Yeah. -And one for the automatic system. -For us, yes. -Yeah. The Mist System was the real reason I was there. When I put a call out for video ideas for this series, the Tyne Tunnels folks got in touch asking if I wanted to look behind the scenes and see their fire suppression system, the first of its kind in Britain. And they suggested that I might want to stand underneath it when they were doing a test. (laughs) So,I had that to look forward to! Also, the new person on camera there is Barry who’s worked at the Tyne Tunnels for 27 years. He wasn’t actually wearing a mic because he was just going to be behind the camera, but turns out he was great. He had a lot of interesting things to say, and he said he was okay to appear now and then. Now, when they’re gonna turn the mist system on, these are the valves that turn on. He did take the box off to show those valves. To lock it off for maintenance, you have to turn the valve handle at the bottom right, and if you do that, you can’t put the red box back on. You have to have that in that position before… If you accidentally left it locked out… -This won’t go on. -Right. And then you’d know that it’s not working. You’d know there’s a problem. -Yep. It also alarms in the control room... -Right, okay. -...as well, so there’s lots of fallbacks. -Okay! Yeah, that’s fair enough. The air in the service gallery was clean. I was expecting to be able to taste petrol fumes, but no: air is forced in, just like the escape gallery, which, by the way, wasn’t part of the original construction. -The escape gallery was put on later on. -Right. Because of the Mont Blanc tunnel disaster. In 1999, a truck carrying flour and margarine caught fire in a tunnel under Mont Blanc on the French-Italian border. Flour is highly combustible, margarine is basically oil. The fire burned for days, 39 people died in the heat and smoke, and that disaster prompted safety upgrades to tunnels around the world: cameras, ventilation, fire suppression systems, proper co-ordination and control, even closing the tunnel to other traffic for the couple of minutes that those petrol tankers were passing through. There’s a saying that safety regulations are written in blood. And while you can never prevent every risk, you can certainly make a difference. We walked along the tunnel for quite a while, past the lowest point where any water would drain into a sump to be pumped back up and out. And then Barry led us into one of the crawlspaces to where the fresh air was being pumped in from the other chimney. (groans) Well, I’m now old enough that I make “ugh” noises while I’m doing that, that’s fun. Oh, I can feel the fresh air from that! That’s what makes it nice and fresh in here. Right. (clanking) -Do you want to come in? -Absolutely! Come on then. It’s really difficult to explain what that feels like. It is a literal breath of fresh air in a place that was pretty fresh anyway, but it’s sudden cold, outdoor air coming... Oh my word! Alright. So, that is the duct that’s bringing all the air in. Yes, yes. Wow. (helmet hit) Ow! And that’s why we have the helmets! (laughter) There was some clambering up ladders made of rungs that were just set into the wall. I feel like a bloody six-foot-five mole! Oh, wow. That is a long way up. -We’re not going up there now. -We are definitely not going up that. Good heavens. So that’s right to the top of the chimney? Yeah. That’s right to the top. Then it was back down, through the service gallery, up to the escape gallery which does run all the way along, obviously. We’d just gone underneath instead. There’s a lot of mind-your-head signs here. Oop-ah! And watch your head coming here, ’cos it’s pretty bad. Yep. There were two lanes of live traffic right next to us as we headed back out the tunnel and into the night, having made it to the historic county of Durham. These days, the whole area on both sides is called Tyne and Wear, but I’m going by historical definitions, and the equipment for the mist system, the thing I’m there to see: that’s in Durham. And to get there, we had to cross the road again. So we’ve got red lights there and the traffic has stopped. Red lights there and the traffic has stopped. So, this traffic lane is actually open right now. But we know that this is all safe because someone is blocking all the traffic on a rolling roadblock. There was a tunnel patrol vehicle coming through for the evening maintenance, travelling slowly and straddling both lanes southbound, which meant there was a big gap for us to walk across. Northbound, I think we delayed two cars by about one minute? Sorry if that was you. Control’s put the red lights on, stopped the traffic. -Yep. -Make that pause. Monitor through CCTV. We know where the gap is. -Yep. On the radio to us, safe to cross the road. -Yep. -Release the traffic straight away, get everybody moving again. -And away they go. It’s as efficient as that. Then, into the South Extract Building, which has pumps, maintenance supplies, some of the backup power, and the water tank for the mist system. 200 tonnes of water, ready to go. And we’ll see that in just a moment. Here’s some audio of me in the shower. (water running, surprised "huh") I had a new razor, and that was my first time shaving with it. I didn't take video, ’cos I was in the shower. I’ve been using disposable razors all my life, because that’s what I started with, and I never thought to change that. I thought razor burn was just an inevitable part of shaving, but it's a design flaw. Most modern razors flex a bit and bend slightly, so the angle’s a bit wrong, and it feels like you have to push harder. The manufacturers use workarounds, like lubrication strips and double or triple or septuple blades, but the root cause is that the razor flexes. Henson Shaving sent me over their razor. It’s made by aerospace engineers, built to very precise tolerances, and machined out of aluminium, so it's solid. First time using it, first time ever using a razor like it, it was as good as the disposables. After a few shaves, when I got used to it, it was better. I’m getting less razor burn, and I’m not buying expensive disposables that I just throw away. And I assumed Henson was going to be some subscription lock-in thing, but no: the blades are the international standard double-edge ones. If you want to use a different brand of blade, you can. Henson just make a very solid, very good razor that will last a long time. If you scan the QR code, or follow the link in the description, you can buy one, and they’ll also add in a hundred free blades, which will last most people years. Enough dallying around: the South Extract Building, and the mist system’s water tank. -Oh, that is the tank just there. -That’s the tank there. Because people could get hit with it. It’s so big. There’s a nice life ring there. -There’s a life ring! Oh, I like the life ring. -Yeah, because someone could fall in. -Absolutely. And they might not know how to swim. I mean, hopefully if they’re here, they do. Well, look at the height of it. (laughter) Just to be clear, that was a joke! They do not have pool parties in the water tank! It’s very cold, dark, deep mains water. And because that tank has to be kept filled at all times, it has a float valve, and… look, there is an alternative term for a float valve, and we’re all just going to have to agree not to laugh at the next few seconds of this video, okay? We’re all gonna be professional. -Oh. -So, the water comes in… That’s just a ballcock for your cistern. -That’s a ballcock. -It’s a ballcock. -That’s the same as you find in a toilet. It’s the biggest ballcock you’ve probably ever seen in your life. (laughter) Don’t, don’t...just don’t. It’s just a height measurer. When that drops, this valve opens. -Absolutely. -Yeah. When the mist system’s activated, they turn on three fire zones: the one the fire’s in and one either side for a total of about 75 metres. And for that, the mist system uses 2,000 litres of water per minute, which is a lot, but not as much as sprinklers or a deluge system. It’s designed to suppress a fire, give people time to get out, and give emergency services time to arrive. And to pump that water… How much…oh! (pump humming) I thought the pump had just started there. -That’s your jockey pump. -That’s the little one, yeah. The jockey pump keeps everything at a safe pressure, ready to go. It’s meant to fire up regularly. For a moment, I thought the system had actually been activated, but, no, that would have been much louder. There’s a pipe from the tank… 2,000 litres of water a minute through a single filter, or about a bathtub every six seconds. That’s a good filter. And then thirty-three pumps. Those aren’t for different parts of the system. When that thing fires off, all of those pumps are ready to run. That’s not 140 PSI. They’re using metric units, 140 bar. Mains water pressure, the stuff that comes out of your tap, is maybe three or four. 140 bar is what you get out of a pressure washer, the ones that blast walls and driveways clean, except those pressure washers use 10 or 20 litres a minute instead of 2,000. And it runs to destruction, because the alternative is a fire. Right. Tonight, they were testing Fire Zone 4 northbound, which conveniently for me was also in County Durham, just a little way into the tunnel. This was scheduled maintenance and it was all gonna happen very quickly. They’d be closing one lane overnight, 10pm to 5am, creating what they call a “dead lane”. So, 10 o’clock is about the time where traffic’s light, least disruption to the north-east. We can manage all the traffic in one lane. That allows us to do cleaning works and maintenance works whilst we keep the tunnel open. And one lane open during the night is enough. -Yep. -So we don’t get any traffic jams. It’s still free flowing. To close that one lane and make it safe for the workers overnight, a vehicle has to go through the tunnel dropping cones all the way along, and that’s a process that takes ten minutes or so. The tunnel is closed for those ten minutes, all traffic stopped, and while that happens, they will also have… A fire test in, I think, Zone 4. (chuckles) I’m gonna be standing under it! This was their suggestion. I didn’t come to them for this. They suggested this. So, we followed the maintenance vehicle into the tunnel. It feels so odd to be walking through the tunnel. Yeah, it’s magnificent. And I don’t know if you can see this, but there’s your two lines there. -Yeah. -They’re your mist system. -Oh. -See your mist heads there? -Yeah, okay. -So, either side. I mean, that’s…let me just make sure I’ve got that shot. So, both sides of your lanes are covered with separate mist lines. So, we’re looking at the line that’s going along… -See this here? See the head there? -Yeah. That’s your nozzle jets. Right, and it’s not like a sprinkler head. It’s not like you find in a hotel or something like that. -Very, very fine. -It’s very fine mist. That’s really important. The “mist” in mist system isn’t an acronym, it doesn’t stand for anything. It isn’t an old style sprinkler that just dumps as much water as possible. It uses superfine mist that will evaporate very, very quickly, sucking heat out of the air and turning to water vapour, cooling down fires and even scrubbing out some of the smoke. And remember those jet fans that we talked about back in the control room? I don’t know how much the camera can pick up… there’s sort of a gentle rotation in there? That’s something called the piston effect. As the vehicles come through, they’re dragging air through constantly. Right. I feel like I can still feel that now. There’s still a little bit of a draught, but when the trucks come through, you’ll feel that suction of air come right the way through the tunnel. “There’s still a little bit of a draught,” said David. He was right, and in hindsight, we should have remembered that. I don’t want to lower your expectations too much, believe me, I was about to get soaked, but the plan was to stand in the middle of Fire Zone 4, hold there for about ten seconds, and then walk forward out of it. Just beyond that speaker there, this is your fire zone. One thing I’d change if I filmed this again: I’d stand a bit further back, because half the mist was gonna be pushed away from me. A single fire zone is only 25 metres long, and that mist is very, very light. We all forgot the piston effect would make a difference. If I’d stood further back, or maybe not central in the road, I could have disappeared into the mist entirely. That said, it was still…an experience. When it happens, I want you to look for three things: First, just how much high-pressure water is coming out of the nozzles in the roof, and how fast it’s moving. Second, how light and gentle that seems, at least by comparison, by the time it hits the ground. And third, while it might not be obvious until you see the close-ups, just how absolutely soaked I am in a couple of seconds. I’d brought every camera that I could. I handed them to various members of the team. Rolling. Okay, folks, if everyone can roll their cameras, please? And then I tried to look composed. And I think I was, at least until I was startled by one of the loudest, most terrifying industrial noises I’ve ever heard. So, this is what it comes down to. There’s no microphone on me. And to be clear, this was their suggestion. I didn’t ask for this. I put a call out for ideas...! (loud screeching hiss) Good grief! (hissing roar) I almost managed to stay for ten seconds. Almost. (hissing roar) And the most startling thing about it… is just how incredibly cold that is! (hissing roar) (laughter) I felt frozen, because that’s what mist does. It hits your skin and then your body heat evaporates it, cooling you down and leaving space for more mist to do the same. That’s why it’s such a good fire suppressant. That is incredible! (hissing) I should have stayed in there for longer. I intended to, I thought I had! But it was so cold and so intense that the survival parts of my brain went, “Get out now, get warm!” I absolutely believe that works on fires. That may not have looked like much, but that soaked me immediately. Every single bit of me. I’m so cold. And that’s the point of it. The point of it is it’s mist! (sighs) Transport down there, do you want it? Yeah, I think I should probably get in there and get dry. I’m gonna start shivering. Thank you very much, everybody. (laughter) Next time, or right now on Nebula: I take a long walk on a dangerous beach and meet someone with a 500-year-old job.
On the northern edge of Los Angeles, fresh water spills down two stark concrete chutes perched on the foothills of the San Gabriel Mountains, a place simply called The Cascades. It’s a deceptively simple-looking finish line: the end of a roughly 300-mile (or 500 km) journey from the eastern slopes of the Sierra Nevada into the city. On November 5, 1913, tens of thousands of people climbed these hills to watch the first water arrive. When the gates finally opened, water trickled through, but that trickle quickly became a torrent. The project’s chief engineer, William Mulholland, leaned over to the mayor and shouted the line that’s been repeated ever since: “There it is, Mr. Mayor. Take it!” That moment was profound for a lot of reasons, depending on where you live and how you feel about water rights. LA didn’t become LA by living within the limits of its local resources. Its meteoric growth into the metropolis we know was enabled by an early and extraordinary decision to reach far beyond its own watershed and pull a whole new river into town. Today, roughly a third of LA’s water comes from the Eastern Sierra through the Los Angeles Aqueduct system. That share swings with snowpack, drought, and environmental constraints, but this one piece of infrastructure helped turn a water-limited town into a world city. It’s one of the most impressive and controversial engineering projects in American history. But to really appreciate that water in the cascades, you have to look way upstream and see what it took to get it there. It’s gravity, geology, politics, and human ambition all in a part of the state that most people never see. Let’s take a little tour so you can see what I mean. I’m Grady and this is Practical Engineering. When most people think about aqueducts, this is what they picture: a bridge carrying water over a valley or river. And, just to be clear, these are aqueducts. But engineers often use the term more broadly to describe any type of conveyance system that carries water over a long distance from a source to a distribution point. Could be a canal, a pipe, a tunnel, or even just a ditch. In the case of the LA aqueduct, it’s all of them, plus a lot of supporting infrastructure as well. From the center of the city, it’s about a four hour drive to the Owens River Diversion Weir. It’s not accessible to the public, but it is the official start of the LA Aqueduct, at least when it was originally built. Here, all the snowmelt and rain from a huge drainage system between the Sierra Nevada and Inyo Mountains funnel down into the Owens River, where a large concrete diversion weir peels nearly all of it out of its natural course and into a canal. This point is roughly 2,500 feet (or 750 meters) higher in elevation than the bottom of the Cascades at the downstream end, which makes it obvious why LA chose it as a source. The entire aqueduct is a gravity machine. There are no pumps pushing the water toward the city. Half a mile of elevation change feels like a lot until you realize you have to spread it out over 300 miles. It’s all achieved through careful grading and managing elevations along the way to keep the flow moving. That care is particularly important in this upper section of the aqueduct, where the water flows in an open canal. To do this efficiently, you need a relatively constant slope from start to finish. That’s a tough thing to achieve on the surface of a bumpy earth. Following a river valley makes this easier, but you can see the twists and turns necessary to keep the aqueduct on its gentle slope toward LA. If it seems kind of wild that a city would buy up the land and water rights from somewhere so far away, it did to a lot of the people who lived in the Owens Valley, too. A lot of the acquisitions and politics of the original LA Aqueduct were carried out in bad faith, souring relationships with landowners, ranchers, farmers, and communities in the area. The saga is full of broken promises and shady dealings. Then when the diversion started, the area dried up, disrupting the ecology of the region, making agriculture more difficult and residents even more resentful. Many resorted to violence, not against people but against the infrastructure. They vandalized parts of the aqueduct, a conflict that later became known as the California Water Wars. In one case in 1924, ranchers used dynamite to blow up a part of the canal. Later that year, they seized the Alabama Gates. About 20 miles or 35 kilometers downstream from the diversion weir, a set of gates sits on the eastern bank of the aqueduct canal. Because it runs beside the river valley, the aqueduct captures some of the water that flows down from the surrounding mountains in addition to what’s diverted out of the Owens River, particularly during strong storms. That means it’s actually possible for the canal to overfill. The Alabama Gates serve as a spillway, allowing operators to divert water back down to the river. This also helps drain the canal for maintenance or repairs when needed. Those Owens Valley ranchers understood exactly what the Alabama Gates controlled. Open them, and the water would run back where it had always run, down the Owens River, instead of south to Los Angeles. The resistance simmered and flared for years, but it didn’t end in the dramatic showdown at the aqueduct. Instead, it ended at a bank counter. The Inyo County Bank was run by two brothers who were also key organizers and financiers of the resistance campaign. In August 1927, an audit revealed major shortfalls and ongoing embezzlement, and the bank quickly collapsed. Residents across the valley saw their savings wiped out or frozen overnight, shattering what was left of the community’s ability to keep fighting. The Alabama Gates weren’t just a political flashpoint though. They also marked an important dividing line in the aqueduct’s design. LA knew that even if the ranchers didn’t release the water to the river in protests, a lot of it would end up there anyway through seepage. As the canal climbed away from the valley floor and crossed more porous soil, it would naturally lose its water through the ground. So, at the Alabama Gates, the aqueduct transitions from an unlined canal to a concrete-lined channel. It’s still open to the air, so there’s no protection against evaporation or contamination, but the losses to the ground are a lot less. This design continues for about 35 miles (or 55 kilometers) through the valley. Along the way, the aqueduct passes the remains of Owens Lake. Once a large body of water, it quickly dried up with the diversion of the Owens River. Of course, there were impacts to wildlife from the loss of water, but the bigger problem came later: dust. All the fine sediment that settled on the lakebed over thousands of years was now exposed to the hot desert sun. When the wind picked up, it filled the air with fine particulates that are dangerous to breathe. Over the years, there have been times when Owens Lake is the single largest source of dust pollution in the entire country, and LA has spent more than a billion dollars just trying to fix this problem alone. The aqueduct passing along the hillside past the lake and its challenges is a reminder that the true cost of water is often a lot more than the infrastructure it takes to deliver it. So far, it might be obvious that this aqueduct system is pretty fragile to be making up a major part of a city’s fresh water supply. Even beyond the vandalism and political resistance, there are a lot of things that could go wrong along the way, from bank collapses, earthquakes, diversion failures, and more. That’s why Haiwee Reservoir was originally built in a narrow saddle between two hills as a kind of buffer. With a dam on either side, it stored water up so the aqueduct could keep running even during a disruption upstream. It also slowed the water down, exposing it to the hot desert sun as a natural form of UV disinfection. In the 1960s, the reservoir was reconfigured into two basins to add some flexibility. That’s because, around that time, the LA aqueduct became two. While the open-topped canal section was large enough to meet demands, the underground conduit in the next section wasn’t. So, LA built a second one in 1970 to increase the flow. If you look at this map of the Haiwee Reservoirs, you can see that water has two paths: it can flow into the second aqueduct here from the north basin, or it can pass through the Merritt Cut to the south reservoir, through the intake there, and into the first aqueduct. This setup allows for some redundancy, along with regulation and balancing of the flows between the two aqueducts. Haiwee marks the start of the long desert run, with both systems no longer in open-topped lined canals, but running underground in concrete conduits. There are a lot of advantages to running an aqueduct in a closed conduit underground, especially one this long through a desert landscape. There’s far less evaporation and less potential for contamination. It doesn’t divide the landscape at the surface level, so there’s no need for bridges, culverts, and wildlife crossings. Going underground also offers more flexibility when it comes to topography. You don’t have to follow the contours of the surface so carefully because if you come to a hill, you can just dig a little deeper to keep the constant slope. Of course, those benefits come with a cost. An underground conduit is more expensive than a simple channel on the surface, and not all the problems with topography are solved. This is Jawbone Canyon, one of the biggest drops for the first aqueduct. Rather than taking a major detour around it, the aqueduct descends 850 feet (or 250 meters) and then ascends back up. This type of structure is often called an inverted siphon. I’ve done a video on how these work for sewer systems, and I’ve also done a video on flood tunnels that work in a similar way, if you want to learn more after this. Unlike the concrete conduit, which really just acts like an underground canal with a roof, this is one of the places where the water in the aqueduct is pressurized. 850 feet of water column is about 370 psi, 26 bar, or two-and-a-half Megapascals. It’s a lot of pressure. These sections of pipe had to be specially manufactured on the East Coast, where the major steel facilities were, and transported by ship because of their size. They travelled all the way around Cape Horn, since the Panama Canal was still under construction. There are actually quite a few of these siphons crossing canyons in this section of the aqueduct, but Jawbone Canyon is the biggest one. A little further downstream, the LA aqueduct crosses the California Aqueduct, part of the State Water Project. That system has a connection to LA as well, but this branch at the crossing actually heads to Silverwood Lake. However, there is a transfer facility, recently completed, that can pump water out of the California Aqueduct directly into the first LA aqueduct. This creates opportunities for LA to buy water that moves through the state system and offers some flexibility in where that water ends up. There’s also a turn-in that can move water from the LA aqueduct into the California aqueduct for situations where trades make sense. The second LA aqueduct passes underneath the state canal here. And this is a good example of the differences between the first project (built in the 1910s) and the second one, built in the 1960s. Over that time, the price of labor went up a lot more than the price of materials. Where the first one carefully followed the existing topography with bends and turns to minimize the need for expensive pressurized pipe, the second one could take a more direct path, reducing labor in return for the more specialized conduit materials. After wandering more than a hundred miles (or 160 kilometers) apart, the two Los Angeles Aqueducts come back together at Fairmont Reservoir, in the northern foothills of the Sierra Pelona Mountains. This is the last major topographic barrier on the way to Los Angeles. There was no way to go up and over without pumps, so instead they went straight through. The largest project was the Elizabeth tunnel. Here, the two aqueducts come together again into a single watercourse. About 5 miles or 8 kilometers of excavation through everything from hard rock to loose, wet ground became one of the most difficult parts of the entire project. The tunnel required continuous temporary supports along most of its length, followed by a permanent concrete lining. It was a monumental effort for its time and essential not only to cross the range. The Elizabeth Tunnel also delivers that water under pressure to the San Francisquito Power Plant Number 1. This is the largest of the eight hydroelectric plants that run along the aqueduct, capturing some of the energy from the water as it flows downward toward LA. These plants are a major part of how the project paid for itself, and they continue to serve as an important source of electricity in the region today. Continuing downstream, Bouquet Canyon reservoir adds another layer of operational flexibility. It helps regulate flow through the power plants and provides additional storage, a sort of insurance policy since this whole reach depends on a single major tunnel crossing the San Andreas Fault. In case of a major earthquake, it’d be best if Angelinos could avoid a simultaneous water shortage. The aqueduct splits again just upstream of the San Francisquito Plant Number 2, which was famously destroyed by the St. Francis Dam failure. That reservoir project was designed to supplement the storage capacity along the aqueduct, but the dam failed catastrophically in 1928, just 2 years after it was completed, killing more than 400 people and destroying several parts of the aqueduct as well. The tragedy was one of the worst engineering disasters in American history. It put another stain on the aqueduct project, and it effectively ruined the reputation of William Mulholland, who was largely considered a hero in LA for all his work on the aqueduct and the city’s water system. The dam was never rebuilt, but workers restored the aqueduct to functioning service in only 12 days. At Drinkwater Reservoir, the two aqueducts run roughly parallel through the Santa Clarita area, sometimes aboveground and sometimes below, before finally reaching the terminal structures that carry water into LA. Usually, the water stays in the conduits, which feed the two hydropower plants at the foot of the mountains. If the plants are out of service or there’s more flow than they can handle, you see excess water thundering through the cascade structures instead. From here, the aqueduct drops out of the mountains and into the north end of the San Fernando Valley, where the water is treated and prepared for distribution. After filtration and disinfection, it’s stored in the Los Angeles Reservoir, the system’s terminal reservoir, so the city can smooth out day-to-day swings in demand even while the aqueduct’s inflow stays relatively steady. For most of Los Angeles' history, that “finished water storage” was out in the open air. But in the 2000s, drinking-water rules pushed utilities to add stronger protection for treated water held in uncovered reservoirs. There’s a good chance you’ve seen their solution on the Veritasium channel or elsewhere: 96 million plastic shade balls that act like a floating cover, blocking sunlight to prevent water-chemistry problems and helping keep wildlife out. They’re the final protection for this water that traveled so long to reach the city. While the LA Reservoir is, in a sense, the end of the journey for this water, the original diversion way back at Owen’s River isn’t even technically the start anymore! In 1940, LA extended the aqueduct system upstream northward by connecting the Mono basin and funneling its water through tunnels to the Owens River basin. Like Owens Lake downstream, Mono Lake began drying out as well. And also like Owens Lake, lawsuits, court orders, and environmental regulations have tempered the value of this water source, forcing LA to significantly reduce diversions and implement costly restoration projects. That’s kind of the story of the LA aqueduct in a nutshell. The project seemed obvious from an engineering perspective. There was lots of snowmelt in the mountains; the city had the technical prowess, the funding, the elevation, and the political power to reach out and take it. The result was one of the most impressive works of infrastructure of the early 20th century. And continued efforts to expand and improve the system have made it even more efficient, flexible, and valuable to the many millions of people who live in one of the most populous cities in America, delivering not only water but also hundreds of megawatts of hydropower. But it many ways, it was not only unscrupulous, but also short-sighted. Residents of the Owens Valley watched ranchland and farmland dry up as the water that had shaped their home was rerouted south. Native communities saw their homeland transformed with access to gathering areas disrupted, places made unrecognizable, and cultural ties strained by changes they didn’t choose. Wind picked up alkaline dust from dried lakebeds. Habitats were disrupted, and the birds that depended on these waters and wetlands lost part of what made this migration corridor work. It’s easy to see why the aqueduct remains controversial, and why what we sometimes dismiss as “red tape” around major infrastructure is often completely justified due diligence. As engineers, and really, as humans, we have to try and account for costs that don’t show up on a balance sheet, but can come back later as decades of lawsuits, mitigation, and restoration. And even the aqueduct’s original thesis (that there’s reliable snowmelt up there, and a growing city down here) is starting to falter. In recent decades, the mountains have delivered less predictable runoff: more swings, more years when the timing is wrong, and more uncertainty about what “normal” even means anymore. California’s climate has always moved in long cycles, but the margin for error is thinner now, and no one can say with much confidence when or if the moisture the state depends on will return to its old pattern. The hopeful part is that this is exactly where engineering makes a difference: at the messy intersection of geology, climate, culture, politics, and human need. The Los Angeles Aqueduct is a case study in what we can build when we’re ambitious, but also what happens when we treat a landscape like a machine with only one output. The next era of water engineers can learn a lot from it. I mentioned the California Aqueduct as another of the large systems that brings water to LA, but there’s actually a third long-distance aqueduct that delivers water to Angelinos, this one coming all the way from Lake Havasu on the Colorado River. Like the Owens River project, that one came with its own set of challenges, controversy, and impressive feats of engineering. My friend Sam from the Wendover Productions channel talks about that, plus all the interplay between politics, growth, drought, and engineering in his incredible documentary, The Colorado Problem. It’s basically an hour-and-a-half Wendover Productions video with excellent graphics and interviews. And if you want to watch it, it’s only available on Nebula. You probably know about Nebula now, even if you’re not subscribed. It’s a streaming service built by and for independent creators. No studio executives deciding what gets the green light, and no advertisements either. It’s just independent creators making stuff they're excited about with as few barriers and distractions as possible between you and us. My videos go live on Nebula before they come out here, and my Practical Construction series was specifically produced for Nebula viewers who want to see deeper dives into specific topics. I know there are a lot of streaming platforms out there right now, and no one wants another monthly cost to keep track of, but I also know that if you’re watching a show like this to end, Nebula’s library of videos is right up your alley. 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(zipper whirring) - How does a zipper actually work? Like try to push down on a zipper from above and it probably won't budge, but if you just use the pull tab, suddenly it's buttery smooth. So how does it do this? We've made more zippers than there are stars in the Milky Way. You probably used one 10 times today without even noticing, except the only time you do is when one breaks. I'll show you what to do when this happens, but what is actually going on inside this thing? I mean, obviously the teeth come together inside the slider, but it turns out there is a surprising amount of engineering to this thing. All of this is too small to see on a real zipper, which is why we made this one. This is a video about the surprising genius of zippers. What is that? - This is a device that basically started it all. The idea was just to take a bunch of hooks and eyes and try to put them together in some fashion to make them quote, "automatic", unquote. - [Gregor] The hooks seemed very sharp, like I don't think I'd want this on my fly. - No, no. Oh, definitely not there. (Robert laughing) - By the 1800s, clothes were typically fastened using laces, buttons, brooches, and hooks and eyes. These got the job done, but they all shared the same flaw. If you had a series of these fasteners on a piece of clothing, well, you have to close them one by one. Most people were satisfied with the state of affairs, but one man, American engineer, Whitcomb Judson, thought the world deserved something better. - The idea, primarily, it appears to be that he would put them in shoes and people who had to lace up would be able to do it in one quick motion. So that was the device that he had in mind, and it didn't work. - Judson was a pretty bad inventor. Most of his patents had never gotten much traction, but he was a great salesman. In 1893 at the Chicago World's Fair, he presented this fastening device as the next big thing, claiming that in no time at all this would replace buttons and laces, and not just on shoes, but on all sorts of garments. A few wealthy investors actually believed it. So with their backing, the Universal Fastener Company was born. A decade later, the company managed to carve out a small niche, primarily selling its fasteners for women's skirts. "A pull and its done!" said their ads, but that was a lie. Judson's fastener design was a mess. It jammed constantly. And because it was delicate and made from rust prone steel, it actually had to be removed from the garment before you could wash it. So literally unsewn from your skirt. Moreover, if a single hook and eye were out of place, the whole fastener became unstable, so simply bend over and the whole thing could pop open. Naturally, the Universal Fastener Company had very few repeat customers, and they fell into debt. But in 1906, a new engineer joined the team, 25-year-old Gideon Sundback, who had just moved to the US from Sweden. Why does someone like Gideon Sundback with a good degree in electrical engineering decide to join this failing company? - Well, it's a great story. - [Gregor] See, one of the managers at the company had- - An absolutely drop-dead gorgeous daughter. (Gregor chuckles) And that daughter came into the eye of Gideon Sundback, and he was completely smitten. So he ends up working for the fastener manufacturer so that he can cozy up to that daughter and they marry. - [Gregor] For the next few years, Sundback made minor improvements to Judson's hook and eye design, but none were ever enough to make the product truly functional. Then soon after giving birth to a daughter, his wife Elvira, fell ill and died. - [Robert] And Sundback was absolutely devastated. So the romantic tale is that he threw himself into his work at that point out of fighting the grief from the loss of his wife. - This dark period in his life led to a major breakthrough. Sundback realized that this (paper crinkling) was never gonna work. So after years of tinkering, he submitted a patent of his own. This is a patent from 1914, but if you take a look, it is nearly identical to a zipper from today. Sundback's modern zipper starts with two rows of teeth, and the teeth are shaped so that they're wider at the end than the opening on the other side. So if you try to push them together, it's pretty hard. Now, this is especially true on a real size zipper where it's practically impossible. But if I add this slider to the bottom and try pulling on the pull tab here, suddenly it's effortless. So how does it do it? Well, I can remove the cover from the slider to reveal that it's just a Y-shaped cavity. That's it. See, as you zip up, the Y-shaped cavity tilts the teeth at just the right angle so that the tooth has enough space to slot into its groove without bumping into the tooth above. And as you zip down, this wedge shaped piece separates the teeth, allowing you to unzip. This results in one awkward design quirk. At the top, no zipper is ever fully zipped up because the wedge is always there. It has to remain between the teeth. Now, Sundback's original design was a little different to this big guy. It sported rectangular teeth with a bump on the top called the nib, and an equivalently shaped indent on the bottom called the scoop. That way, when the teeth would align, each nib would fit neatly into its neighbor's scoop, forming a strong connection. But there was a problem, even though Sundback had a new design and a patent, manufacturing a zipper like this in the 1910s was very impractical. Each of the tiny teeth needed to be precisely shaped for the fastener to work, but at the time, there were simply no tools around that could do this reliably. - So he had to come up with some extraordinarily clever machineries that allowed them to automate the production of the zipper from the very beginning. - Sundback's machine worked like this. It took Y-shaped wire made from a nickel alloy as an input. First, it sliced pieces off the wire to serve as individual teeth, and then it stamped the scoop and nib into each tooth. Finally, the machine would clamp the two arms of the Y-shape together onto a piece of fabric called the tape. This tape held all the teeth in place, and it was the part of the zipper that would later get stitched onto clothes and other products. Sundback's machines worked wonders. Even in their earliest forms, they could already make 150 meters of zippers per day, and these zippers were incredibly strong. That's because for a tooth to become unpaired, it needs to get some distance between itself, and its neighbors, enough for the nibs on either side to pop out. But since the machines spaced the teeth so precisely, there was simply no room for that to happen. Now, you might think you could just stretch the zipper vertically to separate the teeth, but the zipper tape itself is made from strong inelastic fabric. So even if the garment itself is stretchy, the teeth are connected to the tape which is designed not to stretch, so they won't come loose. But there is a way for this mechanism to fail. If even a single tooth falls off, well, then its neighbors have enough space to come loose, and then their neighbors come loose, and this causes a cascading effect, and the whole zipper pops open. This isn't something you had to worry about with buttons, which can only fail one at a time. But even with this flaw, Sundback's employers thought this patent was a gold mine. So the Universal Fastener Company decided to launch the product under their new name, 'The Hookless Hooker'. They abandoned that name pretty quickly and decided to call it 'The Hookless Fastener' instead. This new fastener was a successful product, but not a mainstream one. Its first applications were pretty niche. You'd find it on money belts, essentially the fanny packs of the 1910s, as well as tobacco pouches and rubber boots. Now, those rubber boots were particularly important. They were manufactured by the B.F. Goodrich Company. - When they got a hold of the device, they were convinced that, yes, this will give us a leg up on our competitor. We will introduce this automatic fastener, but we need a name for it. - [Gregor] Then the company's president had an idea. - Well, you know, it worked really well. They're pretty nice. You can just sort of... It just goes sort of zip when you're closing it and when you're opening it. So, B.F. Goodrich came out in the early 1920s with their zipper boots. - The boots were such a hit that the name zipper transcended the shoe and became the name for the fastener itself. Soon, consumers wanted the zipper on everything. By the 1930s, the Universal Fastener Company became very, very successful. They got a new name, too: Talon, since their fasteners had a secure grip, it was kind of like the talons of an eagle. Talon's new zippers were way sturdier than Judson's hook and eye design, because their parts were way simpler, and they were also made out of rust resistant nickel alloy instead of steel, which meant you could leave them on in the wash. By the way, if you are putting something with a zipper inside the washing machine, you should always zip it up. That will prevent the zipper from snagging on your other clothes, and it'll also protect the zipper itself. Now, even though zippers rapidly became popular, there was pushback among the older and more conservative consumers, especially about putting them on the fly. And urban legends began to spread. - One of the most famous ones is the myth of the fellow who has come to his fiance's parents for dinner, he's seated down at the table, he looked down and realized, "Oh my God, I haven't zipped up my fly," so he zips it up. But then when he gets up a few minutes later to leave the table, he has caught the tablecloth in the fly of his trousers, and so ends up sending the entire table tumbling after him as he gets up and leaves. - I'm still a bit confused by... zippers are more expensive, more temperamental than buttons and laces, and anyways, they become huge regardless of that. - Well, now you see the heart of the mystery. The novelty of a zipper itself was something that took hold of people. People wanted to be modern, and it came to be closely identified with being modern. I found it very interesting that looking at World War II, the zipper manufacturer in Germany was one of the protected industries, despite the fact that it used fairly precious metal, metals that were very important for munition and the like. But the zippers were protected because they were closely identified with modern prosperity, with the idea that if we can have zippers then everything must be okay. - But a more obvious reason for the zipper's popularity is that it's just so easy to use. How much quicker is a zipper than a series of buttons? I have a jacket that has both, so let's time it. Three, two, one. There we go. I mean, it's a pretty great example of how a more automatic approach can save you a bunch of time, which is what today's sponsor Hostinger is all about. Say you have a bunch of projects spread out over Slack, and email, and WhatsApp. If you're trying to keep track of everything manually, it's a pretty slow process, and it's super easy to miss something. But with Hostinger and their virtual private server or VPS, you can quickly zip up your entire project and bring everything together in one place. In just a single click, a Hostinger VPS lets you deploy OpenClaw, which is a multi-channel personal assistant. And OpenClaw's declutter feature will go through all of your inboxes and pluck out all the most important messages and zip them together into one central hub. It's also a great research tool, so if there's a topic you're interested in, you can use OpenClaw to scrape the web for relevant sources, and also keep up to date on posts everywhere from Reddit to Nature. And because it runs 24/7, you can wake up each morning to a fresh batch of articles delivered straight to your inbox. So if you're interested in running OpenClaw, head to hostinger.com/veritasiumopenclaw, or click on the link in the description, and make sure you use code Veritasium to get an exclusive discount on all yearly VPS plans. So I wanna thank Hostinger for sponsoring this part of the video. And now back to zippers. Fast forward to today, Sundback's design is still the one we most associate with zippers. Besides the classic metal variant, which was sturdy and reliable, zippers also started being produced from plastic, which was cheaper and more flexible. But you'd probably be surprised to know that these two zipper types aren't the most common zipper in the world. In fact, the world's most popular zipper doesn't have teeth at all, and it's this thing. (dramatic music) Okay, at first it just looks like other zippers, but if I pull out the threads, you can see that everything here is just a single weird piece of plastic. Imagine you have a coil of plastic that you somewhat flatten. You can mold the plastic such that one side of every loop bulges out more on the top and bottom. If you do this a second time with a second piece of plastic, you'll notice that you now have ridges that fit perfectly together, much like zipper teeth. Stitch these two coils onto fabric, then add a slider, and bam, you have a functional zipper. This is known as a coil zipper. It showed up around the 1940s as a cheap alternative to the original design, and now you can find it everywhere, especially on things like suitcases and backpacks where the zipper needs flexibility to maneuver around corners. Coil zippers also have another benefit. Since all of their teeth are one interconnected piece of plastic, there's no way for a single tooth to fall off, so that itself cannot cause that cascading failure. But there was still a problem zippers had to solve, and that was that they were kind of too good, especially zippers that have been used a lot and are kind of worn down in the slider, they can... just unzip on their own. To prevent that, Gideon Sundback himself actually designed a locking mechanism like a break. Under the piece that connects the pull tab to the slider, there is a small metal pin. When the pull tab is in its typical resting position, one end of the pin sticks through a hole in the bottom face of the slider, lodging itself between the zipper's teeth or coils. That way the slider is stuck in place. But when the pull tab is pulled forward, this releases the pin, allowing the slider to move. Now you can see that there's like this little tiny gap through which you can see light, and that's because the zipper stop is now engaged. But if I grab the pull tab and start pulling, you can see that because of the way that it's shaped, it's actually gonna end up pushing that part up, even though I'm pulling to the side, and that's gonna disengage. You can try to pull apart the fly on your pants, but unless you actually grab the pull tab and pull it down, it is not gonna open. These locking mechanisms aren't on every zipper, but they're more common than you might think. I counted up 65 zippers in this room in total, 33 of those 65 had stopping mechanisms, which is over 50%. Which is also something I never noticed on a zipper. But as I was hunting for zippers in my room. I noticed something else, on pull tab after pull tab there's no mention of Talon, but I kept finding the same three letters instead: YKK, YKK, YKK. YKK. Even on clothes and objects from completely different brands. If you look at your zipper now, you'll probably see the same thing. So at first, I thought this might refer to a particular style of zipper or something, but then I Googled it, and it turns out that YKK is a company, the biggest zipper company in the world. If Talon has the original patent rights and they own the original zipper, how don't I have a single Talon zipper in my room, and how did YKK end up dominating the zipper world? Well, Talon pretty much ruled the zipper market until the 1930s, but in 1934, Sundback's original patent expired, so the playing field was wide open to competitors. That same year, Japanese businessman, Tadao Yoshida, founded a new fastener company, the Yoshida Manufacturing Corporation, or YKK. It began as a single workshop in Tokyo where each zipper was made by hand. Then in 1945, that workshop was completely destroyed by allied bombs, but Yoshida was undaunted. He rebuilt the plant, and after the war, he started buying zipper making machines from the US. - They improved the machine, particularly they improved the speed. - [Gregor] They then also decided to switch to manufacturing everything in-house from the zippers themselves to the machines, to even the boxes that the zippers were shipped in. - And YKK emphasizes quality above everything else. So they make a real point of saying that if you have a YKK zipper, you can depend on it utterly. And that turned out to be an enormously successful sale tactic. - [Gregor] Around 1980, YKK surpassed Talon as the world's biggest zipper maker, and by the early 2000s, Talon's US market share had fallen to a mere 7%, while YKK's surged to around 45%. - YKK surpassed the 10 billion annual zipper unit sales last year. - I mean, that's a very impressive number. Like 10 billion is crazy. - It's equivalent to more than 3 million kilometers in length. It could be like around like 80 trips around the world. - [Gregor] And not all of these are regular everyday zippers either. - So this is an airtight, watertight zipper, and this relies on rigid metal to metal sealing, where nickel teeth are forced tightly together against a rubber tape providing an extreme pressure resistance. - I mean, that looks like a mean zipper. What's an extreme use case for a zipper like this? - Deep sea diving, submarine escape suits. - Submarine escape suits sound really cool. In case of an emergency evacuation of a submarine, you need a suit that can balloon up with air to counteract the pressure of the deep ocean, and that can provide buoyancy helping you shoot up to the surface. But you also need to be able to put it on super quickly. And the best option seems to be this suit with a giant watertight and airtight zipper on the front. Airtight zippers like these even made it onto spacesuits. And that's the zipper. It's this surprisingly genius invention that no one really asked for. I hate when this happens. - I think a zipper slider may get stuck if fabric becomes caught in the chain. So if dirt or debris enters in the zipper, the best fix is to carefully remove any trapped fabric or debris, or move the slider gently. - Okay, so carefully removing stuff from the zipper. - Yeah, yeah. - Because my first reaction is just like, try and jam over the slider. So you're saying I shouldn't do that? - Yeah, no. Move it carefully. - Carefully! And if there isn't any visible debris causing the zipper to get stuck, you can try lubricating the area with graphite from a pencil in order to get the slider moving again, because it's a great dry lubricant. But probably the most annoying zipper problem is when a zipper unzips on both sides of the slider. - This usually happens when the slider becomes worn or bent, and can no longer apply enough pressure to properly interlock the zipper elements as a result. So the zipper chain separates behind the slider. - A worn down slider is something you might be able to fix at home. Just take some pliers and crimp the slider together from the sides. That will make the inner cavity more narrow, just like when it was new, which should make it bring the teeth together again. Just don't crimp it too tightly. I just can't get over the fact that the first patent Gideon Sundback submitted was around 1914, and in those 112 years so many other devices that we've invented have been completely transformed, got better, got faster, cheaper, but it seems like the zipper is mostly just the same. So it's just that Sundback's design was that good? - It's that good. (Robert laughing) I don't have any better explanation. It really is. It's that good.
This is the Puente Hills Landfill outside of Los Angeles, California. The first truckload of trash was dumped here in 1957, and the trucks just kept coming. For more than five decades, if you threw something away in LA County, there’s a good chance it’s buried somewhere inside this mountain of waste. At its peak, Puente Hills was accepting around four million tons of trash every year, making it one of the largest landfills in the country. It closed in 2013, creating a time capsule of everyday life and consumption patterns over a span of 56 years. But Puente Hills is also a time capsule of landfill engineering itself. In 1976, right in the middle of its lifespan, sweeping federal regulations changed how we deal with solid waste forever. You probably don’t think too much about where your trash goes, and that’s kind of the whole point of the solid waste industry: to make sure you have the ability to throw something away without it having a serious negative consequence on the environment or public health. There’s a larger conversation to be had about the amount of waste we generate and how much of it can be recycled or reused, but there is always going to be stuff that just doesn’t hold enough value to be kept. Trash is an inescapable element of the human condition. And, I think you’re going to be surprised how complicated that really is. When Puente Hills opened in the 50s, a landfill was pretty much just a hole in the ground where trash was dumped. By the time it closed, landfills were highly engineered holes where trash gets dumped. And I have a scale model of a landfill in the garage to show you how it all works. I’m Grady, and this is Practical Engineering. There are lots of kinds of waste in this crazy world, but one of the biggest sources is just you and me throwing stuff in the trash. The technical term is municipal solid waste, since its collection is usually coordinated at the city level. There are a lot of ways to manage it once collected, but the most common by far is disposal in a landfill. And, one of the biggest parts of landfill engineering is just deciding where to put one in the first place. The main goal of a landfill is to maximize the volume of waste that can be stored there while minimizing the cost and the environmental impacts too, which turns choosing a suitable site into a giant geometry problem. Digging a hole sounds like an obvious choice, but consider this: digging a hole is expensive, and not digging a hole is free. There are costs of excavating tons and tons of soil just to get it out of the way so it can be replaced with trash and costs of hauling away all that soil (since your goal is to maximize the volume on the site). Plus, you have to avoid the water table, any unsuitable geology, and the challenges of building and working deep below the surface of the earth. That’s why most landfills mostly build up into what sanitation professionals call the “air space.” Looking upward, it may seem like the sky is the limit, but anyone who’s built a tower of anything, let alone trash, knows better. The waste pile gets less stable as its height increases, requiring shallower slopes. And the pressures at the bottom go up too, which can lead to settlement and damage of facilities. Plus, there are visual impacts. The bigger the garbage heap, the bigger the eyesore, and people are only willing to look at a landfill so tall. They can’t be too close to airports, because they attract birds that can interfere with planes. And they can’t be too close to homes, parks, playgrounds, and other places people congregate for obvious reasons. Of course, there’s floodplains and wildlife habitat to avoid as well. And you don’t just need a place to put the trash. You also need a scale house to weigh the trucks coming in and out, a shop and storage for the equipment, and sometimes a place for ordinary citizens to drop stuff off. Finally, you need a spot that can handle the huge increase in truck traffic coming and going, practically nonstop. Pretty much, if you can get a college degree in it, it’s going to come into play when siting a landfill: geology, geography, politics, archaeology, public relations, biology, every kind of engineering, and lot more. But once you have your landfill, you can’t just start dumping trash. Let me show you why with a demonstration. And I have some help from my shop assistants. I have my hole dug, and we’ll start adding some trash. So far, no major problems. But eventually, it’s going to rain. And you can’t immediately see the issue. Granted, this is more of a flood than a drizzle, but it gets the point across. All that water is going to filter through the garbage to the bottom of the hole, and, eventually, into the underlying soil. It might go without saying, but I’m going to say it anyway: We really don’t want garbage juice percolating into our soils. Mainly because it can contaminate sources of groundwater, but also because it can migrate well beyond the limits of the landfill, causing all sorts of environmental troubles. So, modern landfills use a bottom liner to keep waste separate from the underlying soils. Often this consists of a thick sheet of plastic, carefully tested and welded together into an impermeable membrane. Even the area between the plastic welds is tested using air pressure to make sure there are no leaks. Another option is thick clay soil compacted to create a watertight layer. In many cases, the two options are combined, so you end up with this intricate structure of different impermeable layers stacked together. Maybe you still see a problem with this solution on its own. Now when it rains, the landfill just fills up with water. This causes issues with stability and settlement. It causes garbage to decompose more quickly, leading to odor and temperature problems. Plus, you just can’t work on top. There’s no way for trucks to unload trash on top of a garbage swamp. So we need a way to get the garbage juice out, without letting it flow into the soil below. By the way, garbage juice isn’t a technical term. It’s actually called leachate, so I’ll use that from here on out. And all modern landfills have sophisticated leachate collection systems to keep the waste as dry as possible and avoid the issues I mentioned. Usually, this consists of a system of perforated pipes covered in a layer of sand, draining to sumps, and eventually leading out of the waste. I built a little leachate collection system in my model landfill using a small tube so you can see this in action. Now my clay bathtub has a drain. When the rain comes, the water that makes its way into the waste is able to flow out of the landfill, keeping it from becoming a swampy mess. This is a little simplified compared to a real landfill. I’ve made a video all about French Drains, which is much closer to what a leachate collection system consists of if you want to learn more after this. Obviously, in my example, the leachate system has to penetrate the bottom liner, which can be a potential source of leaks. So these penetrations are sealed really carefully in the real world, or the collection system just uses pumps and risers that run up the slope of the landfill to the top, so no penetration system is necessary. Of course, now you have a stream of leachate you have to deal with. Actually, leachate management is one of the biggest costs of running a facility like this. Some landfills send it off to a treatment plant that can clean it up. Some have ways to treat it on-site with settling ponds, evaporation, biological treatment, and even plants that can consume and convert landfill leachate into waste that’s easier to dispose of (maybe even back into the landfill itself). Finally, the bottom of our landfill has all the necessary pieces, but the work doesn’t stop there. Remember that volume is everything in a landfill. For as much effort goes into finding a location and building the infrastructure, it’s essential that we get the most trash in here as possible. You probably know this, but municipal garbage just isn’t that dense. Maybe you’ve had to smash a few more bags in the can because you missed the collection one week. If so, you know there’s usually a lot of room for densification. The trucks that collect garbage usually have a way to compact it to make more room in the box before needing to be emptied. But once the trash is at the landfill, there’s still an opportunity for compaction. Landfills often use massive roller compactors with enormous teeth and giant blades to grade out and compress waste and get as much as possible into the site. It saves money, and it’s good stewardship of the space. But density isn’t the only challenge with day-to-day operations. Despite what you’ve heard, landfills are kind of gross. I mean, that’s their whole point is to accept the stuff we don’t want to put anywhere else. But putting it all in one place creates a lot of problems: pests, odors, windblown waste, fires, birds, and more. So to mitigate some of that, most places require that the garbage be covered up at the end of every day. This “daily cover” can take a lot of forms. The basic approach is just to put a layer of soil over the top of the working face at the end of the day. When I do this in my model, you get a sense of the problem. All that clean daily cover is taking up precious space in the landfill. One option is to trim it back off each morning before trucks start arriving, but that’s a sisyphean task of just moving tons and tons of soil around each day. Other alternatives for daily cover are tarps, or just holding back certain types of waste that are more inert like foundry sand, foam, paper, and shredded tires. They’re going in anyway, so you might as well use them on top to cover the more disagreeable stuff overnight. Those alternatives can also help avoid leachate getting perched within the waste, encouraging it to continue downward to the collection system. Ideally, a landfill will last for decades, slowly filling up by packing as much waste as possible. Throughout the course of operating a landfill, there’s constant testing of groundwater, surface water, leachate, air quality and more to make sure they’re not exceeding limits. Landfills are usually built in smaller cells so you don’t have to manage this huge area of waste all at once. A cell fills up, you put soil over the top (called interim cover), and start a new one within the landfill. But eventually, you reach the top of the airspace, and the landfill reaches the end of its useful life. And closing a landfill is not an easy job. Of course, you have to cover all that waste up, creating a mountainous sealed tomb of garbage. That final cover has to keep water out, to reduce the volume of leachate you’re having to collect and treat over time. But it also has to keep the garbage in, and not just the garbage itself, but anything else that comes with it like smells and leachate and pests. And it has to do it basically forever. So, just like the bottom liner, the final cover over a landfill is usually a system of multiple layers, including compacted soil, membranes, and fabrics. And then you have to get the grass to grow, to protect the soil from erosion and damage over time. I don’t have time to wait for grass to grow in my demo, so I’m cheating a little bit. But the fun isn’t quite over yet. The waste may be sealed up, but that doesn’t mean it’s inert. In fact, there’s a lot of chemistry and biology happening inside a landfill, and a lot of those reactions generate gases like methane and hydrogen sulfide that can create pressure, heat, smells, greenhouse effects in the atmosphere, and the potential for explosions. So, one of the steps in landfill closure is to install wells that can collect the gases from the waste. Usually, these consist of vertical pipes connected to a blower that constantly draws air to a collection point. There’s a lot that goes into these systems too. You can’t pull too hard, or you might draw oxygen into the landfill, changing the reactions and microbiological processes, and creating a potential for a fire within the waste. Plus the gas includes a lot of humidity, so managing condensation creates another liquid stream that has to be collected and treated. Once it’s collected, the landfill gas can be flared, combusting it into less environmentally harmful constituents. Another option is to put it to beneficial use to create heat or even electricity. The Puente Hills landfill I showed earlier has a gas-to-energy facility that’s been running since 1987, and even though the landfill is now closed, it currently provides enough electricity to power around 70,000 homes. Once a landfill is closed, there’s not a lot you can do with it after that. It’s a big, sealed up, mountain of trash, after all. Owners are generally required to look after a closed landfill for at least 30 years afterwards, inspecting for leaks, monitoring the air and water, and repairing any damage. Those costs have to be built into the rates they charge, since there’s not a lot of benefit (or revenue) after closure. But, with all that open space and carefully-maintained landscaping, one option that many landfill operators are trying out is parks. And I love this idea. They say, “We’re willing to put our money where our mouth is and invite the public to spend time here, to enjoy this place that used to be, you know, one of the least enjoyable places you can imagine.” Puente Hills in California has big plans, including trails on the slopes, biking, slides, gardens and more. It looks like it will be a really nice place to visit when it’s done. And it also puts the whole concept of landfills in perspective. Of course, we have a lot of room for improvement in how we think about and manage solid waste in this world. Landfills seem like an environmental blight, but really, properly designed ones play a huge role in making sure waste products don’t end up in our soil or air or water. It’s not possible to landfill waste everywhere. Many places are too densely populated or just don’t have enough space. But where they are, the environmental impacts are relatively small. Just consider the resources that go into them. I pay about 20 dollars a month, probably a little on the low end of the national average, and that buys me 64 gallons (about a quarter of a cubic meter) of space in a municipal landfill per week. Of course, I don’t fill the can every week, and that trash gets compacted. But still, do that for a decade, and your 20 bucks a month has paid for the volume of a modest apartment. It’s covered the cost of building the lining and collection systems, the environmental monitoring, the daily operations, the closure, the gas collection, and the maintenance for at least three decades afterwards and for your trash to stay there effectively forever. It’s (almost) free real estate, not that you’d want to live there. But my point is: landfills are a surprisingly low-impact way to manage solid waste in a lot of cases. I hope the future is a utopia where all the stuff we make maintains its beneficial value forever, but for now, I am thankful for the sanitary engineers and the other professions involved in safely and economically dealing with our trash so we don’t have to. I could spend hours talking about the engineering that goes into landfills. There are so many practical challenges that you just really don’t face anywhere else in engineering. And it’s kind of a small club of people who work on them and know a lot about them. I love that kind of stuff, and I have to assume you probably do too, which is why I want to recommend another video series that I think you’ll find really interesting: The Logistics of X. This was produced by my friend Sam from the Wendover Productions channel. It’s a series that takes a peek behind the curtain of stuff that we kind of take for granted. This episode on coal mining is so good. It covers all the things I would naturally wonder about: the heavy equipment, surface and underground methods, processing, transportation, and the major shifts happening in the industry. The graphics they use remind me of the old History and Discovery Network shows I used to love. And if you want to check it out, it’s available to watch on Nebula. You’ve probably heard of Nebula. It’s a streaming service built by and for independent creators. I don’t know about you, but that’s most of what I watch these days. I just like the authenticity and thoughtfulness of videos that haven’t been through a writers room and ten levels of studio executives. Someone said Nebula’s like Netflix for people who love trains. And I like that comparison, not just because I also love trains. Nebula’s totally ad-free, with tons of excellent channels and lots of original series and specials like the Logistics of X. It’s also a great gift, especially because a yearly membership is 40% of the link in the description. My videos go live on Nebula before they come out on YouTube. If you’re with me that independent creators are the future of great video, I hope you’ll consider subscribing. That’s go.nebula.tv/Practical-Engineering. Thank you for watching, and let me know what you think!
We talk about a lot of big structures on this channel. But, it takes a lot of big tools to build the roads, dams, sewage lift stations, and every other part of the constructed environment. To me, there’s almost nothing more fun than watching something get built, and that’s made all the better when you know what all those machines do. So, in this episode, we’re going to try something a little bit different. I’m Grady, and this is Practical Engineering. Let’s get started! [musical transition] A big part of construction is just shifting around soil and rock. If you’ve ever had to dig a hole, you know how limited human effort is in moving earth. Almost no major job site is complete without at least one excavator because they’re just so versatile. Depending on size, the heavy steel bucket of an excavator can match an entire day’s digging of one guy or girl with a single scoop. But excavators get used for more than just digging. They are a lifter, pusher, crane, and hammer all in one. A skid steer is second only to an excavator when it comes to versatility. These little machines are often equipped with a bucket, but you can attach almost any type of tool as well. While there are often purpose built machines that can do the same job, none of them can convert from loader to mower to forklift to drill rig quite so quickly, and in tight confined spaces, a skid steer is the perfect tool. A loader is one in many machines meant to carry soil and rock across a distance. They’re often articulated in the center for tighter turns and use a large bucket on the front for lifting and dumping. They’re meant to carry materials over short distances, like the length of a construction site. Longer hauls use a dump truck. These trucks feature a large open-topped tub meant to withstand repeated loading with various heavy materials. A typical dump truck features a hydraulic cylinder that can lift the bed, tilting it at a steep angle and allowing material to dump out of the back.. Since dump trucks carry heavy loads, lots of them have auxiliary axles that can be lowered to distribute the weight over more tires and keep the truck in compliance with roadway and bridge weight limits. Articulated haulers are dump trucks used in off-road and difficult terrain. If you want to move a lot of soil around a large construction site, another option is a scraper. Rather than loading from the ground into a dump truck, these machines do it all in one. A huge blade scrapes directly from the ground into a hopper. It’s carried directly to where it’s needed and unloaded with a hydraulic ejector, and these are often used on large embankments like for highways and dams. Another Swiss army knife of the construction yard is the backhoe that is kind of a combination excavator and loader. Great for small sites where it doesn’t make sense to have two pieces of equipment. And don’t forget the bulldozer that specializes in moving material at ground level. They can’t move material over large distances, but they can spread out literal tons with their tank-like tracks. The last stop on the digging train is the trencher. There are a huge variety of styles and sizes, but ultimately they all specialize in digging long holes for pipes and utilities. Many use a tooth chain like a giant chainsaw for the Earth! By the way, there are about a hundred different colloquial names for almost every piece of large equipment. Different sites, suppliers, regions, and countries use different words for the same machine; it’s part of the fun. One easy tip to sound like a pro is just to add the drive style to the front of the name. It’s not a loader, it’s a wheel loader, or a tracked excavator and so on. Now let’s hit the road. Roadwork is something we’ve all seen, and while it can be a bit frustrating if you’re stuck in a traffic jam from it, roads might be the largest engineered structures on earth. Our modern lives depend on them, and it takes some pretty cool tools to get them built. A grader is technically an earthwork tool, but it’s used mostly on roadways. The extra long wheelbase makes it well suited for precisely leveling surfaces and evening out bumps, leaving a nice even grade. Once all that soil is in the right place, it needs to be solidified so it doesn’t settle over time. A roller compactor is the main tool for this job. There are a few varieties of these depending on the material being compacted. Smooth drums are used for most soils and asphalt. Sheep’s foot and padded drums have protrusions that work best on clay and silt. Pneumatic tire rollers are best to knead and seal the surface. And a lot of roller compactors have a vibration feature to shake the soil into place. An asphalt paver is the machine where the road meets the road. Hot asphalt is loaded into the machine, which spreads it into an even layer onto the subgrade using a screed. Many paving machines have a wand that follows a stringline as a reference to the exact elevation required for the roadway. If we’re talking about making a road out of concrete, then the tool for the job is a slip former. It’s usually more efficient and produces better quality of work when paving, curbs, and highway barriers are installed continuously rather than building forms and casting them in batches. Careful control of the mix makes it possible for a slip form machine to create long concrete structures without any formwork at all. If we just added another layer of pavement to the road every time it started to wear out, pretty soon, we’d have walls! Roads are designed to be extraordinarily tough, so removing the top layer isn’t easy. That’s a job for an asphalt mill or planer. These specialized tools grind and remove the surface with a large rotating drum. The material is routed up a conveyor system and can be loaded into a following dump truck. It’s actually fairly common to see multiple vehicles following one another in roadwork like this. An interesting example is the so-called paving train. On one end, we have a dump truck full of asphalt fresh from the plant. This is loaded into the asphalt paver, which continuously lays a layer of asphalt that is then compacted by one or more rollers. Workers on the ground also continuously monitor the process to ensure a nice even road surface. Not everything at a construction site is a machine with wheels or tracks. A lot of equipment gets hauled in on a trailer, or is a trailer itself. A light tower lets you work outside of daylight hours, illuminating the site so you can work at night or underground. An air compressor enables the use of lots of tools on a job site, like jackhammers, sandblasters, and painting rigs. If you need electric power instead of compressed air, diesel generators offer access to power when grid service isn’t available. So far, the actual material we’ve seen is in bulk like earth or asphalt. Often in construction, the materials we need to lift or move are objects like girders or concrete pipes. For that you need a crane or similar material-handling equipment. This is a pipe layer. The name is a bit confusing since the workers that operate them are also often called pipe layers. And it's no surprise what kind of jobs they do. They specialize in handling large sections of pipe and precisely lowering them and placing them into trenches. A telescopic handler, or a telehandler or teleporter is like an all-terrain forklift. The boom can have attachments like a bucket, pallet forks, or a winch, and it telescopes to make it easy to deliver materials and equipment exactly where you need it. If you happen to be the load that needs elevating, then you’ll need a boom lift or its cousin, the scissor lift. The operator of these controls the platform while standing on it, allowing for very positioning of people that’s much more precise, and usually safer, than a ladder. Another relative of the boom lift is a bucket truck which has a boom lift in the back, used a lot of electric and utility work on poles. Stepping up in size, we have road-rated all-terrain cranes. If you’ve passed a giant crane driving down the highway, it was one of these, since most other types of cranes have to be hauled to a site in pieces and assembled. As the name implies, all-terrain cranes don’t require perfectly level, paved surfaces to get to work. However, if your job site is particularly rough, you need a rough-terrain crane. The giant rubber tires on these mean you’ll need to have them transported, but once rolling, they can go where highway-rated vehicles might struggle. If the crane you’re looking at is mounted on tracks, you’ve got a crawler crane. These heavy-duty cranes, while slower and bulkier than all-terrain cranes and also requiring modular transport to job sites, can carry immense loads and extend to even greater heights than any of the cranes we’ve seen so far. Most crawler cranes can be configured according to the job with different lengths of booms, amounts of counterweight, and extensions called jibs. A particularly fun configuration is for demolition where a crawler crane might be fitted with a wrecking ball. Most can move from place to place, but not all. Tower cranes use large counterbalanced horizontal booms with an integrated operator cab on top of a large, well… tower. Like most of the cranes we’ve seen so far, these come in a wide range of sizes but can be absolutely enormous, almost a construction project themselves requiring other cranes for assembly. One way to build bridges uses a specialized crane called a launching gantry. You may have heard the term gantry before for a bridgelike overhead crane. These are in all kinds of industries. A launching gantry uses the existing structure of the bridge as a base and often lifts whole pre-built sections of the bridge. Turning from the sky and looking underground, let’s talk about a few foundation-specific machines. The biggest and heaviest structures are supported on bedrock or some deeper geological layer. Even if the usable soil is just clay for hundreds of feet, sinking deep subterranean columns or piles below a heavy structure can keep it from settling too much over time. One way to install a pile is to dig a very deep hole, place a reinforcing steel cage in the hole, then fill the whole thing with concrete. This is the exact job that a pile drill rig is designed to do. These large-scale drills are pretty closely related to the machines used for oil and gas exploration. Another way to install piles is to drive them into the earth, the job of a pile driver. Just like the name implies, they repeatedly strike wooden, steel, or concrete piles to sink them to the required depth. Speaking of concrete, there’s a whole subset of construction machines that are specifically designed to handle, transport, and place this important material. You’ve probably seen a mixer truck before, and I’ll forgive you for calling them cement trucks, even though cement is just one of the ingredients of a concrete mix. The truck can be loaded with dry materials and water, and the mixing occurs en route to the job site, since concrete generally has a limited time before it begins to cure. Concrete is often placed directly from the truck using a chute, but that’s not always the easiest way. Concrete pumps are used to pump concrete to job site locations that are hard to access with a truck, often with a huge overhead boom. Since concrete is more than twice as dense as water, these pumps operate at extremely high pressures, sometimes over 100 times atmospheric pressure! Finishing concrete is mostly a hand-tool job, but there are some machines for big jobs, like ride-on trowels, that speed up the job of floating a slab smooth once it has started to set up. Big jobs with lots of concrete might just mix it onsite with a mobile batching plant. This is helpful if you need to produce vast volumes of concrete over a long period in a way that would be too inconvenient or maybe even impossible for mixer trucks to handle. Sometimes concrete needs to be placed on a sloped or vertical surface to stabilize a rock face, shore up a tunnel, or even just install a pool! The catch-all term for the various varieties of sprayed concrete is shotcrete (although some pool installers might disagree). Shotcrete machines use compressed air to apply concrete to all kinds of surfaces in the construction world. When projects require the installation of new or additional utility lines in areas that are already built up, the traditional method of digging trenches isn’t feasible. This kind of job calls for a directional drilling machine. While these are technically boring tools, they are anything but uninteresting. I actually have a dedicated video just to talk about how they work, and specifically how they steer that bit below the ground. Go check that out after this if you want to learn more. Hopefully there have been a few machines in the list so far that are new to you, but if not, I have a few more specialized machines you might be lucky enough to spot on a site: Fans of the channel might recognize a soil nail rig, a specialized machine that drills out more or less horizontal shafts in an earthen slope and then adds soil nails to greatly enhance stability. Jobs that require grout often use mobile batch plants, called grout plants. You can even inject ground into the ground at high pressures using a hydraulic pump to fill voids and stabilize soils. A wick drain machine installs prefabricated vertical drains into the soil at regular intervals to speed up drainage of water in clay soils which helps speed up the inevitable settling of the soil so construction can get started faster. One option for repairing existing pipelines in place without trenching is cured-in-place pipe lining. Inverting a liner impregnated with epoxy-resin into an existing pipeline using air pressure essentially puts a brand new pipe inside an old or damaged line. One of the least boring machines that you’d be really lucky to see above ground is a tunnel boring machine. These behemoths use a complicated face of various cutting tools followed by a material removal and shoring installation apparatus to efficiently bore full scale tunnels! Obviously I can’t be exhaustive here. The construction industry is just full of machines. There is such a variety in the type and scale of projects that manufacturers are always coming up with new and improved equipment that can get a particular job done better. And lots of industries outside of construction use heavy machinery, including mines, oil and gas, and railroads. Let me know what you think I missed or if you want a similar list within a different industry. But I think this is a good starting point for any burgeoning construction spotter, and I hope it’s exhaustive enough that if you see something that didn’t make the list, you can puzzle out its purpose on your own. That part of the satisfaction of construction spotting anyway, so get out there and see what kinds of machines you can find. I am obviously fascinated by the machines that both build and make up our constructed environment, from the oldest to the most modern. I think it’s interesting that a lot of the differences we see in vehicles comes down to how efficient they are at doing a very specific task. For example, my friend Brian from the Real Engineering channel just released a video all about maglev trains, and he explains why there is only one commercial high speed maglev line in the world, even though the technology seems ready to revolutionize train travel. I had no idea how travel time factors into the economics of these projects. Maybe you’ve noticed what I have over the past few years: my old favorite TV networks are just running reality shows, and the best video content that I actually enjoy watching is being made my independent creators. There’s just something different about a small team who is passionate about their topic instead of being told what to do by some studio executive looking at ratings numbers. You can catch the Real Engineering video on maglev trains on YouTube when it comes out eventually, but if you want to watch it right now (with no ads), you’ll have to head over to Nebula. You’ve heard me talk about Nebula before. 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