[Note that this article is a transcript of the video embedded above.] There’s a new trend in high-rise building design. Maybe you’ve seen this in your city. The best lots are all taken, so developers are stretching the limits to make use of space that isn’t always ideal for skyscrapers. They’re not necessarily taller than buildings of the past, but they are a lot more slender. “Pencil tower” is the term generally used to describe buildings that have a slenderness ratio of more than around 10 to 1, height to width. A lot of popular discussion around skyscrapers is about how tall we can build them. Eventually, you can get so tall that there are no materials strong enough to support the weight. But, pencil towers are the perfect case study in why strength isn’t the only design criterion used in structural engineering. Of course, we don’t want our buildings to fall down, but there’s other stuff we don’t want them to do, too, including flex and sway in the wind. In engineering, this concept is called the serviceability limit state, and it’s an entirely separate consideration from strength. Even if moderate loads don’t cause a structure to fail, the movement they cause can lead to windows breaking, tiles cracking, accelerated fatigue of the structure, and, of course, people on the top floors losing their lunch from disorientation and discomfort. So, limiting wind-induced motions is a major part of high-rise design and, in fact, can be such a driving factor in the engineering of the building that strength is a secondary consideration. Making a building stiffer is the obvious solution. But adding stiffness requires larger columns and beams, and those subtract valuable space within the building itself. Another option is to augment a building’s aerodynamic performance, reducing the loads that winds impose. But that too can compromise the expensive floorspace within. So many engineers are relying on another creative way to limit the vibrations of tall buildings. And of course, I built a model in the garage to show you how this works. I’m Grady, and this is Practical Engineering. One of the very first topics I ever covered on this channel was tuned mass dampers. These are mechanisms that use a large, solid mass to counteract motion in all kinds of structures, dissipating the energy through friction or hydraulics, like the shock absorbers in vehicles. Probably the most famous of these is in the Taipei 101 building. At the top of the tower is a massive steel pendulum, and instead of hiding it away in a mechanical floor, they opened it to visitors, even giving the damper its own mascot. But, mass dampers have a major limitation because of those mechanical parts. The complex springs, dampers, and bearings need regular maintenance, and they are custom-built. That gets pretty expensive. So, what if we could simplify the device? This is my garage-built high-rise. It’s not going to hold many conference room meetings, but it does do a good job swaying from side to side, just like an actual skyscraper. And I built a little tank to go on top here. The technical name for this tank is a tuned liquid column damper, and I can show you how it works. Let’s try it with no water first. Using my digitally calibrated finger, I push the tower over by a prescribed distance, and you can see this would not be a very fun ride. There is some natural damping, but the oscillation goes on for quite a while before the motion stops. Now, let’s put some water in the tank. With the power of movie magic, I can put these side by side so you can really get a sense of the difference. By the way, nearly all of the parts for this demonstration were provided by my friends at Send-Cut-Send. I don’t have a milling machine or laser cutter, so this is a really nice option for getting customized parts made from basically any material - aluminum, steel, acrylic - that are ready to assemble. Instead of complex mechanical devices, liquid column dampers dissipate energy through the movement of water. The liquid in the tank is both the mass and the damper. This works like a pendulum where the fluid oscillates between two columns. Normally, there’s an orifice between the two columns that creates the damping through friction loss as water flows from one side to the other. To make this demo a little simpler, I just put lids on the columns with small holes. I actually bought a fancy air valve to make this adjustable, but it didn’t allow quite enough airflow. So instead, I simplified with a piece of tape. Very technical. Energy transferred to the water through the building is dissipated by the friction of the air as it moves in and out of the columns. And you can even hear this as it happens. Any supplemental damping system starts with a design criterion. This varies around the world, but in the US, this is probability-based. We generally require that peak accelerations with a 1-in-10 chance of being exceeded in a given year be limited to 15-18 milli-gs in residential buildings and 20-25 milli-gs in offices. For reference, the lateral acceleration for highway curve design is usually capped at 100 milli-gs, so the design criteria for buildings is between a fourth and a sixth of that. I think that makes intuitive sense. You don’t want to feel like you’re navigating a highway curve while you sit at your desk at work. It’s helpful to think of these systems in a simplified way. This is the most basic representation: a spring, a damper, and mass on a cart. We know the mass of the building. We can estimate its stiffness. And the building itself has some intrinsic damping, but usually not much. If we add the damping system onto the cart, it’s basically just the same thing at a smaller scale, and the design process is really just choosing the mass and damping systems for the remaining pieces of this puzzle to achieve the design goal. The mass of liquid dampers is usually somewhere between half a percent to two percent of the building’s total weight. The damping is related to the water’s ability to dissipate energy. And the spring needs to be tuned to the building. All buildings vibrate at a natural frequency related to their height and stiffness. Think of it like a big tuning fork full of offices or condos. I can estimate my model’s natural frequency by timing the number of oscillations in a given time interval. It’s about 1.3 hertz or cycles per second. In an ideal tuned damper, the oscillation of the damping system matches that of the building. So tuning the frequency of the damper is an important piece of the puzzle. For a tuned liquid column damper, the tuning mostly comes from the length of the liquid flow path. A longer path results in a lower frequency. The compression of the air above the column in my demo affects this too, and some types of dampers actually take advantage of that phenomenon. I got the best tuning when the liquid level was about halfway up the columns. The orifice has less of an effect on frequency and is used mostly to balance the amount of damping versus the volume of liquid that flows through each cycle. In my model, with one of the holes completely closed off, you can see the water doesn’t move, and you get minimal damping. With the tape mostly covering the hole, you get the most frictional loss, but not all the fluid flows from one side to the other each cycle. When I covered about half of one hole, I got the full fluid flow and the best damping performance. The benefit of a tuned column damper is that it doesn’t take up a lot of space. And because the fluid movement is confined, they’re fairly predictable in behavior. So, these are used in quite a few skyscrapers, including the Random House Tower in Manhattan, One Wall Center in Vancouver (which actually has many walls), and Comcast Center in Philadelphia. But, tuned column liquid dampers have a few downsides. One is that they really only work for flexible structures, like my demo. Just like in a pendulum, the longer the flow path in a column damper, the lower the frequency of the oscillation. For stiffer buildings with higher natural frequencies, tuning requires a very short liquid column, which limits the mass and damping capability to a point where you don’t get much benefit. The other thing is that this is still kind of a complex device with intricate shapes and a custom orifice between the two columns. So, we can get even simpler. This is my model tuned sloshing damper, and it’s about as simple as a damper can get. I put a weight inside the empty tank to make a fair comparison, and we can put it side by side with water in the tank to see how it works. As you can see, sloshing dampers dissipate energy by… sloshing. Again, the water is both the mass and the damper. If you tune it just right, the sloshing happens perfectly out of phase of the motion of the building, reducing the magnitude of the movement and acceleration. And you can see why this might be a little cheaper to build - it’s basically just a swimming pool - four concrete walls, a floor, and some water. There’s just not that much to it. But the simplicity of construction hides the complexity of design. Like a column damper, the frequency of a sloshing damper can be tuned, first by the length of the tank. Just like fretting a guitar string further down the neck makes the note lower, a tank works the same way. As the tank gets longer, its sloshing frequency goes down. That makes sense - it takes longer for the wave to get from one side to the other. But you can also adjust the depth. Waves move slower in shallower water and faster in deeper water. Watch what happens when I overfill the tank. The initial wave starts on the left as the building goes right. It reaches the right side just as the building starts moving left. That’s what we want; it’s counteracting the motion. But then it makes it back to the left before the building starts moving right. It’s actually kind of amplifying the motion, like pushing a kid on a swing. Pretty soon after that, the wave and the building start moving in phase, so there’s pretty much no damping at all. Compare it to the more properly tuned example where most of the wave motion is counteracting the building motion as it sways back and forth. You can see in my demo that a lot of the energy dissipation comes from the breaking waves as they crash against the sides of the tank. That is a pretty complicated phenomenon to predict, and it’s highly dependent on how big the waves are. And even with the level pretty well tuned to the frequency of the building, you can see there’s a lot of complexity in the motion with multiple modes of waves, and not all of them acting against the motion of the building. So, instead of relying on breaking waves, most sloshing dampers use flow obstructions like screens, columns, or baffles. I got a few different options cut out of acrylic so we can try this out. These baffles add drag, increasing the energy dissipation with the water, usually without changing the sloshing frequency. Here’s a side-by-side comparison of the performance without a baffle and with one. You can see that the improvement is pretty dramatic. The motion is more controlled and the behavior is more linear, making this much simpler to predict during the design phase. It’s kind of the best of both worlds since you get damping from the sloshing and the drag of the water passing through the screen. Almost all the motion is stopped in this demo after only three oscillations. I was pretty impressed with this. Here’s all three of the baffle runs side by side. Actually, the one with the smallest holes worked the best in my demo, but deciding the configuration of these baffles is a big challenge in the engineering of these systems because you can’t really just test out a bunch of options at full scale. Devices like this are in service in quite a few high-rise buildings, including Princess Tower in Dubai, and the Museum Tower in Dallas. With no moving parts and very little maintenance except occasionally topping it off to keep the water at the correct level, you can see how it would be easy to choose a sloshing damper for a new high-rise project. But there are some disadvantages. One is volumetric efficiency. You can see that not all the water in the tank is mobilized, especially for smaller movements, which means not all the water is contributing to the damping. The other is non-linearity. The amount of damping changes depending on the magnitude of the movement since drag is related to velocity squared. And even the frequency of the damper isn’t constant; it can change with the wave amplitude as well because of the breaking waves. So you might get good performance at the design level, but not so much for slower winds. Dampers aren’t just used in buildings. Bridges also take advantage of these clever devices, especially on the decks of pedestrian bridges and the towers of long-span bridges. This also happens at a grand scale between the Earth and moon. Tidal bulges in the oceans created by the moon’s tug on Earth dissipate energy through friction and turbulence, which is a big part of why our planet’s rotation is slowing over time. Days used to be a lot shorter when the Earth was young, but we have a planet-scale liquid damper constantly dissipating our rotational energy. But whether it’s bridges or buildings, these dampers usually don’t work perfectly right at the start. Vibrations are complicated. They’re very hard to predict, even with modern tools like simulation software and scale physical models. So, all dampers have to go through a commissioning process. Usually this involves installing accelerometers once construction is nearing completion to measure the structure’s actual natural frequency. The tuning of tuned dampers doesn’t just happen during the design phase; you want some adjustability after construction to make sure they match the structure’s natural frequency exactly so you get the most damping possible. For liquid dampers, that means adjusting the levels in the tanks. And in many cases, buildings might use multiple dampers tuned to slightly different frequencies to improve the performance over a range of conditions. Even in these two basic categories, there is a huge amount of variability and a lot of ongoing research to minimize the tradeoffs these systems come with. The truth is that, relatively speaking, there aren’t that many of these systems in use around the world. Each one is highly customized, and even putting them into categories can get a little tricky. There are even actively controlled liquid dampers. My tuning for the column damper works best for a single magnitude of motion, but you can see that once the swaying gets smaller, the damper isn’t doing a lot to curb it. You can imagine if I constantly adjusted the size of the orifice, I could get better performance over a broader range of unwanted motion. You can do this electronically by having sensors feed into a control system that adjusts a valve position in real-time. Active systems and just the flexibility to tune a damper in general also help deal with changes over time. If a building’s use changes, if new skyscrapers nearby change the wind conditions, or if it gets retrofits that change its natural frequency, the damping system can easily accommodate those changes. In the end, a lot of engineering decisions come down to economics. In most cases, damping is less about safety and more about comfort, which is often harder to pin down. Engineers and building owners face a balancing act between the cost of supplemental damping and the value of the space those systems take up. Tuned mass dampers are kind of household names when it comes to damping. A few buildings like Shanghai Center and Taipei 101 have made them famous. They’re usually the most space-efficient (since steel and concrete are more dense than water). But they’re often more costly to install and maintain. Liquid dampers are the unsung heroes. They take up more space, but they’re simple and cost-effective, especially if the fire codes already require you to have a big tank of water at the top of your building anyway. Maybe someday, an architect will build one out of glass or acrylic, add some blue dye and mica powder, and put it on display as a public showcase. Until then, we’ll just have to know it’s there by feel.

[Note that this article is a transcript of the video embedded above.] Wichita Falls, Texas, went through the worst drought in its history in 2011 and 2012. For two years in a row, the area saw its average annual rainfall roughly cut in half, decimating the levels in the three reservoirs used for the city’s water supply. Looking ahead, the city realized that if the hot, dry weather continued, they would be completely out of water by 2015. Three years sounds like a long runway, but when it comes to major public infrastructure projects, it might as well be overnight. Between permitting, funding, design, and construction, three years barely gets you to the starting line. So the city started looking for other options. And they realized there was one source of water nearby that was just being wasted - millions of gallons per day just being flushed down the Wichita River. I’m sure you can guess where I’m going with this. It was the effluent from their sewage treatment plant. The city asked the state regulators if they could try something that had never been done before at such a scale: take the discharge pipe from the wastewater treatment plant and run it directly into the purification plant that produces most of the city’s drinking water. And the state said no. So they did some more research and testing and asked again. By then, the situation had become an emergency. This time, the state said yes. And what happened next would completely change the way cities think about water. I’m Grady and this is Practical Engineering. You know what they say, wastewater happens. It wasn’t that long ago that raw sewage was simply routed into rivers, streams, or the ocean to be carried away. Thankfully, environmental regulations put a stop to that, or at least significantly curbed the amount of wastewater being set loose without treatment. Wastewater plants across the world do a pretty good job of removing pollutants these days. In fact, I have a series of videos that go through some of the major processes if you want to dive deeper after this. In most places, the permits that allow these plants to discharge set strict limits on contaminants like organics, suspended solids, nutrients, and bacteria. And in most cases, they’re individualized. The permit limits are based on where the effluent will go, how that water body is used, and how well it can tolerate added nutrients or pollutants. And here’s where you start to see the issue with reusing that water: “clean enough” is a sliding scale. Depending on how water is going to be used or what or who it’s going to interact with, our standards for cleanliness vary. If you have a dog, you probably know this. They should drink clean water, but a few sips of a mud puddle in a dirty street, and they’re usually just fine. For you, that might be a trip to the hospital. Natural systems can tolerate a pretty wide range of water quality, but when it comes to drinking water for humans, it should be VERY clean. So the easiest way to recycle treated wastewater is to use it in ways that don’t involve people. That idea’s been around for a while. A lot of wastewater treatment plants apply effluent to land as a disposal method, avoiding the need for discharge to a natural water body. Water soaks into the ground, kind of like a giant septic system. But that comes with some challenges. It only works if you’ve got a lot of land with no public access, and a way to keep the spray from drifting into neighboring properties. Easy at a small scale, but for larger plants, it just isn’t practical engineering. Plus, the only benefits a utility gets from the effluent are some groundwater recharge and maybe a few hay harvests per season. So, why not send the effluent to someone else who can actually put it to beneficial use? If only it were that simple. As soon as a utility starts supplying water to someone else, things get complicated because you lose a lot of control over how the effluent is used. Once it's out of your hands, so to speak, it’s a lot harder to make sure it doesn’t end up somewhere it shouldn’t, like someone’s mouth. So, naturally, the permitting requirements become stricter. Treatment processes get more complicated and expensive. You need regular monitoring, sampling, and laboratory testing. In many places in the world, reclaimed water runs in purple pipes so that someone doesn’t inadvertently connect to the lines thinking they’re potable water. In many cases, you need an agreement in place with the end user, making sure they’re putting up signs, fences, and other means of keeping people from drinking the water. And then you need to plan for emergencies - what to do if a pipe breaks, if the effluent quality falls below the standards, or if a cross-connection is made accidentally. It’s a lot of work - time, effort, and cost - to do it safely and follow the rules. And those costs have to be weighed against the savings that reusing water creates. In places that get a lot of rain or snow, it’s usually not worth it. But in many US states, particularly those in the southwest, this is a major strategy to reduce the demand on fresh water supplies. Think about all the things we use water for where its cleanliness isn’t that important. Irrigation is a big one - crops, pastures, parks, highway landscaping, cemeteries - but that’s not all. Power plants use huge amounts of water for cooling. Street sweeping, dust control. In nearly the entire developed world, we use drinking-quality water to flush toilets! You can see where there might be cases where it makes good sense to reclaim wastewater, and despite all the extra challenges, its use is fairly widespread. One of the first plants was built in 1926 at Grand Canyon Village which supplied reclaimed water to a power plant and for use in steam locomotives. Today, these systems can be massive, with miles and miles of purple pipes run entirely separate from the freshwater piping. I’ve talked about this a bit on the channel before. I used to live near a pair of water towers in San Antonio that were at two different heights above ground. That just didn’t make any sense until I realized they weren’t connected; one of them was for the reclaimed water system that didn’t need as much pressure in the lines. Places like Phoenix, Austin, San Antonio, Orange County, Irvine, and Tampa all have major water reclamation programs. And it’s not just a US thing. Abu Dhabi, Beijing, and Tel Aviv all have infrastructure to make beneficial use of treated municipal wastewater, just to name a few. Because of the extra treatment and requirements, many places put reclaimed water in categories based on how it gets used. The higher the risk of human contact, the tighter the pollutant limits get. For example, if a utility is just selling effluent to farmers, ranchers, or for use in construction, exposure to the public is minimal. Disinfecting the effluent with UV or chlorine may be enough to meet requirements. And often that’s something that can be added pretty simply to an existing plant. But many reclaimed water users are things like golf courses, schoolyards, sports fields, and industrial cooling towers, where people are more likely to be exposed. In those cases, you often need a sewage plant specifically designed for the purpose or at least major upgrades to include what the pros call tertiary treatment processes - ways to target pollutants we usually don’t worry about and improve the removal rates of the ones we do. These can include filters to remove suspended solids, chemicals that bind to nutrients, and stronger disinfection to more effectively kill pathogens. This creates a conundrum, though. In many cases, we treat wastewater effluent to higher standards than we normally would in order to reclaim it, but only for nonpotable uses, with strict regulations about human contact. But if it’s not being reclaimed, the quality standards are lower, and we send it downstream. If you know how rivers work, you probably see the inconsistency here. Because in many places, down the river, is the next city with its water purification plant whose intakes, in effect, reclaim that treated sewage from the people upstream. This isn’t theoretical - it’s just the reality of how humans interact with the water cycle. We’ve struggled with the problems it causes for ages. In 1906, Missouri sued Illinois in the Supreme Court when Chicago reversed their river, redirecting its water (and all the city’s sewage) toward the Mississippi River. If you live in Houston, I hate to break it to you, but a big portion of your drinking water comes from the flushes and showers in Dallas. There have been times when wastewater effluent makes up half of the flow in the Trinity River. But the question is: if they can do it, why can’t we? If our wastewater effluent is already being reused by the city downstream to purify into drinking water, why can’t we just keep the effluent for ourselves and do the same thing? And the answer again is complicated. It starts with what’s called an environmental buffer. Natural systems offer time to detect failures, dilute contaminants, and even clean the water a bit—sunlight disinfects, bacteria consume organic matter. That’s the big difference in one city, in effect, reclaiming water from another upstream. There’s nature in between. So a lot of water reclamation systems, called indirect potable reuse, do the same thing: you discharge the effluent into a river, lake, or aquifer, then pull it out again later for purification into drinking water. By then, it’s been diluted and treated somewhat by the natural systems. Direct potable reuse projects skip the buffer and pipe straight from one treatment plant to the next. There’s no margin for error provided by the environmental buffer. So, you have to engineer those same protections into the system: real-time monitoring, alarms, automatic shutdowns, and redundant treatment processes. Then there’s the issue of contaminants of emerging concern: pharmaceuticals, PFAS [P-FAS], personal care products - things that pass through people or households and end up in wastewater in tiny amounts. Individually, they’re in parts per billion or trillion. But when you close the loop and reuse water over and over, those trace compounds can accumulate. Many of these aren’t regulated because they’ve never reached concentrations high enough to cause concern, or there just isn’t enough knowledge about their effects yet. That’s slowly changing, and it presents a big challenge for reuse projects. They can be dealt with at the source by regulating consumer products, encouraging proper disposal of pharmaceuticals (instead of flushing them), and imposing pretreatment requirements for industries. It can also happen at the treatment plant with advanced technologies like reverse osmosis, activated carbon, advanced oxidation, and bio-reactors that break down micro-contaminants. Either way, it adds cost and complexity to a reuse program. But really, the biggest problem with wastewater reuse isn’t technical - it’s psychological. The so-called “yuck factor” is real. People don’t want to drink sewage. Indirect reuse projects have a big benefit here. With some nature in between, it’s not just treated wastewater; it’s a natural source of water with treated wastewater in it. It’s kind of a story we tell ourselves, but we lose the benefit of that with direct reuse: Knowing your water came from a toilet—even if it’s been purified beyond drinking water standards—makes people uneasy. You might not think about it, but turning the tap on, putting that water in a glass, and taking a drink is an enormous act of trust. Most of us don’t understand water treatment and how it happens at a city scale. So that trust that it’s safe to drink largely comes from seeing other people do it and past experience of doing it over and over and not getting sick. The issue is that, when you add one bit of knowledge to that relative void of understanding - this water came directly from sewage - it throws that trust off balance. It forces you not to rely not on past experience but on the people and processes in place, most of which you don’t understand deeply, and generally none of which you can actually see. It’s not as simple as just revulsion. It shakes up your entire belief system. And there’s no engineering fix for that. Especially for direct potable reuse, public trust is critical. So on top of the infrastructure, these programs also involve major public awareness campaigns. Utilities have to put themselves out there, gather feedback, respond to questions, be empathetic to a community’s values, and try to help people understand how we ensure water quality, no matter what the source is. But also, like I said, a lot of that trust comes from past experience. Not everyone can be an environmental engineer or licensed treatment plant operator. And let’s be honest - utilities can’t reach everyone. How many public meetings about water treatment have you ever attended? So, in many places, that trust is just going to have to be built by doing it right, doing it well, and doing it for a long time. But, someone has to be first. In the U.S., at least on the city scale, that drinking water guinea pig was Wichita Falls. They launched a massive outreach campaign, invited experts for tours, and worked to build public support. But at the end of the day, they didn’t really have a choice. The drought really was that severe. They spent nearly four years under intense water restrictions. Usage dropped to a third of normal demand, but it still wasn’t enough. So, in collaboration with state regulators, they designed an emergency direct potable reuse system. They literally helped write the rules as they went, since no one had ever done it before. After two months of testing and verification, they turned on the system in July 2014. It made national headlines. The project ran for exactly one year. Then, in 2015, a massive flood ended the drought and filled the reservoirs in just three weeks. The emergency system was always meant to be temporary. Water essentially went through three treatment plants: the wastewater plant, a reverse osmosis plant, and then the regular water purification plant. That’s a lot of treatment, which is a lot of expense, but they needed to have the failsafe and redundancy to get the state on board with the project. The pipe connecting the two plants was above ground and later repurposed for the city’s indirect potable reuse system, which is still in use today. In the end, they reclaimed nearly two billion gallons of wastewater as drinking water. And they did it with 100% compliance with the standards. But more importantly, they showed that it could be done, essentially unlocking a new branch on the skill tree of engineering that other cities can emulate and build on.

[Note that this article is a transcript of the video embedded above.] The original plan to get I-95 over the Baltimore Harbor was a double-deck bridge from Fort McHenry to Lazaretto Point. The problem with the plan was this: the bridge would have to be extremely high so that large ships could pass underneath, dwarfing and overshadowing one of the US’s most important historical landmarks. Fort McHenry famously repelled a massive barrage and attack from the British Navy in the War of 1812, and inspired what would later become the national anthem. An ugly bridge would detract from its character, and a beautiful one would compete for it. So they took the high road by building a low road and decided to go underneath the harbor instead. Rather than bore a tunnel through the soil and rock below like the Channel Tunnel, the entire thing was prefabricated in sections and installed from the water surface above - a construction technique called immersed tube tunneling. This seems kind of simple at first, but the more you think about it, the more you realize how complicated it actually is to fabricate tunnel sections the length of a city block, move them into place, and attach them together so watertight and safe that, eventually, you can drive or take a train from one side to the other. Immersed tube construction makes tunneling less like drilling a hole and more like docking a spacecraft. Materials and practices vary across the world, but I want to try and show you, at least in a general sense, how this works. I’m Grady, and this is Practical Engineering. One of the big problems with bridges over navigable waterways is that they have to be so tall. Building high up isn’t necessarily the challenge; it’s getting up and back down. There are limits to how steep a road can be for comfort, safety, and efficiency, and railroads usually have even stricter constraints on grade. That means the approaches to high bridges have to be really long, increasing costs and, in dense cities, taking up more valuable space. This is one of the ways that building a tunnel can be a better option; They greatly reduce the amount of land at the surface needed for approaches. But traditional tunnels built using boring have to be installed somewhat deep into the ground, maintaining significant earth between the roof of the tunnel and the water for stability and safety. Since they’re installed from above, immersed tube tunnels don’t have the same problem. It’s basically a way to get the shortest tunnel possible for a given location, which often means the cheapest tunnel too. That’s a big deal, because tunnels are just about the most expensive way to get from point A to point B. Anything you can do to reduce their size goes a long way. And there are other advantages too. Tunnel boring machines make one shape: a circle. It’s not the best shape for a tunnel, in a lot of ways. Often there’s underutilized space at the top and bottom - excavation you had to perform because of the machinery that is mostly just a waste. Immersed tubes can be just about any shape you need, making them ideal for wider tunnels like combined road and rail routes where a circular cross-section isn’t a good fit. One of the other benefits of immersed tubes is that most of the construction happens on dry land. I probably don’t have to say this, but building stuff while underground or underwater is complex and difficult work. It requires specialty equipment, added safety measures, and a lot of extra expense. Immersed tube sections are built in dry docks or at a shipyard where it's much easier to deliver materials and accomplish the bulk of the actual construction work. Once tunnel sections are fabricated, they have to be moved into place, and I think this is pretty clever. These sections can be enormous - upwards of 650 feet or 200 meters long. But they’re still mostly air. So if you put a bulkhead on either side to trap that air inside, they float. You can just flood the dry dock, hook up some tugboats, and tow them out like a massive barge. Interestingly, the transportation method means that the tunnel segments have to be designed to work as a watercraft first. The weight, buoyancy, and balance of each section are engineered to keep them stable in the water and avoid tipping or rolling before they have to be stable as a structure. Once in place, a tunnel segment is handed over to the apparatus that will set it into place. In most cases, this is a catamaran-style behemoth called a lay barge. Two working platforms are connected by girders, creating a huge floating gantry crane. Internal tanks are filled with water to act as ballast, allowing the segment to sink. But when it gets to the bottom, it doesn’t just sit on the sea or channel floor below. And this is another benefit of immersed tube construction. Especially in navigable waterways, you need to protect a tunnel from damage from strong currents, curious sea life, and ship anchors. So most immersed tube tunnels sit in a shallow trench, excavated using a clamshell or suction dredger. Most waterways have a thick layer of soft sediment at the surface - not exactly ideal as a foundation. This is another reason most boring machines have to be in deeper material. Drilling through soft sediment is prone to problems. Imagine using a power drill to make a nice, clean hole through pudding. But, at least in part due to being full of buoyant air, immersed tubes aren’t that heavy; in fact, in most cases, they’re lighter than the soil that was there in the first place, so the soft sediment really isn’t a problem. You don’t need a complicated foundation. In many cases, it’s just a layer of rock or gravel placed at the bottom of the trench, usually using a fall pipe (like a big garden hose for gravel) to control the location. This layer is then carefully leveled using a steel screed that is dragged over the top like an underwater bulldozer. Even in deep water, the process can achieve a remarkably accurate surface level for the tunnel segments to rest on. The lowering process is the most delicate and important part of construction. The margins are tight because any type of misalignment may make it impossible for the segment to seal against its neighbor. Normally, you’d really want to take your time with this kind of thing, but here, the work usually has to happen in a narrow window to avoid weather, tides, and disruption to ship traffic. The tunnel section is fitted with rubber seals around its face, creating a gasket. Sometimes, the segment will also have a surveying tower that pokes above the water surface, allowing for measurements and fine adjustments to be made as it’s set into place. In some cases, the lowering equipment can also nudge the segment against its neighbor. In other cases, hydraulic jacks are used to pull the segments together. Divers or remotely operated submersibles can hook up the jacks. Or couplers, just like those used on freight trains, can do it without any manual underwater intervention. The jacks extend to couple the free segment to the one already installed, then retract to pull them together, compressing the gasket and sealing the area between the two bulkheads. This joint is the most important part of an immersed tunnel design. It has to be installed blindly and accommodate small movements from temperature changes, settlement, and changes in pressure as water levels go up and down. The gasket provides the initial seal, but there’s more to it. Once in place, valves are opened in the bulkheads to drain the water between them. That actually creates a massive pressure difference between one side of the segment and the other. Hydrostatic force from the water pushes against the end of the tunnel, putting it in even firmer contact with its neighbor and creating a stronger seal. Once in its final place, the segment can be backfilled. The tunnel segment connection is not like a pipe flange, where the joints are securely bolted together, completely restraining any movement. The joints on immersed tunnels have some freedom to move. Of course, there is a restraint for axial compression since the segments butt up against each other. In addition, keys or dowels are usually installed along the joint so that shear forces can transfer between segments, keeping the ends from shifting during settlement or small sideways movements. However, the joints aren’t designed to transfer torque, called moments. And there’s rarely much mechanical restraint to axial tension that might pull one joint away from the other. So you can see why the backfill is so important. It locks each segment into place. In fact, the first layer of backfill is called locking fill for that exact reason. I don’t think they make underwater roller compactors, and you wouldn’t want strong vibrations disturbing the placement of the tunnel segments anyway. So this material is made from angular rock that self-compacts and is placed using fall pipes in careful layers to secure each segment without shifting or disturbing it. After that, general backfill - maybe even the original material if it wasn’t contaminated - can be used in the rest of the trench, and then a layer is placed over the top of everything to protect the backfill and tunnel against currents caused by ships and tides. Sometimes this top layer includes bands of large rock meant to release a ship’s anchor from the bottom, keeping it from digging in and damaging the tunnel. Once a tunnel segment is secured in place, the bulkhead in the previous segment can be removed from the inside, allowing access inside the joint. The usual requirement is that access is only allowed when there are two or more bulkheads between workers and the water outside. A second seal, called an omega seal (because of its shape), then gets installed around the perimeter of the joint. And the process keeps going, adding segments to the tunnel until it’s a continuous, open path from one end to the other. When it reaches that point, all the other normal tunnel stuff can be installed, like roadways, railways, lights, ventilation, drainage, and pumps. By the time it’s ready to travel through, there’s really no obvious sign from inside that immersed tube tunnels are any different than those built using other methods. This is a simplification, of course. Every one of these steps is immensely complicated, unique to each jobsite, and can take weeks to months, to even years to complete. And as impressive as the process is, it’s not without its downsides. The biggest one is damage to the sea or river floor during construction. Where boring causes little disturbance at the surface, immersed tube construction requires a lot of dredging. That can disrupt and damage important habitat for wildlife. It also kicks up a lot of sediment into suspension, clouding the water and potentially releasing buried contaminants that were laid down back when environmental laws were less strict. Some of these impacts can be mitigated: Sealed clamshell buckets reduce turbidity and mobilization of contaminated sediment. And construction activities can be scheduled to avoid sensitive periods like migration of important species. But some level of disturbance is inevitable and has to be weighed against the benefits of the project. Despite the challenges, around 150 of these tunnels have been built around the globe. Some of the most famous include the Øresund Link between Denmark and Sweden, the Busan-Geoje tunnel in South Korea, the Marmaray tunnel crossing the Bosphorus in Turkey, of course, the Fort McHenry tunnel in Baltimore I mentioned earlier, and the BART Transbay Tube between Oakland and San Francisco. And some of the most impressive projects are under construction now, including the Fehmarn Belt between Denmark and Germany, which will be the world’s longest immersed tunnel. My friend Fred produced a really nice documentary about that project on The B1M channel if you want to learn more about it, and the project team graciously shared a lot of very cool clips used in this video too. There’s something about immersed tube tunnels that I can’t quite get over. At a glance, it’s dead simple - basically like assembling lego blocks. But the reality is that the process is so complicated and intricate, more akin to building a moon base. Giant concrete and steel segments floated like ships, carefully sunk into enormous trenches, precisely maneuvered for a perfect fit while completely submerged in sometimes high-traffic areas of the sea, with tides, currents, wildlife, and any number of unexpected marine issues that could pop up. And then you just drive through it like it’s any old section of highway. I love that stuff.

[Note that this article is a transcript of the video embedded above.] In December of 2024, a huge sinkhole opened up on I-80 near Wharton, New Jersey, creating massive traffic delays as crews worked to figure out what happened and get it fixed. Since then, it happened again in February 2025 and then again in March. Each time, the highway had to be shut down, creating a nightmare for commuters who had to find alternate routes. And it’s a nightmare for the DOT, too, trying to make sure this highway is safe to drive on despite it literally collapsing into the earth. From what we know so far, this is not a natural phenomenon, but one that’s human-made. It looks like all these issues were set in motion more than a century ago when the area had numerous underground iron mines. This is a really complex issue that causes problems around the world, and I built a little model mine in my garage to show you why it’s such a big deal. I’m Grady and this is Practical Engineering. We’ve been extracting material and minerals from the earth since way before anyone was writing things down. It’s probably safe to say that things started at the surface. You notice something shiny or differently colored on the side of a hill or cliff and you take it out. Over time, we built up knowledge about what materials were valuable, where they existed, and how to efficiently extract them from the earth. But, of course, there’s only so much earth at the surface. Eventually, you have to start digging. Maybe you follow a vein of gold, silver, copper, coal or sulfur down below the surface. And things start to get more complicated because now you’re in a hole. And holes are kind of dangerous. They’re dark, they fill with water, they can collapse, and they collect dangerous gases. So, in many cases, even today, it makes sense to remove the overburden - the soil and rock above the mineral or material you’re after. Mining on the surface has a lot of advantages when it comes to cost and safety. But there are situations where surface mining isn’t practical. Removing overburden is expensive, and it gets more expensive the deeper you go. It also has environmental impacts like habitat destruction and pollution of air and water. So, as technology, safety, and our understanding of soil and rock mechanics grew, so did our ability to go straight to the source and extract minerals underground. One of the major materials that drove the move to underground mining was coal. It’s usually found in horizontal formations called seams, that formed when vast volumes of paleozoic plants were buried and then crushed and heated over geologic time. At the start of the Industrial Revolution, coal quickly became a primary source of energy for steam engines, steel refining, and electricity generation. Those coal seams vary in thickness, and they vary in depth below the surface too, so many early coal mines were underground. In the early days of underground mining, there was not a lot of foresight. Some might argue that’s still true, but it was a lot more so a couple hundred years ago. Coal mining companies weren’t creating detailed maps of their mines, and even if they did, there was no central archive to send them to. And they just weren’t that concerned about the long-term stability of the mines once the resources had been extracted. All that mattered was getting coal out of the ground. Mining companies came and went, dissolved or were acquired, and over time, a lot of information about where mines existed and their condition was just lost. And even though many mines were in rural areas, far away from major population centers, some weren’t, and some of those rural areas became major population centers without any knowledge about what had happened underneath them decades ago. An issue that confounds the problem of mine subsidence is that in a lot of places, property ownership is split into two pieces: surface rights and mineral rights. And those rights can be owned by different people. So if you’re a homeowner, you may own the surface rights to your land, while a company owns the right to drill or mine under your property. That doesn’t give them the right to damage your property, but it does make things more complicated since you don’t always have a say in what’s happening beneath the surface. There are myriad ways to build and operate underground mines, but especially for soft rock mining, like coal, the predominant method for decades was called “room and pillar”. This is exactly what it sounds like. You excavate the ore, bringing material to the surface. But you leave columns to support the roof. The size, shape, and spacing of columns are dictated by the strength of the material. This is really important because a mine like this has major fixed costs: exploration, planning, access, ventilation, and haulage. It’s important to extract as much as possible, and every column you leave supporting the roof is valuable material you can’t recover. So, there’s often not a lot of margin in these pillars. They’re as small as the company thought they could get away with before they were finished mining. I built a little room and pillar mine in my garage. I’ll be the first to admit that this little model is not a rigorous reproduction of an actual geologic formation. My coal seam is just made of cardboard, and the bright colors are just for fun. But, I’m hoping this can help illustrate the challenges associated with this type of mine. I’ve got a little rainfall simulator set up, because water plays a big role in these processes. This first rainfall isn’t necessarily representative of real life, since it’s really just compacting the loose sand. But it does give a nice image of how subsidence works in general. You can see the surface of the ground sinking as the sand compacts into place. But you can also see that as the water reaches the mine, things start to deform. In a real mine, this is true, too. Stresses in the surrounding soil and rock redistribute over time from long-term movements, relaxation of stresses that were already built up in the materials before extraction, and from water. I ran this model for an entire day, turning the rainfall on and off to simulate a somewhat natural progression of time in the subsurface. By the end of the day, the mine hadn’t collapsed, but it was looking a great deal less stable than when it started. And that’s one big thing you can learn from this model - in a lot of cases, these issues aren’t linearly progressive. They can happen in fits and starts, like this small leak in the roof of the mine. You get a little bit of erosion of soil, but eventually, enough sand built up that it kind of healed itself, and, for a while, you can’t see any evidence of any of it at the surface. The geology essentially absorbed the sinkhole by redistributing materials and stresses so there’s no obvious sign at the surface that anything wayward is happening below. In the US, there were very few regulations on mining until the late 19th century, and even those focused primarily on the safety of the workers. There just wasn’t that much concern about long-term stability. So as soon as material was extracted, mines were abandoned. The already iffy columns were just left alone, and no one wasted resources on additional supports or shoring. They just walked away. One thing that happens when mines are abandoned is that they flood. Without the need to work inside, the companies stop pumping out the water. I can simulate this on my model by just plugging up the drain. In a real soft rock mine, there can be minerals like gypsum and limestone that are soluble in water. Repeated cycles of drying and wetting can slowly dissolve them away. Water can also soften certain materials and soils, reducing their mechanical strength to withstand heavy loads, just like my cardboard model. And then, of course, water simply causes erosion. It can literally carry soil particles with it, again, causing voids and redistribution of stresses in the subsurface. This is footage from an old video I did demonstrating how sinkholes can form. The ways that mine subsidence propagates to the surface can vary a lot, based on the geology and depth of the mine. For collapses near the surface, you often see well-defined sinkholes where the soil directly above the mine simply falls into the void. And this is usually a sudden phenomenon. I flooded and drained my little mine a few times to demonstrate this. Accidentally flooded my little town a few times in the process, but that’s okay. You can see in my model, after flooding the mine and draining it down, there was a partial failure in the roof and a pile of sand toward the back caved in. And on the surface, you see just a small sinkhole. In 2024, a huge hole opened right in the center of a sports complex in Alton, Illinois. It was quickly determined that part of an active underground aggregate mine below the park had collapsed, leading to the sinkhole. It’s pretty characteristic of these issues. You don’t know where they’re going to happen, and you don’t know how the surface soils are going to react to what’s happening underneath. Subsidence can also look like a generalized and broader sinking and settling over a large area. You can see in my model that most of the surface still looks pretty flat, despite the fact that it started here and is now down here as the mine supports have softened and deformed. This can also be the case when mines are deeper in the ground. Even if the collapse is sudden, the subsidence is less dramatic because the geology can shift and move to redistribute the stresses. And the subsidence happens more slowly as the overburden settles into a new configuration. In all cases, the subsidence can extend laterally from the mine, so impacted areas aren’t always directly above. The deeper the mine, the wider the subsidence can be. I ran my little mine demo for quite a few cycles of wet and dry just to see how bad things would get. And I admit I used a little percussion at the end to speed things along. Let’s say this is a simulation of an earthquake on an abandoned mine. [Beat] You can see that by the end of it, this thing has basically collapsed. And take a look at the surface now. You have some defined sinkholes for sure. And you also have just generalized subsidence - sloped and wavy areas that were once level. And you can imagine the problems this can cause. Structures can easily be damaged by differential settlement. Pipes break. Foundations shift and crack. Even water can drain differently than before, causing ponding and even changing the course of rivers and streams for large areas. And even if there are no structures, subsidence can ruin high-value farm land, mess up roads, disrupt habitat, and more. In many cases, the company that caused all the damage is long gone. Essentially they set a ticking time bomb deep below the ground with no one knowing if or when it would go off. There’s no one to hold accountable for it, and there’s very little recourse for property owners. Typical property insurance specifically excludes damage from mine subsidence. So, in some places where this is a real threat, government-subsidized insurance programs have been put in place. Eight states in the US, those where coal mining was most extensive, have insurance pools set up. In a few of those states, it is a requirement in order to own property. The federal government in the US also collects a fee from coal mines that goes into a fund that helps cover reclamation costs of mines abandoned before 1977 when the law went into effect. That federal mining act also required modern mines to use methods to prevent subsidence, or control its effects, because this isn’t just a problem with historic abandoned mines. Some modern underground soft rock mining doesn’t use the room and pillar method but instead a process called longwall mining. Like everything in mining, there are multiple ways to do it. But here’s the basic method: Hydraulic jacks support the roof of the mine in a long line. A machine called a shearer travels along the face of the seam with cutting drums. The cut coal falls onto a conveyor and is transported to the surface. The roof supports move forward into the newly created cavity, intentionally allowing the roof behind them to collapse. It’s an incredibly efficient form of mining, and you get to take the whole seam, rather than leaving pillars behind to support the roof. But, obviously, in this method, subsidence at the surface is practically inevitable. Minimizing the harm that subsidence creates starts just by predicting its extent and magnitude. And, just looking at my model, I think you can guess that this isn’t a very easy problem to solve. Engineers use a mix of empirical information, like data from similar past mining operations, geotechnical data, simplified relationships, and in some cases detailed numerical modeling that accounts for geologic and water movement over time. But you don’t just have to predict it. You also have to measure it to see if your predictions were right. So mining companies use instruments like inclinometers and extensometers above underground mines to track how they affect the surface. I have a whole video about that kind of instrumentation if you want to learn more after this. The last part of that is reclamation - to repair or mitigate the damage that’s been done. And this can vary so much depending on where the mine is, what’s above it, and how much subsidence occurs. It can be as simple as filling and grading land that has subsided all the way to extensive structural retrofits to buildings above a mine before extraction even starts. Sinkholes are often repaired by backfilling with layers of different-sized materials, from large at the bottom to small at top. That creates a filter to keep soil from continuing to erode downward into the void. Larger voids can be filled with grout or even polyurethane foam to stabilize the ground above, reducing the chance for a future collapse. I know coal - and mining in general - can be a sensitive topic. Most of us don’t have a lot of exposure to everything that goes into obtaining the raw resources that make modern life possible. And the things we do see and hear are usually bad things like negative environmental impacts or subsidence. But I really think the story of subsidence isn’t just one of “mining is bad” but really “mining used to be bad, and now it’s a lot better, but there are still challenges to overcome.” I guess that’s the story of so many things in engineering - addressing the difficulties we used to just ignore. And this video isn’t meant to fearmonger. This is a real issue that causes real damages today, but it’s also an issue that a lot of people put a great deal of thought, effort, and ultimately resources into so that we can strike a balance between protection against damage to property and the environment and obtaining the resources that we all depend on.