The Bizarre Flaw in the New Orleans Levees

[Note that this article is a transcript of the video embedded above.]

What happened in New Orleans in August 2005 wasn’t a natural disaster. Don’t get me wrong; Hurricane Katrina was a big storm. By many measures it was one of the strongest hurricanes to ever crash into a United States coastline. It’s in the top 10 for largest diameter, lowest central pressure, highest integrated kinetic energy and more. Storm surge along parts of the Gulf Coast crested at nearly 28 feet or 8.5 meters, the highest in US history.

There really are no words to describe the devastation Katrina caused, particularly in New Orleans and the surrounding areas. Roughly four-fifths of the city was flooded. In some areas the water rose above the rooflines of houses. The storm shredded the fabric of the city, and to this day, more than 20 years later, the population of New Orleans has not recovered to its pre-Katrina level.

If this were just a story about a major storm overwhelming the flood defenses, it wouldn’t undercut the tragedy, but it would be understandable. I think we generally recognize that these kinds of systems don’t offer absolute protection. The levees and floodwalls around New Orleans were a sprawling collection of structures built over many decades. It wouldn’t be out of question for a storm of the century to simply exceed what those protections were designed to handle. But that’s not what happened.

There were about 50 individual locations where levees or floodwalls were breached during Hurricane Katrina. Many of those were situations where the structure was overtopped by storm surge. But out of those fifty, only three of those breaches accounted for the majority of the flows that submerged the heart of New Orleans and led to nearly half of the disaster’s total fatalities and economic damage. All three of those breaches happened at surge levels below what the floodwalls were designed to manage. And I built a model in the garage to show you why.

This video is based on a chapter from my new Book, Disasters By Design: How Engineering Failures Shaped the Modern World, which you can pre-order right now at the links below. The book is all about engineering disasters through history and what we learned from them, and I really hope you’ll consider picking up a copy. I’ll tell you more about it at the end, but first, I want to show you this model. I’m Grady, and this is Practical Engineering.

The land we call New Orleans has a history stretching way beyond its 1718 founding or the birth of the United States. Originally inhabited by tribes like the Chitimacha, this area has always been a complex intersection of human settlement and flooding. It is strategically located on the Gulf Coast at the mouth of the Mississippi, the largest river in North America. But that prime location also comes with challenges, namely that the city’s exposure to tropical storms is extreme, and, being a river delta, much of its development happened on top of historically soft, marshy, and low-lying terrain.

In 1965 Hurricane Betsy rocked New Orleans, flooding more than 150,000 houses. At the time, it was one of the worst hurricanes in American history, earning the nickname “Billion Dollar Betsy,” because it was the first storm with damages in excess of a billion dollars. In response, the federal government, through the US Army Corps of Engineers, made a massive investment into the existing hurricane protection system around New Orleans. But they ran into an issue.

A levee is an earthen embankment meant to hold floodwaters back - kind of like a dam but parallel to coastline or a river instead of across it. We know that soil is naturally unstable on steep slopes. So levees have this trapezoidal shape in cross-section with gentle, stable slopes on the waterside and landside. Many of the levees in New Orleans were built decades before Betsy, and over that time, development encroached right up the toes of the structures. You can see the problem. If you want to raise the embankment to provide more protection, you need more space.

The solution they came up with is the “I-Wall” (not related to the eye wall of a hurricane). Steel sheet piles were driven into the top of the levee, creating a vertical floodwall on top. They got more height and more protection without the need to condemn property for a wider footprint, which was a major win.

All this flood protection infrastructure turned New Orleans into essentially a bowl. The levees keep the floodwaters out, but they also trap rainwater (or any other water) in. So in addition to the levees and floodwalls, the city is dotted with pump stations that move drainage within the city out of the bowl. The heart of this system is the trio of primary outfall canals that run south to north, carrying rainwater from New Orleans’ interior basins out to Lake Pontchartrain: the 17th Street Canal, the Orleans Avenue Canal, and the London Avenue Canal. Where most canals, rivers, and creeks in cities represent a topographic low point, these canals are somewhat of a high point in New Orleans. Runoff within the city naturally flows in street gutters and underground storm drains, eventually reaching one of the massive pumping stations associated with each canal. Those stations raise the water just a little higher than Lake Pontchartrain, and then gravity takes care of the rest, with the water flowing northward out of the city and into the lake.

The problem is that Lake Pontchartrain isn’t really a lake; it’s an estuary with a direct hydraulic connection to the Gulf of Mexico. It functions as an arm of the sea, which means it’s subject to the same tidal forces and catastrophic storm surges as the open ocean. When Hurricane Katrina pushed up a wall of water, the outfall canals stopped acting like drainage ditches and turned into conduits penetrating deep into the heart of New Orleans, inviting the weight and pressure of the sea miles inland. You can see why the breaches along these levees were particularly serious.

In the morning on August 29, just after landfall as storm surge began to peak, three failures along the outfall canals happened in quick succession. Maybe the most well-known was the breach on the 17th Street Canal levee and I-wall. That morning, water was several feet below the top of the wall. This structure should have been more than capable of holding back the surge, but a huge chunk of the levee shifted, leaving a 450-foot- (or 140-meter-wide) gap for a torrent of storm surge to flood the Lakeview neighborhood and surrounding areas.

It’s a little tricky to understand how something like this could happen, so I’ve built a model of a levee and I-wall in my garage to show you how this works. This is just a narrow tank that I built out of sheets of acrylic. On the left side, I have a pump that will move water into the system. This area represents the canal side where storm surge flowed in from Lake Pontchartrain. I have an overflow that will keep the water level constant. I don’t know how well this shows up but the soil that I’m using for the levee and its foundation is a blend of ground up plastic and sand. And then on the right I have a drain that kind of represents the pump stations that move surface water out of protected areas to keep them dry.

This took a few tries to get to work right. I’m sure anybody who’s built a garage storm surge simulation can relate. First the colored liquid dyes weren’t persistent enough. Then my I-wall sprang a leak. So I’ve got solid dye tablets in the soil and the leak hopefully fixed. And you can see that there’s not much happening when the water level in the canal is low. But watch what happens when I turn on the pump. The levee and I-wall hold the water from the canal back, keeping the homes and businesses on the land side protected. But keep an eye on the dye in the soil. I’ll speed this up a bit to make it clearer. Water is still making its way toward the landside of the levee, not above ground, but in the subsurface. It might seem obvious that this would happen, but it’s not a phenomenon we get to see with our eyes very often. And it makes one thing about flood protection systems abundantly clear: these levees aren’t just piles of dirt. They are engineered structures that have to consider some fairly unusual and unintuitive loading conditions. And the behavior of this subsurface flow is absolutely critical to their performance.

It’s easy to follow the flow in the model, but it’s harder to visualize the water pressure, so I coded this little companion toy simulator to go with my model. Thanks to Dan Shiffman at the Coding Train for teaching me javascript and P5. It takes a minute to converge on a solution, but when it does, it plots the pressure contours in the subsurface. Differences in pressure are what drive flow, so these contour lines are perpendicular to the flow lines you see in my model, and I gave the pressure a color gradient to make this more clear. You can see that, even though it’s not as high as the areas closer to the canal, there’s still water pressure in the subsurface on the protected side of the levee. And that’s a problem for a structure trying to hold back an immense wall of storm surge.

Soil grains work kind of like a pile of marbles. Squeeze them and they lock together, increasing their strength against shear. You can imagine if you were able to inject water between those marbles, it would push them away from each other. As a mass altogether, they would be a lot less strong, and it works the same way for soil. Engineers have to take this into account. The calculations when designing a levee aren’t just about how hard the water pushes against the structure, but also how hard it pushes inside the foundation. If the soil doesn’t have enough strength to resist the force of the water pushing sideways while the subsurface water pressure is also pushing the soil particles away from each other, it shears along a plane of weakness. That’s exactly what happened at the 17th Street Canal. The thing is, it happened at a lower water level than anybody expected, And here’s why:

It is no small task to assess the complex subsurface conditions along miles of levees and floodwalls. During the improvements made in the wake of Hurricane Betsy, geotechnical engineers drilled hundreds of boreholes, collected samples, and tested their strengths in labs. Here’s a look at that data plotted as a function of depth.

Now, when you’re designing miles and miles of levee, it’s not feasible to analyze every single inch. Instead, you pick representative sections, and you really want those sections to be the worst ones. That’s just efficient engineering. You know if the design works in the weakest spots, it will work everywhere. But for some reason, that’s not what was done. Knowing that kind of fundamental principle and looking at this data, what would you choose as your representative soil strength for design? If it were me, I might choose something like this - assuming a few of the test values are outliers that don’t represent the bulk properties, but for the most part, being conservative by assuming strengths at or below what was measured in the field. For some reason, the Corps of Engineers drew their line here, assuming the soil was stronger than a majority of the measurements. And what’s worse is that those measurements were taken along the centerline of the existing levees, where the weight of those structures had artificially consolidated and strengthened the soil. Outside of that centerline, the soil was actually much weaker. These design decisions led to the levee failing at water levels lower than the structure was designed to handle.

What made things even worse was a critical flaw of the “I-Wall” idea. It seemed elegant on paper: an efficient way to get more height out of a levee without widening its footprint. But when they were actually put to a true test during Katrina, those walls bowed out, opening a narrow gap deep into the levee’s foundation.

Let’s look back at the simulator to see the difference in pressure. You can see that with the so-called “water-filled gap,” you get more pressure in the foundation of the levee. This also shortens the length of soil shear needed to cause the levee to fail. All these factors resulted in the levee on the 17th Street Canal failing at a level below what it was designed to withstand. The same thing happened nearby on the north side of the London Avenue Canal. And actually, that failure was exclusively the result of the flaw in the I-Wall design. If the wall wouldn’t have deflected to open the gap, that failure wouldn’t have happened at all. But that wasn’t the only failure that happened on the London Avenue Canal, and the one on the south side was even more egregious.

This levee was largely the same as on the 17th street canal with the embankment and I-wall. And it had the same situation with water seeping through the foundation below. But there was one critical difference. The levee rested on a thin layer of marshy clay above a thick bed of permeable sand. Again, it’s hard to show pressure in the garage model, so let’s take a look at the simulation. You can see when I use a homogenous material for the soil of the levee and foundation, you get fairly nice, even spacing of the pressure contours. Let me make a change to add a layer of less permeable material like the London Avenue Canal south and run them side-by-side. With the marsh layer, you get a really different result. The pressure contours stack up right where the water comes out on the protected side. Geotechnical engineers would say there’s a steep exit gradient. And here’s why that matters.

Let’s just take a block of soil and look at it more closely. Because of the steep gradient, the pressure at the bottom of this block is different from the pressure at the top. That difference in pressure means there is a net force pushing it upward. If that net force exceeds the weight of the block itself, it’s going to move. And I can show it in my demonstration, even without the marsh layer.

I’ve reset the demo, but I added an extension to my overflow so the water level will be higher on the canal side now. Even though the pressure gradient through the subsurface is evenly spaced (since I’m using a homogenous material unlike the layers in New Orleans), it’s still steeper just because there’s a bigger difference between the water levels on either side. And because I’m using a lightweight plastic blended into the soil, the so-called “critical” exit gradient is much lower than it would be for denser sand. And it’s easy to see that we’ve exceeded that critical gradient. The particles are dislodging as the seepage comes out of the ground on the protected side. The erosion just keeps getting worse until the levee on the protected side fails. Eventually, the I-wall loses support, and you get a total failure.

This is exactly what happened at the London Avenue Canal South. As the storm surge rose, water seeped through the sand, building upward pressure on that marsh layer. Eventually the pressure got high enough that it literally blew a hole through it. With the sudden release of all that built up pressure, the subsurface water was able to erode a mountain of sand from the foundation. There are pictures of this - a neighborhood covered in what’s basically beach sand. All that erosion caused the I-wall to collapse, opening a wider hole in the levee and letting the full force of Lake Pontchartrain through.

This was not some unknown issue. The Army Corps of Engineers analyzed the seepage forces through the levee when they built the I-walls. But, inexplicably, they left that marsh layer out of the calculations. One of the investigation reports said it clearly: “if a more rigorous analysis had been performed at the time of design, the potential problem would have been predicted and corrective action taken.”

Those three breaches created a majority of the problems in the center of New Orleans, but there were a lot of levees and floodwalls that overtopped in the surrounding area during Hurricane Katrina. One of the worst cases was at the Industrial Canal, where the wall was lower than intended because of ground subsidence and confusion about elevation benchmarks. In that case, the water spilling over the top of the wall eroded the soil, causing the levee and wall to fail. And that gets to the heart of a very complicated problem with flood protection: you can’t build a 100-foot tall invincible wall around the city. You have to draw a line somewhere below absolute safety to a level that’s economically feasible. That means there are going to be potential storms that overwhelm the system, which makes designing levees even more complicated.

Let’s make a graph of depth versus damage. In an area with no flood protection, you might get a curve like this. Just a little damage at first, when depths are low, and those damages increase as flood depth grows. If you build a levee, of course you basically eliminate those damages below the protected level. And above the level, the curve stays the same. Once a levee overtops, even the protected areas flood, so there’s no change there. But over time, a funny thing happens. Before the levee was installed, the lowest places weren’t heavily developed because flooding happened regularly and people generally knew and understood the risk. But once those low areas are protected, and some time with no flooding at all passes, people build stuff in them: homes, business, et cetera. We put a lot of valuable stuff on that previously mostly empty land. So where the depth-damage curve with the levee started like this, now it looks like this. Just a tiny bit of overtopping means an enormous amount of damage because the former floodplains have all been built out. So if you look out toward the long-term, the damages you can expect from flooding over many decades is actually much more than what would have happened if the levees were never built. This concept is sometimes known as hazard creep, and it’s the thing that no one recognized in New Orleans. The shadow of the flood protection system had, in some ways, only made things worse.

In that way, not only did the engineering (particularly along the outfall canals) fail to protect the city (The Corps of Engineers estimated that roughly two-thirds of the deaths during Katrina would not have occurred had the levees and floodwalls not failed.), they generally created a situation where the damage and loss of life was much worse than it would have been in an unprotected area.

Of course, that is really just one tiny piece of the story of Hurricane Katrina. The investigations that followed filled volumes and volumes, tens of thousands of pages, just on the engineering issues alone. When you add in the challenges and controversies associated with the evacuation, rescue operations, and recovery efforts, you could spend your entire life trying to wrap your head around this one event. I know, because I spent a good chunk of my life researching it while I was writing my new book, Disasters By Design. I think there is a lot we can learn from past mistakes, and I really hope you’ll click on the link below or head to your local book store and pre-order a copy.

That’s the thing about engineering disasters: When they first happen, journalists scramble to tell the story with whatever information they can find; politicians and pundits offer premature reactions; and then, slowly, public interest wanes. By the time rigorous investigations are carried out and a consensus has formed around what really happened, most people have already moved on. So, in lots of cases, the documentation of an engineering disaster is this rigorous and complex forensic report produced months or years after the fact that a handful of experts read, and not much else.

Here’s the problem with that: For better or worse, a great deal of humanity’s most important lessons in engineering have been learned the hard way. Humans are capable of, and maybe even prone to, making stupendous errors in judgment, calculation, and prognostication. The only way we have reached this era of remarkable advancement and connectivity is through learning from those errors and striving to avoid repeating them. All the people who studied, investigated, and documented the engineering failures of history did so because they wanted, in some way, to help prevent us from making the same mistakes again. But they often wrote to an audience of other engineers, meaning many of humanity’s most important lessons are tough to wrap your head around unless you have the context, background, resources and education to parse and understand those sometimes impenetrable reports.

I wrote this book because I wanted to bridge that gap. I’ve spent a career working on infrastructure projects as a civil engineer and another career breaking down engineering concepts to people like you on YouTube. I wanted to combine those experiences and curate a collection of stories throughout history that I’ve found to be the most enlightening.

But these aren’t just stories about disasters and tragedy. They’re also stories about what we learned as a result, and how we transform tragedy into hope. Because some of the most important aspects of the built environment and the industry that surrounds it came as a result of something terrible. In that way, the stories of engineering failures are the stories of engineering itself and how it’s evolved over time.

New Orleans is a perfect example. The city’s flood protection system looks a lot different than before Katrina: There are huge gates on the canals to keep Pontchartrain from backflowing into the city. Levees have been raised and strengthened. The flawed I-walls were taken out. There’s a huge new storm surge barrier in Lake Borgne. But it wasn’t just steel and concrete structures that resulted from the storm. The Corps of Engineers completely rewrote the manual for levee engineering, incorporating the lessons learned. They did a nationwide review of existing I-wall projects. The federal government implemented the National Levee Safety Program to support owners and enhance public awareness. Lots of those lessons were incorporated into the International Levee Handbook that is used around the world. It changed so much in engineering, disaster response, and even government. It’s tricky to see how all this plays out, because it really just means quiet. There won’t be any news stories that some levee didn’t fail today. And that’s why I wrote the book.

To really get a sense of how the field of engineering has evolved over time, from antiquity to today, you have to look at the disasters that precipitated the big changes. And I really think having those major events laid out in order and explained so curious people can understand them, with lots of illustrations, is the right way to comprehend engineering history. So yeah, the book will be published in March of next year. I’m super excited to tell you about it, and really proud of how it came out. I know that’s a long time from now, but pre-orders are really important with books so they know how many to print. I know I ask you to click a link at the end of all my videos to support the channel via our sponsors, but this time, it means so much more. Grab a first edition for yourself. And if you have friends, professors, a significant other, or family members who are interested in engineering, it would make a great gift. Your support and encouragement has made this book possible, and I’m so grateful to have you along for the ride. Thank you for watching, and let me know what you think.

How to Demolish a Bridge

[Note that this article is a transcript of the video embedded above.]

In 2022, the world got a very cool new bridge. Two bridges, actually. The Iowa-Illinois Memorial Bridges carries Interstate Highway 74 over the Mississippi River between Moline, Illinois and Bettendorf, Iowa in the “Quad Cities” area. It’s a gorgeous pair of structures with the basket handle arches carrying each deck over the main span of the river. But even after they were finished, Iowa DOT had a problem. The two old bridges were still right there, also crossing the Mississippi River. And even though they were kind of cool looking, they just couldn’t stay. The bridges were already in poor condition, and without extensive ongoing maintenance, they would continue to deteriorate, posing a danger to the public, affecting the sensitive environment along the river, and even disrupting this critical shipping artery. The old bridges would have to come down.

Demolition, on its face, seems kind of easy. For a billion-dollar-bridge replacement project, the demo part feels almost like housekeeping. Smash the structure down or blow it up, then just pick up the pieces. No engineering needed. The truth is that it’s anything but. Demolition engineering, in many ways, is even more complicated than designing a new structure, and the I-74 bridge is the perfect case study in why. And don’t worry, there are explosions at the end. I’m Grady, and this is Practical Engineering.

The original I-74 bridges, with just two lanes each, were way overdue for an upgrade in capacity. This is actually a problem they faced and managed to overcome once before, many decades back. Despite looking like twins, the original pair of bridges were built a generation apart. The first span was completed in the mid 1930s, but car ownership and traffic exploded after World War II. Engineers decided that the best way to increase capacity was to build a nearly identical bridge right next to it. That new bridge was opened in 1959.

Neither bridge was ever intended to meet interstate standards; they predate the interstate system altogether. And yet, they found themselves carrying interstate-levels of traffic, way beyond what the designers in the 1950s, and especially the designers back in the 1930s, had considered. The lanes were narrow, there were no shoulders so they required a lower speed limit, which bottlenecked traffic on I-74.

Size isn’t everything, though; the bridges were also just physically wearing out. Like an old car, it eventually got to the point where the cost of replacing the bridges was outweighed by the constant maintenance and threat of disaster. In 2012, Transportation Secretary Ray LaHood toured this structure, reporting back that it was, quote, “one of the worst bridges I’ve seen in America.”

You would think that already being close to falling down would be to their advantage when it comes to demolition, but it’s quite a bit more complicated than that. These were big bridges with 3 types of structural designs. There’s these three span continuous truss units over the old non-navigable part of the river. There’s the deck trusses that kind of act as connectors. And then you have the big 3 span suspension section.

I’m sure you want to see the explosion, and I promise we’ll get there, but it’s basically the last step. Of course, there are lots of cases where it makes sense to just blast a structure down right away. You end up with a pile of rubble that you can manage with regular construction equipment. It can be much quicker, easier, and safer than dismantling a structure piece by piece, but it’s rarely true for bridges. Of course you’ve got the water that complicates things. Removing debris from below the water line is challenging. Long reach excavators can sometimes handle the smaller stuff, but you often need divers to rig the big stuff to be lifted out by cranes. That’s dangerous and difficult work. You also have shipping traffic to consider. This stretch of the Mississippi is a busy part of the inland waterway system, and closing it to clean debris out of the channel is a disruptive task. The other thing making it tricky, in this case, is the environment. There are endangered mussels living in the non-navigable channel below the continuous truss spans, so the demolition team couldn’t use blasting or even temporary supports in that part of the river. The only option was to dismantle the bridges more carefully and thoughtfully. Step one is to get the deck off.

The strategy here was to sawcut all the concrete into pieces small enough to move with construction equipment. An excavator with a slab crab attachment could lift each panel off the steel structure, swing around, and pass to a wheel loader to carry it off the end. Sounds simple, but it had to be done pretty carefully. Cutting the concrete into panels like this means that the reinforcement is cut too. And this is just a cool part of demolition engineering: using calculations and analysis not to design something new but to answer a tricky question like, “Can these concrete panels support the weight of a 35,000-pound excavator?” The answer was more complicated than just a simple “yes.” So the engineers imposed pretty strict positional requirements for the demolition equipment, in most cases making sure that the tracks of the excavator were always directly above the stringers instead of relying on the concrete deck panels to act like beams and transfer the weight.

Another challenge on the suspension span was asymmetric loading. It’s easiest to use the bridge to dismantle the bridge, systematically working your way toward either end. The problem is that if you take all the weight off one section of the bridge while it’s still remaining on other parts, the trusses are going to bow, the towers are going to deflect, and you could actually fail the bridge prematurely. It’s just like re-racking weights at the gym. If, instead of alternating, you take everything off one side of the barbell, you might have a bad time. So, on the suspension bridges alone, the deck removal was this multi-stage process.

Some slabs were removed with the excavator and loader. Others were popped up and left in place as counterweight to be removed by a crane later. It’s a lot more work with a crane (and slower), but it was the only way to get the deck off in a symmetrical way to avoid overstressing any part of the bridge.

Once the concrete deck was off, the contractor could start removing the steel trusses, beams, and stringers that make up the bridge structures. And this gets pretty tricky too. You can’t just go cutting up a bridge willy nilly. This is like jenga on hard mode with very high stakes, and that requires some structural engineering. On the continuous truss section, the demolition team wasn’t allowed to install temporary supports to avoid disturbing the mussels. So instead, they floated in the support on a barge. This allowed them to safely cut the trusses into pieces small enough for the crane to handle without causing a collapse.

The suspension spans were even more complicated. You can imagine how dangerous it is to cut a piece of steel that’s under significant stress. As soon as the member is severed, it could cause sudden movements and load redistributions within the bridge. Those cuts have to be carefully sequenced. When you’re actively weakening a structure, each step changes the stresses, shifting them around and altering their magnitudes and directions. You have to check each step before you do it to make sure it’s not going to endanger workers, ships below, or the environment. And there’s no way to know how much stress is in a member just by looking at it.

Instead, they had to create a structural computer model. But that’s not as simple as recreating the bridge in 3D. The order also matters. One cool example of this: the rivets in the connections for the top chords of each truss weren’t installed until after the concrete deck was poured. So, most of the load was being carried in the bottom chords in tension. When they removed the deck, the whole truss responded by going into what engineers call “negative bending.” The top chords were in tension and the bottom chords in compression, the exact opposite of what you would expect. That’s something that could have derailed the demolition plan without having gone through the exercise of modeling the bridge exactly how it was originally constructed, modified, and retrofitted over the years. It was almost half engineering, half a history exercise. The engineer even used old magazine articles to understand exactly where those stresses would be.

Here’s another tricky part. To lift the truss sections off the suspension bridges, they had to put a crane on a barge. Anyone who’s been on a boat knows that they’re not the most stable platforms for high-stakes, high-center-of-gravity work, especially when you add in huge loads and the need for precision. They did it for the original construction of these bridges, but that doesn’t mean it’s easy. Operating a crane from a barge involves dynamic loads from lifting and swinging. Barges often use these spud legs to help keep them in place, but there’s still a lot of engineering that goes into checking the stability of a barge for the variety of loading conditions. Those calculations help you pick the right crane, its configuration, limitations on pick weights and movements, et cetera, to make sure the work can be done safely. And just like the concrete deck, these truss segments had to be removed in a staggered manner to keep the towers from deflecting too much in one direction.

Interestingly, sometimes to demolish a structure, you have to add parts first. The original lateral load system had to be removed because of how the bridge would flex during demolition, but the engineers didn’t want people working on a bridge with no way to withstand wind loads. So they had to design and build these steel bumpers that could transfer lateral loads from the superstructure during the demolition process. In another case, they had to install bearing restraints on the trusses of the continuous spans, again to manage wind loads during the time those trusses were partially demolished. And in another example, they had to build an entire stiffening truss, made from pieces of the bridge that had already been removed, so the last deck truss could be lifted and removed as a single piece. All this steel was brought to a location on the shore where it could be cut down using this hydraulic shear and then sent off for recycling.

At this point, basically all that was left of each bridge was the suspension towers and cables. Since those cables are essentially one long structural member, there’s really no way to safely cut them loose. Imagine getting snapped by an enormous rubber band. There’s a lot of stored up energy there, and you don’t want any humans nearby when they come down. This is where the explosives come in. And like every other part of the process, this is tricker than you think.

Explosives used for demolition aren’t really like the ones you see in the movies. You’re not trying to use them to blow everything into tiny pieces. On buildings, you get a lot of breakup anyway because of the kinetic energy of falling. But really, the explosives are just strategically severing columns and beams quickly to start the falling process in a more controlled way. On a bridge, usually what you want is big pieces that can easily be removed from the water. So the explosives are more like small, very exciting saws that can cut quickly, simultaneously, and remotely.

Demolition contractors use shaped charges that sever structural members in a relatively controlled manner, and more importantly, a specific location. It matters a lot what these pieces look like after the blast. They have to be small enough to be lifted and transported out of the water. And you want them to fall in specific locations where they’re accessible without blocking the navigation channel. So, before the explosives are placed, there’s an entire process of precutting. The goal is to reduce each location where explosives will be placed down to flat plates or smaller sections so the blasts are sure to completely cut through. A worst case scenario is an incomplete explosive demolition that doesn’t fully bring the structure down. When that happens, the whole process becomes much more dangerous and difficult because you have to finish the job using workers on a structure where it’s not entirely clear where the stresses are or where it’s safe to cut.

On the I-74 bridges, workers cut the outer strands of the main cable, leaving only 7 of the 37 strands holding. This was done in four locations on each cable to break it up into manageable pieces. The towers were also cut in strategic locations to allow the shaped charges to sever completely and control which direction they would fall into the water. And, by the way, this leaves the bridge extremely vulnerable. You’re basically marching right up to the line of stability so that the explosives can kind of carry you over the finish line to bring the structure down. But it means the clock is ticking. You’re checking the weather. Working long shifts. You don’t want a storm bringing the bridge down before you get to push the button. But once you do…

I’ve mentioned a lot of pros and cons of explosive demolition, but there’s one thing they do better than anything else: the spectacle. You just can’t get around how cool it is to blow stuff up.

They got perfect shots for both bridges, bringing them down safely so that the barges could come out quickly to pick up the pieces. The road and navigation channel only had to be closed for a short period of time. Here’s a good look at how clean the cuts are from the shaped charges on the cables. And actually, they used explosives to demo the substructures later on. The piers in the more sensitive areas were taken out using conventional jackhammers as the final step. These sheet pile containment structures kept the debris from spreading out in the water. And of course they did a scan to make sure all the debris was picked up. It turns out one of those piers has become a great habitat for those endangered mussels, so you can see it still standing next to the new bridges. But other than that, there’s almost no sign that those old bridges were there at all.

I don’t just love bridges; I’m actually a licensed engineer, and part of that means every year I have to take a certain number of classes to stay current in the field. And that’s how I first heard of this job. Some of the engineers and contractors put the story together in a professional development class. So huge thanks to them for sharing so many interesting details of the project. And another thanks to Iowa DOT who answered our questions and shared photos and videos. I am so impressed with this bridge demolition. No shade at all to the new bridges, but honestly I think taking the old ones down was the coolest part of the entire replacement project. Bridge demolition is specialized, challenging work that takes a lot of engineering to get right. Hats off to the entire team on this project for getting it done so safely and efficiently.

Why Roads Get Washboards

[Note that this article is a transcript of the video embedded above.]

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.

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. [Break for real-time footage.] 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.

Fixing the Most Dangerous Dam in the World

[Note that this article is a transcript of the video embedded above.]

Mosul Dam rises 370 feet or 113 meters above the Tigris River in northern Iraq as one of the tallest dams in the Middle East. The dam was built in the 1980s, but, in a way, construction never really stopped. That’s because ever since the reservoir filled behind Mosul Dam, the ground has literally been dissolving, nonstop, below the structure. Almost immediately on filling, water started flowing through the foundation of the dam and back out on the downstream side. Just a year later, the volume of seepage was measured at 800 liters or about 200 gallons per second.

I usually hate to use the olympic-sized swimming pool equivalent, but in this case it makes sense because it was enough to fill one every hour of every day. And the issue is that, once a process like this gets started, it’s pretty hard to stop. So, for the past 40 years or so, the problem at Mosul Dam has been ongoing, scrutinized by some of the most preeminent engineers across the world and complicated by politics, bureaucracy, and, of course, armed conflict. Failure of a structure this large would be catastrophic; towns along the Tigris River would be fully wiped off the map, and some estimate that the breach wave would be so massive that even major parts of Baghdad, hundreds of miles downstream, would be submerged. In 2006, the US Army Corps of Engineers called it, unequivocally, “the most dangerous dam in the world.” That was 20 years ago, and Mosul Dam is still standing, in better shape than ever. And the story of how it got there is fascinating. I’m Grady, and this is Practical Engineering.

Mosul Dam is an earthen embankment dam not far from the City of Mosul in Iraq, built to generate hydropower and store water for irrigation and drinking. The hydro plant is on the west side of the dam with four turbine generators. You can see the massive surge tanks sticking up from the plant that absorb changes in pressure when the units are started and stopped. The dam has an outlet structure through the embankment here. It has a service spillway with radial gates here. And an auxiliary spillway with earthen fuse plugs here. Check out my videos on spillway gates and fuse plugs if you want to learn more about those types of structures after this.

The dam itself is impressive, but the rock that serves as its foundation is extremely complex, and in many ways, far from ideal. The geology of northern Iraq includes a lot of gypsum, a sedimentary rock that is widely used for things like fertilizer, plaster, and drywall. What it’s not widely used for is the foundations of dams. In fact, the consensus of experts involved on Mosul Dam throughout the years is that it was, all around, a terrible idea. One consulting group said that, quote, “the decision to locate such a major and important dam on the foundation rock mass which exists at the Mosul Dam site was fundamentally flawed.” That’s because of a critical property of gypsum, one that it doesn’t share with many other types of rock formations: it dissolves in water.

You might be familiar with limestone caves and karst geology, where water creates voids in the subsurface. Some of these can be quite dramatic like Carlsbad Caverns in New Mexico or Mammoth Cave in Kentucky. They’re formed because the limestone is just a tiny bit soluble in water, as long as it’s a bit acidic, which rainwater usually is. So over the course of millions of years, that water kind of carves away the earth from the inside. Gypsum, on the other hand, is roughly 200 times more soluble in water than limestone. It’s not quite like a spoonful of sugar or salt that dissolves almost instantly, but processes that usually take centuries in limestone are accelerated to human timescales in gypsum. And that’s especially true in the subsurface, because dissolution isn’t a linear process. More dissolving means more space for water which means more dissolving and so on. It’s a positive feedback loop.

Many dam failures have resulted from internal erosion, where water seeping through the soil or rock carries away particles, leaving voids. This process is what led to the demise of Teton Dam, which I covered in an earlier video. But where internal erosion can be combatted by designing filtration systems that catch waterborne particles before they escape the subsurface, you can’t easily filter dissolved gypsum out of seepage.

The designers of the dam knew the gypsum was going to be an issue, and they had a few ideas to address it. One was to install a blanket of bentonite clay lining the bottom of part of the reservoir. This would block seepage from flowing into the subsurface, at least in the dam’s immediate vicinity, lengthening the flow paths and thus reducing the total volume of the flow. However, the volume of material would be enormous, and the blanket layer would be fairly fragile to damage from boats or even strong currents. Another idea was to use a cutoff wall, basically a continuous subsurface diaphragm of some impervious material. The problem was that there were no machines that could trench deep enough to get below the worst of the gypsum. The idea they landed on was the same as at Teton Dam: a grout curtain.

Mosul Dam’s design included a continuous concrete tunnel running along the bottom of the structure. It had one purpose: to provide access to the dam’s foundation for drilling rigs and grout pumps. Political and schedule pressures pushed the government to finish the dam before the grouting was complete, but they knew they would have the access to the gallery tunnel to continue that process after the dam was in operation. Unfortunately, they underestimated how serious and complex a challenge they were setting themselves up to face.

As soon as the reservoir filled up, the problem became obvious. I mentioned the olympic swimming pools of seepage in the intro, but it wasn’t just that. Sinkholes opened up downstream of the dam as caverns formed in the geology below causing the surface to collapse. As time went on, those sinkholes started appearing closer to the dam, an aboveground hint at how the solution cavities were migrating in the subsurface. Essentially since its construction, operators have maintained a continuous grouting program, injecting a mixture of sand, cement, bentonite, and water into the rock below through drilled holes to try and plug up the voids. It’s basically a nonstop race between logistics and chemistry, because grout doesn’t fare well in flowing water and the foundation rock is constantly dissolving.

Recognizing the hazard they had created in the 1980s, the Iraqi government came up with a backup solution. Since it was clear that there really was no permanent fix for Mosul Dam, they would just build another dam downstream that would capture the flood if (and maybe when) Mosul Dam failed. Badush Dam started construction in the late 1980s. It would have a hydropower plant and store water for irrigation, but also include a huge empty storage pool to protect downstream cities from a breach of Mosul Dam. The project got about halfway finished before the geopolitical situation in Iraq ground it to a halt.

In 2003, a US-led coalition invaded Iraq as part of a larger war on terror in response to the September 11th attacks. As a major piece of infrastructure in the country, Mosul Dam had the coalition worried. Some early reports hinted that Iraqi forces might detonate the structure as an act of sabotage. But it didn’t take long to realize that the dam might fail on its own accord. They started coordinating with the US Army Corps of Engineers to assess the structure, whose report concluded that the risk was astronomical. That’s the source of the “most dangerous dam in the world” quote that has plagued the structure ever since. The truth is that the “danger” of a dam is a pretty complicated thing to characterize, and it’s not a statistic that’s widely tracked, especially at a global scale. But the fact that a government agency was willing to say it means a lot. And Iraq’s Ministry of Water Resources took the situation seriously and started working with a panel of experts to review the conditions of the dam. That panel largely came to the same conclusion: Mosul Dam needed serious help.

Coalition forces had bases and equipment along the Tigris River. The situation was concerning enough that they decided to move everything out of the potential inundation area if the dam were to breach. At the same time, a major part of the war effort was helping the new Iraqi government shore up the country’s infrastructure, including improving the grouting program at Mosul Dam. Even though it was really only considered a temporary solution, the consensus seemed to be that it was the only feasible way to address the foundation problems beyond the stalled Badush Dam project downstream.

Initial efforts by the US government to help at Mosul Dam turned into somewhat of a disaster. A few notable examples: The winning contractor for the grout plants submitted a concrete (not grout) mixing plant design, and somehow the review committee didn’t notice, despite it being printed on the front page of the submittal. By the time someone realized it, the concrete plants had already been delivered, and the US government had to pay the contractor to try and convert them into grout mixing plants. The material silos were poorly designed, with no ladders or braces. Some weren’t even bolted to the foundation. The loading ramp for the hoppers had no retaining walls, causing the slopes to slough off. Drills and pumping equipment couldn’t even fit into the grouting galleries below the dam. And the dam operations staff meant to run all this new high-tech equipment had only received a few weeks of training. The oversight report about the project was scathing. Millions of dollars had been spent on 21 contracts for almost no benefit to the dam.

Coalition forces continued efforts to improve the situation at Mosul Dam, but by 2010, the US was withdrawing troops from the country and handing off the reconstruction projects back to the Iraqi government. Unfortunately, that handoff was only temporary, as sectarian violence continued to plague the region. In mid-2014, the Islamic State (also known as ISIS, ISIL, and Daesh) took over several cities in Northern Iraq, disrupting the supplies of materials to Mosul Dam, which was still relying on nearly 24/7 grouting operations to keep the structure safe. That August, ISIS seized control of Mosul Dam, sparking new fears that the structure would collapse. For more than a week, the dam was out of the hands of the Iraqi government, and no one knew what the militants might do (or what they might not do). It was the same situation as before: Even short-term neglect presented a serious safety risk. Fortunately, the dam was recaptured by Kurdish and Iraqi forces, with the help of US air support, 8 days later. The dam was back in Iraqi hands, but the surrounding areas weren’t. With equipment looted during the brief seizure, the disruption of the workforce at the dam, and without regular shipments of cement, the grouting operation wasn’t being maintained. Equipment installed during the Iraq war wasn’t being used. Voids were going untreated, and concerns about the dam’s failure continued to grow.

Realizing that the Iraqi government was too fractured to manage the situation alone, the US decided to stay involved as Mosul Dam’s de facto engineer. In 2015, the Army Corps of Engineers led a task force to assess the condition of the dam, and the results were alarming. The US Embassy released a fact sheet based on their findings, saying that the dam had an “unprecedented risk of catastrophic failure” endangering between half-a-million and 1.5 million people along the Tigris River. A collapse would be a humanitarian crisis unlike almost anything in modern history. The situation was further complicated by the ongoing occupation by the Islamic State, making it difficult or impossible for residents to be able to evacuate to safer areas. Electrical blackouts, lack of government coordination, and poor communication would make things even worse in the event of failure.

The Iraqi government tried to downplay the alarm a bit. In an interview on TV, the Minister of Water Resources said, quote, “The looming danger to Mosul Dam is one in a thousand. This risk level is present in all the world’s dams.” I don’t know if he made that number up, or if it was actually supported by some kind of analysis, but anyone involved in risk management would find it hilarious if it weren’t such a serious situation. Assuming that’s an annual probability, which is what we normally use, and multiplying it by the consequences of failure estimated by the Corps of Engineers, you get an expected annual fatality rate of 500 to 1500 people. Nowhere in the world would anybody consider that acceptable. This is a graph often used to communicate tolerable risks on large dam projects. This green area generally means there’s not a lot of justification for making a structure safer. Yellow, you have to be more thoughtful. Red means unacceptable. Taking the minister’s estimate of probability, and the embassy's estimates of fatalities at face value, Mosul Dam would plot somewhere around here on the chart. That “most dangerous dam in the world” moniker doesn’t seem like hyperbole when you look at it like that. To quote Lieutenant-General Sean MacFarland, “If this dam were in the United States, we would have drained the lake behind it.”

The urgency finally spurred action in 2016. Iraq awarded a contract to an Italian company to rehabilitate the structure, including a massive operation to expand the foundation grouting program. It was one of the most unique civil engineering projects on the globe, with participation from the Iraqi government, the US (through the Corps of Engineers), the Italian military, and a number of international consultants. I actually talked with a few of the engineers involved on the project, and some of their stories are pretty wild. In the early days of the project, they were inserting engineers at night, by helicopter, to support the Iraqis who were operating the dam and install equipment that would let them monitor the situation remotely while ISIS was operating only a short distance away.

The entire project had to happen near the front lines as the conflict with the Islamic State continued to unfold in Iraq. Security forces were needed for the entire duration to protect the dam and supply routes for materials and equipment. That took some time to get set up, but eventually, the project team was able to establish a permanent camp at the dam. Over the next few years, all the grouting infrastructure, including batch plants, piping, electrical systems and drill rigs were replaced with modern equipment. Crews drilled more than 5,000 boreholes with a total length of drilling at more than 400 kilometers or 250 miles. 41,000 cubic meters (50,000 cubic yards) of grout were injected into the foundation along the entire length of the dam. Generally the way it works is this: you can inflate a rubber device called a packer using air or hydraulic pressure, creating a seal between the borehole and injection pipe. Or you just grout the injection pipe directly into the borehole. Then you can pump grout at very high pressure into the borehole, forcing it into voids, cracks, fissures. You just keep pumping until you reach a refusal criterion, a certain maximum pressure that you hold until the grout stops flowing. And you just keep doing it over and over and over.

All this work was done using a sophisticated computer system to keep track of pressure, depth, mix design, flow rate, and quantity of grout for every borehole, allowing the team to track progress, identify issues, and visualize the performance of the operation. From material delivery to batching to drilling and injection, every step of the process became a data point.

I love unique measurement units, and this project had a good one: As a quality control test, the contractor would try to inject water into the foundation rock after it was grouted up. A Lugeon is the loss of water of one liter per minute per meter of borehole length at an overpressure of 1 megapascal or about 145 psi. For all the permeability tests performed for the project, 98 percent had values below 3 Lugeons, a massive improvement over the conditions beforehand.

The project finished in 2019. It was a 3-year effort that cost more than half-a-billion dollars, but Mosul Dam lost its most dangerous dam title as a result. By all accounts, the dam is in a much less precarious position. The project won an award from the Deep Foundations Institute in 2022, highlighting the complexity and the danger of the work.

But this wasn’t like a typical construction project, because the work isn’t over. The goal was to get the Iraqi government set up to continue the process of maintenance grouting. The rock below Mosul Dam may have a lot more grout than it used to, but the gypsum is still soluble, and there’s still a massive reservoir constantly trying to push water through it. A major part of the rehabilitation project was training Iraqi staff to continue the fight. In that way, despite its magnitude, the project was sort of a half-a-billion-dollar bandaid. The grouting has never been considered a permanent solution, and even though this project resulted in an enormous improvement in the long-term prospects of the structure, it’s still a major, ongoing obligation.

Iraq is still planning for a more permanent fix. You can still see the half-finished Badush Dam on the map, downstream from Mosul, and finishing the job is still on the table if anyone can figure out how to come up with the billions of dollars it would take. Another option is that deep foundation cutoff wall considered during the original design. It would provide a continuous barrier for seepage passing through the porous rock below the dam. These are used on a lot of dams across the world, but it’s never been done on the scale and depth as would be required at Mosul. In 2018, the estimated cost for a cutoff was between 3 and 5 billion dollars, an almost unimaginable investment into a dam that already exists and functions today. Whether the electricity and water from Mosul Dam is even worth that scale of capital is something that will probably take a long time to decide. Until then, the government will keep pumping grout and Dinars into the rocks below in the nonstop race against a flawed foundation, but now with much more confidence that they can keep up the pace.