[0:01] Mosul Dam rises 370 feet or 113 meters above 
the Tigris River in northern Iraq as one of  
[0:09] the tallest dams in the Middle East. The dam was 
built in the 1980s, but, in a way, construction  
[0:16] never really stopped. That’s because ever since 
the reservoir filled behind Mosul Dam, the ground  
[0:22] has literally been dissolving, nonstop, below 
the structure. Almost immediately on filling,  
[0:29] water started flowing through the foundation of 
the dam and back out on the downstream side. Just  
[0:35] a year later, the volume of seepage was measured 
at 800 liters or about 200 gallons per second.
[0:42] I usually hate to use the olympic-sized 
swimming pool equivalent, but in this case  
[0:46] it makes sense because it was enough 
to fill one every hour of every day.  
[0:52] And the issue is that, once a process like this 
gets started, it’s pretty hard to stop. So,  
[0:58] for the past 40 years or so, the problem at 
Mosul Dam has been ongoing, scrutinized by  
[1:04] some of the most preeminent engineers across the 
world and complicated by politics, bureaucracy,  
[1:09] and, of course, armed conflict. Failure of a 
structure this large would be catastrophic;  
[1:15] towns along the Tigris River would be fully wiped 
off the map, and some estimate that the breach  
[1:20] wave would be so massive that even major parts of 
Baghdad, hundreds of miles downstream, would be  
[1:27] submerged. In 2006, the US Army Corps of Engineers 
called it, unequivocally, “the most dangerous dam  
[1:34] in the world.” That was 20 years ago, and Mosul 
Dam is still standing, in better shape than ever.  
[1:40] And the story of how it got there is fascinating. 
I’m Grady, and this is Practical Engineering.
[1:55] Mosul Dam is an earthen embankment dam 
not far from the City of Mosul in Iraq,  
[2:00] built to generate hydropower and store water 
for irrigation and drinking. The hydro plant  
[2:05] is on the west side of the dam with four turbine 
generators. You can see the massive surge tanks  
[2:11] sticking up from the plant that absorb changes in 
pressure when the units are started and stopped.  
[2:16] The dam has an outlet structure through the 
embankment here. It has a service spillway with  
[2:21] radial gates here. And an auxiliary spillway with 
earthen fuse plugs here. Check out my videos on  
[2:27] spillway gates and fuse plugs if you want to learn 
more about those types of structures after this.
[2:32] The dam itself is impressive, but the rock that 
serves as its foundation is extremely complex,  
[2:38] and in many ways, far from ideal. The geology 
of northern Iraq includes a lot of gypsum,  
[2:45] a sedimentary rock that is widely used 
for things like fertilizer, plaster,  
[2:49] and drywall. What it’s not widely used 
for is the foundations of dams. In fact,  
[2:55] the consensus of experts involved on Mosul 
Dam throughout the years is that it was,  
[2:59] all around, a terrible idea. One 
consulting group said that, quote,  
[3:04] “the decision to locate such a major and important 
dam on the foundation rock mass which exists at  
[3:10] the Mosul Dam site was fundamentally flawed.” 
[3:15] That’s because of a critical property of gypsum,  
[3:18] one that it doesn’t share with many other types 
of rock formations: it dissolves in water.
[3:24] You might be familiar with limestone caves and 
karst geology, where water creates voids in the  
[3:30] subsurface. Some of these can be quite dramatic 
like Carlsbad Caverns in New Mexico or Mammoth  
[3:36] Cave in Kentucky. They’re formed because the 
limestone is just a tiny bit soluble in water,  
[3:42] as long as it’s a bit acidic, which rainwater 
usually is. So over the course of millions of  
[3:48] years, that water kind of carves away the earth 
from the inside. Gypsum, on the other hand,  
[3:54] is roughly 200 times more soluble in water than 
limestone. It’s not quite like a spoonful of  
[4:00] sugar or salt that dissolves almost instantly, but 
processes that usually take centuries in limestone  
[4:07] are accelerated to human timescales in gypsum. 
And that’s especially true in the subsurface,  
[4:13] because dissolution isn’t a linear process. 
More dissolving means more space for water  
[4:19] which means more dissolving and so 
on. It’s a positive feedback loop.
[4:23] Many dam failures have resulted from internal 
erosion, where water seeping through the soil or  
[4:28] rock carries away particles, leaving voids. This 
process is what led to the demise of Teton Dam,  
[4:34] which I covered in an earlier video. But 
where internal erosion can be combatted  
[4:39] by designing filtration systems that 
catch waterborne particles before they  
[4:44] escape the subsurface, you can’t easily 
filter dissolved gypsum out of seepage.
[4:49] The designers of the dam knew the 
gypsum was going to be an issue,  
[4:53] and they had a few ideas to address it. One was 
to install a blanket of bentonite clay lining  
[4:58] the bottom of part of the reservoir. This would 
block seepage from flowing into the subsurface,  
[5:03] at least in the dam’s immediate vicinity, 
lengthening the flow paths and thus reducing  
[5:08] the total volume of the flow. However, the volume 
of material would be enormous, and the blanket  
[5:14] layer would be fairly fragile to damage from 
boats or even strong currents. Another idea  
[5:19] was to use a cutoff wall, basically a continuous 
subsurface diaphragm of some impervious material.  
[5:26] The problem was that there were no machines that 
could trench deep enough to get below the worst  
[5:31] of the gypsum. The idea they landed on was 
the same as at Teton Dam: a grout curtain.
[5:37] Mosul Dam’s design included a continuous concrete 
tunnel running along the bottom of the structure.  
[5:43] It had one purpose: to provide access to the 
dam’s foundation for drilling rigs and grout  
[5:48] pumps. Political and schedule pressures 
pushed the government to finish the dam  
[5:53] before the grouting was complete, but 
they knew they would have the access  
[5:56] to the gallery tunnel to continue that process 
after the dam was in operation. Unfortunately,  
[6:02] they underestimated how serious and complex a 
challenge they were setting themselves up to face.
[6:08] As soon as the reservoir filled up, the problem 
became obvious. I mentioned the olympic swimming  
[6:13] pools of seepage in the intro, but it wasn’t 
just that. Sinkholes opened up downstream of  
[6:19] the dam as caverns formed in the geology below 
causing the surface to collapse. As time went on,  
[6:25] those sinkholes started appearing closer 
to the dam, an aboveground hint at how  
[6:30] the solution cavities were migrating in the 
subsurface. Essentially since its construction,  
[6:36] operators have maintained a continuous grouting 
program, injecting a mixture of sand, cement,  
[6:42] bentonite, and water into the rock below 
through drilled holes to try and plug up  
[6:47] the voids. It’s basically a nonstop 
race between logistics and chemistry,  
[6:53] because grout doesn’t fare well in flowing water 
and the foundation rock is constantly dissolving.
[6:59] Recognizing the hazard they had created 
in the 1980s, the Iraqi government came up  
[7:04] with a backup solution. Since it was clear that 
there really was no permanent fix for Mosul Dam,  
[7:10] they would just build another dam downstream 
that would capture the flood if (and maybe  
[7:16] when) Mosul Dam failed. Badush Dam 
started construction in the late 1980s.  
[7:22] It would have a hydropower plant and store water 
for irrigation, but also include a huge empty  
[7:28] storage pool to protect downstream cities from 
a breach of Mosul Dam. The project got about  
[7:35] halfway finished before the geopolitical 
situation in Iraq ground it to a halt.
[7:43] In 2003, a US-led coalition invaded Iraq as 
part of a larger war on terror in response  
[7:50] to the September 11th attacks. As a major piece 
of infrastructure in the country, Mosul Dam had  
[7:56] the coalition worried. Some early reports hinted 
that Iraqi forces might detonate the structure  
[8:02] as an act of sabotage. But it didn’t take long to 
realize that the dam might fail on its own accord.  
[8:09] They started coordinating with the US Army 
Corps of Engineers to assess the structure,  
[8:14] whose report concluded that the risk 
was astronomical. That’s the source of  
[8:18] the “most dangerous dam in the world” quote 
that has plagued the structure ever since.  
[8:23] The truth is that the “danger” of a dam is 
a pretty complicated thing to characterize,  
[8:28] and it’s not a statistic that’s widely tracked, 
especially at a global scale. But the fact that  
[8:33] a government agency was willing to say it means a 
lot. And Iraq’s Ministry of Water Resources took  
[8:39] the situation seriously and started working with 
a panel of experts to review the conditions of  
[8:45] the dam. That panel largely came to the same 
conclusion: Mosul Dam needed serious help.
[8:52] Coalition forces had bases and equipment 
along the Tigris River. The situation was  
[8:57] concerning enough that they decided to 
move everything out of the potential  
[9:01] inundation area if the dam were to breach. At 
the same time, a major part of the war effort  
[9:07] was helping the new Iraqi government 
shore up the country’s infrastructure,  
[9:11] including improving the grouting program at 
Mosul Dam. Even though it was really only  
[9:16] considered a temporary solution, the consensus 
seemed to be that it was the only feasible way  
[9:22] to address the foundation problems beyond 
the stalled Badush Dam project downstream.
[9:27] Initial efforts by the US government to help at 
Mosul Dam turned into somewhat of a disaster.  
[9:33] A few notable examples: The winning contractor 
for the grout plants submitted a concrete (not  
[9:39] grout) mixing plant design, and somehow 
the review committee didn’t notice,  
[9:44] despite it being printed on the front page of 
the submittal. By the time someone realized it,  
[9:48] the concrete plants had already been delivered, 
and the US government had to pay the contractor  
[9:53] to try and convert them into grout mixing 
plants. The material silos were poorly designed,  
[9:59] with no ladders or braces. Some weren’t even 
bolted to the foundation. The loading ramp  
[10:05] for the hoppers had no retaining walls, 
causing the slopes to slough off. Drills  
[10:10] and pumping equipment couldn’t even fit into 
the grouting galleries below the dam. And the  
[10:15] dam operations staff meant to run all this 
new high-tech equipment had only received  
[10:20] a few weeks of training. The oversight 
report about the project was scathing.  
[10:25] Millions of dollars had been spent on 21 
contracts for almost no benefit to the dam.
[10:31] Coalition forces continued efforts to improve 
the situation at Mosul Dam, but by 2010,  
[10:37] the US was withdrawing troops from the country 
and handing off the reconstruction projects  
[10:43] back to the Iraqi government. Unfortunately, that 
handoff was only temporary, as sectarian violence  
[10:49] continued to plague the region. In mid-2014, 
the Islamic State (also known as ISIS, ISIL,  
[10:56] and Daesh) took over several cities in Northern 
Iraq, disrupting the supplies of materials to  
[11:03] Mosul Dam, which was still relying on nearly 
24/7 grouting operations to keep the structure  
[11:09] safe. That August, ISIS seized control of Mosul 
Dam, sparking new fears that the structure would  
[11:16] collapse. For more than a week, the dam was out of 
the hands of the Iraqi government, and no one knew  
[11:22] what the militants might do (or what they might 
not do). It was the same situation as before:  
[11:28] Even short-term neglect presented a serious 
safety risk. Fortunately, the dam was recaptured  
[11:34] by Kurdish and Iraqi forces, with the help of US 
air support, 8 days later. The dam was back in  
[11:40] Iraqi hands, but the surrounding areas weren’t. 
With equipment looted during the brief seizure,  
[11:47] the disruption of the workforce at the dam, and 
without regular shipments of cement, the grouting  
[11:52] operation wasn’t being maintained. Equipment 
installed during the Iraq war wasn’t being  
[11:58] used. Voids were going untreated, and concerns 
about the dam’s failure continued to grow.
[12:04] Realizing that the Iraqi government was too 
fractured to manage the situation alone,  
[12:09] the US decided to stay involved as 
Mosul Dam’s de facto engineer. In 2015,  
[12:15] the Army Corps of Engineers led a task 
force to assess the condition of the dam,  
[12:19] and the results were alarming. The US Embassy 
released a fact sheet based on their findings,  
[12:25] saying that the dam had an “unprecedented risk 
of catastrophic failure” endangering between  
[12:30] half-a-million and 1.5 million people along the 
Tigris River. A collapse would be a humanitarian  
[12:37] crisis unlike almost anything in modern history. 
The situation was further complicated by the  
[12:43] ongoing occupation by the Islamic State, making 
it difficult or impossible for residents to be  
[12:49] able to evacuate to safer areas. Electrical 
blackouts, lack of government coordination,  
[12:55] and poor communication would make things 
even worse in the event of failure.
[13:00] The Iraqi government tried to downplay the alarm a 
bit. In an interview on TV, the Minister of Water  
[13:06] Resources said, quote, “The looming danger to 
Mosul Dam is one in a thousand. This risk level  
[13:12] is present in all the world’s dams.” I don’t 
know if he made that number up, or if it was  
[13:17] actually supported by some kind of analysis, but 
anyone involved in risk management would find it  
[13:23] hilarious if it weren’t such a serious situation. 
Assuming that’s an annual probability, which is  
[13:29] what we normally use, and multiplying it by the 
consequences of failure estimated by the Corps  
[13:34] of Engineers, you get an expected annual fatality 
rate of 500 to 1500 people. Nowhere in the world  
[13:43] would anybody consider that acceptable. This is a 
graph often used to communicate tolerable risks on  
[13:49] large dam projects. This green area generally 
means there’s not a lot of justification for  
[13:55] making a structure safer. Yellow, you have to be 
more thoughtful. Red means unacceptable. Taking  
[14:01] the minister’s estimate of probability, and the 
embassy's estimates of fatalities at face value,  
[14:07] Mosul Dam would plot somewhere around here on 
the chart. That “most dangerous dam in the world”  
[14:13] moniker doesn’t seem like hyperbole when you 
look at it like that. To quote Lieutenant-General  
[14:18] Sean MacFarland, “If this dam were in the United 
States, we would have drained the lake behind it.”
[14:24] The urgency finally spurred action in 2016. 
[14:29] Iraq awarded a contract to an Italian company  
[14:32] to rehabilitate the structure, including a 
massive operation to expand the foundation  
[14:38] grouting program. It was one of the most 
unique civil engineering projects on the globe,  
[14:43] with participation from the Iraqi government, 
the US (through the Corps of Engineers), the  
[14:48] Italian military, and a number of international 
consultants. I actually talked with a few of the  
[14:54] engineers involved on the project, and 
some of their stories are pretty wild.  
[14:58] In the early days of the project, they were 
inserting engineers at night, by helicopter,  
[15:03] to support the Iraqis who were operating 
the dam and install equipment that would  
[15:08] let them monitor the situation remotely while 
ISIS was operating only a short distance away.
[15:14] The entire project had to happen near the front 
lines as the conflict with the Islamic State  
[15:20] continued to unfold in Iraq. Security forces 
were needed for the entire duration to protect  
[15:25] the dam and supply routes for materials and 
equipment. That took some time to get set up,  
[15:31] but eventually, the project team was able 
to establish a permanent camp at the dam.  
[15:35] Over the next few years, all the grouting 
infrastructure, including batch plants,  
[15:40] piping, electrical systems and drill 
rigs were replaced with modern equipment.  
[15:45] Crews drilled more than 5,000 boreholes with 
a total length of drilling at more than 400  
[15:51] kilometers or 250 miles. 41,000 cubic meters 
(50,000 cubic yards) of grout were injected  
[15:59] into the foundation along the entire length of the 
dam. Generally the way it works is this: you can  
[16:05] inflate a rubber device called a packer using air 
or hydraulic pressure, creating a seal between the  
[16:11] borehole and injection pipe. Or you just grout 
the injection pipe directly into the borehole.  
[16:17] Then you can pump grout at very high pressure 
into the borehole, forcing it into voids, cracks,  
[16:23] fissures. You just keep pumping until you reach 
a refusal criterion, a certain maximum pressure  
[16:29] that you hold until the grout stops flowing. And 
you just keep doing it over and over and over.
[16:35] All this work was done using a sophisticated 
computer system to keep track of pressure,  
[16:41] depth, mix design, flow rate, and quantity of 
grout for every borehole, allowing the team to  
[16:46] track progress, identify issues, and visualize 
the performance of the operation. From material  
[16:53] delivery to batching to drilling and injection, 
every step of the process became a data point.
[16:59] I love unique measurement units, and this project 
had a good one: As a quality control test,  
[17:05] the contractor would try to inject water into the 
foundation rock after it was grouted up. A Lugeon  
[17:11] is the loss of water of one liter per minute 
per meter of borehole length at an overpressure  
[17:18] of 1 megapascal or about 145 psi. For all the 
permeability tests performed for the project,  
[17:26] 98 percent had values below 3 Lugeons, a massive 
improvement over the conditions beforehand.
[17:34] The project finished in 2019. It was a 3-year 
effort that cost more than half-a-billion dollars,  
[17:40] but Mosul Dam lost its most dangerous 
dam title as a result. By all accounts,  
[17:45] the dam is in a much less precarious position. 
The project won an award from the Deep  
[17:51] Foundations Institute in 2022, highlighting 
the complexity and the danger of the work.
[17:57] But this wasn’t like a typical construction 
project, because the work isn’t over. The  
[18:02] goal was to get the Iraqi government set up to 
continue the process of maintenance grouting.  
[18:08] The rock below Mosul Dam may have a lot more grout 
than it used to, but the gypsum is still soluble,  
[18:15] and there’s still a massive reservoir constantly 
trying to push water through it. A major part of  
[18:20] the rehabilitation project was training Iraqi 
staff to continue the fight. In that way,  
[18:27] despite its magnitude, the project was sort of a 
half-a-billion-dollar bandaid. The grouting has  
[18:33] never been considered a permanent solution, and 
even though this project resulted in an enormous  
[18:39] improvement in the long-term prospects of the 
structure, it’s still a major, ongoing obligation.
[18:46] Iraq is still planning for a more permanent 
fix. You can still see the half-finished  
[18:51] Badush Dam on the map, downstream from 
Mosul, and finishing the job is still on  
[18:56] the table if anyone can figure out how to come 
up with the billions of dollars it would take.  
[19:00] Another option is that deep foundation cutoff 
wall considered during the original design. It  
[19:06] would provide a continuous barrier for seepage 
passing through the porous rock below the dam.  
[19:11] These are used on a lot of dams across the 
world, but it’s never been done on the scale  
[19:16] and depth as would be required at Mosul. In 2018, 
the estimated cost for a cutoff was between 3 and  
[19:23] 5 billion dollars, an almost unimaginable 
investment into a dam that already exists  
[19:29] and functions today. Whether the electricity and 
water from Mosul Dam is even worth that scale of  
[19:36] capital is something that will probably 
take a long time to decide. Until then,  
[19:41] the government will keep pumping grout and Dinars 
into the rocks below in the nonstop race against  
[19:47] a flawed foundation, but now with much more 
confidence that they can keep up the pace.
[19:54] One of the trickiest  parts of Mosul Dam is that you can’t just see what the subsurface looks like. The Army Corps  
[20:01] of Engineers did a really detailed investigation, 
but even then, a lot of it is guesswork based  
[20:06] on very limited observations from individual 
boreholes scattered across the site. This is  
[20:12] a challenge for all kinds of engineering projects 
too: understanding the things we can’t easily see.  
[20:19] My friend Brian at the Real Engineering 
channel has a solution in his new series,  
[20:24] “The Anatomy of.” He’s putting everyday 
objects and devices into a CT scanner,  
[20:29] so we can literally see inside. This is 
such a cool exploration of what makes  
[20:35] up our favorite gadgets, and if you want to 
check it out, it’s only available on Nebula.
[20:40] You probably know about Nebula now, even if 
you’re not subscribed. It’s a streaming service  
[20:44] built by and for independent creators. No studio 
executives deciding what gets the green light, no  
[20:51] advertisements driving the content into a single 
style. It’s just independent creators making stuff  
[20:57] they’re excited about with as few barriers and 
distractions as possible between you and us.
[21:03] My videos go live on Nebula before they come 
out here, and my Practical Construction series  
[21:08] was specifically produced for Nebula viewers who 
want to see deeper dives into specific topics.  
[21:14] I know there are a lot of streaming platforms 
out there right now, and no one wants another  
[21:18] monthly cost to keep track of, but I also know 
that if you’re watching a show like this to end,  
[21:23] there is a ton of other stuff on Nebula that 
you’re going to enjoy as well. So I’ve made  
[21:28] it dead simple: click the link below and you’ll 
get 50% off an annual plan. Pay just one time,  
[21:35] 30 dollars, for an entire year’s access 
at go.nebula.tv/practical-engineering. Or  
[21:42] if you have subscription fatigue, but 
still want to support what I’m doing,  
[21:45] you can get a lifetime membership. Pay once 
and have access for as long as you and Nebula  
[21:50] last. If you’re with me that independent 
creators are the future of great video,  
[21:55] I hope you’ll consider subscribing. Thank you 
for watching, and let me know what you think!