---
title: 'Fixing the Most Dangerous Dam in the World'
source: 'https://youtube.com/watch?v=DWLcmq_DiaA'
video_id: 'DWLcmq_DiaA'
date: 2026-06-28
duration_sec: 1343
---

# Fixing the Most Dangerous Dam in the World

> Source: [Fixing the Most Dangerous Dam in the World](https://youtube.com/watch?v=DWLcmq_DiaA)

## Summary

Mosul Dam, one of the tallest dams in the Middle East, has been plagued by a critical design flaw since its construction in the 1980s: it was built on a foundation of gypsum, a rock that dissolves in water. This has led to continuous seepage and the formation of subsurface voids, necessitating a constant, multi-decade grouting effort to prevent catastrophic failure. The video by Practical Engineering details the engineering, political, and conflict-driven challenges of maintaining this 'most dangerous dam in the world.'

### Key Points

- **Mosul Dam: A flawed foundation** [0:01] — Mosul Dam is an earthen embankment dam built on gypsum rock, which is 200 times more soluble in water than limestone. This causes the foundation to dissolve, leading to continuous seepage and sinkholes.
- **Immediate seepage after filling** [0:29] — Soon after the reservoir filled, water began seeping through the foundation at a rate of 800 liters per second (about 200 gallons per second), enough to fill an Olympic-sized swimming pool every hour.
- **Labeled 'most dangerous dam in the world'** [1:27] — In 2006, the US Army Corps of Engineers called Mosul Dam “the most dangerous dam in the world” due to the high risk of catastrophic failure, which could submerge cities along the Tigris River, including parts of Baghdad.
- **Terrible site selection** [2:32] — The dam's foundation is composed largely of gypsum, a sedimentary rock that dissolves in water. Experts agree that locating such a major dam on this rock was 'fundamentally flawed,' as dissolution creates a positive feedback loop of more voids and more seepage.
- **Grout curtain as primary defense** [4:49] — The designers originally planned a grout curtain to seal the foundation. A concrete gallery tunnel allows crews to drill and inject grout (a mixture of sand, cement, bentonite, and water) into the rock to fill voids, but this must be done continuously because the gypsum keeps dissolving.
- **Plan B: Badush Dam never finished** [7:10] — A second dam, Badush Dam, was started in the 1980s to capture the flood wave if Mosul Dam failed, but construction was halted by geopolitical issues and remains unfinished.
- **2003 Iraq War and coalition efforts** [10:31] — After the 2003 invasion, the US Army Corps of Engineers assessed the dam, confirmed the extreme risk, and moved coalition assets out of the potential flood zone. Initial US-funded grouting contracts were plagued by errors (e.g., wrong equipment delivered).
- **ISIS seizure and renewed crisis** [12:04] — In 2014, ISIS seized the dam, disrupting grouting operations. The dam was recaptured 8 days later, but the damage (looted equipment, missing cement shipments) compounded the risk. The US Embassy warned of 'unprecedented risk of catastrophic failure' endangering 0.5-1.5 million people.
- **Major rehab project (2016-2019)** [14:24] — Iraq contracted an Italian company to rehabilitate the dam. Over 3 years, crews drilled 5,000 boreholes (250 miles of drilling) and injected 50,000 cubic yards of grout. A sophisticated computer system tracked every borehole, improving foundation permeability significantly. The project cost over $500 million.
- **Ongoing maintenance and future solutions** [18:46] — The rehabilitation was a massive but temporary fix. The Iraqi government continues maintenance grouting. Permanent solutions (finishing Badush Dam or building a deep cutoff wall) cost billions and remain uncertain. The dam is safer but still requires constant vigilance.

### Conclusion

Mosul Dam's story is a cautionary tale about engineering choices that force a never-ending, resource-intensive battle against a flawed foundation. While the massive 2019 rehabilitation project significantly reduced the risk of catastrophic failure, the dam remains a permanent obligation requiring continuous grouting and billions in potential future investment for a permanent fix.

## Transcript

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.
One of the trickiest  parts of Mosul Dam is that you can’t just see what the subsurface looks like. The Army Corps  
of Engineers did a really detailed investigation, 
but even then, a lot of it is guesswork based  
on very limited observations from individual 
boreholes scattered across the site. This is  
a challenge for all kinds of engineering projects 
too: understanding the things we can’t easily see.  
My friend Brian at the Real Engineering 
channel has a solution in his new series,  
“The Anatomy of.” He’s putting everyday 
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