Original subtitles

[narrator] High in the Colorado Rockies, an elite team of engineers is

taking on a seemingly impossible challenge.

This is gonna be one for the record books

and the resume.

They're racing to build an extraordinary new mega dam.

We're under a lot of pressure to get this done on schedule.

The clock is ticking.

A chronic drought puts the water supply

for millions of people at risk.

If we don't do something now, we will be too late.

To succeed, they must safely fuse

70-year-old construction methods

with cutting-edge engineering.

This mega dam

ranks in a league of ambitious new engineering wonders that

are bigger, faster, taller, and more advanced

than anything ever constructed before.

This is the inside story of the extraordinary challenge of

building these giants.

Colorado is facing a crisis.

It has long relied on melting mountain snowpack

for its water supply.

Climate change is reducing snowfall and shortening

the melt season.

This means more water runs off

before it can be stored in reservoirs.

Colorado must capture a greater amount of

the dwindling snowmelt.

If these efforts fail, reports suggest the state

could run out of water within 25 years.

To meet this challenge, engineers in Colorado are now

working on a series of megastructures to boost

the state's reserves of water.

The Gross Dam was built in the 1950s

to help supply the growing population

of Colorado's state capital, Denver.

Constructed from over 1,400 concrete blocks,

it rose to an impressive 335 feet

and was designed to hold back

over 13 billion gallons of water.

But that's not enough to meet the demand of the 21st century.

The plan today is to triple its capacity.

Jeff Martin leads the team tasked with making it happen.

[Jeff] The original Gross Dam is just an amazing structure.

Just, unfortunately, now, it doesn't hold

as much water as we need.

So, we're gonna solve our problem,

by raising the Gross Dam

to create more water storage behind it.

Engineers must build a completely new dam on top of

the old one.

To succeed, they must beat extreme weather,

tough geology, and a tight deadline

set by the Federal government.

The engineers will build up the height of the dam

by a third to a colossal 471 feet.

They will double the length of the crest of the dam

to 2,040 feet,

and build a brand-new spillway to release surplus water.

The team will pour over 21 million cubic feet of

special concrete to make the wall's 118 steps,

and expand the width of the dam's base to 300 feet.

The new gross dam will be the tallest dam in Colorado

and hold back a staggering 38 billion gallons of water.

This megaproject transforms the structure from

a gravity dam into an arched dam.

This will be the first time in the world that engineers

have attempted to do this.

Engineer of record, Felipe Garcia, and his team

have been brought on to make it happen.

So, the original Gross Dam was designed as

a curved gravity structure.

What I mean by a curved gravity dam is that it has enough

weight to maintain stability

against the pressure of the reservoir.

But relying on their sheer mass to hold back a reservoir

makes gravity dams expensive and inefficient.

A arch dam achieves its stable factor of safety by

transferring the loads from

the reservoir laterally towards the abutment.

And in turn, you're able to make the arch thinner.

Not only that reduces cost and schedule, but it also

provides a more sustainable project.

Engineering teams must build solid foundations on both sides

of the canyon to support

the greater width of the new dam.

But the geology in this part of Colorado presents

a big problem.

What we have here

is pretty typical of about 90% of the foundation

of we have out there at Gross.

It is very strong rock, but from this particular slope,

this material is highly weathered.

You see? I -- I can just peel it

with my hand -- that means that it doesn't have

enough stiffness to support the loads from the dam.

So, this material would need to be removed.

The teams use a fleet of excavators to scrape away

the loose weathered rock.

Next, the team pours nearly 705,000 cubic feet of concrete

to build a super strong and perfectly level foundation

that bonds tightly to the bedrock.

It takes around 20 months

to prepare the canyon walls for the new dam.

The engineers' next task is to prepare the concrete of

the old dam to make sure that it bonds

to the concrete of the new one.

[Jeff] A concrete dam moves. It lives.

So, the new dam and old dam have to hold together.

They have to bond. They have to move together.

That's critical to the performance of the dam.

One of the first things we need to do to build this dam is

roughen up the face of the old dam

so we can get the new dam to stick to it.

To create this crucial bond,

a blaster fires water at a pressure of nearly

20,000 pounds per square inch

to strip away the top layer of concrete.

A large saw cuts slices into the crest of the dam.

A hydraulic hammer then breaks up the concrete

before another water blaster finishes the job.

These processes leave the dam with a rough surface to allow

the new concrete to form a super strong bond.

[Felipe] The activity that's going on right now is what we call doing

a hydro demolition of the downstream face

of the existing dam.

We're removing the outer layers of the skin of

the existing dam, but we want to make sure

and be careful not to remove too much of it

for a large extended area,

because at the end of the day, right,

we're raising Gross Dam while it's still in operation.

While the water blaster roughs up the concrete, a team at

the top of the dam wrestles with the next challenge.

Here is the crest of the old dam's spillway,

the structure that allows the safe release

of excess water.

It's Casey Dick's responsibility to remove it

before the team raises the height of the dam.

We couldn't just build right on top of the existing spillway

as it sits right now,

because the top of it is rounded.

If we tried to just start placing concrete right on this

rounded surface, it would just fall right off

and we'd never be able to get it started.

Deconstructing this critical part of the old dam is

a major operation.

The concrete is very heavy by its nature, so if we cut it off

in one giant piece, we wouldn't be able to actually

remove it from the dam.

So we have to slice it up into smaller pieces,

about the size of a vehicle, so that the cranes can then

hoist it off one piece at a time.

The team deploys a special cutting system to get

the job done.

To remove the old spillway, we have this apparatus that

uses diamond-encrusted rope, wire rope, so it's really hard

and it's abrasive, and as you drag that through

the concrete, those crystals are stronger than the concrete

and it's able to cut right through the concrete.

So basically, this rope is just going through the dam,

cutting the concrete like a cheese cutter.

Over the next two years, the team blasts,

cuts, and cracks apart the old dam.

Now the engineers can start the next phase,

an innovative concrete pouring process to ensure

its record-breaking rise.

Engineers in Colorado are racing to build a mega dam to

help secure the state's water supply.

They must raise the height of the old Gross Dam by 131 feet

to triple the capacity of the reservoir behind it.

To do this, engineers must build a supersized wall made

from 118 giant steps to sit on top of the existing structure.

For many large construction projects,

engineers traditionally pour concrete into

a steel framework.

But cutting and welding thousands of feet of steel for

the new Gross Dam will significantly raise costs

and risk delays.

To stay on schedule, the engineers use a material

called roller compacted concrete, or RCC.

A conveyor belt system delivers concrete to a fleet of

specially tracked dump trucks at the base of the dam.

These tip the concrete into predetermined positions.

Bulldozers smooth it out.

Heavy rollers then compress this layer

before an excavator with a vibrating pad compacts it

tightly together to increase its strength.

Each step rises about 50 inches by 24 inches.

The team will build 118 steps in total

to make the new dam 471 feet tall.

It's like watching an orchestra where there's a conductor

and the conductor knows what every single one of

the musicians are doing,

and everybody's doing their part.

This is a 24/7 operation.

There's two shifts every day,

and then there's seven days continuously.

The dam extension needs over

17 million cubic feet of concrete.

That's over 15% more than the old dam.

It's up to project manager Jeff Martin to make sure that

a steady supply of concrete reaches the build

to keep it on schedule.

We are gonna create a massive amount of concrete to

build the dam itself.

We have over 300 people here working every day

and we need to make sure we can always have the product

where our people are to put that in and build the dam.

Trucking in building materials would clog up the narrow

mountain roads for years

and put the project over budget.

So Jeff and the team build their own

concrete factory on site.

[Jeff] We couldn't build this dam

without having this concrete plant right here

next to the dam.

The plant operates 24/7 to supply 16,000 cubic feet of

concrete an hour.

But building quickly with concrete creates a problem.

[Jeff] Any time you mix water and cement together,

it creates heat. Heat creates expansion.

Once it cools, it shrinks and cracks.

These cracks could become weak points in the future.

In a dam, that's critical.

Water can force its way in, building pressure and weakening

the structure over time.

To combat this,

engineers cool the concrete to around 50 degrees Fahrenheit.

This limits heat buildup as it cures, reduces expansion,

and helps to prevent cracks from forming.

As you can see, we're in our ice plant now.

This is where we create all the ice that we put in

the concrete -- 450 tons of ice a day

to go into our concrete mix to keep

our concrete as cold as possible.

With this project, we're using 80% ice in the concrete.

That's a key ingredient, a key project recipe

to keep this really cold concrete

that we have to make for this project.

It's critical that the concrete arrives cold to the site,

so the team builds a custom conveyor system to transport it

quickly over the dam and down to the waiting dump trucks.

These tip the concrete onto predesignated spots.

Bulldozers then spread it,

and rollers level it.

Finally, a compactor hammers the concrete.

[Felipe] The reason we use a compactor is to achieve the right density

that it needs to have the compressive strength

and the properties that we need for the raised dam.

The compaction process forces out a cement paste that helps

to strengthen the layers of the dam.

[Felipe] After the compaction, paste comes up to the surface,

and that paste going to the surface is what

allows us to, when we place the next layer,

to get the right bond between the two

so that it's like it was placed monolithically.

You can't even tell that it was placed in two different layers.

The density of the concrete makes the wall of the new

Gross Dam strong enough

to hold back billions of gallons of water.

But it's vulnerable to the harsh climate

of the Rocky Mountains.

Roller compacted concrete itself is not super durable

against freeze-thaw and the elements.

If we just left the roller compacted concrete exposed,

over time, moisture would seep into it, and it would go

through freeze-thaw cycles during our cold seasons,

and that could degrade the concrete and chip it away,

and it could break down over time.

The engineers must weatherproof this layer.

So, they encase it in a nearly

10-inch-thick layer of regular concrete.

Workers pour the weatherproofing layer of

concrete in tandem with the RCC concrete.

It's critical that every layer is the same thickness.

[Casey] It's really important as we go that each step is built to

the exact correct dimensions so that when we get to the top of

the dam, 118 steps up, that we end up at

the correct elevation.

Using GPS, the team continually

monitors the steps to make sure

they're constructed at the correct height.

Nearby, a second team is tackling the water storage

problem in a different way, constructing an entirely

new reservoir from the ground up.

Engineers in Colorado are racing to secure the state's

water supplies in the face of continued droughts.

Their solution is to store more of the ice melt every year.

Nearly 30 miles north of the new Gross Dam project,

a second team is working on

another mega water storage solution.

Engineers here are constructing an entirely new reservoir

from the ground up.

The Chimney Hollow Reservoir will store over

29 billion gallons of water,

held back by a unique dam built from rock and asphalt.

Combined with the Gross Dam, this mega build will help

stabilize Colorado's fragile water supply.

Jeff Drager and Becky Brush lead the project.

There hasn't been anything done like this before.

Chimney Hollow is the first reservoir built within

the last 25 years, which is a pretty incredible feat.

To hold back the vast amounts of liquid, the engineers are

building a dam that's 350 feet tall and 3,700 feet long.

First, workers will blast the weathered rock to expose

the stable bedrock underneath.

Next, they'll lay an 18-inch-thick concrete plinth

across the valley.

A giant drilling machine will then bore 2,000 holes 230 feet

into the bedrock and fill them with grout to make

a watertight curtain beneath the ground.

Above it, a special machine will lay 512 layers of

an impermeable asphalt concrete

to form the core of the Chimney Hollow Dam.

Earthmovers will shift over 315 million cubic feet of rockfall

to support it.

The first challenge the engineers face is building

a solid foundation for the megastructure.

The rock here is too tough for machines to dig away.

The excavators can dig rock that's up to a certain hardness.

But once you get down further into the rock, it's just

too hard -- an excavator just can't dig it.

So, Jeff and his team detonate

millions of pounds of explosives

to smash through the rock.

It's a lot of explosions, probably one a day

for about a year.

[explosion blasts]

[explosion blasts]

Once they've exposed the bedrock, the engineers

move on to the next challenge,

constructing the foundation plinth.

This 3,700-foot-long wall

locks the Chimney Hollow Dam to the rock.

[Becky] It's really important to get the plinth right because that

is the foundation for the main dam, so everything is built on

top of that, and if that goes wrong,

the whole dam will be off.

Engineers fabricate boxes from

thousands of feet of steel rebar

and place them inside wooden forms that extends

the full length of the plinth.

The boxes are then pumped full with super strong concrete.

It takes the team almost two years to complete the plinth.

Engineers can now build the main dam.

But they face a new challenge.

Most of the dams here in Colorado are built with

earthwork and rock.

Most of them have what we call a clay core.

Clay is impermeable, and it prevents water from

leaking through the clay so

that your dam actually holds water.

On our site, we don't have very much clay.

So we needed to find another way to keep the water from

leaking out of the dam.

Jeff and the team make the bold decision to build

the core of the dam from asphalt.

The technique is common in Europe

but rare in the United States.

My first thought was, I don't want to do an asphalt core dam.

It's not been done in Colorado.

I don't want to be a guinea pig.

But over time, I was convinced that that was a good way to go.

[horn honking]

The engineers bring in a special paving machine

to do the job.

It works like a 3D printer.

As it advances, it lays down

a three-foot band of flexible asphalt

and two bands of gravel on either side that lock

the waterproof core into position.

For almost three years, the machine travels up and down

the dam over 500 times to build

it up to its full height of 350 feet.

The asphalt and gravel core is too weak on its own to

hold back billions of gallons of water.

To make the dam strong, engineers must build two large

rock embankments on either side of the waterproof core.

To feed the operation, the engineers transform

a part of Chimney Hollow

into one of the largest quarries in Colorado.

At its peak, it produces over

68,000 tons of material every day.

Supersized dump trucks haul the rock to the dam.

The quarrying operation runs 24/7

to keep the dam builders supplied

with a steady stream of rock.

But the reservoir is just one part of the water storage

solution in Colorado.

The challenge for engineers is to reliably deliver thousands

of gallons of water to where it's needed.

The steep terrain of the Rocky Mountains

makes that difficult.

So the team builds a vast new network

of infrastructure around the dam.

First, they excavate a 2,000-foot-long access tunnel

through solid rock.

This will house the giant pipelines that carry water into

and out of the reservoir.

Next, they construct a valve house to control the flow

of water through the system.

The team also builds a huge concrete spillway capable of

releasing excess water safely during extreme weather.

And because this site sits high in the Rockies, engineers must

construct new roads and bridges to reach it.

All of these systems must operate together.

After four years of construction, the giant dam

and its supporting infrastructure

are finally complete.

What engineers must now do is protect the dam from

an unexpected but common threat --

flooding.

Engineers are racing to guarantee

Colorado's future water supply.

Rebuilding Gross Dam will increase the reservoir's

capacity by over 183%.

But that creates a major problem.

As the water rises, it could exploit natural low points in

the surrounding landscape.

If we have a flood and the reservoir comes up,

the water is actually going to go out a different area that

we don't want it to go out.

Enlarging Gross Dam will raise the reservoir's water level

by nearly 140 feet.

A sudden flood could increase the water level even higher.

There's a risk that the rising water could escape through

a low point in the surrounding valley.

To stop that from happening,

engineers build a second barrier known as

a saddle dam.

First, crews excavate down to solid bedrock.

Then they backfill the cut with thousands of tons

of roller-compacted.

The finished structure stands 40 feet high and stretches

roughly 590 feet across the landscape.

Covered with natural vegetation, the saddle dam

seals the gap to prevent water from escaping from

the reservoir during extreme floods.

The huge new Gross Dam will be an arch dam.

The curved shape of the structure transfers

the pressure of the water sideways into

the surrounding canyon walls.

The saddle dam is different.

So unlike the main dam, which is an arch dam,

the Saddle Dam is a gravity structure, so it holds itself

in place under its own weight.

Crews must build the saddle dam

from layer after layer of dense roller-compacted concrete

to give it the strength it needs.

Right here, we're transitioning from

one layer to the next.

You can see this layer, another layer here,

and they just keep coming in all day long, dumping out

roller-compacted concrete, spreading it into these layers,

vibrating it, rolling it into place,

making sure it's hard and set up.

The saddle dam's location presents engineers

with a challenge.

It's over half a mile from the on-site plant that makes

the special cold concrete the dam needs.

Their solution is to deploy a fleet of dump trucks.

These work in rotation to deliver the concrete

at the correct temperature.

This operation over here is continuous.

We have one truck right after another.

One truck is backing in.

There's another truck out there

waiting to back in right behind them and dump their load.

Once dumped, it's critical that the concrete

is spread quickly and to the right height.

So, the engineers equip each bulldozer with a special

GPS system that tells the driver where to spread it.

[equipment beeping]

It uses, basically, satellites to know exactly

where, what elevation they're at, where they are in space.

Then they can place the material to exact tolerances.

Next, crews compress the concrete.

This reduces its volume by 15% and forces out air to create

a dense, impermeable structure.

[Casey] This piece of equipment here that rolls it into place,

and then you can see how it gets smooth.

The concrete is hard enough to walk on

after only a few minutes.

Once complete, the saddle dam will prevent overflow from

threatening the valley below the reservoir.

We've been planning this work for a really long time.

It's great to see it finished off so that we can move on

and continue the rest of the project

and get things wrapped up around here.

Now, engineers must tackle something that could threaten

the stability of the entire structure.

Engineers in Colorado are supersizing the Gross Dam to

help secure the state's future water supply.

Reservoirs need a way to safely release excess water when

water levels rise too high.

To do that, engineers build them with spillways.

These channels often direct flood water down the face of

the dam and safely back into the river below.

But at Gross Dam, enlarging the spillway creates

a dangerous problem.

The overflowing water strikes the bottom of the dam,

a zone called the stilling basin,

with enormous force,

after it plunges down the stepped concrete chute.

That energy could threaten the stability of

the structure itself.

The concern is with the new steps, the force of the water

could just create too much energy at the base, which then

erodes the foundation and creates a hole

right underneath our new dam.

To solve the problem, Gross Dam's designers turn to

hydro engineers at Colorado State University.

The team builds a scale model

of the new spillway and stilling basin.

Then, they simulate extreme floods to measure how the water

behaves as it crashes down the dam.

The tests reveal a solution.

Engineers must build giant triangular-shaped concrete

structures called baffle blocks inside the stilling basin at

the base of the spillway.

These structures break up the flow of water and reduce

its destructive energy before it reaches the river below.

We learned from that model how large to make these

baffle blocks, how far to space them

from the dam, how long to make

the stilling basin itself, and how tall to make

these stilling basin walls so that all the water would stay

inside this structure.

Now engineers must build the system in real life.

First, crews construct the stilling basin

at the foot of the dam.

Then, they install 11 massive concrete baffle blocks

across its floor.

Next, crews pour towering reinforced-concrete walls

around the basin itself.

To prevent the turbulent water from escaping and eroding

the surrounding ground, the walls must be built

strong enough to withstand the immense force

of the falling water.

[Casey] So these walls are what we're building today.

They're full of reinforcing steel, and these guys are

pumping conventional concrete into these walls.

You can see the workers above

vibrating the concrete into place and making sure it's all

consolidated and properly installed so that when we pull

this formwork off, we have a nice,

smooth, finished product.

Together, the spillway and stilling basin form

a giant safety valve

to protect Gross Dam during the most extreme floods.

The safety features of the dam extend beyond the spillway.

It's critical that the new extension moves slightly

upstream as the reservoir pushes against the structure.

That movement helps to lock the dam

into its final super-strong arch shape.

But if the dam moves the wrong way, the concrete could crack.

Dam safety engineer Erin Gleason

oversees a vast network of sensors

that monitor both structures.

[Erin] We have lots of sensors --

hundreds, in the new dam and the existing dam,

and all of them work together

so that as we construct the new dam, we're ensuring that

it's compatible with the existing dam and that at

the end of construction, we'll have a safe structure.

Joint meters measure the movement between

the original concrete blocks of the old dam.

The engineers expect to see a change as the new dam's

roller compacted concrete pushes on it.

So as the RCC is placed and heats up and cures on

the downstream side of the dam,

that heat expands the concrete

and then pushes the existing dam upstream.

And so we're trying to figure out how much of that expansion

and contraction of the joints is due to the new RCC.

If the joints widen, engineers know

the new extension is pushing the dam upstream as designed.

If the joints contract, the dam could be moving

in the wrong direction.

[Erin] And that could make it prone to cracking.

And cracking is not what we want to see on --

on a arch dam or any dam.

Survey prisms mounted on the dam monitor

vertical movement across the structure.

Any upward shift could indicate

the dam is pulling away from its foundations.

Back in the control room, Erin analyzes the data

in real time.

Over on this screen, we have the survey prisms,

the vertical movement of the survey prisms.

And that's showing no movement, no vertical movement,

which is good.

This specifically is the joint meter instrumentation,

and it shows that further upstream movement is occurring.

So that's exactly what we were expecting to see

and hoping to see, and so that just confirms

that we have concrete that is not going to crack.

The readings confirm the structure remains stable

and the expanded Gross Dam is performing exactly as designed.

Now, engineers must solve one final problem before

they can complete the project.

Ensuring the dam is watertight.

Engineers in Colorado are racing to finish

the mighty Gross Dam.

They must solve one final problem

before they can complete the project.

The concrete walls of the dam are designed to

hold back the reservoir.

But the rock surrounding the structure

is not completely watertight.

Water can seep through tiny cracks and fissures in

the canyon walls beside the dam.

Over time, that leakage could weaken the surrounding rock

and threaten the stability of the structure itself.

Specialist Jessie Strobel has been working

on the project for four years.

This is by far the biggest project I have ever worked on.

I have never seen anything this big being built.

This is gonna be one for the record books

and the resume.

The solution is to build a giant

underground waterproof barrier inside the rock itself.

Known as a grout curtain, it seals cracks deep beneath

the surface and blocks water from passing through.

[Jessie] The old dam did have a grout curtain

that extended up to the top of it,

but this dam is going to be over 140 meters high,

so we have to extend the grout curtain the rest of

the way up to the top of the dam so it can hold back

the reservoir water once it's filled.

To extend the height of the grout curtain,

the crews first drill deep into

the canyon walls around the dam.

The machine drills holes tens of meters into the ground to

make sure that we are getting as far down as we believe any

reservoir seepage can go through.

Then, after it's drilled the hole,

it continues to stay in place and then injects the grout

through tubing all the way in to make sure that

we are filling those tens of meters with all of the grout

and really forming that strong barrier.

We don't know exactly what all the rocks look like

100 feet down, so we just keep feeding grout

in through the machine until it stops taking any more.

Together, these injections form a vast underground curtain

designed to stop water from escaping around the dam.

But seepage creates another danger beneath

the structure itself.

Groundwater can still force its way underneath the dam.

When you're engineering a dam, you don't just need to think

about holding the water back behind the reservoir,

but you also have to think about how the groundwater is

going to want to move as well.

Because if pressure is put on the dam from the groundwater,

it's gonna start lifting the dam up, and that is just

not what you want to be able to hold back a reservoir.

That movement could separate the dam

from the bedrock holding it in place.

The result could be catastrophic.

The team needs another solution.

Deep inside the dam, crews construct

a vast drainage system designed to relieve

the pressure beneath the structure.

From here, crews drill hundreds of long drainage holes deep

into the foundation below.

The drainage holes intercept the ground water,

and channel it into the galleries inside the dam.

From here, the dam can release it into the river downstream.

The existing Gross Dam was pretty watertight,

but you could still fill up a glass of water in about

a minute from the seepage that was coming through.

So we would hope with the raised dam, to keep that

same level of watertightness, but still have some seepage.

So that's why we have the foundation drains being

built in, it's for any water that does come in,

it goes where we want it to go and doesn't start moving around

and moving things that we don't want to be moved.

This super drainage system, together with

the grout curtain, will help to keep Gross Dam

locked securely to the canyon.

To complete Colorado's mega water projects,

engineers excavated deep in the Rocky Mountains...

[explosion blasts]

...raised one of the state's largest dams

to record-breaking heights...

and built a vast new spillway and underground drainage

systems to protect it from the immense pressure of

the reservoir behind it.

What we've created is really a testament not only

to engineering, but of construction innovation.

Now, the new and expanded dams will help provide fresh water

for millions of Colorado residents,

and safeguard the state's water supply against future droughts

for generations to come.