The Assassin (Sha ren zhe Tang Zhan)

In this episode...

This is some of the most incredible engineering

that I've ever seen.

This is really something extraordinary.

...The planet's only floating railroad bridge...

We're essentially putting a rail on a marine vessel.

It's extremely exciting.

...And the pioneering historic innovations...

It's impressive. It's really cool to see this.

...That made the impossible possible.

captions paid for by discovery communications

king county, Washington...

Home to Seattle and Bellevue,

hubs for the nation's booming tech industry...

...Where the population is exploding

and traffic is gridlocked.

Engineer John Sleavin lives

and works in a city pushed to its limits.

There's a lot of major corporations in Seattle.

The traffic's getting worse.

The need for transportation is increasing,

and the need for choices are increasing.

The solution could be to connect the cities by train,

but king county's unique environment

can make travel difficult.

One of the unique features of Seattle

is its geographic terrain.

There are a number of lakes that all restrict where

and how you can place transportation services.

In particular, lake Washington sits between

downtown Seattle and Bellevue,

both of which are high-tech areas that need to be connected.

In a landscape known for vast bodies of water,

lake Washington is the largest and deepest.

Here, traditional bridges just aren't possible.

But engineers in Seattle have the answer.

This once impassible lake has now been conquered

by the incredible I-90 floating bridges...

...A concrete mega structure that actually sits

on the surface of the water unsupported by columns.

This project is incredibly unusual in that

we're applying systems that has not been done

by anybody else in the world ever before.

These extraordinary buoyant bridges are capable

of carrying 142,000 cars a day.

Nowhere else in the world has this ever been done.

It's floating. It moves.

With a massive 357,000 tons of reinforced concrete,

the twin floating bridges weigh more than 52,000 elephants.

All that weight is floating on 38 monster pontoons

with nothing but 210 feet of water below the surface,

crossing a span of over 1.5 miles

and capable of supporting the weight of rush-hour traffic.

And now engineers are entering

the most challenging phase of construction,

adding a state-of-the-art train line

and creating the planet's only floating railroad bridge.

Once this system is commissioned and in operation,

this will be an engineering feat like no other.

But this ambitious project

poses huge engineering challenges.

Is it possible to connect a railroad from land

onto a floating moving bridge,

if those rails were just attached on either side,

that continuous connection would experience

all those movements at one point

and would probably snap the rail.

What happens when a high-voltage current

is introduced to a structure in water?

There is a risk of stray current escaping from the rails,

which could get into the critical bridge structure.

And will the bridge be strong enough

to support 300-ton trains?

We can eccentrically load the bridge

and potentially crack it.

That would not be good.

But the biggest challenge is keeping

this concrete superstructure afloat.

It is very important

that if there's any water intrusion,

it won't sink the whole bridge.

The first step for Seattle’s engineers

was to decide whether they had to build a floating bridge

or if they could go with a more traditional design.

Engineer Jim Stonecipher is very familiar

with the daunting complications of building on this lake.

The lake is deep

and, being in earthquake country,

we need a good material to set our foundations in,

and that's just not available on the bottom of lake Washington.

Even if engineers were to sink support columns

through 213 feet of water,

they would then hit a soft lakebed made of silt and clay.

Pillars would need to go through another

164 feet of sediment

to reach a solid footing.

Add the column length needed above the water

and this becomes an incredibly expensive

and unstable structure.

On the engineering side,

it would be difficult to build the standard cable stay

or other type of bridge in that area.

It takes a unique kind of bridge span

to span lake Washington.

So the engineers' only option

is to float the bridges.

But how can they ensure

the giant concrete structure doesn't sink?

On the Caribbean island of Curaçao,

local engineer Albert Zwueste is exploring

how a clever piece of engineering

could help the team at lake Washington.

The island's main town, Willemstad, was a perfect port,

but by the mid-1800s,

the deep natural harbor was creating a problem.

But the channel is 492-feet wide

and 49-feet deep with a soft sandy seabed,

making most bridges impossible to build,

especially one that allows the passage of ships

into the harbor.

But when American ice merchant Leonard Burlington smith

sailed into Curaçao in 1876,

he had the answer.

Affectionately known to the locals

as the swinging old lady,

it's one of the oldest pontoon bridges in existence.

But the brilliant pontoon design

doesn't just allow for transit between each side.

The floating bridge has another trick up its sleeve.

The impressive 548-foot bridge span

is hinged at one corner

and swings open to allow boats into the harbor.

Smith's design was brilliant in its simplicity.

And just beneath the pedestrian walkway lie the vital components

that will prove significant to the engineers in Seattle.

Pontoon bridges have been around for millennia,

but few can compare to the swinging old lady.

Now, on lake Washington,

engineers are taking the idea of the pontoon bridge

and supersizing it.

King county, Washington,

is one of the nation's fastest-growing regions.

There's a constant battle to keep the population connected.

But with the massive lake Washington in the way,

engineers have been forced

to come up with an innovative solution...

A pair of gigantic concrete floating bridges

supported by pontoons.

The pontoons are large enough to support a highway

carrying 50 million cars a year

and the first ever floating bridge railroad.

Engineer Jim Stonecipher is responsible

for maintaining the bridge.

So our solution to crossing lake Washington

was building these pontoon bridges.

We make a concrete pontoon out of very dense concrete

with hollow cavities inside.

The concrete has enough buoyancy in it

to support the bridge and the traffic on it.

During construction,

38 giant pontoons are positioned end to end,

giving the illusion of one massive bridge base,

each pontoon is divided into cells and sealed

with watertight hatches.

Two overhanging bridge decks

provide enough space for eight lanes of traffic

and two train tracks.

One of the reasons we have so many pontoons

is for redundancy,

so that if one fails, it won't sink the whole bridge.

Each compartment has its own door

and sealed off, kind of like a ship,

and that way, we don't lose the pontoon bridge

and we can maintain traffic.

Keeping these mega bridges afloat

is an impressive feat,

and it takes even more incredible engineering

to keep them from floating away.

Down below us, you're going to see the anchor cables

that help stabilize the bridge

and keep them in place.

And here comes one now.

You can see it just below the water.

The longest anchor cable is about 739 feet

in about 165 feet of water.

Buried in the lake bed,

movements from the bridges

put pressure on these anchor cables,

causing them to fray.

I got a cable here.

To prevent catastrophe,

a team of divers working at depths

of up to 165 feet

are currently replacing damaged components.

The anchor cables are very heavy,

and it takes a real big team

to get those anchor cables in place.

So far, 32 huge new cables have been installed.

But as the seasons change,

so can the tension of the cables.