I'm standing outside one of the
largest diameter bored tunnels in the world. Behind me is a double-deck roadway
under construction
inside that very tunnel. Now let me take you inside and show you how a road
and a tunnel can be built at the same time so Bertha's not just mining and
removing dirt but she's actually building the tunnel.
She installs the concrete ring - the circle that's made up of two foot thick
concrete segments. So the segments all have to go in really like a jigsaw
puzzle so they have to fit just perfectly. When enough of the tunnel is built, the
contractor can start building the road inside because it's a circle and they
need to develop a foundation a point from which to place the walls and the
decks and that foundation is called the corbel and the corbels are built as
all the roadway structures are in 54 foot sections and all that all the
concrete work out here is reinforced with what's called rebar reinforcing
steel to help deal with expansion and contraction of the concrete. So that
steel is placed up against the wall of the tunnel and then the workers come
along and construct formwork.
So after the formworks been installed they come in and begin to pour the
concrete into the corbel formwork. After just a few days
when everythings set up, they remove that formwork and they place
moisture and blankets over the concrete and then allow it to cure for another 28
days. The concrete will actually gain in strength with time.
So while that crew is on the corbels, the crew behind them will begin bringing in
the formwork to actually build the walls and the walls will sit on top of
each of those two corbels. The wall is built like an upright sandwich with
forms on either end and steel and concrete in the middle.
There's 13 tons of steel in each of these 54-foot panels.
Once the reinforcing steel is set, the forms are placed in to create this sandwich.
Then that same pump truck comes along and pours the concrete into that form.
Along the way the workers are making sure that it's flowing and then
verifying at the very end
the depth of that concrete till it reaches the very top of the formwork
and they're finished with the pour. The concrete is allowed to set for a few
days -
long enough that they can remove the form work and then similarly with the
corbels they'll spray it with water and they'll wrap it in plastic blankets. Again 28
days until its reach the design strength and the next step is to set that upper
roadway on top of the two walls that it just built.
There's plenty of room underneath that upper roadway deck to access the tunnel
boring machine.
The crews need to be able to reach the machine, bring in the precast segments
and underneath that upper roadway deck to continue this mining operation.
So the formwork will be placed between the two walls will actually apply an
oil-based product to prevent the concrete from sticking to the formwork.
The reinforcing steel will be placed on top of that formwork and this is a
special type of reinforcing steel. It's covered with an epoxy coating. Those
reinforcing bars would be more susceptible to corrosion
if they were not wrapped in an epoxy and then the crew will come in and pour
the concrete on that upper deck and this is quite an operation.
Everybody has a job to do whether it's managing the concrete that's flowing out
of the nozzle or the placement of the nozzle or smoothing the concrete as its
building in certain areas.
The public won't see the walls very well but you'll feel that roadway surface as
you're driving along and you want to get the absolute highest quality of concrete
and finishing as you can.
The contractor will build walls above that upper deck and at the very end of
the project we'll come in and we'll finish that lower roadway that nestles
on top of those two corbels in the interest of time and efficiency
this is the one piece of the whole operation that is precast which means
it's made off site and then brought in and set into place at the very end.
The amount of concrete used to build the tunnel roadway structure is enough
concrete to build nine Seattle football stadiums. When it's complete, the SR 99
tunnel will be one of the longest roadway tunnels in the US and pave the
way to transform Seattle's waterfront.