Why a Giant Machine Is Digging a Tunnel Under D.C. | National Geographic

we’ve had about three billion gallons of

we’ve had about three billion gallons of raw or combined sewage that would

raw or combined sewage that would

raw or combined sewage that would discharge on an annual basis from our

discharge on an annual basis from our

discharge on an annual basis from our combined sewer systems about two-thirds

combined sewer systems about two-thirds

combined sewer systems about two-thirds overflows into the Anacostia River about

overflows into the Anacostia River about

overflows into the Anacostia River about a third of it goes into the Potomac

a third of it goes into the Potomac

a third of it goes into the Potomac River our program is to capture that so

River our program is to capture that so

River our program is to capture that so we’re about to go into the third of the

we’re about to go into the third of the

we’re about to go into the third of the tunnel here we’re about to go up onto a

tunnel here we’re about to go up onto a

tunnel here we’re about to go up onto a mile to the heavier machine

mile to the heavier machine

mile to the heavier machine overhead you can see the big air

overhead you can see the big air

overhead you can see the big air ventilation system bringing fresh air in

so it’s a fairly simple process that the

so it’s a fairly simple process that the shield has a main bearing unit that has

shield has a main bearing unit that has

shield has a main bearing unit that has drive motors that turn this cutter head

drive motors that turn this cutter head

drive motors that turn this cutter head and that cutter head is digging into the

and that cutter head is digging into the

and that cutter head is digging into the ground simultaneously the machine is

ground simultaneously the machine is

ground simultaneously the machine is advancing into the ground so as this is

advancing into the ground so as this is

advancing into the ground so as this is spinning machines advancing forward and

spinning machines advancing forward and

spinning machines advancing forward and ingesting material then it stops and the

ingesting material then it stops and the

ingesting material then it stops and the crew builds a ring inside of the tail

crew builds a ring inside of the tail

crew builds a ring inside of the tail shield that tail she’ll has a seal

shield that tail she’ll has a seal

shield that tail she’ll has a seal that’s on the backside of it and that

that’s on the backside of it and that

that’s on the backside of it and that keeps the ground and water and grout and

keeps the ground and water and grout and

keeps the ground and water and grout and stuff from invading the tunnel

so as it advances its pulling material

so as it advances its pulling material out of this group conveyor for that

out of this group conveyor for that

out of this group conveyor for that complete advance that’s getting mixed up

complete advance that’s getting mixed up

complete advance that’s getting mixed up into a mixture of soil and water and

into a mixture of soil and water and

into a mixture of soil and water and solution and then getting pulled up all

solution and then getting pulled up all

solution and then getting pulled up all the way to the conveyor belt and that

the way to the conveyor belt and that

the way to the conveyor belt and that conveyor belt then takes the material

conveyor belt then takes the material

conveyor belt then takes the material out to the shaft where it’s dumped into

out to the shaft where it’s dumped into

out to the shaft where it’s dumped into big boxes and wasted up the shaft now

big boxes and wasted up the shaft now

big boxes and wasted up the shaft now it’s ready to excavate again so it

it’s ready to excavate again so it

it’s ready to excavate again so it starts the cycle all over

starts the cycle all over

starts the cycle all over [Music]

[Music]

[Music] this machine has around 100 millimeters

this machine has around 100 millimeters

this machine has around 100 millimeters and minutes that’s about four inches per

and minutes that’s about four inches per

and minutes that’s about four inches per minute so it can theoretically do a six

minute so it can theoretically do a six

minute so it can theoretically do a six foot push in about 18 minutes we have

foot push in about 18 minutes we have

foot push in about 18 minutes we have sewage overflows into the Anacostia

sewage overflows into the Anacostia

sewage overflows into the Anacostia River Potomac and Rock Creek that causes

River Potomac and Rock Creek that causes

River Potomac and Rock Creek that causes water quality problems we’re going to

water quality problems we’re going to

water quality problems we’re going to reduce the overflow volume by 96 percent

reduce the overflow volume by 96 percent

reduce the overflow volume by 96 percent [Music]