More than 100 feet under the Potomac River, scores of construction workers tend to Lady Bird, D.C. Water's answer to cleaner H2O, day and night as she works to reroute polluted water away from area rivers. (Marina Cracchiolo/The Washington Post)

More than 100 feet under the Potomac River, scores of construction workers tend to Lady Bird, D.C. Water's answer to cleaner H2O, day and night as she works to reroute polluted water away from area rivers. (Marina Cracchiolo/The Washington Post)

A massive machine — longer than a football field — is munching away beneath Washington like a giant earthworm. Before it’s done, it will devour about 2 million cubic yards of soil that has been sitting under the city since the days of the dinosaurs.

It is the most amazing and expensive construction project that no one ever will see.

It will come within a center fielder’s throw of Nationals Park, within a corner kick of RFK Stadium, nibble at the deepest roots from the National Arboretum, pass below the Love Nightclub and the United House of Prayer for All, go under railroad tracks that carry 1 million-pound trains into Union Station and a six-lane roadway used by 60,000 cars a day, gnaw its way under Home Depot’s doorstep and then chomp more than a mile and a half down Rhode Island Avenue toward Logan Circle.

Like the creature from a sci-fi thriller, the machine will tunnel along — six feet at a time — beneath a city largely oblivious to its existence.

“That’s the way we like it,” said James Wonneberg, DC Water’s resident engineer.

Cars race from Italy to France through a famous tunnel under the Alps. Bullet trains rocket through a tunnel under the English Channel. One day, raw sewage will roar through Washington’s tunnel.

Not so romantic, perhaps, but vital to a city that now pumps 2 billion gallons a year from its sewers and toilets directly into the Potomac, the Anacostia and Rock Creek.

With a little help from upstream neighbors, those three tributaries may one day run closer to pure. But for now, there is only that dream and a hungry machine.

The machine itself is a marvel of technology, an underground factory 443 feet long and almost six times the weight of the Statue of Liberty. It does about a dozen things at once, and it moves.

Consider just one aspect of that movement: More than 100 feet below ground, how does it know where it’s going?

With a circular face three times the width of a Metrobus, what keeps the machine always within a few millimeters of its intended path?

Separating the streams

Washington needs this new, gargantuan 13-mile long, $2.6 billion sewer tunnel because of what might be called, in hindsight, a dumb decision. Were they still alive to defend it, the city’s forefathers might respond much like the people who were wearing bell-bottoms in the 1970s or who dye their hair electric green today: It was the fashion of the day.

The “it,” in this case, was something called a combined sewage system. They were all the rage in 19th-century America. The District has one, as do more than 770 other places where a total of 40 million people live.

That is a lot of flushes, and on a rainy day, that matters.

Here’s why: In a combined sewer system, your bath water, your laundry water and whatever you flush goes into a network of sewers that also handles all the rainwater that flows down sewer grates from the street.

On a dry day, or one with a slow but steady rain, all of that combined wastewater heads obediently to the sewage-treatment plant. In the case of the District, that is the sprawling facility called Blue Plains that sits beside the Potomac River in the southeast quadrant of the city.

But in a gully-washing downpour, a serious thunderstorm or when 10 inches of rapidly melting snow gushes down the sewer grate, the system gets unruly. The path to the treatment plant becomes overwhelmed, and a filthy mix spews from 53 different outlets into the three tributaries.

Not by accident, but by design.

It’s not an occasional thing. It happens hundreds of times a year, contributing 2 billion gallons of untreated waste to Rock Creek, the Potomac and the Anacostia, which gets the worst of it. All that unhealthy mess, of course, flows down into Chesapeake Bay on its way to the Atlantic Ocean.

“By 2032, our stated goal is to have water in the Anacostia that’s swimmable and fishable,” said George Hawkins, DC Water’s general manager.

Having the Potomac turned into an open sewer appealed to no one, and perhaps least of all to Lady Bird Johnson, who is said to have berated Lyndon about it when they flew into National Airport in daylight. That factoid has relevance even today.

Though Lyndon B. Johnson had a war and civil rights on his plate in those days, the rumblings about pollution that began on his watch led his successor, Richard M. Nixon, to create the Environmental Protection Agency in 1970.

Fast-forward 35 years. Everybody agreed that something had to be done about combined sewer systems, and the EPA and U.S. Justice Department ordered Washington and several other cities — including New York, Philadelphia and Seattle — to stop dumping combined sewer overflow into rivers.

That gave birth to the District’s tunnel plan, and last year, the massive machine began to dig.

They named it “Lady Bird.”

Underground action

Lady Bird keeps busy, at once softening the soil in its path, lurching forward to gnaw at it, sliding the stones that will be the tunnel’s wall into place, blasting fresh surface air to the work crew, laying railroad track for the cars that carry the stone, turning the dirt into muck and dumping it on a conveyor belt headed for the tunnel mouth.

The muck will fill 205,820 huge dump trucks before the project is completed in 2025.

Though all of that is going on pretty much at once — 24 hours a day, six days a week — here’s how each facet takes place.

The round face of the machine is 26 feet across and studded with tungsten carbide scraper bits and cutting wheels. Before the device shoulders forward, a barrage of chemical mix shoots from nozzles to loosen up the earth ahead.

Then the machine heaves forward by six feet, its circular face rotating as the bits and wheels slash into the soil. What they dislodge is sucked into the machine, mixed with a compound that turns it to the consistency of toothpaste and is plopped blob by blob onto a conveyor belt that runs along the ceiling to the tunnel end.

While this is underway, massive curved slabs of concrete — they call them “stones” — have arrived on a small rail car that trundles back and forth from the tunnel mouth. They are stacked in the order in which they will be put in place. When they arrive, an electric device that looks like something you might see in an automatic carwash swoops down from above, presses its big rubber gasket against the stone’s face and literally sucks it up.

Stone in hand, the machine then rumbles to the leading edge of the tunnel, where a crew of five in hard hats uses levels and laser guidance to ease each piece into place. The slabs are connected in front and back by big plastic dowels and on top and bottom by arms that are driven into place by a worker with a compressor-fired rivet gun.

Once the precise placement is achieved, the suction machine scoots back to get the next slab until the six-foot, 80,000-pound ring is complete.

But there’s more.

The machine’s boring face is 26 feet across. The interior tunnel walls are 23 feet in diameter. The stones are 14 inches thick. There is, by design, a gap between the new tunnel exterior and the hole that Lady Bird has created.

Into the gap oozes an epoxy-like substance that will harden into a six-inch-thick casing to become the tunnel’s outer defense.

As Lady Bird rides forward through the tunnel on wheels, it lays track in its wake for the rail car that feeds stones to the growing tunnel.

Now it is time for the next push. Eighteen four-ton jacks powered by the 13.8 kilovolts of electricity nudge up against the edges of the tunnel ring that has just been set in place.

Before they fire, the operator who sits in a narrow booth watching six computer monitors studies one that shows crosshairs. Using a target fixed just behind the face of the machine, and a second target behind the machine that provides global coordinates, the operator uses a laser to line up Lady Bird’s next chomp.

“Right now, he’s within an inch of being right on the mark,” said Brett R. Zernich, construction manager, pointing to the crosshairs earlier this month.

At the push of a button, the operator sends Lady Bird surging six feet farther into the soggy soil.

Chewing down the Potomac

Lady Bird said goodbye to daylight several months ago and was lowered into a deep hole on the grounds of the Blue Plains treatment plant. Right now, the machine has chewed its way more than a quarter of a mile and is about 70 feet beneath the floor of the Potomac, skirting around the Naval Research Laboratory because the Navy wasn’t keen on having a tunnel under its testing facility.

In a few months, it will curve to the right, penetrate some rocky soil and pass under Anacostia to connect with a sewage pumping station at Poplar Point. Then it will dip under the river by the same name and make its way to another pumping station near Nationals Park.

A new tunnel will begin at the Poplar Point pumping station, cross under the river just north of the 11th Street Bridge and head toward RFK Stadium. It will pass the arboretum and make a sharp left 120 feet under Rhode Island Avenue.

Just as a river has its tributaries, so does the tunnel, with the largest going up First Street from Rhode Island. And it will have diversion chambers where waste can be stored temporarily so the system isn’t overwhelmed.

All that, for $2.6 billion. Where does the money come from?

Customers. Finding a way to pay to restore other decrepit infrastructure — notably roads and bridges — has become a knotty issue, but water utilities send out monthly bills. Although DC Water services wholesale customers in Maryland and Virginia, most of the burden will fall on their customers in the District.

The average water and sewer bill has gone up by more than 50 percent in recent years, to more than $65 a month for a single-family home.

“Our ratepayers are paying for all this,” Hawkins said. “We estimate [there will be] rate increases for the next 10 years, and maybe for 20, and most of that’s for the tunnel.”

Parts of it will begin opening in 2016, with big sections to follow in 2018 and 2022. With steel filaments embedded in its concrete walls, the tunnel should last for 100 years, they say.

“I want our ratepayers to understand that we have to do this, but it’s more important that they recognize the benefits of it,” Hawkins said. “No one will ever see this tunnel, but they’ll see that the river’s cleaner, and down stream in the Chesapeake, it will be a significant difference.”