(Jonathan Newton, Washington Post)

Sunday’s story on the big dig that will create 13 miles of tunnel under Washington to handle household sewage and storm runoff, generated a lot of reaction and several questions that we’d like to answer.

Where are they taking the [2-million cubic yards of] material that’s being dug out of the tunnel?

It’s being trucked about 18 miles southeast of the current tunnel mouth to an Army Corp of Engineers dump site in Brandywine, Md.

What chemicals are being used to soften the soil in front of the tunneling machine and to reduce it to what you described as toothpaste-like once it’s inside the machine?

The contractor who is operating the machine, called “Lady Bird,” says that’s proprietary for competitive reasons, but the company says it is “environmentally friendly.”

After the current network of 13 miles of tunnel is complete, will additional tunnels be dug to serve the rest of the District?

DC Water General Manager George Hawkins says the tunnels being built now will capture 98 percent of the waste water that’s now diverting into the District’s creek and rivers.

Who is footing the bill for this $2.6 billion project?

As Hawkins relates in the story, the water authority’s rate payers will pay for the vast majority of it through rate increases that he anticipates will continue for the next 10-20 years. The authority has received $196.8 million in federal dollars since 2003 for the project.

Will buildings sink as a result of the tunnel, as I heard they did after a tunnel project in Milwaukee?

DC Water and its contractors don’t anticipate a problem. Unlike digging a trench from the surface and then putting the earth back in place afterward (which could lead to that earth settling), they are tunneling underground and replacing the soil they remove immediately with a tunnel, so the risk of something collapsing is remote (unless the tunnel were to cave in). Nonetheless, the tunnel project has been using sensors to detect any settlement along the way so far, and will continue to use them as the tunnel progresses. The results to date have been good. “We feel much more confident we can accomplish this without causing settling,” said Carlton Ray, director of the DC Clean Rivers Project.

Is DC Water in charge of this project? Or are there contractors handling it?

Yes to both questions. DC Water is managing the project. The contractors who are driving Lady Bird and constructing the first 4.5 mile tunnel are joint venture of Traylor- Skanska-Jay Dee.

This machine they’re using isn’t unique, is it?

Far from it. Similar tunnel boring machines are being used elsewhere in the U.S. and abroad. Perhaps the most famous tunnel created by one runs under the English Channel between Great Britain and France.

Who built the machine?

It was constructed specifically for the D.C. project in Schwanau, Germany, at the Herrenknecht Factory.

If this is so wonderful machine, why hasn’t it been used to built more underground Metro stations and tunnels to alleviate the horrendous D.C. traffic?

A machine very similar to Lady Bird is in play in London for the Crossrails project. There’s a video of it.

We heard from reader Alan Drake today, who writes: “I am working with one of the original designers of DC Metro on an expansion plan that would more than double ridership while reducing the operating subsidy (i.e. the expansion would operate at a marginal profit). For Phase I, we propose setting up two TBMs near Wisconsin & M. One goes to Columbus Circle in front of Union Station and the other TBM to Connecticut and Beltway (then on the surface west to HQ for Lockheed-Martin, Marriott and NIH infectious Diseases).”

The question, of course, if such a proposal were to move forward, would be who would pay for the project? The first phase of Metro’s Silver Line cost $5.6 billion, with $975 million coming from the federal government. While a largely above-ground rail line that eventually will reach Dulles International Airport is expensive, so too would be tunneling about under Washington. As mentioned above, DC Water will pass on most of the cost of the sewage tunnel to its ratepayers. Is that the model for any new Metro tunnels — higher fares — or would the money come from government?