But two millimeters is more than analysts said they would have expected via normal annual subsidence, or sinking.
If confirmed, it would not be much of a drop — .08 inch, or about the thickness of a wedding band. But it’s a further illustration of the relatively soft, reclaimed ground, dredged from the Potomac River in the 1880s and ’90s, on which the western half of the Mall rests.
The finding comes soon after the completion of a $12 million repair of the Jefferson Memorial’s seawall, which had sunk much more dramatically, almost a foot in places, in recent years.
Other sunken sections of the seawall, which rims the Tidal Basin, are still in need of repair and are completely underwater at high tide.
The 91,000-ton, 555-foot-tall Washington Monument is the only such structure on the Mall that does not rest on pilings driven down to bedrock far below the surface for stability. It sits instead on a 37,000-ton foundation called a “spread footer,” said the National Park Service’s Steve Lorenzetti, deputy superintendent of the National Mall and Memorial Parks.
“It’s basically just a giant flat slab,” he said. “A spread footer is very normal in many areas. Today we would probably not do that on such a tall building.”
The marble and granite monument, while fundamentally sound, officials said, was extensively damaged during the Aug. 23 earthquake that struck the Mid-Atlantic region. Chunks of stone shook loose from the inner and outer walls, the elevator was damaged, and weatherstripping between the stone blocks fell out.
The monument, which about 600,000 visitors enter each year, has been closed since then. Its $15 million repair job will probably keep it shuttered until well into next year, Park Service officials have said.
Experts are drawing up a detailed repair plan, Lorenzetti said. But bids for the work probably won’t be requested until late summer.
He said the Park Service also plans to do soil borings around the monument to check the consistency of the soil since the earthquake.
David Doyle, the National Geodetic Survey’s chief geodetic surveyor, said: “We know that most of this area down here — basically everything from the Washington Monument westward — is constructed on fill. And we know that there is some form of settling, subsidence.
“The exact rates are small, but over time they can add up and be significant,” he said.
The National Geodetic Survey is a federal agency that compiles precise data on latitude, longitude and elevation above sea level to establish official place locations.
The focus on the Mall is elevation, Doyle said. Using a process called “digital bar code leveling,” he and a surveying crew have been walking the Mall with high-tech optical equipment, measuring current elevations to compare with past measurements.
The measuring started in early March and will continue through the end of the month.
Doyle said the early data seem to suggest that there has been two millimeters of sinking where they were expecting subsidence of less than one millimeter.
“That’s way more than we can live with,” he said. “Almost one millimeter could be accounted for just in normal settlement. Something beyond that might be a result of the earthquake.
“What we’re looking for . . . is any kind of either settlement that’s just an ongoing process, and/or anything that occurred because of the earthquake,” he said.
The Park Service has “some serious issues going on with the monument,” he said. “They’ve brought in a lot of contractors to do a lot of different work. One of the things that we’re providing for them is this height measurement difference.
“From that, they will get just a ton of information about what kind of motion may be going on,” he said. “I say ‘may,’ and I highlight ‘may,’ because we just don’t exactly know yet.
“We know there has been some settlement . . . since 1884,” when early measurements were taken, he said. “It is quite modest. I’ve heard people talk about, ‘Oh, it’s going to fall over.’ Nah.”
He said he theorizes that the monument has probably settled about two inches since then.
“We’re very happy that [the National Geodetic Survey] is going to work with us on this,” Lorenzetti said. “We don’t know if anything has happened, but it’s seems to be very prudent to keep an eye on it.”