Workers try to repair the sinking sea wall at the Jefferson Memorial
Wednesday, June 2, 2010
The brown goo oozed from the drill hole like a primordial porridge -- from 60 feet beneath the Jefferson Memorial, it was some of the muck that's under the Mall and part of the stuff that has been slowly swallowing the memorial's sea wall for years.
Centuries of Potomac River sediment and layers of dredged fill, it is the material engineers are drilling through to reach bedrock and anchor the famed memorial's sea wall, for the first time, on a solid foundation.
On Tuesday, crews working in a dewatered section of the Tidal Basin prepared to demolish the old concrete sections of the sea wall as part of the $12.4 million repair project that the National Park Service has been planning since it realized the wall was sinking in 2006.
The work is expected to keep the photogenic north face of the memorial partially obscured by construction equipment through the rest of the tourist season.
In a bit of engineering detective work, experts have discovered that the wall has been slipping away from the memorial's north plaza because the timber pilings that were used to support the wall were probably not long enough to reach bedrock when the memorial was built in the 1930s and '40s.
Studying old photos, engineers were able to determine that the piles were about 65 to 75 feet long, although bedrock starts about 80 feet down, National Park Service civil engineer Steven D. Sims said.
Did the workers cut corners? Did they make a mistake or incorrect assumptions?
"We don't know," Sims said.
The 32,000-ton Jefferson Memorial, on an 18-acre site, is solid, officials said, although it has shifted some since its construction and is monitored for movement.
Dedicated in 1943, the marble and limestone memorial was originally supposed to be in the middle of the Tidal Basin. It wound up along the south shore, and a man-made promontory was added to the man-made shoreline to accommodate construction.
The memorial, which honors the nation's third president and main author of the Declaration of Independence, rests on concentric rings of 634 concrete pilings and pillarlike caissons sunk to bedrock. At least one goes down 138 feet.