By Michael E. Ruane
Washington Post Staff Writer
Wednesday, June 2, 2010; B01
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.
But the pilings under the sea wall, where the memorial meets the basin, are set only in dirt. As a result, the sea wall has sunk almost a foot in places since the memorial was built, and the plaza out front has also slipped and moved laterally.
One day last week, engineers with the Park Service and Clark construction company, the general contractor, explained that much of the work must take place below the water level of the Tidal Basin.
Funded by the American Recovery and Reinvestment Act, the 18-month effort is scheduled for completion next spring, officials said. "We'll all measure this as [a] successful project [if], when we're done, no one will know we were ever here," said Philip J. Sheridan, a vice president with Clark.
The work includes removal of the old sea wall and some of the old timber piles. Forty-one new concrete caissons, each four feet in diameter, will be sunk through the mud and 10 feet into bedrock, along with concrete-filled pipe piles that are 18 inches in diameter and set at an angle for extra support.
The sea wall will then be rebuilt on that foundation. "We hope to have zero movement" when the work is done, Sims said.
Because most of the sea wall is submerged, engineers had to build what is called a coffer dam -- essentially a steel bathtub inside the Tidal Basin. The water inside the dam was then pumped out. Workers, protected by the dam, have begun the arduous process of dismantling the concrete and granite sea wall.
Once the old wall has been removed and the historic stone facings preserved, the new foundations can be drilled, Sheridan said.
Six test holes are being sunk to sample the consistency of the mud and the nature of the rock underneath, he said. "The surface [of the rock] is decomposed," Sheridan said. "That's what we're checking, is how quickly does it get to solid, competent rock."
The original builders failed to rest the wall's foundation on that rock, he said, "which is the principal cause of settlement."
There had been a theory that the old wooden piles had simply deteriorated over time. But the engineers said that is unlikely.
"If the pile stays submerged below the water all the time, it stays in very, very good shape," Sheridan said. "These are below the water table all the time, so we expect that they're going to be like the day they put them in."
Sheridan said engineers are not exactly sure how long the old pilings were, but workers will find out when some of them are removed. "I think everyone is going to be very interested to see just how long the piles are when we start pulling them out," he said. "Very few of the original design documents [are] available."
Workers back then had the materials and the technology to put in longer piles, he said. Why they didn't is a mystery. "My guess is, looking at everything else out there, they built what they were told to build," he said.
Many of the monuments on the Mall are set on pilings driven into bedrock; almost 500 underpin the World War II Memorial. Much of the area was once river bottom or marshland that was filled in with dredged river mud in the late 1800s.
On May 24, as some workers patched leaks in the coffer dam and others worked to dismantle the sea wall, two men ran the drill that bored into the mud, taking core samples as it went.
Nearby, boxes containing jars of mud samples sat in the back of a truck, and wooden boxes sat waiting for samples of rock. The drill was still about 40 feet above bedrock.
As the drill spiraled down, water began to seep out of the hole, followed by smooth, brown mud that looked like cake batter and was part of the substance on which at least some of monumental Washington is built.
It was no matter, Sheridan said, "with the right foundation, you can build almost anywhere."