|Page 2 of 2 <|
Workers try to repair the sinking sea wall at the Jefferson Memorial
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."