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SEWER PROJECT

Davenport tunnel builder runs into aquifer

March 18, 2012 4:00 am  • 

The dirty job of drilling Davenport’s westside diversion tunnel recently became a very wet and muddy one after workers hit what is described as a “perched” aquifer.

Millions of gallons of water have been pumped from the aquifer, briefly raising concerns about the possibility of the ground settling in the area of the construction site at Wilkes Avenue and Dover Court in west Davenport.

But after two weeks of monitoring, city officials say those worries have faded.

Public Works Director Mike Clarke said elevations will continue to be monitored in a 200-foot radius around the construction site.

“We find it interesting,” he said. “We don’t find it alarming.”

Soil borings done ahead of the construction of a drop shaft that meets up with the major sewer tunnel being built in that area showed a soggy mix of clay about 30 feet underground, with heavy clay layers above and below.

When they reached that depth two weeks ago, workers found much more water than expected.

More than 4.5 million gallons of watery sludge have been pumped from the site in the past two weeks. By draining the water, officials feared a void was being created that could cause settling. The sludge and water are separated before the water is sent into the storm sewer system. The sludge is taken to the city’s landfill.

Engineers determined, however, the surface pressure of the houses above was dissipated at the depth where the water was found, so shifting seemed unlikely.

As a precaution, public works employees have been using laser-guided survey equipment to monitor building elevations since the problem arose. No changes have been recorded.

Residents are keeping a close eye on what is happening, and one resident for sure is unhappy about the situation.

The trash bin into which the groundwater is flowing, and from which it is leaking, is sitting right in front of Nancy Bulkely’s house at 2321 Wilkes Ave. Not only does it not look good, it doesn’t smell good, she said.

“It’s horrible to have to look out your front window and see that and smell the sewage,” Bulkely said. “And when the summer time comes, the mosquitoes are going to be terrible.

“I’m going to have to look at that next summer, too.”

Bulkely said she has been living in the house for six years.

When they were talking about the project, Bulkely said, she had no idea “it would be right in front of my house.”

“With all the smarts we have, this is the best they could come up with?” she asked, pointing at the water-filled trash bin.

Mark and Denise Oberhaus have lived in their home at 2405 Wilkes Ave. for 28 years. The messy part of the construction is a few houses to their south and across Dover Court.

Like most homeowners along the street where the construction is being done, they can’t use their sidewalk.

“And we can’t park on the street,” Denise Oberhaus said.

It makes it nearly impossible to have guests, she added, as there is no place for guests to park.

But they did get a new sidewalk along the side of their house out of it, she said.

“The alley is our street now,” she said. It was a safety issue, she added.

“They first offered to put down gravel, and I told them as long as they can shovel it, that’d be fine,” she said. “They came back and said they’d provide us a sidewalk.”

While they support the construction, neither had any idea it would take up so much space.

Still, Mike Oberhaus said, “it doesn’t make a lot of noise. The workers are gone by 5 p.m. But we got the lucky spot. If we had a corner house, it would be a different matter.”

Both think the construction will take longer than the two years they’ve been told.

“When it’s done, I’m going to park in front of the house and prance inside,” Denise Oberhaus said.

The watery level was hit at about 30 feet and returns to hard clay at about 45 feet, according to the city. The diversion tunnel is being dug in that area at a depth of 65 feet.

Clarke said city workers have been diligent in talking to neighbors and being proactive toward their concerns but did not send out a letter about the aquifer situation because they did not want to alarm them needlessly.

“Before we started digging, we had a big town hall meeting and said, ‘There is a big hole coming to your front door,’” Clarke said. “We told neighbors, ‘We’re going to work with you to make it work.’ We have done that, and we continue to do that.”

Alderman Ray Ambrose, 4th Ward, who chairs the city’s public works committee and represents the ward where the work is being done, said the response was the correct one.

“We knew doing this project there would be challenges,” he said. “This is a non-issue.”

Bill Simpkins, a professor of hydrogeology at Iowa State University, Ames, said the situation the city encountered might simply be that the water table sits higher in that area or it is a saturated zone in the soil, rather than a “perched aquifer.”

“In our part of the country, they use this term, but they don’t really prove it,” he said. “What happens is the water table mounds and has a high spot above a clay layer. We see that a lot in our part of the country.”

Clarke said the city wasn’t going to pay to investigate whether it is a true aquifer or to determine its size. He did note the topography is somewhat humped in that area.

He said that despite the recent water problem, the project remains on schedule.