The city of Davenport will use its $1.5 million contingency fund to help keep the $36 million Westside Diversion Tunnel project moving along.

Thanks to what workers are calling an underground “lake” of water that no one knew existed, engineers are having to get creative to finish about 1,000 feet or so of the tunnel.

Discovered in early March, the water was believed to be a manageable obstacle.

When work crews turned off their pumps, however, the water rose right back into a giant cavity under the city.

“We began to sense this was not a water pocket,” said Mike Clarke, head of Public Works. “We brought close to 1,000 years of engineering expertise into one room and began thinking our way out of this thing.”

With the size of the collection of water estimated at 2,000 feet by 2,000 feet, one thing is clear, Clarke said: “We’re not going to drain the bathtub. It’s too big.”

The solution, instead, is a collection of “de-watering wells.” The plan is to use wells and pumps to control the water in specific areas where the tunnel is being dug. When the tunnel drill reaches certain areas, the wells will be used to keep the water out of its way.

“How many wells?” Clarke asked. “We don’t know.”

He also does not know what the final extra cost will be.

“I’m hearing it’s going to be somewhere around $4 million,” Mayor Bill Gluba said. “I do know that we paid $1.7 million for a design, which included borings to show what’s underground, and we expected it to show if there’s a lake down there.

“I’m looking into the possibility, with the council’s approval, of having someone review the work of the engineer we paid.”

Clarke said the cost likely will be somewhere north of $2 million, “but hopefully not over $4 million.”

“We’ve got one location where we’re going to try out our engineering and see how that works,” he said. “We may have to alter our hydraulic model a little bit. But once we know what that will cost, then we’ll be able to extrapolate the cost out to the rest of the distance.”

Clarke said he will meet with the City Council in a month when he has a more solid handle on the cost.

Aldermen unanimously approved of Clarke’s use of the contingency funds at Wednesday’s Committee of the Whole meeting so that the project can continue to move forward.

He added that a contingency fund of 10 percent to 15 percent is written into every construction project because unforeseen issues usually arise.

The total cost of the three phases of the Westside Diversion Tunnel, currently in Phase II, is estimated to be about $36 million because some bids have come in lower than expected. “That’s a good ballpark figure,” Clarke said.

Phase I was awarded at $12 million and changes still are being cleared out in that contract. Phase II was awarded at $15 million. Phase III has not gone to bid.

Clarke added that so far the project is ahead of schedule and is on budget.

The city paid Stanley Consultants, whose Quad-City office is in Muscatine, for the tunnel’s design.

Vice President and Senior Project Manager Pat Mullin said more soil borings were performed on the project than is typical. The studies showed pockets of sand and gravel, which contained water, and the design included a process for dewatering them.

The discovery of a much larger pool was “an unforeseen condition,” he said, adding, “Any time work is performed in underground conditions, there is an element of uncertainty. The quantities of water now found were unexpected and could not have been determined by standard engineering investigations.”

Mullin also said the boring samples were not taken by Stanley, because the company hired another firm for some of the tunnel work. The company, which he did not name, specializes in geotechnical work, and another firm that specializes in tunnel design also was on the job, he said.

“Stanley Consultants will continue to work with the city to resolve the matter,” he said.

Although the mayor acknowledges having no engineering background, he said he wants proof that enough studies were done before the digging started. He said the costly probe of the path should have revealed the water.

“They didn’t find, essentially, a lake down there,” he said. “How come they didn’t find the water, and we did?”

Clarke said he thinks the engineer used a reasonable number of borings at appropriate locations along the tunnel’s route. It is not unusual, he said, for boring samples to miss what cannot be seen below the earth’s surface.

Fortunately, the delay in finding a solution for dealing with the water was well-timed, he said. The equipment being used to dig the tunnel happened to be scheduled for routine maintenance and has been out of commission for several weeks.

He said he is confident the new strategy will work, adding that he cannot be certain, because the solution is a hydraulic model that has not been tested.

Meanwhile, Alderman Ray Ambrose, chairman of the council’s Public Works Committee, said he wishes the city had committed money to the tunnel two decades ago when the cost was about one-third what it is today.

He said too many government regulations, along with normal cost-of-doing business increases, are taking the price tag ever higher. He said he is not convinced the plan for dealing with all the underground water will be successful, but he hopes the estimated costs are accurate.

“We’re looking at the top end (of cost) and praying for the bottom end,” he said. “We’re not going to know anything until we dig some wells.

“The poor taxpayers are holding the bag.”

(Thomas Geyer contributed to this report.)