Engineers got a surprise recently when they began probing the ground under a soccer field as the site for a new outdoor hockey rink next to Edina's Braemar Arena.

A few feet underground were big slabs and chunks of concrete, probably decades-old debris from construction of nearby highways.

It isn't the strangest thing RJM Construction of Minneapolis has discovered on a building site. When a St. Paul lot was excavated for school construction in the past couple of years, engineers discovered soil that smelled like gasoline, then buried tires, and finally the rusting remains of a car.

"In this case, the site was a former auto site, unknown to anybody," said Paul Kolias, a project executive with RJM. "You don't find out until you want to do a project that debris is in the way."

For cities, the discovery of unstable soil or mystery objects underground creates delays and can add significantly to project costs. In the case of the Braemar soccer field, Edina had the choice of spending $250,000 to remove the concrete or spend $112,000 to strip off and replace 5 feet of topsoil above the debris to create a solid base for the covered hockey rink. It chose the cheaper option.

This winter in Richfield, construction on an addition at the rear of its ice arena uncovered part of a mattress, bedspring and debris that City Manager Steve Devich said may have been tied to highway construction. The city delayed other planned improvements in and around the arena to more thoroughly investigate what's underground before doing more construction.

And this month, RJM was doing a "soil boring test" to determine the soil's contents around the Plymouth Ice Center when it found that the soil was not stable enough to build on, Kolias said. More investigation will determine the exact issue, but Kolias said there may be underground debris from previous construction or decayed organic matter that makes soil unstable for building purposes.

The problem isn't a rare one. Kolias said four of the nine projects he's working on now have some kind of issue with soil.

Josh Van Abel, senior engineer with Braun Intertec, said soil engineers almost expect to find things underground in cities and suburbs.

"Anytime you're dealing with a site that was developed beyond the recent past, you find things you're surprised by," he said. Years ago, he said, "It wasn't a big deal to put things underground then. We're a little more careful now."

In some cases, cities may actually have encouraged dumping of some types of fill to raise boggy spots that were planned as parks.

"Certain types of municipal projects, like a city park with green space, walking paths, maybe a fountain or a small warming house, they don't really need quality material or soil to support what's above," Kolias said.

"A lot of times when they built a park … they would fill it with debris, construction stuff, various things, put black dirt over it and plant grass seed.

"Voilà! You have a park."

Putting fill in the ground may have been necessary in some places. Much of Edina was platted on swampy ground. Drainage is still a problem on many city athletic fields.

While Van Abel considers finding an underground mattress unusual, finding building debris is not.

"When they knocked down a business, they just buried the inert debris or wood," he said.

The surprise find

Sometimes projects are in full swing before a problem is found. When Edina began removing the old Edina Realty building near 50th Street and France Avenue last year, which it bought for possible parking expansion, it discovered a buried tank that no one knew about. The tank was still full of dry cleaning fluid, a hazardous waste, apparently from the 1970s.

Multiple environmental studies had been done on the site in recent years, but the tank went unnoticed despite two rounds of subsurface testing, said Bill Neuendorf, Edina's economic development manager. The tank was discovered only after the building was demolished and crews began removing the concrete floor and foundation.

Two months of delay followed. The dry cleaning fluid was pumped out, the tank was removed and about 16 feet of soil had to be removed before the Minnesota Pollution Control Agency was convinced the site was clean, Neuendorf said.

The work cost $75,000 to $100,000. He said the extra costs will be paid for with escrow money that was withheld at closing when the city bought the building.

Not all soil issues are so complicated. At Braemar, where hockey players are eager to have the covered outdoor rink available for next winter, the concrete chunks and slabs that were buried decades ago were deposited in an organized fashion and will be left there, city officials say.

Kolias said that 5 feet of soil above the debris will be removed and the dirt 5 feet down will be compacted to create a firm bridge over the debris. The soil that was removed will be mixed with other good fill and compacted again.

"This building type doesn't need deep foundations," he said. "It would cost hundreds of thousands of dollars to [remove the debris.] … We'll reuse everything that's there and create a firmer base for the structure."

The debris issue, combined with other complications, is expected to push back the opening of the Braemar rink and other improvements to December, a month later than scheduled.