Testing of soil conditions in the St. Croix River could help decide whether fewer piers are needed in the water to support a new four-lane bridge, which would reduce traffic noise, said engineering manager Jon Chiglo of the Minnesota Department of Transportation.

Depending on the findings, Chiglo said last week, the bridge could be built on five sets of piers in the water -- two legs to each set -- instead of six. A seventh pier set on land will support the bridge on the Wisconsin side of the St. Croix, where it will climb through a crevice in the bluff.

"A lot of the [public] concern is with the noise of this bridge," Chiglo said during a boating tour of the testing site about two miles south of Stillwater. Although the concrete "box girder" style of the bridge will absorb sound, he said, eliminating a pier would reduce the number of joints that allow the roadway surface to expand and contract. Fewer joints mean less traffic noise, he said.

Preliminary work on the $676 million bridge project began in June as Carl Bolander and Sons, a St. Paul company, began moving barges onto the river for pier testing. The two-state project, aimed for completion in late 2016, includes extensive highway work on both sides of the St. Croix as well as environmental protections and construction of a loop pedestrian trail over the Stillwater Lift Bridge once it closes to traffic.

Minnesota and Wisconsin will share costs in a joint project with the Wisconsin Department of Transportation. MnDOT is the lead agency.

Crews hammered two 24-inch piles and two 42-inch piles into the river bottom to calculate how much weight can be supported from above and also from the sides if hit by a runaway barge or other obstruction, said Paul Kvisto, the MnDOT engineer in charge of the pier testing.

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“It’s a living river,” MnDOT’s Dwayne Stenlund said of how the St. Croix River’s ecology changes from day to day. Stenlund is responsible for preventing the river from being contaminated by the bedrock testing for a new bridge, done via barges.

The largest pile weighs 50,000 pounds, he said. The piles -- different from final piers that will support the bridge -- will be cut off below layers of sand and mud once the testing ends.

Noise from pounding piers into place has concluded, said Todd Clarkowski, the MnDOT engineer coordinating the overall project. The "load testing" will provide information about soil conditions that will determine the bridge's foundation design. MnDOT should have the results by the end of August, he said.

Fighting contamination

The transportation agency also is taking precautions to prevent contamination of the St. Croix, one of 203 rivers classified as "wild and scenic" and protected under federal law.

"We don't want to contaminate this resource or any resource around the St. Croix," said Dwayne Stenlund, a MnDOT scientist responsible for environment protection. "We're trying to prevent any changes in this river as a result of this project."

Several conservation organizations have been monitoring the pier testing independent of MnDOT, but none has complained about water problems, he said. "There are a lot of concerned groups and rightly so," Stenlund said.

Several measures have been taken to prevent contamination, such as placing plastic liners under the barge cranes to collect spilled fuel and oil, he said. Water that collects on the barges is filtered before it's returned to the river. Workers spit sunflower seeds -- a popular snack on the barges -- into a plastic swimming pool.

Stenlund said he also takes water samples on all sides of the barges to watch for contaminants. MnDOT doesn't want to alter the river's ecology whatsoever, although "it's a living river" subject to rainfall and other influences, he said.

Clarkowski said that the Stillwater Lift Bridge, built in 1931, drains water, oil and other traffic contaminants straight into the St. Croix from openings in the deck. By comparison, the new bridge will pipe runoff into stormwater ponds on land. Monitoring water quality around the barges gives MnDOT experience for construction of the bridge, which will run about 5,080 feet, Stenlund said. "Starting small is really a blessing," he said.

Details are available at www.mndot.gov/stcroixcrossing.

Kevin Giles • 651-925-5037 Twitter: @stribgiles