The first visual evidence of a new St. Croix River bridge will appear by June when workers begin testing bedrock below the water to determine how much weight it can hold.

Barges laden with heavy equipment will hover over two test sites in the bridge's path, driving pilings equipped with sensors to measure whether piers can support thousands of tons of weight.

"It will be pile-driving, so there will be noise," said Paul Kivisto, the metro region bridge engineer for the Minnesota Department of Transportation (MnDOT). "We are using a hammer to drive big steel shells down into fairly dense material at the bottom of the river."

Data produced from the tests will help determine the final design for the bridge, the centerpiece of a $676 million project that includes extensive road building in Minnesota and Wisconsin.

The four-lane bridge will have six sets of piers in the water -- two legs to each set. A seventh pier set will support the bridge on the Wisconsin side of the river, Kivisto said. The hammering noise, while loud, will be limited to short intervals over a period of 20 days, he said.

"This will help us optimize the design as well as reducing the risks during construction," Kivisto said. The new bridge, he said, "will have a significant amount of substructure in the river."

The mammoth construction project drew more than 100 prospective contractors, all hoping to corner a share of the spending, to a recent informational meeting at the MnDOT bridge office in Oakdale.

MnDOT will award four major contracts in the coming months, for the load testing, final bridge design, independent peer review of that final design, and relocating the historic Shoddy Mill buildings from Oak Park Heights to a new home along the river in Stillwater. Proposals must be delivered to MnDOT no later than April 27.

The contract for the bedrock work, known as foundation load testing, will be awarded by May 15, Kivisto said. Results from that testing should be available in September, he said.

"When we look at all these contracts together, there's $20 million that MnDOT has committed to over the next month," project leader Jon Chiglo told Washington County commissioners last week.

Until now, most public debate over the mammoth undertaking has centered on a new bridge, but the project also includes a sprawling package of highway makeovers and environmental safeguards. Contractors wading into the project will have to navigate numerous historic, cultural and landscape considerations in the vicinity of historic Stillwater, the site of the state's territorial convention in 1848.

Traffic congestion related to the two-lane Stillwater Lift Bridge, opened in 1931, was cited as the leading reason for a new four-lane bridge.

"Mainly our core problem sits in downtown Stillwater," project engineer Todd Clarkowski told about 200 engineers and contractors who attended the informational meeting.

The bedrock testing will involve two types of pilings at each of the two sites -- a steel shell hammered into the river floor and a concrete shaft drilled into place. Pilings will sink through water about 25 feet deep, then through several dozen feet of river muck that's accumulated over generations, and finally through more than 50 feet of loose and dense sand to reach sandstone below that.

Once the test pilings have done their job, producing data that will assist in the final bridge design, they'll be cut off two feet below the river bottom. Water pumped out of the shaft will be restored to "the same quality as other water in the river" before it's discharged, Kivisto said.

An earlier MnDOT attempt to conduct the load tests sputtered in April 2010 when the National Park Service, the agency that manages the St. Croix River, withdrew its endorsement of the borings during a federal court review of the construction. The bid for that job was awarded to a St. Paul construction company, Carl Bolander and Sons, but Kivisto said bidding now starts anew with everyone considered.

The testing will require a permit from the U.S. Army Corps of Engineers.

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