Load tests begin for St. Croix Crossing bridge project
By ERIK SANDIN – Stillwater Gazette
The work is "boring," but the first sign that construction of the new St. Croix Crossing bridge in Stillwater will happen.
Two barges anchored in the river south of the Historic Lift bridge are the sites of $3 million load tests being performed on the river bed this summer to provide engineers with information on soil conditions under the river.
Officials with the Minnesota and Wisconsin departments of Transportation took media members, local officials and business leaders out on two boats to view the barges and explain the load tests and mitigation work on the project.
The new four-lane bridge will six sets of piers in the water and a seventh set on land on the Wisconsin side of the river. The span will run from Oak Park Heights to St. Joseph Township, Wis.
Aboard one barge near the Wisconsin side of the river, workers were driving 90 feet of pile into a 50-foot base, eventually reaching 140 feet below the river to dense soil, said Paul Kivisto, MnDOT’s St. Croix Bridge construction engineer. He added that the contractor’s crew would drive four piles in one location.
"This test is only for the piles," he said.
Aboard a second barge in the middle of the river, a second crew was drilling into the river bottom to install a 24-inch diameter pipe filled with concrete, Kivisto said. He added that crews aboard both barges would perform pressure tests on the piles and drill pipe.
Load tests are done for several reasons, MnDOT officials said. Engineers must consider types of loads when designing and building bridges: the bridge itself, utilities and vehicles. They also must determine that the span will stand up to forces from water, wind, snow, ice, temperature chance and possible barge impact.
Load testing gathers date for more refined foundation designs; helps contractors prepare bids that include fewer contingencies; provides information on whether the proposed foundations can be efficiently constructed in the St. Croix, and reduces risk and project costs.
Although the testing is a small part of the estimated $690 million cost of the new bridge, a MnDOT handout said the estimated benefit of the tests is that it reduces foundation costs by $10 to $15 million.
Kivisto said load testing on the river is expected to be finished in late summer.
"They’ll be completely out in August," he said. "We hope to have a report sometime in (late) August."
Along with performing load tests, MnDOT tracks how that work affects the river environment.
"We’ve got park properties on both sides that we’ve got to protect," said MnDOT spokesman Todd Clarkowski.
Clarkowski noted that the St. Croix is a designated wild and scenic river with historic properties and archeological sites.
"The mitigation impact, it’s unique to this project," he said.
Among the environmental concerns regarding the bridge project are the impact on Higgins’ pearly eye mussel habitat along the Wisconsin shoreline, zebra mussels, fish habitat during spawning, issues that arise during construction, barge decontamination by the contractor and water quality protection.
Clarkowski said Minnesota Department of Natural Resources divers found on mussel species on the federal protected list and several other mussel species on the Minnesota and Wisconsin state lists.
"There are good mussels. When they (DNR divers) find those mussels, they’ll try to relocate them," he said.
As for zebra mussels, Clarkowski and Dwayne Stenlund of MnDOT’s Office of Environmental Stewardship both said MnDOT follows the DNR’s recommendations for zebra mussel decontamination. Stenlund said any equipment pulled from the river would be left to sit on land for at least seven days or more.
Stenlund also conducts daily tests of river water at the barge sites for clarity, turbidity and pH.
The barges in the river also have primary and secondary containment systems to prevent any possible contaminants from either getting into or spreading in the river, Stenlund said
"Whatever happens on the barge, stays on the barge," he said. "When you bring in brand new steel, it might have oil on it."
Other MnDOT housekeeping practices for the project include dust control, spill prevention and response program, preventing spread of aquatic organisms and noxious weed control, refueling management, trash control, Porta-facility tethering, chemical lockdown and lockout and waste storage.
As the tour was ending, Clarkowski pointed to the Xcel Energy King plant’s inactive coal barge unloading area that’s being used as a staging area for bridge project barges. He said as part of the project’s mitigation work, large concrete cells in the river and the unloading system will be removed and any rubble in the cells will be allowed to sink to the river bottom.
That mitigation work occurs at the end of the project. Clarkowski said the bridge and approaches would be constructed in stages, with the Minnesota approach work bid in February, followed by the bridge bids in fall 2013 and finally, the approach on the Wisconsin side bid in September 2014.