An amended U.S. Army Corps of Engineers operating permit has cleared the way for dumping tons of rock into the St. Croix River to speed up construction of the superstructure portion of a new four-lane bridge.

The rock, used for a raised road known as a causeway, will allow contractor Lunda/Ames to start building atop Pier 8 this winter, rather than waiting for spring to start the work from a barge, said Jon Chiglo, who's managing the project for the Minnesota Department of Transportation.

"Logistically it makes it easier for a contractor," Chiglo said. "It gives them an opportunity to get ahead of the curve and before the ice is out. One of the challenges we've always had for this project is access to the river."

The Jan. 7 permitting letter, signed by Corps of Engineer regulatory chief Tamara Cameron, specifies that the causeway must be completely removed from the river within 60 days of completion of the bridge project. Her decision modifies an earlier permit, issued in March 2013, that authorized construction of the bridge.

Under current projections, the new St. Croix River bridge is scheduled to open at Oak Park Heights in late 2016.

Additional restrictions for the causeway came from the Minnesota Pollution Control Agency (MPCA), which certified the permitting request "because there is assurance that the activities identified will be conducted in a manner that will not violate applicable water quality standards."

Required mitigations include using "washed" rock and not "clean" rock. The latter, the MPCA said, "often has a lot of material stuck to the riprap that will wash off when placed in the river and can potentially cause a large plume of turbid water for several weeks after placement. Using washed riprap should avoid this potential water quality violation."

Also required is a "floating silt curtain" that will be installed around the causeway to contain sediments disturbed during construction. "This should limit potential impacts to the river to a confined area," wrote Catherine Neuschler, supervisor of the MPCA's unit that determines resource management rules.

Chiglo said the area where the causeway is being constructed has been cleared of sensitive mussels — as required before construction began. The causeway itself will be contained within that "footprint" to avoid harm to the river's ecology. It will extend about 165 feet to Pier 8, with a driving surface 36 to 50 feet wide.

"It's not a unique approach when you're working in a river," Chiglo said of the causeway. Construction atop the four remaining piers in the river will be accomplished from barges once the ice melts, he said.

The St. Croix River is one of 203 nationwide that falls under the protection of the U.S. Wild and Scenic Rivers Act. In 2012, Congress granted an exemption to that federal law, allowing bridge construction to proceed after years of debate and legal challenges. □