A years-long project will be completed when a new bridge planned near Stillwater, Minn., starts crossing the St. Croix River. But first, crews have to put the pieces together. At more than 5,000 feet wide, the crossing has required a specific approach, as Jeffrey Thompson, MPR News noted:
Crews have been at work all winter at a plant in Cottage Grove, building dozens of giant rectangular concrete tubes, 18 feet tall and 48 feet wide. They're like open-ended boxes 10 feet thick that will line up for almost a mile across the St. Croix from Oak Park Heights in Minnesota to St. Joseph, Wis ... Packed with concrete and reinforced steel, each of the blocks weighs about as much as a diesel locomotive.They start out as steel skeletons. Then workers box up the steel in temporary shells to hold the concrete. They use survey equipment to get the measurements just right on the massive pieces, which can't vary by more than about the thickness of three sheets of notebook paper. They're cast in a giant concrete fabrication building, kept at 50 degrees so the fresh concrete doesn't freeze. When they're cured and strong enough, the bridge segments get moved outside on train-car sized carts to a giant storage yard beside the Mississippi River.
This process is meant to speed up construction, which would have taken years longer if the bridge had been poured concrete.The $600 million project is now expected to be completed in 2016.