The Blue Line (nee the Dearborn Subway) actually runs beneath the city's namesake river twice — once under Lake Street and once under Congress Parkway.
The Blue Line's below-river crossings are different than the Red Line's, which sunk enormous tubes into the river at the construction site at State Street. Those tubes are under the bottom of the Chicago River. The bottom of the river at the time of construction was 24 feet from the surface of the water, and the bottom of the bed that was dredged out for the prefabricated tube structure was 52 feet from the surface of the water.
The Red Line's tube structure was sunk by pouring concrete onto the top to weigh it down and sink it. Steel cables attached to two barges on the river controlled the drop into the dredged bed, and divers were used to help guide the tubes to meet workers who were performing hand mining from North State Street and a "biscuit cutter" from the south part of the river.
Both of the Blue Line crossings were hand-mined (as opposed to being tunneled with the "biscuit cutter" shield) and dug much deeper than the Red Line where it crossed under the river, according to CTA historian Graham Garfield, also the organization's general manager for customer information.
Garfield said the Blue Line river crossings could be dug with normal methods, not requiring the prefabricated tubes used for the Red Line crossing, because the nearest stations are farther away from the river crossing.
This added distance "allowed for the room for the tubes to descend at an acceptable grade to a depth sufficient to allow them to be mined under the river without threat of collapse or cave-in during construction," said Garfield, who has a bachelor's degree from University of Illinois and a master's from University of Illinois at Chicago.
The Dearborn Subway was started at the same time as the State Street (Red Line) Subway, but construction was halted in 1942 due to manpower and materials shortages caused by World War II, Garfield said. The project eventually was completed in 1951, he said.
For more neighborhood news, listen to DNAinfo Radio here.