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How Did The Red Line Get Under The Chicago River?

Ever wonder exactly how workers tunneled under the Chicago River for the Red Line?

State Street Subway construction, 1940.

DOWNTOWN — How did the Red Line get under the Chicago River?

“Even today, placing the subway tube into the Loop would be an amazing feat,” said Peter Alter, historian and director for the Chicago History Museum‘s Studs Terkel Center for Oral History.

So how was it done almost eight decades ago?

“Workers not only had to put the enormous tubes in the Chicago River, but had to ensure they were properly aligned so that they could be connected to the dry tubes on either side,” said Bruce Moffat, the CTA‘s manager of signage and wayfinding, and a transit historian. “This is the only time an engineering project of this magnitude and complexity has been done in Chicago.”

Credit: CTA
Shields for tunneling Loop subways, 1939.

The tubes — which are actually built as a single entity — were constructed in the late 1930s in South Chicago in a dry dock at 101st Street and the Calumet River and then floated up Lake Michigan and into the Chicago River to the construction site at State Street, according to CTA spokeswoman Irene Ferradaz. The twin-tube structure is 40-by-200-by-23 feet.

The 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, Ferradaz said.

Credit: CTA
State Street Subway construction, 1940.

The 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.

Credit: CTA
Starting excavation of SB tube southward from shaft dug from surface, 1939

Biscuit cutters are large, circular machines that moved slowly through the ground, grinding up earth, clay and rock, and leaving an open tunnel behind them.

The actual connection would be done later.

After the tubes had been successfully lowered, 5 feet of fill was placed on top of them to create a new riverbed.

After the new riverbed had been restored, the tubes were connected. To avoid water rushing in from both sides, the connection to the tubes was done by using cofferdams that were built on both the north and south sides of the tubes. The ends of the prefabricated tubes were fitted into the cofferdam and sealed watertight by concrete collars.

Credit: CTA
Construction of State Street Subway tube, shield-dug section, 1940.

After that connection had been made, digging from both sides of the river finished to complete the connection in 1940.

Ferradaz would not be specific, but said routine maintenance is done across the CTA’s entire system, including the portion under the Chicago River.

The subway under the Chicago River was completed in Oct. 17, 1943 — on time — according to Encyclopedia of Chicago and the CTA.

Credit: CTA
Completed subway tubes and crossover – Division-Clybourn 1940

“The installation of the subway system exemplifies a city and country fighting the Great Depression with lasting infrastructure improvement projects,” Alter said.

Other fun facts of the Red Line’s construction, according to Ferradaz:

• The tunnel-boring machines were 25 feet and 3½ inches in diameter, exerting a 200-ton push and 4,800 tons of pressure. Following the machines were miners, who lopped off the clay forced through the six holes of the machine. You can tell where cutter machines were used, because the subway tube has the circular shape in those areas. Cutter machines were used from 11th Street to the river.

Credit: CTA
The corner of State and Randolph.

• Traditional mining was also used, where workers used knives to slap into the clay and carve out small areas. Larger areas and deeper tunnels were created using a power-operated winch, which would pull the wire through and curl off a long slab of mud for mockers to heave into cars for transport. This type of mining was used in areas where the subway is horseshoe-shaped. Hand-mining was used from the river to north of North/Clybourn.

• Cut-and-cover — where a structure is built inside an excavation dug from the surface downward and covered over with backfill material when construction of the structure is complete — was used for the station mezzanines, tubes near the entrance portals, the crossover at Grand and the 13th Street “Y” connection. Note: The Clybourn/Division crossover was hand-mined.

Credit: CTA
River xing tubes being towed by tug up to Downtown 1939
Credit: CTA
State Street Subway construction, shield tunneling.
Credit: CTA
Preparing to submerge prefab tubes in trench in bed of Chicago River, 1939.

The story first ran in DNAinfo in 2016.