Cause
Rehabilitation work on the Kinzie Street Bridge crossing the Chicago River required new pilings. Unbeknownst to work crews aboard a barge operated by the Great Lakes Dredge and Dock Company, beneath the river was an abandoned Chicago Tunnel Company tunnel that had been used in the early 20th century to transport coal and goods. One of the pilings on the east bank was driven into the bottom of the river alongside the north wall of the old tunnel. Although the piling did not actually punch through the tunnel wall, it caused pressure that cracked the wall, and mud began to ooze in. After some weeks, all the soft mud had passed, opening a leak. The situation was very serious because the flood doors had been removed from the old tunnels after they fell into disuse.
Read more about this topic: Chicago Flood