On April 13, 1992, adjacent construction tore a 20-foot-long hole through the wall of a tunnel 20 feet beneath the bed of the Chicago River, some 50 feet beneath downtown Chicago. Over 200 million gallons of water surged through an extensive series of underground tunnels, affecting more than 30 major buildings, including City Hall and the financial markets. Lower levels of major office high-rises held up to 40 feet of water, and the city center was evacuated out of fear that electrical or utility connection failures could endanger lives.
Exponent geotechnical, structural and materials engineers performed an investigation to help determine the cause of the accident. It was determined that, in 1991, workers were replacing wooden pile clusters at a bridge pier and apparently had no knowledge of the existing tunnel system beneath them. Pressure from soil displaced by pile driving collapsed the concrete tunnel wall inwards, exposing clay and new wood piles and allowing mud and water to seep slowly into the tunnel. Many months after the initial damage, the breach abruptly destabilized, allowing the river to flood downtown Chicago and causing estimated damages of $2 billion.