Chicago has had more than two days of record setting rain with many areas receiving over 7 inches yesterday alone. That can be a problem around here because many parts of Chicago are prone to serious flooding. There has been some flooding and some areas have been evacuated, but it could have been worse without Chicago's massive sewage overflow system, the Deep Tunnel and Resevoir Project (TARP).
Construction of TARP began in the 1970s and will be completed in 2019. It will include 109 miles of huge underground tunnels that intercept sewer overflow and convey it to large storage reservoirs (Map). Overflow during rainstorms can be held for later treatment in sewage plants. The first parts of the system became operational in 1985. In terms of size, cost and time to complete, the tunnel is one of the largest civil engineering projects ever undertaken. The EPA is picking up 75% of the estimated $2.5 billion cost for the system.
The main tunnel is 35 feet in diameter, bored in solid limestone 240 to 350 feet below ground. Dr. X personally shoveled many tons of that limestone as crushed gravel during a couple of summers in the 1970s when he worked for a paving contractor. We used the gravel as a base for large parking lots. They had to do something with all that limestone.
Workers inside the Deep Tunnel.