Diameter of the tunnels range from 17 to 32 feet, and are roughly 275 to 325 feet underground in dolomite limestone, with shale underneath and softer rock, clay and soil above. The tunnels can hold up to 405 million gallons of wastewater. If the same amount of wastewater were stored in railroad tank cars, it would take two trains stretching from Milwaukee to Chicago to contain it. The tunnels comprise smooth rock surfaces that are plugged or lined in places with epoxy, concrete and grout. About 40 percent of the tunnel system is lined.
Junction boxes intercept overflows before they spill into the waterways.
Gates, controlled by a computer system, allow overflows to enter the dropshafts. Twenty-four dropshafts, six to 15 feet across, convey overflows to the tunnel while controlling the force of the moving wastewater. The tunnel system's official name is the Inline Storage System. The tunnels accept the excess at those 24 points to relieve demand on the existing system. The Central Control System - The Central Control System accumulates and measures 2,500 pieces of information every ten seconds.