The Denny Way / Lake Union Combined Sewer Overflow Control Project is a joint effort of King County and the City of Seattle. When complete in 2004, it will significantly reduce overflows of combined sewage and storm water that occur in Lake Union and Elliott Bay. The “centerpiece” of the project is the Mercer Street Tunnel, constructed by Frank Coluccio Construction Company, which provides storage and conveyance of combined flows during storm events.

The contract included four tunnels, all driven from a common shaft. This 50-ft. deep shaft was 150-ft. long by 70-ft. wide. The shaft would also serve as the principal excavation for the future Elliot West CSO (combined sewer outflow) facility.

Three short tunnels were driven west from the shaft and under multiple railroad tracks. The crown of the first and smallest tunnel excavated was just over five feet below the operating rail tracks. It had a 72-in. diameter and was 125-ft. long. The deeper effluent and CSO tunnels were pipe jacked 50 ft. below the rail grade. Both tunnels were about 350 ft. in length, with 96-in. inside diameters.