| March 6, 2002
News Release
Digging is now complete for the 15-foot in diameter, 1.2 mile long Mercer Street Tunnel, the widest pipe in King County's wastewater collection system. The tunnel will provide storage and conveyance for combined sewage flows as part of the Denny Way/Lake Union Combined Sewer Overflow Control Project. The joint effort by King County and Seattle will greatly reduce pollution in Lake Union and Elliott Bay. "This tunneling technology allowed us to build a major stormwater storage pipe while causing minimal disruption to businesses and neighborhoods above ground," said King County Executive Ron Sims. "The precision of the machine is amazing and the result is a successful tunneling project that benefits the people and the water quality of the region."
When the project is completed, it will provide 7.2 million gallons of storage capacity for combined sewage and stormwater. During dry weather, the tunnel will be empty and flows will travel through the existing collection system to the treatment plant. As flows rise during storms, the high flows will be diverted into the tunnel so that overflows to Lake Union and Elliott Bay will not occur. Since May 2001, workers built about 40 to 50 feet of tunnel each day, five days a week. Concrete segments were installed behind the tunnel-boring machine to form the permanent tunnel lining. Crews used a laser guidance system to ensure proper alignment of the tunnel. Tunneling technology was well suited to this project, given the area geology and the urban character of the project site. Noise, traffic and related construction impacts are confined to areas near major arterial streets, thus minimizing the impact to residential and business neighborhoods. Tunneling was able to proceed under the street for many months without disrupting the busy Mercer Street corridor. The $146 million project is a cooperative effort between the King County Department of Natural Resources and Parks and the City of Seattle to control sewage overflows during storms. The entire project will be completed in 2004. An Infrastructure Grant from the United States Environmental Protection Agency funds the project, in part. Frank Coluccio Construction Company is the general contractor for the Mercer Street Tunnel portion of the Denny Way/Lake Union Combined Sewer Overflow Control Project. As part of this contract, the company also built three tunnels under the Burlington Northern Sante Fe railroad tracks near the grain terminal on Elliott Bay. Coluccio built the 102-inch Elliott Bay Interceptor, a pipe that carries wastewater to King County’s West Point Treatment Plant. The company will connect the new tunnel to the Elliott Bay Interceptor. Refer to the project Web site at http://dnr.metrokc.wtd/dennyway/. Related Information
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The state-of-the art tunnel-boring machine dug its way to daylight this morning after tunneling under Mercer Street on lower Queen Anne Hill for 10 months. 
Discharges of untreated sewage and stormwater currently occur in Lake Union between 10 and 115 times per year during storms. About 50 such discharges occur in Elliott Bay at Myrtle Edwards Park each year. The completion of the project in 2004 will mean the number of untreated discharges will be nearly eliminated.
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