AWWA DSS65664

In 2005, construction began on the Southwest Water Main for Charlotte-Mecklenburg Utilities. When complete, this 72-inch water main will be the largest potable water main in the Carolinas. It will reinforce Charlotte's existing water main system by allowing larger volumes of water into the system and allow the city to serve the rapidly growing southwest areas of Mecklenburg County. When implementing large infrastructure into an existing city, alternate installation techniques become necessary. Open-cut trenching through airport runways, airports, and highways is not an option. This project includes 12 trenchless crossings of 96-inch and larger which were complete or under construction when this paper was written. These tunnels cross airport runways, state highways, railroads, utility crossings, and city streets, and each has unique challenges. This paper presents a case study of the Southwest Water Main tunneling and trenchless crossings. Topics discussed include: permitting tunnels in today's climate and regulators concerns; sharing the risk and balancing the cost of large crossings against a utilities' budget and willingness to take on risk; building tunnel bids; tunneling options; and, lessons learned from the Southwest Water Main project. Of particular interest is the use of a rock tunnel boring machine to construct 2,500 feet of 97-inch "bald" tunnels (no liner plates or casing pipe) through competent rock. This technique is used to cross an airport runway safety zone and two North Carolina Department of Transportation highways. Includes reference, table, figures.