Texas & SoutheastTexas & Southeast Construction NewsBest Projects2015

Best Highway/Bridge: Engineers Take Different Approach for Unique N.C. Interchange

Charlotte's Interstate-485 Outer Loop — *PHOTOS: AEROPHOTO*

Charlotte’s Interstate-485 Outer Loop, one of the state’s most vital urban interchanges, links I-77 to I-85 as well as other major arterials. In 2010, the North Carolina Dept. of Transportation (NCDOT) initiated a design-build-finance advertisement for the $124-million interchange modification as part of the agency’s effort to close the last six-mile gap of the newly constructed I-485 outer loop to the north. The project’s scope included the reconstruction of the existing I-485/I-85 interchange into a fully directional four-legged interchange.

During the RFP phase, the design-build team of contractor Lane Construction Corp. and engineer STV proposed replacing the four-level stack with a two-level turbine interchange, a configuration rarely seen in the U.S. Lane/STV concluded that the turbine layout—also called a “whirlpool” exchange—could enhance safety for both motorists and construction workers. The design choice—never before used on a U.S. interstate highway—also significantly lowered the project’s cost, earning Lane/STV’s unique proposal a “best value” determination from NCDOT.

The project consisted of widening two miles of I-85 to accommodate additional ramp lanes, widening/construction of 1.4 miles of I-485 and the turbine interchange, which comprised eight ramps/loops and 18 precast concrete girder bridges. Other advantages included the elimination of high-level flyovers, including 1,000-ft-long bridges, which helped to avoid major bridge construction 75 ft in the air—and directly over I-85, which is used by 110,000 vehicles per day. Lowering the interchange’s height by approximately 50 ft allowed for the elimination of long, steep flyover profiles, and negated the need to import 2 million cu yd of fill material, reducing the number of trucks needed.