Urban Diamond SC Interchange Project Sparkles

Fri July 11, 2003 - Southeast Edition
Tracy Carbasho



A $16-million urban diamond configuration project will improve the traffic flow at the U.S. 378 and I-26 interchange in Lexington County, SC.

The project, which began in January and is slated for completion in October 2004, consists of a bridge replacement and interchange modifications in West Columbia. South Carolina Department of Transportation (SCDOT) officials said the improvements are necessary to ease the traffic congestion in the heavily-traveled area.

“Traffic control has been the most difficult part of the project because it’s a high-traffic area with many businesses and hotels in the vicinity. The Lexington County Medical Center is located less than 200 ft. from the ramp,’’ said Brian Keys, program manager of SCDOT. “We’re replacing the U.S. 378 overpass bridge, which goes over I-26. The current bridge is a traditional slab-and-pier structure, but we’re replacing it with an urban diamond bridge that will be twice as wide as it is long. The ramps will actually be part of the bridge structure.’’

The single-point urban interchange design, also known as an X-interchange or an urban diamond, is being used extensively in the construction of freeways in large metropolitan areas. This will mark the second interchange of its type in South Carolina. The first urban diamond in the state is under construction in Spartanburg. Ninety percent of the funding for the Lexington County project is federally allocated and the remaining 10 percent comes from state coffers.

The biggest advantage of using the urban diamond is its compact layout, which requires less right-of-way acquisition and the ability to have concurrent left turns for greater capacity. Aerial views show the urban diamond resembles a slim, classical diamond design. The single-point urban diamond ramps are positioned close together to make them effectively part of the same intersection, while the traditional diamond interchange has two ramp intersections at the surface street with one on each side of the freeway.

“The urban diamond design is often used when you have limited space to do the construction. It reduces traffic congestion because there is only one set of traffic lights on the bridge and the width allows for turn lanes,’’ said Robbie Isgett, the DOT’s resident construction engineer. “We’re widening the bridge from five to eight lanes. There will be two lanes going in each direction and four turning lanes.’’

Keys said the new bridge will be 196.5 ft. (57.9 m) long with a width of 290 ft. (88.4 m) at its widest point. The existing bridge is 150 ft. (45.7 m) long. Instead of having two sets of traffic signals at either end of the bridge, one set will be located at the center of the bridge.

The name “single-point” means that all through-traffic and all motorists turning left onto or off of the interchange can be controlled from a single set of traffic signals.

U.S. Group, of Columbia, SC, is serving as the primary contractor on the project, which Keys said is actually the final part of a two-phase effort. The first phase, completed in November, consisted of right-of-way property acquisition.

Isgett said the grade on U.S. 378 has been raised approximately 7.9 ft. (2.4 m) and the ramps will be modified to meet the same specifications. At press time, half of the bridge had been closed for demolition and temporary access ramps had been built to accommodate traffic. Isgett said crews are several weeks away from starting construction on the new bridge.

Before construction of the span can start, a temporary lane must be built along the six-lane interstate so that traffic can be shifted during peak work periods. The project does not call for I-26 to be permanently widened from its existing six lanes.

Isgett said the SCDOT estimates the project, which covers slightly more than 2.7 mi. (4.3 km), is about a month behind schedule as a result of additional excavation, which was necessary to construct the temporary lane and ramps.

Bob Price, project manager of U.S. Group, said the job should be back on schedule once the unexpected work is completed.

“The construction of two new temporary ramps have been delayed due to delays in utilities being relocated,’’ said Price. “This project also has been delayed due to the tremendous amount of rainfall since the project began on Jan. 6. There also has been a very significant amount of muck excavation and pipe underdrain that had to be placed. This work was not anticipated in the original project schedule.’’

Price said approximately 20 employees of U.S. Group are working to get the project back on schedule with subcontractors providing an estimated 15 workers. Lee Construction Co. of Carolinas Inc., in Charlotte, NC, is serving as the bridge subcontractor, while Earth Structures Inc., of Spartanburg, SC, is the mechanically stabilized earth (MSE) wall subcontractor.

U.S. Group has used primarily all of its own equipment, including bulldozers, motorgraders, trackhoes, rollers and loaders, on the project. The most noteworthy piece of specialty equipment is a Komatsu 400 trackhoe used in the mass excavation for the MSE walls.

Price said the interchange job calls for the use of approximately 75,000 tons (67,500 t) of asphalt for paving, approximately 32,292 sq. yds. (27,000 sq m) of pavement removal and approximately 2,224 cu. yds. (1,700 cu m) of concrete for the bridge construction. An estimated 125,563 cu. yds. (96,000 cu m) of dirt was brought by dump truck to the job site.

U.S. Group has completed several large projects for the SCDOT, including the construction of rest area facilities, ramps and new parking lots in Newberry County. The company’s itinerary of current work for the DOT also includes two large road and bridge projects in Charleston County and a major widening project on State Route 802 in Beaufort County.