MYRTLE BEACH INTERCHANGE PROJECT OVERCOMES GEOTECHNICAL LIMITATIONS

The design of a recently completed traffic improvement project in Myrtle Beach, South Carolina, had to account for significant geotechnical shortcomings to enable the installation of a 1,250 ft long bridge, the key step in replacing a busy at-grade intersection with a grade-separated interchange:-

A recently completed $120-million project entailed a significant reconfiguration of one of the busiest intersections in Myrtle Beach, South Carolina—the junction of the U.S. 17 Bypass with S.C. 707 and Farrow Parkway, the latter two of which converge at U.S. 17. The junction is known locally as the "back gate" because of its location near what was once the rear entrance to the former Myrtle Beach Air Force Base. Today the roadways offer access to a mixed-use development known as Market Commons.

As the "major north-south artery" for traffic in Myrtle Beach and the surrounding Horry County, U.S. 17 typically experiences traffic volumes of 70,000 to 80,000 vehicles per day at the intersection, says Mike Barbee, P.E., the regional production engineer for the South Carolina Department of Transportation (SC DOT). Another 25,000 to 30,000 vehicles use S.C. 707 and Farrow Parkway each day. Because Myrtle Beach is a popular vacation destination, the project site receives significant traffic associated with tourism as well local traffic seeking to access nearby businesses. Before the completion of the recent interchange project, the north-south traffic on U.S. 17 "became increasingly congested," Barbee says, causing long delays at the intersection. Seeking relief from this bottleneck, Horry County voters approved a plan in 2006 to dedicate local sales tax funding to pay to convert the busy at-grade intersection into a grade-separated interchange.

The solution entailed adding a bridge at the intersection to enable U.S. 17 traffic to pass above S.C. 707 and Farrow Parkway. In 2008, the SC DOT hired the Rock Hill, South Carolina, office of STV Group, Inc.—which has its headquarters in New York City—to lead a team that would provide project management, environmental documentation, permitting, traffic analysis, roadway and bridge design, utility coordination, hydraulic analysis, and geotechnical investigations for the project. The finished bridge consists of a single substructure with two separate superstructures, one of which carries northbound traffic while the other carries southbound traffic. This approach was selected because U.S. 17 will have to be expanded from four lanes to six lanes at some point in the future, and having to widen the substructure again would prove "pretty costly," says Richard Capps, P.E., a senior vice president of STV. Both superstructures comprise six spans ranging in length from 140 to 300 ft.

Situated along the coast of the Atlantic Ocean, Myrtle Beach was built on former marshland, and the coastal soils provide less than ideal geotechnical conditions for a bridge project of this magnitude. Because of the flat terrain at the project site, the planned interchange required the construction of large ramps and embankments to facilitate the grade separation. At the site, the soils consist of approximately 35 ft of very soft to firm clay and pockets of loose sand. After reviewing the results of the borings conducted as part of the geotechnical investigation, one of the geotechnical engineers involved in the project described the native soils at the site as "nothing more than strong water," Capps says. 

Faced with these conditions, the design team had to implement innovative measures to support the bridge embankments adequately and account for the significant settlement that the embankments were expected to experience. Ranging in height from 10 to 42 ft, the embankments were anticipated to settle on the order of 9 to 67 in., Capps says. Deep soil-cement mixing was used to strengthen the ground in the vicinity of the bridge abutments to enable it to support the heavy features. To this end, approximately 35,600 cu yd of cement was used to create adequate foundations for the bridge abutments. Depending on location, cement mixing depths ranged from 49 to 77.5 ft below the surface.

The embankments themselves were constructed in conjunction with mechanically stabilized earth (MSE) walls. Because various project components had to be constructed at different stages, the MSE walls were also constructed in phases. "What we had to do was build the ramps for the interchange first, and then we came back to construct the bridge and the bridge approach embankments," Capps explains. For this reason, the design called for the construction of both temporary and permanent MSE walls, he says. Used to construct the ramp embankment while traffic remained in its original pattern, the temporary MSE walls were embedded within the project by the subsequent construction of the mainline embankment. "For the permanent MSE walls, we allowed settlement to take place, and then went back and put the final facing on the wall after the settlement had dissipated," Capps notes. Ultimately, the project involved the construction of 51,500 sq ft of temporary MSE walls and 30,500 sq ft of permanent MSE walls.

Because of the critical nature of the project, the design team implemented measures that would expedite the settlement process within the embankments so as to accelerate project completion. For example, thousands of prefabricated vertical drains were installed in the embankments, Capps says, to facilitate moisture removal and hasten settlement. All told, approximately 3 million linear ft of prefabricated vertical drains were used. Another solution involved the use of lightweight aggregate borrow material as fill for the embankments. Approximately half the weight of normal material, the lightweight aggregate settles faster and settles less than typical fill.

Additional improvements conducted as part of the project included the addition of turn lanes on S.C. 707 and Farrow Parkway and a slight change in the configuration of both roadways so that they would align better with the new interchange. As for the aesthetic appearance of the bridge, the design team included bent caps and a color scheme that would complement the nearby Market Commons development, Capps says. At the same time, the city logo for Myrtle Beach and the crescent moon and Palmetto tree that appear on the South Carolina flag adorn various features of the bridge.

Mead and Hunt designed the bridge, while GeoStellar Engineering LLC, of Baton Rouge, Louisiana, conducted the geotechnical design. Civil Engineering Consulting Services, Inc., of Columbia, South Carolina, participated in utility surveys and signal and signing design. Construction was performed by the Wilmington, North Carolina, office of Balfour Beatty Infrastructure, Inc., which has its headquarters in Atlanta. The Columbia, South Carolina, office of CDM Smith, Inc.—which is headquartered in Boston—served as construction manager for the South Carolina DOT.