The ACE Basin — one of the largest undeveloped estuaries on the East Coast — is where the Ashepoo, Combahee and Edisto rivers converge and flow past cypress swamps, historic plantation homes, old rice fields and tidal marshes. It is also home to bald eagles, shortnosed sturgeon, loggerhead turtles, and other threatened or endangered species.
U.S. 17, the Ocean Highway, winds its way through 211 miles of South Carolina and through the historic and environmentally sensitive region of the ACE Basin. A part of the country's National Highway System and the Strategic Highway Network for Defense Preparedness, U.S. 17 provides a direct route between the coastal cities of Myrtle Beach, Charleston and Beaufort.
The widening of U.S. 17 ACE Basin Segment 1 is a 6.5-mile project in Beaufort County between the Combahee River and Gardens Corner, S.C. This segment of U.S. 17 is a two-lane ditch section with narrow to no outside shoulders. On an average day, 13,500 vehicles travel along this route, currently one of the most dangerous two-lane roads in South Carolina and the nation.
Because of the need to complete this project as quickly as possible and within budget, SCDOT chose to manage it with a design-build contract. The design-build team of Phillips & Jordan, Inc. and Davis & Floyd broke down the project into three phases.
Phase 1 is a 3-mile section between the Combahee River and Big Estate road. The roadway will include two 12-foot lanes in each direction, 10-foot shoulders on each side, recoverable slopes to enable correction time for those who may run off of the road, and a 100-foot median separating northbound and southbound traffic. To protect as many significant trees as possible while maintaining high safety standards for the roadway, the 100-foot median includes 30-foot clear zones on either side of a 40-foot area of naturally occurring growth.
Phase 2, which runs between Big Estate Road and Gardens Corner, will offer a five-lane section that includes two northbound lanes, two southbound lanes, and center turning lanes with intermittent planted medians. Combining the use of center turning lanes with the planted medians offers the opportunity to make necessary left turns while minimizing the possibility of head-on collisions. A 10-foot-wide multipurpose pathway is also included on this stretch of roadway to allow bicyclists and hikers to enjoy this scenic area.
Phase 3 involves a major interchange at U.S. 17 and U.S. 21. The preliminary design is still under way on this portion of the project.
Phillips & Jordan, Inc. began construction on August 7. According to Chris Hernandez, SCDOT project manager, the southbound lane for the first phase is primarily new pavement structure. The southbound lanes on Phase 2 and the northbound lanes of Phase 1 will incorporate the old pavement structure, which will be modified. There are four bridges on the project, all of which are wetlands related.
Protecting these fragile wetlands has been a major emphasis along the project.
Hernandez explains that the silt fence layers have been doubled and even quadrupled along most areas of the project "to ensure 100-percent containment within the project limits. To date we've lost zero sediment. The contractor is very proactive in performing daily monitoring and maintenance."
Slope drains are also being used to control erosion. However, the design-build team came up with a new design for these South Carolina wetlands.
"(A slope drain) is essentially an 8-inch plastic pipe that goes down the slope," Hernandez explains. "But instead of letting the water freefall down the pipe into a rip rap section, they have actually put a trap at the bottom (of the pipe) that forces the water to percolate up through another layer of rock. This disperses the water instead of having a constant velocity that can cause further erosion."
Also new for the SCDOT is the use of Earthquake Drains on the project. Installed by Ellington Cross LLC, Indian Trail, N.C., the Earthquake Drains are a cost-effective method for mitigating liquefaction.
According to Ellington Cross's website, Earthquake Drains consist of a polypropylene core wrapped with a geotextile filter fabric. The cores are extruded into a highly flexible configuration with nominal diameters of 3 inches to 6 inches. The geotextile filter fabric is selected to allow free access of pore water into the drain while preventing the piping of fines from adjacent soils. The geotextile is durable enough to withstand handling and abrasion during installation.
Using patented, proprietary installation methods, the drain is installed by inserting a tubular steel mandrel containing the drain into the ground with a combination of static crowd and vibration. For retrofitting projects, Earthquake Drains can be installed using low overhead, restricted-access equipment. In those cases where vibration would be detrimental to existing structures and foundations, Earthquake Drains are installed with drilling equipment or static crowd.
Once the appropriate depth of treatment has been reached, the Earthquake Drains is anchored with a specially designed anchor plate. Earthquake Drains can be installed to depths over 70 feet. Installation rates depend on the general resistance of the soils being improved. Ground improvement with Earthquake Drains typically takes one-quarter to one-third of the time for vibro-replacement. Several machines can be mobilized to increase production.
In terms of cost, Earthquake Drains can be installed at approximately 50 percent to 60 percent of the cost of traditional vibro-replacement stone columns for undeveloped sites. On retrofitting projects, the savings may be even more substantial.
Hernandez points out that a crane would typically be used to vibrate the Earthquake Drains into the ground. "But with equipment like that — and being in the wetlands — we would have to go through a permit process. To simplify this (Ellington Cross) has modified a front end loader with a boom."
With the fatality rate on the two-lane section of U.S. 17 between Gardens Corner and Jacksonboro, S.C., 77 percent higher than the fatality rate reported on the Interstate System and 2.3 times higher than that of multilane portions of U.S. 17, the ACE Basin Widening Project is a welcome site for area residents and regular travelers along this scenic historic highway. Although hampered by recent rains, the $80.3-million project is well under way and is scheduled for completion in 2010.
Editor's Note: Additional material was provided by the South Carolina Department of Transportation and Ellington Cross LLC and is used with permission.