Plan B for HDD - Customizing a Challenging Installation
- Leon Fanning, Troy Maciaszek PE, and Andrew S. Vane PE - Hazen and Sawyer
One challenging section of the Lake Marion Regional Water System (LMRWS) expansion was the 16-inch diameter, six-mile long Harleyville Water Transmission Line project. Its installation would require crossing the Four Holes Swamp wetland on the Orangeburg/Dorchester County border. This particular 8,500 LF section across the Swamp consisted of overhead electrical lines, buried high pressure steel gas mains, CSX railroad, 72-pairs of fiber optic cables, and State Highway 453 all within the 200-foot-wide corridor. To exacerbate the situation, the surrounding wooded property is owned by the Audubon Society.
Hazen and Sawyer initially planned to cross the Swamp with one single horizontal directional drilled (HDD) pipe. However, the calculated pull load and tensile stress exceeded the short-term allowable strength of the high density polyethylene (HDPE) pipe. As a result, other alternatives were considered and evaluated.
The ultimate solution consisted of installing the pipe within the sloped shoulder of Highway 453 and installing shorter HDD’s under three creek channels. This required continuous traffic control along Highway 453 during open-cut construction and temporary fill areas for staging the HDD rig, pit, and mud recycler. The longest HDD was reduced to 1,134 LF. This not only reduced the size of the equipment and footprint, but also the overall construction cost. Each HDD staging area was designed to be 150 feet long by 20 feet wide. A vertical radius of 1,600 feet for the 16-inch diameter HDPE pipe was designed at each location to keep the anticipated pull load as low as possible.
Following construction, the HDD subcontractor indicated they had no issues with the installation or pullback – remarkable considering there was less than 15 feet of space for the pipe to be drilled between the bridge abutments and the edge of the gas line easement while paralleling the fiber optic cable bundle.
This project was a success as the new main will provide over 2 mgd of potable water to Dorchester County, Berkeley County, and the future Volvo manufacturing facility. Utilizing both open-cutting and trenchless technologies provided to be the winning combination for cost effectively crossing Four Holes Swamp.
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