Pushing pipe replacement
As part of its routine maintenance activities in November, the Clayton County, Ga., Water Authority (CCWA) replaced a damaged sewer line that was located in a deep embankment along a heavily traveled freeway. To avoid disrupting traffic and to constrain costs, CCWA used trenchless technology — a method that pushes new pipe into place without cutting an open trench — to repair the sewer pipe.
In the early 1980s, Interstate 675 was built through a corner of Clayton County, and the design for the freeway included an overpass to accommodate Grant Road. Because the elevation of the freeway was at or near the existing grade, Grant Road was raised to the elevation of the bridge, resulting in the construction of an embankment about 40 feet thick on the freeway’s west side. An eight-inch-diameter, vitrified clay sewer line ran parallel to the freeway and was buried about eight feet deep when the embankment was built.
A few years ago, while CCWA crews were inspecting pipes with a video camera, they discovered the pipe was cracked longitudinally and needed to be replaced. It was unclear whether the pipe was damaged by a lack of bedding, poor installation or too great a load, but CCWA designers decided to replace the clay pipe with eight-inch-diameter ductile iron piping manufactured by Birmingham, Ala.-based American Ductile Iron Pipe. They planned to replace approximately 440 feet of the pipe that ran between an existing manhole just north of the Grant Road pavement surface, under the road and into a low swampy area to a shallow manhole.
Because the grade at the surface of the road was close to 50 feet above the grade of the sewer line, the cost and disruption of open cut construction was prohibitive. Instead, the designers decided to install the replacement pipe using trenchless technology developed by Dallas-based Tenbusch, allowing heavy traffic on the two-lane highway to continue uninterrupted. The county contracted with Atlanta-based Environmental Consortium and Kansas City, Mo.-based Garney Companies to complete the job.
The contractors dug one small work pit as close to the embankment as possible, approximately 240 linear feet from the shallow downstream manhole and approximately 185 linear feet from the deep upstream manhole. Then, jacking equipment was installed in the pit and aligned with the old sewer line. Lead pieces were then lowered into the pit and inserted into the old sewer. The lead assembly became a single rigid structure that straightened misaligned joints in the existing pipe.
The new pipe segments were lowered into the jacking frame, and lubricant was applied to the leading end of the new pipe column. Using hydraulic pressure, the jacking unit pushed the new pipe through the existing old line at an average rate of one foot per minute. The pipe segments were pushed to the manholes, where the new pipe was connected to the old.
The $113,000 project was completed in eight days. It included the cost of building an access road into the swamp area as well as the work necessary to gain access to the 40-foot-deep manhole on a steep slope on the side of the highway. Because of the satisfactory completion of the project, the county is planning to use the technology in other areas of its infrastructure.