Two years ago, Anchorage Water and Wastewater Utility (AWWU) began to upgrade water service capacity for a residential area with five homes built approximately 50 years ago. Over the years, three of the homes were converted to multi-family rentals and therefore, demand on the small water main serving them increased greatly. This led to frequent water pressure problems.
“For several years, these properties were served by a 2-in. copper water line running under their back yards, which had become grossly undersized for meeting residents’ water needs,” said Joshua Shackelford, project engineer for AWWU. “We wanted to upsize that line to meet our current standards and move it to the front of the homes to allow easier access for the installation and future maintenance.”
Though the utility originally bid the 260-ft water main project as an open trench installation, thinking it would be the most cost-effective, it was obvious to the engineering staff that this method wouldn’t come without a fair amount of logistical challenges. First, to prevent freezing in Anchorage’s subarctic climate, water mains must be installed at a minimum depth of 10 ft. With the new water main’s close proximity to an old quadruplex residential building, heavy shoring would be needed in an open-trench installation. In addition, factors such as narrow streets, a line of old-growth trees and the quadruplex’s encroachment into the right of way constricted space.
“This is one of Anchorage’s older neighborhoods, and because of that, the land was more unconventionally platted,” said Shackelford. “For unknown reasons, the quadruplex was built outside the current property lines and sticks out into the right of way approximately 6 ft. Because of this and the other space constraints, the only place where we could locate the new water main was about 3.5 ft from the building. We were constricted on both sides, with only 15 ft of available workspace between the building and a line of trees we couldn’t take out.
“Because of the constricted space, we considered directional boring as an installation method during the early stages of the design, but dismissed it because we thought the open trench with shoring would ultimately be less expensive,” Shackelford added.
Roger Hickel Contracting, Inc., of Anchorage, won the general contractor bid for the open trench project and persuaded AWWU to change the project specifications to a horizontal directional drilling (HDD) installation. After crunching the numbers, the contractor concluded that using HDD would cost about the same as open trench and shoring, but would be less labor-intensive and disruptive to surroundings.
“It was a choice between a more challenging, disruptive installation with a lot of shoring in a tight space and the less-invasive directional drilling method,” Shackelford said. “After talking to our contractor, we viewed directional drilling the path of least resistance.”
For pipe, Roger Hickel Contracting recommended CertainTeed C-900/RJ restrained-joint PVC pipe with Certa-Lok Restrained Joint System, a product the contractor had used successfully in previous open-trench projects. The product’s patented restrained-joint technology allows for faster and easier assembly in congested areas without the need for joint fusion. Crew members can assemble the pipe system in 20-ft lengths while concurrently securing it with spline-locked couplings as pullback continues. Though this would be the first time the product was installed in Alaska using a trenchless installation method, it made good sense to the project team.
“The utility wanted to use a thermoplastic pipe, but not a fusible product,” said Scott Dunlap, project manager for Roger Hickel Contracting. “With fusible pipes, like HDPE, you have to fuse the whole pipe string before installation, so you need a lot more room. With the CertainTeed restrained-joint PVC pipe, we only needed an insertion pit and a receiving pit, and we could add pipe sections, piece by piece as we pulled back. In a small neighborhood with confined workspace, that was about the best way to do it.”
Roger Hickel Contracting subcontracted the directional boring work to Frawner Corp., based in Anchorage and one of Alaska’s leading HDD contractors, who began work last September. Operating with a crew of two, the general contractor excavated the insertion and receiving pits for the subcontractor with a Caterpillar 320 excavator and dug potholes for the individual service tie-ins. A Caterpillar 950 loader and D4 bulldozer were utilized for dirt removal. Since the street bordering the worksite was only 12 ft wide, it was closed off to traffic.
Working with a four-person crew, Frawner Corp. made one bore, ranging from 10 to 12 ft in depth, through sandy silt soil with a Ditch Witch JT4020 All-Terrain Directional Drill and pulled back 260 ft of 8-in. restrained-joint PVC pipe. Drilling fluid was supplied from a Ditch Witch mud mixer and the crew removed bore slurry from the pits using a Ditch Witch 500-gallon vacuum excavator trailer. Following installation, the new pipe was tied into the existing system with ductile iron fittings and promptly passed pressure testing. All told, the project took two weeks. The job ran very smoothly, and both contractors were pleased with their first experience installing CertainTeed restrained-joint PVC pipe in an HDD application.
“It’s definitely a good product, and it serves its purpose when you’re working in congested areas like this one, where you’re boring from a deep pit with very little extra space around,” said Jay Frawner, owner of Frawner Corp.
The utility was also happy with the performance and smooth assembly of the restrained-joint PVC pipe, as well as the benefits of the HDD installation method in this application.
“If we’d stayed with the original open-trench plan, our contractor would have had to do all that shoring and store the pipe and other materials somewhere offsite, trucking them in each day,” Shackelford added. “There was no extra room. By going the directional boring route, we were able to use space more efficiently and minimize disturbance to the neighborhood. As far as reducing our risks and re-landscaping, I believe we ended up with a great project.”
John Coogan is channel marketing and communications manager for the CertainTeed Pipe & Foundations Group.
