Kailua Bay lies on the windward side of the island of Oahu, about 12 miles northeast of the City of Honolulu, Hawaii. Primarily a residential community, it is widely known for its crescent ocean beach, considered by many to be among the finest in the Islands.
Deteriorating sewers, including combined gravity flow and stormwater lines, posed the threat of contamination in the bay area, and the city and county of Honolulu launched an infrastructure project in 2008 to repair and replace the lines.
In one phase of the project, under Keolu Drive, Fusible PVC™ pipe was selected over conventional HDPE (high density polyethylene pipe) pipe for a multi-pipe siphon installation to facilitate a combined sewer crossing beneath Kaelepulu stream. Underground Solutions, Inc. (UGSI) of Poway, CA, contracted with Frank Coluccio Construction (Honolulu) to supply three different diameters of Fusible PVC™ pipe and fusion services. The siphon bundle was installed via horizontal direction drilling (HDD) by Michels Directional Crossings (Brownsville, WI).
The project required different sized pipes so that the siphoning process could adapt to widely varying hydraulic conditions, especially the high flows common during Oahu’s rainy season. A siphon, in the context of gravity sewer operations, is a section of piping installed in a ‘dip’ or valley to cross beneath an obstacle. The upstream side of the crossing is maintained at higher elevation than the downstream side to provide the head required to push wastewater through the siphon to the downstream side. Wastewater stays in the siphon until there is more flow from upstream to force it out on the downstream side.
To maintain flow velocity through the pipe and prevent solids settling in the siphons’ low sections, the contractor used three different diameters of UGSI pipe: 8-inch Fusible C-900® pipe, 10-inch Fusible C-900® pipe, and 20-inch Fusible C-905® pipe. Fusible PVC pipe was selected over HDPE because it could be downsized while maintaining the required ID, pressure class, and critical buckling pressure. All pipes were 660 linear feet in length.
The area is environmentally sensitive, so the county elected to horizontally directional drill under the stream, and pull all three pipes as a bundle through a single borehole. The thermal joining procedure for Fusible PVC pipe ensures that there is no mechanical coupling or gasket seal that may leak over time, while also providing the required tensile capacity to be installed via HDD. The combined reduction in pipe OD for the bundle allowed the borehole diameter to be just 42 inches. HDPE pipe would have required a 48-inch diameter hole.
This lowered both the cost and risk of the installation, and the reconnection of the PVC pipe bundle to the up- and downstream diversion and manifold structures was easily accomplished.
About the Company:
Underground Solutions Inc. is an infrastructure technology and pipeline rehabilitation company. It has developed and continues to develop unique and proprietary technologies focused on the underground infrastructure industry, including its line of Fusible PVC pipe products.
Figure 2: Bundled multi-pipe is pulled through a single borehole by horizontal directional drilling.
