In Fall 2009, The HDD Co. of Cameron Park, Calif., passed hydrostatic testing on 2,600 ln ft of 20-in. fusible polyvinyl chloride (FPVC) pipe installed via on-grade horizontal directional drilling (HDD) in Mukilteo, Wash. Underground Solutions Inc. (UGSI) provided the FPVC pipe and fusion services.
In the late 1960s, a concrete sanitary sewer pipe was installed via direct bury adjacent to the Big Gulch Creek, a salmon spawning waterway. Due to population growth in the area and a shift in environmental policies, the sewer line could no longer meet capacity requirements.
Consulting engineering firm Hammond, Collier, Wade and Livingstone (HCWL) was hired to design a replacement for the line. Several construction options were evaluated, including:
- Conventional direct bury;
- HDD with a steel casing and thermoplastic carrier pipe; and
- HDD with an uncased thermoplastic pipe.
Due to recent advancements in HDD methods and tight site constraints (a narrow, steep, heavily wooded gulch), directional drilling with an uncased thermoplastic pipe was selected as the option with the least expense and lowest environmental impact.
Pipe Selection & Installation
With a design depth exceeding 190 ft in some locations, HCWL selected UGSI’s FPVC pipe over other thermoplastic pipe options due to its ability to resist the external soil load as well as the internal hydrostatic load generated from the 140 ft elevation change over the length of the bore hole. FPVC pipe provided the necessary buckling pressure with the smallest outside diameter, decreasing borehole size and cost.
“After much consideration of a variety of pipeline products, the chosen FPVC pipe material met our design constraints while limiting project costs,” said Ian Hunter, project engineer with HCWL.
The bore profile was designed with compound curves, and the gravity flow required a minimum grade of 1.5%. The location, depth and on-grade requirements made for an extremely challenging bore. An intermediate fusion joint was completed during pullback due to the constrained layout area for the fused pipeline. Drill mud management was a challenge because of the elevation change over the length of the bore.
The pipeline was pulled back without any ballasting at a pull force of approximately 80,000 lb, well below the 215,300 lb-rated safe pull force of the pipe. A hydrostatic pressure test was completed shortly after pullback, and a flow test was completed.
“Underground Solutions performed a great job for us on the Mukilteo project,” said Rick Evans, project manager at HDD Co. “They gave us great support throughout the duration of the job. [They were] able to lay out 1,800 ln ft of pipe in very rugged terrain, and they fused on an additional 800 ln ft as we pulled back the pipe. They worked very well with HDD in a very tight and limited working area.”
Staheli Trenchless Consultants (STC) monitored the construction activities. Kim Staheli, president of STC, said: “We were fortunate to work with a knowledgeable owner on this challenging project who was willing to work through the challenges to successfully complete the project. FPVC pipe proved to be an excellent product, and the field support was outstanding. I would not hesitate to specify the pipe on future projects.”