Challenging 2,600 LF On-Grade HDD Sanitary Sewer Project Completed with FPVC® Pipe in Mukilteo, WA

In fall 2009, the HDD Company of Cameron Park, CA passed hydrostatic testing on 2,600 LF of 20" Fusible PVC® pipe installed via on-grade horizontal directional drilling (HDD) in Mukilteo, WA. Underground Solutions, Inc. (UGSI) provided the FPVC® pipe and fusion services.

Pipeline Details and Project Summary
Project: Big Gulch Trunk Sewer Emergency Repair & Stream Restoration Project
Owner: Mukilteo Water and Wastewater District
Location: Mukilteo, WA
Engineer: Hammond, Collier, Wade & Livingstone
Contractor: The HDD Company, Inc.
Construction Mgr: Staheli Trenchless Consultants
Length 2,600 LF 20" DR18 FPVC®

In the late 1960's, 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 investigate and design a replacement for the line. Several construction options were evaluated, including: 1) conventional direct bury, 2) micro-tunneling, 3) horizontal directional drilling (HDD) with a steel casing and thermoplastic carrier pipe, and 4) 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.

With a design depth exceeding 190' 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' 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. Ian Hunter, Project Engineer with HCWL, stated: "After much consideration of a variety of pipeline products, the chosen FPVC® pipe material met our design constraints while limiting project costs."

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 lbs., 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 in September.

Rick Evans, Project Manager with The HDD Company, stated: "Underground Solutions performed a great job for us on the Mukilteo project. They gave us great support throughout the duration of the job. UGSI was able to lay out 1,800 LF of pipe in very rugged terrain and they fused on an additional 800 LF as we pulled back the pipe. They worked very well with The HDD Company in a very tight and limited working area. We look forward to working with Underground Solutions again."

Staheli Trenchless Consultants monitored the construction activities. Kim Staheli, President of STC, noted: "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."



Pipe Delivery


Pipe Layout in Gulch


Intermediate Fused Pipe Strings


HDD Rig at Bottom of Gulch


Insertion Pit


Successful Pull Through