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Characteristics of Hobas pipe prove perfect fit for King County.

Located on the Puget Sound, King County, WA, is home to many things including an extensive wastewater treatment division that serves 18 cities, 16 local sewer agencies and more than 1.4 million residents.

Typically, the public rarely considers sewer needs as such systems are largely "out of sight, out of mind." Prior to 2005, that might have also been true of the 348 miles of interceptor sewer lines that run beneath streets in the King County service area. In an effort to improve the system, this year the 40-year-old regional utility will complete several sewer construction projects. One of these is the Lakeland Hills Force Main, a 24 to 42-inch diameter gravity sewer line that begins at the end of one of their force main pump stations

One of the greatest concerns during the design phase was the corrosion resistance of the piping material because gases released by the sewage could attack many traditional materials. "Since this project starts at a terminus of one of our force mains, we needed a pipe material that was resistant to corrosion from acid attack," said John Abdalkhani, PE, project engineer with King County's Wastewater Treatment Division. Because of the concerns, over 8,000 linear feet of Hobas centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipes were chosen for this project.

Hobas CCFRPM pipes are tested in accordance with ASTM D3681, and meet the requirements of ASTM D3262 for sanitary sewer pipes. The test is performed in a normal H2SO4 environment (pH near 0.5) to mimic a very septic sanitary sewer. The projected design life in this environment has been calculated at more than 100 years.

Design considerations

Other considerations included the pipe's ability to resist the dynamic live loads and static overburden. The soils were varied and poor in some areas with anywhere from four to 50 blows per foot. Luckily, they were generally good and consistent at embedment cover depths of nine to 17 leer.

"Our project also involved the installation of the gravity sewer main under existing railroad tracks, one through tunneling and a few via open cut. We needed a pipe that had the right structural characteristics," explained Abdalkhani. Approved crossings included tracks owned by Union Pacific, GSA and The Boeing Company. To meet the qf structural requirements, the pipe had a minimum of 46 psi pipe stiffness and, where required by site conditions, a stiffness of 72 psi. Hobas pipes are structurally proven to safely handle E-80 rail and HS-20 roadway loading.

The overall engineering design, which included hydraulic analysis, plan and specification development and other civil work, was done in-house by King County. CH2M Hill consulting engineers of Bellevue, WA, provided support and expertise in a variety of areas including geotechnical engineering, traffic engineering and pipe tunneling.

With 200 days to substantial completion, the job was fast paced. Mike McGinley, project manager for Frank Coluccio Construction Company of Seattle, WA, elaborated on the installation, "Most of the job was at moderate to shallow cover in good ground. Installation through this part went really well. Our best day was 13 pipes extending 260 feet. A portion of the job was deep and wet with poor soil conditions. Production rates were slower due to the depth.

In late June, the project was wrapped up and the county took possession of the new line which is currently in service.

Hobas pipe is manufactured in sizes from 18 inches to 110 inches in pressure and non-pressure classes. For more information, please contact Kimberly Paggioli, P.E., marketing manager, Hobas Pipe USA, (800) 856-7473 of 281-821-2200;
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Publication:Underground Construction
Date:Apr 1, 2006
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