The usual description of many sewers is "out of sight and out of mind." Prior to 2005, that may have been true of the 348 miles of interceptor sewer lines that run beneath streets in the King County, Wash., service area, which includes 18 cities, 16 local sewer agencies and more than 1.4 million residents. In an effort to improve the system, the 40-year-old regional utility completed several sewer construction projects, one of which was the Lakeland Hills Force Main, a 24- to 42-in. diameter gravity sewer line that begins at the end of the pump station. More than 8,000 linear ft of Hobas centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipes were used for this project.
One of the greatest concerns during the design phase was the corrosion resistance of the piping material because gases released by the sewage could corrode many traditional materials. Hobas CCFRPM pipes are stringently tested in accordance with ASTM D3681 and meet the requirements of ASTM D3262 for
sanitary sewer pipes. 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 4 to 50 blows per ft. Luckily, they were generally good and consistent at embedment cover depths of 9 to 17 ft.
To meet the structural requirements, Hobas supplied pipes with 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.
"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," said John Abdalkhani, PE, project engineer with King County's wastewater treatment division. CH2M Hill consulting engineers who supported the project, recommended using Hobas pipe due to design conditions that didn't allow the use of other pipe materials on the gravity portion.
With 200 days to substantial completion, the job was fast paced.
"The assembly on site went faster than any pipe I've seen installed for sewer," said Bruce Herman, the project inspector assigned for King County. "The push-together joint installation was rapid, and the joints were perfectly mated every time. The factors that permitted quick assembly were the pipe's light weight plus the fact that the joints don't leak or need repairs."
According to Herman, testing was easy with air pressure because the whole run could be tested at once. No loss was seen during testing, and the new line is currently in service.