Pipe Corrosion When Extending Sewer Lines

The City of Burlington, N.C., is centrally located in the Piedmont area with the Blue Ridge Mountains on the west and the Atlantic seashore on the east. Its growing population is a little more than 43,000 with about 135,500 residents in the surrounding Alamance County.

When the city recently extended its sanitary sewers, the Great Alamance Creek projects, their primary emphasis was on selecting a pipe with the corrosion resistance necessary to stand up to the septic conditions. The critical issues also included having a leak-free system with the structural capability to safely withstand deep covers up to 30 feet. With these criteria in mind, the specifications for buried pipe prepared by the project design firm, Alley, Williams, Carmen & King of Burlington, allowed only two materials: ductile iron with ceramic epoxy lining and centrifugally cast, fiberglass reinforced, polymer mortar (CCFRPM) pipe because of its inherent corrosion resistance.

Bidders were asked to submit an installed price for each of the specified pipes. The bidding contractors differed as to whether the job would cost less with HOBAS CCFRPM pipe or ductile iron. The winning proposal from Beers Construction Company of Winston-Salem actually quoted ductile iron slightly less. Regardless of the difference, Burlington officials decided that HOBAS pipe was worth the small premium because of its optimal corrosion resistance. Consequently, the contract was awarded to Beers based on using CCFRPM pipes for the buried portion of the line.

HOBAS pipe was selected for most of the new interceptors in all phases of the Great Alamance Creek projects. Phase 1 utilized more than three miles of 42-inch, SN 46 (min. pipe stiffness of 46 psi) pipes with HOBAS FWC gasket-sealed, push-together couplings.

Although this was the first time Beers had installed HOBAS pipe, they reported a trouble-free installation. They noted that assembly was easy because they could use a backhoe equipped with a lifting sling to choke the spigots home. The same equipment simplified handling of the 20-foot long, 1.2-ton sections.

Pipe zone backfill was crushed rock with a complete surround for deep covers but placed only to springline for depths up to 18 feet. This burial method was very effective in minimizing deflections with measurements ranging from nil to 2.5 percent. Most checks were below one percent and the deepest location had only a 0.5 percent value.

At completion, the entire system was air tested with no leaks. This included the FWC couplings that connected the buried HOBAS pipes to the ductile iron aerials. The banded rubber boot connections to the concrete manholes also proved successful because of the smooth, constant O.D. of the CCFRPM pipes. Beers representatives said they were delighted with the installation, and the city of Burlington decided to upgrade Phase 2 with an additional $35,000 for 11,300 feet of 30-inch HOBAS pipe so the flattest portions of that line, where corrosive flow will remain the longest, would also be CCFRPM instead of lined ductile iron.