Rapid Response Minimizes Pipe Failure at Encina Wastewater Treatment Plant

March 2011, Vol. 66 No. 3
Rapid response was critical to contain the spill and keep the line in operation by utilizing bypass pumping.

A sewage spill is a disaster that no city wants to experience, but as America’s wastewater infrastructure ages, these types of accidents will be occurring more frequently. The city of Carlsbad, CA, recently had the opportunity to test their preparedness for such a situation.

Rapid response by their crews, the installation contractor and a pipe manufacturer achieved the best case scenario during this catastrophic event.

On Oct. 7, 2010, the primary influent line leading into the Encina Wastewater Treatment Plant collapsed, creating a sinkhole and discharging raw sewage. The failure and resulting sinkhole occurred entirely within the Encina Wastewater Treatment Plant grounds. This plant treats approximately 22 million gallons of sewage per day and serves the communities of Carlsbad, Encinitas, San Marcos and Vista in north San Diego County.

The Carlsbad Utilities Department responded quickly that Thursday morning to contain and divert the discharge from public areas and install a temporary line to maintain service for residents and businesses in the service area. Even before dusk, repairs were being planned. City engineers and the city of Vista engineering staff, with assistance from Brown and Caldwell civil engineers, began designing repairs to the collapsed 54-inch reinforced concrete pipe (RCP) that dated from the early 1980s.

Design considerations
“The engineers wanted a large diameter pipe that is chemically resistant to H2S,” said Terry Smith, senior civil engineer for the Carlsbad Utilities Department. “We were confident that Hobas met these criteria. Also, the O.D. of the 60-inch Hobas pipe was nearly the same as the existing 54-inch RCP pipe, which would make it easier to connect to the existing structures. And lastly, Hobas was able to meet our critical timeline for replacing the pipe.”

“We first used Hobas pipe in Carlsbad about eight years ago when we installed about a mile of 42-inch pipe by microtunneling,” said Smith. “We like the ability of the pipe to resist hydrogen sulfide-related corrosion and installation went smoothly during the microtunneling operation.

By Monday morning, the Carlsbad Utilities department contacted Hobas Pipe’s West Coast division manager, Bijan Khamanian, ready to order 320 linear feet of 60-inch SN46 FWC centrifugally cast, fiber reinforced, polymer-mortar pipe (CCFRPMP) for the repairs.

“The 60-inch diameter CCFRPMP had a similar pipe OD to the existing 54-inch diameter concrete yet, increases the flow capacity substantially,” stated Kimberly Paggioli, vice president of Marketing and Quality Control for Hobas Pipe USA.

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