Pipe Maintenance & Repair: Saving a Sanitary Sewer Interceptor

About the author: Zoltan Fekete, P.E., is senior engineer for the city of Lubbock’s Water Utilities Department. Fekete can be reached a [email protected]. Drew Hardin, P.E., is vice president and regional director for Lockwood, Andrews & Newnam Inc. Hardin can be reached at [email protected].

Zoltan Fekete & Drew Hardin

The Canyon Lakes sanitary sewer inter­ceptor serves northwest and northeast Lubbock, Texas, and is one of the major interceptors in the city’s sewer collection system. The main, which consists of approximately 43,000 ln ft of 24- to 30-in. gravity pipeline, conveys peak wastewater flows of almost 10 million gal per day from thousands of people and key businesses in Lubbock, including Texas Tech University, Lubbock Lake Landmark, Joyland Amusement Park, American Wind Power Center and the Bayer Museum of Agriculture. 

The majority of the interceptor is made of vitrified clay pipe, while the road, railroad and aerial crossing pipe sections are composed of ductile iron pipe or steel. With the interceptor nearing its apparent design life, the city experienced occasional blockages and overflow problems due to corrosion and tuberculation on the pipe wall.

In response to this challenge, the city hired Lockwood, Andrews & Newnam Inc. (LAN), a planning, engineering and program management firm, in January 2014 to perform a condition assessment to identify the root cause of overflows and subsequently design cost-effective improvements for the 45-year-old pipeline. Other members of the project team included CleanServe Inc., Hugo Reed & Associates, aci Consulting and environmental professional Paige Ginn.

“The project will improve the reliability of the Canyon Lakes sanitary sewer interceptor and minimize possible overflows and infiltration problems along the sewer line,” said John Turpin, P.E., the city of Lubbock’s chief water utilities engineer.

Condition Assessment

For the inspection and assessment phase, the project team used several methods to provide a comprehensive evaluation for the city given the varying interceptor conditions. 

To enable optimal observation of pipe conditions, the team primarily used CCTV equipment at night during low-flow periods. In addition to CCTV inspections, field evaluations of aerial crossings were performed, which included visual observations of the pipe exterior and the use of ultrasonic thickness testing to detect internal pipe corrosion and determine whether the pipeline could
withstand mechanical cleaning.

After evaluating inspection results, the team concluded that more than 70% of the existing interceptor could be utilized and, consequently, the entire interceptor would not have to be replaced. Inspections also revealed myriad problems at different locations, including tuberculated metallic underground and aerial pipe, longitudinal cracks, holes in the pipe wall, thinning metallic pipe wall, failing joints and deteriorated manholes. 

“We found a consistent pattern with the clay pipe generally [being] in good shape, but the metal portion of the pipe at roadway, railroad and aerial crossings had numerous issues,” said Harrison Steed, P.E., senior project engineer and condition assessment expert for LAN. “The fact that only 12,000 ln ft of pipe require attention is huge, as it will save the city a substantial amount of money and greatly minimize stakeholder impacts.” 

Design Improvements

The interceptor travels adjacent to the city’s Canyon Lakes system, one of the most scenic areas of Lubbock that is used extensively for outdoor activities. In addition, much of this region is the site of both prehistoric and historic human occupation and is considered a high-probability area for archeological sites. As such, the team designed improvements to limit surface disturbance to the existing Canyon Lakes system and associated parks to the maximum extent possible once under construction.

Coordination at the outset of design with the Texas Historical Commission was critical to maintaining the schedule and minimizing the risk for construction delays if artifacts were found. Archeological investigation during the design phase included a pedestrian survey of all proposed improvement locations in addition to backhoe test trenches and shovel tests by archeological specialist aci Consulting. Efforts concluded with no significant artifacts discovered.

Based on the type of defect and geographical location along the alignment, the team designed a wide range of rehabilitation and replacement solutions that minimized impact, cost and risk of failure. These included open-cut replacement, auger boring, cured-in-place pipe (CIPP), point repairs, heavy mechanical cleaning, fiberglass manhole inserts, and full pipeline and manhole replacement.

Under major roadways, interstates and railroad crossings, the city and project team considered the impact of mechanically cleaning tuberculated sections of the pipeline as well as the option to pipe burst some areas. After evaluating the risks associated with these options for pipelines in this diameter and the location of the pipe, the city and the project team elected to install a separate parallel pipe using auger boring methods. 

In certain locations, auger boring could not be utilized due to utility conflicts or because the length exceeded boring limits. In those areas, the team recommended cleaning the pipe mechanically, followed by CIPP rehabilitation depending on pipe conditions. Final recommendations included more than 12,000 ln ft of improvements using five different pipeline rehabilitation and replacement methods, along with a combination of manhole replacement and rehabilitation.

To minimize costs and re­duce traffic disruptions, the city and project team are pack­aging the construction in two different contracts based on method of construction rather than geographic location. As such, all trenchless im­provements will be implemented in one contract while other rehabilitation methods will be implemented in another contract. The first construction package began in the summer of 2015, with the second package following soon after. The project will be completed in late 2016.

At the onset of the Canyon Lakes Interceptor Project, it seemed almost a certainty that much, if not all, of the existing interceptor would need to be replaced due to age and the inability of the interceptor to convey existing flows. However, the city’s foresight in locating a new treatment facility at the upstream end of the interceptor to divert some flows, combined with the project team’s solutions, will enable the use of the existing interceptor while still addressing key problem areas. These efforts resulted in an estimated $5 million savings to the city, a projected reduction of more than 200 construction days and an 85% reduction in land disturbance.

“Rehabilitation of the Canyon Lakes sanitary sewer line in tandem with a new water reclamation plant is critical to ensure system reliability into the future as north and northwest Lubbock continue to develop in both residential and industrial areas,” Turpin said.

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