The line's location under Grant Road, a major east-west artery through the city, and the large volume of sewage it transports both contributed to the project's complexity, according to Jon Schladweiler, deputy director of engineering for Pima County Wastewater Management, which oversaw the rehabilitation project.
The trunk sewer, which serves a total of 35,000 residents of southern and southwestern Tucson, transports between 10 and 12 million gallons of sanitary sewage each day. Video inspections showed extensive deterioration in a 2,500-foot-long section of the 42-inch-diameter pipe.
"We had to be able to keep everyone in service, with zero tolerance for any leakage of sewage out of the system during the rehabilitation process," said Schladweiler. "We also had the needs of the people who live and work near Grant Road to keep in mind. Tearing up one of the city's main streets for months and disrupting business and traffic was simply not an option." Pima County Wastewater Management awarded Insituform a $1.8 million contract to repair the line using the Insituform process, a nondisruptive technology that restores structural integrity and removes infiltration from underground pipes. To maintain service to customers, Pima County also required Insituform to develop a temporary sewage bypass pumping system capable of conveying the 12 million gallons of daily sewage flow normally transported by this reach of pipe during construction.
Insituform's first challenge was to dry up the damaged sewer and reroute the flow. Workers began by cutting into the interceptor and releasing the raw sewage into a wet well they had created just upstream from where the rehabilitation was to be completed. Using 10 portable diesel-driven pumps, Insituform pumped sewage from the wet well into four 18-inch diameter HDPE pressure pipes. Secured against the concrete-lined bank of an existing drainage ditch, these mains carried the sewage downstream approximately 3,000 feet, where it was returned to the interceptor.
The 10 pumps used to pump the sewage into the bypass line were far more than needed for the sewer's daily average or even peak flow, according to Mike York, technical representative for Insituform.
"We needed a certain comfort zone in case any storm water got in the line. That's why we sized the pumps for peak wet weather flow," explained York. "Plus, we needed the appropriate number of pumps and safeguards so that if for any reason something stopped working, at any time of the day or night, we'd have the excess capacity to take over right away." After several weeks of preparation, Insituform put the bypass pumping system into operation in late August. Despite the heavy rain and the large volume of sanitary sewage it had to accommodate, it operated very well.
Over the next four weeks, on-site technicians worked around-the-clock overseeing the system's operation, while the installation crew set about the task of rehabilitating the deteriorated line.
Insituform divided the rehabilitation project into two parts, rehabilitating 881 linear feet of the nearly one-half-mile-long trunk sewer in the first inversion, and the remaining 2,115 linear feet in the second.
For each installation, a custom made felt tube was impregnated with a liquid thermosetting resin. Insituform's installers then inverted the tube (turned it inside out) into the structurally deteriorated pipe from an existing manhole access point. Water pressure was used to propel the tube through the sewer, as it navigated bends and offsets along the way. Installers then circulated hot water through the tube, curing the resin and forming a jointless and corrosion-resistant Insituform pipe. After the pipe had cured, the service connections were restored.
To cause the least possible disruption to traffic, Insituform installed the resin-saturated tubes in the early evening hours - a time when the fewest number of commuters would be traveling Grant Road and to take advantage of the cooler nighttime temperatures. The curing process continued throughout the night, with the laterals restored by morning. By rush hour, there was little noticeable activity left.
By late September, Insituform had completed the sewer rehabilitation and dismantled the pumping system, and met all of Pima County Wastewater Management's objectives.
Today, according to Schladweiler, Pima County uses cured-in-place construction on 80 percent to 85 percent of its sewer rehabilitation projects, including some that do not run under busy city streets.
"When you consider the noise, the dust, the growing number of utilities and municipal policies on traffic control, we're just much better off going in with a cured in place solution," he says. "Our primary mission here is to ensure the community has a safe and effective wastewater conveyance system. But we also do whatever we can not to disrupt people's lives," said Schladweiler. "That's part of our job, too."
