The Sand Creek Interceptor: Trenchless Rehabilitation in Aurora, Colorado Conservation Area
In April 2019, a 40-year-old sewage pipe showed signs of decay in Denver, Colorado’s largest suburb, Aurora. The 36-in. reinforced concrete sewer pipe interceptor needed to be rehabilitated in order to continue to service the city’s population of more than 360,000.
Although the requirements to repair a manhole and rehab that was just over 6,500 ft of decaying pipe were straightforward, the specific location of this project presented a unique challenge.
The portion of the interceptor to be repaired ran through Sand Creek Regional Greenway, a protected park in the area. Home to a unique abundance of wildlife, the Sand Creek Regional Greenway is a 14-mile trail and natural area that connects the Sand Creek ecosystem to the growing urban world. In addition, the greenway serves as a shared home to two wildlife refuges: Bluff Lake Nature Center and the Morrison Nature Center. Currently, the trail is open to runners, bicyclists, horseback riders, and dog walkers, but future plans include doubling the trail’s length and adding multiple trailheads to accommodate motorized vehicles as well.
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Because of the natural benefits provided by the greenway, the city of Aurora was adamant about protecting the park’s wildlife and sanctuaries. In order to avoid the potential threats posed by construction sites, including air and water pollution and disruption to the natural behaviors of plants and animals, careful thought and planning were taken prior to the project’s start.
Planning & Pre-Bid
Before releasing the project for bid, Aurora Water conducted flow studies and analyzed the length of pipe to be repaired and the project’s environmental requirements to ensure resultant bids would be thorough and meet the unique demands dictated by the site. After a careful vetting process, Insituform, a worldwide provider of cured-in-place pipe (CIPP) rehabilitation and installation, was awarded the job. Sunbelt Rentals, North America’s premier equipment rental provider, was then selected as the subcontractor for the bypass portion of the project. Together, the teams worked to ensure preventative practices were put into place to protect the immediate work zone and surrounding areas.
In order to provide the dry work environment required for Insituform to begin the CIPP process, Sunbelt Rentals first had to get the temporary pumping system operational. Bypass construction began on April 22, and the system went live on May 8.
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Because of the depth of the manhole – 27 ft – and the high suction lift requirement, the Sunbelt Rentals team had to get creative on pump selection and placement. The team removed the cone and barrel section of the manhole and excavated a bench to lower the pumps to an elevation where the net positive suction head (NPSHa) met the specifications for the system to work. The design created by Sunbelt Rentals called for the use of two 12-in. high volume solids handling pumps to handle the 7.4 MGD flow requirement. This decision minimized the number of pumps required – one primary and one backup – while achieving 100 percent redundancy. By limiting the number of pumps on site, Sunbelt Rentals was able to lessen site restoration at the end of the project. In addition, the pumps were sound attenuated, resulting in minimal disruption to the surrounding environment.
From the pumps, the Sunbelt Rentals team used 12-in. DR26 HDPE pipe through 12-in. knife gate valves into a common tee fitting. From the tee, the team was able to increase to 18-in. DR26 HDPE for the 7,200-ft run of discharge piping through the greenway to the discharge manhole on the same interceptor. The HDPE was all cut and fused on site with every effort made to keep disruption to a minimum.
To limit their footprint on the environment, the teams stayed as close to the bike and walking trail as possible. Crews from both Insituform and Sunbelt Rentals also installed rope barriers to delineate the areas where crew members could and could not work. According to Sunbelt Rentals strategic customer representative Joe Cerretani, this process worked without any notable issues, leaving the wildlife and their habitat undisturbed.
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As with many bypass projects, the path of the discharge piping was not without obstacles. In order to stay as close as possible to the trail, the Sunbelt Rentals team installed additional 45-degree and 90-degree bends to the discharge piping. The piping also crossed the trail at three separate locations, requiring trenching and burial of the pipe at each of those points. In addition, the discharge piping crossed two major four-lane roads. To avoid road cuts or any construction that may cause disruption or delays, the Sunbelt Rentals team routed the discharge pipe beneath the roadways via the trail and kept the pipe tight to the side for continued pedestrian use.
Pumping operations ended on May 23. At peak flows, the total dynamic head (TDH) on the system was 98 ft, requiring the primary pump to run at an RPM (revolutions per minute) of 1,450 to accomplish the duty point.
The City of Aurora planned for an eight-week project and was most impressed when Sunbelt Rentals and Insituform completed it on time, restoring the area like no construction ever took place. Bypass operations started and ended according to schedule, and utility service to residents was uninterrupted.
Thanks to the use of trenchless methods, the interceptor was successfully rehabbed while impact to the surrounding environment was minimized.