Has This Been Tried Before? Using Trenchless Technology in a Revolutionary Sewer Tunnel Rehabilitation

The 7,600-foot long Colsman Tunnel, operated by the Southgate Sanitation District in Centennial, CO, is approximately 69-in. wide by 75-in. tall and is inconsistent in cross section due to the hand-tunneled nature of construction. The only manned access points are the east and west portals, with limited intermediate access through four existing ventilation shafts (18-to-24 in. in diameter) spaced approximately 1,600-ft apart. Flow from the District’s entire wastewater collection system (approximately 80,000 residents) is gathered and conveyed through the tunnel. Flows range up to 10 MGD and cannot be shutoff or bypassed. Based on the tunnel’s age and the continuous nature of the wastewater environment, the District became increasingly concerned with the reliability of the tunnel as even a partial collapse could result in a significant spill. Two manned entry inspections were carried out in 1981 and 1991, as well as a robotic inspection in 2015. The results of these assessments showed continuing degradation. The District was determined to take a proactive approach to extend the service life of the tunnel by fully rehabilitating or replacing it. The District determined that the lack of a clear methodology and complicated nature of the work warranted a collaborative approach and chose a Design-Build delivery method. The District selected the Garney Construction/Dewberry Engineers team to develop and execute a comprehensive solution to their high-risk facility. They also engaged Burns & McDonnell Engineering Company, Inc. as their Owners Engineer. What followed was an 18-month long process of developing and discarding conventional methodologies due to constructability, cost, risk, or public impact factors. This process ultimately led to the previously untried method of slip lining the 7600-foot tunnel with no intermediate access by continuously fusing a 48-inch HDPE pipe and pulling it in using a horizontal direction drilling rig through live flow. The Colsman project offers many unique insights and lessons learned on collaborative design-build delivery approach when undertaking a project with a clear goal but with uncertain feasibility or methodologies. The insights gained from this project include: structuring a Design Build team before the construction methodology(ies) are known; developing and vetting alternative approaches with input from the Owner; engaging specialty trades/resources to vet/refine/cost untried methodologies; maintaining Owner engagement and endorsement on design progression when the proposed unconventional construction techniques are in-progress and evolving; and shifting gears and achieving success when a selected construction approach proves to be unfeasible.