C+S March 2021 Vol. 7 Issue 3 (web)

HDD technology has rarely been used in sliplining projects for water/wastewater pipeline rehabilitation, and never for a pipe as long as the Colsman Tunnel. In this photo, the upstream east portal HDD machine is in place with the drill stem extending over a stream into the tunnel.

Rehabbing the Colsman Tunnel Design-Build Sliplining Approach Saves Time and Money By Konnor Bursaw, PE

the district sought to minimize impact on the community, including vehicular traffic. Despite these concerns, Southgate recognized that a standard tunneling project with a redundant parallel system would be a tried and tested solution. If necessary, the district was prepared to commit to a major capital improvement project—in the range of $30 million—that would take two to three years to complete. First, however, Southgate asked the design-build team to carefully consider all of the available engineering options and determine if other cost-effective remedies were feasible. Dewberry and Garney Construction assessed several alternatives, including the traditional approach to bore a new tunnel as well as trenchless alternatives such as cured-in-place pipe, cementitious and geopolymer sprays, rehabilitation using epoxies and poly-hybrid prod- ucts, and rehabilitation using plastic and composite liners. Sliplining options included the installation of FPVC, fiberglass, or high-density polyethylene (HDPE) carrier pipes. After carefully weighing the benefits and calculated risks, the team ultimately recommended a sliplining option with a 48-inch HDPE con- tinuous pipe using a horizontal directional drilling (HDD) system. The district would avoid boring a second tunnel, and, with opportunities for bypass pumping limited, the sliplining option would also facilitate installation during live sewer flows. The team recognized the challenges: the HDD sliplining approach had rarely been used for water and wastewater projects, and never for a tunnel of this length. This would be a breakthrough project; there were

The Colsman Tunnel runs underneath the community of Centennial, Colorado, a suburb located approximately 15 miles south of Denver. The tunnel is a critical infrastructure component in the Southgate Water and Sanitation District’s sanitary sewer system, which serves more than 80,000 customers. Constructed in the mid-1970s, the pipe runs 7,614 lineal feet through hard claystone to collect and convey 100 percent of the district’s 12.4-MGD sanitary sewer peak flow. In 2015, Southgate deployed a multi-sensor robotic inspection camera to verify suspected defects in the tunnel such as lining failures, spall- ing, and visible surface aggregate. The floating camera confirmed that the semi-elliptical tunnel, which reaches a maximum depth of 90 feet, had deteriorated due to decades of conveying corrosive wastewater. A replacement or major rehabilitation was clearly in order. An Innovative Design-Build Solution Southgate engaged the design-build team of Dewberry and Garney Construction, with Burns & McDonnell serving as owner’s representa- tive, to evaluate options and move forward with the project as quickly as possible. In addition to concerns about the budget and schedule,


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