Design-Build Sliplining Approach Saves Time and Money
By Konnor Bursaw, PE
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, spalling, 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 representative, to evaluate options and move forward with the project as quickly as possible. In addition to concerns about the budget and schedule, 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 products, 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 continuous 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 no prior case studies to review and little in the way of industry guidelines. The design-build approach, with construction beginning before the design was complete, created additional pressure but also yielded a clear benefit: the team had worked closely together for many years on design-build water/wastewater projects and had confidence in its ability to problem-solve on site.
Challenges early on included determining the exact geometry of the existing tunnels without reliable records. The team also focused on identifying pipe loading and stresses that would occur as the HDPE pipeline was installed in a 1.5-mile-long tunnel with no intermediate access in live flow.
The fused HDPE pipe was fed into the tunnel at the western portal, with an HDD rig set up at the eastern portal for the pull-in. The pulling of the new carrier pipe into the host pipe proceeded smoothly, as pull forces remained low, and this phase of the project was completed in just over two months.
The most imposing challenge arrived with the next step: grouting the annular space between the pipe and the old tunnel. A barometric loop was used to keep the pipeline full during grouting to mitigate thermal expansion and pipeline buoyancy. The grout placement was verified with specially built cameras. As this step carried the risk of collapse or water infiltration, the team selected a low-density cellular grout, a form of extremely lightweight concrete that had never been used for this application over such a long length before.
As the grout was pushed through the tunnel, however, air bubbles began to pop, decreasing the volume of the grout. The team paused the grouting process and considered remedies, ultimately selecting a specialty drilling company with lightweight equipment to install new injection points at approximately 500-foot intervals. Although there were concerns about drilling the verticals and potentially puncturing the pipe, the drillers were able to stop within a foot of the pipe, and the grout was safely installed along the length of the entire pipe.
The rehabilitation of the tunnel was completed in just six months with an $8 million price tag. The compressed construction timeline minimized the impact on the community, with just one temporary road closure. Although the project was complex from a construction standpoint, the approach offered a much smaller footprint and saved the district from the expense of boring a new tunnel and installing a redundant sanitary sewer line.
Konnor Bursaw is a project engineer in Dewberry’s Denver office.